Optical Ground--based Images of BL Lac Objects Scheduled for HST Observations

R. Falomo, R. Scarpa
Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, Padova, Italy

J. Pesce, C.M. Urry
Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218 USA

A. Treves
SISSA/ISAS, Via Beirut 2--4, 34014 Trieste, Italy

Abstract:

We present sub-arcsec optical ground-based images of three BL Lac objects (PKS 2254+07, S4 1823+56 and B2 1308+32 ) that are scheduled for WFPC2 observations with Hubble Space Telescope during cycle 5. The observations are aimed at studying the properties of the host galaxies and close environment for sources with different characteristics.

Keywords: BL Lac objects

Introduction

BL Lac objects are AGNs characterized by high variability, polarization, and modest emission lines. Recently it was shown that they may be, at least sporadically, strong gamma ray sources. It is generally accepted that BL Lacs should be interpreted within a picture where they are associated with relativistic jets pointing in the direction of the observer. The population hosting BL Lacs should, therefore, be much more abundant than the population of BL Lacs, and it was suggested that it consists of FR--I radio galaxies (see e.g., Urry & Padovani 1995 and references therein).

The nature of the parent population can be explored focusing on properties which are essentially independent of the beaming. In particular, the morphological study of the hosts, their clustering properties, and the possible presence of close companions, perhaps related to the triggering of the BL Lac activity, seem to be of key importance for interpreting the BL Lac phenomenology. All these arguments go into the study of BL Lac environments.

Several groups have presented results based on ground-based observations (Abraham et al. 1991, Stickel et al. 1993, Pesce et al. 1995, Wurtz et al. 1995, Falomo 1996 and references therein). In most cases, host galaxies are giant ellipticals, but in a few cases spirals are claimed. BL Lacs are members of poor clusters (Abell class 0), which may indicate that they are associated with only a subgroup of FR--I radio sources. Moreover, some evidence is accumulating that BL Lacs are frequently associated with close companions, a fact which has been recently discussed extensively for close-by quasars.

It is obvious that the exploration of the environment of BL Lacs can greatly profit from the use of HST since, for instance, the angular resolution can allow the extension of the studies to larger z. We are involved with two cycle 5 HST proposals (for low z and high z BL Lacs), and one cycle 6 proposal (snapshots of BL Lacs).

As shown by the studies of the environment of QSOs, it is of particular importance to understand the merits of HST vs ground-based observations, and to complement the use of the two sets of data (see e.g., McLeod & Rieke 1995). In this spirit we are presenting here images of three BL Lacs: PKS 2254+07, S4 1823+56 and B2 1308+32 -- obtained with high quality ground-based telescopes during sub-arcsec seeing conditions. The first two sources have been observed with HST in October 1995, but the data analysis is only at a preliminary stage. The third source is scheduled for early 1996.

Observations and Data Analysis

Optical images were obtained using both the 3.5m New Technology Telescope (NTT), at the European Southern Observatory, and the 2.5m Nordic Optical Telescope (NOT) at La Palma. The data were acquired using the Superb Seeing Imager (SUSI; Melnick et al. . 1992) installed at one of the Nasmyth foci of the NTT and the BroCam direct imaging camera mounted at the NOT. In both cases, an R-band filter (Cousins system) and a CCD (TK 1024) was used. The scale is of 0.13 and 0.176 arcsec/pixel for NTT and NOT, respectively. Conditions were photometric and seeing was always sub-arcsec.

Surface photometry analysis was performed using an interactive package (AIAP, Fasano 1990). After proper masking of isophotes, ellipses were fitted with free parameters. These yield the photometrical and structural parameters of the objects (surface brightness, ellipticity, position angle, center of ellipses and Fourier coefficients describing deviations of isophotes from pure ellipse).

The Point Spread Function (PSF) was derived independently for each observed frame averaging many stars detected in the frame. When bright (saturated) and faint (unsaturated) stars are available we combined them in order to properly determine the core and the wings of the PSF. This is particularly important for the marginally resolved objects because the reliability of the derived properties significantly depends on assumptions of the PSF shape. The combination of stars of different magnitude permits, in fact, an increase of the dynamical range of the PSF.

