Abstract: Observations with the Five-Hundred-Meter Aperture Spherical Telescope have revealed the presence of a marginally-resolved source of 21 cm emission from a location $\sim50'$ from the M94 galaxy, without a stellar counterpart down to the surface brightness limit of the DESI Imaging Legacy Survey ($\sim29.15$ mag arcsec$^{-2}$ in the $g$ band). The system (hereafter Cloud-9) has round column density isocontours and a line width consistent with thermal broadening from gas at $T\sim2\times10^4$ $K$. These properties are unlike those of previously detected dark HI clouds and similar to the expected properties of REionization-Limited-HI Cloud (RELHICs), namely, starless dark matter (DM) halos filled with gas in hydrostatic equilibrium and in thermal equilibrium with the cosmic ultraviolet background. At the distance of M94, $d\sim4.7$ Mpc, we find that Cloud-9 is consistent with being a RELHIC inhabiting a Navarro-Frenk-White (NFW) DM halo of mass, $M_{200}\sim5\times10^{9}$ $M_{\odot}$, and concentration, $c_{\rm NFW}\sim13$. Although the agreement between the model and observations is good, Cloud-9 appears to be slightly, but systematically, more extended than expected for $\Lambda$CDM RELHICs. This may imply either that Cloud-9 is much closer than implied by its recessional velocity, $v_{\rm CL9}\sim300$ km s$^{-1}$, or that its halo density profile is flatter than NFW, with a DM mass deficit greater than a factor of $10$ at radii $r\lesssim1$ kpc. Further observations may aid in constraining these scenarios better and help elucidate whether Cloud-9 is the first ever observed RELHIC, a cornerstone prediction of the $\Lambda$CDM model on the smallest scales.
A thermal-kinetic subgrid model for supernova feedback in simulations of galaxy formation
Monthly Notices of the Royal Astronomical Society,
523,
3709, (2023).
Authors:
Chaikin, Evgenii
Schaye, Joop
Schaller, Matthieu
Benítez-Llambay, Alejandro;
Nobels, Folkert S. J.
Ploeckinger, Sylvia
Abstract: We present a subgrid model for supernova feedback designed for cosmological simulations of galaxy formation that may include a cold interstellar medium (ISM). The model uses thermal and kinetic channels of energy injection, which are built upon the stochastic kinetic and thermal models for stellar feedback used in the OWLS and EAGLE simulations, respectively. In the thermal channel, the energy is distributed statistically isotropically and injected stochastically in large amounts per event, which minimizes spurious radiative energy losses. In the kinetic channel, we inject the energy in small portions by kicking gas particles in pairs in opposite directions. The implementation of kinetic feedback is designed to conserve energy, linear and angular momentum, and is statistically isotropic. To test the model, we run simulations of isolated Milky Way-mass and dwarf galaxies, in which the gas is allowed to cool down to 10 K. Using the thermal and kinetic channels together, we obtain smooth star formation histories and powerful galactic winds with realistic mass loading factors. Furthermore, the model produces spatially resolved star formation rates (SFRs) and velocity dispersions that are in agreement with observations. We vary the numerical resolution by several orders of magnitude and find excellent convergence of the global SFRs and wind mass loading. We show that large thermal energy injections generate a hot phase of the ISM and modulate the star formation by ejecting gas from the disc, while the low-energy kicks increase the turbulent velocity dispersion in the neutral ISM, which in turn helps suppress star formation.
The impact and response of minihalos and the inter-halo medium on cosmic reionization
arXiv e-prints,
arXiv:2305.04959, (2023).
Authors:
Chan, Tsang Keung
Benitez-Llambay, Alejandro;
Theuns, Tom
Frenk, Carlos
Bower, Richard
Abstract: An ionization front (I-front) that propagates through an inhomogeneous medium is slowed down by self-shielding and recombinations. We perform cosmological radiation hydrodynamics simulations of the I-front propagation during the epoch of cosmic reionization. The simulations resolve gas in minihalos (halo mass $10^4\lesssim M_h[{\rm M}_\odot]\lesssim 10^8)$ that could dominate recombinations, in a computational volume that is large enough to sample the abundance of such halos. The numerical resolution is sufficient (gas particle mass $\sim 20{\rm M}_\odot$, spatial resolution $< 0.1\;{\rm ckpc}$) to allow accurate modelling of the hydrodynamic response of gas to photo-heating. We quantify the photo-evaporation time of minihalos as a function of $M_h$ and its dependence on the photo-ionization rate, $\Gamma_{-12}$, and the redshift of reionization, $z_i$. The recombination rate can be enhanced over that of a uniform medium by a factor $\sim 10-20$ early on. The peak value increases with $\Gamma_{-12}$ and decreases with $z_i$, due to the enhanced contribution from minihalos. The clumping factor, $c_r$, decreases to a factor of a few at $\sim 100\;{\rm Myr}$ after the passage of the I-front when the minihalos have been photo-evaporated; this asymptotic value depends only weakly on $\Gamma_{-12}$. Recombinations increase the required number of photons per baryon to reionize the Universe by 20-100 per cent, with the higher value occurring when $\Gamma_{-12}$ is high and $z_i$ is low. We complement the numerical simulations with simple analytical models for the evaporation rate and the inverse Strömgren layer. The study also demonstrates the proficiency and potential of SPHM1RT to address astrophysical problems in high-resolution cosmological simulations.
The diversity of rotation curves of simulated galaxies with cusps and cores
Monthly Notices of the Royal Astronomical Society,
521,
1316, (2023).
Authors:
Roper, Finn A.
Oman, Kyle A.
Frenk, Carlos S.
Benítez-Llambay, Alejandro;
Navarro, Julio F.
Santos-Santos, Isabel M. E.
Abstract: We use ΛCDM cosmological hydrodynamical simulations to explore the kinematics of gaseous discs in late-type dwarf galaxies. We create high-resolution 21-cm 'observations' of simulated dwarfs produced in two variations of the EAGLE galaxy formation model: one where supernova-driven gas flows redistribute dark matter and form constant-density central 'cores', and another where the central 'cusps' survive intact. We 'observe' each galaxy along multiple sightlines and derive a rotation curve for each observation using a conventional tilted-ring approach to model the gas kinematics. We find that the modelling process introduces systematic discrepancies between the recovered rotation curve and the actual circular velocity curve driven primarily by (i) non-circular gas orbits within the discs; (ii) the finite thickness of gaseous discs, which leads to overlap of different radii in projection; and (iii) departures from dynamical equilibrium. Dwarfs with dark matter cusps often appear to have a core, whilst the inverse error is less common. These effects naturally reproduce an observed trend which other models struggle to explain: late-type dwarfs with more steeply rising rotation curves appear to be dark matter-dominated in the inner regions, whereas the opposite seems to hold in galaxies with core-like rotation curves. We conclude that if similar effects affect the rotation curves of observed dwarfs, a late-type dwarf population in which all galaxies have sizeable dark matter cores is most likely incompatible with current measurements.
The cosmic UV background and the beginning and end of star formation in simulated field dwarf galaxies
Monthly Notices of the Royal Astronomical Society,
519,
1425, (2023).
Authors:
Pereira-Wilson, Matthew
Navarro, Julio F.
