The Global Rate and Efficiency of Star Formation in Spiral Galaxies as a Function of Morphology and Environment

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Abstract

CCD images of Hα and R-band emission in 120 spiral galaxies were obtained using the now-retired No. 1-0.9 m telescope of Kitt Peak National Observatory. These images were used to derive the distribution and total flux of continuum-subtracted Hα line emission, and therefore the Hα surface brightnesses and high mass star formation rates in these galaxies. We find a small but significant variation in the mean Hα surface brightness for spiral galaxies along the Hubble sequence; the Sd-Ir galaxies exhibit a mean Hα surface brightness 1.4 times higher than the Sbc-Scd galaxies, and 2-3 times higher than the Sa-Sb galaxies. Estimates for the total formation rate for high mass stars have been compared with global molecular gas masses to determine the global efficiency of high mass star formation (= L_*_/M_gas_) as a function of morphological type and environment. We find that the mean efficiency of high mass star formation in this sample of spiral galaxies shows little dependence on morphological type for galaxies of type Sa through Scd, although there is a wide range in star formation efficiencies within each type. Galaxies in disturbed environments (i.e., strongly interacting systems) are found to have a mean star formation efficiency ~4 times higher than in isolated spiral galaxies, uncorrected for extinction. This confirms previous findings (Young et al. 1986a,b; Sanders et al. 1986; Solomon & Sage 1988; Tinney et al. 1990), based on the far-infrared luminosity rather than the Hα luminosity to trace the rate of high mass star formation, that the mean star formation efficiency among isolated galaxies is significantly lower than that among interacting systems. This result provides further confirmation that the rate of high mass star formation is reasonably well traced by both the Hα and the IR luminosity in spiral galaxies.

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