Hubble Space TelescopeObservations of the Host Galaxies of BL Lacertae Objects

Abstract

Six BL Lac objects from the complete 1 Jy radio-selected sample of 34 objects were observed in Cycle 5 with the HST WFPC2 camera to an equivalent limiting flux of µ I ∼ 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 previously by Falomo et al. (1997). In addition, we have analyzed in the same way HST images of three X-ray-selected BL Lacs observed by Jannuzi et al. (1997). The ensemble of 9 BL Lac objects spans the redshift range from z = 0.19 to ∼ 1. Host galaxies are clearly detected in seven cases, while the other two, at z ∼ 0.258 (redshift highly uncertain) and z = 0.997, are not resolved. The HST images constitute a homogeneous data set with unprecedented morphological information between a few tenths of an arcsecond and several arcseconds from the nucleus, allowing us in 6 of the 7 detected host galaxies to rule out definitively a pure disk light profile. The host galaxies are luminous ellipticals with an average absolute magnitude of M I ∼ −24.6 mag (with dispersion 0.7 mag), more than a magnitude brighter than L * and comparable to brightest cluster galaxies. The morphologies are generally smooth and have small ellipticities (ǫ ∼ < 0.2). Given such roundness, there is no obvious 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 show no strong spatial gradients and are as expected for K-corrected passively evolving elliptical galaxies. The host galaxies of the radio-selected and X-ray-selected BL Lacs for this very limited sample are comparable in both morphology and luminosity.

R-band imaging of the host galaxies of RGB BL Lacertae objects

“…Urry et al (1999) found R = 19.0 and r eff = 2.4 arcsec for the host galaxy and SU00 R = 20.2 and r eff = 2.5 arcsec, whereas we find R = 19.9 and r eff = 3.0 arcsec. P02 found R = 19.9 and r eff = 4.0 arcsec using the same data as in this study.…”

“…The χ 2 is almost the same for both models, however. Urry et al (1999) also found 1540+147 equally well fitted by the two models 1626+352: The core plus a de Vaucouleurs fit for this object gives R = 19.73 ± 0.12 and r eff = 0.8 ± 0.2 arcsec for the host galaxy. Because of the errors in r eff this object is denoted as unresolved in Table 4.…”

“…We use the same data as P02 and our results agree well with each other (R = 19.75 ± 0.10, r eff = 2.6 arcsec and R = 20.02 ± 0.06, r eff = 2.5 arcsec in P02 and this study, respectively). The two HST studies (Urry et al 1999, SU00) find similar host magnitudes, but smaller host galaxy (∼1 arcsec).…”

“…values in Table 4 and the fact that not a single clear case of a disk host was detected strongly support the view that BL Lac objects are almost exclusively found in elliptical galaxies. Urry et al (1999) found that for 58 objects among the 72 that were resolved the de Vaucouleurs model was preferred over the disk model with a high statistical significance and concluded that at most 8% of the BL Lacs are hosted by disk type galaxies.…”

“…We have a very deep exposure (6000 s) taken under good seeing conditions (0.9 arcsec), yet no sign of the host galaxy is seen. Urry et al (1999) found the object also unresolved in their deep HST exposure. Stickel et al (1993) noted that the outer isophotes are extended more to the west than to the east.…”