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Color-magnitude relations of disk galaxies: observations vs. model predictions

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

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Color-magnitude relations of disk galaxies: observations vs. model predictions

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

  2. 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.

  3. 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.

  4. 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

  5. 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: 40987face-on disk galaxies, SDSS DR4 Sample B: 9930face-on disk galaxies, SDSS DR4 + 2MASS

  6. 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.

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

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

  9. 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).

  10. 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.

  11. 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.

  12. No dust correction • 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? more data ? ?

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

  14. 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.

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