Molecular gas and dust in Arp 94: the formation of a recycled galaxy in an interacting system
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American Astronomical Society; Institute of Physics (IOP)
Molecular dataGalaxiesActiveInteractionsISMArp 94J1023+1952NGC 3227
Lisenfeld, U.; et al. Molecular gas and dust in Arp 94: the formation of a recycled galaxy in an interacting system. Astrophysical Journal, 685(1): 181-193 (2008). [http://hdl.handle.net/10481/28395]
PatrocinadorU. L. acknowledges financial support from the research project AYA 2005-07516-C02-01 and ESP 2004-06870-C02-02 from the Spanish Ministerio de Ciencia y Educación and from the Junta de Anaducía. C. G. M. acknowledges financial support from the Royal Society and the RCUK.
We present new results for the molecular gas, dust emission and the ionized gas in J1023+1952, an HI 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 30m telescope, emission from CO(1-0) and CO(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$ x 5.9 kpc, as well as in the nuclear region and outer disk of NGC 3227. The molecular gas is found to be abundant over the entire HI cloud, with H2-to-HI 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 24mu show that young SF is restricted to the southern part of the cloud. Despite the relatively uniform H$_2$ and HI column density across the cloud, young SF occurs only where the velocity dispersion in the CO and HI is a factor of $\sim$two lower (FWHM of 30 - 70 \kms) than elsewhere in the cloud (FWHM of 80 - 120 \kms). 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 Halpha photometry confirm that all the knots are young, with a tentative age sequence from the south-west (oldest knots) to the north-east (youngest). 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 pre-existing dwarf galaxy. Instead, it must be formed from recycled, metal-enrichd gas, expelled from NGC 3227 or NGC 3226 in a previous phase of the interaction.