Small-Scale Structure of O VI Interstellar Gas in the Direction of the Globular Cluster NGC 6752

In order to study the small-scale structure of the hot interstellar gas, we obtained Far Ultraviolet Spectroscopic Explorer interstellar O VI interstellar absorption spectra of 4 four post-extreme horizontal branch stars in the globular cluster NGC 6752 [(l,b) = (336.50,-25.63), d = 3.9 kpc, z= -1.7 kpc]. The good quality spectra of these stars allow us to measure both lines of the O VI doublet at 1031.926 \AA and 1037.617 \AA. The close proximity of these stars permits us to probe the hot interstellar gas over angular scale of only 2\farcm2 - 8\farcm9, corresponding to spatial scales \la 2.5-10.1 pc. On these scales we detect no variations in the O VI column density and velocity distribution. The average column density is log = 14.34 \pm 0.02 (log = 13.98). The measured velocity dispersions of the O VI absorption are also indistinguishable. Including the earlier results of Howk et al., this study suggests that interstellar O VI is smooth on scales \Delta \theta \la 12\arcmin, corresponding to a spatial scale of less than 10 pc, and quite patchy at larger scales. Although such small scales are only probed in a few directions, this suggests a characteristic size scale for the regions producing collisionally-ionized O VI in the Galaxy.Comment: Accepted for publication in the PASP (to appear in the October 2004 issue

A Reservoir of Ionized Gas in the Galactic Halo to Sustain Star Formation in the Milky Way

Without a source of new gas, our Galaxy would exhaust its supply of gas through the formation of stars. Ionized gas clouds observed at high velocity may be a reservoir of such gas, but their distances are key for placing them in the Galactic halo and unraveling their role. We have used the Hubble Space Telescope to blindly search for ionized high-velocity clouds (iHVCs) in the foreground of Galactic stars. We show that iHVCs with 90 < |v_LSR| < 170 km/s are within one Galactic radius of the sun and have enough mass to maintain star formation, while iHVCs with |v_LSR|>170 km/s are at larger distances. These may be the next wave of infalling material.Comment: This paper is part of a set of three papers on circumgalactic gas observed with COS and STIS on HST, to be published in Science, together with related papers by Tripp et al. and Tumlinson et al., in the November 18, 2011 edition. This version has not undergone final copyediting. Please see Science online for the final printed versio

The First Data Release of the KODIAQ Survey

We present and make publicly available the first data release (DR1) of the Keck Observatory Database of Ionized Absorption toward Quasars (KODIAQ) survey. The KODIAQ survey is aimed at studying galactic and circumgalactic gas in absorption at high-redshift, with a focus on highly-ionized gas traced by OVI, using the HIRES spectrograph on the Keck-I telescope. KODIAQ DR1 consists of a fully-reduced sample of 170 quasars at 0.29 < z_em < 5.29 observed with HIRES at high resolution (36,000 <= R <= 103,000) between 2004 and 2012. DR1 contains 247 spectra available in continuum normalized form, representing a sum total exposure time of ~1.6 megaseconds. These co-added spectra arise from a total of 567 individual exposures of quasars taken from the Keck Observatory Archive (KOA) in raw form and uniformly processed using a HIRES data reduction package made available through the XIDL distribution. DR1 is publicly available to the community, housed as a higher level science product at the KOA. We will provide future data releases that make further QSOs, including those with pre-2004 observations taken with the previous-generation HIRES detectors.Comment: 14 pages, 9 figures, Submitted to AJ. All data products available at the Keck Observatory Archive beginning May 15, 2015. URL: https://koa.ipac.caltech.edu/applications/KODIA

An HST/COS legacy survey of high-velocity ultraviolet absorption in the Milky Way's circumgalactic medium and the Local Group

To characterize the absorption properties of this circumgalactic medium (CGM) and its relation to the LG we present the so-far largest survey of metal absorption in Galactic high-velocity clouds (HVCs) using archival ultraviolet (UV) spectra of extragalactic background sources. The UV data are obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST) and are supplemented by 21 cm radio observations of neutral hydrogen. Along 270 sightlines we measure metal absorption in the lines of SiII, SiIII, CII, and CIV and associated HI 21 cm emission in HVCs in the velocity range |v_LSR|=100-500 km s^-1. With this unprecedented large HVC sample we were able to improve the statistics on HVC covering fractions, ionization conditions, small-scale structure, CGM mass, and inflow rate. For the first time, we determine robustly the angular two point correlation function of the high-velocity absorbers, systematically analyze antipodal sightlines on the celestial sphere, and compare the absorption characteristics with that of Damped Lyman alpha absorbers (DLAs) and constrained cosmological simulations of the LG. Our study demonstrates that the Milky Way CGM contains sufficient gaseous material to maintain the Galactic star-formation rate at its current level. We show that the CGM is composed of discrete gaseous structures that exhibit a large-scale kinematics together with small-scale variations in physical conditions. The Magellanic Stream clearly dominates both the cross section and mass flow of high-velocity gas in the Milky Way's CGM. The possible presence of high-velocity LG gas underlines the important role of the local cosmological environment in the large-scale gas-circulation processes in and around the Milky Way (abridged).Comment: 37 pages, 25 figures, 8 tables, accepted for publication in A&

Properties and Origin of the High-Velocity Gas Toward the Large Magellanic Cloud

In the spectra of 139 early-type Large Magellanic Cloud (LMC) stars observed with FUSE and with deep radio Parkes HI 21-cm observations along those stars, we search for and analyze the absorption and emission from high-velocity gas at +90<v<+175 km/s. The HI column density of the high-velocity clouds (HVCs) along these sightlines ranges from <10^18.4 to 10^19.2 cm^-2. The incidence of the HVC metal absorption is 70%, significantly higher than the HI emission occurrence of 32%. We find that the mean metallicity of the HVC is [OI/HI] = -0.51 (+0.12,-0.16). There is no strong evidence for a large variation in the HVC metallicity, implying that thes e HVCs have a similar origin and are part of the same complex. The mean and scatter of the HVC metallicities are more consistent with the present-day LMC oxygen abundance than that of the Small Magellanic Cloud or the Milky Way. We find that on average [SiII/OI] = +0.48 (+0.15,- 0.25) and [FeII/OI] = +0.33 (+0.14,-0.21), implying that the HVC complex is dominantly ionized. The HVC complex has a multiphase structure with a neutral (OI, FeII), weakly ionized (FeII, NII), and highly ionized (OVI) components, and has evidence of dust but no molecules. All the observed properties of the HVC can be explained by an energetic outflow from the LMC. This is the first example of a large (>10^6 M_sun) HVC complex that is linked to stellar feedback occurring in a dwarf spiral galaxy.Comment: Accepted for publication in the Ap