Brian A. Keeney

THE COSMIC ORIGINS SPECTROGRAPH

The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in 2009 May, during HST Servicing Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux F ? 1.0 ? 10?14?erg?cm?2?s?1 ??1, COS can achieve comparable signal to noise (when compared to Space Telescope Imaging Spectrograph echelle modes) in 1%-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (2009 September-2011 June) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is nine times than sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of 2011 June. COS has measured, for the first time with high reliability, broad Ly? absorbers and Ne VIII in the intergalactic medium, and observed the He II reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy.

AN HST/COS SURVEY OF THE LOW-REDSHIFT INTERGALACTIC MEDIUM. I. SURVEY, METHODOLOGY, AND OVERALL RESULTS*

We use high-quality, medium-resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV-bright active galactic nuclei (AGNs) at redshifts z(AGN) \textless 0.85 to construct the largest survey of the low-redshift intergalactic medium (IGM) to date: 5138 individual extragalactic absorption lines in H I and 25 different metal-ion species grouped into 2611 distinct redshift systems at z(abs) \textless 0.75 covering total redshift pathlengths Delta z(HI) = 21.7 and Delta z(O VI) = 14.5. Our semi-automated line-finding and measurement technique renders the catalog as objectively defined as possible. The cumulative column density distribution of H I systems can be parametrized dN (\textgreater N)/dz = C-14 (N/10(14) cm(-2))(-(beta-1)), with C-14 = 25 +/- 1 and beta = 1.65 +/- 0.02. This distribution is seen to evolve both in amplitude, C-14 infinity (1+z)(2.3 +/- 0.1), and slope beta(z) = 1.75-0.31 z for z \textless= 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N-H I. The distribution of O VI absorbers appears to evolve in the same sense as the Ly alpha forest. We calculate contributions to Omega(b) from different components of the low-z IGM and determine the Ly alpha decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function in velocity space. We find substantial clustering of H I absorbers on scales of Delta v = 50-300 km s(-1) with no significant clustering at Delta(v) greater than or similar to 1000 km s(-1). Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N-H I greater than or similar to 10(13.5) cm(-2), metal-bearing IGM systems. The full catalog of absorption lines and fully reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high-level science product at http://archive.stsci.edu/prepds/igm/.

COSMIC ORIGINS SPECTROGRAPH DETECTION OF Ne VIII TRACING WARM-HOT GAS TOWARD PKS 0405–123*

We report on the detection of Ne vm in the Hubble Space Telescope/Cosmic Origins Spectrograph (COS) spectrum of the intervening absorption system at z = 0.495096 toward PKS 0405―123 (z em = 0.5726). The high signal-to-noise COS spectrum also covers absorption from H I, C III, O III, O IV, and O VI associated with this multiphase system. The Ne VIII is detected with high significance in both lines of the doublet, with integrated column densities of log N a (Ne VIII 770) = 13.96 ± 0.06 and log N a (Ne VIII 780) = 14.08 ± 0.07. We find the origin of Ne VIII consistent with collisionally ionized gas at T ∼ 5 × 10 5 K with a large baryonic column density of N(H) ∼ 10 19 ―10 20 cm ―2 . The metallicity in the Ne VIII gas phase is estimated to be [Ne/H] ∼ -0.6 ± 0.3 dex. The intermediate ions such as Cm, O III, O IV, and H I are consistent with photoionization in lower ionization gas at T ∼ 10 4 K. The O V and O VI in this absorber can have contributions from both the photoionized and collisionally ionized gas phases. The absorber is at |Δv| = 180 km s ―1 systematic velocity and ρ = 110h ―1 70 kpc projected separation from an M R = ―19.6 galaxy of extended morphology. The collisionally ionized gas at T ∼ 5 x 10 5 K detected in Ne VIII and O VI points to an origin in multiphase gas embedded in the hot halo of the galaxy, or in a nearby WHIM structure. The high-sensitivity UV spectroscopy afforded by COS has opened up new opportunities for discovering large reservoirs of missing baryons in the low-z universe through the detection of Ne vm systems.

Does the Milky Way Produce a Nuclear Galactic Wind

We detect high-velocity absorbing gas using Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer medium-resolution spectroscopy along two high-latitude active galactic nucleus (AGN) sight lines (Mrk 1383 and PKS2005� 489)aboveandbelowtheGalacticcenter(GC).Theseabsorptionsaremoststraightforwardlyinterpreted as a wind emanating from the GC that does not escape from the Galaxy’s gravitational potential. Spectra of four comparisonBstarsareusedtoidentifyandremoveforegroundvelocitycomponentsfromtheabsorption-lineprofiles of O vi ,N v ,C ii ,C iii ,C iv ,S iii ,S iiii, and Si iv. Two high-velocity (HV) absorption components are detected along each AGN sight line, three redshifted and one blueshifted. Assuming that the four HV features trace a large-scale Galactic wind emanating from the GC, the blueshifted absorber is falling toward the GC at a velocity of 250 � 20 km s � 1 , which can be explained by ‘‘Galactic fountain’’ material that originated in a bound Galactic wind. The otherthreeabsorbersrepresentoutflowingmaterial;thelargestderivedoutflowvelocityis+250 � 20 km s � 1 ,which is only 45% of the velocity necessary for the absorber to escape from its current position in the Galactic gravitational potential. All four HV absorbers are found to reach the same maximum height above the Galactic plane ( zmax jj ¼ 12 � 1 kpc), implying that they were all ejected from the GC with the same initial velocity. The derived metallicity limits of k10%–20% solar are lower than expected for material recently ejected from the GC unless these absorbers also contain significant amounts of hotter gas in unseen ionization stages.

