11.7.2. Observations at the Owens Valley Radio Observatory
The two-element interferometer at the Owens Valley Radio Observatory has been used to map a number of late-type galaxies to an angular resolution of 2 minutes are and a velocity resolution of 21 km sec-1. The Owens Valley interferometer is able to track sources over only a limited hour angle range but both E-W and N-S spacings are available so that good coverage of the (u - v) plane can still be obtained. Fixed observing stations are positioned at 100-foot intervals, giving a grating sidelobe response at 20 minutes arc radius and enabling sources smaller than this to be mapped without confusion.
For larger galaxies special techniques must be used to eliminate the sidelobe response. A powerful technique, first used in the case of M101 (Rogstad and Shostak, 1971), is to iteratively subtract the synthesized beam together with its sidelobes from the maximum of the source region of the synthesized map. The iteration stops when the sources being subtracted are a small percentage of the original maximum or when the residual map approaches the noise level. A map, corrected for the sidelobe response, is then reconstituted by convolving the subtracted sources with a clean sidelobe-free beam and adding them back into the residual map.
This procedure may not result in a unique map, but there are some tests on the validity of the procedure, which is discussed in more detail in Chapter 10.
b) Integrated Hydrogen Distribution of M101
Figure 11.10 shows the integrated HI brightness of M 101 observed with a 4-minute-arc beam. Most remarkable in Figure 11.10 is the ring distribution and the marked asymmetry in the hydrogen distribution. If the gas is assumed to be optically thin, then the contour interval corresponds to an HI surface density of 1.4 atom cm-2. For an assumed distance of 6.9 Mpc the integrated HI mass is 9 × 109 M. If the HI distribution is really symmetric and the asymmetry in the integrated brightness is due to variations in the spin temperature of the gas, then this mass would be increased by 25%. Assuming that the gas is optically thin, the central depression in the integrated brightness represents a surface density of ~ 3.5 × 1020 atom cm-2. Monnet (1971) has observed a weak background of H radiation in several galaxies. In M101 the estimated plasma density is nearly. sufficient to account for the missing hydrogen, suggesting that the gas is highly ionized.
Figure 11.10. Integrated HI brightness in M101 to an angular resolution of 4 minutes arc.
c) Velocity Field and Streaming Motions in M101
The velocity field of M101 (Figure 11.11) derived from the HI observations (Rogstad and Shostak, 1971) displays many deviations of 10 ~ 20 km sec-1 from circular rotation of the gas. Many of these deviations are in the form of ridges in the vicinity of the luminous spiral arms. Rogstad (1971) has interpreted this as evidence for the density wave theory of spiral structure (see Chapter 4).
Figure 11.11. Isovelocity contours in M101.
There are further deviations in the isovelocity contours near the nucleus of M101 which could indicate expansion motions 40 km sec-1 in the vicinity of an inner spiral arm and coincident with a central nonthermal radio source. The outflow of HI is sufficient to account for the even deeper hole in the HI distribution of the nuclear region.