arXiv subjects

Lucio Mayer

Publications and source records attributed to Lucio Mayer.

At least 181 records · Page 10Linked to original sources

arXiv2013

The circumgalactic medium of massive galaxies at z~3: a test for stellar feedback, galactic outflows, and cold streams

We present new results on the kinematics, thermal and ionization state, and spatial distribution of metal-enriched gas in the circumgalactic medium (CGM) of massive galaxies at redshift 3, using the "Eris" suite of cosmological "zoom-in" simulations. The reference run adopts a blastwave scheme for supernova feedback that produces galactic outflows, a star formation recipe based on a high gas density threshold, metal-dependent radiative cooling, and a model for the diffusion of metals and thermal energy. Synthetic spectra through the multiphase CGM produce interstellar absorption line strengths of Lya, CII, CIV, SiII, and SiIV as a function of galactocentric impact parameter (scaled to the virial radius) that are in broad agreement with those observed at high-redshift by Steidel et al. (2010). Only about one third of all the gas within R_vir is outflowing. The fraction of sightlines within one virial radius that intercept optically thick material is 27%, in agreement with recent observations by Rudie et al. (2012). Such optically thick absorption is shown to trace inflowing "cold" streams that penetrate deep inside the virial radius. The streams, enriched to metallicities above 0.01 solar, give origin to strong (log N > 13) CII absorption with a covering factor of 22% within R_vir and 10% within 2 R_vir. Galactic outflows do not cause any substantial suppression of the cold accretion mode. The central galaxy is surrounded by a large OVI halo, with a typical column density log N>14 and a near unity covering factor maintained all the way out to 150 kpc. This matches the trends recently observed in star-forming galaxies at low redshift by Tumlinson et al. (2011). Our zoom-in simulations of this single system appear to reproduce quantitatively the complex baryonic processes that determine the exchange of matter, energy, and metals between galaxies and their surroundings. (Abridged)

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arXiv2013

An Off-center Density Peak in the Milky Way's Dark Matter Halo?

We show that the position of the central dark matter density peak may be expected to differ from the dynamical center of the Galaxy by several hundred parsec. In Eris, a high resolution cosmological hydrodynamics simulation of a realistic Milky-Way-analog disk galaxy, this offset is 300 - 400 pc (~3 gravitational softening lengths) after z=1. In its dissipationless dark-matter-only twin simulation ErisDark, as well as in the Via Lactea II and GHalo simulations, the offset remains below one softening length for most of its evolution. The growth of the DM offset coincides with a flattening of the central DM density profile in Eris inwards of ~1 kpc, and the direction from the dynamical center to the point of maximum DM density is correlated with the orientation of the stellar bar, suggesting a bar-halo interaction as a possible explanation. A dark matter density offset of several hundred parsec greatly affects expectations of the dark matter annihilation signals from the Galactic Center. It may also support a dark matter annihilation interpretation of recent reports by Weniger (2012) and Su & Finkbeiner (2012) of highly significant 130 GeV gamma-ray line emission from a region 1.5 degrees (~200 parsec projected) away from Sgr A* in the Galactic plane.

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arXiv2013

Tidal Stirring of Disky Dwarfs with Shallow Dark Matter Density Profiles: Enhanced Transformation into Dwarf Spheroidals

