ALMA multiline observations toward the central region of NGC 613

Abstract

We report ALMA observations of molecular gas and continuum emission in the 90 and 350 GHz bands toward a nearby Seyfert galaxy NGC 613. Radio continuum emissions were detected at 95 and 350 GHz from both the circum-nuclear disk (CND) ($r\leq90$ pc) and a star-forming ring (250 pc $\leq r\leq 340$ pc), and the 95 GHz continuum was observed to extend from the center at a position angle of $20^{\circ} \pm 8^{\circ}$. The archival 4.9 GHz data and our 95 GHz data show spectral indices of $\alpha\leq -0.6$ and $-0.2$ along the jets and in the star-forming ring; these can be produced by synchrotron emission and free-free emission, respectively. In addition, we detected the emission of CO(3-2), HCN(1-0), HCN(4-3), HCO$^+$(1-0), HCO$^+$(4-3), CS(2-1), and CS(7-6) in both the CND and ring. The rotational temperatures and column densities of molecules derived from $J=1-0$ and $4-3$ lines of HCN and HCO$^+$ and $J=2-1$ and $7-6$ of CS in the CND and ring were derived. Furthermore, a non-LTE model revealed that the kinetic temperature of $T_{\rm k}=350-550$ K in the CND is higher than $T_{\rm k}=80-300$ K in the ring, utilizing the intensity ratios of HCN, HCO$^+$, and CS. The star-formation efficiency in the CND is almost an order of magnitude lower than those at the spots in the star-forming ring, while the dominant activity of the central region is the star formation rather than active galactic nuclei. We determined that the large velocity dispersion of CO extending toward the north side of the CND and decomposing into blueshifted and redshifted features is probably explained by the effect of the radio jets. These results strongly suggest that the jets heat the gas in the CND, in which the feedback prevents star formation.