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 $\Lambda$CDM HAHs, sharing a final virial mass of $M_{h}(z=0)=10^{12.25}M_\odot$. Hosts with poor/rich ASHs assemble following orthogonal growth-patterns. Hosts with rich ASHs experience accretion events (AEs) with high virial mass ratios (HVMRs, $M_s/M_h\gtrsim 0.1$) at $0.5\lesssim z_{infall}\lesssim1.5$, in a phase of fast growth. This maximizes the accreted stellar mass under the condition these satellites are disrupted by $z=0$. 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 $\log M_{s,*}/M_\odot\gtrsim 9$). 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 $\Lambda$CDM framework and have invoked cored dark matter haloes, processed by tides. I show that the tidal evolution of $\Lambda$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 $\rho\sim r^{-\alpha}$ with $\alpha\gtrsim 0.2$. 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 $\alpha\lesssim 0.2$. 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 $L_V\sim5\times 10^5 L_\odot$ and $L_V\sim6\times10^4 L_\odot$, in which stellar feedback is unlikely to be effective.Comment: accepted versio

On feathers, bifurcations and shells: the dynamics of tidal streams across the mass scale

I present an organic description of the regimes of collisionless tidal streams and define the orderings between the physical quantities that shape their morphology. Three fundamental dichotomies are identified in the form of dimensionless inequalities. These govern i) the speed of the stream's growth, ii) its internal coherence, iii) its thickness or opening angles. The mechanisms that regulate such main properties are analysed. The slope of the host's density profile influences the speed of the stream's growth, in both length and width, as steeper profiles enhance differential streaming. Internal coherence is the requirement for the appearance of substructure in tidal debris, and I concentrate on the `feathering' typical of GC streams. Overdensities are associated with minima in the relative streaming velocity of the stream members. For streams with high circularity, these are caused by the epicyclic oscillations of stars; however, for highly non-circular progenitor's orbits, substructure is caused by the oscillating differences in energy and actions with which material is shed at different orbital phases of the progenitor. This modulation results in different streaming speeds: the streakline of material shed between two successive apocentric passages is folded along its length, pulled at its centre by the faster streaming of particles released near pericenter, which are therefore more widely scattered. When the stream is coherent enough, this mechanism is potentially capable of generating a bimodal profile in the density distributions of the longer wraps of more massive progenitors, which I dub `bifurcations'. The conditions for internal coherence are explored and I comment on the cases of Palomar 5, Willman 1, the Anticenter and Sagittarius' streams. Analytical methods are accompanied by numerical experiments, performed using a purposely built generative model, also presented here.Comment: 18 pages, submitted to MNRA

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

Contributions to the accreted stellar halo: an atlas of stellar deposition

The accreted component of stellar halos is composed of the contributions of several satellites, falling onto their host with their different masses, at different times, on different orbits. This work uses a suite of idealised, collisionless N-body simulations of minor mergers and a particle tagging technique to understand how these different ingredients shape each contribution to the accreted halo, in both density and kinematics. I find that more massive satellites deposit their stars deeper into the gravitational potential of the host, with a clear segregation enforced by dynamical friction. Earlier accretion events contribute more to the inner regions of the halo; more concentrated subhaloes sink deeper through increased dynamical friction. The orbital circularity of the progenitor at infall is only important for low-mass satellites: dynamical friction efficiently radialises the most massive minor mergers erasing the imprint of the infall orbit for satellite-to-host virial mass ratios $\gtrsim1/20$. The kinematics of the stars contributed by each satellite is also ordered with satellite mass: low-mass satellites contribute fast-moving populations, in both ordered rotation and radial velocity dispersion. In turn, contributions by massive satellites have lower velocity dispersion and lose their angular momentum to dynamical friction, resulting in a strong radial anisotropy.Comment: 15 pages, accepted version, discussion extende

The core size of the Fornax dwarf Spheroidal

We exploit the detection of three distinct stellar subpopulations in the red giant branch of the Fornax dwarf Spheroidal to probe its density distribution. This allows us to resolve directly the evolution with radius of the dark matter mass profile. We find that a cored dark matter halo provides a perfect fit to the data, being consistent with all three stellar populations well within 1-sigma, and for the first time we are able to put constraints on the core size of such a halo. With respect to previous work, we do not strengthen the statistical exclusion of a dark matter cusp in Fornax, but we find that Navarro-Frenk-White haloes would be required to have unrealistically large scale radii in order to be compatible with the data, hence low values of the concentration parameter. We are then forced to conclude that the Fornax dwarf Spheroidal sits within a dark matter halo having a constant density core, with a core size of between 0.6 and 1.8 kpc.Comment: MNRAS Letters, submitte

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 ($\mathrm{M}_{\mathrm{200crit}}>6\times10^{12}\,\mathrm{M}_\odot$) 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 $\mu_{\mathrm{stars}}\gtrsim1:10$). 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 $z=0$ 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 $2$-$4\,\mathrm{R}_{\mathrm{eff}}$ 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 ($\mathrm{M}_{\mathrm{200crit}}>6\times10^{12}\,\mathrm{M}_{\odot}$), $18\%\pm3\%$ 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 $z=0$ form preferentially through relatively major mergers ($\gtrsim$1:10 in stellar mass ratio). Progenitors are accreted on low angular momentum orbits, in a preferred time-window between $\sim$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