Involved people at OAB: Sancisi and Fraternali.
Sancisi is continuing a number of neutral hydrogen studies, which he initiated while still at the University of Groningen.
The recent 21-cm line study of NGC 891 by Swaters, Sancisi and Van der Hulst (ApJ 491,140, 1997) has shown the presence of an extended HI halo which seems to rotate more slowly than the disk. Schaap, Sancisi and Swaters (Astron. Astrophys. 356, L49, 2000) have just completed a similar study of the vertical distribution and the kinematics of HI in the spiral galaxy NGC 2403 and have reached for this galaxy conclusions similar to those obtained for NGC 891. They have used 21-cm line observations with the Westerbork Synthesis Radio Telescope and have discovered a remarkably asymmetric structure, a tail towards the systemic velocity -the 'beard'- in the 21-cm line profiles. They have interpreted this as evidence for the presence of gas in the halo of NGC 2403. The 3-D models indicate that this gas has a somewhat lower (about 25 km/s) rotational velocity than the gas in the plane. A vertical gradient in the rotational velocity may mean a gradient in the gravitational potential and serve, therefore, to investigate the vertical distribution of dark matter.
As a follow-up of this study, new observations of NGC 2403 with higher sensitivity and resolution have been obtained with the VLA by Sancisi, Oosterloo and van Moorsel. A PhD student at the Bologna Astronomy department, Filippo Fraternali, has joined the team for the data reduction and in particular for the model analysis of the 3-D density distribution and kinematics. The preliminary results confirm the conclusions of Schaap et al. regarding the presence of a slowly rotating HI halo around NGC 2403. In addition, these new VLA data also indicate that the halo HI has radial motions (probably an overall inflow of order 25 km/s) and other peculiar high vertical motions as well. These striking new results present a real puzzle as to the origin of the halo gas: a galactic fountain or infall of primeval intergalactic gas?
Swaters, Sancisi, Van Albada and Van der Hulst have studied the dark matter content of about 80 late-type dwarf galaxies which are part of the WHISP sample (see PhD thesis of Swaters, Groningen University, 1999). High quality HI rotation curves have been derived for a subsample of 40 objects. The derived rotation curves are found to be more steeply rising than hitherto found for dwarf galaxies. Mass modeling indicates that these dwarf galaxies do not need large amounts of dark matter in the inner parts. At larger radii dark matter is still needed to explain the shape of the rotation curves.
Swaters, Schoenmakers, Sancisi and Van Albada (MNRAS 304, 330, 1999) have studied the kinematical lopsidedness of spiral galaxies. They have pointed out the frequency of occurrence of the phenomenon and discussed the possible relationship with asymmetries in the potential.
M. Kregel (Kapteyn Institute, Groningen) and Sancisi have completed their study of the HI structure and kinematics of the peculiar starburst galaxy NGC 3310 (UGC 5786). The optical image of this galaxy (see Arp 217) is dominated in the outer parts by the so-called 'bow-and-arrow' structure. Kregel and Sancisi have found new evidence bearing on the origin of the starburst. The main bulk of the HI coincides with the bright optical disk and shows differential rotation like in normal spiral galaxies. But its velocity dispersion is unusually large ( 20 km/s) for a spiral galaxy, indicating that the disk is highly perturbed. There are, in addition, two prominent HI tails, one extending to the north-west and the other to the south. The newly found HI structures and perturbed kinematics and the already known optical peculiarities strongly suggest a recent major merger between two gas-rich galaxies. NGC 3310 may be the example of a major merger in which most of the gas in the inner parts has been preserved in neutral atomic form and either one of the progenitor disks has survived or a new disk has formed.
Balcells (I.A.C., Tenerife), Van Gorkom (Columbia University) and Sancisi have studied the gas distribution and kinematics in the recent merger NGC 3656, a perturbed elliptical galaxy. They have followed up with new 21-cm observations with the VLA an earlier HI study with the WSRT by Balcells and Sancisi (AJ. 111, 1053, 1996). The earlier detection of HI is confirmed and the new, higher sensitivity observations have revealed a wealth of new features in the HI distribution. In the inner parts the HI coincides with the dust lane and shows rapid rotation like a warped disk seen edge-on. Further out, it follows the prominent optical shell-like structure to the south. In addition, the new data reveal extended HI tails in the outer parts and the presence of nearby HI-rich companions supporting the earlier suggestion of a possible galaxy merger.
NGC 3656 is a useful laboratory to study the evolution of neutral gas in early-type galaxies. NGC 3656 is reminiscent of Cen-A for its warped HI configuration in the inner parts. The presence, however, of a prominent optical-HI shell, or disrupted satellite, to the south suggests that NGC 3656 is in an accretion state earlier than that of Cen-A.