ROSAT OBSERVATIONS OF THE X-RAY NEBULA AROUND ARP-220

We report on the X-ray properties of Arp 220-the most luminous object in the local universe and the nearest, brightest, and best-studied exa mple of the class of ''ultraluminous'' infrared galaxies. New X-ray im ages and spectra obtained with ROSAT show that the X-ray emission in t he 0.1-2.4 keV band is spatially extended with a size of similar to 30 x 11 kpc, has a luminosity of between 4.3 x 10(40) and 2.3 x 10(41) e rgs s(-1) (depending on the amount of X-ray absorption), and can be fi tted by a thermal spectrum with T similar to 10(7) K. The total therma l energy associated with the hot gas is similar to 2 x 10(57) ergs. At most similar to 20% of the observed X-ray emission can be associated with any single point source (e.g., a ''buried'' QSO). The X-ray nebul a 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 ''supe rwind'' 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(7) yr by a mechanical luminosity of similar to 10(43 ) ergs s(-1) into a galactic halo with mean density similar to 10(-2) cm(-3) satisfactorily accounts for ail the observed global X-ray and o ptical properties of the nebula. Such a mechanical luminosity agrees w ith estimates for an ultraluminous starburst; however, a QSO-driven wi nd 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 mediu m at early epochs. Alternative models for the nebula are less attracti ve. Gas shock-heated as a result of a galactic merger will probably be too cool (T less than a few times 10(6) K) and would not be expected to have the regular, symmetric morphology we observe in H alpha. Elect ron scattering of X-rays from a hidden QSO by warm (T < 10(6) 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 pr oduce a bright central pair of pointlike X-ray sources that straddle t he nucleus, which together account for similar to 40% of the total X-r ay emission. 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 duster of galaxies at an est imated redshift of similar to 0.1. This source contributes about half the emission seen in earlier Einstein X-ray observations of Arp 220.