Benítez-Llambay, Alejandro;
Santos-Santos, Isabel
Abstract: We use the APOSTLE cosmological simulations to examine the role of the cosmic UV background in regulating star formation (SF) in low-mass Lambda cold dark matter (ΛCDM) haloes. In agreement with earlier work, we find that after reionization SF proceeds mainly in haloes whose mass exceeds a redshift-dependent 'critical' mass, Mcrit, set by the structure of the haloes and by the thermal pressure of UV-heated gas. Mcrit increases from $\sim 10^{8}\, \mathrm{M}_\odot$ at z ~ 10 to $M_{\rm crit}\sim 10^{9.7}\, \mathrm{M}_\odot$ at z = 0, roughly following the average mass growth of haloes in that mass range. This implies that haloes well above or below critical at present have remained so since early times. Haloes of luminous dwarfs today were already above-critical and star forming at high redshift, explaining naturally the ubiquitous presence of ancient stellar populations in dwarfs, regardless of luminosity. The SF history of systems close to the critical boundary is more complex. SF may cease or reignite in dwarfs whose host halo falls below or climbs above the critical boundary, suggesting an attractive explanation for the episodic nature of SF in some dwarfs. Also, some subcritical haloes today may have been above critical in the past; these systems should at present make up a sizable population of faint field dwarfs lacking ongoing star formation. Although few such galaxies are currently known, the discovery of this population would provide strong support for our results. Our work indicates that, rather than stellar feedback, it is the ionizing UV background and mass accretion history what regulates SF in the faintest dwarfs.
The present-day gas content of simulated field dwarf galaxies
Monthly Notices of the Royal Astronomical Society,
518,
6305, (2023).
Authors:
Herzog, Georg
Benítez-Llambay, Alejandro;
Fumagalli, Michele
Abstract: We examine the gas content of field dwarf galaxies in a high-resolution cosmological simulation. In agreement with previous work, we find that galaxies inhabiting dark matter haloes with mass below a critical value, M200 ≲ Mcrit ≈ 5 × 109 M⊙, are quiescent at the present day. The gas content of these galaxies is thus insensitive to feedback from evolving stars. Almost half of these quiescent systems today have gas masses much smaller than that expected for their mass. We find that gas-deficient galaxies originate from (1) past interactions with massive hosts, in which a dwarf loses gas and dark matter via tidal and ram-pressure forces; and (2) from hydrodynamic interactions with the gaseous filaments and sheets of the cosmic web, in which a dwarf loses gas via ram pressure. We refer to these systems as 'flybys' and 'COSWEBs'. Flybys locate in high-density regions, tracing the location of the most massive galaxies in the simulation. In contrast, COSWEBs are dispersed throughout the volume and trace the cosmic web. For sub-critical systems, M200 < Mcrit, the fraction of COSWEB galaxies can be as high as $35 {{\ \rm per\ cent}}$, and much higher for flybys, which make up 100 per cent of the galaxies with $M_{200}\lt 3\times 10^8 \ \rm M_{\odot }$. The deficit of gas caused by these mechanisms may preclude the detection of a large fraction of field dwarfs in future H I surveys. For galaxies inhabiting haloes with mass M200 > Mcrit, we find that cosmic web stripping, on average, shuts down star formation in more than $70{{\ \rm per\ cent}}$ of the affected systems.
Simulations of the reionization of the clumpy intergalactic medium with a novel particle-based two-moment radiative transfer scheme
The Predictive Power of Computational Astrophysics as a Discover Tool,
362,
15, (2023).
Authors:
Chan, Tsang Keung
Benitez-Llambay, Alejandro;
Theuns, Tom
Frenk, Carlos
Abstract: The progress of cosmic reionization depends on the presence of over-dense regions that act as photon sinks. Such sinks may slow down ionization fronts as compared to a uniform intergalactic medium (IGM) by increasing the clumping factor. We present simulations of reionization in a clumpy IGM resolving even the smallest sinks. The simulations use a novel, spatially adaptive and efficient radiative transfer implementation in the SWIFT SPH code, based on the two-moment method. We find that photon sinks can increase the clumping factor by a factor of ∼10 during the first ∼100 Myrs after the passage of an ionization front. After this time, the clumping factor decreases as the smaller sinks photoevaporate. Altogether, photon sinks increase the number of photons required to reionize the Universe by a factor of η ∼2, as compared to the homogeneous case. The value of η also depends on the emissivity of the ionizing sources.
Galactic satellite systems in CDM, WDM and SIDM
Monthly Notices of the Royal Astronomical Society,
517,
5627, (2022).
Authors:
Forouhar Moreno, Victor J.
Benítez-Llambay, Alejandro;
Cole, Shaun
Frenk, Carlos
Abstract: We investigate the population of bright satellites ($M_{*} \ge 10^{5} \, \mathrm{M}_{\odot }$) of haloes of mass comparable to that of the Milky Way in cosmological simulations in which the dark matter (DM) is either cold, warm, or self-interacting (CDM, WDM, and SIDM, respectively). The nature of the DM gives rise to differences in the abundance and structural properties of field haloes. In WDM, the main feature is a reduction in the total number of galaxies that form, reflecting a suppression of low-mass DM haloes and lower galaxy formation efficiency compared to CDM. For SIDM, the changes are structural, restricted to the central regions of haloes and dependent on the assumed self-interaction cross-section. We also consider different baryonic subgrid physics models for galaxy formation, in which supernova gas blowouts can or cannot induce the formation of a core in dwarf galaxies. Overall, the inclusion of baryons lessen the differences in the halo properties in the different DM models compared to DM-only simulations. This affects the satellite properties at infall and therefore their subsequent tidal stripping and survival rates. None the less, we find slightly less concentrated satellite radial distributions as the SIDM cross-section increases. Unfortunately, we also find that the satellite populations in simulations with baryon-induced cores in CDM and WDM can mimic the results found in SIDM, making the satellite stellar mass and maximum circular velocity functions heavily degenerate on the assumed nature of the DM and the adopted subgrid modelling. These degeneracies preclude using the brightest satellites of the Milky Way to constrain the nature of DM.
Baryon-driven decontraction in Milky Way-mass haloes
Monthly Notices of the Royal Astronomical Society,
511,
3910, (2022).
Authors:
Forouhar Moreno, Victor J.
Benítez-Llambay, Alejandro;
Cole, Shaun
Frenk, Carlos
Abstract: We select a sample of Milky Way (MW) mass haloes from a high-resolution version of the EAGLE simulation to study their inner dark matter (DM) content and how baryons alter it. As in previous studies, we find that all haloes are more massive at the centre compared to their dark matter-only (DMO) counterparts at the present day as a result of the dissipational collapse of baryons during the assembly of the galaxy. However, we identify two processes that can reduce the central halo mass during the evolution of the galaxy. First, gas blowouts induced by active galactic nuclei feedback can lead to a substantial decrease of the central DM mass. Secondly, the formation of a stellar bar and its interaction with the DM can induce a secular expansion of the halo; the rate at which DM is evacuated from the central region by this process is related to the average bar strength, and the time-scale on which it acts determines how much the halo has decontracted. Although the inner regions of the haloes we have investigated are still more massive than their DMO counterparts at z = 0, they are significantly less massive than in the past and less massive than expected from the classic adiabatic contraction model. Since the MW has both a central supermassive black hole and a bar, the extent to which its halo has contracted is uncertain. This may affect estimates of the mass of the MW halo and of the expected signals in direct and indirect DM detection experiments.
