We estimate the mean dust-to-gas ratio of MgII absorbers as a function of rest equivalent width W_0 and redshift over the range 0.5<z<1.4. Using the expanded SDSS/HST sample of low-redshift Lyman-alpha absorbers we first show the existence of a 8-sigma correlation between the mean hydrogen column density <N_HI> and W_0, an indicator of gas velocity dispersion. By combining these results with recent dust-reddening measurements we show that the mean dust-to-gas ratio of MgII absorbers does not appreciably depend on rest equivalent width. Assuming that, on average, dust-to-gas ratio is proportional to metallicity, we find its redshift evolution to be consistent with that of L^star galaxies from z=0.5 to 1.4 and we show that our constraints disfavor dwarf galaxies as the origin of such absorbers. We discuss other scenarii and favor galactic outflows from ~L^star galaxies as the origin of the majority of strong MgII absorbers. Finally, we show that, once evolutionary effects are taken into account, the Bohlin et al. relation between A_V and N_H is also satisfied by strong MgII systems down to lower column densities than those probed in our Galaxy.
Recent observations suggest the incidence of strong intervening MgII absorption systems along the line-of-sight to gamma ray burst (GRB) afterglows is significantly higher than expected from analogous quasar sightlines. One possible explanation is a geometric effect, arising because MgII absorbers only partially cover the quasar continuum regions, in which case MgII absorbers must be considerably smaller than previous estimates. We investigate the production of abnormal absorption profiles by partial coverage and conclude that the lack of any known anomalous profiles in observed systems, whils…
We measure the mean halo-mass of z=0.5 MgII absorbers using the cross-correlation (over co-moving scales 0.05-13h^{-1}Mpc) between 1806 MgII quasar absorption systems and ~250,000 Luminous Red Galaxies (LRGs), both selected from the SDSS DR3. The MgII systems have rest-frame equivalent widths W_r(2796)>=0.3A. From the ratio of the MgII-LRG cross-correlation to the LRG-LRG auto-correlation, we find that the bias ratio between MgII absorbers and LRGs is 0.65+/-0.08, which implies that the absorber host-galaxies have a mean halo-mass 20-40 times smaller than that of the LRGs; the MgII absorbers h…
We present the results of photoionization modeling of nine weak MgII (W_r<0.3 Ang) quasar absorption line systems with redshifts 1.4<z<2.4 obtained with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope. These systems have been chosen because they provide access to a regime of red-shift space that previous weak MgII studies have not looked at. The densities, metallicities, Doppler parameters, and column densities of these systems are compared to those of other weak MgII systems at lower redshift. There is no significant statistical variation in the properties of the a…
Using a large sample of MgII absorbers with 0.4<z<2.2 detected by Nestor et al (2005) in the Early Data Release of the SDSS, we present new constraints on the physical properties of these systems based on two statistical analyses: (i) By computing the ratio between the composite spectra of quasars with and without absorbers, we measure the reddening effects induced by these intervening systems; and (ii) by stacking SDSS images centered on quasars with strong MgII absorption lines and isolating the excess light around the PSF, we measure the mean luminosity and colors of the absorbing galaxies.…
We present results from a survey of weak MgII absorbers in the VLT/UVES spectra of 81 QSOs obtained from the ESO archive. In this survey, we identified 112 weak MgII systems within the redshift interval 0.4 < z < 2.4 with 86% completeness down to a rest-frame equivalent width of W_r(2796) = 0.02A, covering a cumulative redshift path length of deltaZ=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 <z>=0.9 and 1.06 +/- 0.10 at <z>=0.6. Thu…
The Damped Lyman-alpha systems (DLAs), seen in absorption in the spectrum of quasars, are believed to contain a large fraction of the neutral gas in the Universe. Paradoxically, these systems are more difficult to observe at z_abs<1.7, since they are rare and their HI feature then falls in UV spectra. Rao & Turnshek (2000) pioneered a method based on MgII-selected DLAs, that is absorbers discovered thanks to our knowledge of their MgII feature in optical spectra. We use new observations undertaken at the TNG as well as a careful literature & archival search to build samples of low redshift abs…