The Peculiar Balmer Decrement of SN 2009ip: Constraints on Circumstellar Geometry

We present optical and near-IR spectroscopic observations of the luminous blue variable SN 2009ip during its remarkable photometric evolution of 2012. The spectra sample three key points in the SN 2009ip light curve, corresponding to its initial brightening in August (2012-A) and its dramatic rebrightening in early October (2012-B). Based on line fluxes and velocities measured in our spectra, we find a surprisingly low I(H?)/I(H?) ratio (~1.3-1.4) in the 2012-B spectra. Such a ratio implies either a rare Case B recombination scenario where H?, but not H?, is optically thick, or an extremely high density for the circumstellar material of ne > 1013?cm?3. The H? line intensity yields a minimum radiating surface area of 20,000?AU2 in H? at the peak of SN 2009ips photometric evolution. Combined with the nature of this objects spectral evolution in 2012, a high circumstellar density and large radiating surface area imply the presence of a thin disk geometry around the central star (and, consequently, a possible binary companion), suggesting that the observed 2012-B rebrightening of SN 2009ip can be attributed to the illumination of the disks inner rim by fast-moving ejecta produced by the underlying events of 2012-A.

Discovery of a Dwarf Poststarburst Galaxy near a High Column Density Local Lyα Absorber

We report the discovery of a dwarf (MB = � 13.9) poststarburst galaxy coincident in recession velocity (within uncertainties) with the highest column density absorber (NH i =1 0 15.85 cm � 2 at cz ¼ 1586 km s � 1 ) in the 3C 273 sight line. This galaxy is by far the closest galaxy to this absorber, projected just 71 h � 1 70 kpc on the sky from the sight line. The mean properties of the stellar populations in this galaxy are consistent with a massive starburst � 3.5 Gyr ago, whose attendant supernovae, we argue, could have driven sufficient gas from this galaxy to explain the nearby absorber. Beyond its proximity on the sky and in recession velocity, the further evidence in favor of this conclusion includes both a match in the metallicities of absorber and galaxy and the fact that the absorber has an overabundance of Si/C, suggesting recent Type II supernova enrichment. Thus, this galaxy and its ejecta are in the expected intermediate stage in the fading dwarf evolutionary sequence envisioned by Babul & Rees to explain the abundance of faint blue galaxies at intermediate redshifts. While this one instance of a QSO metal-line absorber and a nearby dwarf galaxy is not proof of a trend, a similar dwarf galaxy would be too faint to be observed by galaxy surveys around more distant metal-line absorbers. Thus, we cannot exclude the possibility that dwarf galaxies are primarily responsible for weak (NH i =1 0 14 ‐10 17 cm � 2 ) metal-line absorption systems in general. If a large fraction of the dwarf galaxies expected to exist at high redshift had a similar history (i.e., they had a massive starburst that removed all or most of their gas), these galaxies could account for at least several hundred high-z metal-line absorbers along the line of sight to a high-z QSO. The volume-filling factor for this gas, however, would be less than 1%.

Cosmic Origins Spectrograph and FUSE Observations of T ~ 105?K Gas in a Nearby Galaxy Filament

We present a clear detection of a broad Lyα absorber (BLA) with a matching Ovi line in the nearby universe. The BLA is detected at z(Lyα) = 0.01028 in the high signal-to-noise ratio spectrum of Mrk 290 obtained using the Cosmic Origins Spectrograph. The Lyα absorption has two components, with b(Hi) = 55 ± 1k m s −1 and b(Hi) = 33 ± 1k m s −1 , separated in velocity by v ∼ 115 km s −1 .T he Ovi, detected by the Far-Ultraviolet Spectroscopic Explorer at z(Ovi) = 0.01027, has a b(Ovi) = 29 ± 3k m s −1 and is kinematically well aligned with the broader Hi component. The non-detection of other ions such as Cii ,S iii ,F eii ,C iii ,S iiii ,C iv ,S iiv, and Nv at the same velocity as the BLA and the Ovi implies that the absorber is tracing highly ionized gas. The different line widths of the BLA and Ovi suggest a temperature of T = 1.4 × 10 5 K in the absorber. Photoionization, collisional ionization equilibrium as well as non-equilibrium collisional ionization models do not explain the ion ratios at this temperature. The observed line strength ratios and line widths favor an ionization scenario in which both ion–electron collisions and UV photons contribute to the ionization in the gas. Such a model requires a low metallicity of ∼− 1.7 dex, ionization parameter of log U ∼− 1.4, a large total hydrogen column density of N (H) ∼ 4 × 10 19 cm −2 , and a path length of ∼400 kpc. The line of sight to Mrk 290 intercepts at the redshift of the absorber, a megaparsec scale filamentary structure extending over ∼20 ◦ in the sky, with several luminous galaxies distributed within ∼1.5 h −1 Mpc projected distance from the absorber. The collisionally ionized gas phase of this absorber is most likely tracing a shock-heated gaseous structure, consistent with a few different scenarios for the origin including an overdense region of the warm-hot intergalactic medium in the galaxy filament or highly ionized gas in the extended halo of one of the galaxies in the filament. In general, BLAs with metals provide an efficient means to study T ∼ 10 5 –10 6 K gas in galaxy halos and in the intergalactic medium. A substantial fraction of the baryons missing from the present universe is predicted to be in such environments in the form of highly ionized plasma.