Abstract: We use a robust analytical model together with a high-resolution hydrodynamical cosmological simulation to demonstrate that in a Lambda cold dark matter (ΛCDM) universe, a small fraction of dwarf galaxies inhabiting dark matter (DM) halos in the mass range 3 × 109 ≲ M200/M⊙ ≲ 1010 form unusually late (z < 3) compared to the bulk population of galaxies. These galaxies originate from the interplay between the stochastic growth of DM halos and the existence of a time-dependent DM halo mass below which galaxies do not form. The formation epoch of the simulated late-forming galaxies traces remarkably well the time when their host DM halos first exceeded a nontrivial (but well-understood) time-dependent critical mass, thus making late-forming dwarfs attractive cosmological probes with constraining power over the past growth history of their host halos. The agreement between our model and the simulation results demonstrates that the population of simulated late-forming dwarfs is a robust cosmological outcome and largely independent of the specific galaxy formation model included in the simulations provided: (1) the universe underwent cosmic reionization before ${z}_{\mathrm{re}}\sim 8;$ (2) star formation proceeds in gas that self-gravitates; and (3) galaxy formation is largely restricted to atomic-cooling halos before zre. The scarcity of massive late-forming dwarfs expected in ΛCDM implies that the great majority of bright, metal-poor, and actively star-forming dwarfs observed in our local universe-the most obvious candidates for these late-forming galaxies-cannot be undergoing their formation for the first time at the present day in a ΛCDM universe.
The detailed structure and the onset of galaxy formation in low-mass gaseous dark matter haloes
Monthly Notices of the Royal Astronomical Society,
498,
4887, (2020).
Authors:
Benitez-Llambay, Alejandro;
Frenk, Carlos
Abstract: We present a model for the formation of the first galaxies before and after the reionization of hydrogen in the early universe. In this model, galaxy formation can only take place in dark matter haloes whose mass exceeds a redshift-dependent critical value, which, before reionization, is equal (in the simplest case) to the mass at which atomic hydrogen cooling becomes effective and, after reionization, is equal to the mass above which gas cannot remain in hydrostatic equilibrium. We define the Halo Occupation Fraction (HOF) as the fraction of haloes that host a luminous galaxy as a function of halo mass. The HOF is established by the interplay between the evolution of the critical mass and the assembly history of haloes and depends on three factors: the minimum halo mass for galaxy formation before reionization, the redshift of reionization, and the intensity of the (evolving) external photoheating rate. Our fiducial model predicts a cutoff in the galaxy mass function at a present-day halo mass, $M_{200} \sim 3\times 10^{8} \, \mathrm{M}_{\odot }$ ; 100 per cent occupation at $M_{200} \gt 5\times 10^9 \, \mathrm{M}_{\odot }$ ; and a population of starless gaseous haloes of present-day mass in the range 106 ≲ M200/M⊙ ≲ 5 × 109, in which the gas is in thermal equilibrium with the ultraviolet background radiation and in hydrostatic equilibrium in the gravitational potential of the halo. The transition between HOF = 0 and HOF = 1 reflects the stochastic nature of halo mass growth. We explore how these characteristic masses vary with model assumptions and parameter values. The results of our model are in excellent agreement with cosmological hydrodynamic simulations of galaxy formation.
To β or not to β: can higher order Jeans analysis break the mass-anisotropy degeneracy in simulated dwarfs?
Monthly Notices of the Royal Astronomical Society,
498,
144, (2020).
Authors:
Genina, A.
Read, J. I.
Frenk, C. S.
Cole, S.
Benítez-Llambay, A.;
Ludlow, A. D.
Navarro, J. F.
Oman, K. A.
Robertson, A.
Abstract: We test a non-parametric higher order Jeans analysis method, GravSphere, on 32 simulated dwarf galaxies comparable to classical Local Group dwarfs like Fornax. The galaxies are selected from A Project Of Simulating The Local Environment (APOSTLE) suite of cosmological hydrodynamics simulations with cold dark matter (CDM) and self-interacting dark matter (SIDM) models, allowing us to investigate cusps and cores in density distributions. We find that, for CDM dwarfs, the recovered enclosed mass profiles have a bias of no more than 10 per cent, with a 50 per cent scatter in the inner regions and a 20 per cent scatter near the half-light radius, consistent with standard mass estimators. The density profiles are also recovered with a bias of no more than 10 per cent and a scatter of 30 per cent in the inner regions. For SIDM dwarfs, the mass and density profiles are recovered within our 95 per cent confidence intervals but are biased towards cuspy dark matter distributions. This is mainly due to a lack of sufficient constraints from the data. We explore the sources of scatter in the accuracy of the recovered profiles and suggest a χ2 statistic to separate successful models from biased ones. Finally, we show that the uncertainties on the mass profiles obtained with GravSphere are smaller than those for comparable Jeans methods and that they can be further improved if stronger priors, motivated by cosmological simulations, are placed on the velocity anisotropy. We conclude that GravSphere is a promising Jeans-based approach for modelling dark matter distributions in dwarf galaxies.
Baryonic clues to the puzzling diversity of dwarf galaxy rotation curves
Monthly Notices of the Royal Astronomical Society,
495,
58, (2020).
Authors:
Santos-Santos, Isabel M. E.
Navarro, Julio F.
Robertson, Andrew
Benítez-Llambay, Alejandro;
Oman, Kyle A.
Lovell, Mark R.
Frenk, Carlos S.
Ludlow, Aaron D.
Fattahi, Azadeh
Ritz, Adam
Abstract: We use a compilation of disc galaxy rotation curves to assess the role of the luminous component ('baryons') in the rotation curve diversity problem. As in earlier work, we find that rotation curve shape correlates with baryonic surface density: high surface density galaxies have rapidly rising rotation curves consistent with cuspy cold dark matter haloes; slowly rising rotation curves (characteristic of galaxies with inner mass deficits or 'cores') occur only in low surface density galaxies. The correlation, however, seems too weak to be the main driver of the diversity. In addition, dwarf galaxies exhibit a clear trend, from 'cuspy' systems where baryons are unimportant in the inner mass budget to 'cored' galaxies where baryons actually dominate. This trend constrains the various scenarios proposed to explain the diversity, such as (I) baryonic inflows and outflows during galaxy formation; (II) dark matter self-interactions; (III) variations in the baryonic mass structure coupled to rotation velocities through the 'mass discrepancy-acceleration relation' (MDAR); or (IV) non-circular motions in gaseous discs. Together with analytical modelling and cosmological hydrodynamical simulations, our analysis shows that each of these scenarios has promising features, but none seems to fully account for the observed diversity. The MDAR, in particular, is inconsistent with the observed trend between rotation curve shape and baryonic importance; either the trend is caused by systematic errors in the data or the MDAR does not apply. The origin of the dwarf galaxy rotation curve diversity and its relation to the structure of cold dark matter haloes remains an open issue.
The milky way total mass profile as inferred from Gaia DR2
Monthly Notices of the Royal Astronomical Society,
494,
4291, (2020).
Authors:
Cautun, Marius
Benítez-Llambay, Alejandro;
Deason, Alis J.
Frenk, Carlos S.
Fattahi, Azadeh
Gómez, Facundo A.
Grand, Robert J. J.
Oman, Kyle A.
Navarro, Julio F.
Simpson, Christine M.
