21 research outputs found
The accreted stellar halo as a window on halo assembly in L* galaxies
Theory and observations agree that the accreted stellar halos (ASHs) of Milky
Way-like galaxies display significant scatter. I take advantage of this
stochasticity to invert the link between halo assembly history (HAH) and ASH,
using mock ASHs corresponding to 750 CDM HAHs, sharing a final virial
mass of . Hosts with poor/rich ASHs assemble
following orthogonal growth-patterns. Hosts with rich ASHs experience accretion
events (AEs) with high virial mass ratios (HVMRs, ) at
, in a phase of fast growth. This maximizes
the accreted stellar mass under the condition these satellites are disrupted by
. At similar times, hosts with poor ASHs grow slowly through minor
mergers, with only very recent HVMR AEs: this results in a globally more
abundant satellite population and in distinctive surviving massive satellites
(stellar mass ). Several properties of the Milky
Way are in agreement with the predictions of this framework for hosts with
poor, concentrated ASHs, including: i) the recent infall of Sagittarius and
Magellanic Clouds, ii) the likely higher-than-average concentration of its dark
halo, iii) the signatures of fast chemical enrichment of a sizable fraction of
its halo stellar populations.Comment: accepted version, minor change
Giant cold satellites from low-concentration haloes
The dwarf satellite galaxies of the Milky Way Crater II and Antlia II have
uncommonly low dynamical mass densities, due to their large size and low
velocity dispersion. Previous work have failed to identify formation scenarios
within the CDM framework and have invoked cored dark matter haloes,
processed by tides. I show that the tidal evolution of CDM NFW haloes
is richer than previously recognised: tidal heating causes the innermost
regions of haloes that fall short of the mass-concentration relation to expand
significantly, resulting in the formation of giant, kinematically cold
satellites like Crater II and Antlia II. Furthermore, while the satellite is
reaching apocenter, extra-tidal material can cause an even more inflated
appearance. When present, as likely for the larger Antlia II, nominally unbound
material can be recognised thanks to its somewhat hotter kinematics and
line-of-sight velocity gradient. Contrary to other formation scenarios, Crater
II and Antlia II may well have experienced very little mass loss, as in fact
hinted by their observed metallicity. If indeed a satellite of NGC1052, tidal
evolution of a low-concentration halo may similarly have led to the formation
of NGC1052-DF2.Comment: MNRAS Letters submitted, comments welcom
Deadly dark matter cusps vs faint and extended star clusters: Eridanus II and Andromeda XXV
The recent detection of two faint and extended star clusters in the central
regions of two Local Group dwarf galaxies, Eridanus II and Andromeda XXV,
raises the question of whether clusters with such low densities can survive the
tidal field of cold dark matter haloes with central density cusps. Using both
analytic arguments and a suite of collisionless N-body simulations, I show that
these clusters are extremely fragile and quickly disrupted in the presence of
central cusps with . Furthermore, the
scenario in which the clusters where originally more massive and sank to the
center of the halo requires extreme fine tuning and does not naturally
reproduce the observed systems. In turn, these clusters are long lived in cored
haloes, whose central regions are safe shelters for . The
only viable scenario for hosts that have preserved their primoridal cusp to the
present time is that the clusters formed at rest at the bottom of the
potential, which is easily tested by measurement of the clusters proper
velocity within the host. This offers means to readily probe the central
density profile of two dwarf galaxies as faint as
and , in which stellar feedback is unlikely to be
effective.Comment: accepted versio
The virial mass distribution of ultra-diffuse galaxies in clusters and groups
We use the observed abundances of ultra-diffuse galaxies (UDGs) in clusters
and groups and \Lambda CDM subhalo mass functions to put constraints on the
distribution {of present-day halo masses of satellite} UDGs. If all of the most
massive subhaloes in the cluster host a UDG, UDGs occupy all subhaloes with
\log M_{sub}/M_\odot\gtrsim11. For a model in which the efficiency of UDG
formation is higher around some characteristic halo mass, higher fractions of
massive UDGs require larger spreads in the UDG mass distribution. In a cluster
with a virial mass of 10^{15}M_\odot, the 90% upper limit for the fraction of
UDGs with \log M_{sub}/M_\odot>12 is 7%, occupying 70% of all cluster subhaloes
above the same mass. To reproduce the observed abundances, however, the mass
distribution of satellite UDGs has to be broad, with >30% having \log
M_{sub}/M_\odot<10.9. This strongly supports that UDGs are part of a continuous
distribution in which a majority are hosted by low mass haloes. The abundance
of satellite UDGs may fall short of the linear relation with the cluster/group
mass M_{host} in low-mass hosts, \log M_{host}/M_\odot\sim 12. Characterising
these deviations -- or the lack thereof -- will allow for stringent constraints
on the UDG virial mass distribution.Comment: matches accepted versio
Galaxies with Shells in the Illustris Simulation: Metallicity Signatures
Stellar shells are low surface brightness arcs of overdense stellar regions,
extending to large galactocentric distances. In a companion study, we
identified 39 shell galaxies in a sample of 220 massive ellipticals
() from the
Illustris cosmological simulation. We used stellar history catalogs to trace
the history of each individual star particle inside the shell substructures,
and we found that shells in high-mass galaxies form through mergers with
massive satellites (stellar mass ratios ).
