[Previous] | [Session 45] | [Next]
J.S. Ulvestad (NRAO), L.C. Ho (Carnegie Observatories, Pasadena)
Current wisdom is that massive black hole formation and evolution in galaxy nuclei are fundamentally tied to bulge formation and evolution. Dynamical studies suggest that 100% of bulges in nearby galaxies have black holes, with masses proportional to their bulge mass and velocity dispersion. However, we have very little knowledge of the demography of black holes and associated active galactic nuclei (AGNs) in late-type galaxies, those with low-mass or undetected bulges.
We have used the Very Large Array (VLA) at 5 GHz to search for sub-arcsecond radio nuclei (indicative of AGNs) in 40 bright Sc galaxies that are closer than 30 Mpc and have ``H II'' spectra. Twenty-six of these galaxies were observed in 3-minute VLA snapshots, while the other 14 were imaged using archival VLA data. Only 6 of the 40 galaxies contain detectable radio cores. Typical upper limits for the undetected objects are P(5\ {\rm GHz}) < 4 \times 1019 W Hz-1. (These limits can be reduced by an order of magnitude once the Expanded VLA is completed.) The current radio limits and detections are considerably lower than the powers found for a sample of 45 early-type galaxies at similar distances whose spectra show weak Seyfert characteristics. However, a comparison radio sample of early-type H II galaxies does not yet exist.
The four galaxies with the highest H\alpha luminosities all have detected radio cores. Using the unresolved radio cores alone, the value of the parameter q that characterizes the logarithmic ratio of far-infrared to radio emission ranges from 1.4 to 1.7; this is considerably below the canonical value of 2.3 for galaxies whose radio emission is dominated by starbursts. The excess radio luminosities, combined with the relatively high H\alpha luminosities, imply that these four galaxies most likely harbor previously undetected AGNs and central massive black holes.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
The author(s) of this abstract have provided an email address for comments about the abstract: julvesta@nrao.edu
[Previous] | [Session 45] | [Next]
Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.