AAS 205th Meeting, 9-13 January 2005
Session 60 Secular Evolution Potpourri: Star Formation to Galactic Structures
Poster, Tuesday, January 11, 2005, 9:20am-6:30pm, Exhibit Hall

## [60.11] Stellar Populations in Spiral Galaxies

L.A. MacArthur (University of British Columbia), S. Courteau (Queen's University), E.F. Bell (Max-Planck-Institut für Astronomie), J.A. Holtzman (New Mexico State University)

We investigate optical and near-IR color gradients in a sample of 172 low-inclination galaxies spanning Hubble types S0--Irr. The colors are compared to stellar population synthesis models from which luminosity-weighted average ages and metallicities are determined. We explore the effects of different underlying star formation histories and additional bursts of star formation. Because the observed gradients show radial structure, we measure inner'' and outer'' disk age and metallicity gradients. Relative trends in age and metallicity and their gradients are explored as a function of Hubble type, rotational velocity, total near-IR galaxy magnitude, central surface brightness, and scale length. We find strong correlations in age and metallicity with Hubble type, rotational velocity, total magnitude, and central surface brightness in the sense that earlier-type, faster rotating, more luminous, and higher surface brightness galaxies are older and more metal-rich, suggesting an early and more rapid star formation history for these galaxies. The increasing trends level off for T \la 4 (Sbc and earlier), V\mathrm{rot} \ga 120 km s-1, MK \la -23 mag, and \mu0 \la 18.5 mag arcsec-2. Outer disk gradients are weaker than the inner gradients as expected for a slower variation of the potential and surface brightness in the outer parts. We find that stronger age gradients are associated with weaker metallicity gradients. Relative trends in gradients with galaxy parameters do not agree with predictions of semi-analytic models of hierarchical galaxy formation, possibly as a result of bar-induced radial flows. However, the observed trends are in agreement with chemo-spectro photometric models of spiral galaxy evolution based on CDM-motivated scaling laws but including none of the hierarchical merging characteristics. This implies a strong dependence of the star formation history of spiral galaxies on the galaxy potential and halo spin parameter.

L.A.M. and S.C acknowledge support from the National Science and Engineering Research Council of Canada, E.F.B. acknowledges the financial support provided through the European Community's Human Potential Program under contract HPRN-CT-2002-00316, SISCO, and J.H. acknowledges support from the Research Corporation and the New Mexico Space Grant Consortium.