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

3. Results

In Fig. 1 we report the observed radial brightness profile of the objects together with the best fit with the two components (point source plus elliptical galaxy) for the objects resolved. Parameters of the fit and absolute quantities for host galaxies and the nuclei are given in Table 2. In this table Columns 4-8 we give the results from this paper. The redshift in Column 2 is drawn from literature except that for 0301-24 and two cases where a photometric redshift (given in parenthesis) is derived from the observed host properties. In the following discussion absolute quantities are given including corrections for galactic extinction and redshift (K-correction). Optical spectra of the BL Lacs or companion galaxies are reported in Fig. 2 together with the main identifications of observed spectral features.

Fig. 1. The observed radial luminosity profiles of each BL Lac object (filled squares), superimposed to the fitted model (solid line) consisting of the PSF (short-dashed line), de Vaucouleurs bulge (medium-dashed line). In the cases of unresolved sources only the scaled PSF profile is shown.

Fig. 1. (continued)

Fig. 2. Optical spectra of BL Lac objects and companion galaxies. Fluxes are in relative units.

Table 2. Properties of host galaxies and nuclei.

3.1. Comments for individual objects

PKS 0138-097. 

This object was observed under 1.2" seeing and it looks unresolved. Heidt et al. 1996 have presented deep sub-arcsec images of this source that indicate the presence of close companion objects. These could be responsible for the intervening absorption system at z = 0.501 (Stickel et al. 1993) seen in the spectrum of the BL Lac object. Our image was taken under relatively poor seeing but nevertheless some evidence of the southern feature at 1.5" from the center of the source is present in our image. This object has also been imaged by HST and found to be unresolved (Scarpa et al. 2000) but the presence of a companion galaxy at 1.5" South from the nucleus is clearly apparent.

Recent spectroscopy (Stocke & Rector 1997) detects for the first time the emission-line redshift of z=0.733 based upon weak Mg II and [O II] emission features. At this relatively high redshift our image result is consistent with this object being in a luminous (not detected) host galaxy at z = 0.733.

0301-243. 

We took a 20 minute image under good seeing (0.8") of this BL Lac object that clearly shows an extended nebulosity (ellipticity = 0.3; = 1- ) with a complex close environment (see Fig. 3).

Fig. 3. Contour plot of the central part of 0301-24 before (left ) and after (right ) subtraction of a 2D model of the galaxy and the nucleus. After subtraction of the model a faint galaxy 3.5" South is visible together with some residual light from the bright nucleus. The spacing between isophotes is 0.5 magnitudes while faintest shown level is = 25.5 mag arcsec-2. Galaxies G1 and G2 are at redshift z = 0.26.

The immediate region around the object is rich with faint galaxies and there is a marked enhancement of the galaxy density within 60" from the BL Lac object.

The spectra of two galaxies (G1 and G2; see Fig. 3) at 6" and 20" from 0301-243 indicate that they are at suggesting a cluster of galaxies of Abell richness class 0 might be associated with the BL Lac source at this redshift (Pesce et al. 1995). The radial profile is adequately well represented by a point source plus the elliptical model while the fit with an exponential disk is not acceptable. Fig. 3 (right panel) shows the field after subtraction of the BL Lac model (nucleus plus host galaxy) revealing the faint galaxy 3.5" South of the nucleus. After masking out the companion from the image we find that the surrounding nebulosity is very well centered on the nucleus within an accuracy of 0.2".

We took three optical spectra of the nebulosity with the slit off the nucleus by 2". They are still dominated by the signal from the non-thermal source but all three show one weak emission line at = 6303 Å (see Fig. 2). The most plausible identification for this emission is [O III] 5007 Å that yields a redshift of 0.26. Fainter emissions like [O III] 4959 Å or could be present at this z but not detectable in our spectrum because the features are lost in the noise. Other possible identifications like MgII 2800 (at z = 2.25) are not acceptable because the host galaxy would be too luminous (M -30). The redshift of 0.26 is very similar to the redshift of the companion galaxies G1 and G2 (respectively of M_R = -20.7 and M_R = -22.3) and supports the idea that the host of the BL Lac is the dominant member of a cluster of galaxies. We note that few other examples have been reported in the literature of BL Lacs in clusters whose membership has been proved spectroscopically. H0414+00 (Falomo et al. 1993a) is in a cluster of Abell class 0; PKS 0548-32 (Falomo et al. 1995) is in a cluster of Abell class 1-2.