(Abridged) The origin of dSphs in the Local Group (LG) remains an enigma. The tidal stirring model posits that late-type, rotationally-supported dwarfs resembling present-day dwarf irregular (dIrr) galaxies can transform into dSphs via interactions with Milky Way-sized hosts. Using collisionless N-body simulations, we investigate for the first time how tidal stirring depends on the dark matter (DM) density distribution in the central stellar region of the progenitor disky dwarf. Specifically, we explore various asymptotic inner slopes gamma of the dwarf DM density profiles (rho \propto r^{-gamma} as r -> 0). For a given orbit inside the primary, rotationally-supported dwarfs embedded in DM halos with core-like density distributions (gamma = 0.2) and mild density cusps (gamma = 0.6) demonstrate a substantially enhanced likelihood and efficiency of transformation into dSphs compared to their counterparts with steeper DM density profiles (gamma = 1). Such shallow DM distributions are akin to those of observed dIrrs, highlighting tidal stirring as a plausible model for the LG morphology-density relation. When gamma <1, a single pericentric passage can induce dSph formation and disky dwarfs on low-eccentricity or large-pericenter orbits are able to transform into dSphs; these new results allow the tidal stirring model to explain the existence of virtually all known dSphs across a wide range of distances from their hosts. A subset of rotationally-supported dwarfs with gamma <1 are eventually disrupted by the primary; those that survive as dSphs are generally on orbits that are biased towards lower eccentricities and/or larger pericenters relative to those of typical CDM satellites. The latter could explain the rather peculiar orbits of several classic LG dSphs such as Fornax, Leo I, Tucana, and Cetus.

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arXiv2012

Hydrodynamics of galaxy mergers with supermassive black holes: is there a last parsec problem ?

We study the formation of a supermassive black hole (SMBH) binary and the shrinking of the separation of the two holes to sub-pc scales starting from a realistic major merger between two gas-rich spiral galaxies with mass comparable to our Milky Way. The simulations, carried out with the Adaptive Mesh Refinement (AMR) code RAMSES, are capable of resolving separations as small as 0.1 pc. The collision of the two galaxies produces a gravo-turbulent rotating nuclear disk with mass (10^9 Msun) and size (60 pc) in excellent agreement with previous SPH simulations with particle splitting that used a similar setup (Mayer et al. 2007) but were limited to separations of a few parsecs. The AMR results confirm that the two black holes sink rapidly as a result of dynamical friction onto the gaseous background, reaching a separation of 1 pc in less than 10^7 yr. We show that the dynamical friction wake is well resolved by our model and we find good agreement with analytical predictions of the drag force as a function of the Mach number. Below 1 pc, black hole pairing slows down significantly, as the relative velocity between the sinking SMBH becomes highly subsonic and the mass contained within their orbit falls below the mass of the binary itself, rendering dynamical friction ineffective. In this final stage, the black holes have not opened a gap as the gaseous background is highly pressurized in the center. Non-axisymmetric gas torques do not arise to restart sinking in absence of efficient dynamical friction, at variance with previous calculations using idealized equilibrium nuclear disk models. (abridged)

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arXiv2012

Massive black hole seeds born via direct gas collapse in galaxy mergers: their properties, statistics and environment

We study the statistics and cosmic evolution of massive black hole seeds formed during major mergers of gas-rich late-type galaxies. Generalizing the results of the hydro-simulations from Mayer et al. 2010, we envision a scenario in which a supermassive star can form at the center of galaxies that just experienced a major merger owing to a multi-scale powerful gas inflow, provided that such galaxies live in haloes with masses above 10^{11} Msun, are gas-rich and disc-dominated, and do not already host a massive black hole. We assume that the ultimate collapse of the supermassive star leads to the rapid formation of a black hole of 10^5 Msun following a quasi-star stage. Using a model for galaxy formation applied to the outputs of the Millennium Simulation, we show that the conditions required for this massive black hole formation route to take place in the concordance LambdaCDM model are actually common at high redshift, and can be realized even at low redshift. Most major mergers above z~4 in haloes with mass > 10^{11} Msun can lead to the formation of a massive seed and, at z~2, the fraction of favourable mergers decreases to about half. Interestingly, we find that even in the local universe a fraction (~20%) of major mergers in massive haloes still satisfy the conditions for our massive black hole formation route. Those late events take place in galaxies with a markedly low clustering amplitude, that have lived in isolation for most of their life, and that are experiencing a major merger for the first time. We predict that massive black hole seeds from galaxy mergers can dominate the massive end of the mass function at high (z>4) and intermediate (z~2) redshifts relative to lighter seeds formed at higher redshift, for example, by the collapse of Pop III stars. Finally, a fraction of these massive seeds could lie, soon after formation, above the MBH-MBulge relation.

