Kinematic Evidence for Superbubbles in I Zw 18: Constraints on the Star Formation History and Chemical Evolution

We have combined measurements of the kinematics, morphology, and oxygen abundance of the ionized gas in \IZw18, one of the most metal-poor galaxies known, to examine the star formation history and chemical mixing processes.Comment: 31 pages including 6 figures. Accepted for publication in the Astrophysical Journa

Resolving Gas Flows in the Ultraluminous Starburst IRAS23365+3604 with Keck LGSAO/OSIRIS

Keck OSIRIS/LGSAO observations of the ultraluminous galaxy IRAS~23365+3604 resolve a circumnuclear bar (or irregular disk) of semimajor axis 0.42" (520 pc) in Paschen-alpha emission. The line-of-sight velocity of the ionized gas increases from the northeast toward the southwest; this gradient is perpendicular to the photometric major axis of the infrared emission. Two pairs of bends in the zero-velocity line are detected. The inner bend provides evidence for gas inflow onto the circumnuclear disk/bar structure. We interpret the gas kinematics on kiloparsec scales in relation to the molecular gas disk and multiphase outflow discovered previously. In particular, the fast component of the outflow (detected previously in line wings) is not detected, adding support to the conjecture that the fast wind originates well beyond the nucleus. These data directly show the dynamics of gas inflow and outflow in the central kiloparsec of a late-stage, gas-rich merger and demonstrate the potential of integral field spectroscopy to improve our understanding of the role of gas flows during the growth phase of bulges and supermassive black holes.Comment: 14 pages with 7 figures accepted to the astrophysical journa

Resolving 3D Disk Orientation using High-Resolution Images: New Constraints on Circumgalactic Gas Inflows

We constrain gas inflow speeds in star-forming galaxies with color gradients consistent with inside-out disk growth. Our method combines new measurements of disk orientation with previously described circumgalactic absorption in background quasar spectra. Two quantities, a position angle and an axis ratio, describe the projected shape of each galactic disk on the sky, leaving an ambiguity about which side of the minor axis is tipped toward the observer. This degeneracy regarding the 3D orientation of disks has compromised previous efforts to measure gas inflow speeds. We present HST and Keck/LGSAO imaging that resolves the spiral structure in five galaxies at redshift $z\approx0.2$. We determine the sign of the disk inclination for four galaxies, under the assumption that spiral arms trail the rotation. We project models for both radial infall in the disk plane and circular orbits onto each quasar sightline. We compare the resulting line-of-sight velocities to the observed velocity range of Mg II absorption in spectra of background quasars, which intersect the disk plane at radii between 69 and 115 kpc. For two sightlines, we constrain the maximum radial inflow speeds as 30-40 km s$^{-1}$. We also rule out a velocity component from radial inflow in one sightline, suggesting that the structures feeding gas to these growing disks do not have unity covering factor. We recommend appropriate selection criteria for building larger samples of galaxy--quasar pairs that produce orientations sensitive to constraining inflow properties.Comment: 15 pages with 8 figures and 2 tables; accepted for publication in Ap

The ring around SN1987A

Stars in the 9-40 solar mass range play a prominent role in the hydrodynamical and chemical evolution of galaxies. Their stellar winds and supernova explosions are believed to create the hot component of the interstellar medium (ISM). In some galactic disks, the kiloparsec sized super bubbles formed around clusters of massive stars may blow out of the disk plane and release hot, metal enriched gas into the galaxy's halo. Additionally, the expanding shock front of a super bubble in the disk may trigger additional star formation. Furthermore, similar processes probably drive the galactic winds associated with star burst nuclei that enrich the intracluster and intergalactic mediums. Nonetheless, the explosion of a blue super giant in the Large Magellanic Cloud (LMC), SN1987A, illuminated the incompleteness of our understanding of massive stars. Evolutionary models of massive stars do not synthesize the observed super giant populations in either the Milky Way or LMC. Our modeling of the formation of SN1987A's ring will improve our knowledge of both the post-main-sequence evolution of massive stars and their coupling to the ISM in galaxies

Temperature-based metallicity measurements at z=0.8: direct calibration of strong-line diagnostics at intermediate redshift

