Probing Circumnuclear Environments with the HCN(J = 3-2) and HCO+(J =
  3-2) lines: Case of NGC 1097
  We present the first interferometric HCN(J = 3-2) and HCO+(J = 3-2) maps in
the circumnuclear region of NGC 1097, obtained with the Submillimeter Array.
The goal is to study the characteristics of the dense gas associated with the
starburst ring/Seyfert nucleus. With these transitions, we suppress the diffuse
low density emission in the nuclear region. We detect and resolve the
individual compact giant molecular cloud associations (GMAs) in the 1.4 kpc
circumnuclear starburst ring and within the 350 pc nuclear region. The nucleus
is brighter than the ring in both lines, and contributes to ~20% and ~30% to
the total detected HCO+(J = 3-2) and HCN(J = 3-2) flux, within the central 1.4
kpc. The intensity ratios of HCN(J = 3-2)/HCO+(J = 3-2) are roughly unity in
the GMAs of the starburst ring. However, this ratio is up to ~2 in the nuclear
region. From the HCN(J = 3-2)/HCN(J = 1-0) ratio of <0.2 in the nucleus, we
infer that the nuclear HCN(J = 3-2) emission might be optically thin. The
HCO+(J = 3-2) and HCN(J = 3-2) show correlations with 12CO(J = 3-2) and the
24{\mu}m emission. The tight correlations of HCN(J = 3-2), HCO+(J = 3-2) and
24{\mu}m emission in the starburst ring suggest that the dense molecular gas
and the dust are from the same origins of star forming regions. On the other
hand, the HCN(J = 3-2) emission of the nucleus is significantly enhanced,
indicating mechanisms other than star formation, such as AGN activities. A
self-consistent check of the fractional abundance enhanced by X-ray ionization
chemistry of the nucleus is possible with our observations.
