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We dissect giant Sc-Scd galaxies with Hubble Space Telescope photometry andHobby-Eberly Telescope spectroscopy. We use HET's High Resolution Spectrograph(resolution = 15,000) to measure stellar velocity dispersions in the nuclearstar clusters and pseudobulges of the pure-disk galaxies M33, M101, NGC 3338,NGC 3810, NGC 6503, and NGC 6946. We conclude: (1) Upper limits on the massesof any supermassive black holes are MBH <= (2.6+-0.5) * 10**6 M_Sun in M101 andMBH <= (2.0+-0.6) * 10**6 M_Sun in NGC 6503. (2) HST photometry shows that theabove galaxies contain tiny pseudobulges that make up <~ 3 % of the stellarmass but no classical bulges. We inventory a sphere of radius 8 Mpc centered onour Galaxy to see whether giant, pure-disk galaxies are common or rare. In thisvolume, 11 of 19 galaxies with rotation velocity > 150 km/s show no evidencefor a classical bulge. Four may contain small classical bulges that contribute5-12% of the galaxy light. Only 4 of the 19 giant galaxies are ellipticals orhave classical bulges that contribute 1/3 of the galaxy light. So pure-diskgalaxies are far from rare. It is hard to understand how they could form as thequiescent tail of a distribution of merger histories. Recognition ofpseudobulges makes the biggest problem with cold dark matter galaxy formationmore acute: How can hierarchical clustering make so many giant, pure-diskgalaxies with no evidence for merger-built bulges? This problem dependsstrongly on environment: the Virgo cluster is not a puzzle, because >2/3 of itsstellar mass is in merger remnants.

Bulgeless Giant Galaxies Challenge our Picture of Galaxy Formation by Hierarchical Clustering