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We present the first 100 pc scale view of the dense molecular gas in thecentral ~ 1.3 kpc region of the type-1 Seyfert NGC 1097 traced by HCN (J=4-3)and HCO+ (J=4-3) lines afforded with ALMA band 7. This galaxy shows significantHCN enhancement with respect to HCO+ and CO in the low-J transitions, whichseems to be a common characteristic in AGN environments. Using the ALMA data,we study the characteristics of the dense gas around this AGN and search forthe mechanism of HCN enhancement. We find a high HCN (J=4-3) to HCO+ (J=4-3)line ratio in the nucleus. The upper limit of the brightness temperature ratioof HCN (v2=1^{1f}, J=4-3) to HCN (J=4-3) is 0.08, which indicates that IRpumping does not significantly affect the pure rotational population in thisnucleus. We also find a higher HCN (J=4-3) to CS (J=7-6) line ratio in NGC 1097than in starburst galaxies, which is more than 12.7 on the brightnesstemperature scale. Combined from similar observations from other galaxies, wetentatively suggest that this ratio appears to be higher in AGN-host galaxiesthan in pure starburst ones similar to the widely used HCN to HCO+ ratio. LTEand non-LTE modeling of the observed HCN and HCO+ lines using J=4-3 and 1-0data from ALMA, and J=3-2 data from SMA, reveals a high HCN to HCO+ abundanceratio (5 < [HCN]/[HCO+] < 20: non-LTE analysis) in the nucleus, and that thehigh-J lines (J=4-3 and 3-2) are emitted from dense (10^{4.5} < n_H2 [/cc] <10^6), hot (70 < Tkin [K] < 550) regions. Finally we propose that the hightemperature chemistry is more plausible to explain the observed enhanced HCNemission in NGC 1097 than the pure gas phase PDR/XDR chemistry.

Submillimeter ALMA Observations of the Dense Gas in the Low-Luminosity Type-1 Active Nucleus of NGC 1097