We present a search for "" weak ÏÏ Mg II absorbers [those with in the HIRES/Keck W r (2796) \ 0.3 A ] spectra of 26 QSOs. We found 30, of which 23 are newly discovered. The spectra are 80% complete to and have a cumulative redshift path of D17.2 for the redshift range 0.4 ¹ z ¹ 1.4. The W r (2796) \ 0.02 A number of absorbers per unit redshift, dN/dz, is seen to increase as the equivalent width threshold is decreased ; we obtained dN/dz \ 1.74^0.10 for our 0.02 sample. The equivalent width W r (2796) \ 0.3 A distribution follows a power law, N(W ) P W~d, with d D 1.0 ; there is no turnover down to W r (2796) \ 0.02 at SzT \ 0.9. Weak absorbers comprise at least 65% of the total Mg II absorption population, A which outnumbers Lyman limit systems (LLSs) by a factor of 3.8^1.1 ; the majority of weak Mg II absorbers must arise in sub-LLS environments. Tentatively, we predict that D5% of the Lya forest clouds with will have detectable Mg II absorption to and that this W r (Lya) º 0.1 A W r min(2796) \ 0.02 A is primarily a high-metallicity selection e †ect This implies that Mg II absorbing struc-. tures Ðgure prominently as tracers of sub-LLS environments where gas has been processed by stars. We compare the number density of absorbers with that of both high and low surface W r (2796) º 0.02 A brightness galaxies and Ðnd a Ðducial absorber size of 35 h~1È63 h~1 kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing "" clouds ÏÏ have W r (2796) ¹ 0.15 and their narrow (often unresolved) line widths imply temperatures of D25,000 K. We measured A , from C IV in Faint Object Spectrograph/Hubble Space T elescope archival spectra and, based W r (1548) upon comparisons with Fe II, found a range of ionization conditions (low, high, and multiphase) in absorbers selected by weak Mg II.

We present an investigation of Mg ii absorbers characterized as single-cloud "weak systems" (defined by W r (2796) < 0.3Å) at z ∼ 1. We measured column densities and Doppler parameters for Mg ii and Feii in 15 systems found in HIRES/Keck spectra at 6.6 km s −1 . Using these quantities and Civ, Lyα and Lyman limit absorption observed with the Faint Object Spectrograph on the Hubble Space Telescope (resolution ∼ 230 km s −1 ) we applied photoionization 1 Based in part on observations obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among Caltech, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Dynamic interactions between the two Magellanic Clouds have flung large quantities of gas into the halo of the Milky Way. The result is a spectacular arrangement of gaseous structures including the Magellanic Stream, the Magellanic Bridge, and the Leading Arm (collectively referred to as the Magellanic System). In this third paper of a series studying the Magellanic gas in absorption, we analyze the gas ionization level using a sample of 69 Hubble Space Telescope/Cosmic Origins Spectrograph sightlines that pass through or within 30 • of the 21 cm-emitting regions. We find that 81% (56/69) of the sightlines show UV absorption at Magellanic velocities, indicating that the total cross section of the Magellanic System is ≈11 000 square degrees, or around a quarter of the entire sky. Using observations of the Si III/Si II ratio together with Cloudy photoionization modeling, we calculate the total gas mass (atomic plus ionized) of the Magellanic System to be ≈2.0×10 9 M ⊙ (d/55 kpc) 2 , with the ionized gas contributing around three times as much mass as the atomic gas. This is larger than the current-day interstellar H I mass of both Magellanic Clouds combined, indicating that they have lost most of their initial gas mass. If the gas in the Magellanic System survives to reach the Galactic disk over its inflow time of ∼0.5-1.0 Gyr, it will represent an average inflow rate of ∼3.7-6.7 M ⊙ yr −1 , potentially raising the Galactic star formation rate. However, multiple signs of an evaporative interaction with the hot Galactic corona indicate that the Magellanic gas may not survive its journey to the disk fully intact, and will instead add material to (and cool) the corona.

Compact groups of galaxies provide a unique environment to study the mechanisms by which star formation occurs amid continuous gravitational encounters. We present 2MASS (JHK), Spitzer IRAC (3.5-8 micron) and MIPS (24 micron) observations of a sample of twelve Hickson Compact Groups (HCGs 2, 7, 16, 19, 22, 31, 42, 48, 59, 61, 62, and 90) that includes a total of 45 galaxies. The near-infrared colors of the sample galaxies are largely consistent with being dominated by slightly reddened normal stellar populations. Galaxies that have the most significant PAH and/or hot dust emission (as inferred from excess 8 micron flux) also tend to have larger amounts of extinction and/or K-band excess and stronger 24 micron emission, all of which suggest ongoing star formation activity. We separate the twelve HCGs in our sample into three types based on the ratio of the group HI mass to dynamical mass. We find evidence that galaxies in the most gas-rich groups tend to be the most actively star forming. Galaxies in the most gas-poor groups tend to be tightly clustered around a narrow range in colors consistent with the integrated light from a normal stellar population. We interpret these trends as indicating that galaxies in gas-rich groups experience star formation and/or nuclear actively until their neutral gas consumed, stripped, or ionized. The galaxies in this sample exhibit a ``gap'' between gas-rich and gas-poor groups in infrared color space that is sparsely populated and not seen in the Spitzer First Look Survey sample. This gap may suggest a rapid evolution of galaxy properties in response to dynamical effects. These results suggest that the global properties of the groups and the local properties of the galaxies are connected.Comment: 34 pages, 26 figures, accepted for publication in AJ, higher quality images available in publicatio

We use Keck/HIRES spectra of 37 optically bright quasars at z = 2-4 to study narrow absorption lines that are intrinsic to the quasars (intrinsic NALs, produced in gas that is physically associated with the quasar central engine). We identify 150 NAL systems, that contain 124 C IV, 12 N V, and 50 Si IV doublets, of which 18 are associated systems (within 5,000 km s −1 of the quasar redshift). We use partial coverage analysis to separate intrinsic NALs from NALs produced in cosmologically intervening structures. We find 39 candidate intrinsic systems, (28 reliable determinations and 11 that are possibly intrinsic). We estimate that 10-17% of C IV systems at blueshifts of 5,000-70,000 km s −1 relative to quasars are intrinsic. At least 32% of quasars contain one or more intrinsic C IV NALs. Considering N V and Si IV doublets showing partial coverage as well, at least 50% of quasars host intrinsic NALs. This result constrains the solid angle subtended by the absorbers to the background source(s). We identify two families of intrinsic NAL systems, those with strong N V absorption, and those with negligible absorption in N V, but with partial coverage in the C IV doublet. We discuss the idea that these two families represent different regions or conditions in accretion disk winds. Of the 26 intrinsic C IV NAL systems, 13 have detectable low-ionization absorption lines at similar velocities, suggesting that these are two-phase structures in the wind rather than absorbers in the host galaxy. We also compare possible models for quasar outflows, including radiatively accelerated disk-driven winds, magnetocentrifugally accelerated winds, and pressure-driven winds, and we discuss ways of distinguishing between these models observationally.