Browsing by Subject "disk-dominated galaxies"

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Bulgeless Giant Galaxies Challenge Our Picture Of Galaxy Formation By Hierarchical Clustering
(2010-11) Kormendy, John; Drory, Niv; Bender, Ralf; Cornell, Mike E.; Kormendy, John; Cornell, Mike E.
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.
The Structure Of Classical Bulges And Pseudobulges: The Link Between Pseudobulges And Sersic Index
(2008-08) Fisher, David B.; Drory, Niv; Fisher, David B.
In this paper, we study the properties of pseudobulges (bulges that appear similar to disk galaxies) and classical bulges (bulges which appear similar to E-type galaxies) in bulge-disk decompositions. We show that the distribution of bulge Sersic indices, (n)b, is bimodal, and this bimodality correlates with the morphology of the bulge. Pseudobulges have n(b) less than or similar to 2 and classical bulges have n(b) less than or similar to 2 with little to no overlap. Also, pseudobulges do not follow the correlations of Sersic index with structural parameters or the photometric projections of the fundamental plane in the same way that classical bulges and elliptical galaxies do. We find that pseudobulges are systematically flatter than classical bulges and thus more disk-like in both their morphology and shape. We do not find significant differences between different bulge morphologies which we are collectively calling pseudobulges (nuclear spirals, nuclear rings, nuclear bars, and nuclear patchiness); they appear to behave similarly in all parameter correlations. In the Sersic index, flattening, and bulge-to-total ratio, the distinction appears to be between classical bulges and pseudobulges, not between different pseudobulge morphologies. The Sersic index of the pseudobulges does not correlate with B/T, in contrast to classical bulges. Also, the half-light radius of the pseudobulge correlates with the scale length of the disk; this is not the case for classical bulges. The correlation of Sersic index and scale lengths with bulge morphology suggests that secular evolution is creating pseudobulges with low-Sersic index and that other processes (e. g., major mergers) are responsible for the higher Sersic index in classical bulges and elliptical galaxies.