We present a statistical analysis of the photometric properties and spatial distribution of more than 2,800 MgII absorbers with 0.37<z<1 and rest equivalent width W_0(\lambda2796)>0.8Ådetected in SDSS quasar spectra. Using an improved image stacking technique, we measure the cross-correlation between MgII gas and light (in the g, r, i and z-bands) from 10 to 200 kpc and infer the light-weighted impact parameter distribution of MgII absorbers. Such a quantity is well described by a power-law with an index that strongly depends on W_0, ranging from ~-1 for W_0<~1Åto ~-2 for W_0>~ 1.5Å. At redshi…
We study the cross-correlation between 716 MgII quasar absorption systems and about 100,000 Luminous Red Galaxies (LRGs) selected from the Sloan Digital Sky Survey Data Release 3 in the redshift range 0.4<z<0.8. The MgII systems were selected to have 2796 & 2803 rest-frame equivalent widths greater than 1.0 Åand identifications confirmed by the FeII 2600 or MgI 2852 lines. Over co-moving scales 0.2--13/h Mpc, the MgII--LRG cross-correlation has an amplitude 0.69+/-0.09 times that of the LRG--LRG auto-correlation. Since LRGs have halo-masses of 10^{13} \msun, this strong cross-correlation impli…
The exact nature of weak MgII absorbers (those with W_r(2796) < 0.3 A) is a matter of debate, but most are likely related to areas of local star formation or supernovae activity outside of giant galaxies. Using 18 QSO spectra obtained with the Ultra-Violet Echelle Spectrograph (UVES) on the Very Large Telescope (VLT), we have conducted a survey for weak MgII absorbers at 1.4 < z < 2.4. We searched a redshift path length of 8.51, eliminating regions badly contaminated by atmospheric absorption so that the survey is close to 100% complete to W_r(2796) = 0.02 A. We found a total of 9 weak absorbe…
We study the cross-correlation between 212 MgII quasar absorption systems and \~20,000 Luminous Red Galaxies (LRGs) selected from the Sloan Digital Sky Survey Data Release 1 in the redshift range 0.4<z<0.8. The MgII systems were selected to have 2796 & 2803 rest-frame equivalent widths >=1.0 Angstrom and identifications confirmed by the FeII 2600 or MgI 2852 lines. Over comoving scales 0.05--13 h^-1 Mpc, the MgII--LRG cross-correlation has an amplitude 0.69+/-0.09 times that of the LRG--LRG auto-correlation. Since LRGs have halo-masses greater than 3.5 x 10^12 solar masses for M_R<-21, this re…
We challenge the conventional view that the majority of MgII/Lyman limit absorbers are extended halos of galaxies comprised of ``clouds'' with near-unity covering factor. The gaseous disks of spiral galaxies are known to extend to large radii and likely contribute a significant cross-section for absorption. We perform a Monte-Carlo survey of QSO fields in which the model galaxies have MgII absorbing ``clouds'' in a spherical halo or in a randomly oriented disk. For both geometries, models that recover the observed properties of MgII absorbers have only a 70-80% covering factor. Therefore, rega…
We consider the evolution of strong (W_r(2796) > 0.3A) MgII absorbers, most of which are closely related to luminous galaxies. Using 20 high resolution quasar spectra from the VLT/UVES public archive, we examine 33 strong MgII absorbers in the redshift range 0.3 < z < 2.5. We compare and supplement this sample with 23 strong MgII absorbers at 0.4 < z < 1.4 observed previously with HIRES/Keck. We find that neither equivalent width nor kinematic spread (the optical depth weighted second moment of velocity) of MgII2796 evolve. However, the kinematic spread is sensitive to the highest velocity com…
We report on a survey for strong (rest equivalent width W_r >= 1A), intervening MgII systems along the sightlines to long-duration gamma-ray bursts (GRBs). The GRB spectra which comprise the survey have a heterogeneous mix of resolution and wavelength coverage, but we implement a strict, uniform set of search criteria to derive a well-defined statistical sample. We identify 15 strong MgII absorbers along 14 GRB sightlines (nearly every sightline exhibits at least one absorber) with spectra covering a total pathlength Delta z = 15.5 at a mean redshift <z> = 1.1. In contrast, the predicted incid…
We present an investigation of MgII absorbers characterized as single-cloud weak systems at z~1. We measured column densities and Doppler parameters for MgII and FeII in 15 systems found in HIRES/Keck spectra at 6.6 km/s. Using these quantities and CIV, Lyman alpha and Lyman limit absorption observed with FOS/HST (resolution ~230 km/s) we applied photoionization models to each system to constrain metallicities, densities, ionization conditions, and sizes. We find that: (1) Single-cloud weak systems are optically thin in neutral hydrogen and may have their origins in a population of objects dis…