Abstract: We determine the Milky Way (MW) mass profile inferred from fitting physically motivated models to the Gaia DR2 Galactic rotation curve and other data. Using various hydrodynamical simulations of MW-mass haloes, we show that the presence of baryons induces a contraction of the dark matter (DM) distribution in the inner regions, r ≲ 20 kpc. We provide an analytic expression that relates the baryonic distribution to the change in the DM halo profile. For our galaxy, the contraction increases the enclosed DM halo mass by factors of roughly 1.3, 2, and 4 at radial distances of 20, 8, and 1 kpc, respectively compared to an uncontracted halo. Ignoring this contraction results in systematic biases in the inferred halo mass and concentration. We provide a best-fitting contracted NFW halo model to the MW rotation curve that matches the data very well.1 The best-fit has a DM halo mass, $M_{200}^{\rm DM}=0.97_{-0.19}^{+0.24}\times 10^{12}\,\mathrm{M}_\odot$ , and concentration before baryon contraction of $9.4_{-2.6}^{+1.9}$ , which lie close to the median halo mass-concentration relation predicted in ΛCDM. The inferred total mass, $M_{200}^{\rm total}=1.08_{-0.14}^{+0.20} \times 10^{12}\,\mathrm{M}_\odot$ , is in good agreement with recent measurements. The model gives an MW stellar mass of $5.04_{-0.52}^{+0.43}\times 10^{10}\,\mathrm{M}_\odot$ and infers that the DM density at the Solar position is $\rho _{\odot }^{\rm DM}=8.8_{-0.5}^{+0.5}\times 10^{-3}\,\mathrm{M}_\odot \,\mathrm{pc}^{-3}\equiv 0.33_{-0.02}^{+0.02}\,\rm {GeV}\,\rm {cm}^{-3}$ . The rotation curve data can also be fitted with an uncontracted NFW halo model, but with very different DM and stellar parameters. The observations prefer the physically motivated contracted NFW halo, but the measurement uncertainties are too large to rule out the uncontracted NFW halo.
Cusp or core? Revisiting the globular cluster timing problem in Fornax
Monthly Notices of the Royal Astronomical Society,
491,
3336, (2020).
Authors:
Meadows, Noah
Navarro, Julio F.
Santos-Santos, Isabel
Benítez-Llambay, Alejandro;
Frenk, Carlos
Abstract: We use N-body simulations to revisit the globular cluster (GC) 'timing problem' in the Fornax dwarf spheroidal (dSph). In agreement with earlier work, we find that, due to dynamical friction, GCs sink to the centre of dark matter haloes with a cuspy inner density profile but 'stall' at roughly 1/3 of the core radius (rcore) in haloes with constant-density cores. The time-scales to sink or stall depend strongly on the mass of the GC and on the initial orbital radius, but are essentially the same for either cuspy (Navarro-Frenk-White) or cored haloes normalized to have the same total mass within rcore. Arguing against a cusp on the basis that GCs have not sunk to the centre is thus no different from arguing against a core, unless all clusters are today at ∼(1/3) r_core. This would imply a core radius exceeding ∼3 kpc, much larger than seems plausible in any core-formation scenario. (The average projected distance of Fornax GCs is <RGC, Fnx> ∼ 1 kpc and its effective radius is ∼700 pc.) A simpler explanation is that Fornax GCs have only been modestly affected by dynamical friction, as expected if clusters started orbiting at initial radii of the order of ∼1-2 kpc, just outside Fornax's present-day half-light radius but well within the tidal radius imprinted by Galactic tides. This is not entirely unexpected. Fornax GCs are significantly older and more metal-poor than most Fornax stars, and such populations in dSphs tend to be more spatially extended than their younger and more metal-rich counterparts. Contrary to some earlier claims, our simulations further suggest that GCs do not truly 'stall' at ∼ 0.3 r_core, but rather continue decaying towards the centre, albeit at reduced rates. We conclude that dismissing the presence of a cusp in Fornax based on the spatial distribution of its GC population is unwarranted.
The distinct stellar metallicity populations of simulated Local Group dwarfs
Monthly Notices of the Royal Astronomical Society,
488,
2312, (2019).
Authors:
Genina, Anna
Frenk, Carlos S.
Benítez-Llambay, Alejandro;
Cole, Shaun
Navarro, Julio F.
Oman, Kyle A.
Fattahi, Azadeh
Abstract: A number of Local Group dwarf galaxies are known to have two spatially segregated stellar metallicity populations, a centrally concentrated metal-rich population, and a more extended metal-poor population. In this work we discuss mechanisms that lead to the formation of two spatially segregated metallicity populations. Using a set of high-resolution hydrodynamical simulations of Local Group-like environments, we select a sample of satellite and field galaxies, spanning the stellar mass range 106-109 M⊙, that exhibit bimodality in their metallicity distributions. Among those, we identify a subsample with a strong spatial segregation in the two populations. We find three distinct mechanisms for their formation. In field dwarfs and in a small fraction of satellites, a merger causes the metal-poor stars to migrate to larger radii and encourages the available gas to sink to the centre of the dwarf. Most of the gas is subsequently blown out of the halo through star formation feedback, but the remaining gas is consumed in the formation of a metal-rich population. In the exclusive case of satellites that have retained some of their gas at infall, it is the compression of this gas by ram pressure near pericentre that triggers the formation of metal-rich stars, whilst simultaneously preventing star formation at larger radii through stripping. Additionally, in a small number of field and satellite dwarfs, interactions with gaseous filaments and other galaxies can result in the formation of a metal-rich population. Regardless of the formation mechanism, a history of mergers typically enhances the spatial segregation.
Baryon-induced dark matter cores in the EAGLE simulations
Monthly Notices of the Royal Astronomical Society,
488,
2387, (2019).
Authors:
Benítez-Llambay, Alejandro;
Frenk, Carlos S.
Ludlow, Aaron D.
Navarro, Julio F.
Abstract: We examine the formation of dark matter (DM) cores in dwarf galaxies simulated with the EAGLE model of galaxy formation. As in earlier work, we find that the star formation (SF) gas density threshold (ρth) plays a critical role. At low thresholds (LT), gas is unable to reach densities high enough to dominate the gravitational potential before being dispersed by feedback from supernovae. LT runs show little effect on the inner DM profile, even in systems with extended and bursty SF, two ingredients often cited as critical for core formation. For higher thresholds, gas is able to dominate the gravitational potential before being ejected by feedback. This can lead to a substantial reduction in the inner DM content, but only if the gas is gravitationally important over an extended period of time, allowing the halo to contract before gas removal. Rapid assembly and removal of gas in short SF bursts is less effective at altering the inner DM content. Subsequent gas accretion may draw DM back in and reform a cusp, unless SF is bursty enough to prevent it, preserving the core. Thus, for the EAGLE SF + feedback model, there is no simple relation between core formation and SF history, contrary to recent claims. The dependence of the inner DM content of dwarfs on ρth hinders robust predictions and the interpretation of observations. A simulation of a (12 Mpc)^3 volume with high ρth results in dwarfs with sizeable cores over a limited halo mass range, but with insufficient variety in mass profiles to explain the observed diversity of dwarf galaxy rotation curves.
Fluorescent rings in star-free dark matter haloes
Monthly Notices of the Royal Astronomical Society,
487,
609, (2019).
