Deeply Buried Nuclei in the Infrared-Luminous Galaxies NGC 4418 and Arp 220: II. Line Forests at $\lambda = $1.4--0.4 mm and Circumnuclear Gas Observed with ALMA

TL;DR

This study uses ALMA to analyze dense molecular line forests in the nuclei of NGC 4418 and Arp 220, revealing gas dynamics, outflows, and chemical conditions at high resolution.

Contribution

First detailed spectral line survey of deeply buried galactic nuclei revealing complex gas motions and chemical environments.

Findings

Detected dense line forests indicating complex molecular environments.

Confirmed counter-rotation of nuclear disks and galactic disks.

Imaged bipolar outflows and identified gas inflow or collimated outflows.

Abstract

We present the line observations in our ALMA imaging spectral scan toward three deeply buried nuclei in NGC 4418 and Arp 220. We cover 67 GHz in $f_{\rm rest}$=215-697 GHz at about 0.2$"$ (30, 80 pc) resolution. All the nuclei show dense line forests; we report our initial line identification using 55 species. The line velocities generally indicate gas rotation around each nucleus, tracing nuclear disks of $\sim$100 pc sizes. We confirmed the counter-rotation of the nuclear disks in Arp 220 and that of the nuclear disk and the galactic disk in NGC 4418. While the brightest lines exceed 100 K, most of the major lines and many $^{13}$C isotopologues show absorption against even brighter continuum cores of the nuclei. The lines with higher upper-level energies, including those from vibrationally-excited molecules, tend to arise from smaller areas, indicating radially varying conditions in these nuclei. The outflows from the two Arp 220 nuclei cause blueshifted line absorption below the continuum level. The absorption mostly has small spatial offsets from the continuum peaks to indicate the outflow orientations. The bipolar outflow from the western nucleus is also imaged in multiple emission lines, showing the extent of $\sim$1$"$ (400 pc). Redshifted line absorption against the nucleus of NGC 4418 indicates either an inward gas motion or a small collimated outflow slanted to the nuclear disk. We also resolved some previous confusions due to line blending and misidentification.