Open AccessAn X‐ray investigation of powerful far‐infrared galaxies
Author(s) -
Iwasawa K.
Publication year - 1999
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.1999.02084.x
Subject(s) - physics , astrophysics , galaxy , active galactic nucleus , luminosity , infrared , astronomy , spectral line , emission spectrum , luminous infrared galaxy , x ray , nebula , thermal emission , thermal , stars , quantum mechanics , meteorology
We present ASCA results on four prototype powerful far‐infrared galaxies, Mrk 231, Mrk 273, Arp 220 and NGC 6240. The soft X‐ray spectra show signatures of thermal emission with temperatures of (0.5‐‐1),×10 7 K, which is probably produced in starbursts. Their soft X‐ray (0.5‐‐2 keV) luminosities range from 4×10 40 to 7×10 41 erg s ‐1 . The X‐ray properties are examined in the context of a starburst. Evidence for a heavily obscured active nucleus is found in Mrk 273 and NGC 6240. The ASCA spectra of both galaxies show strong iron K emission‐line features. The hard X‐ray emission (> 3 keV) in NGC 6240 is most likely the reflected light of a hidden QSO, the intrinsic luminosity of which is suspected to be ∼ 10 45 erg s ‐1 , comparable to the far‐IR luminosity. The 2‐‐10 keV emission, possibly related to an active galactic nucleus (AGN), is found in Mrk 231. The observed 2‐‐10 keV luminosity is only ∼ 2×10 42 erg s ‐1 , and the origin of the hard X‐ray emission is uncertain because of the low quality of the present data. No evidence for an AGN is found in Arp 220 in the X‐ray data. However, the soft X‐ray emission originating in the starburst is also as weak as the Hα nebula and near‐IR continuum, despite the large far‐IR excess. The possible existence of a powerful but heavily obscured (Compton‐thick) AGN is discussed.