Hierarchical clustering in a cold dark matter universe (White & Rees 1978) is a remarkably successful theory of galaxy formation. The struggle now is with baryonic physics. The most serious problem has been emphasized many times, both by observers (e. g., Freeman 2000; KK04; Kormendy & Fisher 2005; Carollo et al. 2007; Kormendy 2008) and by modelers (e. g., Steinmetz & Navarro 2002; Abadi et al. 2003). Given so much merger violence, how can hierarchical clustering produce so many pure disk galaxies with no signs of merger-built bulges? This problem gets harder when we realize that many of what we used to think are small bulges are really pseudobulges made by secular evolution. We know of no Sc or later-type galaxy with a classical bulge (KK04). So the solution to the above problem is not to hope that bulgeless disks are rare enough that they can be explained as the tail of a distribution of formation histories that included a few fortuitously mergerless galaxies.
This section provides new examples and better statistics on bulgeless disks.
The bulgeless disks that most constrain our formation picture are those that live in high-mass dark halos - say, ones in which circular-orbit rotation velocities are Vcirc ~ 200 km s-1. Kormendy et al. (2009) have used the Hobby-Eberly Telescope to obtain high-resolution (instrumental dispersion instr 8 km s-1) spectroscopy of the nuclear star clusters in M 101 and NGC 6946. M 101 is an Scd galaxy with Vcirc = 210 ± 15 km s-1 (Bosma et al. 1981). But its nucleus has a velocity dispersion = 25 ± 7 km s-1 like that of a big globular cluster. NGC 6946 is a similar Scd with Vcirc = 210 ± 10 km s-1 (Tacconi & Young 1986; Sofue 1996) and = 38 ± 3 km s-1. IC 342 is a third such galaxy with Vcirc = 192 ± 5 km s-1 (Rogstad, Shostak, & Rots 1973; Sofue 1996) and = 33 ± 3 km s-1 (Böker et al. 1999; instr = 5.5 km s-1). All three galaxies show small central upturns in their JHK brightness profiles (Jarrett et al. 2003) and NGC 6946 and IC 342 also show rapid rises in their central CO rotation curves V(r) (Sofue 1996). But their small dispersions << V show that these are pseudobulges. How did these halos grow so large with no signs of major mergers?
Could bulgeless disks be rare enough to have formed as the quiescent tail of a distribution of merger histories? We believe that the answer is "no". Consider first the Local Group. Only our Galaxy has uncertainty in its bulge classification. The box-shaped structure implies a pseudobulge. The low velocity dispersion of the bulge merges seamlessly with that of the disk (Lewis & Freeman 1989). The central profile derived by Tremaine et al. (2002) implies a pseudobulge. Only the old, -element-enhanced stellar population is suggestive of a classical bulge (KK04 discusses these caveats). In agreement with Freeman (2008), we conclude that there is no photometric or dynamical evidence for a classical bulge. Then the Local Group contains one elliptical, M 32, and one classical bulge, in M 31. In the most massive three galaxies, there is only one classical bulge.
Looking beyond the Local Group, the most distant bulgeless disk discussed above is M 101. Its Cepheid distance modulus is m - M = 29.34 ± 0.10; i. e., distance = 7.4 ± 0.3 Mpc (Ferrarese et al. 2000), and Vcirc = 210 ± 15 km s-1. We will be conservative and look for all galaxies with Vcirc > 150 km s-1 or central > 106 km s-1 and m - M < 29.5. HyperLeda and Tonry et al. (2001) provide 19 such galaxies. M 101, NGC 6946, and IC 342, are 3/19 of the big galaxies in our sample volume. Of the rest, 8 are dominated by pseudobulges with no sign of a classical bulge. One more, NGC 2787, has a dominant pseudobulge but could also have a small classical bulge component. Three galaxies in the above volume are ellipticals, Maffei 1, NGC 3077 (probably), and NGC 5128. Three galaxies are known to have classical bulges, M 31, M 81, and NGC 4258. NGC 5195, the companion of M 51, has an uncertain classification but = 157 km s-1; we include it among the classical bulges. This leaves us with the following statistics: Within 8 Mpc of us, 11 of 19 galaxies with Vcirc > 150 km s-1 show no evidence for a classical bulge, one may contain both a classical bulge and a pseudobulge, and 7 of 19 are either ellipticals or contain classical bulges. Big galaxies with evidence for a major merger are less than half of the sample.
In contrast, in the Virgo cluster, about 2/3 of the stellar mass is in elliptical galaxies and some additional mass is in classical bulges (KFCB). So the above statistics are a strong function of environment.
We therefore restate the theme of this section: What is special about galaxy formation in low-density, Local-Group-like environments that allows > 1/2 of the galaxies with halo Vcirc > 150 km s-1 to form with no signs of major mergers?
Acknowledgements We thank Ralf Bender, Mark Cornell, Niv Drory and Reynier Peletier for permission to quote results before publication. Our work used the HyperLeda database at http://leda.univ-lyon1.fr/search.html. Support from the National Science Foundation under grant AST-0607490 is gratefully acknowledged.