Very Small Grains and the Infrared Colors of Galaxies

August 1991 • 1991ApJ...376..505H

Authors • Helou, G. • Ryter, C. • Soifer, B. T.

Abstract • We use IRAS data to study the contribution of very small grains with fluctuating temperature to the infrared emission of disk galaxies and to estimate the variation among galaxies of the abundance of these small grains relative to the larger "classical," thermally stable grains. We argue that the 12 micron emission from spiral galaxies originates in their interstellar medium and compare the integrated IRAS colors of galaxies to the IRAS colors of local emission within Galactic nebulae. {GAMMA} = vf_v_(12 micron)/F(40-120 micron) is advanced as an estimator of the small- to-large grain ratio, and the interpretation of its statistical behavior in samples of galaxies is discussed. In a color-color diagram of the emission from Galactic nebulae, {GAMMA} depends only on {THETA} = f_v_(60 micron)/f_v_(100 micron), decreasing slowly as {THETA} increases. The same trend is observed for the integrated colors of galaxies and is seen in three different samples: optically selected "normal" spirals in the Virgo cluster, infrared-luminous galaxies from the IRAS bright galaxy sample and "cold" galaxies selected for their low ratios of f_v_(60 micron) to f_v_(100 micron). The dispersion on {GAMMA} is small (rms~ 40%) for {THETA}< 1/3, and increases to about a factor of 2 at {THETA}~1. We argue that the low dispersion in the "cold" galaxies represents the true dispersion in the small-to-large grain abundance ratio among galaxies, while the larger dispersion among warmer galaxies is a result of increasing effects of optical depth heating of dust by active galactic nuclei, and destruction of small dust grains at high radiative energy densities. The small-to-large grain ratio is found to be roughly constant among normal galaxies with similar metallicities, with a rms dispersion of 40% or less.

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George Helou

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