11.4.4. Galaxies with Smaller Angular Diameters

With more distant galaxies, where the beam size is comparable with the angular diameter of the galaxy, it is still possible to measure the line profile and estimate the mass of neutral hydrogen. The velocity width of the profile allows an estimate of the rotation or random velocities within the galaxy and, with the usual assumption of gravitational equilibrium, the mass of the galaxy may be estimated.

When the hydrogen distribution subtends 2 or 3 beamwidths, an estimate of the large-scale HI distribution may be made and compared with the optical features. An example of this sort of observation is the work done by the Meudon group with the Nançay radio-telescope. This instrument has a beamwidth of 4 minutes arc E-W by 24 minutes arc N-S at 21-cm, which allows an estimate of the E-W hydrogen distribution. Using this instrument Bottinelli (1971) finds that the hydrogen distribution is asymmetrical with respect to the optical distribution in about 40% of the galaxies observed. The ratio of the neutral hydrogen diameter to the optical diameter [measured on the Holmberg (1958) system] is observed to be a function of the morphological type, increasing toward the later-type galaxies. This characteristic gives a mean HI surface density independent of the galactic type. Details of the neutral hydrogen distribution may be deduced through a model-fitting procedure, and Bottinelli finds that a ring-like model for the HI distribution (as in M31) is consistent with about 30% of the galaxies observed and that the neutral hydrogen seems to be more strongly concentrated toward the center in the early-type galaxies.