Spatially resolved CO SLED of the Luminous Merger Remnant NGC 1614 with
  ALMA
  We present high-resolution (1".0) Atacama Large Millimeter/submillimeter
Array (ALMA) observations of CO (1-0) and CO (2- 1) rotational transitions
toward the nearby IR-luminous merger NGC 1614 supplemented with ALMA archival
data of CO (3-2), and CO (6-5) transitions. The CO (6-5) emission arises from
the starburst ring (central 590 pc in radius), while the lower-$J$ CO lines are
distributed over the outer disk ($\sim$ 3.3 kpc in radius). Radiative transfer
and photon dominated region (PDR) modeling reveal that the starburst ring has a
single warmer gas component with more intense far-ultraviolet radiation field
($n_{\rm{H_2}}$ $\sim$ 10$^{4.6}$ cm$^{-3}$, $T_{\rm{kin}}$ $\sim$ 42 K, and
$G_{\rm{0}}$ $\sim$ 10$^{2.7}$) relative to the outer disk ($n_{\rm{H_2}}$
$\sim$ 10$^{5.1}$ cm$^{-3}$, $T_{\rm{kin}}$ $\sim$ 22 K, and $G_{\rm{0}}$
$\sim$ 10$^{0.9}$). A two-phase molecular interstellar medium with a warm and
cold ($>$ 70 K and $\sim$ 19 K) component is also an applicable model for the
starburst ring. A possible source for heating the warm gas component is
mechanical heating due to stellar feedback rather than PDR. Furthermore, we
find evidence for non-circular motions along the north-south optical bar in the
lower-$J$ CO images, suggesting a cold gas inflow. We suggest that star
formation in the starburst ring is sustained by the bar-driven cold gas inflow,
and starburst activities radiatively and mechanically power the CO excitation.
The absence of a bright active galactic nucleus can be explained by a scenario
that cold gas accumulating on the starburst ring is exhausted as the fuel for
star formation, or is launched as an outflow before being able to feed to the
nucleus.
