11.7.3. Comparison of the HI Distributions of Late-Type Galaxies
Several other galaxies have recently been mapped at the Owens Valley. NGC 6946 and IC 342 (Rogstad et al. 1973) ; NGC 2403, NGC 4236, (Shostak and Rogstad, 1973), and IC 10 (Shostak, 1974); M51, M81, and M82 (Weliachew and Gottesman, 1973); and Mafei 2 (Wright and Seielstad, 1973); IC 2574, and NGC 7640 (Seielstad and Wright, 1973). These observations have an angular resolution of 2 minutes arc and a velocity resolution of 10 or 21 km sec-1.
NGC 2403 is an Sc- (Holmberg system) galaxy and is very similar in morphological form and stellar content to M33 [see the Hubble Atlas of Galaxies (Sandage, 1961)]. The integrated brightness distribution has a marked depression in the central region, similar to that in M101 but quite different from the HI distribution in M33. The three highest peaks in the integrated HI brightness distribution do not correspond well with the luminous spiral features, but rather lie in the dust lanes between the luminous spiral arms. A number of bright HII regions can be seen around the edges of these HI peaks. This can also be seen in places in M33. The rotation curve, as in M33, is very steep at the center and has a large constant-velocity region extending to the boundary of the detected hydrogen (as suggested in Section 11.5).
NGC 4236 is classified Sc+ by Holmberg (1958) and SB 7 by de Vaucouleurs (1964). A number of hydrogen concentrations are found, the largest peak in the north corresponding to a bright HII complex at the end of the "bar." The HII regions in the south, on the other hand, tend to lie around the HI peaks. All the rotation curves of NGC 4236, NGC 2403, and M33 have the same characteristic shape if scaled to common rotation curve maxima, Rmax and Vmax,
IC 10 is an irregular galaxy in a heavily obscured region of the sky. The integrated HI distribution shows a number of peaks and a large depression where the integrated brightness falls to zero. The peak brightnesses of all three galaxies are very similar ( 1200°K km sec-1), although the inclinations are different.
b) Five Scd Galaxies
In a comparison of the five Scd galaxies, M33, NGC 2403, IC 342, M101, and NGC 6946, Rogstad and Shostak (1972) find sub stantial similarities in the hydrogen distributions and rotation curves. The integral HI parameters are nearly constant or scale with the Holmberg (1958) diameter. Eighty percent of the observed HI mass lies within the Holmberg diameter for each galaxy. The mean hydrogen content is 11% of the total mass within this same boundary, and the central HI surface density is also nearly constant ( 8 × 1020 atom cm-2).
All the rotation curves are observed to be flat-topped, with a maximum velocity proportional to the Holmberg diameter. Because of the flat-topped rotation curves, the total masses extrapolated to infinite radius are indeterminate, but the total mass within the Holmberg radius is proportional to RH03 Vmax2, and therefore scales as RH03 for these five galaxies. The constancy of the integral properties within one morphological class is an important justification for the use of the morphological classification system for spiral galaxies.