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Register a new userAverage Properties of a Large Sample of z_abs ~ z_em associated Mg II Absorption Line Systems
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We have studied a sample of 415 associated (z_ab z_em; relative velocity with respect to QSO <3000km/s) Mg II absorption systems with 1.0<=z_ab<=1.86, in the spectra of SDSS DR3 QSOs, to determine the dust content and ionization state in the absorbers. We studied the dependence of these properties on the properties of the QSOs and also, compared the properties with those of a similarly selected sample of 809 intervening systems (apparent relative velocity with respect to the QSO of >3000km/s), so as to understand their origin. From the analysis of the composite spectra, as well as from the comparison of measured equivalent widths in individual spectra, we conclude that the associated Mg II absorbers have higher apparent ionization, measured by the strength of the C IV absorption lines compared to the Mg II absorption lines, than the intervening absorbers. The ionization so measured appears to be related to apparent ejection velocity, being lower as the apparent ejection velocity is more and more positive. There is clear evidence, from the composite spectra, for SMC like dust attenuation in these systems; the 2175AA absorption feature is not present. The extinction is almost twice that observed in the similarly selected sample of intervening systems. We reconfirm that QSOs with non-zero FIRST radio flux are intrinsically redder than the QSOs with no detection in the FIRST survey. The incidence of associated Mg II systems in QSOs with non-zero FIRST radio flux is 1.7 times that in the QSOs with no detection in the FIRST survey. The associated absorbers in radio-detected QSOs which comprise about 12% of our sample, cause 3 times more reddening than the associated absorbers in radio-undetected QSOs. This excess reddening possibly suggests an intrinsic nature for the associated absorbers in radio-detected QSOs.
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We investigate the effect of Fe II equivalent width ($W_{2600}$) and fibre size on the average luminosity of [O II]$lambdalambda$3727,3729 nebular emission associated with Mg II absorbers (at $0.55 le z le 1.3$) in the composite spectra of quasars obtained with 3 and 2 arcsec fibres in the Sloan Digital Sky Survey. We confirm the presence of strong correlations between [O II] luminosity (L$_{[rm O~II]}$) and equivalent width ($W_{2796}$) and redshift of Mg II absorbers. However, we show L$_{[rm O~II]}$ and average luminosity surface density suffers from fibre size effects. More importantly, for a given fibre size the average L$_{[rm O~II]}$ strongly depends on the equivalent width of Fe II absorption lines and found to be higher for Mg II absorbers with $R equiv$ $W_{rm 2600}/W_{rm 2796}$ $ge 0.5$. In fact, we show the observed strong correlations of L$_{[rm O~II]}$ with $W_{2796}$ and $z$ of Mg II absorbers are mainly driven by such systems. Direct [O II] detections also confirm the link between L$_{[rm O~II]}$ and $R$. Therefore, one has to pay attention to the fibre losses and dependence of redshift evolution of Mg II absorbers on $W_{2600}$ before using them as a luminosity unbiased probe of global star formation rate density. We show that the [O II] nebular emission detected in the stacked spectrum is not dominated by few direct detections (i.e., detections $ge 3 sigma$ significant level). On an average the systems with $R$ $ge 0.5$ and $W_{2796}$ $ge 2$ AA are more reddened, showing colour excess E($B-V$) $sim$ 0.02, with respect to the systems with $R$ $< 0.5$ and most likely traces the high H I column density systems.
We have studied a sample of 809 Mg II absorption systems with 1.0 < z_abs < 1.86 in the spectra of SDSS QSOs, with the aim of understanding the nature and abundance of the dust and the chemical abundances in the intervening absorbers. Normalized, composite spectra were derived, for abundance measurements, for the full sample and several sub-samples, chosen on the basis of the line strengths and other absorber and QSO properties. Average extinction curves were obtained for the sub-samples by comparing their geometric mean spectra with those of matching samples of QSOs without absorbers in their spectra. There is clear evidence for the presence of dust in the intervening absorbers. The 2175 A feature is not present in the extinction curves, for any of the sub-samples. The extinction curves are similar to the SMC extinction curve with a rising UV extinction below 2200 A. The absorber rest frame colour excess, E(B-V), derived from the extinction curves, depends on the absorber properties and ranges from < 0.001 to 0.085 for various sub-samples. The column densities of several ions do not show such a correspondingly large variation. The depletion pattern is similar to halo clouds in the Galaxy. Assuming an SMC gas-to-dust ratio we find a trend of increasing abundance with decreasing extinction; systems with N_H I ~ 10^{20} cm^{-2} show solar abundance of Zn. The large velocity spread of strong Mg II systems seems to be mimicked by weak lines of other elements. The ionization of the absorbers, in general appears to be low. QSOs with absorbers are, in general, at least three times as likely to have highly reddened spectra as compared to QSOs without any absorption systems in their spectra.
