Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...209.0413f&link_type=abstract
2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #04.13; Bulletin of the American Astronomical Society, V
Other
Scientific paper
The global colors and structural properties of galaxies have recently been shown to follow bimodal distributions. Also, recent work has established that there are two types of bulges: “classical bulges'' that are dynamically hot systems resembling ellipticals, and “pseudobulges'', dynamically cold, flattened, disk-like structures that could not have formed via violent relaxation. At intermediate Hubble types (Sa-Sc) does the galaxy dichotomy separates galaxies at a specific bulge-to-total ratio, B/T, or does it separate galaxies of different bulge type?
Detailed surface photometry reveals that: (1) The red -blue dichotomy is a function of bulge type: at the same B/T pseudobulges are in globally blue galaxies and classical bulges are in globally red galaxies. (2) Bulge type also predicts where the galaxy lies in other dichotomous global structural parameters: global Sersic index and central surface brightness. (3) Hence, the red -blue dichotomy is not due to decreasing bulge prominence alone, and the bulge type of a galaxy carries significance for the galaxy's evolutionary history. Classical bulges indicate that a galaxy has suffered a major merger (of smaller fragments) in its past. This is more likely to have happened early, in higher-density environments. Therefore, these galaxies are likely to be red today. Pseudobulges form via cold internal evolution; such a galaxy has not suffered a major merger since its disk formed. This is more likely at later epochs, when the merger rate is lower and in low-density environments. Therefore, these are likely to be younger, blue galaxies.
Drory Niv
Fisher David B.
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