We present a systematic study of weak intervening CaII absorbers at low redshift (z<0.5), based on the analysis of archival high-resolution (R>=45000) optical spectra of 304 quasars and active galactic nuclei observed with VLT/UVES. Along a total redshift path of {DELTA}z~~100 we detected 23 intervening CaII absorbers in both the CaII H & K lines, with rest frame equivalent widths W_r,3934_=15-799m{AA} and column densities logN(CaII)=11.25-13.04 (obtained by fitting Voigt-profile components).
We develop a method for separating quasars from other variable point sources using Sloan Digital Sky Survey (SDSS) Stripe 82 light-curve data for ~10000 variable objects. To statistically describe quasar variability, we use a damped random walk (DRW) model parametrized by a damping timescale, {tau}, and an asymptotic amplitude (structure function), SF_{infinite}_. With the aid of an SDSS spectroscopically confirmed quasar sample, we demonstrate that variability selection in typical extragalactic fields with low stellar density can deliver complete samples with reasonable purity (or efficiency, E). Compared to a selection method based solely on the slope of the structure function, the inclusion of the {tau} information boosts E from 60% to 75% while maintaining a highly complete sample (98%) even in the absence of color information. For a completeness of C=90%, E is boosted from 80% to 85%. Conversely, C improves from 90% to 97% while maintaining E=80% when imposing a lower limit on {tau}. With the aid of color selection, the purity can be further boosted to 96%, with C=93%. Hence, selection methods based on variability will play an important role in the selection of quasars with data provided by upcoming large sky surveys, such as Pan-STARRS and the Large Synoptic Survey Telescope (LSST). In summary, given an adequate survey cadence, photometric variability provides an even better method than color selection for separating quasars from stars.
Using the 2MASS Second Incremental Data Release (Cat. <B/2mass>), we have searched for near-infrared counterparts to 13214 quasars from the Veron-Cetty & Veron (Cat. <VII/215>) catalog. We have detected counterparts within 4" for 2277 of the approximately 6320 quasars within the area covered by the 2MASS Second Incremental Data Release. Only 1.6% of these are expected to be chance coincidences. Though this sample is heterogeneous, we find that known radio-loud quasars are more likely to have large near-infrared-to-optical luminosity ratios than radio-quiet quasars are, at a statistically significant level. This is consistent with dust-reddened quasars being more common in radio-selected samples than in optically selected samples, due to stronger selection effects against dust-reddened quasars in the latter. We also find a statistically significant dearth of optically luminous quasars with large near-infrared-to-optical luminosity ratios. This can be explained in a dust obscuration model but not in a model where synchrotron emission extends from the radio into the near-infrared and creates such large ratios. We also find that selection of quasar candidates from the B-J/J-K color-color diagram, modeled on the V-J/J-K selection method of Warren, Hewett, & Foltz, is likely to be more sensitive to dust-obscured quasars than selection using only infrared-infrared colors.
We present results from a variability and proper motion (VPM) search for QSOs in the field around M3. The VPM search is demonstrated to be powerful technique for efficiently finding QSOs without major selection effects with regard to the spectral energy distribution. Remarkably, the properties of the VPM QSOs do not significantly differ from those of samples from more conventional optical search techniques. The lightcurves of the QSOs from the resulting sample provide an interesting data set for the statistical investigation of QSO long-term variability.
Arguably the best hope of understanding the tail end of the re-ionization of the intergalactic medium (IGM) at redshift z>6 is through the detection and characterization of the Gunn-Peterson damping wing absorption of the IGM in bright quasar spectra. However, the use of quasar spectra to measure the IGM damping wing requires a model of the quasar's intrinsic Lyman{alpha} emission line. Here we quantify the uncertainties in the intrinsic line shapes, and how those uncertainties affect the determination of the IGM neutral fraction. We have assembled a catalogue of high-resolution Hubble Space Telescope spectra of the emission lines of unobscured low-redshift quasars, and have characterized the variance in the shapes of their lines. We then add simulated absorption from the high-redshift IGM to these quasar spectra in order to determine the corresponding uncertainties in re-ionization constraints using current and future samples of z>6 quasar spectra. We find that, if the redshift of the Lyman{alpha} emission line is presumed to coincide with the systemic redshift determined from metal lines, the inferred IGM neutral fraction is systematically biased to low values due to a systematic blueshift of the Lyman{alpha} line relative to the metal lines.
