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

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4. Summary and conclusions

We have presented optical images of a number of BL Lacs that were not previously well studied. For several of these objects the first detection of the host galaxy is presented here. The properties of the hosts are consistent with them being luminous ellipticals as found in previous similar studies. For two of the resolved objects that have not a spectroscopic redshift we derive a photometric redshift based on the observed properties of the surrounding nebulosity.

A bright and boxy elliptical

We find that the external isophotes of the luminous host galaxy of 1101-23 are significantly boxy while the inner disky region suggests the presence of a small disc component. This is the first clear example of a boxy galaxy hosting a BL Lac object. Boxy isophotes are observed in a fraction of luminous ellipticals (Bender 1988) and could be ascribed to merging events from equal mass galaxies (e.g. Naab et al. 1999). It would be interesting to know what fraction of hosts of BL Lacs exhibit boxy isophotes as compared with non active ellipticals. Very little data are, however, available on isophote shapes of BL Lacs host galaxies because the presence of the bright nucleus and the quality of data often hinder a reliable estimation of this parameter especially at high redshift. For relatively low redshift objects with high resolution images it should be possible to investigate the isophote shape in a systematic way.

The immediate environment of BL Lacs

Our spectroscopy has allowed us to derive a redshift for 0301-24 (z = 0.26) and possibly for 0754+10 (z=0.28). Both objects have companion galaxies at redshifts very similar to that of the BL Lacs. The companions and the BL Lacs are thus very likely to be gravitationally bound. A third case is PKS 0829+04 for which we took the spectra of two galaxies in the immediate environment and found that one is at the same redshift as the BL Lac object. These spectroscopic results improve the scanty data on redshifts of companion galaxies of BL Lacs. Together with previous findings (Falomo et al. 1993a,b; Pesce et al. 1994,1995; Heidt et al. 1999) our new results yield convincing evidence that galaxies around BL Lacs are (often) gravitationally bound with the BL Lacs. On the other hand only in very few cases do these interactions lead to significantly (observable) disturbed morphology (see e.g. Falomo et al. 1995, Heidt et al. 1999).

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000