Structure of Disk‐dominated Galaxies. II. Color Gradients and Stellar Population Models

Abstract

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 with stellar population synthesis models from which luminosityweighted average ages and metallicities are determined. We explore the effects of different underlying star formation histories and additional bursts of star formation. Our results are robust in a relative sense under the assumption that our galaxies shared a similar underlying star formation history and that no bursts involving more than $10% of the galaxy mass have occurred in the past 1-2 Gyr. Because the observed gradients show radial structure, we measure ''inner'' and ''outer'' disk age and metallicity gradients. 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 with rotational velocity and total magnitude level off for V rot k120 km s À1 and M K P À23 mag, respectively. This effect is stronger for metallicity (than age), which could reflect a threshold potential above which all metals are retained and thus metallicity saturates at the yield. Outer disk gradients are found to be 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. Trends in gradients with galaxy parameters are compared with model predictions: these trends do not agree with predictions of semianalytic models of hierarchical galaxy formation, possibly owing to the effect of bar-induced radial flows. The observed trends are in agreement with chemo-spectrophotometric models of spiral galaxy evolution based on CDM-motivated scaling laws but none of the hierarchical merging characteristics, implying a strong dependence of the star formation history of spiral galaxies on the galaxy potential and halo spin parameter.

THE STRUCTURE AND STELLAR CONTENT OF THE OUTER DISKS OF GALAXIES: A NEW VIEW FROM THE Pan-STARRS1 MEDIUM DEEP SURVEY

“…These typically have shown color profiles that become bluer with increasing radius (e.g., MacArthur et al 2004). It is only recently that systematic studies of the outer disks have shown the trend toward redder colors that is seen in our deep images.…”

Radial Distribution of Stars, Gas, and Dust in Sings Galaxies. Ii. Derived Dust Properties

“…This limitation not only applies to integrated data, but also to surface brightness profiles and color gradients. For instance, the so-called inside-out scenario for the formation of galactic disks predicts that the timescale of gas infall and conversion into stars increases with radius, leading to radial variations of the star formation history (SFH), which are in turn observationally translated into color gradients (de Jong 1996; Bell & de Jong 2000;MacArthur et al 2004;Taylor et al 2005). Given that the dust content also changes with radius, it is key to properly quantify that variation with direct tracers of dust extinction; otherwise, derived parameters such as the radial growth rate of disks might be biased (see Muñoz-Mateos et al 2007 and references therein).…”

Color Profiles of Disk Galaxies since z ~ 1: Probing Outer Disk Formation Scenarios

“…When we can apply these tools to objects at different redshifts, we can see how changes in the spatial distribution of stellar mass in the disks actually take place. Color gradients have been extensively used in the literature as a way to extract information on both ages and metallicities through the galaxy radial profiles in the local universe (e.g., MacArthur et al 2004). However, an interesting point these previous works missed was the connection between the color distribution and the break phenomenology.…”