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Register a new userThe stellar content of low redshift BL Lac host galaxies from multicolour imaging
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We present B-band imaging of 18 low redshift (z<0.3) BL Lac objects for which their host galaxies were previously resolved in the R-band and the near-infrared H-band. For a subset of the objects, also U- and V-band imaging is presented. These data are used to investigate the blue-red-near-infrared colours and the colour gradients of the host galaxies of BL Lacs in comparison with other elliptical galaxies with and without nuclear activity. In all cases galaxies are well represented by an elliptical model, with average absolute magnitude M_B=-21.6+-0.7 and average scale length R_e=7.6+-3.2 kpc. The best-fit B-band Kormendy relation is in reasonable agreement with that obtained for normal ellipticals and radio galaxies. This structural and dynamical similarity indicates that all massive elliptical galaxies can experience nuclear activity without significant perturbation of their global structure. The distributions of the integrated blue/near-infrared colour (with average B-H=3.5+-0.5) and colour gradient (with average Delta(B-R)/Delta(log r)=-0.14+-0.75) of the BL Lac hosts are much wider than those for normal ellipticals, and most BL Lac objects have bluer hosts and/or steeper colour gradients than those in normal ellipticals. The blue colours are likely caused by a young stellar population component, and indicates a link between star formation caused by an interaction/merging event and the onset of the nuclear activity. This result is corroborated by stellar population modelling, indicating a presence of young/intermediate age populations in the majority of the sample, in agreement with low redshift quasar hosts. The lack of strong signs of interaction may require a significant time delay between the event with associated star formation episodes and the start of the nuclear activity.
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(abridged) We present near-infrared Ks-band imaging of 13 high redshift (0.6 < z < 1.3) BL Lac objects. We clearly detect the host in eight objects, and marginally in three others. In all cases, the host galaxy is well represented by an r^1/4 surface brightness law. The host galaxies of high redshift BL Lacs are large (<R(e)> ~7 kpc) and very luminous (<M(K)> = -27.9+-0.7), ~3 mag brighter than L*, and ~1 mag brighter than brightest cluster galaxies. They are also ~1 mag brighter than low redshift radio galaxies and appear to deviate from their K-z relationship. On the other hand, the high luminosities agree with the few optical studies of high redshift BL Lac hosts. The nuclear luminosity and the nucleus-galaxy luminosity ratio of the high redshift BL Lacs are much larger than those in low redshift BL Lacs. This may be due to either a higher intrinsic nuclear luminosity, or enhanced luminosity because of strong beaming. Contrary to what is observed in low redshift BL Lacs, the luminosities of the host galaxy and of the nucleus are fairly well correlated, as expected from the black hole mass - bulge luminosity relationship. High redshift BL Lacs radiate with a wide range of power with respect to their Eddington luminosity, and this power is intermediate between those in nearby BL Lacs and in luminous radio-loud quasars. The high redshift BL Lac host galaxies appear to be ~2 mag brighter than those at low redshift. This is likely due to a strong selection effect in the surveys of BL Lacs that makes observable only the most luminous sources at z > 0.5 and produces a correlation between the nuclear and the host luminosity. However, this may also suggest strong luminosity evolution which is inconsistent with a simple passive evolution of the host galaxies, and requires a contribution from relatively recent star formation episodes.
Near-infrared and optical imaging of BL Lac host galaxies is used to investigate their colour properties. We find that the R-H colour and colour gradient distributions of the BL Lac hosts are much wider than those for normal ellipticals, and many objects have very blue hosts and/or steep colour gradients. The blue colours are most likely caused by recent star formation. The lack of obvious signs of interaction may, however, require a significant time delay between the interaction event with associated star formation episodes and the onset of the nuclear activity.
Since the last meeting on BL Lac objects 10 years ago, BL Lac host galaxies and their cluster environment have gained much attention. Hence, our current knowledge of the properties of BL Lac host galaxies and their cluster environment has improved considerably, which will be reviewed. The importance of future observing programs using (very) large telescopes is briefly outlined.
We present medium spectral resolution near-infrared (NIR) HK-band spectra for 8 low redshift (z<0.06) radio galaxies to study the NIR stellar properties of their host galaxies. As a homogeneous comparison sample, we used 9 inactive elliptical galaxies that were observed with similar resolution and wavelength range. The aim of the study is to compare the NIR spectral properties of radio galaxies to those of inactive early-type galaxies and, furthermore, produce the first NIR HK-band spectra for low redshift radio galaxies. For both samples spectral indices of several diagnostic absorption features, SiI(1.589microns), CO(1.619microns), NaI(2.207microns), CaI(2.263microns), CO(>2.29microns), were measured. To characterize the age of the populations, the measured EWs of the absorption features were fitted with the corresponding theoretical evolutionary curves of the EWs calculated by the stellar synthesis model. On average, EW(CO 2.29) of radio galaxies is somewhat greater than that of inactive ellipticals. Most likely, EW(CO 2.29) is not significantly affected by dilution, and thus indicating that elliptical galaxies containing AGN are in a different stage in their evolution than inactive ellipticals. This is also supported by comparing other NIR features, such as CaI and NaI, with each other. Absorption features are consistent with the intermediate age stellar population, suggesting that host galaxies contain both an old and intermediate age components. It is consistent with previous optical spectroscopy studies which have shown evidence on the intermediate age (~2 Gyr) stellar population of radio galaxies, and also in some of the early-type galaxies. The existence of intermediate age population is a link between the star formation episode, possibly induced by interaction or merging event, and the triggering of the nuclear activity.
The correlation between black hole mass M(BH) and stellar velocity dispersion sigma in nearby elliptical galaxies affords a novel way to determine M(BH) in active galaxies. We report on measurements of sigma from optical spectra of 7 BL Lac host galaxies. The derived values of sigma are in the range of 160 - 290 km/s corresponding to M(BH) of 5 x 10^7 to 1 x 10^9 Msun. The average ratio of M(BH) to the host galaxy mass is 1.4 x 10^-3, consistent with that estimated in other active and inactive galaxies. The velocity dispersions and the derived values of M(BH) of the BL Lacs are similar to those obtained for low redshift radio galaxies, in good agreement with the predictions of the unified models for radio-loud active galaxies.
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