At this redshift (z = 0.26) the absolute magnitude of the host galaxy of 0301-24 is M_R = -24.1

0338-21. 

Our image obtained with 0.6" seeing shows the object to be resolved. This is the first detection of the nebulosity for the source. However its magnitude is not consistent with the redshift of the object published twenty years ago (z = 0.048; Wright et al. 1977). The strongest absorption line identified in Wright et al. is indeed a telluric band at 6870 Å. Subsequent spectroscopy of the source has failed to confirm this redshift and a pure featureless optical spectrum has been observed (Falomo et al. 1994). In fact at this redshift of z = 0.048 the nebulosity would correspond to an unreasonably faint and small host galaxy (M_R -18.5). Assuming a typical host galaxy (see Sect. 1) we can well fit the radial brightness profile with a nucleus plus host galaxy obtaining a photometric redshift z 0.45.

REX 0353-36. 

The source was identified as a BL Lac object in the REX survey of AGN (Wolter et al. 1997). Its optical spectrum is featureless (Wolter et al. 1998). We obtained an image under very good seeing (0.6 arcsec) and are able to detect the surrounding nebulosity and measure its luminosity and R_e. This is the first detection of its host. There is no spectroscopic redshift but we can estimate a photometric redshift from the image decomposition assuming the host galaxy has average properties for BL Lacs hosts. The value of the photometric redshift so obtained is z 0.4.

PKS 0548-322. 

This is a very well know BL Lac object at z = 0.068 (Fosbury & Disney, 1976) with a very large host galaxy in a rich environment (Falomo et al. 1995). We took one relatively short exposure but good signal-to-noise spectrum centered in the nucleus to search for possible emission lines as have been reported in a number of nearby BL Lacs (e.g. BL Lac itself, Vermeulen et al. 1995).The spectrum, shown in Fig. 2, exhibits a substantial contribution from the stellar population of the host galaxy. The MgI 5175 Å and Na blend 5892 Å are well detected with equivalent widths of 12 Å and 6.5 Å, respectively. We could not find any emission down to a limit of equivalent with of 2 Å. This limit corresponds to line luminosity L() 5 10⁴⁰ erg s^-1 which is about a factor 10 lower than the line detected in BL Lac (Vermeulen et al. 1995).

PKS 0735+178. 

This BL Lac object is bright and strongly variable. It has been extensively studied in the radio range and several moving components have been detected in VLBI. The optical spectrum shows the absorption line due to an intervening system at 3980 Å, which if identified with Mg II gives z 0.424 (Carswell et al. 1974) Our images were obtained under seeing of 0.8" but the source remains unresolved. Previous images were presented by Hutchings et al. (1988) who also found this source unresolved. There is no sign in our image (see Fig. 4) that the galaxy 7" NW is distorted by interaction with 0735+178 as suggested by previous lower resolution images (Hutchings et al. 1988). The object was also imaged by Stickel et al. (1993) who are not able to detect the surrounding nebulosity. They obtained a spectrum of the galaxy 7" NW and found z = 0.645. This BL Lac object is unresolved also in a short exposure image obtained with HST (Scarpa et al. 2000). In addition to the two well resolved companion galaxies we detect a faint emission at 3.5" East from the BL Lac (see Fig. 4). Given its projected distance from the BL Lac (25 kpc at z = 0.424) it could be related to the intervening absorption at z = 0.424 but we cannot exclude that it is just a faint background source.

Fig. 4. The BL Lac object PKS 0735+17 (brightest object in the center) imaged by NTT+SUSI (R filter). The two bright objects at both sides of 0735+17 are stars. Field shown is 52 arcsec and North-East is at the top-left side.

From our image we can set a lower limit to the redshift (again assuming the typical properties for the host) of z 0.5, consistent with the limit derived from intervening absorption.

PKS 0736+017. 

