Molecular gas and Star Formation Properties in Early Stage Mergers: SMA
  CO(2-1) Observations of the LIRGs NGC 3110 and NGC 232
  Mergers of galaxies are an important mode for galaxy evolution because they
serve as an efficient trigger of powerful starbursts. However, observational
studies of the molecular gas properties during their early stages are scarce.
We present interferometric CO(2-1) maps of two luminous infrared galaxies
(LIRGs), NGC 3110 and NGC 232, obtained with the Submillimeter Array (SMA) with
~ 1 kpc resolution. While NGC 3110 is a spiral galaxy interacting with a minor
(14:1 stellar mass) companion, NGC 232 is interacting with a similarly sized
object. We find that such interactions have likely induced in these galaxies
enhancements in the molecular gas content and central concentrations, partly at
the expense of atomic gas. The obtained molecular gas surface densities in
their circumnuclear regions are $\Sigma_{\rm mol}~\gtrsim10^{2.5}$ M$_\odot$
pc$^{-2}$, higher than in non-interacting objects by an order of magnitude. Gas
depletion times of ~ 0.5 - 1 Gyr are found for the different regions, lying in
between non-interacting disk galaxies and the starburst sequence. In the case
of NGC 3110, the spiral arms show on average 0.5 dex shorter depletion times
than in the circumnuclear regions if we assume a similar H$_2$-CO conversion
factor. We show that even in the early stages of the interaction with a minor
companion, a starburst is formed along the circumnuclear region and spiral
arms, where a large population of SSCs is found (~350), and at the same time a
large central gas concentration is building up which might be the fuel for an
active galactic nucleus. The main morphological properties of the NGC 3110
system are reproduced by our numerical simulations and allow us to estimate
that the current epoch of the interaction is at ~ 150 Myrs after closest
approach.