We present the observations of a weak MgII absorption system detected at z~0.452 in the UVES high resolution spectrum of the QSO HE0001-2340. The weaker of the two MgII components forming the system shows associated absorptions due to SiI, CaI and FeI observed for the first time in a QSO spectrum. We investigate the nature of this absorber by comparing its properties with those of different classes of absorbers (weak MgII, Damped Ly-alpha systems and local interstellar clouds) and reproducing its ionization conditions with photoionization models. The observed absorber belongs to the class of w…
A new and mysterious class of object has been revealed by the detection of numerous weak MgII doublets in quasar absorption line spectra. The properties of these objects will be reviewed. They are not in close proximity to luminous galaxies, yet they have metallicities close to the solar value; they are likely to be self-enriched. A significant fraction of the weak MgII absorbers are constrained to be less than 10 parsecs in size, yet they present a large cross-section for absorption, indicating that there are more than a million times more of them than there are luminous galaxies. They could …
We have initiated a survey of MgII 2796, 2803 absorbers near known galaxies at projected distances rho<~100 h^-1 kpc to a background QSO, in order to constrain the extent and covering fraction of Mg^+ ions around galaxies of different luminosity and mass. The current sample consists of 13 galaxy and absorber pairs and 10 galaxies that do not produce MgII absorption lines to within sensitive upper limits. The redshifts of the galaxy and absorber pairs range from z = 0.2067 to 0.892 with a median of z = 0.3818. We find that galaxies at larger impact parameters produce on average weaker MgII abso…
We present the results of a survey of the analogs of weak MgII absorbers (rest frame equivalent width W(2796) < 0.3 A) at 0 < z < 0.3. Our sample consisted of 25 HST/STIS echelle quasar spectra (R = 45,000) which covered SiII 1260 and CII 1335 over this redshift range. Using those similar transitions as tracers of MgII facilitates a much larger survey, covering a redshift pathlength of g(z) = 5.3 for an equivalent width limit of MgII corresponding to W(2796) > 0.02 A, with 30% completeness for the weakest lines. We find the number of weak MgII absorber analogs with 0.02 < W(2796) < 0.3 to be d…
Using a sample of almost 7000 strong MgII absorbers with 0.4 < z < 2.2 detected in the SDSS DR4 dataset, we investigate the gravitational lensing and dust extinction effects they induce on background quasars. After carefully quantifying several selection biases, we isolate the reddening effects as a function of redshift and absorber rest equivalent width, W_0. We find the amount of dust to increase with cosmic time as (1+z)^(-1.1 +/- 0.4), following the evolution of cosmic star density or integrated star formation rate. We measure the reddening effects over a factor 30 in E(B-V) and we find th…
The largest known structure in the high redshift universe is mapped by at least 18 quasars and spans ~5 deg x 2.5 deg on the sky, with a quasar spatial overdensity of 6-10 times above the mean. This large quasar group provides an extraordinary laboratory ~100 x 200 x 200 h^-3 comoving Mpc^3 in size (q0=0.5, Lambda=0, H0=100h km/s/Mpc) covering 1.20<z<1.39 in redshift. One approach to establish how LQGs relate to mass (galaxy) enhancements is to probe their gas content and distribution via background quasars. We have found the large quasar group to be associated with 11 MgII absorption systems …
The current status and remaining questions of MgII absorbers are reviewed with an eye toward new results incorporating high quality Hubble Space Telescope images of the absorbing galaxies. In the end, we find that our current picture of extended gaseous regions around galaxies at earlier epochs is in need of some revision; MgII absorbing "halos" appear to be patchier and their geometry less regular than previously inferred. We also find that the so-called "weak" MgII absorbers are associated with normal galaxies over a wide range of impact parameters, suggesting that this class of absorber doe…
