盘状星系的颜色和颜色梯度 常 瑞 香 （上海天文台 ） 合作者：沈世银 刘成则 侯金良 邵正义 颜色 - 星等关系 颜色梯度 盘状星系的演化模型

Baum (1959) : established the color-magnitude relation (CMR) of early- type galaxies: more luminous galaxies tend to have redder colors. Bower et al. (1992, MNRAS, 254, 601): compared the colors (U V J K) of early-type galaxies in Virgo and Coma cluster to investigate the universality of the color-magnitude relation. The color-magnitude relation does have a universal form!

Interpretation of the color-magnitude relation: Arimoto & Yoshii (1987); Kodama & Arimoto (1997): A. Ellipticals form monolithically in a single burst of star formation at high redshift. B. More massive galaxies are able to retain their gas for longer and so reach higher metallicity. Semi-analytic models : The CMR arised because large ellipticals form by mergers of massive, metal-rich progenitor disk systems.

Bernardi et al (2005, AJ, 129, 61): The luminosity of ellipiticals is an indictor of its metallicity, whereas residuals in color from the relation are indicators of the luminocity-weighted age of its stars.

The case for late-type galaxies is more complicated. dust on-going star formation semi-analytic model predicts a flat CMR, while observations show more luminous galaxies tend to have redder colors.

Our aims: to obtain the observed CMR for late-type galaxies from a matched sample of SDSS and 2MASS to investigate the color gradients of late-type galaxies using the data in ProfMean in SDSS to construct a simple model for the evolution of disk galaxy

The observed color-magnitude relation the sample: fracPSF < 0.5: to select disk galaxies a/b > 0.75 (face-on): to reduce the effect of dust with photometry data from both SDSS and 2MASS final sample: 3801face-on disk galaxies K-correction: We generate a grid of model spectral energy distribution using the population synthetic models of Bruzual & Charlot (2003, BC03).

The observed color gradients the sample: fracPSF < 0.5: to select disk galaxies a/b > 0.75 (face-on): to reduce the effect of dust Nprof > 9 final sample: 8857 face-on disk galaxies

a simple model for disk galaxy assumptions: The disk is consisted by a set of independent rings. The disk originates and grows from infall of primordial gas. Instantaneous-recycling approximation and instantaneous mixing of the gas with the stellar ejecta are assumed.

Gas infall rate: f in (r,t)=A(r)te -t/  s 0 1 f in (r,t)dt =  tot (0)e -r/rd 2  r d 2  tot (0)=m d M halo The formula of rd and m d is adopted from MMW98 and Mo et al (2005). given  each halo (, v c ) f in (r,t) Model ingredients

star formation law:  (r,t) /  gas 1.4 (r,t) Gas outflow rate: f out (r,t)=B out  (r,t)/v c 2 one free parameter:  Main properties of the star formation history of disk galaxies

Red dash lines: no outflow,  =3Gyr Green dot-dash lines: with outflow,  =3Gyr Blue solid lines: no outflow,  = 36Gyr/lg 2 (m d M halo /10 8 M ¯ ) =0.05

Summary: Our results show that more luminous disk galaxies tend to have redder colors. The CMR of disk galaxies is mainly a consequence of stellar age, i.e., more massive galaxies tend to have older stellar population. The color gradients of the outer disk have very weak correlation with the luminosity, while strongly correlates with the disk half-light radius.