The existence of stellar halos around galaxies is a natural prediction of the hierarchical nature of the ΛCDM model. Recent observations of Milky Way-like galaxies have revealed a wide range in stellar halo mass, including cases with no statistically significant detection of a stellar halo, as in the case of M101, NGC3351 and NGC1042. We use the Illustris simulation to investigate the scatter in stellar halo content and, in particular, to study the formation of galaxies with the smallest fraction of this diffuse component. Stellar halos are far from spherical, which diminishes the surface brightness of the stellar halo for face-on disks. Once accounting for projection effects, we find that the stellar halo fraction f SH correlates strongly with galaxy morphology and star formation rate, but not with environment, in agreement with observations. Galaxies with the lowest stellar halo fractions are disk-dominated, star-forming and assemble their dark matter halos on average earlier than galaxies with similar stellar masses. Accreted satellites are also lower in stellar mass and have earlier infall times than centrals with high f SH . In situ rather than accreted stars dominate the stellar halos of galaxies with the lowest stellar halo fractions, with a transition radius from in situ to accretion-dominated r ∼ 45 kpc. Our results extrapolated to real galaxies such as M101 may indicate that these galaxies inhabit old halos which endured mergers only at higher redshifts and evolved relatively unperturbed in the last ∼ 10 Gyrs.

We use a dedicated 0.7-m telescope to image the halos of 119 galaxies in the Local Volume to µ r ∼ 28 − 30 mag/arcsec 2 . The sample is primarily from the 2MASS Large Galaxy Atlas and extended to include nearby dwarf galaxies and more distant giant ellipticals, and spans fully the galaxy colour-magnitude diagram including the blue cloud and red sequence. We present an initial overview, including deep images of our galaxies. Our observations reproduce previously reported low surface brightness structures, including extended plumes in M 51, and a newly discovered tidally extended dwarf galaxy in NGC 7331. Low surface brightness structures, or "envelopes", exceeding 50 kpc in diameter are found mostly in galaxies with M V < −20.5, and classic interaction signatures are infrequent. Defining a halo diameter at the surface brightness 28 mag/arcsec 2 , we find that halo diameter is correlated with total galaxy luminosity. Extended signatures of interaction are found throughout the galaxy colour-magnitude diagram without preference for the red or blue sequences, or the green valley. Large envelopes may be found throughout the colour-magnitude diagram with some preference for the bright end of the red sequence. Spiral and S0 galaxies have broadly similar sizes, but ellipticals extend to notably greater diameters, reaching 150 kpc. We propose that the extended envelopes of disk galaxies are dominated by an extension of the disk population rather than by a classical population II halo.

Recent studies using Gaia DR2 have identified a massive merger in the history of the Milky Way (MW) whose debris is markedly radial and counterrotating. This event, known as the Gaia-Enceladus/Gaia-Sausage (GE/GS), is also hypothesized to have built the majority of the inner stellar halo. We use the cosmological hydrodynamic simulation Illustris to place this merger in the context of galaxy assembly within ΛCDM. From ∼150 MW analogs, ∼ 80% have experienced at least one merger of similar mass and infall time as GE/GS. Within this sample, 37 have debris as radial as that of the GE/GS, which we dub the Ancient Radial Mergers (ARMs). Counterrotation is not rare among ARMs, with 43% having > 40% of their debris in counterrotating orbits. However, the compactness inferred for the GE/GS debris, given its large β and its substantial contribution to the stellar halo, is more difficult to reproduce. The median radius of ARM debris is r * ,deb 45kpc, while GE/GS is thought to be mostly contained within r ∼ 30 kpc. For most MW analogs, a few mergers are required to build the inner stellar halo, and ARM debris only accounts for ∼ 12% of inner accreted stars. Encouragingly, we find one ARM that is both compact and dominates the inner halo of its central, making it our best GE/GS analog. Interestingly, this merger deposits a significant number of stars (M * 1.5 × 10 9 M ) in the outer halo, suggesting that an undiscovered section of GE/GS may await detection.

The standard cosmological model (Λ cold dark matter, ΛCDM) predicts the existence of the cosmic web: a distribution of matter into sheets and filaments connecting massive haloes. However, observational evidence has been elusive due to the low surface brightness levels of the filaments. Recent deep Multi Unit Spectroscopic Explorer (MUSE)/Very Large Telescope (VLT) data and upcoming observations offer a promising avenue for Lyα detection, motivating the development of modern theoretical predictions. We use hydrodynamical cosmological simulations run with the arepo code to investigate the potential detectability of large-scale filaments, excluding contributions from the haloes embedded in them. We focus on filaments connecting massive ($M_{200c}\sim (1\!-\!3)\times 10^{12}\hbox{$\rm \, M_{\odot }$}$) haloes at z = 3, and compare different simulation resolutions, feedback levels, and mock image pixel sizes. We find increasing simulation resolution does not substantially improve detectability notwithstanding the intrinsic enhancement of internal filament structure. By contrast, for a MUSE integration of 31 h, including feedback increases the detectable area by a factor of ≃5.5 on average compared with simulations without feedback, implying that even the non-bound components of the filaments have substantial sensitivity to feedback. Degrading the image resolution from the native MUSE scale of 0.2 arcsec2 pixel−1 to 5.3 arcsec2 apertures has the strongest effect, increasing the detectable area by a median factor of ≃200 and is most effective when the size of the pixel roughly matches the width of the filament. Finally, we find the majority of Lyα emission is due to electron impact collisional excitations, as opposed to radiative recombination.