Ultra-strong MgII Absorbers as a Signature of Cool Intragroup Gas

We present the results of a spectroscopic survey of galaxies in the vicinity of an ultra-strong MgII ⋋⋋2786, 2803 absorber of rest-frame absorption equivalent width W_r(2796) = 4.2Å at z = 0.5624. This absorber was originally found at projected separation p = 246 kpc of a luminous red galaxy (LRG) at z = 0.5604. Magellan IMACs spectroscopy has revealed two galaxies at p 1 Gyr) and little on-going star formation activity (SFR 3.0 M_☉/yr. Including the two ultra-strong MgII absorbers analyzed by Nestor et al. (2011), this is the third ultra-strong MgII absorber for which a detailed study of the galactic environment is available. All three aborbers are found in galaxy groups. We examine different physical mechanisms giving rise to the absorbing gas including starburst driven-outflows, cold filaments, extended rotating disks, and stripped gas. We argue that the large equivalent width observed in these absorbers is more likely due to the gas dynamics of the intragroup medium rather than driven by starburst outflows

Characterizing Circumgalactic Gas around Massive Ellipticals at z~0.4 - II. Physical Properties and Elemental Abundances

We present a systematic investigation of the circumgalactic medium (CGM) within projected distances d<160 kpc of luminous red galaxies (LRGs). The sample comprises 16 intermediate-redshift (z=0.21-0.55) LRGs of stellar mass M_star>1e11 M_sun. Combining far-ultraviolet Cosmic Origin Spectrograph spectra from the Hubble Space Telescope and optical echelle spectra from the ground enables a detailed ionization analysis based on resolved component structures of a suite of absorption transitions, including the full HI Lyman series and various ionic metal transitions. By comparing the relative abundances of different ions in individually-matched components, we show that cool gas (T~1e4 K) density and metallicity can vary by more than a factor of ten in in an LRG halo. Specifically, metal-poor absorbing components with <1/10 solar metallicity are seen in 50% of the LRG halos, while gas with solar and super-solar metallicity is also common. These results indicate a complex multiphase structure and poor chemical mixing in these quiescent halos. We calculate the total surface mass density of cool gas, \Sigma_cool, by applying the estimated ionization fraction corrections to the observed HI column densities. The radial profile of \Sigma_cool is best-described by a projected Einasto profile of slope \alpha=1 and scale radius r_s=48 kpc. We find that typical LRGs at z~0.4 contain cool gas mass of M_cool= (1-2) x1e10 M_sun at d<160 kpc (or as much as 4x1e10 M_sun at d<500 kpc), comparable to the cool CGM mass of star-forming galaxies. Furthermore, we show that high-ionization OVI and low-ionization absorption species exhibit distinct velocity profiles, highlighting their different physical origins. We discuss the implications of our findings for the origin and fate of cool gas in LRG halos.Comment: Accepted for publication in MNRAS after a minor revision. 23 pages, 14 figures, and a 29-page Appendix with 27 additional figure

Spatially resolved velocity maps of halo gas around two intermediate-redshift galaxies

Absorption-line spectroscopy of multiply-lensed QSOs near a known foreground galaxy provides a unique opportunity to go beyond the traditional one-dimensional application of QSO probes and establish a crude three-dimensional map of halo gas around the galaxy that records the line-of-sight velocity field at different locations in the gaseous halo. Two intermediate-redshift galaxies are targeted in the field around the quadruply-lensed QSO HE 0435−1223 at redshift z = 1.689, and absorption spectroscopy along each of the lensed QSOs is carried out in the vicinities of these galaxies. One galaxy is a typical, star-forming L* galaxy at z = 0.4188 and projected distance of ρ = 50 kpc from the lensing galaxy. The other is a super-L* barred spiral at z = 0.7818 and ρ = 33 kpc. Combining known orientations of the quadruply-lensed QSO to the two foreground galaxies with the observed Mg ii λλ2796, 2803 absorption profiles along individual QSO sightlines has for the first time led to spatially resolved kinematics of tenuous halo gas on scales of 5–10 kpc at z > 0.2. A Mg ii absorber is detected in every sightline observed through the haloes of the two galaxies, and the recorded absorber strength is typical of what is seen in previous close QSO–galaxy pair studies. While the multisightline study confirms the unity covering fraction of Mg ii absorbing gas at ρ < 50 kpc from star-forming discs, the galaxies also present two contrasting examples of complex halo gas kinematics. Different models, including a rotating disc, collimated outflows and gaseous streams from either accretion or tidal/ram-pressure stripping, are considered for comparisons with the absorption-line observations, and infalling streams/stripped gas of width ≳10 kpc are found to best describe the observed gas kinematics across multiple sightlines. In addition, the observed velocity dispersion between different sightlines offers a crude estimate of turbulence in the Mg ii absorbing halo gas. The observations presented here demonstrate that multiple-QSO probes enable studies of spatially resolved gas kinematics around distant galaxies, which provide key insights into the physical nature of circumgalactic gas beyond the nearby Universe

