A massive dense gas cloud close to the nucleus of the Seyfert galaxy NGC 1068

Abstract

Abstract Using the ALMA archival data of both 12CO (6–5) line and 689-GHz continuum emission towards the archetypical Seyfert galaxy, NGC 1068, we identified a distinct continuum peak separated by 15 pc from the nuclear radio component S1 in projection. The continuum flux gives a gas mass of ∼2 × 105 M⊙ and bolometric luminosity of ∼108 L⊙, leading to a star formation rate of ∼0.1 M⊙ yr−1. Subsequent analysis on the line data suggest that the gas cloud has a size of ∼10 pc, yielding to a mean H2 number density of ∼105 cm−3. We therefore refer to the gas as a “massive dense gas cloud”: the gas density is high enough to form a “protostar cluster” with a stellar mass of ∼104 M⊙. We found that the gas stands at a unique position between galactic and extraglactic clouds in the diagrams of start formation rate (SFR) vs. gas mass proposed by Lada et al. (2012, ApJ, 745, 190) and surface density of gas vs. SFR density by Krumholz and McKee (2005, ApJ, 630, 250). All the gaseous and star-formation properties may be understood in terms of the turbulence-regulated star formation scenario. Since there are two stellar populations with ages of 300 Myr and 30 Myr in the 100 pc scale circumnulear region, we discuss that NGC 1068 has experienced at least three episodic star-formation events with the likelihood that the inner star-forming region is the younger. Together with several lines of evidence that the dynamics of the nuclear region is decoupled from that of the entire galactic disk, we discuss that the gas inflow towards the nuclear region of NGC 1068 may be driven by a past minor merger.