We study the dark matter (DM) assembly in the central regions of massiveearly-type galaxies up to $z\sim 0.65$. We use a sample of $\sim 3800$ massive($\log M_{\rm \star}/M_{\rm \odot} > 11.2$) galaxies with photometry andstructural parameters from 156 sq. deg. of the Kilo Degree Survey, andspectroscopic redshifts and velocity dispersions from SDSS. We obtain centraltotal-to-stellar mass ratios, $M_{\rm dyn}/M_{\rm \star}$, and DM fractions, bydetermining dynamical masses, $M_{\rm dyn}$, from Jeans modelling of SDSSaperture velocity dispersions and stellar masses, $M_{\rm \star}$, from KiDSgalaxy colours. We first show how the central DM fraction correlates withstructural parameters, mass and density proxies, and demonstrate that most ofthe local correlations are still observed up to $z \sim 0.65$; at fixed $M_{\rm\star}$, local galaxies have larger DM fraction, on average, than theircounterparts at larger redshift. We also interpret these trends with a nonuniversal Initial Mass Function (IMF), finding a strong evolution withredshift, which contrast independent observations and is at odds with theeffect of galaxy mergers. For a fixed IMF, the galaxy assembly can beexplained, realistically, by mass and size accretion, which can be physicallyachieved by a series of minor mergers. We reproduce both the $R_{\rm e}-M_{\rm\star}$ and $M_{\rm dyn}/M_{\rm \star}-M_{\rm \star}$ evolution with stellarand dark mass changing at a different rate. This result suggests that the mainprogenitor galaxy is merging with less massive systems, characterized by asmaller $M_{\rm dyn}/M_{\rm \star}$, consistently with results from haloabundance matching.

A major question in galaxy formation is how the gas supply that fuelsactivity in galaxies is modulated by their environment. We use spectroscopy ofa set of well characterized clusters and groups at $0.4<z<0.8$ from the ESODistant Cluster Survey (EDisCS) and compare it to identically selected fieldgalaxies. Our spectroscopy allows us to isolate galaxies that are dominated byold stellar populations. Here we study a stellar-mass limited sample($\log(M_*/M_\odot)>10.4$) of these old galaxies with weak [OII] emission. Weuse line ratios and compare to studies of local early type galaxies to concludethat this gas is likely excited by post-AGB stars and hence represents adiffuse gas component in the galaxies. For cluster and group galaxies thefraction with EW([OII])$>5$\AA\ is $f_{[OII]}=0.08^{+0.03}_{-0.02}$ and$f_{[OII]}=0.06^{+0.07}_{-0.04}$ respectively. For field galaxies we find$f_{[OII]}=0.27^{+0.07}_{-0.06}$, representing a 2.8$\sigma$ difference betweenthe [OII] fractions for old galaxies between the different environments. Weconclude that a population of old galaxies in all environments has ionized gasthat likely stems from stellar mass loss. In the field galaxies also experiencegas accretion from the cosmic web and in groups and clusters these galaxieshave had their gas accretion shut off by their environment. Additionally,galaxies with emission preferentially avoid the virialized region of thecluster in position-velocity space. We discuss the implications of our results,among which is that gas accretion shutoff is likely effective at group halomasses (log~${\cal M}/$\msol$>12.8$) and that there are likely multiple gasremoval processes happening in dense environments.

Galaxy formation depends critically on the physical state of gas in thecircumgalactic medium (CGM) and its interface with the intergalactic medium(IGM), determined by the complex interplay between inflows from the IGM andoutflows from supernovae or AGN feedback. The average Lyman-alpha (Ly-a)absorption profile around galactic halos represents a powerful tool to probetheir gaseous environments. We compare predictions from Illustris and Nyxhydrodynamical simulations with the observed absorption around foregroundquasars, damped Ly-a systems, and Lyman-break galaxies. We show how large-scaleBOSS and small-scale quasar pair measurements can be combined to preciselyconstrain the absorption profile over three decades in transverse distance20kpc$\lesssim b\lesssim$20Mpc. Far from galaxies $\gtrsim2$Mpc, thesimulations converge to the same profile and provide a reasonable match to theobservations. This asymptotic agreement arises because the $\Lambda$CDM modelsuccessfully describes the ambient IGM, and represents a critical advantage ofstudying the mean absorption profile. However, significant differences betweenthe simulations, and between simulations and observations are present on scales20kpc$\lesssim b\lesssim$2Mpc, illustrating the challenges of accuratelymodeling and resolving galaxy formation physics. It is noteworthy that thesedifferences are observed as far out as $\sim2$Mpc, indicating that the`sphere-of-influence' of galaxies could extend to approximately $\sim20$ timesthe halo virial radius ($\sim100$kpc). Current observations are very precise onthese scales and can thus strongly discriminate between different galaxyformation models. We demonstrate that the Ly-a absorption profile is primarilysensitive to the underlying temperature-density relationship of diffuse gasaround galaxies, and argue that it thus provides a fundamental test of galaxyformation models.

We present our catalog of extended low-surface-brightness galaxies (LSBGs)identified in the Wide layer of the Hyper Suprime-Cam Subaru Strategic Program(HSC-SSP). Using the first ${\sim}$200 deg$^2$ of the survey, we have uncovereda rich diversity of LSB phenomena, including red ($g-i\geq0.64$) and blue($g-i<0.64$) LSBGs with a wide range of morphologies, tidal debris from galaxyinteractions, and cirrus emission from Galactic dust. We publish a catalog of781 LSBGs, which, because we focus on angularly extended galaxies($r_\mathrm{eff}=2.5$-$14^{\prime\prime}$), is likely dominated by low-redshiftobjects. We define LSBGs to have mean surface brightnesses$\bar{\mu}_\mathrm{eff}(g)>24.3$ mag arcsec$^{-2}$, which allows nucleatedgalaxies into our sample. As a result, the central surface brightnessdistribution spans a wide range of $\mu_0(g)=18$-$27.4$ mag arcsec$^{-2}$, with50% and 95% of galaxies fainter than 24.3 and 22 mag arcsec$^{-2}$,respectively. Furthermore, the mean surface brightness distribution is a strongfunction of color, with the red distribution being much broader and generallyfainter than that of the blue LSBGs, and this trend shows a clear correlationwith galaxy morphology. Red LSBGs typically have smooth light profiles that arewell-characterized by single-component S\'{e}rsic functions. In contrast, blueLSBGs tend to have irregular morphologies and show evidence for ongoing starformation. We crossmatch our sample with existing optical, HI, and ultravioletcatalogs to gain insight into the physical nature of the LSBGs. We find thatour sample is diverse, with galaxies ranging from dwarf spheroidals andultra-diffuse galaxies in nearby groups to gas-rich (physically large)irregulars to giant LSB spirals, demonstrating the potential of the HSC-SSP toprovide a truly unprecedented view of the galaxy population at low surfacebrightnesses.