Astron. Astrophys. 342, 395-407 (1999)

3. Continuum definition and emission lines in the UV

First of all the UV data of HS 1103+6416 were corrected for interstellar reddening according to Seaton's law (1979) with corresponding to N(H I )=1.03 10²⁰ cm^-2 (Reimers et al. 1995b). This value is from the Stark HI survey (Stark et al. 1992)

We searched for regions in the data apparently free of absorption lines, where we calculated the mean flux and the error of the mean flux to check for consistency with the noise. The continua were then constructed by fitting cubic splines to the sample of mean flux values.

The data show one strong and one weak Lyman edge at and 1800 Å, respectively (see Fig. 1). Due to blending of the highest Lyman series lines the continuum flux level is poorly defined at these edges and we manually modified the continuum in these regions. By modelling the Lyman edge and H I absorption lines calculating Voigt profiles for the first 39 Lyman series lines we find log N(H I )=17.46 cm^-2 and km ^-1 for the LLS at and log N(H I )=16.6 cm^-2 and km s^-1 for the LLS at , respectively.

Fig. 1. Observed fluxes in 10-15 erg s-1 cm-2 Å-1 of HS1103+6416 obtained with the FOS and GHRS onboard the HST. Data have been rebinned for presentation purposes only.

The decrease in flux at Å and  Å cannot be explained by further Lyman edges since the corresponding Lyman series lines are missing.

Since there is a large time gap between FOS and GHRS observations (8 months) we cannot decide if the observed flux increase at Å is intrinsic to the QSO continuum or due to flux variations of the QSO. In the fitted continuum broad emission at Å is apparent which might be due to Ne VIII 774 and/or N IV 765 and/or O IV 788.

3.1. Spectral energy distribution

Ultraviolet spectra of QSOs are still strongly influenced by absorption of intervening absorbers. In order to find the intrinsic spectral energy distribution of the QSOs corrections have to be applied to the observed data. First, the dereddened spectra were corrected for continuum absorption by neutral hydrogen in the identified LLSs.

Corrected fluxes were transferred to luminosities using

for q (Weedman 1986) and H km s^-1 Mpc^-1. Fig. 2 shows the spectral energy distributions log versus frequency in the rest frame of HS 1103+6416.

Fig. 2. Spectral energy distributions log versus frequency in the rest frame of HS 1103+6416. Optical data were obtained at the Calar Alto 2.2 m telesope. In the UV the continua derived for the dereddened spectra were corrected for the neutral hydrogen continuum absorption of the LLSs and transferred to luminosities adopting q and H km s- 1 Mpc-1. An additional correction for the cumulative hydrogen continuum absorption of the numerous Ly clouds with log N(H I ) cm-2 leads to higher luminosities and to changes in the continuum slopes (dotted lines).

Monte Carlo simulations were performed in order to estimate the depression of the quasar spectrum due to the cumulative hydrogen continuum absorption by the numerous Ly clouds. The incidence of LLSs, i.e. absorber clouds with neutral hydrogen column densities greater than log N(H I ) cm^-2, is easily detected by their Lyman edges. Thus only clouds with log N(H I ) cm^-2 are considered. The distribution in redshift and column density of Ly clouds can be described by

We chose A, and for Ly clouds with H I ) (see e.g. Madau 1995 and references therein). Most absorption from Ly lines occurs at column densities of log N(HI) = 14. Five thousand simulations were performed to calculate the mean transmission exp() at the observed wavelengths with given by

and

for Å. This additional correction leads to higher luminosities and - even more important - to changes in the continuum slope (see dotted lines in Fig. 2).

In contrast to HS 1307+4617 and HS 1700+6416 the continuum shape of HS 1103+6416 is much flatter in the optical, but steeper in the ultraviolet range (see Reimers et al. 1998) and compatible to the common assumption of for Å.

© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999
helpdesk.link@springer.de