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

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1. Introduction

In the past decade BL Lac objects have been actively investigated in direct imaging and spectroscopy using ground based telescopes and HST.The imaging effort has been directed towards detecting the host galaxy, and when possible towards measuring its absolute luminosity and colors and determining its morphological properties. The aim of the spectroscopy has been to measure the redshift of the host or to measure the redshift of companions galaxies in order to assess a possible group or cluster membership.

Apart from studies on individual objects a number of papers have presented optical images for samples or lists of objects.

Twenty three objects have been imaged with the William Herschel Telescope in the R filter and 14 are resolved (Abraham et al. 1991). However due to either unknown redshift or poorly detected nebulosity only for 6 sources absolute quantities are derived. Some cases of disc dominated host galaxies are proposed.

Sixteen objects in the southern sky have been studied by Falomo (1996) using sub-arcsec images obtained at the ESO 3.5m New Technology Telescope (NTT). Eleven sources were resolved and the hosts found to be luminous ellipticals (MR [FORMULA] -23.5). For a number of objects close companion galaxies are detected. Due to their small projected distance it is likely that they are associated with the BL Lac but spectroscopy is needed to assess this point.

A larger sample but with poorer average resolution was investigated using the 3.6m CFHT (Wurtz et al. 1996). Fifty objects have been observed and 36 well resolved. For another ten objects the host galaxy has been only marginally detected. No difference of host properties is found between objects discovered in radio surveys (i.e. 1Jy sample) and those derived from X-ray surveys (i.e. EMSS). With very few exceptions all the BL Lac objects investigated are classified as ellipticals based on the surface brightness profiles.

More recently a study of the host galaxies in a large sample of X-ray selected (high frequency peaked) BL Lacs have been presented by (Falomo & Kotilainen 1999). They used high resolution images in the R filter at the Nordic Optical Telescope (NOT) to image 52 targets from EMSS and Einstein Slew samples. All the 45 objects resolved are well represented by elliptical models. On average the hosts are found 1 magnitude more luminous than M* (M[FORMULA] -22.5; Mobasher et al. 1993; assuming R-K = 2.7).

In addition to ground based studies several 0.1 arcsec resolution short exposure images have been obtained with WFPC2 camera on board of HST during a snapshot survey (Scarpa et al. 2000; Urry et al. 2000). Objects from various samples, and in the redshift interval 0.05 [FORMULA] 1.3, were observed and 69 out of 110 observed are resolved. The highest redshift host galaxy detected is [FORMULA] for 1823+568. For 80% of the resolved host galaxies an elliptical model is clearly preferred over a disc galaxy. The median absolute magnitude of these host galaxies ([FORMULA]) is at least one magnitude brighter than [FORMULA]. The nuclei are always well centered over the body of the galaxy and have luminosity similar to that of its host galaxy. From the point of view of the optical morphology the hosts of BL Lacs appear indistinguishable from "normal" (non active) ellipticals.

The main aim of all these observations outlined above was to detect the host galaxies and to determine their structural and photometric properties. The knowledge of the kind of galaxies that host a BL Lac phenomenon in the nucleus is of importance not only for understanding/studying the nuclear activity vs galaxy connection (see e.g. Lawrence 1999) but also as a probe to test unified models of radio loud AGN. In particular if BL Lacs are FR I radio galaxies whose jet is aligned along the line of sight (e.g. Urry & Padovani 1995; Ulrich 1989) their host galaxies should exhibit exactly the same photometrical and morphological properties as the hosts of FR I. The properties of the BL Lacs hosts can also be compared with those of related beamed objects such as FSRQ and HPQ (see e.g. Kotilainen et al. 1998a).

The aim of this work is to complement the existing data on BL Lac host galaxies and close environment with new imaging and spectroscopy for a dozen of (previously not well studied) objects. A general discussion and comparison of the properties of BL Lacs and radio galaxies will be presented elsewhere.

In this paper we therefore present results from optical images of BL Lac objects collected at the NTT with mostly sub-arcsec resolution. Most of the objects presented here were not previously investigated with adequate capabilities. These observations therefore complement the existing data on BL Lac host galaxies.

We also present spectroscopic observations for some of the objects performed with the aim of deriving the redshift of the host galaxies and of some nearby companion galaxies. When no spectroscopic redshift is available we give an estimate of the photometric redshift derived by assuming that the host has MR = -23.85 and Re = 9 kpc (the typical median values found in previous studies of BL Lacs hosts; e.g. Falomo & Kotilainen 1999).

In Sect. 2 we describe the observations and data analysis. Sect. 3 reports the results obtained for each individual objects. Sect. 4 gives a summary of the results and discussion.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000