C IV QSO absorption systems and properties of galactic haloes at high redshift.
Abstract
We have obtained high spectral resolution (FWHM = 0.6 or 0.35 A) observations of six high redshift QSOs, z_em_ > 2.2, containing lines from 31 metal-rich absorption line systems amongst which 20 C IV doublets splitted in subcomponents with velocity differences within a system smaller than 400 km s^-1^. Our survey has been combined with published data at similar resolution and the resulting sample comprises 35 systems, 23 of which are splitted in a total of 87 subcomponents. The mean redshift of the sample is <z> = 2.65. The distribution of velocity separations is best fitted by two Gaussian components with dispersions of 109 and 525 km s^-1^ and is very similar to the distribution observed in Mg II systems at redshift z~1. The clustering on the smaller scale could be due to relative motions of clouds within galaxy haloes. The small observed value of the onedimensional velocity dispersion suggests that most of the systems are associated with potential wells characteristic of dwarf galaxies. The larger scale could reflect clustering of haloes in small groups. There are groups of redshifts spanning up to 1200 km s^-1^; our sample however is not large enough to investigate the clustering up to such velocity differences. As observed for Mg II systems there is a strong correlation between the total C IV equivalent width of the systems and the number of components. The correlation between the total equivalent width and the total velocity spread, {DELTA}V_max_, is strong for {DELTA}Vmax < 400 km s^-1^. For larger velocity spread the equivalent width tends to be smaller than what would be expected from extrapolation of the correlation at smaller velocity differences. Several examples of such highly multiple systems with comparatively small equivalent width have been observed at z ~ 1.5. Although they have not been searched for systematically they may be related to a different class of C IV systems and reveal a particular epoch in galaxy formation. The distribution of C IV column densities for the observed subcomponents is fitted by a power law dn/dN (CIV) is proportional to N^- β^ with β = 1.64 +/-0.10 in the range 13.4 < log N(C IV) < 14.8. We have investigated the possible relation between metal line systems and dark matter haloes in the framework of CDM models. The number of C IV systems per unit redshift is well reproduced for 0.3 < z < 1.5 if the properties of the associated galaxy gaseous haloes do not change much over this redshift range. At higher redshift C IV systems could be associated with the virialized part of haloes with circular velocity V_c_ > 70 km s^-1^. The large number of Lyman limit systems (LLS) observed at redshift larger than 3 suggests that part of the Lyα forest becomes optically thick at large redshift.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- March 1994
- Bibcode:
- 1994A&A...283..759P