We present a survey for optically thick Lyman limit absorbers at z<2.6 using archival Hubble Space Telescope observations with the Faint Object Spectrograph and Space Telescope Imaging Spectrograph. We identify 206 Lyman limit systems (LLSs), increasing the number of cataloged LLSs at z<2.6 by a factor of ~10.
We present an analysis of CCD images (obtained at the Palomar 1.5m telescope) of more than 100 quasars; many of the objects have been or will be observed in the HST Key Project Quasar Absorption-Line Survey. The data set consists of B, g, and i photometry of 117 quasars and deeper g (limiting magnitude of ~22) images of 101 quasar fields. Positions accurate to ~1", measured with the Space Telescope Science Institute's Astrometric Support Program, are listed for all of the quasars in this study. Positions, magnitudes, and classifications of stars and galaxies located within ~100" of the quasars are given for all of the deep g data. The positions of the stars and galaxies relative to the quasar are accurate to ~0.5". The results presented here can be used to prepare spectroscopic programs designed to obtain redshifts of galaxies in the fields of these quasars.
Sensitive H I 21cm emission line spectra have been measured for the directions to 143 quasars and AGNs chosen from the observing lists for the HST Quasar Absorption Line Key Project. Narrow-band and wide-band data were obtained with the NRAO 43m radio telescope for each object. The narrow-band data have a velocity resolution of 1km/s, extend from -220 to +170km/s, and are corrected for stray 21cm radiation. The wide-band data have a resolution of 4km/s and extend from -1000 to +1000km/s. The data are important for the interpretation of ultraviolet absorption lines near zero redshift in Key Project spectra. Twenty-two percent of the quasars lie behind Galactic high-velocity H I clouds with |VLSR|>100km/s whose presence can increase the equivalent width of interstellar absorption lines significantly. This paper contains the emission spectra and measures of the H I velocities and column densities along the sight line to each quasar. We discuss how the measurements can be used to estimate the visual and ultraviolet extinction toward each quasar and to predict the approximate strength of the strong ultraviolet resonance lines of neutral gas species in the HST Key Project spectra.
We present a set of 71 quasars observed in the near ultraviolet with the PR200L prism on the Advanced Camera for Surveys and the G280 grism on the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope. The quasars were selected from the Sloan Digital Sky Survey data to search for intervening Lyman limit system (LLS) absorption. The sample was subjected to the constraints that the quasars have AB magnitude g'<18.5, quasar emission redshift 2.3<z_em_<2.6, and lack strong BAL or z~z_em_ absorption. The median emission redshift of the sample is {bar}z=2.403. The data were all processed using custom data reduction pipelines, and the one-dimensional spectra have sufficient signal-to-noise ratio and resolution to easily identify absorption from LLS over the redshift range 1.2<z<2.5. The WFC3 data presented here are the first non-calibration spectra from the G280 grism.
We present the first science results from our Hubble Space Telescope survey for Lyman limit absorption systems (LLS) using the low dispersion spectroscopic modes of the Advanced Camera for Surveys and the Wide Field Camera 3. Through an analysis of 71 quasars, we determine the incidence frequency of LLS per unit redshift and per unit path length, l(z) and l(X), respectively, over the redshift range 1<z<2.6, and find a weighted mean of l(X)=0.29+/-0.05 for 2.0<z<2.5 through a joint analysis of our sample and that of Ribaudo et al. (2011, J/ApJ/736/42). Through stacked spectrum analysis, we determine a median (mean) value of the mean free path to ionizing radiation at z=2.4 of {lambda}^912^_mfp_=243(252)h^-1^_72_Mpc, with an error on the mean value of +/-43h^-1^_72_Mpc. We also re-evaluate the estimates of {lambda}^912^_mfp_ from Prochaska et al. (2010, J/ApJ/718/392) and place constraints on the evolution of {lambda}^912^_mfp_ with redshift, including an estimate of the "breakthrough" redshift of z=1.6. Consistent with results at higher z, we find that a significant fraction of the opacity for absorption of ionizing photons comes from systems with N_HI_<=10^17.5^cm^-2^ with a value for the total Lyman opacity of {tau}^Lyman^_eff_=0.40+/-0.15. Finally, we determine that at minimum, a 5-parameter (4 power law) model is needed to describe the column density distribution function f(N_HI_,X) at z~2.4, find that f(N_HI_,X) undergoes no significant change in shape between z~2.4 and z~3.7, and provide our best fit model for f(N_HI_,X).
We investigate a sample of 3413 International Celestial Reference Frame (ICRF3) extragalactic radio-loud sources with accurate positions determined by very long baseline interferometry in the S/X band, mostly active galactic nuclei and quasars, which are cross-matched with optical sources in the second Gaia data release (Gaia DR2). The main goal of this study is to determine a core sample of astrometric objects that define the mutual orientation of the two fundamental reference frames, the Gaia (optical) and the ICRF3 (radio) frames. The distribution of normalized offsets between the VLBI sources and their optical counterparts is non-Rayleigh, with a deficit around the modal value and a tail extending beyond the 3{sigma} confidence level. A few filters are applied to the sample in order to discard double cross-matches, confusion sources, and Gaia astrometric solutions of doubtful quality. Panoramic Survey Telescope and Rapid Response System and Dark Energy Survey stacked multicolor images are used to further deselect objects that are less suitable for precision astrometry, such as extended galaxies, double and multiple sources, and obvious misidentifications. After this cleaning, 2643 quasars remain, of which 20% still have normalized offset magnitudes exceeding 3, or a 99% confidence level. We publish a list of 2119 radio-loud quasars of prime astrometric quality. The observed dependence of binned median offset on redshift shows the expected decline at small redshifts, but also an unexpected rise at z~1.6, which may be attributed to the emergence of the CIV emission line in the Gaia's G band. The Gaia DR2 parallax zero-point is found to be color-dependent, suggesting an uncorrected instrumental calibration effect.
