A VLBI Search for AGN Radio Cores in Starburst Galaxies
Abstract
We have searched for compact, high brightness temperature, milliarcsecond synchrotron cores in four starburst galaxies which have compact VLA radio sources and high far-infrared luminosities but no optical spectroscopic evidence for a classical active nucleus. The search, using a sensitive Mk III VLBI array at λ = 18 cm, has resulted in the detection of compact radio sources in two of the four galaxies: Mrk 297 (NGC 6052) and Mrk 171 (NGC 3690/IC 694). In Mrk 297 the source lies in an obscure clump near the edge of the galaxy. This is either the first known case of a detached nucleus (presumably as a result of disruption during a merger) or a radio supernova of unprecedented luminosity. No VLBI-scale emission was detected in the nucleus of the starburst galaxy Mrk 201 (NGC 4194), or in Mrk 928a, although the Seyfert 2 nucleus Mrk 928b was detected. We have also observed two other galaxies with compact VLA radio sources: the Seyfert 2 nucleus Mrk 620 and the previously unclassified galaxy Mrk 520. Mrk 620 was found to have a milliarcsecond core, but Mrk 520 was not detected. We have obtained optical spectroscopy for five of these galaxies, including Mrk 520, which is shown to be a Seyfert 1.9 galaxy. Our VLBI detection rate of starburst galaxies with compact VLA radio cores is therefore 50% (2/4), and of Seyfert 2 galaxies 66% (2/3). If the VLBI sources in Mrk 297 and Mrk 171 are indeed of active galactic nucleus origin, these results demonstrate that, in luminous starburst galaxies, an active nucleus can effectively be completely hidden from view in the optical by dust. They also support a physical or causal connection between classical nuclear activity and the starburst phenomenon. Luminous radio supernovae, associated with the massive starbursts in Mrk 297 and Mrk 171, remain a plausible alternative interpretation.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 1992
- DOI:
- 10.1086/171377
- Bibcode:
- 1992ApJ...391..629L
- Keywords:
-
- Active Galactic Nuclei;
- Brightness Temperature;
- Galactic Radio Waves;
- Starburst Galaxies;
- Very Long Base Interferometry;
- Cosmic Dust;
- Galactic Nuclei;
- Radio Sources (Astronomy);
- Very Large Array (Vla);
- Astrophysics;
- GALAXIES: ACTIVE;
- GALAXIES: STELLAR CONTENT;
- RADIO CONTINUUM: GALAXIES;
- TECHNIQUES: INTERFEROMETRIC