Astron. Astrophys. 357, 91-100 (2000)

2. Observations and data analysis

Optical observations were obtained using the 3.5m New Technology Telescope (NTT) at the European Southern Observatory (ESO), operated via remote control from the ESO headquarters in Garching (Germany). We acquired images using the Superb Seeing Imager (SUSI; Melnick et al. 1992) which is installed at one of the Nasmyth foci of the NTT. Configuration used was R-band filter and a CCD (TK 1024) with 24µm pixel size corresponding to 0.13" on the sky. Conditions were photometric and seeing was ranging from 0.55 to 1.2 arcsec (FWHM), and in most cases 1". Observations of standard stars (Landolt 1992) were used to set the photometric zero point.

We obtained images centered on the BL Lac object with exposure times ranging from 10 to 30 min (see Table 1). For many objects we also secured one short (2 minutes) exposure in order to be sure to get unsaturated images of the nucleus of the targets and to enable us to use bright stars in the field to study the PSF.

Table 1. Journal of the observations.

The images were processed in the standard way (bias subtracted, trimmed, flat fielded, and cleaned of cosmic rays) using the Image Reduction and Analysis Facility (IRAF) procedures. A journal of the observations is given in Table 1.

Spectroscopy of the the objects and/or of galaxies in the field were obtained for some targets in order to determine the redshift of BL Lacs and/or nearby companion galaxies. For this purpose the ESO multi mode instrument EMMI (Melnick et al. 1992) was used with red arm and grism elements. In general the slit has been oriented in order to obtain in a single observation both the BL Lac object and one or more galaxies around the source.

All the images have been analyzed following the methods and procedure described in Falomo (1996). In particular surface photometry analysis was performed down to the surface brightness magnitude 26 mag./arcsec² in order to derive the properties of the host galaxies. A fit of the radial brightness profile was performed assuming a simple two model components: a point source plus a elliptical galaxy described by a de Vaucouleurs law

where is the surface brightness and the effective radius.
Also disc galaxies models were attempted but in no cases they gave a better fit than the elliptical model. This is consistent with what was found in previous studies on a larger number of sources (Falomo & Kotilainen 1999; Urry et al. 1999; Scarpa et al. 2000).

To obtain absolute quantities we applied correction for Galactic extinction and redshift (K-correction). The former was determined using the Bell Lab Survey of neutral hydrogen N_H converted to (Stark et al. 1992; Shull & Van Steenberg 1985), while the latter was computed from the model of Coleman et al. (1980) for elliptical galaxies. Throughout this paper, H₀ = 50 km s^-1 Mpc^-1 and q₀ = 0 are adopted.

© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000
helpdesk.link@springer.de