Astronomy and Astrophysics – Astrophysics
Scientific paper
1994-02-24
Astrophys.J.Suppl.94:17-46,1994
Astronomy and Astrophysics
Astrophysics
(Plain Tex, 46 pages and 21 figures. To get 21 postscript files for figures ftp to 132.239.146.152 and login as ftp in directo
Scientific paper
10.1086/192071
We present optical spectra of QSO HS~1946+7658 with either high resolution (FWHM=10 km/s) or high signal to noise ratio (SNR=40-100). We find 113 Lyman alpha and six metal line systems. The metal systems at Zabs=2.844 and 3.050 have complex velocity structures. We find that the system at 2.844 is a damped Ly-a absorption system, with neutral hydrogen column density of logN(HI)=20.2+/-0.4, and it is the cause of the Lyman limit break at 3520\AA. The metal abundance in the gas phase of the system is [M/H]=-2.6+/-0.3, with a best estimate of [M/H]= -2.8, with ionization parameter Gamma=-2.75, from a photoionization model. We show that the abundance ratios rule out appreciable dust in this DLA system. We do not see the enhancement of O over C reported in Nature by Reimers last years. We see CII*(1335) offset by 15 km/s with respect to CII(1334), presumably because the gas density varies from 2 - 8 cm(-3) with changing velocity in the DLA system. These densities imply that the damped component is 6 - 25 pc thick, which is reasonable for a single cloud in a cold spiral disk. We see a 2.6sigma lack of Ly-a forest lines well away from the QSO redshift, which may be a chance fluctuation. We also see a correlation between column density N(HI) and Doppler parameter b for 96 unsaturated Ly-a forest absorption lines, and although this correlation persists in the 36 Ly-a lines which lie in regions where SNR=8-16, we agree with Rauch etal (1993) that it is probably a bogus effect of low SNR. We show that there are few low-b Ly-a forest lines in moderate SNR data.
Fan X.-M.
Tytler David
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