On the origin of the 40-120 micron emission of galaxy disks A comparison with H-alpha fluxes

ON THE ORIGIN OF THE 40-120 MICRON EMISSION OF GALAXY DISKS:A COMPARISON WITH H-ALPHA FLUXESCarol J. Lonsdale Persson and George HelouInfrared Processing and Analysis CenterCalifornia Institute of Technology, 100-22Pasadena, CA 91125AbstractA comparison of 40-120 micron IRAS fluxes with published H-alpha and UBV photometry shows that the far infrared emission ofgalaxy disks consists of at least two components: a warm oneassociated with OB stars in HII-regions and young star-formingcomplexes, and a cooler one from dust in the diffuse, neutralinterstellar medium, heated by the more general interstellarradiation field of the old disk population (a 'cirrus'-likecomponent). Most spiral galaxies are dominated by emission fromthe cooler component in this model. A significant fraction of thepower for the cool component must originate with non-ionizingstars. For a normal spiral disk there is a substantial uncertaintyin a star formation rate derived using either the H-alpha or thefar infrared luminosity.i. INTRODUCTIONThere is now a general consensus that the far infrared flux ofthe Galactic disk originates in part in the diffuse neutral andionized media, heated by the general interstellar radiation fieldin these regions (eg. Cox and Mezger 1987). In this paper, whichsummarises the results of Lonsdale Persson and Helou _1987), weexplore the extent to which the IRAS-measured 40-120 micron fluxesof a sample of normal disk galaxies can be attributed to thedifferent regimes (young HII-region/GMC complexes, the diffuseneutral medium and low density HII-regions) and stellar populations(OB stars, A and later stars) which contribute to the Galactic farinfrared (i-I000 micron) emission, by comparison of the 40-120micron flux with the H-alpha flux. If the emission of galaxy disksin the IRAS bands is dominated by emission from young OB stars(either optically visible or GMC-embedded), then the IRAS fluxesmight be expected to be highly correlated with H-alpha fluxes.Conversely, a poor correlation of far infrared with H-alpha fluxmight imply large dispersion in H-alpha extinction, far infraredemission from a mixture of HII-regions of varying density anddustiness, or a substantial contribution to the far infraredemission from older and less massive stars.The H-alpha fluxes are taken from Kennicutt and Kent (1983),who made large aperture H-alpha+[Nil] observations of over I00 diskgalaxies. Our restricted sample of 54 galaxies represents allthose in Kennicutt and Kent's list with high quality H-alpha, 60micron and 100 micron fluxes, and with 60 micron flux densityCarol J. Lonsdale Persson (Editor)Star Formation in Galaxies 153