We have characterized a sample of extended X-ray sources in the A1367 galaxycluster that lack optical counterparts. The sources are galaxy size and have anaverage total mass of $1.3\times10^{11}$ solar masses. The average hot gas massis $3.0\times10^{9}$ solar masses and the average X-ray luminosity is$4.3\times10^{41}$ erg cm$^{-2}$ s$^{-1}$. Analysis of a composite sourcespectrum indicates the X-ray emission is thermal, with temperature of 1.25 -1.45 keV and has low metallicity, 0.026 - 0.067 solar. The average hot gasradius (12.7 kpc) is well matched to nominal stripping radius. We argue thatthis optically dark, X-ray bright galaxy population forms by a sequence ofstripping followed by heating and mixing with the intracluster medium.

We present results from deep HI and optical imaging of AGC 229101, an unusualHI source detected at v$_{\rm helio}$ = 7116 km/s in the ALFALFA survey.Initially classified as a candidate "dark" source because it lacks a clearoptical counterpart in SDSS or DSS2 imaging, AGC 229101 has $10^{9.31\pm0.05}$solar masses of HI, but an HI line width of only 43$\pm$9 km/s. Low resolutionWSRT imaging and higher resolution VLA B-array imaging show that the source issignificantly elongated, stretching over a projected length of ~80 kpc. The HIimaging resolves the source into two parts of roughly equal mass. WIYN pODIoptical imaging reveals a faint, blue optical counterpart coincident with thenorthern portion of the HI. The peak surface brightness of the optical sourceis only $\mu_{g}$ = 26.6 mag arcsec$^{-2}$, well below the typical cutoff thatdefines the isophotal edge of a galaxy, and its estimated stellar mass is only$10^{7.32\pm0.33}$ solar masses, yielding an overall neutral gas-to-stellarmass ratio of M$_{\rm HI}$/M$_*=$~98$_{+111}\atop^{-52}$. We demonstrate theextreme nature of this object by comparing its properties to those of otherHI-rich sources in ALFALFA and the literature. We also explore potentialscenarios that might explain the existence of AGC~229101, including a tidalencounter with neighboring objects and a merger of two dark HI clouds.

Star formation in half of massive galaxies was quenched by the time theUniverse was three billion years old. Very low amounts of molecular gas appearresponsible for this, at least in some cases, though morphological gasstabilization, shock heating, or activity associated with accretion onto acentral supermassive black hole is invoked in other cases. Recent studies ofquenching by gas depletion have been based upon upper limits that areinsufficiently sensitive to determine this robustly, or stacked emission withits problems of averaging. Here we report 1.3mm observations of dust emissionfrom six strongly lensed galaxies where star formation has been quenched, withmagnifications of up to a factor of 30. Four of the six galaxies are undetectedin dust emission, with an estimated upper limit on the dust mass of 0.0001times the stellar mass, and by proxy (assuming a Milky Way moleculargas-to-dust ratio) 0.01 times the stellar mass in molecular gas. This is twoorders of magnitude less molecular gas per unit stellar mass than seen in starforming galaxies at similar redshifts. It remains difficult to extrapolate fromthese small samples, but these observations establish that gas depletion isresponsible for a cessation of star formation in some fraction of high-redshiftgalaxies.

The existence of massive galaxies lacking a classical bulge has often beenproposed as a challenge to $\Lambda$CDM. However, recent simulations proposethat a fraction of massive disc galaxies might have had very quiescent mergerhistories, and also that mergers do not necessarily build classical bulges. Wetest these ideas with deep MUSE observations of NGC 5746, a massive ($\sim10^{11}$ M$_\odot$) edge-on disc galaxy with no classical bulge. We analyse itsstellar kinematics and stellar populations, and infer that a massive andextended disc formed very early: 80% of the galaxy's stellar mass formed morethan 10 Gyr ago. Most of the thick disc and the bar formed during that earlyphase. The bar drove gas towards the center and triggered the formation of thenuclear disc followed by the growth of a boxy/peanut-shaped bulge. Around$\sim$ 8 Gyr ago, a $\sim$1:10 merger happened, possibly on a low-inclinationorbit. The satellite did not cause significant vertical heating, did notcontribute to the growth of a classical bulge, and did not destroy the bar andthe nuclear disc. It was however an important event for the galaxy: bydepositing its stars throughout the whole galaxy it contributed $\sim 30$% ofaccreted stars to the thick disc. NGC 5746 thus did not completely escapemergers, but the only relatively recent significant merger did not damage thegalaxy and did not create a classical bulge. Future observations will reveal ifthis is representative of the formation histories of massive disc galaxies.