The startling discovery of Prochter et al. (2006) that the frequency of very strong (W_r(2796)>1 A) MgII absorbers along gamma-ray burst (GRB) lines of sight ([dN/dz]_{GRB} = 0.90) is more than three times the frequency along quasar lines of sight ([dN/dz]_{QSO} = 0.24), over similar redshift ranges, has yet to be understood. We reconsider the possibility that the excess of very strong MgII absorbers toward GRBs is intrinsic either to the GRBs themselves or to their immediate environment, and associated with bulk outflows with velocities as large as v_{max} ~ 0.3c. In order to examine this hyp…
To understand which parts of galaxies give rise to the variety of observed MgII absorption profiles seen in QSO spectra, we have embarked upon a program to generate simulated absorption profiles as they would arise from the many typical galactic structures we see today. Here, we present preliminary results for a clumpy, rotating disk which has been sampled by a variety of QSO lines of sight. The resulting ensemble of simulated profiles is qualitatively similar to those observed in HIRES spectra of 0.4 < z < 1.0 MgII systems (Churchill, this volume). Additionally, we discuss the statistical con…
We describe the first optical survey of absorption systems associated with galaxy clusters at z= 0.3-0.9. We have cross-correlated SDSS DR3 quasars with high-redshift cluster/group candidates from the Red-Sequence Cluster Survey. We have found 442 quasar-cluster pairs for which the MgII doublet might be detected at a transverse (physical) distance d<2 Mpc from the cluster centers. To investigate the incidence (dN/dz) and equivalent-width distribution n(W) of MgII systems at cluster redshifts, two statistical samples were drawn out of these pairs: one made of high-resolution spectroscopic quasa…
We report the detection, based on HST and Keck data, of two peculiar absorbers in the Ly-alpha forest of the quasar PKS 0454+039. These clouds, at redshifts z=0.6248 and 0.9315 respectively, display both MgII and FeII absorption lines in addition to the Ly-alpha line. Based upon photoionization models, these are inferred to be photoionized by the intergalactic UV background, and to have HI column densities in the range 15.8 < N(HI) < 16.8. Furthermore, if one supposes that the relative abundances of heavy elements is similar to that of depleted clouds of our galaxy, the abundances of these two…
Prochter et al. 2006 recently found that the number density of strong intervening 0.5<z<2 MgII absorbers detected in gamma-ray burst (GRB) afterglow spectra is nearly 4 times larger than in QSO spectra. We have conducted a similar study using CIV absorbers. Our CIV sample, consisting of a total of 20 systems, is drawn from 3 high resolution and high to moderate S/N VLT/UVES spectra of 3 long-duration GRB afterglows, covering the redshift interval 1.6< z<3.1. The column density distribution and number density of this sample do not show any statistical difference with the same quantities measure…
The incidence of strong MgII systems in gamma-ray burst (GRB) spectra is a few times higher than in quasar (QSO) spectra. We investigate several possible explanations for this effect, including: dust obscuration bias, clustering of the absorbers, different beam sizes of the sources, multiband magnification bias of GRBs, association of the absorbers with the GRB event or the circumburst environment. We find that: i) the incidence rate of MgII systems in QSO spectra could be underestimated by a factor 1.3-2 due to dust obscuration; ii) the equivalent-width distribution of the MgII absorbers alon…
The study of galaxy evolution using quasar absorption lines requires an understanding of what components of galaxies and their surroundings are contributing to the absorption in various transitions. This paper considers the kinematic composition of the class of 0.4 < z < 1.0 MgII absorbers, particularly addressing the question of what fraction of this absorption is produced in halos and what fraction arises from galaxy disks. We design models with various fractional contributions from radial infall of halo material and from a rotating thick disk component. We generate synthetic spectra from li…
High resolution optical (HIRES/Keck) and UV (STIS/HST) spectra, covering a large range of chemical transitions, are analyzed for three single-cloud weak MgII absorption systems along the line of sight toward the quasar PG 1634+706. Weak MgII absorption lines in quasar spectra trace metal-enriched environments that are rarely closely associated with the most luminous galaxies (>0.05L^*). The two weak MgII systems at z=0.81 and z=0.90 are constrained to have >=solar metallicity, while the metallicity of the z=0.65 system is not as well-constrained, but is consistent with >1/10th solar. These wea…
