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Title

The Millennium Galaxy Catalogue: galaxy bimodality

Author(s)

Driver, Simon P.;  Liske, Jochen;  Graham, Alister W.

Abstract

Galaxy bimodality is caused by the bulge-disc nature of galaxies as opposed to two distinct galaxy classes. This is evident in the colour-structure plane which clearly shows that elliptical galaxies (bulge-only) lie in the red compact peak and late-type spiral galaxies (disc-dominated) lie in the blue diffuse peak. Early-type spirals (bulge plus disc systems) sprawl across both peaks. However after bulge-disc decomposition the bulges of early-type spirals lie exclusively in the red compact peak and their discs in the blue diffuse peak (exceptions exist but are rare, e.g., dust reddened edge-on discs and blue pseudo-bulges). Movement between these two peaks is not trivial because whilst switching off star-formation can transform colours from blue to red, modifying the orbits of ~1 billion stars from a planar diffuse structure to a triaxial compact structure is problematic (essentially requiring an equal mass merger). We propose that the most plausible explanation for the dual structure of galaxies is that galaxy formation proceeds in two stages. First an initial collapse phase (forming a centrally concentrated core and black hole), followed by splashback, infall and accretion (forming a planar rotating disc). Dwarf systems could perhaps follow the same scenario but the lack of low luminosity bulge-disc systems would imply that the two components must rapidly blend to form a single flattened spheroidal system.

Publication type

Conference paper

Research centre

Swinburne University of Technology. Faculty of Information and Communication Technologies. Centre for Astrophysics and Supercomputing

Source

Galaxy evolution across the Hubble time: Proceedings of the IAU Symposium no. 235, Prague, Czech Republic, 14-17 August 2006 / Françoise Combes and Jan Palous (eds.), Vol. 2, pp. 17-18

Publication year

2007

Keyword(s)

Dwarf galaxies;  Early-type galaxies;  Galaxy bimodality;  Galaxy evolution;  Galaxy formation;  Spiral galaxies

Publisher

Cambridge University Press

ISSN

1743-9213 (series ISSN)

ISBN

9780521863445, 0521863449

Publisher URL

http://dx.doi.org/10.1017/S1743921306004960

Copyright

Copyright © International Astronomical Union 2007. Published version of the paper reproduced here in accordance with the copyright policy of the publisher.

Full text

Peer reviewed