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The Millennium Galaxy Catalogue: Galaxy Bimodality

Published online by Cambridge University Press:  01 August 2006

Simon P. Driver
Affiliation:
School of Physics and Astronomy, University of St Andrews, St Andrews, Scotland email: spd3@st-and.ac.uk
Jochen Liske
Affiliation:
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
Alister W. Graham
Affiliation:
Astrophysics and Supercomputing, Swinburne University of Technology, Australia
Corresponding
E-mail address:
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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 coule 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.

Type
Contributed Papers
Copyright
Copyright © International Astronomical Union 2007

References

Allen, P., 2006, MNRAS, 371, 2 (astro-ph/0605699)CrossRefGoogle Scholar
Abazajian, K., 2003, AJ, 126, 2081CrossRefGoogle Scholar
Battaglia, G., 2006, A&A, in press, astro-ph/0608370Google Scholar
Cross, N.J.G., 2004, MNRAS, 349, 576CrossRefGoogle Scholar
Driver, S.P., Liske, J., Cross, N.J.G., De Propris, R., & Allen, P.D., 2005, MNRAS, 360, 81CrossRefGoogle Scholar
Driver, S.P., 2006, MNRAS, 368, 414CrossRefGoogle Scholar
Graham, A.W., & Driver, S.P., 2005, PASA, 22, 118CrossRefGoogle Scholar
Liske, J., Lemon, D., Driver, S.P., Cross, N.J.G., & Couch, W.J., 2003, MNRAS, 344, 307CrossRefGoogle Scholar
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