We have examined the resolved stellar populations at large galactocentric distances along the minor axis (from 10 kpc up to between 40 and 75 kpc), with limited major axis coverage, of six nearby highly-inclined Milky Way-mass disc galaxies using HST data from the GHOSTS survey. We select red giant branch stars to derive stellar halo density profiles. The projected minor axis density profiles can be approximated by power laws with projected slopes of between −2 and −3.7 and a diversity of stellar halo masses of 1 − 6 × 10 9 M , or 2 − 14% of the total galaxy stellar masses. The typical intrinsic scatter around a smooth power law fit is 0.05 − 0.1 dex owing to substructure. By comparing the minor and major axis profiles, we infer projected axis ratios c/a at ∼ 25 kpc between 0.4 − 0.75. The GHOSTS stellar haloes are diverse, lying between the extremes charted out by the (rather atypical) haloes of the Milky Way and M31. We find a strong correlation between the stellar halo metallicities and the stellar halo masses. We compare our results with cosmological models, finding good agreement between our observations and accretion-only models where the stellar haloes are formed by the disruption of dwarf satellites. In particular, the strong observed correlation between stellar halo metallicity and mass is naturally reproduced. Low-resolution hydrodynamical models have unrealistically high stellar halo masses. Current high-resolution hydrodynamical models appear to predict stellar halo masses somewhat higher than observed but with reasonable metallicities, metallicity gradients and density profiles.

Galaxies with Milky Way-like stellar masses have a wide range of bulge and black hole masses; in turn, these correlate with other properties such as star formation history. While many processes may drive bulge formation, major and minor mergers are expected to play a crucial role. Stellar halos offer a novel and robust measurement of galactic merger history; cosmologically-motivated models predict that mergers with larger satellites produce more massive, higher metallicity stellar halos, reproducing the recently-observed stellar halo metallicity-mass relation. We quantify the relationship between stellar halo mass and bulge or black hole prominence using a sample of eighteen Milky Way-mass galaxies with newly-available measurements of (or limits on) stellar halo properties. There is an order of magnitude range in bulge mass, and two orders of magnitude in black hole mass, at a given stellar halo mass (or, equivalently, merger history). Galaxies with low mass bulges show a wide range of quiet merger histories, implying formation mechanisms that do not require intense merging activity. Galaxies with massive 'classical' bulges and central black holes also show a wide range of merger histories. While three of these galaxies have massive stellar halos consistent with a merger origin, two do not -merging appears to have had little impact in making these two massive 'classical' bulges. Such galaxies may be ideal laboratories to study massive bulge formation through pathways such as early gas-rich accretion, violent disk instabilities or misaligned infall of gas throughout cosmic time.

The number, distribution, and properties of dwarf satellites are crucial probes of the physics of galaxy formation at low masses and the response of satellite galaxies to the tidal and gas dynamical effects of their more massive parent. To make progress, it is necessary to augment and solidify the census of dwarf satellites of galaxies outside the Local Group. Müller et al. (2015) presented 16 dwarf galaxy candidates near M83, but lacking reliable distances, it is unclear which candidates are M83 satellites. Using red giant branch stars from the HST/GHOSTS survey in conjunction with ground-based images from VLT/VIMOS, we confirm that one of the candidates, dw1335-29 -with a projected distance of 26 kpc from M83 and a distance modulus of (m−M ) 0 = 28.5 +0.3 −0.1 -is a satellite of M83. We estimate an absolute magnitude M V = −10.1 ± 0.4, an ellipticity of 0.40 +0.14 −0.22 , a half light radius of 656 +121 −170 pc, and [Fe/H] = −1.3 +0.3 −0.4 . Owing to dw1335-29's somewhat irregular shape and possible young stars, we classify this galaxy as a dwarf irregular or transition dwarf. This is curious, as with a projected distance of 26 kpc from M83, dw1335-29 is expected to lack recent star formation. Further study of M83's dwarf population will reveal if star formation in its satellites is commonplace (suggesting a lack of a hot gas envelope for M83 that would quench star formation) or rare (suggesting that dw1335-29 has a larger M83-centric distance, and is fortuitously projected to small radii).

Galaxy mergers are expected to influence galaxy properties, yet measurements of individual merger histories are lacking. Models predict that merger histories can be measured using stellar halos and that these halos can be quantified using observations of resolved stars along their minor axis. Such observations reveal that Milky Way-mass galaxies have a wide range of stellar halo properties and show a correlation between their stellar halo masses and metallicities. This correlation agrees with merger-driven models where stellar halos are formed by satellite galaxy disruption. In these models, the largest accreted satellite dominates the stellar halo properties. Consequently, the observed diversity in the stellar halos of Milky Way-mass galaxies implies a large range in the masses of their largest merger partners. In particular, the Milky Way's low mass halo implies an unusually quiet merger history. We used these measurements to seek predicted correlations between the bulge and central black hole (BH) mass and the mass of the largest merger partner. We found no significant correlations: while some galaxies with large bulges and BHs have large stellar halos and thus experienced a major or minor merger, half have small stellar halos and never experienced a significant merger event. These results indicate that bulge and BH growth is not solely driven by merger-related processes.