Search

VLA observations of large-scale HI and OH absorption in the merging galaxy of NGC6240 are presented with 1 arcsec resolution. HI absorption is found across large areas of the extended radio continuum structure with a strong concentration towards the double-nucleus. The OH absorption is confined to the nuclear region. The HI and OH observations identify fractions of the gas disks of the two galaxies and confirm the presence of central gas accumulation between the nuclei. The data clearly identify the nucleus of the southern galaxy as the origin of the symmetric superwind outflow and also reveal blue-shifted components resulting from a nuclear starburst. Various absorption components are associated with large-scale dynamics of the system including a foreground dust lane crossing the radio structure in the northwest region., Comment: To be published in the ApJ, Vol 661, May 20, 2007, 32 pages, 11 figures

Radio observations using very long baseline interferometry (VLBI) and the Westerbork interferometer have been carried out to study the hydroxyl Megamaser emission in Mrk~273 at different spatial resolutions. Line and continuum observations were carried out by the European VLBI network (EVN) at 1.6 GHz and display a number of distinct structural components in the central arcsec^2 region. The observed continuum emission shows three prominent regions with both flat and steep spectral indexes. The hydroxyl (OH) emission detected by the EVN measurements accounts for only 12 percent of the total OH emission in Mrk 273, but it does show the same dominant 1667 MHz line emission components as the WSRT observations. The spatial distribution of the maser emission provides a high resolution view of the molecular environment in the nuclear region. The OH emission has only been detected toward a distinct radio source in the northern nucleus with a spatial extent of 108 pc. The OH emission is only partially superposed on the radio continuum and is associated with the near-infrared emission source. The low pump efficiency of the maser and the OH main-line ratio suggest that this emission originates in an optically thin and unsaturated maser environment with a complex pumping scheme that cannot be explained by radiative infrared pumping with a single dust temperature. The specific line emission pattern and the line-of-sight velocities indicate the organized structure of an edge-on disk/TORUS with a Keplerian rotation surrounding a central object with a binding mass of (1.39+-0.16)x10^9 M_sun., Comment: 11 pages, including 8 Figures, accepted for publication in A & A 2004 Vol 419 p. 887

Water vapor maser emission in the 6_{16}-5_{23} transition towards the narrow-line Seyfert 1 (NLS1) galaxy NGC 4051 has been discovered during an ongoing single-dish extragalactic water maser survey. The Doppler-shifted maser components appear to bracket maser components lying near the systemic velocity of the galaxy symmetrically. The tentative result of a Very Large Array (VLA) snapshot observation is that the masers are confined within 0.1 arcsec (5 pc at a distance D = 9.7 Mpc) of the radio continuum peak seen at 8.4 GHz. The low luminosity of the maser (~2 Lsun) is not typical for masers that coincide with the radio continuum nucleus and appear associated with Active Galactic Nucleus (AGN) activity. A low-luminosity maser in a Type 1 Seyfert nucleus could be explained by a low maser gain resulting from the lower inclination of an obscuring disk around an active nucleus., Comment: 5 pages, 1 figure, to appear in Mon.Not.Roy.Astron.Soc. Volume 344, Oct 1, 2003

Here we report on observations of powerful hydroxyl (OH) line emissions that trace the obscuring material within the circumnuclear environment of the galaxy Markarian 231. The hydroxyl (mega)-maser emission shows the characteristics of a rotating, dusty, molecular torus (or thick disk) located between 30 and 100 pc from the central engine. We now have a clear view of the physical conditions, the kinematics and the spatial structure of this material on intermediate size scales, confirming the main tenets of unification models., Comment: 10 pages, including 3 Figures, published in Nature Vol 421 2003; the published pdf--file and higher quality images are available at http://www.astro.rug.nl/~hrkloeck/np/pubmrk231.html