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

4. Conclusions

In this paper we have presented the results of a near-infrared imaging study of a sample of 11 BL Lac objects, for most of which the host galaxy is clearly resolved. Consistently with what is found in optical studies, we find that the host galaxies of low redshift BL Lacs are large (average bulge scale length R(e) = 8.89.9 kpc) and luminous (average M(H) = -25.80.5); they are more luminous than L^* galaxies (by 1 mag) but of similar luminosity to or slightly fainter than the brightest cluster galaxies. Our NIR study was able for the first time to address the issue of the optical-NIR colour of BL Lac host galaxies. The average R-H colour and colour gradient of the BL Lac hosts are consistent with those of non-active early-type galaxies, suggesting that the nuclear activity does not have much effect on the star formation history of the host galaxies. The nucleus-to-galaxy ratio of BL Lacs is similar to that found in low redshift RGs and consistent with what found in previous optical studies of BL Lacs. However, it is smaller that that found for the higher redshift FSRQs (KFS98), suggesting there is a difference in the intrinsic brightness of the nuclear source or in the Doppler beaming factor between the two types of blazars. We finally encourage a systematic NIR multiwavelength study of a large, well defined sample of BL Lac objects and their immediate environments with the new generation large NIR arrays.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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