Hubble Space Telescope observations of the host galaxies of BL Lacertae objects

Six BL Lac objects from the complete 1 Jy radio-selected sample of 34 objec ts were observed in cycle 5 with the Hubble Space Telescope (HST) Wide-Fiel d Planetary Camera 2 (WFPC2) to an equivalent limiting flux of mu(I) simila r to 26 mag arcsec(-2). Here we report results for the second half of this sample, as well as new results for the first three objects, discussed previ ously by Falomo and coworkers. In addition, we have analyzed in the same wa y HST images of three X-ray-selected BL Lac objects observed by Jannuzi and coworkers. The ensemble of nine BL Lac objects spans the redshift range fr om z = 0.19 to similar to 1. Host galaxies are clearly detected in seven ca ses, while the other two, at z similar to 0.258 (redshift highly uncertain) and z = 0.997, are not resolved. The HST images make up a homogeneous data set with unprecedented morphological information between a few tenths of a n arcsecond and several arcseconds from the nucleus, allowing us to rule ou t definitively a pure disk light profile in six of the seven detected host galaxies. The host galaxies are luminous ellipticals with an average absolu te magnitude of M-I similar to -24.6 mag (with dispersion 0.7 mag), more th an a magnitude brighter than L* and comparable to the brightest cluster gal axies. The morphologies are generally smooth and have small ellipticities ( epsilon less than or similar to 0.2). Given such roundness, there is no obv ious alignment with the more linear radio structures. In the six cases for which we have HST WFPC2 images in two filters, the derived color profiles s how no strong spatial gradients and are as expected for K-corrected passive ly evolving elliptical galaxies. The host galaxies of the radio-selected an d X-ray-selected BL Lac objects for this very limited sample are comparable in both morphology and luminosity.