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Photometric Study of Galaxies with OH Megamasers in the Infrared

OH megamasers are the most luminous masers in the universe. The total of 109 OH megamaser (OHM) galaxies known so far have been collected, and cross-identifications of those OHM galaxies with Two Micron All Sky Survey (2MASS) sources are made in this paper. Using 2MASS and IRAS data, the infrared properties of OHM-detected and nondetected sources are compared. It is shown that the main difference between those two kinds of sources is the infrared color from 25 to 60 μm. Statistically, the OHM-detected sources have redder colors and steeper spectral indices in the 25-60 μm region than the nondetected sources. In addition, considering the spectral types, the properties of the OHM galaxies in the near-infrared and far-infrared are discussed. It is found that Seyfert 1 and Seyfert 2 sources with hidden broad-line regions (BLRs) have redder colors and steeper spectral indices than other sources, including LINER, composite spectrum nucleus, and H II sources in the near-infrared region, whereas the latter have redder colors and steeper spectral indices than the Seyfert 1 and Seyfert 2 sources with hidden BLRs in the far-infrared. It is also found that almost all sources either with or without an identified spectral type show a starburst-dominated feature in the far-infrared, while a considerable number of sources, in particular Seyfert 1 and Seyfert 2 sources with hidden BLRs, show an active galactic nucleus-dominated feature in the near-infrared. Furthermore, as far-infrared radiation at 35 and 53 μm is likely responsible for the OH maser pumping in OHM galaxies, the pumping efficiencies in the infrared are also analyzed in this paper. It is found that, statistically, the pumping efficiencies increase with far-infrared color. It is also found that, statistically, the OH peak fluxes are not proportional to the related infrared fluxes at either 35 or 53 μm. This result implies that the pump lines are optically thin and the pump sources are unsaturated.