BULGELESS GIANT GALAXIES CHALLENGE OUR PICTURE OF GALAXY FORMATION BY HIERARCHICAL CLUSTERING

To better understand the prevalence of bulgeless galaxies in the nearby field, we dissect giant Sc-Scd galaxies with Hubble Space Telescope (HST) photometry and Hobby-Eberly Telescope (HET) spectroscopy. We use the HET High Resolution Spectrograph (resolution R equivalent to lambda/FWHM similar or equal to 15,000) to measure stellar velocity dispersions in the nuclear star clusters and (pseudo) bulges of the pure-disk galaxies M 33, M 101, NGC 3338, NGC 3810, NGC 6503, and NGC 6946. The dispersions range from 20 +/- 1 km s(-1) in the nucleus of M 33 to 78 +/- 2 km s(-1) in the pseudobulge of NGC 3338. We use HST archive images to measure the brightness profiles of the nuclei and (pseudo) bulges in M 101, NGC 6503, and NGC 6946 and hence to estimate their masses. The results imply small mass-to-light ratios consistent with young stellar populations. These observations lead to two conclusions. (1) Upper limits on the masses of any supermassive black holes are M-center dot less than or similar to (2.6 +/- 0.5) x 10(6) M-circle dot in M 101 and M-center dot less than or similar to (2.0 +/- 0.6) x 10(6) M-circle dot in NGC 6503. (2) We show that the above galaxies contain only tiny pseudobulges that make up less than or similar to 3% of the stellar mass. This provides the strongest constraints to date on the lack of classical bulges in the biggest pure-disk galaxies. We inventory the galaxies in a sphere of radius 8 Mpc centered on our Galaxy to see whether giant, pure-disk galaxies are common or rare. We find that at least 11 of 19 galaxies with V-circ > 150 km s(-1), including M 101, NGC 6946, IC 342, and our Galaxy, show no evidence for a classical bulge. Four may contain small classical bulges that contribute 5%-12% of the light of the galaxy. Only four of the 19 giant galaxies are ellipticals or have classical bulges that contribute similar to 1/3 of the galaxy light. We conclude that pure-disk galaxies are far from rare. It is hard to understand how bulgeless galaxies could form as the quiescent tail of a distribution of merger histories. Recognition of pseudobulges makes the biggest problem with cold dark matter galaxy formation more acute: How can hierarchical clustering make so many giant, pure-disk galaxies with no evidence for merger-built bulges? Finally, we emphasize that this problem is a strong function of environment: the Virgo cluster is not a puzzle, because more than 2/3 of its stellar mass is in merger remnants.