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arXiv2012

Pseudobulge Formation as a Dynamical Rather than a Secular Process

We investigate the formation and evolution of the pseudobulge in "Eris", a high-resolution N-body + smoothed particle hydrodynamics (SPH) cosmological simulation that successfully reproduces a Milky Way-like massive late-type spiral in a cold dark matter (LCDM) universe. At the present epoch, Eris has a virial mass Mvir=8x10^11 Msun, a photometric stellar mass M*=3.2x10^10 Msun, a bulge-to-total ratio B/T = 0.26, and a weak nuclear bar. We find that the bulk of the pseudobulge forms quickly at high redshift via a combination of non-axisymmetric disk instabilities and tidal interactions or mergers both occurring on dynamical timescales, not through slow secular processes at lower redshift. Its subsequent evolution is not strictly secular either, and is closely intertwined with the evolution of the stellar bar. In fact, the structure that we recognize as a pseudobulge today evolves from a stellar bar that formed at high redshift, was destroyed by minor mergers at z~3, reformed shortly after, and weakened again following a steady gas inflow at z<1. The gradual dissolution of the bar ensues at z=1 and continues until the present without increasing the stellar velocity dispersion in the inner regions. In this scenario the pseudobulge is not a separate component from the inner disk in terms of formation path, rather it is the first step in the inside-out formation of the baryonic disk, in agreement with the fact that pseudobulges of massive spiral galaxies have typically a dominant old stellar population. If our simulations do indeed reproduce the formation mechanisms of massive spirals, then the progenitors of late-type galaxies should have strong bars and small photometric pseudobulges at high redshift.

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arXiv2012

The origin of metals in the circum-galactic medium of massive galaxies at z=3

We present a detailed study of the metal-enriched circum-galactic medium of a massive galaxy at z=3 using "ErisMC", a new cosmological hydrodynamic "zoom-in" simulations of a disk galaxy with mass comparable to the Milky Way. The run adopts a blastwave scheme for supernova feedback that generates galactic outflows without explicit wind particles, a star formation recipe based on a high gas density threshold, and high temperature metal cooling. ErisMC's main progenitor at z=3 resembles a "Lyman break" galaxy of mass M_vir=2.4e11 M_sun, virial radius R_vir=48 kpc, and star formation rate 18 M_sun/yr, and its metal-enriched CGM extends to 200 (physical) kpc from its center. Approximately 41, 9, and 50 percent of all gas-phase metals at z=3 are locked in a hot (T> 3e5 K), warm (3e5 >T> 3e4 K), and cold (T< 3e4 K) medium, respectively. We identify three sources of heavy elements: 1) the main host, responsible for 60% of all the metals found within 3R_vir; 2) its satellite progenitors, responsible for 28% of all the metals within 3R_vir, and for only 5% of those beyond 3R_vir; and nearby dwarfs, which give origin to 12% of all the metals within 3R_vir and 95% of those beyond 3R_vir. Late (z<5) galactic "superwinds" account for only 9% of all the metals observed beyond 2R_vir, the bulk having been released at redshifts 5< z < 8 by early star formation and outflows. In the CGM, lower overdensities are typically enriched by `older', colder metals. Heavy elements are accreted onto Eris along filaments via low-metallicity cold inflows, and are ejected hot via galactic outflows at a few hundred km/s. The outflow mass-loading factor is of order unity for the main halo, but can exceed 10 for nearby dwarfs. We stress that our "zoom-in" simulation focuses on the CGM of a single massive system and cannot describe the enrichment history of the IGM as a whole. (abridged)

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arXiv2012

How to make an ultra-faint dwarf spheroidal galaxy: tidal stirring of disky dwarfs with shallow dark matter density profiles

