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Register a new userO VI Absorbers Tracing Hot Gas Associated with a Pair of Galaxies at z = 0.167
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High signal-to-noise (S/N) observations of the QSO PKS 0405-123 (zem = 0.572) with the Cosmic Origins Spectrograph from 1134 to 1796 A 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 S/N 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 h70-1 and 99 h70-1. The broad and symmetric O VI absorption is detected in the z = 0.16716 restframe 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 absence of corresponding H I absorption indicates 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 IGM 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 HVCs where the O VI is likely produced in the conductive and turbulent interfaces between cool and hot gas.
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We report on the detection of two O VI absorbers separated in velocity by 710 km/s at z ~ 0.4 towards the background quasar SBS0957+599. Both absorbers are multiphase systems tracing substantial reservoirs of warm baryons. The low and intermediate ionization metals in the first absorber is consistent with an origin in photoionized gas. The O VI has a velocity structure different from other metal species. The Ly-alpha shows the presence of a broad feature. The line widths for O VI and the broad Ly-alpha suggest T = 7.1 x 10^5 K. This warm medium is probing a baryonic column which is an order of magnitude more than the total hydrogen in the cooler photoionized gas. The second absorber is detected only in H I and O VI. Here the temperature of 4.6 x 10^4 K supports O VI originating in a low-density photoionized gas. A broad component is seen in the Ly-alpha, offset from the O VI. The temperature in the broad Ly-alpha is T < 2.1 x 10^5 K. The absorbers reside in a galaxy overdensity region with 7 spectroscopically identified galaxies within ~ 10 Mpc and delta_v ~ 1000 km/s of the first absorber, and 2 galaxies inside a similar separation from the second absorber. The distribution of galaxies relative to the absorbers suggest that the line of sight could be intercepting a large-scale filament connecting galaxy groups, or the extended halo of a sub-L* galaxy. Though kinematically proximate, the two absorbers reaffirm the diversity in the physical conditions of low redshift O VI systems and the galactic environments they inhabit.
Ultraviolet observations of the QSO 3C 263 (zem = 0.652) with COS and FUSE reveal O VI absorption systems at z = 0.06342 and 0.14072 . WIYN multi-object spectrograph observations provide information about the galaxies associated with the absorbers. The multi-phase system at z = 0.06342 traces cool photoionized gas and warm collisionally ionized gas associated with a L ~ 0.31L* compact spiral emission line galaxy with an impact parameter of 63 kpc. The cool photoionized gas in the absorber is well modeled with log U ~ -2.6, log N(H) ~17.8, log n(H) ~ -3.3 and [Si/H] = -0.14pm0.23. The collisionally ionized gas containing C IV and O VI probably arises in cooling shock heated transition temperature gas with log T ~ 5.5. The absorber is likely tracing circumgalactic gas enriched by gas ejected from the spiral emission line galaxy. The simple system at z = 0.14072 only contains O VI and broad and narrow H I. The O VI with b = 33.4pm11.9 km s-1 is likely associated with the broad H I {lambda}1215 absorption with b = 86.7pm15.4 km s-1. The difference in Doppler parameters implies the detection of a very large column of warm gas with log T = 5.61(+0.16, -0.25), log N(H) = 19.54(+0.26, -0.44) and [O/H] = -1.48 (+0.46, -0.26). This absorber is possibly associated with a 1.6L* absorption line galaxy with an impact parameter of 617 kpc although an origin in warm filament gas or in the halo of a fainter galaxy is more likely.
