An X-ray study of luminous infrared galaxies observed with ASCA

Abstract

The discovery of ultra-luminous infrared galaxies (ULIRGs) has provided a clue to an evolutionary connection between starburst and active galactic nuclei. The IRAS color is suggested to be a possible trace of the evolution. We present the results of ASCA observations of two ULIRGs, IRAS20551-4250 and IRAS23128-5919, which are southern 100 μm bright galaxies with L IR ∼ 10 12 L ⊙. Both are mergers and have a “warm” IRAS color ( 25μm 100μm ≥ 0.15 ). The ASCA spectrum of IRAS20551-4250 can be characterized by two components, one of which is a soft thermal component ( kT ∼ 0.3keV) and the other is a hard power-law component absorbed by a column density of 10 22 cm −2. The observed X-ray luminosity is ∼ 2.5 × 10 42 ergs/s in the rest frame 2–10keV band (assuming H 0 = 50 km/s/Mpc). IRAS23128-5919 also shows a hard spectrum ( L X ∼ 3 × 10 42 ergs/s), but thermal emission is not as clear as that in IRAS20551-4250. Since these targets are similar in infrared luminosity as well as in hard X-rays but not in soft X-rays, L IR would be associated with hard X-rays. In addition to these results, we here compare X-ray properties of ULIRGs with IR properties.