Star Formation Histories of Spiral Galaxies from a Multi-Wavelength Study

Extract

We study the star formation histories of a sample of 113 nearby spiral galaxies using their radio continuum (20cm), FIR (40-120μm), H (1.65μm) and B (4400Å) luminosities. The first two are used as indicators of star formation rate over the past ∼ 10⁸ years, as suggested by the tight and nearly universal FIR/radio correlation (Xu et al. 1994). Compared to other indicators of recent star formation rate such as Hα and UV, FIR and radio continuum have the advantage of being insensitive to extinction. The B luminosity is taken as star formation indicator for the time scale of 3 10⁹ years, and the H luminosity for the time scale of 10¹⁰ years. We find:

1. 1. The long-term star-formation history (from a few billion years to the entire Hubble time), as indicated by the B-to-H luminosity ratio, depends strongly on the Hubble type.

2. 2. The recent star-formation history in the last a few billion years, as indicated by the radio-to-B luminosity ratio and the FIR-to-B luminosity ratio, does not depend on the Hubble type.

3. 3. Galaxies of a given Hubble type have similar long-term star-formation histories. On the other hand, their recent star-formation histories in the last a few billion years can be much different: the ratio between the star-formation rate averaged over the past 10⁸ years to that over the past 3 10⁹ years can be different by two orders of magnitude, as indicated by the scattering of the radio-to-B luminosity ratio and that of the FIR-to-B luminosity ratio. This is not likely to be due to the extinction on the B luminosity, because the scattering of the B-to-H luminosity ratio for a given type is much smaller.