Molecular Gas Kinematics and Line Diagnostics in Early-type Galaxies:
  NGC4710 and NGC5866
  We present interferometric observations of CO lines (12CO(1-0, 2-1) and
13CO(1-0, 2-1)) and dense gas tracers (HCN(1-0), HCO+(1-0), HNC(1-0) and
HNCO(4-3)) in two nearby edge-on barred lenticular galaxies, NGC 4710 and NGC
5866, with most of the gas concentrated in a nuclear disc and an inner ring in
each galaxy. We probe the physical conditions of a two-component molecular
interstellar medium in each galaxy and each kinematic component by using
molecular line ratio diagnostics in three complementary ways. First, we measure
the ratios of the position-velocity diagrams of different lines, second we
measure the ratios of each kinematic component's integrated line intensities as
a function of projected position, and third we model these line ratios using a
non-local thermodynamic equilibrium radiative transfer code. Overall, the
nuclear discs appear to have a tenuous molecular gas component that is hotter,
optically thinner and with a larger dense gas fraction than that in the inner
rings, suggesting more dense clumps immersed in a hotter more diffuse molecular
medium. This is consistent with evidence that the physical conditions in the
nuclear discs are similar to those in photo-dissociation regions. A similar
picture emerges when comparing the observed molecular line ratios with those of
other galaxy types. The physical conditions of the molecular gas in the nuclear
discs of NGC4710 and NGC5866 thus appear intermediate between those of spiral
galaxies and starbursts, while the star formation in their inner rings is even
milder.
