Open AccessAn XMM–Newton hard X‐ray survey of ultraluminous infrared galaxies
Author(s) -
Franceschini A.,
Braito V.,
Persic M.,
Della Ceca R.,
Bassani L.,
Cappi M.,
Malaguti P.,
Palumbo G. G. C.,
Risaliti G.,
Salvati M.,
Severgnini P.
Publication year - 2003
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.1365-8711.2003.06744.x
Subject(s) - physics , astrophysics , galaxy , luminosity , active galactic nucleus , luminous infrared galaxy , infrared , astronomy , star formation
XMM–Newton observations of 10 ultraluminous infrared galaxies (ULIRGs) from a 200‐ks mini‐survey programme are reported. The aim is to investigate in hard X‐rays a complete ULIRG sample selected from the bright IRAS 60‐μm catalogue. All sources are detected in X‐rays, five of which for the first time. These observations confirm that ULIRGs are intrinsically faint X‐ray sources, their observed X‐ray luminosities being typically L 2–10 keV ≤ 10 42 –10 43 erg s −1 , whereas their bolometric (mostly infrared) luminosities are L bol > 10 45 erg s −1 . In all sources we find evidence for thermal emission from hot plasma with a rather constant temperature kT ≃ 0.7 keV , dominating the X‐ray spectra below 1 keV, and probably associated with a nuclear or circumnuclear starburst. This thermal emission appears uncorrelated with the far‐infrared luminosity, suggesting that, in addition to the ongoing rate of star formation, other parameters may also affect it. The soft X‐ray emission appears to be extended on a scale of ∼30 kpc for Mrk 231 and IRAS 19254−7245, possible evidence of galactic superwinds. In these two sources, IRAS 20551−4250 and 23128−5919, we find evidence for the presence of hidden active galactic nuclei (AGNs), while a minor AGN contribution may be suspected also in IRAS 20100−4156. In particular, we have detected a strong (EW ∼ 2 keV) Fe K line at 6.4 keV in the spectrum of IRAS 19254−7245 and a weaker one in Mrk 231, suggestive of deeply buried AGNs. For the other sources, the X‐ray luminosities and spectral shapes are consistent with hot thermal plasma and X‐ray binary emissions of mainly starburst origin. We find that the 2–10 keV luminosities in these sources, most probably due to high‐mass X‐ray binaries, are correlated with L FIR : both luminosities are good indicators of the current global star formation rate in the Galaxy. The composite nature of ULIRGs is then confirmed, with hints for a predominance of the starburst over the AGN phenomenon in these objects even when observed in hard X‐rays.