In recent years the Sloan Digital Sky Survey has unraveled a new population of ultra-faint dwarf galaxies (UFDs) in the vicinity of the Milky Way (MW) whose origin remains a puzzle. Using a suite of collisionless N-body simulations, we investigate the formation of UFDs in the context of the tidal stirring model for the formation of dwarf spheroidal galaxies in the Local Group (LG). Our simulations are designed to reproduce the tidal interactions between MW-sized host galaxies and rotationally supported dwarfs embedded in 10^9 M_sun dark matter (DM) halos. We explore a variety of inner density slopes ρ\propto r^{-α} for the dwarf DM halos, ranging from core-like (α= 0.2) to cuspy (α= 1), and different dwarf orbital configurations. Our experiments demonstrate that UFDs can be produced via tidal stirring of disky dwarfs on relatively tight orbits, consistent with a redshift of accretion by the host galaxy of z \sim 1, and with intermediate values for the halo inner density slopes (ρ\propto r^{-0.6}). The inferred slopes are in excellent agreement with those resulting from both the modeling of the rotation curves of dwarf galaxies and recent cosmological simulations of dwarf galaxy formation. Comparing the properties of observed UFDs with those of their simulated counterparts, we find remarkable similarities in terms of basic observational parameters. We conclude that tidal stirring of rotationally supported dwarfs represents a viable mechanism for the formation of UFDs in the LG environment.

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arXiv2012

The shapes of Milky Way satellites: looking for signatures of tidal stirring

We study the shapes of Milky Way satellites in the context of the tidal stirring scenario for the formation of dwarf spheroidal galaxies. The standard procedures used to measure shapes involve smoothing and binning of data and thus may not be sufficient to detect structural properties like bars, which are usually subtle in low surface brightness systems. Taking advantage of the fact that in nearby dwarfs photometry of individual stars is available we introduce discrete measures of shape based on the two-dimensional inertia tensor and the Fourier bar mode. We apply these measures of shape first to a variety of simulated dwarf galaxies formed via tidal stirring of disks embedded in dark matter halos and orbiting the Milky Way. In addition to strong mass loss and randomization of stellar orbits, the disks undergo morphological transformation that typically involves the formation of a triaxial bar after the first pericenter passage. These tidally induced bars persist for a few Gyr before being shortened towards a more spherical shape if the tidal force is strong enough. We test this prediction by measuring in a similar way the shape of nearby dwarf galaxies, satellites of the Milky Way. We detect inner bars in Ursa Minor, Sagittarius, LMC and possibly Carina. In addition, six out of eleven studied dwarfs show elongated stellar distributions in the outer parts that may signify transition to tidal tails. We thus find the shapes of Milky Way satellites to be consistent with the predictions of the tidal stirring model.

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arXiv2012

Formation and Hardening of Supermassive Black Hole Binaries in Minor Mergers of Disk Galaxies

We model for the first time the complete orbital evolution of a pair of Supermassive Black Holes (SMBHs) in a 1:10 galaxy merger of two disk dominated gas-rich galaxies, from the stage prior to the formation of the binary up to the onset of gravitational wave emission when the binary separation has shrunk to 1 milli parsec. The high-resolution smoothed particle hydrodynamics (SPH) simulations used for the first phase of the evolution include star formation, accretion onto the SMBHs as well as feedback from supernovae explosions and radiative heating from the SMBHs themselves. Using the direct N-body code ϕ-GPU we evolve the system further without including the effect of gas, which has been mostly consumed by star formation in the meantime. We start at the time when the separation between two SMBHs is ~ 700 pc and the two black holes are still embedded in their galaxy cusps. We use 3 million particles to study the formation and evolution of the SMBH binary till it becomes hard. After a hard binary is formed, we reduce (reselect) the particles to 1.15 million and follow the subsequent shrinking of the SMBH binary due to 3-body encounters with the stars. We find approximately constant hardening rates and that the SMBH binary rapidly develops a high eccentricity. Similar hardening rates and eccentricity values are reported in earlier studies of SMBH binary evolution in the merging of dissipation-less spherical galaxy models. The estimated coalescence time is ~ 2.9 Gyr, significantly smaller than a Hubble time. We discuss why this timescale should be regarded as an upper limit. Since 1:10 mergers are among the most common interaction events for galaxies at all cosmic epochs, we argue that several SMBH binaries should be detected with currently planned space-borne gravitational wave interferometers, whose sensitivity will be especially high for SMBHs in the mass range considered here.

