11.4.3. Nearby Spiral Galaxies
Although the Magellanic Clouds are closest and allow the most detailed comparison of optical and radio features, it is important to also observe the external spiral galaxies which more closely resemble the Milky Way. Our galaxy is thought to have a morphological form somewhere between that of M31 and M33 (see Burke, 1967). Both of these galaxies have been mapped with 10 minutes arc resolution, equivalent to 2 kpc at a distance of 680 kpc. At this resolution the most prominent feature of the HI distribution of M31 is a deficiency of hydrogen in the central regions, with the peak hydrogen distribution in the form of a broad ring at a radius of 50 minutes are (Roberts, 1966; Gottesman and Davies, 1970). The OB star associations, HII regions, and radio continuum emission are closely correlated with the ridge of neutral hydrogen, all of which suggests that star formation is proceeding at maximum rate where the neutral hydrogen density is highest (see Section 11.7.5). There are several possibilities for the deficit of neutral hydrogen in the central regions of M31 (a feature it has in common with our galaxy and several other external galaxies; see Section 11.7.4).
A rotation curve has been derived out to 150 minutes arc from the center of M31. The computed mass distribution gives an almost constant M/L (mass-to-light) ratio (11.9M / L) over the entire galaxy. The total mass within 150 minutes arc is some 2 × 1011 M, giving M31 a fractional hydrogen content of 2% - typical for an Sb galaxy. There is a marked asymmetry in the isovelocity contours of adjacent quadrants of the galaxy, and the rotation curves derived separately for the north and south of the galaxy differ in the central regions by some 30 km sec-1. These asymmetries have been interpreted as a tilt in the plane of the galaxy of some 10° and could be caused by the tidal effect of the companion galaxies M32 and NGC 205 in a similar way to the suggested influence of the Magellanic Clouds on the Milky Way (Avner and King, 1967).
M33 is an Sc galaxy and has the second largest angular size in the Northern Hemisphere. At an assumed distance of 690 kpc it lies some 190 kc from M31. Observations with a 10-minutes-arc resolution (Gordon, 1971) reveal a neutral hydrogen distribution with the same overall dimensions as the Holmberg optical size (83 × 53 minutes arc). The neutral hydrogen distribution is asymmetric, with a major concentration in the south-preceding quadrant and a 10% central depression. At the extreme ends of the major axis are two companions, or wings, of the galaxy which have a very different position angle to that of the main body of the galaxy. These wings contain 10% of the total HI mass. The computed rotation curve is rather flat-topped, and the derived total mass within 60 minutes arc of the center is 2.3 × 1010 M. The neutral hydrogen mass is some 7% of this total. The velocities in the wings do not follow those predicted by the rotation curve, and a consistent interpretation is that the wings are stable gaseous companions gravitationally bound to M33 rather like the Magellanic Clouds are to our galaxy. M33 is discussed in greater detail in Section 11.7.1.