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 \equiv \lambda / { FWHM } \simeq 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 \pm 1 km s ^ { -1 } in the nucleus of M 33 to 78 \pm 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 ( BHs ) are M _ { \bullet } \lesssim ( 2.6 \pm 0.5 ) \times 10 ^ { 6 } M _ { \odot } in M 101 and M _ { \bullet } \lesssim ( 2.0 \pm 0.6 ) \times 10 ^ { 6 } M _ { \odot } in NGC 6503 .

( 2 ) We show that the above galaxies contain only tiny pseudobulges that make up \lesssim 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 \sim 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 .