We present a search for "weak" MgII absorbers [those with W_r(2796) < 0.3 A in the HIRES/Keck spectra of 26 QSOs. We found 30, of which 23 are newly discovered. The spectra are 80% complete to W_r(2796) = 0.02 A and have a cumulative redshift path of ~17.2 for the redshift range 0.4 < z < 1.4. The 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 <= W_r(2796) < 0.3 A sample. The equivalent width distribution follows a power law with slope -1.0; there is no turnover down to W_r(2796) = 0…
We present Gemini near-infrared spectroscopic observations of six luminous quasars at z=5.8$\sim$6.3. Five of them were observed using Gemini-South/GNIRS, which provides a simultaneous wavelength coverage of 0.9--2.5 $μ$m in cross dispersion mode. The other source was observed in K band with Gemini-North/NIRI. We calculate line strengths for all detected emission lines and use their ratios to estimate gas metallicity in the broad-line regions of the quasars. The metallicity is found to be supersolar with a typical value of $\sim$4 Z_{\sun}, and a comparison with low-redshift observations shows…
Low-ionization transitions such as the MgII 2796/2803 doublet trace cold gas in the vicinity of galaxies. It is not clear whether this gas is part of the interstellar medium of large proto-disks, part of dwarfs, or part of entrained material in supernovae-driven outflows. Studies based on MgII statistics, e.g. stacked images and clustering analysis, have invoked starburst-driven outflows where MgII absorbers are tracing the denser and colder gas of the outflow. A consequence of the outflow scenario is that the strongest absorbers ought to be associated with starbursts. We use the near-IR integ…
Using data from the Sloan Digital Sky Survey data release 3 (SDSS DR3) we investigate how narrow (<700km/s) CIV and MgII quasar absorption line systems are distributed around quasars. The CIV absorbers lie in the redshift range 1.6 < z < 4 and the MgII absorbers in the range 0.4<z<2.2. By correlating absorbers with quasars on different but neighbouring lines-of-sight, we measure the clustering of absorbers around quasars on comoving scales between 4 and 30Mpc. The observed comoving correlation lengths are r_o~5h^-1Mpc, similar to those observed for bright galaxies at these redshifts. Comparing…
One way to characterize and understand HI-selected galaxies is to study their metallicity properties. In particular, we show that the metallicity of absorbers is a bivariate function of the HI column density (NHI) and the MgII equivalent width (Wr(2796)). Thus, a selection upon Wr(2796) is not equivalent to a HI selection for intervening absorbers. A direct consequence for damped absorbers with NHI>20.3 from the bivariate metallicity distribution is that any correlation between the metallicity [X/H] and velocity width (or Wr(2796)) arises from the HI cut and therefore can not be interpreted as…
[abridged] Using stacked Sloan Digital Sky Survey spectra, we present the detection of [OII]3727,3730 nebular emission from galaxies hosting CaII and MgII absorption line systems. Both samples of absorbers, 345 CaII systems and 3461 MgII systems, span the redshift interval 0.4 < z < 1.3; all of the former and half the latter sample are expected to be bona-fide damped Lyman-alpha (DLA) absorbers. The measured star formation rate (SFR) per absorber from light falling within the SDSS fibre apertures (corresponding to physical radii of 6-9 h^-1 kpc) is 0.11-0.14 Msol/yr for the MgII-selected DLAs …
We analyze high resolution spectra of a multi--cloud weak [defined as W_r(MgII) < 0.3 A] absorbing system along the line of sight to PG 1634+706. This system gives rise to a partial Lyman limit break and absorption in MgII, SiII, CII, SiIII, SiIV, CIV, and OVI. The lower ionization transitions arise in two kinematic subsystems with a separation of ~150 km/s. Each subsystem is resolved into several narrow components, having Doppler widths of 3-10 kms. For both subsystems, the OVI absorption arises in a separate higher ionization phase, in regions dominated by bulk motions in the range of 30-40 …
Using a sample of nearly 700 quasars with strong (W_0(2796)>0.8 Angstrom) MgII absorption lines detected in the Early Data Release of the SDSS, we demonstrate the feasibility of measuring the photometric properties of the absorber systems by stacking SDSS imaging data. As MgII lines can be observed in the range 0.37<z_abs<2.2, the absorbing galaxies are in general not identified in SDSS images, but they produce systematic light excesses around QSOs which can be detected with a statistical analysis. In this Letter we present a 6-sigma detection of this effect over the whole sample in i-band, ri…
