How general are the results of Section 2? There are four reasons why we suggest that secular evolution and pseudobulge building are important in more than the ~ 1/3 of all disk galaxies that look barred at optical wavelengths:
As emphasized at this conference, near-infrared images penetrate dust absorption and are insensitive to the low-M / L frosting of young stars in galaxy disks. They show us the old stars that trace the mass distribution. They reveal that bars are hidden in many galaxies that look unbarred in the optical (Block & Wainscoat 1991; Spillar et al. 1992; Mulchaey & Regan 1997; Mulchaey et al. 1997; Seigar & James 1998; Knapen et al. 2000; Eskridge et al. 2000, 2002; Block et al. 2001; Laurikainen & Salo 2002; Whyte et al. 2002). About two-thirds of all spiral galaxies look barred in the infrared. Measures of bar strengths based on infrared images (Buta & Block 2001; Block et al. 2001; Laurikainen & Salo 2002) should help to tell us the consequences for secular evolution.
Many unbarred galaxies are globally oval. Ovals are less elongated than bars - typical axial ratios are ~ 0.85 compared with ~ 0.2 for bars - but more of the disk mass participates in the nonaxisymmetry. Strongly oval galaxies can be recognized independently by photometric criteria (Kormendy & Norman 1979; Kormendy 1982a) and by kinematic criteria (Bosma 1981a, b). Brightness distributions: The disk consists of two nested ovals, each with a shallow surface brightness gradient interior to a sharp outer edge. The inner oval is much brighter than the outer one. The two "shelves" in the brightness distribution have different axial ratios and position angles, so they must be oval if they are coplanar. But the flatness of edge-on galaxies shows that such disks are oval, not warped. Kinematics: Velocity fields in oval disks are symmetric and regular, but (1) the kinematic major axis twists with radius, (2) the optical and kinematic major axes are different, and (3) the kinematic major and minor axes are not perpendicular. Twists in the kinematic principal axes are also seen when disks warp, but Bosma (1981a, b) points out that warps happen at larger radii and lower surface brightnesses than ovals. Also, observations (2) and (3) imply ovals, not warps. Kormendy (1982a) shows that the photometric and kinematic criteria for recognizing ovals are in excellent agreement.
Oval galaxies are expected to evolve similarly to barred galaxies. Many simulations of the response of gas to "bars" assumed that all of the potential is oval rather than that part of the potential is barred and the rest is not. NGC 4736 is a prototypical oval with strong evidence for secular evolution (Figures 3 and 6 here; Kormendy & Kennicutt 2004).
Bars commit suicide by transporting gas inward and building up the central mass concentration (Hasan & Norman 1990; Freidli & Pfenniger 1991; Friedli & Benz 1993; Hasan, Pfenniger, & Norman 1993; Norman, Sellwood, & Hasan 1996; Heller & Shlosman 1996; Berentzen et al. 1998; Sellwood & Moore 1999). Norman et al. (1996) grew a point mass at the center of an n-body disk that previously had formed a bar. As they turned on the point mass, the bar amplitude weakened. Central masses of 5 - 7% of the disk mass dissolved the bar completely. Shen & Sellwood (2004) find that central masses with small radii, like supermassive black holes, destroy bars more easily than ones with radii of several hundred parsecs, like pseudobulges. A bar can tolerate a soft central mass of 10% of the disk mass. Observations suggest that still higher central masses can be tolerated when the bar gets very nonlinear. The implication is this: Even if a disk galaxy does not currently have a bar, bar-driven secular evolution may have happened in the past.
Late-type, unbarred, but global-pattern spirals are expected to evolve like barred galaxies, only more slowly. Global spirals are density waves that propagate through the disk (Toomre 1977b). 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. As in the bar case, the gas accelerates as it approaches the arms and decelerates as it leaves them. Again, the results are shocks where the gas piles up. This time the shocks have a spiral shape. They are identified with the dust lanes on the concave side of the spiral arms (Figure 4). Gas dissipates at the shocks, but it does so more weakly than in barred galaxies, because the gas meets the shock obliquely. Nevertheless, it sinks. In early-type spirals with big classical bulges, the spiral structure stops at an ILR at a large radius. The gas may form some stars there, but since the bulge is already large, the relative contribution of secular evolution is likely to be minor. In late-type galaxies, the spiral structure extends close to the center. Sinking gas reaches small radii and high densities. We suggest that star formation then contributes to the building of pseudobulges. Moreover, late-type galaxies have no classical bulges. So secular growth of pseudobulges can most easily contribute a noticeable part of the central mass concentration precisely in the galaxies where the evolution is most important.
M 51 and NGC 4321 (Figure 4) are examples of nuclear star formation in unbarred galaxies. Their exceedingly regular spiral structure and associated dust lanes wind down close to the center, where both galaxies have bright regions of star formation (e. g., Knapen et al. 1995a, b; Sakamoto et al. 1995; Garcia-Burillo et al. 1998). They are examples of secular evolution in galaxies that do not show prominent bars or ovals.
 |
Figure 4. Nuclear star formation in the unbarred galaxies M 51 and NGC 4321 (M 100). Dust lanes on the trailing side of the global spiral arms reach in to small radii. As in barred spirals, they are are indicative of gas inflow. Both galaxies have concentrations of star formation near their centers that resemble those in Figure 3. These images are from the Hubble Space Telescope and are reproduced here courtesy of STScI. |