The excellent (seeing 0.55 arcsec) image (see Fig. 1) shows the flat spectrum radio quasar PKS 0736+01 (z = 0.191) as well as two close resolved faint companions that are embedded in the nebulosity of the object. The radial luminosity profile (see Fig. 1) is very well represented by an elliptical galaxy with a bright point source in the nucleus. It is found that the galaxy has M_R = -24.3 and effective radius of 12 kpc.

This host galaxy was previously detected in the optical with lower resolution by Wright et al. (1998). They derive M_R = -22.0, which is substantially fainter than our value. We note that this discrepancy could be due to a problem in the Wright et al. image calibration as their surface brightness goes unbelievably faint. At 5" from the nucleus their surface brightness is about =28 while our value at the same radius is = 24.

The object has been also resolved in the NIR by Taylor et al. (1996) who found M_K = -26.3, and by Kotilainen et al. (1998a) who found M_H = -26.2. The R-H color turns out to be 2.0, consistent with the range of values reported by Kotilainen et al. 1998b for a number of BL Lacs.

PKS 0754+10. 

We took two spectra of this BL Lac object for which no firm value of the redshift is available but whose host galaxy had already been detected (Abraham et al. 1991; Falomo 1996). The tentative redshift () proposed by Persic & Salucci (1986) based on inspection of the photographic spectrum reported by Wilkes et al. (1983) is unlikely as the host galaxy would be extremely luminous (( mag). Our spectra were obtained positioning the slit 2" from the nucleus in order to reduce the contamination of light from the bright nucleus. Therefore the spectrum (see Fig. 2) is noisy and it is still dominated by the nuclear non thermal emission. We are not able to unambiguously identify spectral lines but some hint of the CaII break signature from the host galaxy is possibly apparent at = 5045 Å which corresponds to z = 0.28. At this redshift the detected surrounding nebulosity would be M_R -23. We note that this is consistent with the value of the redshift of the companion galaxy (see Fig. 5) 13.6" north-east of the BL Lac object (z=0.27; Pesce et al. 1995) and could be another case of a companion galaxy physically associated with a BL Lac object. A definitive redshift determination is however still needed for this BL Lac object.

Fig. 5. Image of the BL Lac object (brightest object in the center) PKS 0754+10 obtained with NTT+SUSI (R filter). The galaxy G1 at 13" NE is at z = 0.27. Field shown is 52 arcsec and North-East is at the top-left corner.

PKS 0818-128. 

There is no redshift for this object and its optical spectrum is featureless (Falomo et al. 1994). Our optical images, obtained with seeing of 0.7", are not able to detect the host galaxy. The radial brightness profile is well matched by that of a scaled PSF (see Fig. 1). We can set a lower limit to the redshift assuming its nucleus is hosted by a standard luminous (M_R -23.8) elliptical. The limit of redshift we found for such a galaxy to be undetected in our image is z 0.5.

In order to search for emission or intervening absorption line we gathered spectra in a wide wavelength range. Our spectrum (see Fig. 2) is still dominated by the non-thermal featureless emission. The only feature (in addition to telluric bands) we can detect is an absorption at 6284 Å (e.w. 0.7 Å). The most likely identification of this feature is with an interstellar diffuse absorption band at the same wavelength. This is consistent with the low galactic latitude (b_II 13^o) of the source. Alternatively the absorption line could be identified with MgII 2800 Å and this would yield approximately z 1.2 and, consequentially, the object would be extremely luminous (M_R -29).

PKS 0829+046. 

Previous images obtained at sub-arcsec resolution showed that the host galaxy (z = 0.18) has M_R -23 (Falomo 1996). There is also an excess density of galaxies around this object (Pesce et al. 1994). But our spectroscopy shows that only some of them may be physically associated with the BL Lac object. Pesce et al. 1994 obtained the redshifts of galaxies G1 and G2 (see Fig. 6) at respectively z =0.24 and z = 0.204. We took additional spectra of two other galaxies (G3 and G4, see Figs. 2 and 6). We found that G4 is at significantly higher redshift (z = 0.29) while G3 is at z = 0.175, consistent with being associated with PKS 0829+04 at projected distance of 120 kpc. In fact G3 is the only galaxy which is at the same redshift as the BL Lac. On one hand this is another case of similar redshift of a companion galaxy and its BL Lac. On the other hand the environment of 0829+04 must be less rich than what can be estimated from galaxy counts.

