ROSAT Observations of the X-Ray Nebula around ARP 220

Abstract

We report on the X-ray properties of Arp 22(#the most luminous object in the local universe and the nearest, brightest, and best-studied example of the class of "ultraluminous" infrared galaxies. New X-ray images and spectra obtained with ROSAT show that the X-ray emission in the 0.1-2.4 keV band is spatially extended with a size of ∼30 × 11 kpc, has a luminosity of between 4.3 × 10<SUP>40</SUP> and 2.3 × 10<SUP>41</SUP> ergs s<SUP>-1</SUP> (depending on the amount of X-ray absorption), and can be fitted by a thermal spectrum with T ∼ 10<SUP>7</SUP> K. The total thermal energy associated with the hot gas is ∼2 x 10<SUP>57</SUP> ergs. At most ∼20% of the observed X-ray emission can be associated with any single point source (e.g., a "buried" QSO). <P />The X-ray nebula bears a strong morphological relationship to the expanding "double-bubble" system seen in optical emission-line images. We suggest that the X-ray and optical emission arise as the result of a bipolar "superwind" driven out from the nucleus by a starburst or a dust-shrouded QSO. A simple model of an expanding structure driven over a timescale of a few times 10<SUP>7</SUP> yr by a mechanical luminosity of ∼10<SUP>43</SUP> ergs s<SUP>-1</SUP> into a galactic halo with mean density ∼10<SUP>-2</SUP> cm<SUP>-3</SUP> satisfactorily accounts for all the observed global X-ray and optical properties of the nebula. Such a mechanical luminosity agrees with estimates for an ultraluminous starburst; however, a QSO-driven wind cannot be ruled out. We note that this energetic outflow in Arp 220 represents a local laboratory to study how newly formed galaxies and/or QSOs may have pumped energy and metals into the intergalactic medium at early epochs. <P />Alternative models for the nebula are less attractive. Gas shock-heated as a result of a galactic merger will probably be too cool (T less than a few times 106 K) and would not be expected to have the regular, symmetric morphology we observe in Hα. Electron scattering of X-rays from a hidden QSO by warm (T &lt; 10<SUP>6</SUP> K) halo gas underpredicts the total observed X-ray luminosity by more than an order of magnitude. However, it is possible that this mechanism may produce a bright central pair of pointlike X-ray sources that straddle the nucleus, which together account for ∼40% of the total X-ray emission. <P />Our X-ray images also show a diffuse source located about 2' south-southwest of Arp 220. Optical images show that the source is probably associated with a group or poor cluster of galaxies at an estimated redshift of ∼0.1. This source contributes about half the emission seen in earlier Einstein X-ray observations of Arp 220.