We present an analysis of the FUSE (Far Ultraviolet Spectroscopic Explorer) and HST (Hubble Space Telescope) STIS E140M spectra of HE 0226-4110 (z=0.495). We detect 56 Lyman absorbers and five OVI absorbers.
We present a galaxy survey of the field surrounding PKS 0405-123 performed with the WFCCD spectrometer at Las Campanas Observatory. The survey is comprised of two data sets: (1) a greater than 95% complete survey to R=20mag of the field centered on PKS 0405-123 with 10' radius (L~0.1L* and radius of 1Mpc at z=0.1); and (2) a set of four discontiguous (i.e., non-overlapping), flanking fields covering ~1deg^2^ area with completeness ~90% to R=19.5mag. With these data sets, one can examine the local and large-scale galactic environment of the absorption systems identified toward PKS 0405-123. In this paper, we focus on the O VI systems analyzed in Paper I (Prochaska et al., 2004, Cat. <J/ApJ/617/718>). The results suggest that this gas arises in a diverse set of galactic environments including the halos of individual galaxies, galaxy groups, filamentary-like structures, and also regions devoid of luminous galaxies. In this small sample, there are no obvious trends between galactic environment and the physical properties of the gas. Furthermore, we find similar results for a set of absorption systems with comparable NHI but no detectable metal lines. The observations indicate that metals are distributed throughout a wide range of environments in the local universe. Future papers in this series will address the distribution of galactic environments associated with metal-line systems and the Ly forest based on data for over 10 additional fields. All of the spectra and fits tables are available at http://www.ucolick.org/~xavier/WFCCDOVI/
A systematic investigation of the relationship between different redshift estimation schemes for more than 91000 quasars in the Sloan Digital Sky Survey (SDSS) Data Release 6 is presented. The publicly available SDSS quasar redshifts are shown to possess systematic biases of {Delta}z/(1+z)>=0.002 (600km/s) over both small ({delta}z~=0.1) and large ({delta}z~=1) redshift intervals. Empirical relationships between redshifts based on (i) CaII H&K host galaxy absorption, (ii) quasar [OII] {lambda}{lambda}3728, (iii) [OIII] {lambda}{lambda} 4960, 5008 emission and (iv) cross-correlation (with a master-quasar template) that includes, at increasing quasar redshift, the prominent MgII {lambda}{lambda} 2799, CIII] {lambda}{lambda} 1908 and CIV {lambda}{lambda} 1549 emission lines are established as a function of quasar redshift and luminosity. New redshifts in the resulting catalogue possess systematic biases, a factor of ~=20 lower compared to the SDSS redshift values; systematic effects are reduced to the level of {Delta}z/(1+z) (30km/s) per unit redshift or <=2.5x10^-5^ per unit absolute magnitude. Redshift errors, including components due both to internal reproducibility and to the intrinsic quasar-to-quasar variation among the population, are available for all quasars in the catalogue. The improved redshifts and their associated errors have wide applicability in areas such as quasar absorption outflows, quasar clustering, quasar-galaxy clustering and proximity-effect determinations.
At present, most of the variability studies of active galactic nuclei (AGNs) are based on ensemble analyses. Nevertheless, it is interesting to provide estimates of the individual variability properties of each AGN, in order to relate them with intrinsic physical quantities. A useful dataset is provided by the Catalina Surveys Data Release 2 (CSDR2), which encompasses almost a decade of photometric measurements of ~500 million objects repeatedly observed hundreds of times.We aim to investigate the individual optical variability properties of 795 AGNs originally included in the Multi-Epoch XMMSerendipitous AGN Sample 2 (MEXSAS2). Our goals consist in: (i) searching for correlations between variability and AGN physical quantities; (ii) extending our knowledge of the variability features of MEXSAS2 from the X-ray to the optical.We use the structure function (SF) to analyse AGN flux variations. We model the SF as a power-law, SF(tau)=A(tau/tau_0)^gamma^, and we compute its variability parameters. We introduce the V-correction as a simple tool to correctly quantify the amount of variability in the rest frame of each source.We find a significant decrease of variability amplitude with increasing bolometric, optical and X-ray luminosity. We obtain the indication of an intrinsically weak positive correlation between variability amplitude and redshift, z. Variability amplitude also appears to be positively correlated with alpha_{ox}.The slope of the power-law SF, gamma, is weakly correlated with the bolometric luminosity L_{bol} and/or with the black hole mass M_{BH}. When comparing optical to X-ray variability properties, we find that X-ray variability amplitude is approximately the same for those AGNs with larger or smaller variability amplitude in the optical.On the contrary, AGNs with steeper SF in the optical do present steeper SF in the X-ray, and vice versa.
