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The origin of the far infrared luminosity in spiral galaxies is a subject of continuing speculation. At issue, is whether the far infrared luminosity measured by the Infrared Astronomical Satellite (IRAS) is powered by O and B stars or non-ionizing stars.
New H${\alpha}$ images have been obtained for 4 nearby nearby large spiral galaxies that were well resolved by IRAS. Two early-type spirals; M 31 \& M81, and two late-type spirals; M33 and IC 342. The correspondence between the H${\alpha}$ and far infrared morphology is striking when the H${\alpha}$ images are convolved to the same resolution as the IRAS data. Additionally, the far infrared luminosity agrees quantitatively with that expected from the O and B stars which are required to ionize the Hydrogen gas. The new results indicate that the far infrared luminosity is as good a measure of the high mass star formation rate as the H${\alpha}$ luminosity, which is important because H${\alpha}$ fluxes have been measured for only ${\sim}$ 200 galaxies, whereas the IRAS provided far infrared fluxes for literally thousands of galaxies.
The large database of far infrared measurements has been used to quantify high mass star formation rates for a larger sample of galaxies than has previously been possible. The data reveal a surprising result; the current high mass star formation rate is as high in the early-type spirals as the late-types. The result challenges conventional doctrine and raises new questions about the origin and evolution of the Hubble sequence.
