Luminous Infrared Galaxies with the Submillimeter Array. III. The Dense Kiloparsec Molecular Concentrations of Arp 299
We have used high-resolution (~2".3) observations of the local (D_L = 46 Mpc) luminous infrared galaxy Arp 299 to map out the physical properties of the molecular gas that provides the fuel for its extreme star formation activity. The ^(12)CO J = 3-2, ^(12)CO J = 2-1, and ^(13)CO J = 2-1 lines were observed with the Submillimeter Array, and the short spacings of the ^(12)CO J = 2-1 and J = 3-2 observations have been recovered using the James Clerk Maxwell Telescope single dish observations. We use the radiative transfer code RADEX to estimate the physical properties (density, column density, and temperature) of the different regions in this system. The RADEX solutions of the two galaxy nuclei, IC 694 and NGC 3690, are consistent with a wide range of gas components, from warm moderately dense gas with T_kin > 30 K and n(H_2) ~ 0.3-3 × 10^3 cm^(–3) to cold dense gas with T_kin ~ 10-30 K and n(H_2) > 3 × 10^3 cm^(–3). The overlap region is shown to have a better constrained solution with T_kin ~ 10-50 K and n(H_2) ~ 1-30 × 10^3 cm^(–3). We estimate the gas masses and star formation rates of each region in order to derive molecular gas depletion times. The depletion times of all regions (20-50 Myr) are found to be about two orders of magnitude lower than those of normal spiral galaxies. This rapid depletion time can probably be explained by a high fraction of dense gas on kiloparsec scales in Arp 299. We estimate the CO-to-H_2 factor, α_co to be 0.4 ± 0.3(3 × 10^(–4)/x_CO) M_☉ (K km s^(–1) pc^2)^(–1) for the overlap region. This value agrees well with values determined previously for more advanced merger systems.