People involved at OAB: Sancisi.
High velocity gas and HI halos of spiral galaxies
R. Boomsma (PhD thesis, Groningen), R. Sancisi, T. van der Hulst (Univ. Groningen), F. Fraternali and T. Oosterloo (ASTRON, Dwingeloo) have continued the study of the vertical structure and kinematics of HI gas in spiral galaxies. Two galaxies have been studied in detail: NGC 6946 and NGC 253. In the direction of the inner disk of NGC 6946 large complexes of high-velocity HI have been detected as well as many HI holes. Both are believed to be caused by the active star formation taking place in the disk. HI holes and high-velocity gas complexes have also been found in the outer parts of NGC 6946, where star formation does not seem to be playing a significant role. These may, therefore, have been produced by other mechanisms such as the collision with infalling gas clouds. Also the starburst galaxy NGC 253 shows HI peculiar features and plumes of gas which apparently extend vertically up to 8 kpc from the plane. This may be related to the starburst taking place in the central regions of the galaxy.
C. Barbieri (Tesi di Laurea, Univ. Milano), F. Fraternali and T. Oosterloo
(ASTRON, Dwingeloo), R. Sancisi, and G. Bertin (Univ. Milano) have studied
the distribution of mass and the presence of HI (anomalous velocities) in the
halo of the spiral galaxy NGC 4559.
They have used HI line observations obtained with the Westerbork
radiotelescope. The HI picture obtained from these observations is similar to
that of NGC 2403 as derived by Fraternali et al. in recent years.
NGC 4559 is kinematically lopsided and shows an asymmetric distribution also
in the halo gas.
Galaxy Interactions, accretion, minor mergers
T. van der Hulst (Univ. Groningen) and R. Sancisi have used new HI
observations with the Westerbork Radiotelescope to study the tidal
interaction and gas accretion in the spiral galaxies NGC 3359
and NGC 4565. These galaxies show HI bridges and tails, clear signs
of ongoing tidal interactions with small companions. These are believed to be
phenomena similar to the Magellanic Stream and the stellar streamers
(e.g. Sgr) discovered in the halo of our galaxy.
Luminous and dark matter in spiral galaxies
G. Battaglia, F. Fraternali, T. Oosterloo (ASTRON, Dwingeloo) and R. Sancisi
have completed their study of the HI distribution and dynamics of NGC 5055.
This galaxy shows a very large, symmetric warp and a decline of 25 km s
in the rotation curve, both occurring at about R
. A firm lower limit is
derived for the disk mass-to-light ratio in a model with an isothermal halo.
This galaxy also shows a peculiar dynamical behaviour: at about R
the systemic velocity changes. A striking geometrical and kinematical
symmetrization of the system is obtained by allowing the centre and
the systemic velocity to vary.
Furthermore, also in NGC 5055 the observations show the presence of
HI with anomalous kinematics.
E. Noordermeer (PhD thesis, Groningen), R. Sancisi, T. van der Hulst, T.S. van Albada (Univ. Groningen) and R. Swaters (JHU, Baltimore) have continued the study of early-type disk galaxies (making use of the WHISP database, morphological types Sa and Sb). Rotation curves are derived from the HI data and also from optical spectra. They are found to rise quite steeply in the centre and often to decline significantly in the outer parts. The more recent part of this work consists of the modelling of the contribution of the different mass components in these galaxies to the observed rotation curves. The stellar light distribution is decomposed in a spheroidal bulge and a flat disk component. The stellar disk is needed to explain the rotation curve at intermediate radii, whereas the rotation velocities in the outer regions can only be explained by the presence of a substantial amount of dark matter. The neutral gas is found to have little or no dynamical impact in these systems. There is a degeneracy between the contributions of the stellar disk and the dark matter, but these early-type disks are certainly not as dark matter dominated as most late-type and dwarf galaxies.
R. Sancisi and R. Swaters (JHU, Baltimore) have continued their study of the luminous and dark matter in the central parts of spiral galaxies. Sancisi (see contribution at IAU Symp. 220, Sydney, 2003) has emphasized the close coupling between the luminous matter and the dark matter in the central regions of spiral galaxies (including LSBs and low-luminosity systems).