[Abridged] We present a detailed study of the largest sample of intervening O VI systems in the redshift range 1.9 < z < 3.1 detected in high resolution (R ~ 45,000) spectra of 18 bright QSOs observed with VLT/UVES. Based on Voigt profile and apparent optical depth analysis we find that (i) the Doppler parameters of the O VI absorption are usually broader than those of C IV (ii) the column density distribution of O VI is steeper than that of C IV (iii) line spread (delta v) of the O VI and C IV are strongly correlated (at 5.3sigma level) with delta v(O VI) being systematically larger than delta v(C IV) and (iv) delta v(O VI) and delta v(C IV) are also correlated (at > 5sigma level) with their respective column densities and with N(H I) (3 and 4.5 sigma respectively). These findings favor the idea that C IV and O VI absorption originate from different phases of a correlated structure and systems with large velocity spread are probably associated with overdense regions. The velocity offset between optical depth weighted redshifts of C IV and O VI absorption is found to be in the range 0 < |Delta v (O VI - CIV)| < 48 km/s with a median value of 8 km/s. We compare the properties of O VI systems in our sample with that of low redshift (z < 0.5) samples from the literature and find that (i) the O VI components at low-z are systematically wider than at high-z with an enhanced non-thermal contribution to their b-parameter, (ii) the slope of the column density distribution functions for high and low-z are consistent, (iii) range in gas temperature estimated from a subsample of well aligned absorbers are similar at both high and low-z, and (iv) Omega_{O VI} = (1.0 pm 0.2) times10^{-7} for N(O VI) > 10^{13.7} cm^{-2}, estimated in our high-z sample, is very similar to low-z estimations.
Using a mass-selected ($M_{star} ge 10^{11} M_{odot}$) sample of 198 galaxies at 0 < z < 3.0 with HST/NICMOS $H_{160}$-band images from the COSMOS survey, we find evidence for the evolution of the pair fraction above z ~ 2, an epoch in which massive galaxies are believed to undergo significant structural and mass evolution. We observe that the pair fraction of massive galaxies is 0.15 pm 0.08 at 1.7 < z < 3.0, where galaxy pairs are defined as massive galaxies having a companion of flux ratio from 1:1 to 1:4 within a projected separation of 30 kpc. This is slightly lower, but still consistent with the pair fraction measured previously in other studies, and the merger fraction predicted in halo-occupation modelling. The redshift evolution of the pair fraction is described by a power law F(z) = (0.07 pm 0.04) * (1+z) ^ (0.6 pm 0.5). The merger rate is consistent with no redshift evolution, however it is difficult to constrain due to the limited sample size and the high uncertainties in the merging timescale. Based on the merger rate calculation, we estimate that a massive galaxy undergoes on average 1.1 pm 0.5 major merger from z = 3 to 0. The observed merger fraction is sufficient to explain the number density evolution of massive galaxies, but insufficient to explain the size evolution. This is a hint that mechanism(s) other than major merging may be required to increase the sizes of the massive, compact quiescent galaxies from z ~ 2 to 0.
(Abridged) With the goal of investigating the nature of OVI absorbers at high redshifts, we study the effects of proximity to the background quasar. In a sample of sixteen quasars at z(QSO) between 2.14 and 2.87 observed at 6.6 km/s resolution with VLT/UVES, we detect 35 OVI absorption-line systems lying within 8000 km/s of z(QSO). The systems can be categorized into 9 strong and 26 weak OVI absorbers. The strong absorbers are defined by the presence of either broad, fully saturated OVI absorption or partial coverage of the continuum source, and have log N(OVI)>~15.0; these systems are intrinsic to the AGN. The weak (narrow) systems show no partial coverage or saturation, and are characterized by log N(OVI)<14.5 and have a median total velocity width of only 42 km/s. The incidence dN/dz of weak OVI systems within 2000 km/s of the quasar is 42+/-12. Between 2000 and 8000 km/s, dN/dz falls to 14+/-4, equal to the incidence of intervening OVI absorbers. Whereas the accompanying H I and C IV column densities are significantly lower (by a mean of ~1 dex) in the weak OVI absorbers within 2000 km/s of z(QSO) than in those at larger velocities, the OVI column densities display no dependence on proximity. Furthermore, significant offsets between the HI and OVI centroids in ~50% of the weak absorbers imply that (at least in these cases) the HI and OVI are not formed in the same phase of gas. In summary, we find no firm evidence that quasar radiation influences the OVI-bearing gas, suggesting the OVI is collisionally ionized rather than photoionized, possibly in the multi-phase halos of foreground galaxies. Non-equilibrium collisional ionization models are needed to explain the low temperatures in the absorbing gas, which are implied by narrow line widths (b<14 km/s) in over half of the observed OVI components.
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