We report the finding of two large-scale structures of galaxies in a 40'x35' field embedded in a 25deg^2 area where two 100 Mpc-scale structures of quasars and MgII absorbers at z~0.8 and z~1.2 have been previously detected. Using deep optical (V- and I-band) imaging, we are able to select high-redshift (0.5<z<1.3) early-type galaxies as those redder than the cluster red sequence at z=0.5 and having I<23.5. Through comparison with a 35'x35' control field, we find a 30% excess of these red galaxies, corresponding to 563 galaxies across the field. The colour distribution of the excess galaxies s…
We discuss issues concerning the physical nature of intervening MgII quasar absorption systems in light of results from our recent surveys using SDSS EDR QSO spectra and data obtained at the MMT. These surveys indicate an excess of weak $W_0^{\lambda2796} \lesssim 0.3$Å) systems relative to the exponential dN/dW distribution of stronger systems. The incidence of intermediate-strength lines shows remarkably little evolution with redshift, thereby constraining models for the nature of the clouds comprising these absorbers. The total distribution does evolve, with the incidence decreasing with de…
Sloan Digital Sky Survey (SDSS) quasar spectroscopy is yielding a database of strong low-ionization MgII absorbers over the redshift interval 0.36<z<2.28 which is over two orders of magnitude larger than anything previously assembled. HST UV spectroscopy has been used to measure neutral hydrogen column densities for a small subset of them. These data empirically show that MgII absorbers with rest equivalent widths $W^{\lambda2796}_0 \ge 0.6$ Åhave a mean neutral hydrogen column density that is roughly constant at $N(HI) \approx 4 \times 10^{20}$ atoms cm$^{-2}$, with individual systems lying i…
The goal of the Complete Optical and Radio Absorption Line System (CORALS) survey is to quantify the potential impact on QSO absorber statistics from dust in intervening galaxies. Dust may introduce a selection bias in surveys which are based on magnitude limited QSO samples, leading to an underestimate of absorber number density, n(z). Here we present the results of the second phase of the CORALS survey which extends our previous work on z > 1.8 damped Lyman alpha systems (DLAs) to search for strong metal line systems (candidate DLAs) in the range 0.6 < z < 1.7. We have identified 47 MgII sys…
The z=0.9902 system along the quasar PG 1634+706 line of sight is a strong MgII absorber (W(2796)>0.3A) with only weak CIV absorption (it is ``CIV-deficient''). To study this system, we used high-resolution spectra from both HST/STIS (R=30,000) and Keck/HIRES (R=45,000). These spectra cover key transitions, such as MgI, MgII, FeII, SiII, CII, SiIII, CIII, SiIV, and CIV. Assuming a Haardt and Madau extragalactic background spectrum, we modeled the system with a combination of photoionization and collisional ionization. Based on a comparison of synthetic spectra to the data profiles, we infer th…
We use 29 z>4 quasar spectra to build a homogeneous sample of high-redshift CIV absorbers. We use these data to calculate the number density, n(z), of W_rest(CIV)>0.30 A CIV doublets. We find that n(z) increases with time from z ~ 4.5. In addition, W_rest(CIV)>0.15 A CIV systems are more numerous than the former at all redshifts. On the contrary, n(z) of W_rest(MgII)>0.30 A MgII doublets decreases with time, in agreement with results from studies of LLS number densities. Below z<3, none of all the classes of absorbers show signs of evolution. We interpret this as the formation of galactic enve…
We present the following results of photoionization modeling of six MgII-selected absorption systems, at redshift 0.5 < z 0.9, along lines of sight toward three quasars: PG 1241+176, PG 1248+401, and PG 1317+274. These are part of a larger sample of ~20 MgII absorbers at intermediate redshift, that facilitates a survey of the properties of, and processes active in, the multiple phases of gas, both in and outside of galaxies. We present new high resolution ultraviolet spectra from HST/STIS as well as high resolution optical spectra obtained with HIRES/Keck. Together, these spectra allow simulta…
