3.1.3. Radio 21-cm H I line emission
NGC 1365 has been observed in the 21-cm radio emission line of interstellar neutral atomic hydrogen (H I ) in various single dish radio surveys, and references to these are found in the compilation by Huchtmeier and Richter (1989).
The galaxy has been mapped in the 21-cm line in extensive observations with the VLA. Ondrechen and van der Hulst (1989) mapped the galaxy using the compact C (3 km) array. The H I was found concentrated to the spiral arms but no gas was detected in the bar itself. Deviations from circular motions were found just outside the bar and in the spiral arms. In order to profoundly increase the sensitivity and resolution Jörsäter and van Moorsel (1995) reobserved the galaxy in the H I line using hybrid BnA, CnB and DnC configurations, where the spacings between the antennae varied up to 35 km. In the reduction the previous observations by Ondrechen and van der Hulst were added.
The total hydrogen map published by Jörsäter and van Moorsel is reproduced in Fig. 8. Again we see the prominent spiral structure, now outlined by the H I gas. However, a comparison with the optical picture of Fig. 1 immediately reveals the following differences:
A comparison between the H I and the optical light distributions is seen in Fig. 9. The shift of the H I to the outer parts of the galaxy, the lack of H I emission in the center, and a sharp decline at the outer edge, where also the optical surface brightness shows a steep drop, are clearly seen.
Figure 9. The number of H I atoms per cm2 (solid line) and averaged B surface brightness in magnitudes per square arcsec as a function of radius (dotted line) in NGC 1365. From Jörsäter & van Moorsel (1995).
The total flux of 21-cm line radiation obtained by Jörsäter and van Moorsel was 162 Jy km s-1 in agreement with single dish observations (Fisher & Tully 1981; Reif et al. 1982; Davies et al. 1989). Assuming an optically thin medium Jörsäter and van Moorsel derived a total H I mass of 1.3 x 1010 M (corrected to a distance of 18.6 Mpc).
Jörsäter and van Moorsel claim that the plane of the galaxy is warped in its outer part. We will discuss this in Section 3.5.
The nuclear region of NGC 1365 has been mapped by Sandqvist and co-workers in a number of CO line transitions (Sandqvist et al. 1988, 1995; Kristen et al. 1998; Sandqvist 1999) and was observed in the H92 radio recombination line by Anantharamaiah et al. (1993). We will postpone the discussion of these observations until Section 4.5.