ChandraX-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved HiiRegions

Abstract

We present high spatial resolution X-ray imaging data for the interacting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-ray telescope. In addition to the unresolved starburst nucleus, a variable point source with LX ≈ 1040 ergs s-1 was detected 15 (270 pc) to the northwest of the nucleus, coincident with a blue, extremely optically luminous (MV ≈ -14.1) point source on Hubble Space Telescope images. Eleven other candidate pointlike ultraluminous X-ray sources (ULXs) were also detected in the vicinity of NGC 7714/7715, two of which exceed 1040 ergs s-1. Ten of these appear to be associated with interaction-induced features, but only two are associated with star formation regions. We also found diffuse emission with LX ≈ 3 × 1040 ergs s-1 extending 11'' (1.9 kpc) to the north of the nucleus. Its spectrum can be fitted with either a two-temperature MEKAL function (kT = 0.59 and 8 keV) or a 0.6 keV MEKAL function plus a power law (Γ = 1.8 ± 0.2). The hard component may be due to high-mass X-ray binaries (HMXBs) with possible contributions from inverse Compton radiation, while the soft component is likely from a superwind. Superbubble models imply an expansion age of ≈15 Myr, supporting previous assertions of an intermediate-age nuclear stellar population in addition to a 5 Myr starburst. We also detected extended X-ray emission associated with four extranuclear H II region complexes. The emission from these H II regions and the nuclear starburst could be due to either an enhanced population of HMXBs relative to Local Group galactic averages or to diffuse gas heated by winds from supernovae, if the X-ray production efficiency LX/Lmech is high (≈5%). To estimate LX/Lmech, we collected published data for well-studied H II regions and superbubbles in nearby galaxies. For H II regions with ages less than 3.5 Myr, the median LX/Lmech ≈ 0.02%, while for older star formation regions, LX/Lmech ≈ 0.2%–7%. Thus, it is possible that gas heating by supernovae may be sufficient to account for the observed X-rays from these H II regions. In galaxies much more distant than NGC 7714, for example, the Cartwheel galaxy, H II region complexes similar to those in NGC 7714 will be unresolved by Chandra and will mimic ULXs. No X-ray emission was detected from the Type Ib supernova SN 1999dn, with an upper limit of ≈2 × 1038 ergs s-1.