Authors:
Sykes, Calvin
Fumagalli, Michele
Cooke, Ryan
Theuns, Tom
Benítez-Llambay, Alejandro;
Abstract: Photoheating of the gas in low-mass dark matter (DM) haloes prevents baryons from cooling, leaving the haloes free of stars. Gas in these `dark' haloes remains exposed to the ultraviolet background (UVB), and so is expected to emit via fluorescent recombination lines. We present a set of radiative transfer simulations, which model dark haloes as spherical gas clouds in hydrostatic equilibrium with a DM halo potential, and in thermal equilibrium with the UVB at redshift z = 0. We use these simulations to predict surface brightnesses in H α, which we show to have a characteristic ring-shaped morphology for haloes in a narrow mass range between ≃109.5 and 10^{9.6} {M_⊙}. We explore how this emission depends on physical parameters such as the DM density profile and the UVB spectrum. We predict the abundance of fluorescent haloes on the sky, and discuss possible strategies for their detection. We demonstrate how detailed observations of fluorescent rings can be used to infer the properties of the haloes which host them, such as their density profiles and the mass-concentration relation, as well as to directly measure the UVB amplitude.
Non-circular motions and the diversity of dwarf galaxy rotation curves
Monthly Notices of the Royal Astronomical Society,
482,
821, (2019).
Authors:
Oman, Kyle A.
Marasco, Antonino
Navarro, Julio F.
Frenk, Carlos S.
Schaye, Joop
Benítez-Llambay, Alejandro;
Abstract: We use mock interferometric H I measurements and a conventional tilted-ring modelling procedure to estimate circular velocity curves of dwarf galaxy discs from the APOSTLE suite of Λ cold dark matter cosmological hydrodynamical simulations. The modelling yields a large diversity of rotation curves for an individual galaxy at fixed inclination, depending on the line-of-sight orientation. The diversity is driven by non-circular motions in the gas; in particular, by strong bisymmetric fluctuations in the azimuthal velocities that the tilted-ring model is ill-suited to account for and that are difficult to detect in model residuals. Large misestimates of the circular velocity arise when the kinematic major axis coincides with the extrema of the fluctuation pattern, in some cases mimicking the presence of kiloparsec-scale density `cores', when none are actually present. The thickness of APOSTLE discs compounds this effect: more slowly rotating extra-planar gas systematically reduces the average line-of-sight speeds. The recovered rotation curves thus tend to underestimate the true circular velocity of APOSTLE galaxies in the inner regions. Non-circular motions provide an appealing explanation for the large apparent cores observed in galaxies such as DDO 47 and DDO 87, where the model residuals suggest that such motions might have affected estimates of the inner circular velocities. Although residuals from tilted-ring models in the simulations appear larger than in observed galaxies, our results suggest that non-circular motions should be carefully taken into account when considering the evidence for dark matter cores in individual galaxies.
The innate origin of radial and vertical gradients in a simulated galaxy disc
Monthly Notices of the Royal Astronomical Society,
476,
3648, (2018).
Authors:
Navarro, Julio F.
Yozin, Cameron
Loewen, Nic
Benítez-Llambay, Alejandro;
Fattahi, Azadeh
Frenk, Carlos S.
Oman, Kyle A.
Schaye, Joop
Theuns, Tom
Abstract: We examine the origin of radial and vertical gradients in the age/metallicity of the stellar component of a galaxy disc formed in the APOSTLE cosmological hydrodynamical simulations. Some of these gradients resemble those in the Milky Way, where they have sometimes been interpreted as due to internal evolution, such as scattering off giant molecular clouds, radial migration driven by spiral patterns, or orbital resonances with a bar. Secular processes play a minor role in the simulated galaxy, which lacks strong spiral or bar patterns, and where such gradients arise as a result of the gradual enrichment of a gaseous disc that is born thick but thins as it turns into stars and settles into centrifugal equilibrium. The settling is controlled by the feedback of young stars; which links the star formation, enrichment, and equilibration time-scales, inducing radial and vertical gradients in the gaseous disc and its descendent stars. The kinematics of coeval stars evolve little after birth and provide a faithful snapshot of the gaseous disc structure at the time of their formation. In this interpretation, the age-velocity dispersion relation would reflect the gradual thinning of the disc rather than the importance of secular orbit scattering; the outward flaring of stars would result from the gas disc flare rather than from radial migration; and vertical gradients would arise because the gas disc gradually thinned as it enriched. Such radial and vertical trends might just reflect the evolving properties of the parent gaseous disc, and are not necessarily the result of secular evolutionary processes.
The core-cusp problem: a matter of perspective
Monthly Notices of the Royal Astronomical Society,
474,
1398, (2018).
Authors:
Genina, Anna
Benítez-Llambay, Alejandro;
Frenk, Carlos S.
Cole, Shaun
Fattahi, Azadeh
Navarro, Julio F.
Oman, Kyle A.
Sawala, Till
Theuns, Tom
Abstract: The existence of two kinematically and chemically distinct stellar subpopulations in the Sculptor and Fornax dwarf galaxies offers the opportunity to constrain the density profile of their matter haloes by measuring the mass contained within the well-separated half-light radii of the two metallicity subpopulations. Walker and Peñarrubia have used this approach to argue that data for these galaxies are consistent with constant-density `cores' in their inner regions and rule out `cuspy' Navarro-Frenk-White (NFW) profiles with high statistical significance, particularly in the case of Sculptor. We test the validity of these claims using dwarf galaxies in the APOSTLE (A Project Of Simulating The Local Environment) Λ cold dark matter cosmological hydrodynamic simulations of analogues of the Local Group. These galaxies all have NFW dark matter density profiles and a subset of them develop two distinct metallicity subpopulations reminiscent of Sculptor and Fornax. We apply a method analogous to that of Walker and Peñarrubia to a sample of 50 simulated dwarfs and find that this procedure often leads to a statistically significant detection of a core in the profile when in reality there is a cusp. Although multiple factors contribute to these failures, the main cause is a violation of the assumption of spherical symmetry upon which the mass estimators are based. The stellar populations of the simulated dwarfs tend to be significantly elongated and, in several cases, the two metallicity populations have different asphericity and are misaligned. As a result, a wide range of slopes of the density profile are inferred depending on the angle from which the galaxy is viewed.
HBT+: an improved code for finding subhaloes and building merger trees in cosmological simulations
Monthly Notices of the Royal Astronomical Society,
474,
604, (2018).
Authors:
Han, Jiaxin
Cole, Shaun
Frenk, Carlos S.
Benitez-Llambay, Alejandro;
Helly, John
Abstract: Dark matter subhalos are the remnants of (incomplete) halo mergers. Identifying them and establishing their evolutionary links in the form of merger trees is one of the most important applications of cosmological simulations. The HBT (Hierachical Bound-Tracing) code identifies haloes as they form and tracks their evolution as they merge, simultaneously detecting subhaloes and building their merger trees. Here we present a new implementation of this approach, HBT+ , that is much faster, more user friendly, and more physically complete than the original code. Applying HBT+ to cosmological simulations, we show that both the subhalo mass function and the peak-mass function are well fitted by similar double-Schechter functions. The ratio between the two is highest at the high-mass end, reflecting the resilience of massive subhaloes that experience substantial dynamical friction but limited tidal stripping. The radial distribution of the most-massive subhaloes is more concentrated than the universal radial distribution of lower mass subhaloes. Subhalo finders that work in configuration space tend to underestimate the masses of massive subhaloes, an effect that is stronger in the host centre. This may explain, at least in part, the excess of massive subhaloes in galaxy cluster centres inferred from recent lensing observations. We demonstrate that the peak-mass function is a powerful diagnostic of merger tree defects, and the merger trees constructed using HBT+ do not suffer from the missing or switched links that tend to afflict merger trees constructed from more conventional halo finders. We make the HBT+ code publicly available.