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arXiv2012

Observability of Dual Active Galactic Nuclei in Merging Galaxies

Supermassive black holes (SMBHs) have been detected in the centers of most nearby massive galaxies. Galaxies today are the products of billions of years of galaxy mergers, but also billions of years of SMBH activity as active galactic nuclei (AGNs) that is connected to galaxy mergers. In this context, detection of AGN pairs should be relatively common. Observationally, however, dual AGN are scant, being just a few percent of all AGN. In this Letter we investigate the triggering of AGN activity in merging galaxies via a suite of high resolution hydrodynamical simulations. We follow the dynamics and accretion onto the SMBHs as they move from separations of tens of kiloparsecs to tens of parsecs. Our resolution, cooling and star formation implementation produce an inhomogeneous, multi-phase interstellar medium, allowing us to accurately trace star formation and accretion onto the SMBHs. We study the impact of gas content, morphology, and mass ratio, allowing us to study AGN activity and dynamics across a wide range of relevant conditions. We test when the two AGN are simultaneously detectable, for how long and at which separations. We find that strong dual AGN activity occurs during the late phases of the mergers, at small separations (<1-10 kpc) below the resolution limit of most surveys. Much of the SMBH accretion is not simultaneous, limiting the dual AGN fraction detectable through imaging and spectroscopy to a few percent, in agreement with observational samples.

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arXiv2012

The fate of high redshift massive compact galaxies in dense environments

Massive compact galaxies seem to be more common at high redshift than in the local universe, especially in denser environments. To investigate the fate of such massive galaxies identified at z~2 we analyse the evolution of their properties in three cosmological hydrodynamical simulations that form virialised galaxy groups of mass ~10^13 Msun hosting a central massive elliptical/S0 galaxy by redshift zero. We find that at redshift ~2 the population of galaxies with M_*> 2 10^10 Msun is diverse in terms of mass, velocity dispersion, star formation and effective radius, containing both very compact and relatively extended objects. In each simulation all the compact satellite galaxies have merged into the central galaxy by redshift 0 (with the exception of one simulation where one of such satellite galaxy survives). Satellites of similar mass at z = 0 are all less compact than their high redshift counterparts. They form later than the galaxies in the z = 2 sample and enter the group potential at z < 1, when dynamical friction times are longer than the Hubble time. Also, by z = 0 the central galaxies have increased substantially their characteristic radius via a combination of in situ star formation and mergers. Hence in a group environment descendants of compact galaxies either evolve towards larger sizes or they disappear before the present time as a result of the environment in which they evolve. Since the group-sized halos that we consider are representative of dense environments in the LambdaCDM cosmology, we conclude that the majority of high redshift compact massive galaxies do not survive until today as a result of the environment.

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arXiv2011

Formation of Dwarf Spheroidal Galaxies Via Mergers of Disky Dwarfs

We perform collisionless N-body simulations to investigate whether binary mergers between rotationally-supported dwarfs can lead to the formation of dwarf spheroidal galaxies (dSphs). Our simulation campaign is based on a hybrid approach combining cosmological simulations and controlled numerical experiments. We select merger events from a Constrained Local UniversE (CLUES) simulation of the Local Group (LG) and record the properties of the interacting dwarf-sized halos. This information is subsequently used to seed controlled experiments of binary encounters between dwarf galaxies consisting of exponential stellar disks embedded in cosmologically-motivated dark matter halos. These simulations are designed to reproduce eight cosmological merger events, with initial masses of the interacting systems in the range ~ (5-60) x 10^7 Mo, occurring quite early in the history of the LG, more than 10 Gyr ago. We compute the properties of the merger remnants as a distant observer would and demonstrate that at least three of the simulated encounters produce systems with kinematic and structural properties akin to those of the classic dSphs in the LG. Tracing the history of the remnants in the cosmological simulation to z=0, we find that two dSph-like objects remain isolated at distances larger than 800 kpc from either the Milky Way or M31. These systems constitute plausible counterparts of the remote dSphs Cetus and Tucana which reside in the LG outskirts, far from the tidal influence of the primary galaxies. We conclude that merging of rotationally-supported dwarfs represents a viable mechanism for the formation of dSphs in the LG and similar environments.

