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Astron. Astrophys. 318, 347-360 (1997)


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The shape of the ionizing UV background at z   ~  3.7 from the metal absorption
systems of Q 0000-2619 * **

Sandra Savaglio 1, 2, Stefano Cristiani 3, Sandro D'Odorico 1, Adriano Fontana 4, Emanuele Giallongo 4 and Paolo Molaro 5

1 European Southern Observatory, Karl-Schwarzschildstr. 2, D-85748 Garching bei München, Germany
2 Istituto di Astrofisica Spaziale del CNR, I-00044 Frascati, Italy
3 Dipartimento di Astronomia dell' Università, I-35122 Padova, Italy
4 Osservatorio Astronomico di Roma, Via dell' Osservatorio 2, I-00040 Monteporzio, Italy
5 Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34131 Trieste, Italy

Received 1 March 1996 / Accepted 3 June 1996

Abstract

Spectra of the [FORMULA]   [FORMULA] quasar Q 0000-2619 have been obtained in the range [FORMULA] Å with a resolution of 13 km s-1  and signal-to-noise ratio of [FORMULA] per resolution element. The list of the identified absorption lines is given together with their fitted column densities and Doppler widths. The mode of the distribution of the Doppler parameters for the Ly [FORMULA]  lines is [FORMULA] km s-1. The fraction of lines with [FORMULA] km s-1  is 17%. The Doppler values derived from uncontaminated Ly [FORMULA]  lines are smaller than those obtained from the corresponding Ly [FORMULA] lines, indicating the contribution of non saturated, non resolved components in the Ly [FORMULA]  profiles.

The integrated UV background estimated from the proximity effect is found to be [FORMULA] erg s-1 cm-2  Hz-1 sr-1. This value is consistent with previous estimates obtained at a lower z, implying no appreciable redshift evolution of the UVB up to [FORMULA].

13 metal systems are identified, five of which previously unknown. The analysis of the associated metal systems suggests abundances generally below the solar value with an average [C/H] [FORMULA]. This value is about one order of magnitude higher than that found in intervening systems at about the same redshift.

The analysis of the intervening metal line systems has revealed in particular the presence of three optically thin systems with [FORMULA] showing associated CIV and SiIV absorptions. In order to make the observed column densities consistent with [Si/C] ratios lower than 10 times the solar value, it is necessary to assume a large jump in the spectrum of the ionizing UV background beyond the HeII edge ([FORMULA]). This result, if confirmed in other spectra at the same redshift is suggestive of a possible dominance of a stellar ionizing emissivity over the declining quasar one at [FORMULA].

Key words: galaxies: formation of – intergalactic medium – quasars: Q 0000 - 2619: absorption lines

* Based on observations collected at the European Southern Observatory, La Silla, Chile (ESO No. 2-013-49K).
** Tables 2 and 3 only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html

Send offprint requests to: Sandro D'Odorico

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998

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