EDP Sciences
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Volume 381, Number 3, January III 2002
Page(s) 810 - 824
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20011564

A&A 381, 810-824 (2002)
DOI: 10.1051/0004-6361:20011564

Deep optical imaging of radio selected BL Lacertae objects

T. Pursimo1, K. Nilsson1, L. O. Takalo1, A. Sillanpää1, J. Heidt2 and H. Pietilä1

1  Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
2  Landessternwarte, Königstuhl, 69117 Heidelberg, Germany

(Received 29 March 2000 / Accepted 19 October 2001 )

Deep imaging data on radio-selected BL Lacertae objects are presented. The complete 1 Jy sample consists of 37 objects. Altogether 25 objects were observed which span a large range of redshifts (up to z=1.34) with 12 objects having z>0.5. In addition, three objects are located at an unknown distance. Altogether in eleven objects the host galaxy could be resolved. All nine objects with $z\la 0.4$ were resolved and all eleven objects with $z\ga0.7$ or unknown redshift were unresolved. In all (six) low- z ( $z\la0.2$) objects, an elliptical host galaxy was preferred. Of the remaining objects, only in one case (2007+777) did a disc-type galaxy give better fit than elliptical, but in this case neither can be preferred. The absolute brightness of the host galaxies (median MR=-23.82) is similar to the findings of X-ray selected BL Lacs and low redshift radio galaxies. Also the sizes (median $r_{\rm e}=14.8$ kpc) of the galaxies are similar. This supports the idea that (at least low redshift) BL Lacs belong to one family of objects. These results agree well with other recent studies on BL Lacs. There is a strong correlation between the brightness of the active nucleus and redshift. At low redshift radio and X-ray selected BL Lacs have similar brightnesses. No (or only weak) correlation was found between the brightness of the nucleus and the host galaxy.

Key words: BL Lacertae objects: general -- galaxies: nuclei -- galaxies: structure

Offprint request: T. Pursimo, tpursimo@utu.fi

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