Evolution of BL Lacertae host galaxies

We present and discuss deep, high-resolution I-band images of 24 BL Lac objects between z = 0.3 and 1.3 taken with the Nordic Optical Telescope (NOT) and the ESO-NTT and VLT telescopes. In addition, new redshifts for the BL Lac objects PKS 0406+121, PKS 0426-380 and PKS 1519-273 are reported. In 17/24 (71%) of the BL Lac objects, we detected an underlying nebulosity, in 11/17 for the first time. We assigned the underlying nebulosity to the BL Lac host galaxy in 11 cases spanning the redshift range z = 0.3-1. The remaining 6 BL Lac objects have either intervening galaxies (S4 0218+35, PKS 0426-380), no redshift (MH 2133-449) or are probably misidentified (Q 0230+3429, B2 0937+26, MS 2347.4+1924). Restricting ourselves to the 11 BL Lac objects (< z > = 0.6), where a core and host galaxy was detected, we find that their host galaxies are luminous (M_I = -25.2± 0.8) and large (r_e = 10.5± 7 kpc). They are on average about 0.6 mag brighter than BL Lac host galaxies at z ∼ 0.3 indicative of evolution, whereas their half-light radii are similar. By combining our data with literature data at low-redshift and applying evolutionary models to them, we show that the properties of the host galaxies of BL Lac objects up to z ∼ 1 are compatible with passively evolving elliptical galaxies formed at a redshift of z ∼ 2 (13 Gyrs ago in our adopted cosmology). Our results, however, are affected by an unavoidable luminosity bias and need to be confirmed. Future prospects are described. If they could be confirmed, host galaxies of low-luminosity radio-loud AGN (BL Lac/FR I) have very similar properties to the hosts of radio-quiet QSOs and high-luminosity radio-loud AGN (radio-loud QSO/FR II) over a wide redshift range. This supports the picture of the ``Grand Unification'' in which AGN activity is a transient phenomenon in galaxy evolution.

Based on observations collected with the VLT-UT1 on Cerro Paranal (Chile), the NTT on La Silla (Chile) operated by the European

Southern Observatory in the course of the observing proposals 64.P-0230 and 66.B-0125.

Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.

Appendix A is only available in electronic form at http://www.edpsciences.org