Global properties of infrared bright galaxies

Abstract

We have analyzed the IRAS data for 182 galaxies in order to determine accurate measures of their total flux densities, especially for galaxies that are partially resolved by IRAS. These galaxies are a subset of a complete, magnitude-limited sample whose molecular contents are being measured using the Five College Radio Astronomy Observatory (FCRAO) 14 m millimeter telescope as part of the FCRAO Extragalactic CO Survey. Here, we present IR flux densities at 12, 25, 60, and 100 microns from co-added IRAS data, including results for 50 galaxies in the Virgo cluster. For galaxies with optical diameters between 5' and 8', we find that the Point Source Catalog (PSC) typically underestimates the flux density by a factor of 2 at 60 microns and by a factor of 1.5 at 100 microns. Furthermore, flux densities at 12 and 25 microns are reported for 63 galaxies for which only upper limits are reported in the PSC. IR luminosities, colors, and warm dust masses are derived for the 182 galaxies, and these quantities are compared with the interstellar gas masses and optical luminosities of the galaxies. The H_2_ masses reported here have been derived from models for the source distributions and are corrected for source-beam coupling for our previously published CO observations of 124 galaxies. The IR luminosity is found to correlate better with the molecular mass than with the total H I mass or the total H I+ H_2_ mass for galaxies with L_IR_ above 10^10^ L_sun_. This is consistent with the IR emission arising primarily from dust in molecular clouds for galaxies with L_IR_ > 10^10^ L-sun_ since the interstellar medium (ISM) with the inner disk for these galaxies is primarily molecular. The best correlation we find is that between the warm dust masses inferred from IRAS data and the molecular masses derived from CO observations, such that M(H_2_) is proportional to M^1.0^_dust_. The mean value of M(H_2_)/M_dust_ in this sample is 570+/-50; that this value is higher than 100 probably reflects the fact that IRAS is not sensitive to the cold dust emitting beyond 120 microns. From fits to the comparisons of L_IR_ and L_B_ with M(H_2_) and M(H I), we find that L_IR_ is proportional to M(H_2_)^1.0^ and L_B_ is proportional to M(H_2_)^0.72^, with similar exponents for the comparison of L_IR_ and L_B_ with M(H I). We suggest that extinction may lower the blue luminosities in the most luminous galaxies relative to the IR luminosity, since the luminous galaxies have higher H_2_ surface densities and therefore larger dust column densities in their central regions. We demonstrate that the IR luminosity is a measure of the star formation rate for this sample from the correlation of Hα and IR luminosities. If L_IR_ measures the star formation rate, then the ratio L_IR_/M(H_2_) measures the stellar luminosity per unit H_2_ mass, which we call the star formation efficiency. Furthermore, we find a good correlation between L_IR_/M(H_2_) and the global Hα equivalent widths for 26 late-type spiral galaxies, from which we suggest that galaxies that are forming large numbers of high-mass stars are doing so through efficient conversion of gas into stars.