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arXiv2011

Forming Realistic Late-Type Spirals in a LCDM Universe: The Eris Simulation

Simulations of the formation of late-type spiral galaxies in a cold dark matter LCDM universe have traditionally failed to yield realistic candidates. Here we report a new cosmological N-body/SPH simulation of extreme dynamic range in which a close analog of a Milky Way disk galaxy arises naturally. Termed Eris, the simulation follows the assembly of a galaxy halo of mass Mvir=7.9x10^11 Msun with a total of N=18.6 million particles (gas + dark matter + stars) within the final virial radius, and a force resolution of 120 pc. It includes radiative cooling, heating from a cosmic UV field and supernova explosions, a star formation recipe based on a high gas density threshold (nSF=5 atoms cm^-3 rather than the canonical nSF=0.1 atoms cm^-3), and neglects AGN feedback. At the present epoch, the simulated galaxy has an extended rotationally-supported disk with a radial scale length Rd=2.5 kpc, a gently falling rotation curve with circular velocity at 2.2 disk scale lenghts of V2.2=214 km/s, a bulge-to-disk ratio B/D=0.35, and a baryonic mass fraction that is 30% below the cosmic value. The disk is thin, is forming stars in the region of the Sigma_SFR - Sigma_HI plane occupied by spiral galaxies, and falls on the photometric Tully-Fisher and the stellar mass-halo virial mass relations. Hot (T>3x10^5 K), X-ray luminous halo gas makes only 26% of the universal baryon fraction and follows a flattened density profile proportional to r^-1.13 out to r=100 kpc. Eris appears then to be the first cosmological hydrodynamic simulation in which the galaxy structural properties, the mass budget in the various components, and the scaling relations between mass and luminosity are all consistent with a host of observational constraints. (Abridged)

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arXiv2011

Evolutionary tracks of tidally stirred disky dwarf galaxies

Using collisionless N-body simulations, we investigate the tidal evolution of late-type, rotationally supported dwarfs inside Milky Way-sized host galaxies. Our study focuses on a wide variety of dwarf orbital configurations and initial structures. During the evolution, the disky dwarfs undergo strong mass loss, the stellar disks are transformed into spheroids, and rotation is replaced by random motions of the stars. Thus, the late-type progenitors are transformed into early-type dwarfs as envisioned by the tidal stirring model for the formation of dwarf spheroidal (dSph) galaxies in the Local Group. We determine the photometric properties of the dwarfs, including the total visual magnitude, the half-light radius and the central surface brightness as they would be measured by an observer near the galactic center. Special emphasis is also placed on studying their kinematics and shapes. We demonstrate that the measured values are biased by a number of observational effects including the increasing angle of the observation cone near the orbital pericenter, the fact that away from the pericenter the tidal tails are typically oriented along the line of sight, and the fact that for most of the evolution the stellar components of the dwarfs are triaxial ellipsoids whose major axis tumbles with respect to the line of sight. Finally, we compare the measured properties of the simulated dwarfs to those of dwarf galaxies in the Local Group. The evolutionary tracks of the dwarfs in different parameter planes and the correlations between their different properties, especially the total magnitude and the surface brightness, strongly suggest that present-day dSph galaxies may have indeed formed from late-type progenitors as proposed by the tidal stirring scenario.

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arXiv2011

Formation and evolution of dwarf galaxies in the CDM Universe

We first review the results of the tidal stirring model for the transformation of gas-rich dwarf irregulars into dwarf spheroidals, which turns rotationally supported stellar systems into pressure supported ones. We emphasize the importance of the combined effect of ram pressure stripping and heating from the cosmic ultraviolet background in removing the gas and converting the object into a gas poor system as dSphs. We discuss how the timing of infall of dwarfs into the primary halo determines the final mass-to-light ratio and star formation history. Secondly we review the results of recent cosmological simulations of the formation of gas-rich dwarfs. These simulations are finally capable to produce a realistic object with no bulge, an exponential profile and a slowly rising rotation curve. The result owes to the inclusion of an inhomogeneous ISM and a star formation scheme based on regions having the typical density of molecular cloud complexes. Supernovae-driven winds become more effective in such mode, driving low angular momentum baryons outside the virial radius at high redshift and turning the dark matter cusp into a core. Finally we show the first tidal stirring experiments adopting dwarfs formed in cosmological simulations as initial conditions. The latter are gas dominated and have have turbulent thick gaseous and stellar disks disks that cannot develop strong bars, yet they are efficiently heated into spheroids by tidal shocks.

