The SAURON project - XI. Stellar populations from absorption-line strength maps of 24 early-type spirals

Abstract

We present absorption line strength maps of a sample of 24 representative early-type spiral galaxies, mostly of type Sa, obtained as part of the SAURON survey of nearby galaxies using our custom-built integral-field spectrograph. Using high-quality spectra, spatially binned to a constant signal-to-noise, we measure several key age, metallicity and abundance ratio sensitive indices from the Lick/IDS system over a contiguous two-dimensional field including bulge and inner disc. We present maps of Hβ, Fe 5015, and Mgb, for each galaxy. We find that Sa galaxies on the average have slightly smaller Mgb and Fe 5015 line strengths than ellipticals and S0s, and higher Hβ values, but with a much larger scatter. The absorption line maps show that many galaxies contain some younger populations ( � 1 Gyr), distributed in small or large inner discs, or in circumnuclear star forming rings. In many cases these young stars are formed in circumnuclear mini-starbursts, which are dominating the light in the centres of some of the early-type spirals. These mini-starburst cause a