|Annu. Rev. Astron. Astrophys. 2004. 42:
Copyright © 2004 by Annual Reviews. All rights reserved
3.4. Global Pattern Spirals
Our picture of global spiral structure in galaxies is by now well developed (Toomre 1977b). Global spirals are density waves that propagate through the disk. Like water waves in an ocean but unlike bars, they are not made of the same material at all times. In general, stars and gas revolve around the center faster than the spiral arms, so they catch up to the arms from behind and pass through them. Central to our understanding of why young and bright but short-lived stars are concentrated in the arms is the concept that star formation is triggered when gas passes through the arms. As in the bar case, the gas accelerates as it approaches the arms and decelerates as it leaves them. Again, shocks form where the gas piles up. This time the shocks have a spiral shape. Their observational manifestations are dust lanes located on the concave side of the spiral arms (e.g., NGC 5236 in Figure 7). The strength of the shocks can be predicted from the rotation curve: the mass determines the rotation velocity, and the central concentration determines the arm pitch angle and hence the angle at which the gas enters the arms. The results (Roberts, Roberts, & Shu 1975) provide the basis of our understanding of van den Bergh (1960a, b) luminosity classes of galaxies. More massive galaxies tend to have more differential rotation and stronger shocks, so star formation is enhanced and the arms seen in young stars are thinner and more regular.
Gas loses energy at the shocks and sinks toward the center. The effect is weaker than in barred galaxies, because the pitch angles of spiral arms are much less than 90°. The gas meets the shocks obliquely rather than head-on. Nevertheless, it must sink. Where it stalls depends on the mass distribution. In early-type spirals with big classical bulges, the spiral structure has an ILR at a large radius. The spiral arms become azimuthal at ILR and stop there. As the arm pitch angle approaches 0° and as the arm amplitude gets small, the energy loss drops to zero. The gas stalls near ILR. It may form some stars, but the bulge is already large, so the relative contribution of secular evolution is minor.
In contrast, in late-type galaxies, there is no ILR, or the ILR radius is small. The gas reaches small radii and high densities; the result is expected to be star formation. If the process is fast enough, it can build a pseudobulge. Moreover, galaxies with no ILR are late in type. They have little or no classical bulge. Therefore, secular processes can contribute a central mass concentration that we would notice in just those galaxies in which the evolution is most important.
Is the evolution rapid enough to matter? Theoretical timescales are uncertain but look interestingly short. Gnedin, Goodman, & Frei (1995) measure spiral arm torques from surface photometry of NGC 4321. They estimate that the timescale for the outward transport of angular momentum is 5 - 10 Gyr. Thus, even the stellar distribution should have evolved significantly if the spiral structure has consistently been as strong as it is now. NGC 4321 has unusually regular and high-amplitude spiral arms; weak spiral structure can easily imply angular momentum transport timescales that are an order of magnitude longer (Bertin 1983; Carlberg 1987). However, shocks speed up the sinking of gas; Carlberg (1987) estimates that it takes place on a Hubble timescale even for the weak spiral structure in his simulation. Zhang (1996, 1998, 1999, 2003) derives even shorter timescales. Apart from such disagreements, we do not know how long the spiral structure has been as we observe it, a problem that Gnedin et al. (1995) understood. Despite the uncertainties, more studies like Gnedin et al. (1995) would be valuable.
Whatever the theoretical uncertainties, observations show that star formation takes place. Timescales are discussed in Section 5. Excellent examples of nuclear star formation in unbarred galaxies can be seen in M 51 and NGC 4321 (Kormendy & Cornell 2004 show illustrations). Both galaxies have exceedingly regular global spiral structure. The spiral arms and their dust lanes wind down very close to the center, where both galaxies have bright regions of star formation. NGC 4321 is studied by Arsenault et al. (1988); Knapen et al. (1995a, b), Sakamoto et al. (1995), and García-Burillo et al. (1998). It is classified as Sc in Sandage (1961). The RC3 (de Vaucouleurs et al. 1991) classifies it SAB(s)bc; the spiral arms are distorted similar to pseudo-inner and -outer rings. There are signs of a weak bar in the infrared (see the above references and Jarrett et al. 2003). Nevertheless, NGC 4321 suggests that secular evolution can be important even in galaxies that do not show prominent bars.
Why doesn't every late-type galaxy have a pseudobulge? Calculations of spiral-arm shock strengths show that the shocks are weak if the rotation curve rises too linearly. The lowest-luminosity galaxies have little shear; it is not surprising that they do not make substantial pseudobulges.
In summary, late-type unbarred but global-pattern spirals are likely to evolve in substantially the same way as barred galaxies, only more slowly.