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arXiv2011

The central slope of dark matter cores in dwarf galaxies: Simulations vs. THINGS

We make a direct comparison of the derived dark matter (DM) distributions between hydrodynamical simulations of dwarf galaxies assuming a LCDM cosmology and the observed dwarf galaxies sample from the THINGS survey in terms of (1) the rotation curve shape and (2) the logarithmic inner density slope alpha of mass density profiles. The simulations, which include the effect of baryonic feedback processes, such as gas cooling, star formation, cosmic UV background heating and most importantly physically motivated gas outflows driven by supernovae (SNe), form bulgeless galaxies with DM cores. We show that the stellar and baryonic mass is similar to that inferred from photometric and kinematic methods for galaxies of similar circular velocity. Analyzing the simulations in exactly the same way as the observational sample allows us to address directly the so-called "cusp/core" problem in the LCDM model. We show that the rotation curves of the simulated dwarf galaxies rise less steeply than CDM rotation curves and are consistent with those of the THINGS dwarf galaxies. The mean value of the logarithmic inner density slopes alpha of the simulated galaxies' dark matter density profiles is ~ -0.4 +- 0.1, which shows good agreement with α= -0.29 +- 0.07 of the THINGS dwarf galaxies. The effect of non-circular motions is not significant enough to affect the results. This confirms that the baryonic feedback processes included in the simulations are efficiently able to make the initial cusps with α~ -1.0 to -1.5 predicted by dark-matter-only simulations shallower, and induce DM halos with a central mass distribution similar to that observed in nearby dwarf galaxies.

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arXiv2011

The ACS LCID project. V. The Star Formation History of the Dwarf Galaxy \objectname[]{LGS-3}: Clues for Cosmic Reionization and Feedback

We present an analysis of the star formation history (SFH) of the transition-type (dIrr/dSph) Local Group galaxy \objectname[]{LGS-3} (Pisces) based on deep photometry obtained with the {\it Advanced Camera for Surveys} onboard the {\it Hubble Space Telescope}. Our analysis shows that the SFH of \objectname[]{LGS-3} is dominated by a main episode $\sim 11.7$ Gyr ago with a duration of $\sim$ 1.4 Gyr which formed $\sim 90%$ of the stars. Subsequently, \objectname[]{LGS-3} continued forming stars until the present, although at a much lower rate. The lack of early chemical enrichment is in contrast to that observed in the isolated dSph galaxies of comparable luminosity, implying that the dSphs were more massive and subjected to more tidal stripping. We compare the SFH of \objectname[]{LGS-3} with expectations from cosmological models. Most or all the star formation was produced in \objectname[]{LGS-3} after the reionization epoch, assumed to be completed at $z\sim6$ or $\sim 12.7$ Gyr ago. The total mass of the galaxy is estimated to be between 2 and $4\times 10^8$ M$_\odot$, corresponding to circular velocities between 28 km\ s$^{-1}$ to 36 km\ s$^{-1}$. These values are close to but somewhat above the limit of 30 km\ s$^{-1}$ below which the UV background is expected to prevent any star formation after reionization. Feedback from SNe associated with the initial episode of star formation (mechanical luminosity from SNe $L_w=5.3\times 10^{38}$ erg s$^{-1}$) is probably inadequate to completely blow away the gas. However, the combined effects of SN feedback and UV background heating might be expected to completely halt star formation at the reionization epoch for the low mass of \objectname[]{LGS-3}; this suggests that self-shielding is important to the early evolution of galaxies in this mass range.

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