[Abridged] We examine a sample of 45 Mg II absorption-selected systems over the redshift range 0.4 to 1.4. Mg II and Fe II absorption profiles were observed at a resolution of ~6 km/s with HIRES/Keck. Lyman-alpha and C IV data were measured in archival FOS/HST spectra (~230 km/s). We perform a multivariate analysis of W(MgII), W(FeII), W(CIV) and W(Lya) (rest-frame) equivalent widths and the Mg II kinematic spread. We find five categories of Mg II absorbers: "Classic", "C IV-deficient", "Single/Weak", "Double", and "DLA/HI-Rich". There is a strong connection between low-ionization kinematics a…
Intergalactic MgII absorbers are known to have structures down to scales ~ 10^{2.5} pc, and there are now indications that they may be fragmented on scales <~ 10^{-2.5} pc (Hao et al., astro-ph/0612409). When a lensed quasar is microlensed, the micro-images of the quasar experience creation, destruction, distortion, and drastic astrometric changes during caustic-crossing. I show that quasar microlensing can effectively probe MgII and other absorption "cloudlets" with sizes ~ 10^{-4.0} - 10^{-2.0} pc by inducing significant spectral variability on the timescales of months to years. With numeric…
We report results on the incidence and cosmological mass density of damped Lyman-alpha (DLA) systems at redshifts less that 1.65. We used HST and an efficient non-traditional (but unbiased) survey technique to discover DLA systems at redshifts z<1.65, where we observe the Lyman-alpha line in known MgII absorption-line systems. We uncovered 14 DLA lines including 2 serendipitously. We find that (1) The DLA absorbers are drawn almost exclusively from the population of MgII absorbers which have rest equivalent widths W(2796)>0.6A. (2) The incidence of DLA systems per unit redshift, n(DLA), is obs…
[Abridged] We present 23 quasar absorption line systems selected by the MgII doublet with W_r(2796)>0.3 Ang over the redshift range 0.4<z<1.2. The kinematics are studied at ~6 km/s resolution to a 5-sigma detection threshold of 0.015 Ang. Most absorbers are characterized by a dominant kinematic subsystem with W_r>0.2 Ang and velocity spreads ranging from 10-50 km/s, in proportion to the system equivalent width. Additional kinematic subsystems have velocities out to ~400 km/s. The equivalent widths and velocity spreads of these "outlying", weaker subsystems are anti-correlated with their veloci…
A study of the absorption systems toward the gravitationally lensed quasar APM 08279+5255 is presented. Most of the MgII systems in the redshift range 1.2-2.07, although saturated, show large residuals at the bottom of the lines. The most likely interpretation is that individual clouds within MgII halos do cover only one of the two brightest QSO images. This reveals that MgII halos are made of a collection of clouds of radius smaller than about 1 kpc. Two strong MgII absorbers at z = 1.062 and 1.181 are studied in detail. This is the first time that the NaI3303 doublet is detected in such high…
As part of a larger program to identify low redshift radio analogues of the damped Lyman-alpha (DLA) absorbers seen in the spectra of high redshift quasars, Westerbork Synthesis Radio Telescope (WSRT) observations have discovered two new HI 21cm absorption lines at z=0.394 and z=0.437 in the spectra of the radio sources B 0248+430 and B 1243-072 respectively. These sightlines and redshifts were selected for study on the basis of the previously known low ionization absorption lines of MgII, and neither has been observed in the Lyman-alpha line. The 21cm line observations provide information on …
We consider geometries and possible physical models for weak low ionization absorbers based on the relative incidence of low and high ionization absorption systems. We found a total of 16 metal-line systems, with low and/or high ionization absorption detected in our survey of weak low ionization absorption systems from the archive of HST/STIS data. The weak low ionization absorbers trace an abundant population of metal-enriched regions (close to solar metallicity). Generally, models show that these systems have a ~10pc region of higher density gas and a ~1kpc region of lower density phase of h…
We present results from a spectroscopic and imaging survey of galaxies in the fields of quasars from the HST Quasar Absorption Line Key Project. The aim of this survey is to identify galaxies within 3.5' from the quasar sightline, to a limiting r-band magnitude mr = 22.5. The data are then compared to the HST homogeneous sample of Lyman-alpha-only absorbers in order to put constraints on the nature of these absorbers, and in particular on their relation to galaxies. The analysis of our sample combined with those of previous studies shows that: 1) the redshift agreements of the Ly-alpha absorbe…
