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Astron. Astrophys. 339, 678-686 (1998)

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The size and geometry of the Ly[FORMULA] clouds *

V. D'Odorico 1, S. Cristiani 2, S. D'Odorico 3, A. Fontana 4, E. Giallongo 4 and P. Shaver 3

1 International School for Advanced Studies, SISSA, via Beirut 2-4, I-34014 Trieste, Italy
2 Dipartimento di Astronomia dell'Università di Padova, Vicolo dell'Osservatorio 5, I-35122 Padova, Italy
3 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
4 Osservatorio Astronomico di Roma, via dell'Osservatorio, I-00040 Monteporzio, Italy

Received 14 April 1998 / Accepted 29 July 1998


Spectra of the QSO pair Q0307-195A,B have been obtained in the Ly[FORMULA] forest (3660-3930 Å) and C iv (4720-4850 Å) regions with a FWHM resolution between 0.7 and 0.5 Å. 46 lines have been detected in the spectrum of object A while 36 in the spectrum of object B, of them 29 and 20 were identified as Ly[FORMULA] absorptions respectively. The present observations have been supplemented with data of comparable quality on other 7 QSO pairs available in the literature to give an enlarged sample of 217 Ly[FORMULA] lines with rest equivalent width [FORMULA] Å. The analysis of the hits (i.e. when an absorption line appears in both QSO spectra) and misses (i.e. when a line is seen in any of the QSO spectra, but no line is seen in the other), carried out with an improved statistical approach, indicates that the absorbers have typically a large size 1: [FORMULA] kpc, with 95% confidence limits [FORMULA] kpc and [FORMULA] kpc, with 95% confidence limits [FORMULA] kpc for the radius of idealized spherical and disc geometries, respectively. The present data do not allow to establish any correlation of the typical inferred size with the proper separation or with the redshift of the pairs.

The correlation between the observed equivalent widths of the absorbers in the adjacent lines of sight becomes poorer and poorer with increasing proper separation. A disc geometry with a column density profile [FORMULA], [FORMULA], is found to reasonably reproduce the data with [FORMULA] kpc, but also spherical clouds with the same column density profile and a power-law distribution of radii may give a satisfactory representation of the observations.

Key words: intergalactic medium – quasars: absorption lines – quasars: individual: Q 0307-195A,B)

* Based on observations collected at the European Southern Observatory, La Silla, Chile (ESO No. 149.B-0013).

Send offprint requests to: Stefano Cristiani

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998