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

Abstract

Abstract. We present optical R-band images of 100 BL Lacertae objects in the ROSAT-Green Bank (RGB) sample obtained at the Nordic Optical Telescope (NOT). For 62 objects we could not find any previously published images in the literature, of these 35 are new BL Lacs discovered during the RGB BL Lac survey. We were able to resolve the host galaxy in 62% of the objects. We analyze the host galaxies using a two-dimensional fitting procedure and tabulate the best-fit core magnitude and host galaxy parameters (magnitude, effective radius, ellipticity, position angle and shape parameter β) for each object. With two exceptions, all objects are better fit by a model representing an elliptical galaxy (β = 0.25) than by a disk galaxy model (β = 1.0). In the two exceptions the disk host fit is only marginally better than the elliptical host fit, however, and we do not find a single clear example of a disk-dominated host galaxy. The host galaxies have an average brightness M R = −23.9 ± 0.8 and average effective radius r eff = 13.2 ± 0.8 kpc. Their bulk properties are indistinguishable from normal elliptical and radio galaxies. The median β is 0.18, well below the value 0.25 that is normally used to describe ellipticals. However, the difference may be caused by a selection effect due to the dependence of β on host galaxy luminosity.

Properties of optically selected BL Lacertae candidates from the SDSS

“…In all 19 cases where our simulations showed the host galaxy type to be well determined the bulge model was preferred. This result is in line with previous BL Lac host galaxy imaging surveys (e.g., Urry et al 2000;Nilsson et al 2003), which show that BL Lacs are almost exclusively found in ellipticals. Given this result we use the bulge fit results for all targets from this point on.…”

“…Heidt et al 1999;Nilsson et al 2003Nilsson et al , 2007. A more detailed description of the fitting procedure can be found in Nilsson et al (1999), here we briefly describe the main features of the program and its application to present data.…”

Redshift constraints for RGB 0136+391 and PKS 0735+178 from deep optical imaging

“…Piranomonte et al (2007) derived a lower limit z > 0.193 based on the absence of host galaxy features in the spectrum. In the only imaging attempt so far, Nilsson et al (2003) did not detect the host galaxy in a 1200 s R-band exposure.…”

Optical flux behaviour of a sample ofFermiblazars

“…This object shows a featureless spectrum and its host galaxy can be resolved in the images (Abraham et al 1991;Falomo 1996), but it is rather faint compared to the AGN. Nilsson et al (2003) suggest a host magnitude of R = 18.55 mag and r eff = 1.3 arcsec. Carangelo et al (2003) suggest a redshift z = 0.266 ± 0.001 based on the identification of two faint emission lines.…”