Fig. 6. The field around the BL Lac object PKS 0829+04 obtained with NTT+SUSI (R filter). Field shown is 2.2 arcmin and North-East is at the top-left corner.

H 1101-23. 

This is a BL Lac discovered from X-ray survey and is surrounded by a conspicuous rather elongated nebulosity (see Fig. 7) at the proposed (Remillard et al. 1989) confirmed by Falomo et al. (1994). The radial brightness profile extends to 15 arcsec along the major axis. We found that the luminosity profile is well fitted by an elliptical galaxy model plus a point source. The luminosity of the host galaxy is very high. The absolute magnitude, M_R = -24.45, sets this galaxy among the brightest hosts of BL Lac objects (Falomo & Kotilainen 1999).

Fig. 7. Central portion (52" 52") of the NTT + SUSI image of the BL Lac object H 1101-23 (brightest object in the center). A squared shape of isophotes are well apparent as signature of boxiness. North is up and East to the left

For this object (see Fig. 8) we performed detailed surface photometry analysis using the AIAP package (Fasano 1990) in order to study the structural properties of the galaxy. From this analysis we derived photometric and structural parameters (surface brightness, ellipticity, position angle and Fourier coefficient C₄ describing the deviation of isophotes from the ellipse) as a function of the equivalent radius where a is the semi-major axis and is the ellipticity of the ellipse fitting a given isophote. We found the ellipticity profile is increasing from the center outwards up to = 0.45. The profile of the C₄ (see Fig. 9) shows disky (positive C₄) trend in the inner region while the external isophotes are substantially boxy (negative C₄), possibly due to merging processes (e.g. Bender et al. 1988). This is the only clear evidence of significantly boxy isophotes ever found in a BL Lac host.

Fig. 8. Contour plot of 1101-23. Faintest isophote is = 25.2 and spacing between isophotes is 0.5 mag. External isophotes are significantly boxy.

Fig. 9. The amplitude of the Fourier coefficient C4/100 showing the isophotes of the host galaxy of H 1101-23 being disky around 3-4 arcsec and substantially boxy at larger radii.

Another example of a very luminous host galaxy ( = -24.8, or -24.45 if de Vaucouleurs law is fitted) was reported by Heidt et al. (1999) for 1ES 1741+196. Also in this case the host galaxy isophotes have high ellipticity ( 0.35). There is no information, however, about the detailed shape of the isophotes and the amount of possible boxiness.

MS 1312.1-422. 

This source, drawn from the EMSS of BL Lacs (Maccacaro et al.. 1994) was observed during bad seeing conditions (seeing of 1.4") but since it is at relatively low redshift (z = 0.108; Morris et al. 1991) it is rather well resolved. The host galaxy is indeed dominant with respect to the nuclear source (ratio nucleus/host = 0.1). Our fit of the brightness profile yields M_R = -23.4. No other detection of this host galaxy can be found in the literature.

Note that in the calculation of it is usually the luminosity of the whole object (nucleus + host) which is used in the calculation. Such a procedure if applied to 1312-42 would overestimate the optical flux by a factor 10.

MS 1332.6-293. 

The target belongs to the EMSS sample of BL Lacs although its classification is uncertain. Optical spectra showed either emission lines at z=0.256 or strong CaII break (Stocke et al. 1991) due to a substantial contribution from stellar emission.

Our image shows this object is only marginally resolved. This is in part due to the bad seeing ( 1.5") and also because the host galaxy is substantially under-luminous (M_R = -21.3) with respect to the average of the host galaxies of BL Lacs (M_R = -23.8 Falomo & Kotilainen 1999).

We note that the same object (1ES1322-297) is listed in the Einstein Slew sample of BL Lacs (Perlman et al. 1996) and has a redshift z = 0.512 quite different from the previous finding. Since in neither cases there are spectra published we are not able to make our own judgment of the validity of the redshift values. However, the latter value seems confirmed by another optical spectrum (albeit noisy) reported by Rector (1998). At z = 0.512 the host galaxy and point source would be much more luminous ( = -23.5 and = -23.3) and well within the averages of these types of objects.

© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000
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