A Supernova Factory in the Merger System Arp 299

We have imaged the nearby galaxy merger Arp 299 at arcsecond and milliarcsecond resolution, using both the Very Large Array and the Very Long Baseline Array. The large-scale radio emission from the merger contains 5 bright, compact radio sources embedded in diffuse emission, with diameters less than 200 pc. Supernova rates of 0.1 to 1 per year are required to produce the VLA-detected radio emission in these sources. Two of the compact VLA radio sources, designated Source A and Source D, also have been detected and imaged at milliarcsecond scales. Source A, which is associated with the nucleus of one of the merging galaxies, contains five milliarcsecond-scale sources, each with a radio power between 100 and 1000 times that of the Galactic supernova remnant Cassiopeia A. Four of these have flat or inverted spectra and appear to be young supernovae. Three of the VLBI-scale sources are located within 10 pc (projected) of one another, and two are separated by less than 3 pc, indicating that they all may be within the same super starcluster or complex of such clusters. The brightest VLBI-scale source, A0, has an extremely inverted pectrum, with alpha larger than +2 at gigahertz frequencies. It seems to be the youngest supernova, which has not yet broken out of its circumstellar shell. The milliarcsecond radio sources within Source A appear to constitute a upernova factory, confirming the presence of an extreme starburst that peaked at least a few million years ago.Comment: Accepted for the Astrophysical Journal, 22 pages, 10 figure

Outflow-Dominated Emission from the Quiescent Massive Black Holes in NGC 4621 and NGC 4697

The nearby elliptical galaxies NGC 4621 and NGC 4697 each host a supermassive black hole with a mass more than 1e8 Solar masses. Analysis of archival Chandra data and new NRAO Very Large Array data shows that each galaxy contains a low-luminosity active galactic nucleus (LLAGN), identified as a faint, hard X-ray source that is astrometrically coincident with a faint 8.5-GHz source. The latter has a diameter less than 0.3 arcsec (26 pc for NGC 4621, 17 pc for NGC 4697). The black holes energizing these LLAGNs have Eddington ratios L(2-10 keV) / L(Edd) ~ 1e-9, placing them in the so-called quiescent regime. The emission from these quiescent black holes is radio-loud, with log Rx = log nuLnu(8.5 GHz) / L(2-10 keV) ~ -2, suggesting the presence of a radio outflow. Also, application of the radio-X-ray-mass relation from Yuan & Cui for quiescent black holes predicts the observed radio luminosities nuLnu(8.5 GHz) to within a factor of a few. Significantly, that relation invokes X-ray emission from the outflow rather than from an accretion flow. The faint, but detectable, emission from these two massive black holes is therefore consistent with being outflow-dominated. Observational tests of this finding are suggested.Comment: 11 pages; 4 figures: emulateapj; to appear in Ap

The Origin of Radio Emission in Low-Luminosity Active Galactic Nuclei: Jets, Accretion Flows, or Both?

The low-luminosity active galactic nuclei in NGC 3147, NGC 4203, and NGC 4579 have been imaged at four frequencies with the Very Long Baseline Array. The galaxies are unresolved at all frequencies, with size upper limits of $10^3-10^4$ times the Schwarzschild radii of their central massive black holes. The spectral indices between 1.7 and 5.0 GHz range from 0.2 to 0.4; one and possibly two of the galaxies show spectral turnovers between 5.0 and 8.4 GHz. The high brightness temperatures ($> 10^9$ K) and relatively straight spectra imply that free-free emission and/or absorption cannot account for the slightly inverted spectra. Although the radio properties of the cores superficially resemble predictions for advection-dominated accretion flows, the radio luminosities are too high compared to the X-ray luminosities. We suggest that the bulk of the radio emission is generated by a compact radio jet, which may coexist with a low radiative efficiency accretion flow.Comment: To appear in ApJ (Letters). 4 page

Gigahertz-Peaked Spectrum Radio Sources in Nearby Galaxies

There is now strong evidence that many low-luminosity AGNs (LLAGNs) contain accreting massive black holes and that the nuclear radio emission is dominated by parsec-scale jets launched by these black holes. Here, we present preliminary results on the 1.4 GHz to 667 GHz spectral shape of a well-defined sample of 16 LLAGNs. The LLAGNs have a falling spectrum at high GHz frequencies. Several also show a low-frequency turnover with a peak in the 1-20 GHz range. The results provide further support for jet dominance of the core radio emission. The LLAGNs show intriguing similarities with gigahertz-peaked spectrum (GPS) sources.Comment: 6 pages, to appear in ASP Conference series, 2002, Vol. 25

A Flaring Megamaser in Mrk 348

We report new observations of the H2O megamaser in the Seyfert 2 galaxy Mrk 348. Following our initial detection in 2000 March using the Effelsberg 100 m telescope, re-analysis of previous data on this source indicates that the maser was present but only marginally detectable in late 1997. Monitoring through late 2000 shows that the maser has again decreased to its original level. The H2O line is redshifted by ~130 km/s with respect to the systemic velocity, is extremely broad, with a FWHM of 130 km/s, and has no detectable high velocity components within 1500 km/s on either side of the strong line. Followup VLBA observations show that the maser emission emanates entirely from a region >0.25 pc in extent, toward the base of the radio jet.Comment: 4 pages, 2 figures, to appear in Cosmic Masers: from Protostars to Black Holes, IAU 206, Eds. V. Migenes et al., ASP Conference Serie

The Starburst in the Central Kiloparsec of Markarian 231

We present VLBA observations at 0.33 and 0.61 GHz, and VLA observations between 5 and 22 GHz, of subkiloparsec scale radio emission from Mrk 231. In addition to jet components clearly associated with the AGN, we also find a smooth extended component of size 100 - 1000 pc most probably related to the purported massive star forming disk in Mrk 231. The diffuse radio emission from the disk is found to have a steep spectrum at high frequencies, characteristic of optically thin synchrotron emission. The required relativistic particle density in the disk can be produced by a star formation rate of 220 Msolar/yr in the central kiloparsec. At low frequencies the disk is absorbed, most likely by ionized gas with an emission measure of 8 x 10^5 pc cm-6. We have also identified 4 candidate radio supernovae that, if confirmed, represent direct evidence for ongoing star formation in the central kiloparsec.Comment: in press at ApJ for v. 519 July 1999, 14 page LaTeX document includes 6 postscript figure