Age patterns in a sample of spiral galaxies

Abstract

We present the burst ages for young stellar populations in a sample of six nearby (<10 Mpc) spiral galaxies using a differential pixel-based analysis of the ionized gas emission. We explore this as an alternative approach for connecting large-scale dynamical mechanisms with star formation processes in disc galaxies, based on burst ages derived from the Hα to far-UV (FUV) flux ratio. Images of each galaxy in Hα were taken with Taurus Tunable Filter and matched to FUV imaging from GALEX. The resulting flux ratio provides a robust measure of relative age across the disc which we discuss in terms of the large-scale dynamical motions. Systematic effects such as a variable initial mass function, non-solar metallicities, variable star formation histories (SFHs) and dust attenuation have been used to derive estimates of the systematic uncertainty. The resulting age maps show a wide range of patterns outside of those galaxies with the strongest spiral structure, confirming the idea that star formation is driven one by several processes, largely determined by the individual circumstances of the galaxy. Generally, grand design spirals such as M74, M100 and M51 exhibit age gradients across the main spiral arms, with the youngest star formation regions along the central and inner edges. Likewise, in the dominant star-forming complex of IC 2574 or the ring of M94, the most recent star formation is centrally confined to the regions of star formation activity. In M63 and M74 galaxy-wide trends emerge, contrary to the spiral structure in these galaxies, suggesting that spiral density waves are not the dominant driver in some cases. We argue that despite appearances, galaxy morphology is not an absolute discriminator of the SFH of an individual galaxy, nor of the processes triggering it. We conclude that Hα-to-FUV flux ratios are a relatively direct way to probe burst ages across galaxies and infer something of their dynamical histories, provided that sources of systematics are properly taken into account.