AAS 201st Meeting, January, 2003
Session 52. Galaxy Evolution and Surveys: Observations and Interpretation
Poster, Tuesday, January 7, 2003, 9:20am-6:30pm, Exhibit Hall AB

## [52.12] Properties of galaxies with non-constant star-formation rates

A. D. Quintero, D. W. Hogg, M. R. Blanton (NYU)

Two different simple measurements of galaxy star formation rate with different timescales are compared empirically on ~105 spectra of galaxies with r < 17.77~mag taken from the Sloan Digital Sky Survey in the redshift range of 0.05 < z < 0.20. In the first measurement, a linear sum of an average old stellar poplulation (\textsf{E}) spectrum and average A-star spectrum (\textsf{A}) is fit to the galaxy spectrum. This linear sum is a surprisingly good description of a normal galaxy spectrum. The ratio of \textsf{A/E} is, for each galaxy, a measure of the fraction of stars formed in the last ~1~Gyr which can be calibrated using models of stellar populations. The second measurement is the equivalent width (EW) of the H\alpha emission line; it is a measure of the fraction of stars formed in the last ~ 0.03~Gyr. The ratios \textsf{A/E} and H\alphaEW are strongly correlated. There are outliers from the median correlation that display excess \textsf{A/E} relative to H\alphaEW. These galaxies must have \emph{decreasing} star-formation rates over the last ~1 Gyr. The classical E~+~A'' galaxies are found in this group. There are also outliers with excess H\alphaEW relative to \textsf{A/E}. These must have \emph{increasing} star-formation rates over the last ~1~Gyr. The photometric, morphological and spectroscopic properties of galaxies are measured and discussed in the context of their relative values of \textsf{A/E} and H\alphaEW.