Chris W. Purcell

Shredded Galaxies as the Source of Diffuse Intrahalo Light on Varying Scales

We make predictions for diffuse stellar mass fractions in dark matter halos from the scales of small spiral galaxies to those of large galaxy clusters. We use an extensively tested analytic model for subhalo infall and evolution and empirical constraints from galaxy survey data to set the stellar mass in each accreted subhalo, which is added to the diffuse light as subhalos become disrupted due to interactions within their hosts. We predict that the stellar mass fraction in diffuse, intrahalo light should rise on average from ~0.5% to ~20% from small galaxy halos (~1011 M☉) to poor groups (~1013 M☉). The trend with mass flattens considerably beyond the group scale, increasing weakly from a fraction of ~20% in poor galaxy clusters (~1014 M☉) to roughly ~30% in massive clusters (~1015 M☉). The mass-dependent diffuse light fraction is governed primarily by the empirical fact that the mass-to-light ratio in galaxy halos must vary as a function of halo mass. Galaxy halos have little diffuse light because they accrete most of their mass in small subhalos that themselves have high mass-to-light ratios; stellar halos around galaxies are built primarily from disrupted dwarf-irregular-type galaxies with M* ~ 108.5 M☉. The diffuse light in group and cluster halos is built from satellite galaxies that form stars efficiently; intracluster light is dominated by material liberated from massive galaxies with M* ~ 1011 M☉. Our results are consistent with existing observations spanning the galaxy, group, and cluster scale; however, they can be tested more rigorously in future deep surveys.

Heated disc stars in the stellar halo

ABSTRACT Minor accretion events with mass ratio M sat :M host ≃ 1:10 are common in thecontext of ΛCDM cosmology. We use high-resolution simulations of Galaxy-analoguesystems to show that these mergers can dynamically eject disc stars into a diffuselight component that resembles a stellar halo both spatially and kinematically. Fora variety of initial orbital configurations, we find that ∼ 3 − 5 × 10 8 M ⊙ of primarystellar disc material is ejected to a distance larger than 5 kpc above the galactic plane.This ejected contribution is similar to the mass contributed by the tidal disruption ofthe satellite galaxy itself, though it is less extended. If we restrict our analysis to theapproximate solar neighborhood in the disc plane, we find that ∼ 1% of the initial discstars in that region would be classified kinematically as halo stars. Our results suggestthat the inner parts of galactic stellar halos contain ancient disc stars and that thesestars may have been liberated in the very same events that delivered material to theouter stellar halo.Key words: Cosmology: theory — galaxies: formation — galaxies: evolution

THE DESTRUCTION OF THIN STELLAR DISKS VIA COSMOLOGICALLY COMMON SATELLITE ACCRETION EVENTS

Most Galaxy-sized systems (M host 1012 M ?) in the ?CDM cosmology are expected to have interacted with at least one satellite with a total mass M sat 1011 M ? 3M disk in the past 8 Gyr. Analytic and numerical investigations suggest that this is the most precarious type of accretion for the survival of thin galactic disks because more massive accretion events are relatively rare and less massive ones preserve thin disk components. We use high-resolution, dissipationless N-body simulations to study the response of an initially thin, fully formed Milky Way-type stellar disk to these cosmologically common satellite accretion events, and show that the thin disk does not survive. Regardless of orbital configuration, the impacts transform the disks into structures that are roughly three times as thick and more than twice as kinematically hot as the observed dominant thin disk component of the Milky Way. We conclude that if the Galactic thin disk is a representative case, then the presence of a stabilizing gas component is the only recourse for explaining the preponderance of disk galaxies in a ?CDM universe; otherwise, the disk of the Milky Way must be uncommonly cold and thin for its luminosity, perhaps as a consequence of an unusually quiescent accretion history.

STELLAR POPULATIONS ACROSS THE NGC 4244 TRUNCATED GALACTIC DISK

We use HST/ACS to study the resolved stellar populations of the nearby, nearly edge-on galaxy NGC 4244 across its outer disk surface density break. The stellar photometry allows us to study the distribution of different stellar populations and reach very low equivalent surface brightnesses. We find that the break occurs at the same radius for young, intermediate age, and old stars. The stellar density beyond the break drops sharply by a factor of at least 600 in 5 kpc. The break occurs at the same radius independent of height above the disk, but is sharpest in the midplane and nearly disappears at large heights. These results make it unlikely that truncations are caused by a star formation threshold alone: the threshold would have to keep the same radial position from less than 100 Myr to 10 Gyr ago, in spite of potential disturbances such as infall and redistribution of gas by internal processes. A dynamical interpretation of truncation formation is more likely such as due to angular momentum redistribution by bars or density waves, or heating and stripping of stars caused by the bombardment of dark matter sub-halos. The latter explanation is also in quantitative agreement with the small diffuse component we see around the galaxy. Subject headings: galaxies: evolution — galaxies: halos — galaxies: individual (NGC 4244) — galaxies: spiral — galaxies: stellar content — galaxies: structure

Mergers and Disk Survival in ΛCDM

Disk galaxies are common in our universe and this is a source of concern for hierarchical formation models like LCDM. Here we investigate this issue as motivated by raw merger statistics derived for galaxy-size dark matter halos from LCDM simulations. Our analysis shows that a majority (~70%) of galaxy halos with M = 10^12 M_sun at z=0 should have accreted at least one object with mass m > 10^11 M_sun ~ 3 M_disk over the last 10 Gyr. Mergers involving larger objects should have been rare, and this pinpoints m/M = 0.1 mass-ratio mergers as the most worrying ones for the survival of thin galactic disks. Motivated by these results, we use use high-resolution, dissipationless N-body simulations to study the response of stellar Milky-Way type disks to these common mergers and show that thin disks do not survive the bombardment. The remnant galaxies are roughly three times as thick and twice as kinematically hot as the observed thin disk of the Milky Way. Finally, we evaluate the suggestion that disks may be preserved if the mergers involve gas-rich progenitors. Using empirical measures to assign stellar masses and gas masses to dark matter halos as a function of redshift, we show that the vast majority of large mergers experienced by 10^12 M_sun halos should be gas-rich (f_gas > 0.5), suggesting that this is a potentially viable solution to the disk formation conundrum. Moreover, gas-rich mergers should become increasingly rare in more massive halos > 10^12.5 M_sun, and this suggest that merger gas fractions may play an important role in establishing morphological trends with galaxy luminosity.

GHOSTS: The resolved stellar outskirts of massive disk galaxies

We show initial results from our ongoing HST GHOSTS survey of the resolved stellar envelopes of 14 nearby, massive disk galaxies. In hierarchical galaxy formation the stellar halos and thick disks of galaxies are formed by accretion of minor satellites and therefore contain valuable information about the (early) assembly process of galaxies. We detect for the first time the very small halo of NGC4244, a low mass edge-on galaxy. We find that massive galaxies have very extended halos, with equivalent surface brightnesses of 28-29 V-mag/arcsec^2 at 20-30 kpc from the disk. The old RGB stars of the thick disk in the NGC891 and NGC4244 edge-on galaxies truncate at the same radius as the young thin disk stars, providing insights into the formation of both disk truncations and thick disks. We furthermore present the stellar populations of a very low surface brightness stream around M83, the first such a stream resolved into stars beyond those of the Milky Way and M31.