Compact starburst in the central regions of Seyfert galaxies

TL;DR

This study uses high-resolution millimeter-wave imaging to investigate molecular emissions in Seyfert galaxy centers, revealing that some host compact starbursts while others show elevated HCN emissions likely due to X-ray effects, not star formation.

Contribution

It provides the first detailed high-resolution molecular line survey of Seyfert galaxy centers, distinguishing starburst activity from X-ray influenced molecular chemistry.

Findings

Many Seyfert galaxies host compact nuclear starbursts.

Elevated HCN emission in some Seyferts is due to XDR effects, not star formation.

HCN/HCO+ ratios are higher in X-ray dominated regions than in starbursts.

Abstract

We have conducted a high-resolution ``3D'' imaging survey of the CO(1--0), HCN(1--0), and HCO$^+$(1--0) lines toward the central a few kpc regions of the Seyfert and starburst galaxies in the local universe using the Nobeyama Millimeter Array. We detected luminous HCN(1--0) emissions toward a considerable fraction of these Seyfert galaxies (10 of 12 in our sub-sample), which indicated that some of these Seyfert galaxies, such as NGC 3079, NGC 3227, NGC 4051, NGC 6764, and NGC 7479, are indeed accompanied with compact nuclear starburst, given the tight correlation between the HCN(1--0) luminosity and the star formation rate among star-forming galaxies. However, we suggest that the elevated HCN(1--0) emission from some of these Seyfert galaxies, including NGC 1068, NGC 1097, NGC 5033, and NGC 5194, does not signify the presence of massive starbursts there. This is because these Seyfert nuclei show abnormally high HCN(1--0)/HCO$^+$(1--0) ratios (2--3), which were never observed in the starburst nuclei in our sample. This could be attributed to the overabundance of HCN molecules in the X-ray dominated regions (XDRs) at the centers of these Seyfert galaxies.