Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe luminous host galaxies of high redshift BL Lac objects
104
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
(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.
rate research
Read More
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.
Among active galactic nuclei, BL Lac objects show extreme properties that have been interpreted as the effect of relativistic beaming on the emission from a plasma jet oriented close to the line of sight. The Doppler amplification of the jet emission makes them ideal targets for studying jet physics. In particular, low-power BL Lacs (LPBL) are very interesting because they probe the jet formation and emission processes at the lowest levels of accretion. However, they are difficult to identify since their emission is swamped by the radiation from the host galaxy in most observing bands. In this paper we propose a new LPBL selection method based on the mid-infrared emission, in addition to the traditional optical indices. We considered the radio-selected sample of Best & Heckman (2012, MNRAS, 421, 1569) and cross-matched it with the WISE all-sky survey. In a new diagnostic plane including the W2-W3 color and the Dn(4000) index, LPBL are located in a region scarcely populated by other sources. By filtering objects with small emission line equivalent width, we isolated 36 LPBL candidates up to redshift 0.15. Their radio luminosity at 1.4 GHz spans the range log L_r = 39.2-41.5 [erg/s]. Considering the completeness of our sample, we analyzed the BL Lac luminosity function (RLF), finding a dramatic paucity of LPBL with respect to the extrapolation of the RLF toward low power. This requires a break in the RLF located at log L_r~40.6 [erg/s]. The consequent peak in the BL Lacs number density is possibly the manifestation of a minimum power required to launch a relativistic jet.
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.
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.
Here we present highlights from VERITAS observations of high-frequency-peaked BL Lac objects (HBLs). We discuss the key science motivations for observing these sources -- including performing multiwavelength campaigns critical to understanding the emission mechanisms at work in HBLs, constraining the intensity and spectra shape of the extragalactic background light, and placing limits on the strength of the intergalactic magnetic field.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