Using the same sample of shell galaxies, the current study extends the stellar
history catalogs in order to investigate the metallicity of stellar shells
around massive galaxies. Our results indicate that outer shells are often times
more metal-rich than the surrounding stellar material in a galaxy's halo. For a
galaxy with two different satellites forming shells, we find a
significant difference in the metallicity of the shells produced by each
progenitor. We also find that shell galaxies have higher mass-weighted
logarithmic metallicities ([Z/H]) at -
compared to galaxies without shells. Our results indicate that observations
comparing the metallicities of stars in tidal features, such as shells, to the
average metallicities in the stellar halo can provide information about the
assembly histories of galaxies.Comment: 15 pages, 5 figures. Article published in a special issue of MDPI
Galaxies after the conference "On the Origin (and Evolution) of Baryonic
Galaxy Halos", Galapagos Islands, 201
Formation and Incidence of Shell Galaxies in the Illustris Simulation
Shells are low surface brightness tidal debris that appear as interleaved
caustics with large opening angles, often situated on both sides of the galaxy
center. In this paper, we study the incidence and formation processes of shell
galaxies in the cosmological gravity+hydrodynamics Illustris simulation. We
identify shells at redshift z=0 using stellar surface density maps, and we use
stellar history catalogs to trace the birth, trajectory and progenitors of each
individual star particle contributing to the tidal feature. Out of a sample of
the 220 most massive galaxies in Illustris
(),
of the galaxies exhibit shells. This fraction increases with
increasing mass cut: higher mass galaxies are more likely to have stellar
shells. Furthermore, the fraction of massive galaxies that exhibit shells
decreases with increasing redshift. We find that shell galaxies observed at
redshift form preferentially through relatively major mergers
(1:10 in stellar mass ratio). Progenitors are accreted on low angular
momentum orbits, in a preferred time-window between 4 and 8 Gyrs ago. Our
study indicates that, due to dynamical friction, more massive satellites are
allowed to probe a wider range of impact parameters at accretion time, while
small companions need almost purely radial infall trajectories in order to
produce shells. We also find a number of special cases, as a consequence of the
additional complexity introduced by the cosmological setting. These include
galaxies with multiple shell-forming progenitors, satellite-of-satellites also
forming shells, or satellites that fail to produce shells due to multiple major
mergers happening in quick succession.Comment: 27 pages, 18 figures. Accepted for publication in MNRAS (new figures
3 and D1 + additional minor changes to match accepted version
Dark matter cores in the Fornax and Sculptor dwarf galaxies: joining halo assembly and detailed star formation histories
We combine the detailed Star Formation Histories of the Fornax and Sculptor
dwarf Spheroidals with the Mass Assembly History of their dark matter (DM) halo
progenitors to estimate if the energy deposited by Supernova type II (SNeII) is
sufficient to create a substantial DM core. Assuming the efficiency of energy
injection of the SNeII into DM particles is , we find
that a single early episode, , that combines the
energy of all SNeII due to explode over 0.5 Gyr, is sufficient to create a core
of several hundred parsecs in both Sculptor and Fornax. Therefore, our results
suggest that it is energetically plausible to form cores in Cold Dark Matter
(CDM) halos via early episodic gas outflows triggered by SNeII. Furthermore,
based on CDM merger rates and phase-space density considerations, we argue that
the probability of a subsequent complete regeneration of the cusp is small for
a substantial fraction of dwarf-size haloes.Comment: ApJL accepted versio
Formation and Incidence of Shell Galaxies in the Illustris Simulation
Shells are low surface brightness tidal debris that appear as interleaved
caustics with large opening angles, often situated on both sides of the galaxy
center. In this paper, we study the incidence and formation processes of shell
galaxies in the cosmological gravity+hydrodynamics Illustris simulation. We
identify shells at redshift z=0 using stellar surface density maps, and we use
stellar history catalogs to trace the birth, trajectory and progenitors of each
individual star particle contributing to the tidal feature. Out of a sample of
the 220 most massive galaxies in Illustris
(),
of the galaxies exhibit shells. This fraction increases with
increasing mass cut: higher mass galaxies are more likely to have stellar
shells. Furthermore, the fraction of massive galaxies that exhibit shells
decreases with increasing redshift. We find that shell galaxies observed at
redshift form preferentially through relatively major mergers
(1:10 in stellar mass ratio). Progenitors are accreted on low angular
momentum orbits, in a preferred time-window between 4 and 8 Gyrs ago. Our
study indicates that, due to dynamical friction, more massive satellites are
allowed to probe a wider range of impact parameters at accretion time, while
small companions need almost purely radial infall trajectories in order to
produce shells. We also find a number of special cases, as a consequence of the
additional complexity introduced by the cosmological setting. These include
galaxies with multiple shell-forming progenitors, satellite-of-satellites also
forming shells, or satellites that fail to produce shells due to multiple major
mergers happening in quick succession.Comment: 27 pages, 18 figures. Accepted for publication in MNRAS (new figures
3 and D1 + additional minor changes to match accepted version