IRAS13349+2438 was the first quasar suggested to host a warm absorber with internal dust. We obtained new HRI data for this object and derive constraints on the X-ray emitting region of IRAS13349+2438 by investigating its X-ray extent and lightcurve. Rapid X-ray variability is detected with a factor of two change in count rate within one day and 20% variability within about three hours, which practically rules out any significant contribution of scattered X-rays. We present for the first time a detailed, self-consistent modeling of the ROSAT PSPC and ASCA spectrum of IRAS13349+2438 in terms of…
The effects of the dust on the determination of elemental abundances in damped Ly alpha (DLA) absorbers are investigated. Relations between the observed abundances measured in the gas phase and the overall abundances (gas plus dust) are derived as a function of dust-to-gas ratio, metallicity, element-to-element abundance pattern, average extinction coefficient of dust grains, and chemical composition of dust grains. A method is presented for determining dust-to-gas ratios, dust-to-metals ratios, and dust-corrected relative abundances in DLA absorbers by assuming dust of Galactic type and const…
1.02-1.16 micron spectra (R ~ 7,000) of the gravitationally lensed QSO APM 08279+5255 at z_em=3.911 were obtained during the commissioning run of IRCS, the 1-5 micron near-infrared camera and spectrograph for the Subaru 8.2 m Telescope. Strong MgII doublet at 2976,2800 angstrom and FeII (2600 angstrom), FeII (2587 angstrom) absorption lines at z_abs=2.974 were clearly detected in the rest-frame UV spectra, confirming the presence of a damped Lyman-alpha system at the redshift as suggested by Petitjean et al. Also MgI (2853 angstrom) absorption line is probably detected. An analysis of the abso…
Using a sample of 21 damped Lyman alpha systems (DLAs) and 35 sub-DLAs, we evaluate the D-index = EW / Delta v x 1000 from high resolution spectra of the MgII lambda 2796 profile. This sample represents an increase in sub-DLA D-index statistics by a factor of four over the sample used by Ellison (2006). We investigate various techniques to define the velocity spread (Delta v) of the MgII line to determine an optimal D-index for the identification of DLAs. The success rate of DLA identification is 50 -- 55%, depending on the velocity limits used, improving by a few percent when the column densi…
We have analyzed survey and pointed \ros PSPC observations of the Seyfert galaxy NGC 3227. Large amplitude X-ray variability is detected, with a factor $\sim$ 15 change in count rate within about 3 years. Smaller changes are seen on the timescale of days, the largest being a factor of 3.5. No strong spectral variability is found throughout the pointed observation. The X-ray spectrum is modeled in terms of warm absorption and both, a dust-free warm absorber and one with internal dust, give an excellent spectral fit. Additionally using multi-wavelength observations of NGC 3227, the dusty warm ab…
Absorption line studies of galaxies along the line-of-sight to distant quasars allow a direct observational link between the properties of the extended gaseous disk/halo and of the star forming region of galaxies. In these proceedings we review recent work on CaII absorbers detected in the SDSS at 0.4<z<1.3 which, because of their dust content and chemical properties, may lie spatially closer to the central host galaxy than most DLAs. We present direct evidence for the presence of star formation, through observation of the [OII]3727,3730 emission line, in both CaII absorbers and MgII-selected …
CaII 3934,3969 absorbers, which are likely to be a subset of damped Lyman alpha systems, are the most dusty quasar absorbers known, with an order of magnitude more extinction in E(B-V) than other absorption systems. There is also evidence that CaII absorbers trace galaxies with more ongoing star-formation than the average quasar absorber. Despite this, relatively little is known in detail about these unusual absorption systems. Here we present the first high resolution spectroscopic study of 19 CaII quasar absorbers, in the range 0.6<= z_abs<=1.2, with W3934>=0.2A. Their general depletion patt…
A modification of the popular unification model for Seyfert galaxies is proposed, which takes into account recent observational findings on the statistical properties of both type 1 and type 2 Seyferts. In the proposed scenario, Compton-thick Seyfert 2 galaxies are those sources observed through compact, thick matter (the `torus'), while Compton-thin/intermediate Seyfert galaxies are obscured by dust lanes at larger distances.