The Star Formation History of Luminous Red Galaxies Hosting MgII Absorbers

We present a spectroscopic sample of z \approx 0.5 luminous red galaxies (LRGs) that are located within physical projected distances \rho \leq 350 kpc/h of a QSO sightline. Of the 37 LRGs in our sample, eight have associated MgII absorbers with rest-frame equivalent width Wr(2796)>0.3A and velocity separation |dv| \leq 350 km/s and 29 do not have associated MgII absorbers to a 2-sigma limit of Wr(2796)=0.3A. We perform a stellar population synthesis analysis using stacked spectra of the MgII absorbing and non-absorbing LRG subsamples. We find that LRGs with or without associated MgII absorbers share similar star formation histories and are best described by old stellar population models (\geq 1 Gyr). Younger stellar populations (\leq 1 Gyr) fail to reproduce their spectra. These findings are consistent with the lack of [OII] emission features in the LRG spectra. The primarily old stellar populations in the LRGs indicate that starburst driven outflows are unlikely to explain the observed MgII absorbers at large distances from the LRGs. In addition, the spectroscopic LRG sample allows us to derive a sensitive constraint for the cool gas covering fraction of =14 +/- 6% in the LRG halos for absorbers of Wr(2796)>0.3A. Finally, we speculate on the origin of the observed MgII absorbers around the LRGs.Comment: 7 pages, 3 figures, 1 table. Accepted for publication in MNRA

An Empirical Characterization of Extended Cool Gas Around Galaxies Using MgII Absorption Features

We report results from a survey of MgII absorbers in the spectra of background QSOs that are within close angular distances to a foreground galaxy at z<0.5, using the Magellan Echellette Spectrograph. We have established a spectroscopic sample of 94 galaxies at a median redshift of = 0.24 in fields around 70 distant background QSOs (z_QSO>0.6), 71 of which are in an 'isolated' environment with no known companions and located at rho <~ 120 h^-1 kpc from the line of sight of a background QSO. The rest-frame absolute B-band magnitudes span a range from M_B-5log h=-16.4 to M_B-5log h=-21.4 and rest-frame B_AB-R_AB colors range from B_AB-R_AB~0 to B_AB-R_AB~1.5. Of these 'isolated' galaxies, we find that 47 have corresponding MgII absorbers in the spectra of background QSOs and rest-frame absorption equivalent width W_r(2796)=0.1-2.34 A, and 24 do not give rise to MgII absorption to sensitive upper limits. Our analysis shows that (1) Wr(2796) declines with increasing distance from 'isolated' galaxies but shows no clear trend in 'group' environments; (2) more luminous galaxies possess more extended MgII absorbing halos with the gaseous radius scaled by B-band luminosity according to R_gas=75x(L_B/L_B*)^(0.35+/-0.03) h^{-1} kpc; (3) there is little dependence between the observed absorber strength and galaxy intrinsic colors; and (4) within R_gas, we find a mean covering fraction of ~70% for absorbers of Wr(2796)>=0.3 A and ~80% for absorbers of Wr(2796)>=0.1 A. The lack of correlation between Wr(2796) and galaxy colors suggests a lack of physical connection between the origin of extended MgII halos and recent star formation history of the galaxies. Finally, we discuss the total gas mass in galactic halos as traced by MgII absorbers. We also compare our results with previous studies.Comment: 20 pages, 13 figures; to appear in the Astrophysical Journal 2010 May 10 issue; a version with higher resolution figures can be found at http://lambda.uchicago.edu/public/tmp/mage_apj.pd

The incidence of cool gas in ~ 1e13 Msun halos

We present the first results of an ongoing spectroscopic follow-up of close luminous red galaxy (LRGs) and MgII {\lambda}{\lambda} 2796,2803 absorber pairs for an initial sample of 15 photometrically selected LRGs at physical projected separations {\rho} \le 350 kpc/h from a QSO sightline. Our moderate-resolution spectra confirm a physical association between the cool gas (T ~ 1e4 K) revealed by the presence of MgII absorption features and the LRG halo in five cases. In addition, we report an empirical estimate of the maximum covering fraction (\kappa_max) of cool gas in massive, \ge 1e13 Msun/h dark matter halos hosting LRGs at z ~ 0.5. This study is performed using a sample of foreground LRGs that are located at {\rho} < 400 kpc/h from a QSO sightline. The LRGs are selected to have a robust photometric redshift \sigma_z/(1+z_ph) \approx 0.03. We determine \kappa_max based on the incidence of MgII absorption systems that occur within z_ph +/- 3sigma_z in the spectra of the background QSOs. Despite the large uncertainties in z_ph, this experiment provides a conservative upper limit to the covering fraction of cool gas in the halos of LRGs. We find that \kappa_max \approx 0.07 at W_r(2796) \ge 1.0 A and \kappa_max \approx 0.18 at W_r(2796) \ge 0.5 A, averaged over 400 kpc/h radius. Our study shows that while cool gas is present in \ge 1e13 Msun/h halos, the mean covering fraction of strong absorbers is no more than 7%.Comment: Accepted for publication in the Astrophysical Journal. 8 pages, 3 figures