The vertical structure of gaseous galaxy discs in cold dark matter haloes
Monthly Notices of the Royal Astronomical Society,
473,
1019, (2018).
Authors:
Benítez-Llambay, Alejandro;
Navarro, Julio F.
Frenk, Carlos S.
Ludlow, Aaron D.
Abstract: We study the vertical structure of polytropic centrifugally supported gaseous discs embedded in cold dark matter (CDM) haloes. At fixed radius, R, the shape of the vertical density profile depends weakly on whether the disc is self-gravitating (SG) or non-self-gravitating (NSG). The disc 'characteristic' thickness, zH, set by the midplane sound speed and circular velocity, zNSG = (cs/Vc)R, in the NSG case, and by the sound speed and surface density, $z_{\rm SG} = c_{\rm s}^2/G\Sigma$, in SG discs, is smaller than zSG and zNSG. SG discs are typically Toomre unstable, NSG discs are stable. Exponential discs in CDM haloes with roughly flat circular velocity curves 'flare' outwards. Flares in mono abundance or coeval populations in galaxies like the Milky Way are thus not necessarily due to radial migration. For the polytropic equation of state of the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations, discs that match observational constraints are NSG for Md < 3 × 109 M⊙ and SG at higher masses, if fully gaseous. We test these analytic results using a set of idealized smoothed particle hydrodynamic simulations and find excellent agreement. Our results clarify the role of the gravitational softening on the thickness of simulated discs, and on the onset of radial instabilities. EAGLE low-mass discs are NSG so the softening plays no role in their vertical structure. High-mass discs are expected to be SG and unstable, and may be artificially thickened and stabilized unless gravity is well resolved. Simulations with spatial resolution high enough to not compromise the vertical structure of a disc also resolve the onset of their instabilities, but the converse is not true.
Py-SPHViewer: Cosmological simulations using Smoothed Particle Hydrodynamics
Abstract: Py-SPHViewer visualizes and explores N-body + Hydrodynamics simulations. The code interpolates the underlying density field (or any other property) traced by a set of particles, using the Smoothed Particle Hydrodynamics (SPH) interpolation scheme, thus producing not only beautiful but also useful scientific images. Py-SPHViewer enables the user to explore simulated volumes using different projections. Py-SPHViewer also provides a natural way to visualize (in a self-consistent fashion) gas dynamical simulations, which use the same technique to compute the interactions between particles.
The origin of the mass discrepancy-acceleration relation in ΛCDM
Monthly Notices of the Royal Astronomical Society,
471,
1841, (2017).
Authors:
Navarro, Julio F.
Benítez-Llambay, Alejandro;
Fattahi, Azadeh
Frenk, Carlos S.
Ludlow, Aaron D.
Oman, Kyle A.
Schaller, Matthieu
Theuns, Tom
Abstract: We examine the origin of the mass discrepancy-radial acceleration relation (MDAR) of disc galaxies. This is a tight empirical correlation between the disc centripetal acceleration and that expected from the baryonic component. The MDAR holds for most radii probed by disc kinematic tracers, regardless of galaxy mass or surface brightness. The relation has two characteristic accelerations: a0, above which all galaxies are baryon dominated, and amin, an effective minimum acceleration probed by kinematic tracers in isolated galaxies. We use a simple model to show that these trends arise naturally in Λ cold dark matter (ΛCDM). This is because (I) disc galaxies in ΛCDM form at the centre of dark matter haloes spanning a relatively narrow range of virial mass; (II) cold dark matter halo acceleration profiles are self-similar and have a broad maximum at the centre, reaching values bracketed precisely by amin and a0 in that mass range and (III) halo mass and galaxy size scale relatively tightly with the baryonic mass of a galaxy in any successful ΛCDM galaxy formation model. Explaining the MDAR in ΛCDM does not require modifications to the cuspy inner mass profiles of dark haloes, although these may help to understand the detailed rotation curves of some dwarf galaxies and the origin of extreme outliers from the main relation. The MDAR is just a reflection of the self-similar nature of cold dark matter haloes and of the physical scales introduced by the galaxy formation process.
Mass-Discrepancy Acceleration Relation: A Natural Outcome of Galaxy Formation in Cold Dark Matter Halos
Physical Review Letters,
118,
161103, (2017).
Authors:
Ludlow, Aaron D.
Benítez-Llambay, Alejandro;
Schaller, Matthieu
Theuns, Tom
Frenk, Carlos S.
Bower, Richard
Schaye, Joop
Crain, Robert A.
Navarro, Julio F.
Fattahi, Azadeh
Oman, Kyle A.
Abstract: We analyze the total and baryonic acceleration profiles of a set of well-resolved galaxies identified in the eagle suite of hydrodynamic simulations. Our runs start from the same initial conditions but adopt different prescriptions for unresolved stellar and active galactic nuclei feedback, resulting in diverse populations of galaxies by the present day. Some of them reproduce observed galaxy scaling relations, while others do not. However, regardless of the feedback implementation, all of our galaxies follow closely a simple relationship between the total and baryonic acceleration profiles, consistent with recent observations of rotationally supported galaxies. The relation has small scatter: Different feedback implementations—which produce different galaxy populations—mainly shift galaxies along the relation rather than perpendicular to it. Furthermore, galaxies exhibit a characteristic acceleration g†, above which baryons dominate the mass budget, as observed. These observations, consistent with simple modified Newtonian dynamics, can be accommodated within the standard cold dark matter paradigm.
The properties of `dark' ΛCDM haloes in the Local Group
Monthly Notices of the Royal Astronomical Society,
465,
3913, (2017).
Authors:
Benítez-Llambay, Alejandro;
Navarro, Julio F.
Frenk, Carlos S.
Sawala, Till
Oman, Kyle
Fattahi, Azadeh
Schaller, Matthieu
Schaye, Joop
Crain, Robert A.
Theuns, Tom
Abstract: We examine the baryon content of low-mass Λ cold dark matter (ΛCDM) haloes (108 < M200/M⊙ < 5 × 109) using the APOSTLE cosmological hydrodynamical simulations. Most of these systems are free of stars and have a gaseous content set by the combined effects of cosmic reionization, which imposes a mass-dependent upper limit, and of ram-pressure stripping, which reduces it further in high-density regions. Haloes mainly affected by reionization (RELHICS; REionization-Limited H I Clouds) inhabit preferentially low-density regions and make up a population where the gas is in hydrostatic equilibrium with the dark matter potential and in thermal equilibrium with the ionizing UV background. Their thermodynamic properties are well specified, and their gas density and temperature profiles may be predicted in detail. Gas in RELHICS is nearly fully ionized but with neutral cores that span a large range of H I masses and column densities and have negligible non-thermal broadening. We present predictions for their characteristic sizes and central column densities; the massive tail of the distribution should be within reach of future blind H I surveys. Local Group RELHICS (LGRs) have some properties consistent with observed Ultra Compact High Velocity Clouds (UCHVCs) but the sheer number of the latter suggests that most UCHVCs are not RELHICS. Our results suggest that LGRs (I) should typically be beyond 500 kpc from the Milky Way or M31; (II) have positive Galactocentric radial velocities; (III) H I sizes not exceeding 1 kpc, and (IV) should be nearly round. The detection and characterization of RELHICS would offer a unique probe of the small-scale clustering of CDM.
Mergers and the outside-in formation of dwarf spheroidals
Monthly Notices of the Royal Astronomical Society,
456,
1185, (2016).
Authors:
Benítez-Llambay, A.;
Navarro, J. F.
