Deeply Buried Nuclei in the Infrared-Luminous Galaxies NGC 4418 and Arp 220: I. ALMA Observations at $\lambda =$1.4-0.4 mm and Continuum Analysis

Abstract

We observed with ALMA three deeply buried nuclei in two galaxies, NGC 4418 and Arp 220, at $\sim$0.2${}^{\prime \prime }$ resolution over a total bandwidth of 67 GHz in $f_{\mathrm{rest}}$ = 215 - 697 GHz. Here we (1) introduce our program, (2) describe our data reduction method for wide-band, high-resolution imaging spectroscopy, (3) analyze in visibilities the compact nuclei with line forests, (4) develop a continuum-based estimation method of dust opacity and gas column density in heavily obscured nuclei, which uses the BGN (buried galactic nuclei) model and is sensitive to $\log (N_{{\mathrm{H}}_2}/{\mathrm{cm}}^{-2})\sim$ 25 - 26 at $\lambda \sim 1$ mm, and (5) present the continuum data and diagnosis of our targets. The three continuum nuclei have major-axis FWHM of $\sim$0.1${}^{\prime \prime }$-0.3${}^{\prime \prime }$ (20-140 pc) aligned to their rotating nuclear disks of molecular gas. However, each nucleus is described better with two or three concentric components than with a single Gaussian. The innermost cores have sizes of 0.05${}^{\prime \prime }$-0.10${}^{\prime \prime }$ (8-40 pc), peak brightness temperatures of ~100-500 K at 350 GHz, and more fractional flux at lower frequencies. The intermediate components correspond to the nuclear disks. They have axial ratios of $\approx$0.5 and hence inclinations $\stackrel{>}{\sim }60$ deg. The outermost elements include the bipolar outflow from Arp 220W. We estimate 1 mm dust opacity of ${\tau }_{\mathrm{d},1mm}\approx 2.2$, $1.2$, and $\stackrel{<}{\sim }0.4$ respectively for NGC 4418, Arp 220W, and Arp 220E. The first two correspond to $\log (N_{\mathrm{H}}/{\mathrm{cm}}^{-2})\sim 26$ for conventional dust-opacity laws, and hence the nuclei are highly Compton thick.

59 pages, 18 figures, 15 tables, accepted for publication in ApJ

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• The Rich JWST Spectrum of the Western Nucleus of Arp 220: Shocked Hot Core Chemistry Dominates the Inner Disk
• Investigating starburst-driven neutrino emission from galaxies in the Great Observatories All-Sky LIRG Survey
• Systematic Study of the Inner Structure of Molecular Tori in Nearby U/LIRGs using Velocity Decomposition of CO Rovibrational Absorption Lines
• The Binding Energies of Atoms on Amorphous Silicate Dust: A Computational Study
• Implication of a galaxy-scale negative feedback by one of the most powerful multi-phase outflows in a hyper-luminous infrared galaxy at the intermediate redshift