Halo Masses of MgII absorbers at z ~ 0.5 from SDSS DR7

We present the cross-correlation function of MgII absorbers with respect to a volume-limited sample of luminous red galaxies (LRGs) at z = 0.45 - 0.60 using the largest MgII absorber sample and a new LRG sample from SDSS DR7. We present the clustering signal of absorbers on projected scales r_p = 0.3 - 35 h^(-1) Mpc in four W^(λ2796)_r bins spanning W^(λ2796)_r = 0.4 - 5.6Å. We found that on average Mg II absorbers reside in halos (log M_h) ≈ 12.1, similar to the halo mass of an L_* galaxy. We report that the weakest absorbers in our sample with W^(λ2796)_r = 0.4-1.1Å reside in relatively massive halos with (log M_h) ≈ 12.5^(+0.6)_(-1.3), while stronger absorbers reside in halos of similar or lower masses (log M_h) 11.6^(+0.9). We compared our bias data points, b, and the frequency distribution function of absorbers, f_W_r, with a simple model incorporating an isothermal density profile to mimic the distribution of absorbing gas in halos. We also compared the bias data points with Tinker & Chen (2008) who developed halo occupation distribution models of Mg II absorbers that are constrained by b and f_W_r. The simple isothermal model can be ruled at a ≈ 2.8σ level mostly because of its inability to reproduce f_W_r. However, b values are consistent with both models, including TC08. In addition, we show that the mean b of absorbers does not decrease beyond W^(λ2796)_r ≈ 1.6Å. The flat or potential upturn of b for W^(λ2796)-r ≳ 1.6Å absorbers suggests the presence of additional cool gas in massive halos

Halo masses of Mg ii absorbers at z∼ 0.5 from Sloan Digital Sky Survey Data Release 7

We present the cross-correlation function of Mg ii absorbers with respect to a volume-limited sample of luminous red galaxies (LRGs) at z = 0.45–0.60 using the largest Mg ii absorber sample and a new LRG sample from Sloan Digital Sky Survey Data Release 7. We present the clustering signal of absorbers on projected scales r_p = 0.3-35 h^(−1) Mpc in four W^(λ2796)_(r) bins spanning W^(λ2796)_(r) =0.4−5.6 Å. We found that on average Mg ii absorbers reside in haloes 〈log Mh〉 ≈ 12.1, similar to the halo mass of an L_* galaxy. We report that the weakest absorbers in our sample with W^(λ2796)_(r)=0.4−1.1 Å reside in relatively massive haloes with ⟨logM_h⟩≈12.5+0.6−1.3, while stronger absorbers reside in haloes of similar or lower masses 〈log M_h〉 ≈ 11.6+ 0.9. We compared our bias data points, b, and the frequency distribution function of absorbers, fW_r, with a simple model incorporating an isothermal density profile to mimic the distribution of absorbing gas in haloes. We also compared the bias data points with Tinker & Chen who developed halo occupation distribution models of Mg ii absorbers that are constrained by b and fW_r. The simple isothermal model can be ruled at a ≈2.8σ level mostly because of its inability to reproduce fW_r. However, b values are consistent with both models, including Tinker & Chen. In addition, we show that the mean b of absorbers does not decrease beyond W^(λ2796)_(r)≈1.6 Å. The flat or potential upturn of b for W^(λ2796)_(r)≳1.6 Å absorbers suggests the presence of additional cool gas in massive haloes

What is the Physical Origin of Strong Lyα Emission? II. Gas Kinematics and Distribution of Lyα Emitters

We present a statistical study of velocities of Lyα, interstellar (IS) absorption, and nebular lines and gas covering fraction for Lyα emitters (LAEs) at z ≃ 2. We make a sample of 22 LAEs with a large Lyα equivalent width (EW) of ≳50 Å based on our deep Keck/Low Resolution Imaging Spectrometer (LRIS) observations, in conjunction with spectroscopic data from the Subaru/Fiber Multi Object Spectrograph program and the literature. We estimate the average velocity offset of Lyα from a systemic redshift determined with nebular lines to be Δv_(Lyα) = 234 ± 9 km s^(–1). Using a Kolmogorov-Smirnov test, we confirm the previous claim of Hashimoto et al. that the average Δv Lyα of LAEs is smaller than that of Lyman break galaxies (LBGs). Our LRIS data successfully identify blueshifted multiple IS absorption lines in the UV continua of four LAEs on an individual basis. The average velocity offset of IS absorption lines from a systemic redshift is Δv_(IS) = 204 ± 27 km s^(–1), indicating LAEs' gas outflow with a velocity comparable to typical LBGs. Thus, the ratio R^(Lyα)_(IS) ≡ Δν_(Lyα)/Δν_(IS) of LAEs is around unity, suggestive of low impacts on Lyα transmission by resonant scattering of neutral hydrogen in the IS medium. We find an anti-correlation between Lyα EW and the covering fraction, f_c, estimated from the depth of absorption lines, where f_c is an indicator of average neutral hydrogen column density, N_(H I) . The results of our study support the idea that N_(H I) is a key quantity determining Lyα emissivity