A Privileged View of Low Luminosity Galaxies


Marcello Giroletti, Institute of Radioastronomy and University of Bologna
Gabriele Giovannini, University of Bologna
Greg B. Taylor, National Radio Astronomy Observatory

VLBA project code BG123 (as well as VLA project AG618)
Epoch: February 2002

BL Lac 0706+541
The source 0706+591 is part of a sample of 30 nearby (z < 0.2) BL Lac objects that we have studied at arcsecond and milliarcsecond angular resolution. At the top-left, red contours outline the large scale radio emission as seen with the Very Large Array, while the blue tones reveal the Hubble Space Telescope picture of the host galaxy. The right panel zooms in the core region with the extraordinary angular resolution of the Very Long Baseline Array, and the rainbow colours trace the inner emission, with white/red representing the region of most intense radiation.

BL Lac objects, named after object BL in constellation Lacerta, resemble quasars, being quasi-stellar in the optical and compact in the radio. In current schemes of unification, a BL Lac is interpreted as a result of projection effects for a large radio galaxy. In a radio galaxy, two jets composed of ultra-energetic charged particles emerge from a central core, powered by a supermassive black hole, and end in extended lobes where the particles cool down. When our line of sight is closely aligned to the jet axis, the object looks more compact because of projection, and the approaching jet is boosted as a consequence of effects predicted by the theory of special relativity.

Our VLBA radio observations, as well as the VLA and HST images shown on the top left, agree nicely with this scheme: in the large scale radio image (red contours), the diffuse lobe is visible, with the core apparently sitting at its northwest edge because of projection; the radio core is coincident with the optical nucleus of the host galaxy, a giant elliptical resolved by the Hubble Space Telescope (shades of blue). On parsec scale, the VLBA data reveal a core-jet morphology, with the jet emerging in PA -160 degrees and extending for about 20 mas before getting too faint and confused with the noise (0.13 mJy/beam). Similarly, a parsec scale jet is detected in other 21 objects of our sample, which is composed of nearby sources, on average more than ten times weaker than any other sample ever studied with similar detail.

Detection of a jet in such faint objects is an important result, and the fact we observe only one jet is a direct consequence of relativistic boosting on the charged particles moving away from the core. The comparison of the jet brightness to the limit for the counter-jet allows us to estimate the corresponding velocity and orientation. Further constraints based on the core brightness and the X-ray emission from the source argue for a significant Doppler beaming taking place in this source; we estimate that the jet axis is oriented between 20 to 30 degrees to the line of sight, and that it has a bulk Lorentz factor of about 2.9; i.e., the relativistic particles in it are travelling at about 0.94 times the speed of light.

The viewing angles and jet velocity obtained for 0706+591 are similar to those of the other objects in the sample, which has been selected with no bias on nuclear power and is representative of the BL Lac population at low redshift. For the sample as a whole, the average viewing angle is 18 +/- 5 degrees and the Lorentz factors range between 2 and 7. These values are consistent with the hypothesis that BL Lacs are radio galaxies seen end on. In particular, the resulting Doppler factors imply de-beamed, intrinsic radio powers typical of type I Fanaroff-Riley radio galaxies (FR I), which provides further evidence supporting the Unification Scheme for radio loud active galactic nuclei.


Giroletti, Giovannini, Taylor & Falomo 2004, ApJ in press (scheduled Octrober issue, astro-ph/0406255)

Giroletti, Giovannini, Taylor & Falomo, in press for ASP Conference Series: "Future Directions in High Resolution Astronomy: A Celebration of the 10th Anniversary of the VLBA", June 8 - 12, 2003, Socorro, NM, USA (astro-ph/0309380)