Spiral Structure

Figure 3.10: $B$- (left), and $I$-band (right) images of M51. Taken from Fig. 3 of Elmegreen et al. (1989).

Figure 3.10 shows the $B$- (left) and $I$-band images of M51. $B$-band light originates from the massive early type stars. Although the image taken in $B$-band shows a number of spiral arms, that of $I$-band shows clearly two arms. The $I$-band light seems to come from mainly less-massive long-lived stars, while the $B$-band light is essentially coming from the massive short-lived stars which are formed in the spiral arm. On the contrary, the less-massive stars are not necessarily born in the spiral arm. This suggests that there are two kinds of spiral patterns: one is made by stars (mainly less-massive) and the other is the gaseous spiral arm where massive stars are born and contribute to the $B$-band image.

In this section, first, we will briefly describe the density wave theory which explains the former spiral pattern in the stellar component. You will find the amplitude of the spiral pattern in stellar component is not so large. However, the response of gaseous components (HI and H$_2$ gas) to the spiral density wave potential with small amplitude is much more nonlinear than that of the stellar component and a high-contrast spiral pattern appears in the gaseous component .