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Register a new userAPM z>4 QSO Survey: Spectra and Intervening Absorption Systems
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Lisa Storrie-Lombardi
Publication date
1996
fields
Physics
and research's language is
English
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The APM multicolor survey for bright z > 4 objects, covering 2500 deg^2 of sky to m(R)~19, resulted in the discovery of thirty-one quasars with z > 4. High signal-to-noise optical spectrophotometry at 5A resolution has been obtained for the twenty-eight quasars easily accessible from the northern hemisphere. These spectra have been surveyed to create new samples of high redshift Lyman-limit systems, damped Lyman-alpha absorbers, and metal absorption systems (e.g. CIV and MgII). In this paper we present the spectra, together with line lists of the detected absorption systems. The QSOs display a wide variety of emission and absorption line characteristics, with 5 exhibiting broad absorption lines and one with extremely strong emission lines (BR2248-1242). Eleven candidate damped Ly-alpha absorption systems have been identified covering the redshift range 2.8<z<4.4 (8 with z>3.5). An analysis of the measured redshifts of the high ionization emission lines with the low ionization lines shows them to be blueshifted by 430+/-60 km/s. In a previous paper (Storrie-Lombardi et. al. 1994) we discussed the redshift evolution of the Lyman limit systems catalogued here. In subsequent papers we will discuss the properties of the Ly-alpha forest absorbers and the redshift and column density evolution of the damped Ly-alpha absorbers.
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Eleven candidate damped Lya absorption systems were identified in 27 spectra of the quasars from the APM z>4 survey covering the redshift range 2.8<z(abs)<4.4 (8 with z(abs)>3.5). High resolution echelle spectra (0.8A FWHM) have been obtained for three quasars, including 2 of the highest redshift objects in the survey. Two damped systems have confirmed HI column densities of N(HI) >= 10^20.3 atoms cm^-2, with a third falling just below this threshold. We have discovered the highest redshift damped Lya absorber known at z=4.383 in QSO BR1202-0725. The APM QSOs provide a substantial increase in the redshift path available for damped surveys for z>3. We combine this high redshift sample with other quasar samples covering the redshift range 0.008 < z < 4.7 to study the redshift evolution and the column density distribution function for absorbers with log N(HI)>=17.2. In the HI column density distribution f(N)=kN^-beta we find evidence for breaks in the power law, flattening for 17.2< log N(HI)<21 and steepening for log N(HI)>21.2. The column density distribution function for the data with log N(HI)>=20.3 is better fit with the form f(N)=(f*/N*)(N/N*)^-beta exp(-N/N*). Significant redshift evolution in the number density per unit redshift is evident in the higher column density systems with an apparent decline in N(z) for z>3.5.
We present high signal-to-noise, 5 A resolution (FWHM) spectra of 66 z ga 4 bright quasars obtained with the 4 m Cerro Tololo Inter-American Observatory and 4.2 m William Hershel telescopes. The primary goal of these observations was to undertake a new survey for intervening absorption systems detected in the spectra of background quasars. We look for both Lyman-limit systems (column densities N(HI) > 1.6 * 10^{17} atoms cm-2) and damped Ly-alpha systems (column densities N(HI) > 2 * 10^{20} atoms cm-2). This work resulted in the discovery of 49 Lyman-limit systems, 15 of which are within 3000 km s-1 of the quasar emission and thus might be associated with the quasar itself, 26 new damped Ly-alpha absorption candidates, 15 of which have z>3.5 and numerous metal absorption systems. In addition ten of the quasars presented here exhibit intrinsic broad absorption lines.
We present the spectra, positions, and finding charts for 31 bright (R<19.3) colour-selected quasars covering the redshift range z=3.85-4.78, with 4 having redshifts z>4.5. The majority are in the southern sky (dec<-25 deg). The quasar candidates were selected for their red (B-R > 2.5) colours from UK or POSSII Schmidt Plates scanned at the Automated Plate Measuring facility in Cambridge. Low resolution (~> 10A) spectra were obtained to identify the quasars, primarily at the Las Campanas Observatory. The highest redshift quasar in our survey is at z ~ 4.8 (R = 18.7) and its spectrum shows a damped Lyman-alpha absorption system at z = 4.46. This is currently the highest redshift damped Lyman-alpha absorber detected. Five of these quasars exhibit intrinsic broad absorption line features. Combined with the previously published results from the first part of the APM UKST survey we have now surveyed a total of ~8000 squ deg of sky i.e. 40% of the high galactic latitude (|b|>30 deg) sky, resulting in 59 optically selected quasars in the redshift range 3.85 to 4.78; 49 of which have z>=4.00.
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.
We present the results from a spectroscopic survey for intervening MgII absorption in the spectra of 381 background QSOs conducted at the Multiple Mirror Telescope. This survey complements our earlier SDSS EDR MgII survey, extending our results to lower redshift ($z simeq 0.15$) and weaker MgII $lambda2796$ rest equivalent width ($W_0^{lambda2796} simeq 0.1$AA). We confirm two major results from that survey: the transition in the $W_0^{lambda2796}$ distribution at $W_0^{lambda2796} approx 0.3$AA, and the $W_0^{lambda2796}$-dependent evolution of the incidence of systems. The nature of $partial^2N/partial z partial W_0^{lambda2796}$ is consistent with the idea that multiple physically-distinct components/processes contribute to the incidence of MgII absorption systems in a $W_0$-dependent manner and evolve at different rates. A significant decrease in the total proper absorption cross section is detected in our MMT data for systems as weak as 1.0 AA $le W_0^{lambda2796} < 1.5$AA at $zlesssim 0.4$. We discuss this $W_0$-dependent evolution in the context of the evolution of galaxy structures, processes including superwinds and interactions, and damped-Ly$alpha$ absorbers. We also consider the possibility that the observed redshift and $W_0^{lambda2796}$ dependence of the incidence of absorption in spectroscopic surveys for low-ion/neutral gas results from the effects of dust-induced extinction.
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