Molecular gas and dust in ARP 94: The formation of a recycled galaxy in an interacting system

We present new results for the molecular gas, dust emission, and the ionized gas in J1023+1952, an H I-rich intergalactic star-forming tidal dwarf galaxy candidate. It is located at the projected intersection of two faint stellar tidal streams wrapped around the interacting pair of galaxies NGC 3227/6 (Arp 94). Using the IRAM 30 m telescope, emission from ¹²CO(1-0) and ¹²CO(2-1) was detected across the entire extent of the neutral hydrogen cloud associated with J1023+1952, a region of the size of 8.9 × 5.9 kpc. The molecular gas is found to be abundant over the entire H I cloud, with H₂-to-H I gas mass ratios between 0.5 and 1.7. New Spitzer mid-infrared observations at 3.6, 4.5, 5.8, 8.0, 15, and 24 μm show that young SF is restricted to the southern part of the cloud. Despite the relatively uniform H₂ and H I column density across the cloud, young SF occurs only in those regions where the velocity dispersion in the CO and H I is a factor of ∼2 lower (FWHM of 30-70 km s^-1) than elsewhere in the cloud (FWHM of 80-120 km s^-1). Thus, the kinematics of the gas, in addition to its column density, seems to be a crucial factor in triggering SF. Optical/infrared spectral energy distributions (SEDs) and Hα photometry confirm that all the knots are young. Optical spectroscopy of the brightest SF region allowed us to determine the metallicity [12 + log (O/H) = 8.6 ± 0.2] and the extinction (A_B = 2.4). This shows that J1023+1952 is made from metal-enriched gas which is inconsistent with the hypothesis that it represents a preexisting dwarf galaxy. Instead, it must be formed from recycled, metal-enriched gas, expelled from NGC 3227 or NGC 3226 in a previous phase of the interaction.