Hubble space telescope observations of BL Lacertae environments

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Abstract

We analyze images of BL Lacertae objects obtained with the Hubble Space Telescope WFPC2 and the F814W filter. The nine objects cover a redshift range of 0.19 to 0.997. The relatively deep images are sufficient to detect galaxies at least one magnitude below MI* (−21.4) and in most cases to three magnitudes below M*. Galaxy enhancement over the average background is found around four out of the nine objects. Results for some cases are confirmed by ground-based imaging. In the other cases, the redshifts of the target BL Lac objects may be incorrect or they are truly isolated. These findings reinforce the idea that on average, BL Lac objects are found in regions of above average galaxy density. However, isolated objects apparently can host BL Lac nuclei too, a result that has implications for the processes that trigger/fuel the nuclear activity.

Introduction

BL Lacertae objects (BL Lacs) are an extreme form of active galactic nuclei (AGN) which exhibit rapid flux variability at all frequency, high polarization, and weak or non-existent spectral features. Along with their cousins, the Flat-Spectrum Radio Quasars, they form the blazar class of AGN. The model which best describes blazars is one in which a jet of relativistic material is beamed directly at us (Blandford and Rees, 1978).

Although among the rarest types of AGN, BL Lacs are particularly important precisely because we can look into their jets. In this way, we can see the site of energy production which is presumably close to the central engine, or black hole. Thus, by studying these enigmatic objects we can better understand the central black hole in all other AGN.

One way of studying these objects, which is independent of any assumptions made about the nuclear regions and energy production, is by their environments. There is growing consensus that BL Lacs are found on average in poor clusters of galaxies (Falomo et al., 1993, Falomo et al., 1995, Stickel et al., 1993, Pesce et al., 1994, Pesce et al., 1995, Smith et al., 1995, Wurtz et al., 1997). However, some individual objects (e.g. PKS 0548–322) are found in rich clusters (Falomo et al., 1995).

Work has progressed on the kilo-parsec-scale environments, or host galaxies of BL Lacs as well. BL Lacs are found in giant elliptical galaxies with −21.5≲MV≲−24.5 mag (Abraham et al., 1991, Falomo, 1996, Wurtz et al., 1996, Falomo et al., 1997, Urry et al., 1999). Some individual objects were thought to be hosted by spiral galaxies, but high resolution observations have shown them to be ellipticals (Urry et al., 1999).

High resolution images of the host galaxies of six radio-selected BL Lac objects observed with the Hubble Space Telescope (HST) have been presented in Falomo et al. (1997) and Urry et al. (1999). These are relatively deep WFPC2 images from cycle 5. In addition, we are currently analyzing the host galaxies and extended environments of more than 100 BL Lac objects at redshifts 0.03–1 observed by HST as part of a snapshot project (Pesce et al., 2002; Scarpa et al., 2000, Urry et al., 2000). The combined dataset will allow a thorough investigation of BL Lac environmental properties over a large redshift range.

In this paper we present the analysis of the extended environments of six radio selected objects from our HST cycle 5 GO project (PI URRY). To this sample we have added three X-ray selected objects obtained from the HST archive and observed during cycle 5 (PI JANNUZI). We assume H0=50 km s−1 Mpc1 and q0=0.5.

Section snippets

Data reduction and analysis

The objects were observed with the WFPC2 and the F814W (I-band) filter, the BL Lac was centered in PC camera. Exposure times varied and were approximately 10 min to 1.5 h (see Table 1). More details about the reduction and analysis of these fields can be found in Urry et al. (1999).

We used the FOCAS software, in IRAF, to detect and classify all objects on an image and produce a catalog (Fig. 1). For the central BL Lac, the Point Spread Function (PSF) was created with Tiny Tim (Hasan and

Results

To avoid background contamination, we only count galaxies with mImI*+1 where MI*=−21.4. The PC images subtend 0.3 arcmin2 while the WF chips subtend 1.5 arcmin2 giving a maximum radial extent from our BL Lacs of 0.4 –0.8 Mpc. Most BL Lac clusters are found within 0.5 Mpc of object (Pesce et al., 1994, Pesce et al., 1995). However, because of the WFPC2 geometry, we only observe ∼1/2 of the volume surrounding the BL Lac.

We checked our procedures by analyzing an archival image of the rich cluster

Conclusions

We have analyzed the extended environments of nine BL Lac objects observed with the HST WFPC2 in the F814W filter. The long exposures provide deep (m∼24 mag) images. We find significant enhancement of galaxy density around four of the nine BL Lac objects (1407+599, 1538+149, 1823+568, 2143+070). In some cases (1407+599, 1823+568) the enhancement is extreme, with dozens of galaxies within ∼150 kpc of the BL Lac object.

The remaining five fields show no indication of excess galaxy counts. In fact,

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Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

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