Abstract
Using 10 095 galaxies (B <20 mag) from the Millennium Galaxy Catalogue, we derive B-band luminosity distributions and selected bivariate brightness distributions for the galaxy population subdivided by eyeball morphology; Sersic index (n); two-degree Field Galaxy Redshift Survey (2dFGRS) eta parameter; rest-(u-r) colour (global and core); MGC continuum shape; half-light radius; (extrapolated) central surface brightness; and inferred stellar mass-to-light ratio. All subdivisions extract highly correlated subsets of the galaxy population which consistently point towards two overlapping distributions: an old, red, inert, predominantly luminous, high central-surface brightness subset; and a young, blue, star forming, intermediate surface brightness subset. A clear bimodality in the observed distribution is seen in both the rest->(u-r)colour and log (n) distributions. Whilst the former bimodality was well established from Sloan Digital Sky Survey data, we show here that the rest-(u-r) colour bimodality becomes more pronounced when using the core colour as opposed to global colour. The two populations are extremely well separated in the colour-log(n) plane. Using our sample of 3314 (B <19 mag) eyeball classified galaxies, we show that the bulge-dominated, early-type galaxies populate one peak and the bulge-less, late-type galaxies occupy the second. The early- and mid-type spirals sprawl across and between the peaks. This constitutes extremely strong evidence that the fundamental way to divide the luminous galaxy population (M-BMGC- 5 log h <-16 mag, i.e. dwarfs not included) is into bulges (old red, inert, high concentration) and discs (young, blue, star forming, low concentration) and that the galaxy bimodality reflects the two-component nature of galaxies and not two distinct galaxy classes. We argue that these two components require two independent formation mechanisms/processes and advocate early bulge formation through initial collapse and ongoing disc formation through splashback, infall and merging/accretion. We calculate the B-band luminosity densities and stellar mass densities within each subdivision and estimate that the z approximate to 0 stellar mass content in spheroids, bulges and discs is 35 +/- 2, 18 +/- 7 and 47 +/- 7 per cent, respectively.
Original language | English |
---|---|
Pages (from-to) | 414-434 |
Number of pages | 21 |
Journal | Monthly Notices of the Royal Astronomical Society |
Volume | 368 |
Issue number | 1 |
DOIs | |
Publication status | Published - 1 May 2006 |
Keywords
- surveys
- galaxies
- fundamental parameters
- galaxies : luminosity function, mass function
- galaxies : statistics
- DIGITAL SKY SURVEY
- ARTIFICIAL NEURAL-NETWORKS
- LUMINOSITY FUNCTION
- REDSHIFT SURVEY
- DISK GALAXIES
- STRUCTURAL PARAMETERS
- SPACE DENSITY
- STELLAR MASS
- DATA RELEASE
- BLACK-HOLE