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 (∼ 10 11 M ⊙ ) to poor groups (∼ 10 13 M ⊙ ). The trend with mass flattens considerably beyond the group scale, increasing weakly from a fraction of ∼ 20% in poor galaxy clusters (∼ 10 14 M ⊙ ) to roughly ∼ 30% in massive clusters (∼ 10 15 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-irregulartype galaxies with M * ∼ 10 8.5 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 * ∼ 10 11 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.

Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar, although our knowledge of its structure and origin is incomplete. Traditional attempts to understand our Galaxy's morphology assume that it has been unperturbed by major external forces. Here we report simulations of the response of the Milky Way to the infall of the Sagittarius dwarf galaxy (Sgr), which results in the formation of spiral arms, influences the central bar and produces a flared outer disk. Two ring-like wrappings emerge towards the Galactic anti-Centre in our model that are reminiscent of the low-latitude arcs observed in the same area of the Milky Way. Previous models have focused on Sgr itself to reproduce the dwarf's orbital history and place associated constraints on the shape of the Milky Way gravitational potential, treating the Sgr impact event as a trivial influence on the Galactic disk. Our results show that the Milky Way's morphology is not purely secular in origin and that low-mass minor mergers predicted to be common throughout the Universe probably have a similarly important role in shaping galactic structure.

Recently, Widrow and collaborators announced the discovery of vertical density waves in the Milky Way disk. Here we investigate a scenario where these waves were induced by the Sagittarius dwarf galaxy as it plunged through the Galaxy. Using numerical simulations, we find that the the Sagittarius impact produces North-South asymmetries and vertical wave-like behavior that qualitatively agrees with what is observed. The extent to which vertical modes can radially penetrate into the disc, as well as their amplitudes, depend on the mass of the perturbing satellite. We show that the mean height of the disc is expected to vary more rapidly in the radial than in the azimuthal direction. If the observed vertical density asymmetry is indeed caused by vertical oscillations, we predict radial and azimuthal variations of the mean vertical velocity, correlating with the spatial structure. These variations can have amplitudes as large as 8 km s −1 .

We present an overview of the GHOSTS survey, the largest study to date of the resolved stellar populations in the outskirts of disk galaxies. The sample consists of 14 disk galaxies within 17 Mpc, whose outer disks and halos are imaged with the Hubble Space Telescope Advanced Camera for Surveys (ACS). In the first paper of this series, we describe the sample, explore the benefits of using resolved stellar populations, and discuss our ACS F606W and F814W photometry. We use artificial star tests to assess completeness and use overlapping regions to estimate photometric uncertainties. The median depth of the survey at 50% completeness is 2.7 mag below the tip of the red giant branch (TRGB). We comprehensively explore and parameterize contamination from unresolved background galaxies and foreground stars using archival fields of high-redshift ACS observations. Left uncorrected, these would account for 10 0.65×F814W−19.0 detections per mag per arcsec 2. We therefore identify several selection criteria that typically remove 95% of the contaminants. Even with these culls, background galaxies are a significant limitation to the surface brightness detection limit which, for this survey, is typically V ∼ 30 mag arcsec −2. The resulting photometric catalogs are publicly available and contain some 3.1 million stars across 76 ACS fields, predominantly of low extinction. The uniform magnitudes of TRGB stars in these fields enable galaxy distance estimates with 2%-7% accuracy.