We present the first direct calibration of strong-line metallicity diagnostics at significant cosmological distances using a sample at z=0.8 drawn from the DEEP2 Galaxy Redshift Survey. Oxygen and neon abundances are derived from measurements of electron temperature and density. We directly compare various commonly used relations between gas-phase metallicity and strong line ratios of O, Ne, and H at z=0.8 and z=0. There is no evolution with redshift at high precision ($\Delta \log{\mathrm{O/H}} = -0.01\pm0.03$, $\Delta \log{\mathrm{Ne/O}} = 0.01 \pm 0.01$). O, Ne, and H line ratios follow the same locus at z=0.8 as at z=0 with $\lesssim$0.02 dex evolution and low scatter ($\lesssim$0.04 dex). This suggests little or no evolution in physical conditions of HII regions at fixed oxygen abundance, in contrast to models which invoke more extreme properties at high redshifts. We speculate that offsets observed in the [N II]/H$\alpha$ versus [O III]/H$\beta$ diagram at high redshift are therefore due to [NII] emission, likely as a result of relatively high N/O abundance. If this is indeed the case, then nitrogen-based metallicity diagnostics suffer from systematic errors at high redshift. Our findings indicate that locally calibrated abundance diagnostics based on alpha-capture elements can be reliably applied at z$\simeq$1 and possibly at much higher redshifts. This constitutes the first firm basis for the widespread use of empirical calibrations in high redshift metallicity studies.Comment: 14 pages, 10 figures, accepted to Ap

Spatially Resolved Gas Kinematics within a Lyα\alpha Nebula: Evidence for Large-scale Rotation

We use spatially extended measurements of Ly$\alpha$ as well as less optically thick emission lines from an $\approx$80 kpc Ly$\alpha$ nebula at $z\approx1.67$ to assess the role of resonant scattering and to disentangle kinematic signatures from Ly$\alpha$ radiative transfer effects. We find that the Ly$\alpha$, CIV, HeII, and CIII] emission lines all tell a similar story in this system, and that the kinematics are broadly consistent with large-scale rotation. First, the observed surface brightness profiles are similar in extent in all four lines, strongly favoring a picture in which the Ly$\alpha$ photons are produced in situ instead of being resonantly scattered from a central source. Second, we see low kinematic offsets between Ly$\alpha$ and the less optically thick HeII line ($\sim$100-200 km s$^{-1}$), providing further support for the argument that the Ly$\alpha$ and other emission lines are all being produced within the spatially extended gas. Finally, the full velocity field of the system shows coherent velocity shear in all emission lines: $\approx$500 km s$^{-1}$ over the central $\approx$50 kpc of the nebula. The kinematic profiles are broadly consistent with large-scale rotation in a gas disk that is at least partially stable against collapse. These observations suggest that the Ly$\alpha$ nebula represents accreting material that is illuminated by an offset, hidden AGN or distributed star formation, and that is undergoing rotation in a clumpy and turbulent gas disk. With an implied mass of M(<R=20 kpc)$\sim3\times10^{11}$ $M_{\odot}$, this system may represent the early formation of a large Milky Way mass galaxy or galaxy group.Comment: Accepted to ApJ; 25 pages in emulateapj format; 15 figures, 4 table

Lyα\alpha emission from Green Peas: the role of circumgalactic gas density, covering, and kinematics

We report Hubble Space Telescope/Cosmic Origins Spectrograph observations of the Ly$\alpha$ emission and interstellar absorption lines in a sample of ten star-forming galaxies at $z\sim0.2$. Selected on the basis of high equivalent width optical emission lines, the sample, dubbed "Green Peas," make some of the best analogs for young galaxies in an early Universe. We detect Ly$\alpha$ emission in all ten galaxies, and 9/10 show double-peaked line profiles suggestive of low HI column density. We measure Ly$\alpha$/H$\alpha$ flux ratios of 0.5-5.6, implying that 5% to 60% of Ly$\alpha$ photons escape the galaxies. These data confirm previous findings that low-ionization metal absorption (LIS) lines are weaker when Ly$\alpha$ escape fraction and equivalent width are higher. However, contrary to previously favored interpretations of this trend, increased Ly$\alpha$ output cannot be the result of varying HI covering: the Lyman absorption lines (Ly$\beta$ and higher) show a covering fraction near unity for gas with $N_{HI} \gtrsim 10^{16}$ cm$^{-2}$. Moreover, we detect no correlation between Ly$\alpha$ escape and the outflow velocity of the LIS lines, suggesting that kinematic effects do not explain the range of Ly$\alpha$/H$\alpha$ flux ratios in these galaxies. In contrast, we detect a strong anti-correlation between the Ly$\alpha$ escape fraction and the velocity separation of the Ly$\alpha$ emission peaks, driven primarily by the velocity of the blue peak. As this velocity separation is sensitive to HI column density, we conclude that Ly$\alpha$ escape in these Green Peas is likely regulated by the HI column density rather than outflow velocity or HI covering fraction.Comment: 27 pages, 26 figures; Accepted for publication in the Astrophysical Journa