Quasar outflows carry mass, momentum and energy into the surrounding environment, and have long been considered a potential key factor in regulating the growth of supermassive black holes and the evolution of their host galaxies. A crucial parameter for understanding the origin of these outflows and measuring their influence on their host galaxies is the distance (R) between the outflow gas and the galaxy center. While R has been measured in a number of individual galaxies, its distribution remains unknown. Here we report the distributions of R and the kinetic luminosities of quasars outflows, using the statistical properties of broad absorption line variability in a sample of 915 quasars from the Sloan Digital Sky Surveys. The mean and standard deviation of the distribution of R are 10^{1.4+/-0.5} parsecs. The typical outflow distance in this sample is tens of parsec, which is beyond the theoretically predicted location (0.01 ~ 0.1 parsecs) where the accretion disc line-driven wind is launched, but is smaller than the scales of most outflows that are derived using the excited state absorption lines. The typical value of the mass-flow rate is of tens to a hundred solar masses per year, or several times the accretion rate. The typical kinetic-to-bolometric luminosity ratio is a few per cent, indicating that outflows are energetic enough to influence the evolution of their host galaxies.
We present a generic and fully-automatic method aimed at detecting absorption lines in the spectra of astronomical objects. The algorithm estimates the source continuum flux using a dimensionality reduction technique, nonnegative matrix factorization, and then detects and identifies metal absorption lines. We apply it to a sample of ~100,000 quasar spectra from the Sloan Digital Sky Survey and compile a sample of ~40,000 Mg II & Fe II absorber systems, spanning the redshift range 0.4< z < 2.3. The corresponding catalog is publicly available. We study the statistical properties of these absorber systems and find that the rest equivalent width distribution of strong Mg II absorbers follows an exponential distribution at all redshifts, confirming previous studies. Combining our results with recent near-infrared observations of Mg II absorbers we introduce a new parametrization that fully describes the incidence rate of these systems up to z~5. We find the redshift evolution of strong Mg II absorbers to be remarkably similar to the cosmic star formation history over 0.4<z<5.5 (the entire redshift range covered by observations), suggesting a physical link between these two quantities.
We present the results of a MgII absorption-line survey using QSO spectra from the SDSS EDR. Over 1,300 doublets with rest equivalent widths greater than 0.3AA and redshifts $0.366 le z le 2.269$ were identified and measured. We find that the $lambda2796$ rest equivalent width ($W_0^{lambda2796}$) distribution is described very well by an exponential function $partial N/partial W_0^{lambda2796} = frac{N^*}{W^*} e^{-frac{W_0}{W^*}}$, with $N^*=1.187pm0.052$ and $W^*=0.702pm0.017$AA. Previously reported power law fits drastically over-predict the number of strong lines. Extrapolating our exponential fit under-predicts the number of $W_0 le 0.3$AA systems, indicating a transition in $dN/dW_0$ near $W_0 simeq 0.3$AA. A combination of two exponentials reproduces the observed distribution well, suggesting that MgII absorbers are the superposition of at least two physically distinct populations of absorbing clouds. We also derive a new redshift parameterization for the number density of $W_0^{lambda2796} ge 0.3$AA lines: $N^*=1.001pm0.132(1+z)^{0.226pm0.170}$ and $W^*=0.443pm0.032(1+z)^{0.634pm 0.097}$AA. We find that the distribution steepens with decreasing redshift, with $W^*$ decreasing from $0.80pm0.04$AA at $z=1.6$ to $0.59pm0.02$AA at $z=0.7$. The incidence of moderately strong MgII $lambda2796$ lines does not show evidence for evolution with redshift. However, lines stronger than $approx 2$AA show a decrease relative to the no-evolution prediction with decreasing redshift for $z lesssim 1$. The evolution is stronger for increasingly stronger lines. Since $W_0$ in saturated absorption lines is an indicator of the velocity spread of the absorbing clouds, we interpret this as an evolution in the kinematic properties of galaxies from moderate to low z.
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