Neutral hydrogen studies

People involved at OAB: Sancisi, Fraternali.

I. HI halos of spiral galaxies. High velocity gas.

Sancisi, Oosterloo (NFRA) and Fraternali have completed their study of the vertical distribution and kinematics of neutral hydrogen in the nearby spiral galaxy NGC 2403. Such a study has revealed gas with anomalous kinematics (``anomalous gas") located above the plane of the galaxy and rotating $\sim 20-50$ km s$^{-1}$ more slowly than the gas in the disk. This gas has a total mass of about $3 \times 10^8$ $M_{\odot}$ ($\sim 1/10$ of the total HI mass) and shows a large-scale radial inflow ($10-20$ km s$^{-1}$) towards the centre of the galaxy.

Long-slit spectra of NGC 2403 with the WHT (La Palma) were also obtained. The slits ($3.7'$ long) were placed along the major and minor axis of the galaxy. Wings of emission at lower rotation velocities (anomalous ionised gas) similar to those observed in HI have been detected. Also this ionised anomalous gas is rotating more slowly than the regular disk and shows a weak indication of large scale inflow. Local powerful outflows of ionised gas were also found with velocities reaching up to 200 km s$^{-1}$.

Chandra observations (36 ks) of NGC 2403, obtained in April 2001, show a diffuse soft X-ray emission (0.4$-$1 keV) from the disk of the galaxy. Such emission (with a total luminosity of $\sim$ 2 $\times$ 10$^{38}$ erg s$^{-1}$ and a gas temperature of 2$-$8 $\times$ 10$^{6}$ K) is well separated from the numerous bright point sources and is probably produced by hot gas in the disk of NGC 2403.

All the above results seem to point at a galactic fountain type of phenomenon which may explain the presence and kinematics of the anomalous gas in NGC 2403.

Sancisi and Oosterloo have continued their study of high velocity HI with the Westerbork Synthesis Radiotelescope in a number of nearby spiral galaxies. One of these, NGC 4559, shows a strongly kinematically lopsided disk and the presence of anomalous HI as NGC 2403.

Sancisi, Oosterloo, T. van der Hulst and Boomsma (Kapteyn) have started a project to study the vertical motions of the gas in the nearly face-on spiral galaxy NGC 6946. Very deep (16x12 hours integration) HI observations with the upgraded WSRT are planned. These observations are also useful for the investigation of HI holes in the disk and the associated high velocity gas.

II. Luminous and Dark Matter in Galaxies.

Sancisi and Swaters (DTM, Washington DC) have continued their study of the rotation curves and of the light distribution in the central regions of spiral galaxies. The shapes of the HI and optical rotation curves have been compared with the distribution of luminous matter as inferred from photometric profiles.

Sancisi, T. S. van Albada, T. van der Hulst and E. Noordermeer have started a study of the mass distribution in bright compact Sa and Sb galaxies. These galaxies are part of the WHISP (Westerbork Survey of HI in Irregular and Spiral galaxies) sample of galaxies. They are expected to have declining rotation curves in their outer parts. This project is the continuation of the study of dark matter in spiral galaxies of different luminosities and morphological types. The radio data are complemented with $R$-band CCD photometry.