Self-Interacting Dark Matter (SIDM) has long been proposed as a solution tosmall scale problems posed by standard Cold Dark Matter (CDM). We use numericalsimulations to study the effect of dark matter interactions on the morphologyof disk galaxies falling into galaxy clusters. The effective drag force on darkmatter leads to offsets of the stellar disk with respect to the surroundinghalo, causing distortions in the disk. For anisotropic scatteringcross-sections of 0.5 and 1.0$\,\textrm{cm}^{2}\textrm{g}^{-1}$, we show thatpotentially observable warps, asymmetries, and thickening of the disk occur insimulations. We discuss observational tests of SIDM with galaxy surveys andmore realistic simulations needed to obtain detailed predictions.

We use the ALFALFA HI survey to examine whether the cold gas reservoirs ofgalaxies are inhibited or enhanced in large-scale filaments. Our sampleincludes 9947 late-type galaxies with HI detections, and 4236 late-typegalaxies with well-determined HI detection limits that we incorporate usingsurvival analysis statistics. We find that, even at fixed local density andstellar mass, and with group galaxies removed, the HI deficiency of galaxies inthe stellar mass range 8.5 <log(M/Mo) < 10.5 decreases with distance from thefilament spine, suggesting that galaxies are cut off from their supply of coldgas in this environment. We also find that, at fixed local density and stellarmass, the galaxies that are the most gas-rich are those in small, correlated"tendril" structures within voids: although galaxies in tendrils are insignificantly denser environments, on average, than galaxies in voids, they arenot redder or more HI deficient. This stands in contrast to the fact thatgalaxies in tendrils are more massive than those in voids, suggesting a moreadvanced stage of evolution. Finally, at fixed stellar mass and color, galaxiescloser to the filament spine, or in high density environments, are moredeficient in HI. This fits a picture where, as galaxies enter denser regions,they first lose HI gas and then redden as star formation is reduced.

According to the current understanding of cosmic structure formation, theprecursors of the most massive structures in the Universe began to form shortlyafter the Big Bang, in regions corresponding to the largest fluctuations in thecosmic density field. Observing these structures during their period of activegrowth and assembly - the first few hundred million years of the Universe - ischallenging because it requires surveys that are sensitive enough to detect thedistant galaxies that act as signposts for these structures and wide enough tocapture the rarest objects. As a result, very few such objects have beendetected so far. Here we report observations of a far-infrared-luminous objectat redshift 6.900 (less than 800 Myr after the Big Bang) that was discovered ina wide-field survey. High-resolution imaging reveals this source to be a pairof extremely massive star-forming galaxies. The larger of these galaxies isforming stars at a rate of 2900 solar masses per year, contains 270 billionsolar masses of gas and 2.5 billion solar masses of dust, and is more massivethan any other known object at a redshift of more than 6. Its rapid starformation is probably triggered by its companion galaxy at a projectedseparation of just 8 kiloparsecs. This merging companion hosts 35 billion solarmasses of stars and has a star-formation rate of 540 solar masses per year, buthas an order of magnitude less gas and dust than its neighbor and physicalconditions akin to those observed in lower-metallicity galaxies in the nearbyUniverse. These objects suggest the presence of a dark-matter halo with a massof more than 400 billion solar masses, making it among the rarest dark-matterhaloes that should exist in the Universe at this epoch.

We present a kpc-scale analysis of the relationship between the moleculardepletion time ($\tau_\mathrm{dep}^\mathrm{mol}$) and the orbital time($\tau_\mathrm{orb}$) across the field of 39 face-on local galaxies, selectedfrom the EDGE-CALIFA sample. We find that, on average, 5% of the availablemolecular gas is converted into stars per orbital time, or$\tau_\mathrm{dep}^\mathrm{mol}\sim20\tau_\mathrm{orb}$. The resolved relationshows a scatter of $\sim0.5$ dex. The scatter is ascribable to galaxies ofdifferent morphologies that follow different$\tau_\mathrm{dep}^\mathrm{mol}-\tau_\mathrm{orb}$ relations which decrease insteepness from early- to late-types. The morphologies appear to be linked withthe star formation rate surface density, the molecular depletion time, and theorbital time, but they do not correlate with the molecular gas content of thegalaxies in our sample. We speculate that in our molecular gas rich, early-typegalaxies, the morphological quenching (in particular the disc stabilization viashear), rather than the absence of molecular gas, is the main factorresponsible for their current inefficient star formation.

We determine abundance ratios of 37 dwarf ellipticals (dEs) in the nearbyVirgo cluster. This sample is representative of the early-type population ofgalaxies in the absolute magnitude range -19.0 < Mr < -16.0. We analyze theirabsorption line-strength indices by means of index-index diagrams and scalingrelations and use the stellar population models to interpret them. We presentages, metallicities and abundance ratios obtained from these dEs within anaperture size of Re/8. We calculate [Na/Fe] from NaD, [Ca/Fe] from Ca4227 and[Mg/Fe] from Mgb. We find that [Na/Fe] is under-abundant with respect to solarwhile [Mg/Fe] is around solar. This is exactly opposite to what is found forgiant ellipticals, but follows the trend with metallicity found previously forthe Fornax dwarf NGC 1396. We discuss possible formation scenarios that canresult in such elemental abundance patterns and we speculate that dEs havedisk-like SFH favouring them to originate from late-type dwarfs or smallspirals. Na-yields appear to be very metal-dependent, in agreement with studiesof giant ellipticals, probably due to the large dependence on theneutron-excess in stars. We conclude that dEs have undergone a considerableamount of chemical evolution, they are therefore not uniformly old, but haveextended SFH, similar to many of the Local Group galaxies.