MERLIN observations of Stephan's Quintet

We present MERLIN L-band images of the compact galaxy group, Stephan's Quintet. The Seyfert 2 galaxy, NGC 7319, the brightest member of the compact group, is seen to have a triple radio structure typical of many extra-galactic radio sources which have a flat spectrum core and two steep spectrum lobes with hot spots. The two lobes are asymmetrically distributed on opposite sides of the core along the minor axis of the galaxy. Ultraviolet emission revealed in a high resolution HRC/ACS HST image is strongly aligned with the radio plasma and we interpret the intense star formation in the core and north lobe as an event induced by the collision of the north radio jet with over-dense ambient material. In addition, a re-mapping of archive VLA L-band observations reveals more extended emission along the major axis of the galaxy which is aligned with the optical axis. Images formed from the combined MERLIN and archive VLA data reveal more detailed structure of the two lobes and hot spots.Comment: Completely revised version with new HST data included, to appear in MNRA

A Search for Active Galactic Nuclei in Sc Galaxies with H II Spectra

(Abridged) We have searched for nuclear radio emission from a statistically complete sample of 40 Sc galaxies within 30 Mpc that are optically classified as star-forming objects, in order to determine whether weak AGNs might be present. Only three nuclear radio sources were detected, in NGC 864, NGC 4123, and NGC 4535. These galaxies have peak 6-cm radio powers of 10^{20} W/Hz at arcsecond resolution, while upper limits of the non-detected galaxies typically range from 10^{18.4} to 10^{20} W/Hz. The three nuclear radio sources all are resolved and appear to have diffuse morphologies, with linear sizes of ~300 pc. This strongly indicates that circumnuclear star formation has been detected in these three H II galaxies. Comparison with previous 20-cm VLA results for the detected galaxies shows that the extended nuclear radio emission has a flat spectrum in two objects, and almost certainly is generated by thermal emission from gas ionized by young stars in the centers of those galaxies. The 6-cm radio powers are comparable to predictions for thermal emission that are based on the nuclear H-alpha luminosities, and imply nuclear star formation rates of 0.08-0.8 solar masses/yr, while the low-resolution NRAO VLA Sky Survey implies galaxy-wide star formation rates of 0.3-1.0 solar masses/yr in stars above 5 solar masses. Although the presence of active nuclei powered by massive black holes cannot be definitively ruled out, the present results suggest that they are likely to be rare in these late-type galaxies with H II spectra.Comment: To appear in ApJ. 7 page

Deep MERLIN 5GHz Radio Imaging of Supernova Remnants in the M82 Starburst

The results of an extremely deep, 8-day long observation of the central kpc of the nearby starburst galaxy M82 using MERLIN (Multi-Element Radio Linked Interferometer Network) at 5 GHz are presented. The 17E-06 Jy/beam, rms noise level in the naturally weighted image make it the most sensitive high resolution radio image of M82 made to date. Over 50 discrete sources are detected, the majority of which are supernova remnants, but with 13 identified as HII regions. Sizes, flux densities and radio brightnesses are given for all of the detected sources, which are all well resolved with a majority showing shell or partial shell structures. Those sources within the sample which are supernova remnants have diameters ranging from 0.3 to 6.7 pc, with a mean size of 2.9 pc. From a comparison with previous MERLIN 5 GHz observations made in July 1992, which gives a 9.75 year timeline, it has been possible to measure the expansion velocities of ten of the more compact sources, eight of which have not been measured before. These derived expansion velocities range between 2200 and 10500 km/s.Comment: 34 pages, 10 figures. Accepted by MNRA

The Extreme Compact Starburst in MRK 273

Images of neutral Hydrogen 21cm absorption and radio continuum emission at 1.4 GHz from Mrk 273 were made using the Very Long Baseline Array and Very Large Array. These images reveal a gas disk associated with the northern nuclear region with a diameter of 0.5'' (370 pc), at an inclination angle of 53deg. The radio continuum emission is composed of a diffuse component plus a number of compact sources. This morphology resembles those of nearby, lower luminosity starburst galaxies. These images provide strong support for the hypothesis that the luminosity of the northern source is dominated by an extreme compact starburst. The HI 21cm absorption shows an east-west gradient in velocity of 450 km/s across 0.3'' (220 pc), implying an enclosed mass of 2e9 M_solar, comparable to the molecular gas mass. The brightest of the compact sources may indicate radio emission from an active nucleus (AGN), but this source contributes only 3.8% to the total flux density of the northern nuclear region. The HI 21cm absorption toward the southeast radio nucleus suggests infall at 200 km/s on scales < 40 pc, and the southwest near IR nucleus is not detected in high resolution radio continuum images.Comment: standard AAS format, 23 pages, 5 figures, fixed figure. To appear in ApJ Letter

Origin of the X-ray Quasi-Periodic Oscillations and Identification of a Transient Ultraluminous X-Ray Source in M82

The starburst galaxy M82 contains two ultraluminous X-ray sources (ULXs), CXOM82 J095550.2+694047 (=X41.4+60) and CXOM82 J095551.1+694045 (=X42.3+59), which are unresolved by XMM-Newton. We revisited the two XMM-Newton observations of M82 and analyzed the surface brightness profiles using the known Chandra source positions. We show that the quasi-periodic oscillations (QPOs) detected with XMM-Newton originate from X41.4+60, the brightest X-ray source in M82. Correcting for the contributions of the unresolved sources, the QPO at a frequency of 55.8+/-1.3 mHz on 2001 May 06 had a fractional rms amplitude of 32%, and the QPO at 112.9+/-1.3 mHz on 2004 April 21 had an amplitude of 21%. The QPO frequency may possibly be correlated with the source flux, similar to the type C QPOs in XTE 1550-564 and GRS 1915+105, but at luminosities two orders of magnitude higher. X42.3+59, the second brightest source in M82, displayed a strikingly high flux of 1.4E-11 ergs/cm^2/s in the 2-10 keV band on 2001 May 6. A seven-year light curve of X42.3+59 shows extreme variability over a factor of 1000; the source is not detected in several Chandra observations. This transient behavior suggests accretion from an unstable disk. If the companion star is massive, as might be expected in the young stellar environment, then the compact object would likely be an IMBH.Comment: 9 pages, 6 figures, submitted to ApJ on May 08, 200