In the local (redshift z~0) Universe, collisional ring galaxies make up only~0.01% of galaxies and are formed by head-on galactic collisions that triggerradially propagating density waves. These striking systems provide keysnapshots for dissecting galactic disks and are studied extensively in thelocal Universe. However, not much is known about distant (z>0.1) collisionalrings. Here we present a detailed study of a ring galaxy at a look-back time of10.8 Gyr (z=2.19). Compared with our Milky Way, this galaxy has a similarstellar mass, but has a stellar half-light radius that is 1.5-2.2 times largerand is forming stars 50 times faster. The large, diffuse stellar light outsidethe star-forming ring, combined with a radial velocity on the ring and anintruder galaxy nearby, provides evidence for this galaxy hosting a collisionalring. If the ring is secularly evolved, the implied large bar in a giant diskwould be inconsistent with the current understanding of the earliest formationof barred spirals. Contrary to previous predictions, this work suggests thatmassive collisional rings were as rare 11 Gyr ago as they are today. Ourdiscovery offers a unique pathway for studying density waves in young galaxies,as well as constraining the cosmic evolution of spiral disks and galaxy groups.

We present detections of 21-cm emission from neutral hydrogen (HI) in thecircumgalactic medium (CGM) of the local edge-on galaxies NGC 891 and NGC 4565using the Robert C. Byrd Green Bank Telescope (GBT). With our 5$\sigma$sensitivity of $8.2 \times 10^{16}$ cm$^{-2}$ calculated over a 20 km s$^{-1}$channel, we achieve $>5\sigma$ detections out to $90-120$ kpc along the minoraxes. The velocity width of the CGM emission is as large as that of the disk$\approx 500$ km s$^{-1}$, indicating the existence of a diffuse componentpermeating the halo. We compare our GBT measurements with interferometric datafrom the Westerbork Synthesis Radio Telescope (WSRT). The WSRT maps the HIemission from the disk at high S/N but has limited surface brightnesssensitivity at the angular scales probed with the GBT. After convolving theWSRT data to the spatial resolution of the GBT (FWHM = 9.1$'$), we find thatthe emission detected by the WSRT accounts for $48^{+15}_{-25}$%($58^{+4}_{-18}$%) of the total flux recovered by the GBT from the CGM of NGC891(NGC 4565). The existence of significant GBT-only flux suggests the presenceof a large amount of diffuse, low column density HI emission in the CGM. Forreasonable assumptions, the extended diffuse HI could account for $5.2\pm0.9$%and $2.0\pm0.8$% of the total HI emission of NGC 891 and NGC 4565.

We present a catalog of holes and shells in the neutral atomic hydrogen (\HI)of 41 gas-rich dwarf galaxies in LITTLE THINGS (Local Irregulars That TraceLuminosity Extremes, The \HI Nearby Galaxy Survey). We analyzed theirproperties as part of an investigation into the relation between star formationand structures and kinematics in the \HI of small galaxies. We confirmed 306holes between 38 pc (our resolution limit) and 2.3 kpc, with expansionvelocities up to 30 \kms. The global star formation rates measured by \Ha andFUV emission are consistent with those estimated from the energy required tocreate the cataloged holes in our sample. Although we found no obviouscorrelation between global star-formation rates and the \HI surface and volumeporosities of our sample, two of the four lowest porosity galaxies and the twohighest porosity galaxies have no recent star formation as measured by \Ha andFUV emission.

The Sloan Digital Sky Survey (SDSS) has proved to be a powerful resource forunderstanding the physical properties and chemical composition of star-forminggalaxies in the local universe. The SDSS population of active galactic nuclei(AGN) remains as of yet less explored in this capacity. To extend the rigorousstudy of \ion{H}{ii} regions in the SDSS to AGN, we adapt methods for computingdirect-method chemical abundances for application to the narrow-line regions(NLR) of AGN. By accounting for triply-ionized oxygen, we are able to morecompletely estimate the total oxygen abundance. We find a strong correlationbetween electron temperature and oxygen abundance due to collisional cooling bymetals. Furthermore, we find that nitrogen and oxygen abundances in AGN arestrongly correlated. From the metal-temperature relation and the coupling ofnitrogen and oxygen abundances, we develop a new, empirically and physicallymotivated method for determining chemical abundances from the strong emissionlines commonly employed in flux-ratio diagnostic diagrams (BPT diagrams). Ourapproach, which for AGN reduces to a single equation based on the BPT lineratios, consistently recovers direct-method abundances over a 1.5 dex range inoxygen abundance with an rms uncertainty of 0.18 dex. We have determinedmetallicities for thousands of AGN in the SDSS, and in the process havediscovered an ionization-related discriminator for Seyfert and LINER galaxies.

We present kinematic orientations and high resolution (150 pc) rotationcurves for 67 main sequence star-forming galaxies surveyed in CO (2-1) emissionby PHANGS-ALMA. Our measurements are based on the application of a new fittingmethod tailored to CO velocity fields. Our approach identifies an optimalglobal orientation as a way to reduce the impact of non-axisymmetric (bar andspiral) features and the uneven spatial sampling characteristic of CO emissionin the inner regions of nearby galaxies. The method performs especially wellwhen applied to the large number of independent lines-of-sight contained in thePHANGS CO velocity fields mapped at 1'' resolution. The high resolutionrotation curves fitted to these data are sensitive probes of mass distributionin the inner regions of these galaxies. We use the inner slope as well as theamplitude of our fitted rotation curves to demonstrate that CO is a reliableglobal dynamical mass tracer. From the consistency between photometricorientations from the literature and kinematic orientations determined with ourmethod, we infer that the shapes of stellar disks in the mass range of log($\rmM_{\star}(M_{\odot})$)=9.0-10.9 probed by our sample are very close to circularand have uniform thickness.

We present results from a 577 ks XMM-Newton observation of SPT-CL J0459-4947,the most distant cluster detected in the South Pole Telescope 2500 squaredegree (SPT-SZ) survey, and currently the most distant cluster discoveredthrough its Sunyaev-Zel'dovich effect. The data confirm the cluster's highredshift, $z=1.71 \pm 0.02$, in agreement with earlier, less precise optical/IRphotometric estimates. From the gas density profile, we estimate acharacteristic mass of $M_{500}=(1.8 \pm 0.2) \times 10^{14}M_{Sun}$; clusteremission is detected above the background to a radius of $\sim 2.2 r_{500}$, orapproximately the virial radius. The intracluster gas is characterized by anemission-weighted average temperature of $7.2 \pm 0.3$ keV and metallicity withrespect to Solar of $0.37 \pm 0.08$. For the first time at such high redshift,this deep data set provides a measurement of metallicity outside the clustercenter; at radii $r > 0.3 r_{500}$, we find it to be $0.33 \pm 0.17$, in goodagreement with precise measurements at similar radii in the most nearbyclusters, supporting an early enrichment scenario in which the bulk of thecluster gas is enriched to a universal metallicity prior to cluster formation,with little to no evolution thereafter. The leverage provided by the highredshift of this cluster tightens by a factor of 2 constraints on evolvingmetallicity models, when combined with previous measurements at lowerredshifts.