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Register a new userOn the Cosmic Evolution of Fe/Mg in QSO Absorption Line Systems
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We investigate the variation of the ratio of the equivalent widths of the FeII$lambda$2600 line to the MgII$lambdalambda$2796,2803 doublet as a function of redshift in a large sample of absorption lines drawn from the JHU-SDSS Absorption Line Catalog. We find that despite large scatter, the observed ratio shows a trend where the equivalent width ratio $mathcal{R}equiv W_{rm FeII}/W_{rm MgII}$ decreases monotonically with increasing redshift $z$ over the range $0.55 le z le 1.90$. Selecting the subset of absorbers where the signal-to-noise ratio of the MgII equivalent width $W_{rm MgII}$ is $ge$3 and modeling the equivalent width ratio distribution as a gaussian, we find that the mean of the gaussian distribution varies as $mathcal{R}propto (-0.045pm0.005)z$. We discuss various possible reasons for the trend. A monotonic trend in the Fe/Mg abundance ratio is predicted by a simple model where the abundances of Mg and Fe in the absorbing clouds are assumed to be the result of supernova ejecta and where the cosmic evolution in the SNIa and core-collapse supernova rates is related to the cosmic star-formation rate. If the trend in $mathcal{R}$ reflects the evolution in the abundances, then it is consistent with the predictions of the simple model.
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It is difficult to describe in a few pages the numerous specific techniques used to study absorption lines seen in QSO spectra and to review even rapidly the field of research based on their observation and analysis. What follows is therefore a pale introduction to the invaluable contribution of these studies to our knowledge of the gaseous component of the Universe and its cosmological evolution. A rich bibliography is given which, although not complete, will be hopefully useful for further investigations. Emphasis will be laid on the impact of this field on the question of the formation and evolution of galaxies.
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.
Molecules dominate the cooling function of neutral metal-poor gas at high density. Observation of molecules at high redshift is thus an important tool toward understanding the physical conditions prevailing in collapsing gas. Up to now, detections are sparse because of small filling factor and/or sensitivity limitations. However, we are at an exciting time where new capabilities offer the propect of a systematic search either in absorption using the UV Lyman-Werner H2 bands or in emission using the CO emission lines redshifted in the sub-millimeter.
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 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.
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