Anand Narayanan

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.

COS OBSERVATIONS OF METAL LINE AND BROAD LYMAN-α ABSORPTION IN THE MULTI-PHASE O VI AND Ne VIII SYSTEM AT z = 0.20701 TOWARD HE 0226-4110*

Observations of the QSO HE 0226–4110 (zem = 0.495) with the Cosmic Origins Spectrograph (COS) from 1134 to 1796 A with a resolution of ∼17 km s −1 and signal-to-noise ratio (S/N) per resolution element of 20–40 are used to study the multi-phase absorption system at z = 0.20701 containing Ovi and Neviii. The system was previously studied with lower S/N observations with Far-Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS). The COS observations provide more reliable measures of the Hi and metal lines present in the system and reveal the clear presence of broad Lyα (BLA) absorption with b = 72(+13, −6) km s −1 and log N(Hi) = 13.87 ± 0.08. Detecting BLAs associated with warm gas absorbers is crucial for determining the temperature, metallicity, and total baryonic content of the absorbers. The BLA is probably recording the trace amount of thermally broadened Hi in the collisionally ionized plasma with log T ∼ 5.7 that also produces the Ovi and Neviii absorption. The total hydrogen column in the collisionally ionized gas, log N(H) ∼ 20.1, exceeds that in the cooler photoionized gas in the system by a factor of ∼22. The oxygen abundance in the collisionally ionized gas is [O/H] =− 0.89 ± 0.08 ± 0.07. The absorber probably occurs in the circumgalactic environment (halo) of a foreground L = 0.25L∗ disk galaxy with an impact parameter of 109h70 −1 kpc identified by Mulchaey & Chen.

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.

DETECTION OF Ne VIII IN AN INTERVENING MULTIPHASE ABSORPTION SYSTEM TOWARD 3C 263

We report the detection of Ne VIII in an intervening multiphase absorption line system at z = 0.32566 in the Far Ultraviolet Spectroscopic Explorer spectrum of the quasar 3C 263 (zem = 0.646). The Ne VIII λ770 A detection has a 3.9σ significance. At the same velocity, we also find absorption lines from C IV, O III, O IV, and N IV. The line parameter measurements yield log [N(Ne VIII) cm–2] = 13.98+0.10 –0.13 and b = 49.8 ± 5.5 km s–1. We find that the ionization mechanism in the gas phase giving rise to the Ne VIII absorption is inconsistent with photoionization. The absorber has a multiphase structure, with the intermediate ions produced in cool photoionized gas and the Ne VIII most likely in a warm collisionally ionized medium in the temperature range (0.5-1.0) × 106 K. This is the second ever detection of an intervening Ne VIII absorption system. Its properties resemble the previous Ne VIII absorber reported by Savage and colleagues. Direct observations of H I and O VI are needed to better constrain the physical conditions in the collisionally ionized gas phase of this absorber.

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.

A survey of weak MgII absorbers at 0.4 < z < 2.4

We present results from a survey of weak Mg II absorbers in the VLT/UVES spectra of 81 QSOs obtained from the ESO archive. In this survey, we identified 112 weak Mg II systems within the redshift interval 0.4 < z < 2.4 with 86% completeness down to a rest-frame equivalent width of Wr(2796) = 0.02 A, covering a cumulative redshift path length of ΔZ ~ 77.3. From this sample, we estimate that the number of weak absorbers per unit redshift (dN/dz) increases from 1.06 ± 0.04 at z = 1.9 to 1.76 ± 0.08 at z = 1.2 and thereafter decreases to 1.51 ± 0.09 at = 0.9 and 1.06 ± 0.10 at z = 0.6. Thus, we find evidence for an evolution in the population of weak Mg II absorbers, with their number density peaking at z = 1.2. We also determine the equivalent width distribution of weak systems at z = 0.9 and z = 1.9. At 0.4 < z < 1.4, there is evidence for a turnover from a power law of the form n(Wr) ∝ W at Wr(2796) < 0.1 A. This turnover is more extreme at 1.4 < z < 2.4, where the equivalent width distribution is close to an extrapolation of the exponential distribution function found for strong Mg II absorbers. Based on these results, we discuss the possibility that some fraction of weak Mg II absorbers, particularly single cloud systems, are related to satellite clouds surrounding strong Mg II systems. These structures could also be analogs to Milky Way high-velocity clouds. In this context, the paucity of high-redshift weak Mg II absorbers is caused by a lack of isolated clouds accreting onto galaxies during that epoch.

The Chemical and Ionization Conditions in Weak Mg II Absorbers

We present an analysis of the chemical and ionization conditions in a sample of 100 weak Mg II absorbers identified in the VLT/UVES archive of quasar spectra. In addition to Mg II, we present equivalent width and column density measurements of other low ionization species such as Mg I, Fe II, Al II, C II, Si II, and also Al III. We find that the column densities of C II and Si II are strongly correlated with the column density of Mg II, with minimal scatter in the relationships. The column densities of Fe II exhibit an appreciable scatter when compared with the column density of Mg II, with some fraction of clouds having N(Fe II) ~ N(Mg II)

HST/COS detection of a Ne viii absorber towards PG 1407+265: an unambiguous tracer of collisionally ionized hot gas?

N(Fe ^{t{

A SURVEY OF ANALOGS TO WEAK Mg II ABSORBERS IN THE PRESENT

-->, in which case the density is constrained to -->nH > 0.05 cm−3. Other clouds in which N(Fe II) N(Mg II)

HST/COS observations of a new population of associated QSO absorbers

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