Results and Discussion

2254+07

This is a relatively low redshift (z = 0.19) BL Lac object. We obtained three NTT subsequent images with seeing of 0.95 and then stacked them into an average frame for a total integration time of 420 sec. The nebulosity is well visible (see Fig. 1a) and looks rather round ( 0.1). The galaxy extends up to 10 at surface brightness 26 mag/arcsec. The radial profile (Fig. 1b) is well fitted by an elliptical model plus point source yielding a galaxy of absolute magnitude M = -23.9 (H = 50 kms kpc and q = 0) which is typical for galaxies hosting BL Lacs. However, even a disc model plus point source gives a satisfactory fit to the radial profile. In this case the host galaxy's total magnitude would be 0.3 mag fainter. Our analysis of the Fourier coefficients does not, however, show any significant excess due to the disky isophotes, favouring the elliptical model.

The host galaxy of this source has also been also studied by Stickel, Fried & Kühr (1988) who report a very similar absolute magnitude for the host galaxy although no satisfactory fit with either elliptical or disc models was found. These authors also took spectra of two galaxies close to the BL Lac object finding one at the same z as 2254+07. Higher resolution images obtained with the WFPC2 camera would be able to set stronger constraints to the galaxy morphology since the region close to the nucleus, where the and exponential disc models greatly differ, becomes accessible.

Figure: PKS 2254+07 (z=0.19). XXFigure 2. S4 1823+56 (z=0.664).
Figure 3. B2 1308+32 (z=0.997). a) Contour plot of the central portion of the R-band image; spacing between isophotes is 0.5 mag/arcsec . b) Surface brightness profile ( filled squares). The solid line is the sum of a PSF ( dotted line) plus an elliptical r law ( short-dashed line).

1823+56

This is a moderately high redshift (z=0.664) source whose spectrum exhibits, however, only one confirmed emission line identified as [OII] 3727Å (Stickel et al. . 1993). We secured subsequent images at NOT with seeing of 0.7 arcsec for a total exposure of 1800 sec and combined them into a single frame (see Fig 2a). Despite the high redshift, the object is well resolved (Fig 2b). If the excess of light over the PSF is interpreted as emission of the associated host galaxy (modeled by an elliptical galaxy), this would correspond to an extremely luminous galaxy with M truept.truept < -25.3. This questions the reliability of the redshift.

There are many faint galaxies in the neighborhood of 1823+56 that could be associated with the BL Lac object. However the only redshift known for a galaxy in this group (z = 0.394) is quite different from that proposed for the BL Lac. The closest galaxy (G1) is at 5 (projected distance 46 Kpc at z = 0.664). This object appears highly distorted with an asymmetric faint emission elongated towards South. Observations with WFPC2 would permit both to further constraint the properties of the nebulosity, and to detect possible signs of interaction with the close companion.

1308+32

This is a high redshift (z = 0.997) BL Lac object that exhibits strong optical variability. Due to its high z we do not expect to resolve the nebulosity even at sub-arcsec resolution. However, Stickel (1991) reported the detection of a faint nebulosity at brightness levels below 26 mag/arcsec using images of 1.3 seeing. This was interpreted as evidence for a foreground galaxy. We took two 20 minutes NOT exposure with seeing of 0.8 and derived isophotal analysis down to = 26 mag/arcsec (see Fig 3a,b). The radial profile is perfectly matched by a scaled PSF down to = 25 mag/arcsec and show a small excess below it. We tried to derive the properties of the galaxy that could produce such an excess but found rather unrealistic values. Moreover, we note that this excess is mainly due to some faint emission on the East side of the source and could be produced by a diffuse companion object a few arcsec East from the BL Lac nucleus. This feature could be resolved by WFPC2 observations and be related to the z = 0.879 absorption system seen in the spectrum of 1308+32. The companion object (G1) at 5.6 SW is resolved and its projected distance is 60 Kpc. If associated with 1308+32, it would have M. Several other faint galaxies are detected around the BL Lac and we are testing if some evidence of clustering is present.