The optical-near-infrared colour of the host galaxies of BL Lacertae objects

Abstract

We present H-band imaging of 23 low redshift (z < 0.3) BL Lac objects that were previously investigated by us in the R-band. For all the observed objects, the host galaxy is well represented by an elliptical model, with average scale length R(e) = 7.2 +- 3.6 kpc and average absolute magnitude M(H) = -25.9 +- 0.6. BL Lac hosts are therefore luminous elliptical galaxies indistinguishable from those of low redshift radio galaxies and inactive ellipticals. The best-fit H-band Kormendy relation of mu(e) = 3.7 log r(e) (kpc) + 14.8 mag arcsec^-2 agrees with that obtained for normal ellipticals and radio galaxies. This similarity indicates that all massive elliptical galaxies can experience nuclear activity without significant perturbation of their global structure. The new observations are combined with literature data of 18 objects, and this extended sample is used to investigate the optical-NIR colour and colour gradient properties of the host galaxies of BL Lacs and to perform a direct comparison with other elliptical galaxies with and without nuclear activity. The optical/near-infrared colour (R-H = 2.2 +- 0.4) and colour gradient (Delta(R-H)/Delta(log r) = -0.38 +- 0.60) distributions of the BL Lac hosts are much wider than those for normal ellipticals with old stellar populations, and many BL Lacs have bluer hosts and/or steeper colour gradients than those in normal ellipticals. The blue colours are most likely caused by a young stellar population, and indicates a link between star formation caused by an interaction/merging event and the onset of the nuclear activity. However, the lack of obvious signs of interaction may require a significant time delay between the event with associated SF episodes and the onset of nuclear activity.

arXiv commentA&A, accepted (20 pages). Table 1 and Figure 1 will only be available in the electronic version of A&A

References in corpus (19)

• Unified Schemes for Radio-Loud Active Galactic Nuclei
• The Evolution of Early-Type Galaxies in Distant Clusters
• The HST Survey of BL Lacertae Objects. II. Host Galaxies
• Northern JHK Standard Stars for Array Detectors
• Near-infrared imaging and the K-z relation for radio galaxies in the 7C Redshift Survey
• The HST Survey of BL Lacertae Objects. I. Surface Brightness Profiles, Magnitudes, and Radii of Host Galaxies
• The black hole mass distribution in early-type galaxies: cusps in HST photometry interpreted through adiabatic black hole growth
• The K-band Hubble diagram for the brightest cluster galaxies: a test of hierarchical galaxy formation models
• The Black Hole Masses and Host Galaxies of BL Lac Objects
• HST observations of nearby radio-loud early-type galaxies
• The black hole mass of BL Lac objects from the stellar velocity dispersion of the host galaxy
• Origin of Color Gradients in Elliptical Galaxies
• The Near-Infrared Fundamental Plane of Elliptical Galaxies
• The HST Survey of BL Lacertae Objects: Morphological Properties of Low redshift Host Galaxies
• The Fundamental Plane of Radio Galaxies
• The HST Survey of BL~Lacertae Objects. IV. Infrared Imaging of Host Galaxies
• Black hole masses and the Fundamental Plane of BL Lac objects
• Near-Infrared Observations of BL Lacertae Host Galaxies
• Imaging of the Host Galaxies of Three X-Ray Selected BL Lacertae Objects

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• The stellar content of low redshift radio galaxies from near-infrared spectroscopy
• $R$-band host galaxy contamination of TeV $\gamma$-ray blazar Mrk 501: effects of the aperture size and seeing