We present the results from a time domain study of absorption lines detected in quasar spectra with repeat observations from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7). Beginning with over 4,500 unique time separation baselines of various absorption line species identified in the SDSS DR7 quasar spectra, we create a catalogue of 2,522 quasar absorption line (QAL) systems with two to eight repeat observations, representing the largest, unbiased, and homogeneous collection of multi-epoch absorption systems ever published. To investigate these systems for time-variability of narrow absorption lines, we refine this sample based on the reliability of the system detection, the proximity of pixels with bright sky contamination to individual absorption lines, and the quality of the continuum fit. Variability measurements of this sub-sample based on the absorption line equivalent widths yield a total of 33 systems with indications of significantly variable absorption strengths on time-scales ranging from one day to several years in the rest frame of the absorption system. Of these, at least 10 are from a class known as intervening absorption systems caused by foreground galaxies along the line of sight to the background quasar. This is the first evidence of possible absorption line variability detected in intervening systems, and their short time-scale variations suggest that small-scale structures (~10-100AU) are likely to exist in their host foreground galaxies.
We discuss the statistics of projection on the sky of a simple model for radio sources. The model has a core and two hot spots at unequal distance with an overall structure which is bent at the core. We examine the distributions or the observed sizes and bend angles in 114 quasars and 78 radio galaxies with z<2, from our own data. We find that the observations are well matched by this model with (a) a size distribution matching the observed one, (b) core-hotspot length differences between 0 and 45kpc, and (c) bend angles distributed between 0deg and ~25deg, where quasars are seen within 50deg of the source axis, and radio galaxies are seen at greater angles. We discuss the implications of these fits to unified models and source evolution.
This paper reports the initial results of the Palomar Transit Grism Survey (PTGS). The PTGS was designed to produce a sample of z>2.7 quasars that were identified by well-defined selection criteria. The survey consists of six narrow (~8.5' wide) strips of sky; the total effective area is 61.47deg^2^. Low-resolution slitless spectra, covering the wavelength range from 4400 to 7500A, were obtained for approximately 600000 objects. The wavelength- and flux-calibrated spectra were searched for emission lines with an automatic software algorithm. A total to 1655 emission features in the grism data satisfied our signal-to-noise ratio and equivalent width selection criteria; subsequent slit spectroscopy of the candidates confirmed the existence of 1052 lines (928 different objects). Six groups of emission lines were detected in the survey: Lyman alpha+N V, C IV, C III], Mg II, Hbeta+[O III], and Halpha+[S II]. More than two-thirds of the candidates are low-redshift (z<0.45) emission-line galaxies; ninety objects are high-redshift quasars (z>2.7) detected via their Lyman alpha+N V emission lines. The survey contains three previously unknown quasars brighter than 17th magnitude; all three have redshifts of ~1.3. In this paper we present the observational properties of the survey, the algorithms used to select the emission-line candidates, and the catalog of emission-line objects.
We study the properties of infrared-selected QSOs (IR QSOs), optically selected Palomar-Green QSOs (PG QSOs), and narrow-line Seyfert 1 galaxies (NLS1s). We compare their properties from the IR to the optical and examine various correlations among the black hole mass, accretion rate, star formation rate, and optical and IR luminosities.
We present the spectra and redshifts of 62 quasars, from observations made with the Cerro Tololo Inter-American Observatory (CTIO) 4-m Blanco Telescope. These quasars form part of a total sample of 118 (with 56 having been published previously), which is being used for analysis of structure in the early universe. Quasars of particular interest are noted, including eight broad absorption line (BAL) quasars and two quasars with unusual emission spectra. Finally, we include a short summary of the present status of the large quasar group (LQG) that was discovered by Clowes & Campusano (1991, Cat. J/MNRAS/249/218 and 1994, Cat. J/MNRAS/266/317) from the earlier observations. The quasars are from an area ~25.3deg^2^ of ESO/SERC field 927, which is centred at (B1950) 10h 40m 00s, 05{deg} 00'00".
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