Abadi, M. G.
Gottlöber, S.
Yepes, G.
Hoffman, Y.
Steinmetz, M.
Abstract: We use a cosmological simulation of the formation of the Local Group to explore the origin of age and metallicity gradients in dwarf spheroidal galaxies. We find that a number of simulated dwarfs form `outside-in', with an old, metal-poor population that surrounds a younger, more concentrated metal-rich component, reminiscent of dwarf spheroidals like Sculptor or Sextans. We focus on a few examples where stars form in two populations distinct in age in order to elucidate the origin of these gradients. The spatial distributions of the two components reflect their diverse origin; the old stellar component is assembled through mergers, but the young population forms largely in situ. The older component results from a first episode of star formation that begins early but is quickly shut off by the combined effects of stellar feedback and reionization. The younger component forms when a late accretion event adds gas and reignites star formation. The effect of mergers is to disperse the old stellar population, increasing their radius and decreasing their central density relative to the young population. We argue that dwarf-dwarf mergers offer a plausible scenario for the formation of systems with multiple distinct populations and, more generally, for the origin of age and metallicity gradients in dwarf spheroidals.
Abstract: This is the first official release of Py-SPHViewer. This is a publicly available Python package to visualize and explore N-Body + Hydrodynamical simulations. In particular, it was developed to visualize cosmological simulations, but it can be applied to render any set of particles. The Smoothed Particle Hydrodynamics interpolation scheme is used to interpolate the underlying density field (or any other property) traced by a finite set of particles, thus producing smooth images of a particle distribution that can be easily interpreted. The computation of the images is made fully in parallel using OpenMP, and its is almost straightforward to extend its use within distribute memory systems using mpi4py. Follow the main repository of the code here For a small hands-on tutorial check the link below: Tutorial
The imprint of reionization on the star formation histories of dwarf galaxies
Monthly Notices of the Royal Astronomical Society,
450,
4207, (2015).
Authors:
Benítez-Llambay, A.;
Navarro, J. F.
Abadi, M. G.
Gottlöber, S.
Yepes, G.
Hoffman, Y.
Steinmetz, M.
Abstract: We use a compilation of star formation histories (SFHs) and cosmological simulations to explore the impact of cosmic reionization on nearby isolated dwarf galaxies. Nearby dwarfs show a wide diversity of SFHs; from ancient systems that completed their star formation (SF) ∼10 Gyr ago to young dwarfs that formed the majority of their stars in the past ∼5 Gyr to `two-component' systems characterized by the overlap of old and young stars. As an ensemble, SF in nearby dwarfs dips to lower-than-average rates at intermediate times (4 < t/Gyr < 8), a feature caused in the simulation by cosmic reionization. Reionization heats the gas and drives it out of low-mass haloes, affecting especially systems with virial temperatures of ∼2 × 104 K at zreion. SF begins before zreion in systems above this threshold; its associated feedback compounds the effects of reionization, emptying the haloes of gas and leaving behind old stellar systems. In haloes below the threshold at zreion, reionization leads to a delay in the onset of SF that lasts until the halo grows massive enough to allow gas to cool and form stars, leading to a system with a prominent young stellar component. `Two-component' systems may be traced to late accretion events that allow young stars to form in systems slightly above the threshold at zreion. The dearth of intermediate-age stars in nearby dwarfs might be the clearest signature of the imprint of cosmic reionization on the SFHs of dwarf galaxies.
Historia del contenido bariónico de las galaxias EnanasHistoria del contenido bariónico de las galaxias EnanasThe baryon content history of dwarf galaxies;
Abstract: Using a N-body/gas-dynamical cosmological simulation of the Local Group of galaxies we identify a new mechanism that allows to remove baryons from low-mass halos without appealing to feedback or reionization.
Reddening and age of six poorly studied star clusters of the Large Magellanic Cloud derived from integrated spectra
Astronomy and Astrophysics,
565,
A49, (2014).
Authors:
Minniti, J. H.
Ahumada, A. V.
Clariá, J. J.
Benítez-Llambay, A.;
Abstract: Aims: To increase the number of studied star clusters (SCs) of the Large Magellanic Cloud (LMC), we present flux-calibrated integrated spectra in the optical range (λ = 3700-6800 Å) for six poorly studied LMC SCs of IVA type. This type corresponds to the age range between 200 and 400 Myr. We also aim at creating a new template spectrum representative of this age range at the metallicity level of the LMC. Methods: Foreground reddening E(B - V) values and ages are derived by applying the template matching method that consists of comparing the line strengths and continuum distribution of the cluster spectra with those of template cluster spectra with known properties. The equivalent width (EW) of the Balmer lines and the diagnostic diagrams involving the sum of EWs of selected spectral lines were also employed as age indicators. Results: For the first time, we provide estimates of the clusters' reddenings and ages. As expected, all the clusters appear to be of nearly the same age, their mean value being (400 ± 100) Myr, while the resulting mean E(B - V) values range between 0.00 and 0.10 mag. Conclusions: The present cluster sample complements previous ones in an effort to gather a spectral library with several clusters per age bin. By averaging the reddening-corrected integrated spectra, weighted by their signal-to-noise ratios (S/N), a new high S/N template spectrum of 400 Myr has been created. Integrated spectra for each star cluster are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/565/A49
VizieR Online Data Catalog: Integrated spectroscopy of 6 LMC clusters (Minniti+, 2014)
VizieR Online Data Catalog,
J/A+A/565/A49, (2014).
Authors:
Minniti, J. H.
Ahumada, A. V.
Claria, J. J.
Benitez-Llambay, A.;
Abstract: The observations were carried out with the Jorge Sahade 2.15m telescope at Complejo Astronomico El Leoncito (CASLEO, San Juan Argentina) during eight nights in December 2009 and 2012. We employed the REOSC spectrograph (simple mode). The slit was set in the East-West direction, and the observations were performed by scanning the slit across the objects in the North-South direction. We used a grating of 300 grooves/mm, which produces an average dispersion in the observed region of ~140Å/mm (3.46Å/pix). The useful spectral coverage was λ=3700-6800Å. Here we present the integrated spectra for the six SCs, corrected by radial velocity, and the new created template. (2 data files).
Dwarf Galaxies and the Cosmic Web
The Astrophysical Journal,
763,
L41, (2013).
Authors:
Benítez-Llambay, Alejandro;
Navarro, Julio F.
Abadi, Mario G.
Gottlöber, Stefan
Yepes, Gustavo
Hoffman, Yehuda
Steinmetz, Matthias
Abstract: We use a cosmological simulation of the formation of the Local Group of Galaxies to identify a mechanism that enables the removal of baryons from low-mass halos without appealing to feedback or reionization. As the Local Group forms, matter bound to it develops a network of filaments and pancakes. This moving web of gas and dark matter drifts and sweeps a large volume, overtaking many halos in the process. The dark matter content of these halos is unaffected but their gas can be efficiently removed by ram pressure. The loss of gas is especially pronounced in low-mass halos due to their lower binding energy and has a dramatic effect on the star formation history of affected systems. This "cosmic web stripping" may help to explain the scarcity of dwarf galaxies compared with the numerous low-mass halos expected in ΛCDM and the large diversity of star formation histories and morphologies characteristic of faint galaxies. Although our results are based on a single high-resolution simulation, it is likely that the hydrodynamical interaction of dwarf galaxies with the cosmic web is a crucial ingredient so far missing from galaxy formation models.
