Gas rich galaxies with weak stellar spirals have more interarm gas and star formation than galaxies with strong stellar spirals. In weak-arm spiral galaxies, local swing-amplified instabilities in the gas become prominent and these can occur almost anywhere. In galaxies with strong, global stellar waves, the magneto-Jeans instability forms giant cloud complexes primarily in the spiral arms, where the density is high and the shear is low.
 |
Figure 1. Hubble Space Telescope image of NGC 4414, from multiple passbands. |
Figure 1 shows a Hubble Space Telescope image of the galaxy NGC 4414, which has numerous patches of star formation in the midst of a faint 2-arm structure. This is an example of the first type mentioned in the previous paragraph. Figure 2 shows an HST image of M51, a strong two-arm spiral with little star formation between the stellar arms. This is an example of the second type.
 |
Figure 2. Hubble Space Telescope image of NGC 5194, from multiple passbands. |
Stellar spirals define two scales,
2
G
/
2, which is
the "Toomre length"
(Toomre 1964)
separating the spirals, and
2
2 / G
, which is the Jeans
length separating the
condensations in the spirals. The Jeans length is about three times the
arm width and physically smaller inside the dense dust lanes. The beads
on a string seen in spiral arms are giant star complexes. Each has a
feather or spur of dust from a spiral wave flow downstream. This is
clearly visible in the HST image of M51, shown in
Figure 2. In
the interarm regions of M51, young stars are still in star complexes
that are aging. Lingering star formation and triggered star formation
occur in the interarm fields of cloudy debris. Further downstream, the
cloud envelopes are more diffuse but there is still a little star
formation in some of them (Fig. 3). The
molecular envelopes of GMCs must be long-lived to survive as far as they
do downstream from the arms, ~ 100 Myr or more. This seems to require
magnetic support in the cloud envelopes
(Elmegreen 2007).
Further downstream, almost at the
next spiral arm, the cloudy debris from the previous arm has little
associated star formation. There appears to be a lot of diffuse
molecular gas indicated by these interarm dust features. They coagulate
into a dust lane when they reach the next arm.
 |
Figure 3. Enlargement of the Western interarm region of M51, from the Hubble Space Telescope image. Dark dust clouds with small amounts of star formation, or no evident star formation, are seen. Some are at the edges of old OB associations and may contain triggered star formation. |
At a very basic level, a gravitational instability in a spiral arm, or in a spiral arm dust lane (Elmegreen 1979) can be viewed as an instability in a cylinder. Such instabilities occur when
 |
(5.1) |
where ยต is the mass per unit length in the cylinder and
is the velocity dispersion.
The fastest growing mode has a wavelength of about 3 times the cylinder
width
(Elmegreen &
Elmegreen 1983).
There is also enhanced star formation at the end of some strong bars. This is most likely a crowding effect from the gas that turns a corner there in its orbit relative to the bar (Lord & Kenny 1991). The inner, nearly straight, dust lanes in many bars do not contain much star formation and look non-self gravitating. This is probably because of high shear and radial tidal forces. Inside this dustlane, in the center of the bar, there is often a ring close to the inner Lindblad resonance (Buta & Combes 1996). This ring also has two characteristic scales, the thickness in the radial direction and the Jeans length. ILR rings develop major sites of star formation, or "hot-spots" along them, with a separation of around the Jeans length, probably because of local gravitational instabilities in the gas (e.g., Elmegreen 1994).