Astron. Astrophys. 336, 479-489 (1998)

1. Introduction

BL Lac objects are active galactic nuclei (AGN) characterized by strong and rapidly variable continuum emission and polarization across the electromagnetic spectrum, strong compact flat spectrum radio emission and superluminal motion (see e.g. Kollgaard et al. 1992 for general references). They share many properties with flat spectrum radio quasars (FSRQ) and are often grouped together as blazars. The clearest difference between them is that the latter have strong broad emission lines, while these are very weak or absent in BL Lacs.

Blazar properties are usually explained by the beaming model (Blandford & Rees 1978), where the observed emission is dominated by a synchrotron emitting relativistically boosted jet oriented close to our line-of-sight. This model is supported by the fact that almost all blazars are strong and rapidly variable -ray sources (e.g. von Montigny et al. 1995). The beaming model implies the existence of a more numerous parent population of intrinsically identical objects but with their jet oriented at larger angles to our line-of-sight. In the current unified models for radio-loud AGN (e.g. Urry & Padovani 1995), BL Lac objects are unified with low luminosity core-dominated (F-R I) radio galaxies (RG) seen nearly along the jet axis, while the high luminosity lobe-dominated (F-R II) RGs represent the parents of FSRQs (Padovani & Urry 1990; Urry, Padovani & Stickel 1991). However, for potential problems in this simple unification, see Urry & Padovani (1995). For a direct test of the unification model, we need to compare orientation-independent properties of BL Lac objects with those of the parents, e.g. extended radio emission, host galaxies and environments.

Considerable amount of optical imaging exists for relatively nearby (z0.5) BL Lac hosts (e.g. Abraham, McHardy & Crawford 1991; Stickel, Fried & Kühr 1993; Falomo 1996; Wurtz et al. 1996; Falomo et al. 1997b; Jannuzi, Yanny & Impey 1997). The host galaxies of nearby BL Lacs have turned out to be predominantly giant ellipticals with similar magnitude to F-R I RGs, although there appear to be some cases of disk dominated host galaxies (e.g. McHardy et al. 1991; Abraham et al. 1991; Stocke, Wurtz & Perlman 1995, but see contradicting views by e.g. Romanishin 1992; Stickel et al. 1993; Falomo et al. 1997a). The extended radio power and optical environments of BL Lacs are also consistent with those of F-R I RGs, but suggest a contribution to the parent population from F-R II RGs (Kollgaard et al. 1992; Pesce, Falomo & Treves 1995; Wurtz et al. 1997).

Very little near-infrared (NIR) imaging exists on BL Lac objects. However, NIR wavelengths may offer some advantages. Optical emission from BL Lacs is often dominated by the nuclear source, while the luminosity of the massive old stellar population peaks in the NIR. This leads to a better contrast of the nebulosity with respect to the nuclear source at these wavelengths. One also needs to apply much lower K-correction in the NIR than in the optical. In this paper we present NIR H-band (1.65 µm) images of 11 BL Lac objects and compare the NIR host properties with those of RGs and FSRQs. The BL Lacs were observed during our project to study the host galaxies of a complete sample of FSRQs (Kotilainen, Falomo & Scarpa 1998; hereafter KFS98) and thus they do not satisfy any criteria of completeness. However, all the low redshift BL Lacs in this sample have previously been imaged in the optical by us. The same procedure of analysis was performed on the NIR and optical datasets, thus allowing us to investigate the R-H colour of the host galaxies in a homogeneous manner. Properties of the observed objects are given in Table 1. In Sect. 2, we briefly describe the observations, data reduction and the method of the analysis and refer the reader to a more thorough discussion given in KFS98. Our results are presented in Sect. 3 and conclusions in Sect. 4. Throughout this paper, H₀ = and q₀ = 0 are used.

Table 1. Journal of observations.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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