La abundancia de galaxias y halos de materia oscura en el universo CDM
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina,
56,
33, (2013).
Authors:
Abadi, M. G.
Benítez-Llambay, A.;
Ferrero, I.
Abstract: A long-standing puzzle of CDM cosmological model concerns to the different shape of the galaxy stellar mass function and the halo mass function on dwarf galaxy scales. Dwarf galaxies are much less numerous than halos massive enough to host them; suggesting a complex non-linear relation between the mass of a galaxy and the mass of its surrounding halo. Usually; this is reconciled by appealing to baryonic processes that can reduce the efficiency of galaxy formation in low-mass halos. Recent work applying the abundance matching technique require that virtually no dwarf galaxies form in halos with virial mass below . We use rotation curves of dwarf galaxies compiled from the literature to explore whether their total enclosed mass is consistent with these constraints. Almost one-half of the dwarfs in our sample are at odds with this restriction; they are in halos with masses substantially below . Using a cosmological simulation of the formation of the Local Group of galaxies we found that ram-pressure stripping against the cosmic web removes baryons from low-mass halos without appealing to feedback or reionization. This mechanism may help to explain the scarcity of dwarf galaxies compared with the numerous low-mass halos expected in CDM and the large diversity of star formation histories and morphologies characteristic of faint galaxies. FULL TEXT IN SPANISH
Propiedades espectrales integradas de 27 cúmulos estelares concentrados pertenecientes a la Nube Mayor de Magallanes
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina,
56,
327, (2013).
Authors:
Minniti, J. H.
Ahumada, A. V.
Clariá, J. J.
Benítez-Llambay, A.;
Abstract: We present integrated spectra in the optical range () for 27 concentrated star clusters (SCs) of the Large Magellanic Cloud (LMC). colour excesses and ages of these SCs are determined from two different methods: (1) Measurement of equivalent widths in selected spectral windows. (2) Comparison of the integrated spectra with template spectra. The current sample of integrated spectra complements previous ones in an effort to gather a spectral library with several clusters per age bin. FULL TEXT IN SPANISH
Fast Integrated Spectra Analyzer: A New Computational Tool for Age and Reddening Determination of Small Angular Diameter Open Clusters
Publications of the Astronomical Society of the Pacific,
124,
173, (2012).
Authors:
Benítez-Llambay, Alejandro;
Clariá, Juan J.
Piatti, Andrés E.
Abstract: We present a new algorithm called “Fast Integrated Spectra Analyzer” (FISA) that permits fast and reasonably accurate age and reddening determinations for small angular diameter open clusters by using their integrated spectra in the (3600-7400) Å range and currently available template spectrum libraries. This algorithm and its implementation help to achieve astrophysical results in shorter times than from other methods. A brief review is given of the integrated spectroscopic technique applied to the study of open clusters, as well as the basic assumptions that justify its use. We describe the numerical algorithm employed in detail, show examples of its application, and provide a link to the code. Our method has successfully been applied to integrated spectroscopy of open clusters, both in the Galaxy and in the Magellanic Clouds, to determine ages and reddenings.
Abstract: FISA (Fast Integrated Spectra Analyzer) permits fast and reasonably accurate age and reddening determinations for small angular diameter open clusters by using their integrated spectra in the (3600-7400) AA range and currently available template spectrum libraries. This algorithm and its implementation help to achieve astrophysical results in shorter times than from other methods. FISA has successfully been applied to integrated spectroscopy of open clusters, both in the Galaxy and in the Magellanic Clouds, to determine ages and reddenings.
Las Galaxias Enanas y la Red Cósmica
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina,
55,
245, (2012).
Abstract: Using a cosmological numerical simulation of the Local Group formation, we identify a mechanism able to remove gas from low-mass halos, affecting dramatically their star formation histories. As the Local Group forms, matter bound to it develops an intrincate network of filaments and pancakes, which sweeps a large volume, overtaking and removing the gas of several halos, leaving their dark matter component unaffected. The gas loss is particulary pronunced for low-mass halos, which may help to explain the scarcity of dwarf galaxies observed when compared to the large number of low-mass halos expected in the ΛCDM cosmological model. FULL TEXT IN SPANISH
Determinación de edades y enrojecimientos de cúmulos estelares pertenecientes a la Nube Mayor de Magallanes
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina,
55,
119, (2012).
Authors:
Minniti, J. H.
Ahumada, A. V.
Clariá, J. J.
Benítez-Llambay, A.;
Oddone, M. A.
Bica, E.
Santos, J. F. C., Jr.
Abstract: We present flux-calibrated integrated spectra in the optical range obtained with the REOSC spectrograph (CASLEO, Argentina) for 9 concentrated stellar clusters belonging to the Large Magellanic Cloud, one of which was not previously studied. We estimate simultaneously age and foreground reddening by using the algorithm FISA. For the studied sample, we derive ages between 2 Myr and 350 Myr and foreground reddening E(B-V) values in the range 0.00--0.23 mag. These values are mostly in agreement with the ones found by other authors. FULL TEXT IN SPANISH
EFOSC2/NTT integrated spectroscopy of ten Magellanic Clouds' stellar clusters
Revista Mexicana de Astronomia y Astrofisica Conference Series,
40,
264, (2011).
Authors:
Ahumada, A. V.
Benítez Llambay, A.;
Santos, J. F. C., Jr.
Clariá, J. J.
Bica, E.
Piatti, A. E.
Abstract: We present integrated spectra in the optical range for 10 concentrated Magellanic Clouds' (MCs) stellar clusters (SCs), six of them belonging to the Large Magellanic Cloud (LMC), and the other four to the Small Magellanic Cloud (SMC). We estimate simultaneously age and foreground reddening by comparing the continuum distribution and line strengths of the cluster spectra with those of template spectra. The present cluster sample complements previous ones, in an effort to create a spectral library for the MCs with several clusters per age bin.
Espectros integrados EFOSC2/NTT de 10 cúmulos estelares pertenecientes a las Nubes de Magallanes
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina,
54,
171, (2011).
Authors:
Ahumada, A. V.
Benítez-Llambay, A.;
Santos, J. F. C., Jr.
Clariá, J. J.
Bica, E.
Piatti, A. E.
Abstract: We present integrated spectra in the optical range for 10 concentrated Mag- ellanic Clouds' (MCs) stellar clusters, six of them belonging to the Large Magellanic Cloud, and the other four to the Small Magellanic Cloud. The spectra were obtained with the EFOSC2 spectrograph (NTT-ESO, La Silla, Chile). We estimate simultaneously age and foreground reddening by com- paring the continuum distribution and line strengths of the cluster spectra with those of template spectra. The present cluster sample complements previous ones, in an effort to create a spectral library for the MCs with sev- eral clusters per age bin. FULL TEXT IN SPANISH
Determinación espectroscópica de parámetros fundamentales de 18 cúmulos abiertos de pequeño diámetro angular
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina,
53,
161, (2010).
Authors:
Benítez-Llambay, A.;
Piatti, A. E.
Clariá, J. J.
Palma, T.
Ahumada, A. V.
Abstract: Following specific criteria, we have carefully selected objects catalogued as galactic open cluster candidates. Out of a total of 58 small angular diameter selected objects without previous studies, we obtained integrated spectra of 18 of them at CASLEO (San Juan, Argentina). Using a computational tool specially developed for the analysis of integrated spectra, reddenings and ages of the observed clusters were determined for the first time. FULL TEXT IN SPANISH