Infrared photometric study for the IRAS 1 Jy sample of ultraluminous infrared galaxies

The Two Micron All Sky Survey observation of the IRAS 1 Jy sample of ultraluminous infrared galaxies (ULIRGs) is presented in this paper. Together with the IRAS photometric data, the properties and energy sources of the IRAS 1 Jy sample of ULIRGs in the near- and far-infrared are discussed. It is shown that most sources, not only Seyfert galaxies but also H II and LINER galaxies, have a radiation mechanism with a power-law continuum in the near- infrared. All Seyfert 1 sources and some Seyfert 2 sources with hidden broad-line regions (BLRs) have much redder colors and steeper spectral indices in the near- infrared, indicative of an extremely strong energy source from the central active galactic nucleus. Almost all sources have thermal radiation indicative of starburst domination in the IRAS region; in particular, all Seyfert 1 sources and many Seyfert 2 sources with hidden BLRs have lower [25]-[60] colors, indicative of a flatter spectral continuum and a rather warm color temperature in the 25-60 mu m region. However, all kinds of sources have similar [60]-[100] color distributions, indicative of similar energy sources from the enhanced heating in active star-forming regions and very cold material and also indicative of the quite small influence from the central activity in the 60-100 mu m region. In addition, all Seyfert 1 sources and some Seyfert 2 sources with hidden BLRs have much larger absolute magnitudes in the K band compared with other sources. The results presented in this paper may further help us to understand the physical properties of ULIRGs.