O VI ABSORBERS TRACING HOT GAS ASSOCIATED WITH A PAIR OF GALAXIES AT z = 0.167*

High signal-to-noise observations of the QSO PKS 0405-123 (z em = 0.572) with the Cosmic Origins Spectrograph from 1134 to 1796 ? with a resolution of ~17?km?s?1 are used to study the multi-phase partial Lyman limit system (LLS) at z = 0.16716, which has previously been studied using relatively low signal-to-noise spectra from STIS and FUSE. The LLS and an associated H I-free broad O VI absorber likely originate in the circumgalactic gas associated with a pair of galaxies at z = 0.1688 and 0.1670 with impact parameters of 116 h ?1 70 and 99 h ?1 70. The broad and symmetric O VI absorption is detected in the z = 0.16716 rest frame with v = ?278?? 3?km?s?1, log N(O VI) = 13.90?? 0.03, and b = 52?? 2?km?s?1. This absorber is not detected in H I or other species with the possible exception of N V. The broad, symmetric O VI profile and the absence of corresponding H I absorption indicate that the circumgalactic gas in which the collisionally ionized O VI arises is hot (log T?~ 5.8-6.2). The absorber may represent a rare but important new class of low-z intergalactic medium absorbers. The LLS has strong asymmetrical O VI absorption with log N(O VI) = 14.72?? 0.02 spanning a velocity range from ?200 to +100?km?s?1. The high and low ions in the LLS have properties resembling those found for Galactic highly ionized high-velocity clouds where the O VI is likely produced in the conductive and turbulent interfaces between cool and hot gas.

On the Significance of Absorption Features in HST/COS Data*

We present empirical scaling relations for the significance of absorption features detected in medium-resolution, far-UV spectra obtained with the Cosmic Origins Spectrograph (COS). These relations properly account for both the extended wings of the COS line-spread function and the non-Poissonian noise properties of the data, which we characterize for the first time, and predict limiting equivalent widths that deviate from the empirical behavior by ≤ 5% when the wavelength and Doppler parameter are in the ranges of λ = 1150-1750 A and b > 10 km s-1. We have tested a number of co-addition algorithms and find the noise properties of individual exposures to be closer to the Poissonian ideal than are co-added data in all cases. For unresolved absorption lines, limiting equivalent widths for co-added data are 6% larger than limiting equivalent widths derived from individual exposures with the same signal-to-noise ratio. This ratio scales with b-value for resolved absorption lines, with co-added data having a limiting equivalent width that is 25% larger than individual exposures when b ≈ 150 km s-1.

Very Isolated Early-Type Galaxies

We use the Karachentseva Catalogue of Very Isolated Galaxies to investigate a candidate list of more than 100 very isolated early-type galaxies. Broadband imaging and low-resolution spectroscopy are available for a large fraction of these candidates and result in a sample of 102 very isolated early-type galaxies, including 65 elliptical (E) and 37 S0 galaxies. Many of these systems are quite luminous, and the resulting optical luminosity functions of the E and early-type (E+S0) galaxies show no statistical differences when compared to luminosity functions dominated by group and cluster galaxies. However, whereas S0 galaxies outnumber E galaxies 4 : 1 in the CfA survey, isolated E outnumber S0 galaxies by nearly 2 : 1. We conclude that very isolated elliptical galaxies show no evidence of a different formation and/or evolution process compared to those formed in groups or clusters, but that most S0 galaxies are formed by a mechanism (e.g., gas stripping) that occurs only in groups and rich clusters. Our luminosity function results for elliptical galaxies are consistent with very isolated elliptical galaxies being formed by merger events, in which no companions remain. Chandra observations were proposed specifically to test the merger hypothesis for isolated elliptical galaxies. However, this program has resulted in the observation of only one isolated early-type galaxy, the S0 KIG 284, which was not detected at a limit well below that expected for a remnant group of galaxies. Therefore, the hypothesis remains untested that very isolated elliptical galaxies are the remains of a compact group of galaxies that have completely merged.