A Comparison of Halpha and Stellar Scale Lengths in Virgo and Field Spirals

The scale lengths of the old stars and ionized gas distributions are compared for similar samples of Virgo Cluster members and field spiral galaxies via Halpha and broad R-band surface photometry. While the R-band and Halpha scale lengths are, on average, comparable for the combined sample, we find significant differences between the field and cluster samples. While the Halpha scale lengths of the field galaxies are a factor of 1.14 +- 0.07 longer on average than their R-band scale lengths, the Halpha scale lengths of Virgo Cluster members are on average 20% smaller than their R-band scale lengths. Furthermore, in Virgo, the scale length ratios are correlated with the size of the star-forming disk: galaxies with smaller overall Halpha extents also show steeper radial falloff of star formation activity. At the same time, we find no strong trends in scale length ratio as a function of other galaxy properties including galaxy luminosity, inclination, morphological type, central R--band light concentration, or bar type. Our results for Halpha emission are similar to other results for dust emission, suggesting that Halpha and dust have similar distributions. The environmental dependence of the Halpha scale length places additional constraints on the evolutionary process(es) that cause gas depletion and a suppression of the star formation rate in clusters of galaxies.