A relation between circumnuclear HI, dust, and optical cores in low-power radio galaxies

From new observations and literature data we investigate the presence of HI, dust, and optical cores in the central kiloparsec of low-power radio galaxies. The goal of this pilot study is to identify physical relations between these components, which can help us to study kinematics and feeding mechanisms in future samples of active galaxies. Our results are consistent with neutral gas being associated with dust on sub-kiloparsec scales. Objects that have HI absorption always have significant amounts of dust in their host galaxy. If there is no visible dust in the host galaxy, there is also no HI absorption. The presence of an unresolved optical core correlates with the HI column density, with the core being absent in high column density sources. This work opens a path for studying the kinematics of cold material in the central regions of active galaxies by combining information of HI absorption and molecular lines. Consistent with previous work, we find no evidence for a compact, parsec-scale obscuring torus in low-power radio galaxies.Comment: Accepted for publication in A&

HI on large and small scales in starburst radio galaxies

The study of the optical continuum of radio galaxies shows that about 30% have a young stellar population component. Among them are the most far-IR bright radio galaxies. A further indication of the relatively gas rich environment of these galaxies (possibly related to the recent merger from which they originate) is the high fraction being detected in HI. Here we present recent results obtained from the study of neutral hydrogen (detected either in emission or absorption) in a group of starburst radio galaxies. In some objects, large-scale (tens of kpc) structures involving HI masses exceeding 10^9 M_sun are observed. In these cases, the HI can be used to study the origin and evolution of these systems and the timescales involved. In this respect, the parameters obtained from the study of the stellar populations and from the HI can be complementary. In other objects, very broad (> 1000 km/s), mostly blueshifted HI is detected in absorption. This result shows that, despite the extremely energetic phenomena occurring near an AGN - including the powerful radio jet - some of the outflowing gas remains, or becomes again, neutral. This can give new and important insights in the physical conditions of the gaseous medium around an AGN. The possible origin of the extreme kinematics is discussed.Comment: Invited review to appear in the proceedings of the conference "Neutral ISM in Starburst Galaxies", eds. S.Aalto, S.Huttemeister & A.Pedlar. 12 pages, Figs.2,4,5 are in separate gif files. The full paper with high resolution images can be downloaded from http://www.nfra.nl/~morganti/Papers/starburst.ps.g

A new, kinematically anomalous HI component in the spiral galaxy NGC 2403

We discuss new, high sensitivity HI observations of the spiral galaxy NGC2403 which show extended emission at anomalous velocities with respect to the `cold' disk. This `anomalous' gas component (~1/10 of the total HI mass) is probably located in the region of the halo and rotates more slowly (~20-50 km/s) than the gas in the disk. Moreover, it shows a distortion in the velocity field that we interpret as a large-scale radial motion (10-20 km/s inflow) towards the centre of the galaxy. The most likely explanation for its origin and kinematics seems to be that of a galactic fountain. There is, however, a significant part of the anomalous gas which seems to be moving contrary to rotation and is difficult to understand in such a picture. These anomalous gas complexes discovered in NGC 2403 may be analogous to the High Velocity Clouds of our Galaxy. They may be rather common in spiral galaxies and not have been detected yet for lack of sensitivity.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter

Aperture Synthesis Observations of the Nearby Spiral NGC 6503: Modeling the Thin and Thick HI Disks

We present sensitive aperture synthesis observations of the nearby, late-type spiral galaxy NGC 6503, and produce HI maps of considerably higher quality than previous observations by van Moorsel & Wells (1985). We find that the velocity field, while remarkably regular, contains clear evidence for irregularities. The HI is distributed over an area much larger than the optical image of the galaxy, with spiral features in the outer parts and localized holes within the HI distribution. The absence of absorption towards the nearby quasar 1748+700 yields an upper limit of 5 10^{17} cm^{-2} for the column density of cold HI gas along a line of sight which should intersect the disk at a radius of 29 kpc. This suggests that the radial extent of the HI disk is not much larger than that which we trace in HI emission (23 kpc). The observed HI distribution is inconsistent with models of a single thin or thick disk. Instead, the data require a model containing a thin disk plus a thicker low column-density HI layer that rotates more slowly than the thin disk and that extends only to approximately the optical radius. This suggests that the presence of extra-planar gas in this galaxy is largely the result of star formation in the disk rather than cold gas accretion. Improved techniques for interferometric imaging including multi-scale Clean that were used in this work are also described.Comment: 29 pages, 17 figures, for actual figures see http://www.aoc.nrao.edu/~egreisen/NGC6503.pdf submitted following referee report to A

Neutral hydrogen observations of binary galaxies

In dit proefschrift worden melkwegstelsels die voorkomen in paren bestudeerd met behulp van de Westerbork Synthese Radio Telescoop (WSRT) In de 21 cm lijn van neutrale waterstof (HI) on uit de bewegingen van beide stelsels in zo'n paar inzicht te verkrijgen over de aanwezigheid van donkere materie. Immers, voor de bewegingen van een paar is alle materie over een afstand gelijk aan de onderlinge separatie verantwoordelijk, en niet alleen die materie die overeenkomt met het gebled waarover optische of radio-emissie optreedt. ... Zie: Samenvatting