Bulgeless giant galaxies challenge our picture of galaxy formation by hierarchical clustering

Abstract

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 = λ/FWHM ≡ 15,000) to measure stellar velocity dispersions in the nuclear star clusters and (pseudo)bulges of the pure-disk galaxies M33, M101, NGC 3338, NGC 3810, NGC 6503, and NGC 6946. The dispersions range from 20 ±1 km s-1 in the nucleus of M33 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 M101, 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 Mmiddot; ≲ (2.6 ±0.5) × 106M⊙ in M101 and M · ≲ (2.0 ±0.6) × 106M⊙ in NGC 6503. (2) We show that the above galaxies contain only tiny pseudobulges that make up ≲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 Vcirc > 150 km s-1, including M101, 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 ∼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.