Color-magnitude relations of disk galaxies: observations vs. model predictions Ruixiang Chang （ Shanghai Astronomical Observatory ） Collaborators: Jinliang Hou Shiyin Shen Chenggang Shu Jun Yin

Baum (1959) established the color-magnitude relations (CMRs) of early-type galaxies: more luminous galaxies tend to have redder colors. The CMRs of early-type galaxies have been mainly ascribed to the metallicity effect.

The case for late-type galaxies is more complicated. dust attenuation on-going star formation & emission lines There are only a few papers focused on the comparisons between model predictions and observed CMRs of disk galaxies.

Our aims: to derived observed color-magnitude relations of disk galaxies to construct a simple model for the evolution of disk galaxies to investigate main properties of star formation history of disk galaxies

The observed color-magnitude relation Chang et al. (2006, MNRAS, 372, 199): fracDeV < 0.5: to select disk galaxies a/b > 0.75 (face-on): to reduce the effect of dust 0.01< z< 0.2 Sample A: face-on disk galaxies, SDSS DR4 Sample B: 9930 face-on disk galaxies, SDSS DR4 + 2MASS

Aperture corrections: to correct SDSS magnitudes to the aperture where the 2MASS magnitudes are measured. Emission-line corrections: to compare magnitude difference estimated from the spectra before and after removing the emission lines. Dust corrections: to take A_z from Kauffmann et al. (2003) and plus some kind of attenuation curve.

More luminous galaxy tends to have redder colors, but the optical CMRs have shallower slope at lower mass.

Gil De Paz et al. (2007, ApJSS, 173, 185): GALEX nearby galaxies survey: 1034 nearby galaxies

A simple model for disk evolution Main assumptions: The disk is sheet-like and composed by a set of independent rings. The disk origins and grows by gas infall. The radial profile of stellar mass surface density is exponential in the present-day and the relation between M_* and r_d is taken from Shen et al.(2003).

Gas infall rate: f_{in}=A(r)e^{-t/\tau} Star formation law: Kenicutt star formation law Gas outflow rate: f_{out}=b_{out}*SFR Input parameter: M_*, M_* ~ r_d Free parameter: \tau, b_{out} A(r) is constrained by the stellar mass surface density in the present day.

Model predictions are very sensitive to infall timescale \tau. Gas outflow mainly influences gas metallicity and color (r-J), while almost has no effect on optical color.

Viable model: \tau=100exp(-logM_*+8) b_{out)=0.2(12-logM_*) Massive galaxies tend to have shorter infall time scale than low mass systems. Down-sizing? ? No dust correction more data ?

next step: to compare model predictions with observed color gradients along the disk

Summary The predicted CMRs of disk galaxies are very sensitive to infall timescale \tau. Gas outflow mainly influences gas metallicity and color (r- J), while almost has no effect on optical color. Massive spirals tend to have shorter infall timescale than low mass systems.