5-12 pc resolution ALMA imaging of gas and dust in the obscured compact nucleus of IRAS 17578-0400

TL;DR

This study uses high-resolution ALMA imaging to reveal a compact, obscured nucleus with a dusty torus, dense gas disk, and collimated outflows, providing insights into SMBH feeding and feedback mechanisms.

Contribution

It presents the first detailed ALMA observations of the gas and dust structures in IRAS 17578-0400's nucleus at 0.02-0.04'' resolution, uncovering a dusty torus and complex gas kinematics.

Findings

Discovered a 4 pc dusty torus with flat spectral index indicating millimeter corona emission.

Identified a 7 pc dense gas disk with outflows driven by disk winds.

Estimated SMBH mass of approximately 10^8 solar masses from kinematic analysis.

Abstract

We here present 0.02-0.04'' resolution ALMA observation of the compact obscured nucleus (CON) of IRAS17578-0400. A dusty torus within the nucleus, approximately 4 pc in radius, has been uncovered, exhibiting a usually flat spectral index at ALMA band 3, likely due to the millimeter corona emission from the central supermassive black hole (SMBH). The dense gas disk, traced by $^{13}$CO(1-0), spans 7 pc in radius and suggests an outflow driven by a disk wind due to its asymmetrical structure along the minor axis. Collimated molecular outflows (CMO), traced by the low-velocity components of the HCN(3-2) and HCO$^+$(3-2) lines, align with the minor axis gas disk. Examination of position-velocity plots of HCN(3-2) and HCO$^+$(3-2) reveals a flared dense gas disk extended a radius of $\sim$ 60 pc, infalling and rotating at speeds of about 200 km/s and 300 km/s, respectively. A centrifugal barrier, located around 4 pc from the dynamical center, implies an SMBH mass of approximately 10$^8$ $M_\odot$, consistent with millimeter corona emission estimates. The CMO maintains a steady rotation speed of 200 km/s over the 100 pc scale along the minor axis. The projected speed of the CMO is about 80 km/s, corresponding to around $\sim$ 500 km/s, assuming an inclination angle of 80$^\circ$. Such a kinematics structure of disk-driven collimated rotating molecular outflow with gas supplies from a falling rotating disk indicates that the feedback of the compact obscured nucleus is likely regulated by the momentum transfer of the molecular gas that connects to both the feeding of the nuclear starburst and supermassive black hole.