ROSAT Observations of the X-Ray Nebula around ARP 220

Details ROSAT Observations of the X-Ray Nebula around ARP 220 ROSAT Observations of the X-Ray Nebula around ARP 220

Feb 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...457..616h&link_type=abstract

Astrophysical Journal v.457, p.616

Astronomy and Astrophysics

Astronomy

80

Galaxies: Individual Name: Arp 220, X-Rays: Galaxies

Scientific paper

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 × 1040 and 2.3 × 1041 ergs s-1 (depending on the amount of X-ray absorption), and can be fitted by a thermal spectrum with T ˜ 107 K. The total thermal energy associated with the hot gas is ˜2 x 1057 ergs. At most ˜20% of the observed X-ray emission can be associated with any single point source (e.g., a "buried" QSO).
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 107 yr by a mechanical luminosity of ˜1043 ergs s-1 into a galactic halo with mean density ˜10-2 cm-3 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.
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 < 106 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.
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.

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