IRAS observations of a large sample of normal irregular galaxies

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Abstract

The rarity of dark nebulae in irregular galaxies and the low optical internal reddenings of these galaxies as compared with spiral galaxies suggest a deficiency of dust and are puzzling features of irregulars actively forming stars. In order to explore the dust content in irregular galaxies, we present far-infrared IRAS 12, 25, 60, and 100 micron observations for a large sample of these systems. Dwarf, giant, and amorphous irregulars have similar infrared properties. The low L_IR_/L_Hα_ ratios of these irregular galaxies are consistent with the irregulars being relatively transparent systems without large amounts of hidden star formation. Thus, L_Hα_ is a good measure of the current massive star formation. Compared with spirals, the irregulars have similar L_IR_/L_B_ ratios, warmer S_100_/S_60_ ratios, cooler S_25_/S_12_ ratios, lower L_IR_/L_Hα_ ratios, and lower dust-to-H I gas mass ratios. The temperature, dust-to-H I gas mass ratios, and L_IR_/L_B_ ratios do not correlate with the metallicity of the ionized gas of the irregular galaxies. Distant luminous blue irregular galaxies stand out as having infrared-to-optical flux ratios which are consistent with higher optical thicknesses. These galaxies are also often interacting with other systems. It is hard to disentangle sources of dust heating, such as the general interstellar radiation field and very young stars, from global infrared measurements. This is especially true when the global optical properties themselves are correlated as they are in constant star-formation rate systems. Nevertheless, simple models are presented for two primary sources of heating of dust-H II regions and the UV-dominated general interstellar radiation field. A sample of H II-dominated irregulars is identified, and model L_IR_/L_Hα_ and S_100_/S_60_ ratios are found to agree fairly well with the observations. Irregulars have warmer dust compared with spirals because of their higher UV surface brightnesses and lower optical depths. Star-formation rates derived from L_IR_ Tinsley-Renzini population models are also discussed. L_IR_ is a star-formation rate indicator because a variety of stellar population age groups can contribute to L_IR_. Combining IR and optical star-formation rate indicators, we find that normal irregulars are generally not evolving via bursts of star formation, while some amorphous systems may be. The star-formation histories of luminous irregular galaxies are confused by optical depth effects, but many of these systems are in phases of enhanced star-forming activity.

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