Dr Tom Oosterloo (ASTRON, Dwingeloo, The Netherlands)

HI halos in external galaxies - Dr Tom Oosterloo Colloquium

ATNF Marsfield Lecture Theatre

Abstract

I will discuss deep HI observations of a small number of nearby spiral
galaxies recently done with the WSRT, the VLA and the ATCA. The observations
show that these galaxies have significant amounts of neutral hydrogen in their
halos. Up to 10% of the total HI mass is found in the halo. While most of the
halo HI is in a disk a few kpc thick, it can reach heights above the disk of
up to 15 kpc. The overall kinematics of most of the halo HI is regular. The
rotation of the halo HI is 20-50 km/s lower than that of the HI in the galaxy
disk, while it also shows a small radial inward motion of about 15
km/s. Locally, large vertical motions of more than 200 km/s are observed. The
most likely explanation for these HI halos is that they correspond to the
cooling, inflow phase of galactic fountains that are driven by star formation
in the disk.

Apart from the halo HI related to star formation, large HI complexes of 10^7
Msol or more are detected in the halo of a number of galaxies. They may be
extragalactic analogues of the larger HVC complexes seen in our galaxy. They
are most likely remnants of small companions accreted by the main galaxy.