We have obtained 12 CO (1-0) emission-line observations for a sample of 37 interacting galaxy systems, chosen from a parent sample of optically-selected interacting galaxies. The sample observed here spans a large range of interaction strengths and current star formation rates. Using the standard Galactic CO-to-H 2 conversion factor we find that the interacting galaxies are, on average, marginally more rich in molecular gas than a comparison sample of isolated spiral galaxies, having mean M H2 /L B and M H2 /M HI ratios that are 20-40% higher than that of the isolated spirals. The interacting galaxies have a mean L IR /M H2 ratio that is a factor of ∼1.3 higher than the isolated spirals, indicating that the rate of highmass star formation per unit molecular gas-the star formation efficiency-is also enhanced within the interacting galaxy sample. There is a strong correlation between relative H 2 content and star formation rates derived from both far-infrared and Hα luminosities, indicating that the level of interaction-induced star formation activity is highly dependent upon the available gas supply. There are some individual galaxies with moderate amounts of molecular gas but little or no current star formation activity. Therefore molecular gas is a necessary, but not sufficient, condition for interaction-induced star formation activity. There is a strong correlation between interaction strength and both star formation rate and fractional H 2 content, with merging systems showing the highest star formation rates and fractional H 2 contents. There is also an increase in the molecular to atomic gas ratio with increasing interaction strength. If the increase in derived H 2 content is real, there must be a substantial conversion of H I to H 2 gas taking place in the strongest interactions. At least some portion of these trends, however, may arise due to an increase in cloud heating, brought about by the increased population of young, massive stars in galaxies with high current star formation rates. A reduction in the standard CO-to-H 2 conversion ratio by a factor of two could account for the systematic increase in derived H 2 content as a function of interaction strength. Such a change could not, however, account for the total range of H 2 contents within the entire samples of galaxies. Therefore the correlation between molecular gas content and star formation rate must be real.

We present the first results from 60ks of observations of Arp 220 using the ACIS-S instrument on Chandra. We report the detection of several sources near the galaxy's nucleus, including a point source with a hard spectrum that is coincident with the western radio nucleus B. This point source is mildly absorbed (N H ∼ 3 × 10 22 cm −2 ) and has an estimated luminosity of 4 × 10 40 erg/s. In addition, a fainter source may coincide with the eastern nucleus A. Extended hard X-ray emission in the vicinity raises the total estimated nuclear 2-10 keV X-ray luminosity to 1.2 × 10 41 erg/s, but we cannot rule out a hidden AGN behind columns exceeding 5 × 10 24 cm −2 . We also detect a peak of soft X-ray emission to the west of the nucleus, and a hard point source 2.5 kpc from the nucleus with a luminosity of 6 × 10 39 erg/s.

We resolve the extended X-ray emission from the prototypical ultraluminous infrared galaxy Arp 220. Extended, faint, edge-brightened, soft X-ray lobes outside the optical galaxy are observed to a distance of 10-15 kpc on each side of the nuclear region. Bright plumes inside the optical isophotes coincide with the optical line emission and extend 11 kpc from end to end across the nucleus. The data for the plumes cannot be fitted by a single-temperature plasma and display a range of temperatures from 0.2 to 1 keV. The plumes emerge from bright, diffuse circumnuclear emission in the inner 3 kpc centered on the H peak, which is displaced from the radio nuclei. There is a close morphological correspondence between the H and soft X-ray emission on all spatial scales. We interpret the plumes as a starburst-driven superwind and discuss two interpretations of the emission from the lobes in the context of simulations of the merger dynamics of Arp 220.

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