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

Journal article, 2021

We observed with Atacama Large Millimeter/submillimeter Array three deeply buried nuclei in two galaxies, NGC 4418 and Arp 220, at similar to 0.'' 2 resolution over a total bandwidth of 67 GHz in f(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 buried galactic nuclei (BGN) model and is sensitive to log (NH2/cm(-2)) similar to 25-26 at lambda similar to 1 mm, and (5) present the continuum data and diagnosis of our targets. The three continuum nuclei have major-axis FWHMs of similar to 0.'' 1-0.'' 3 (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.'' 05-0.'' 10 (8-40 pc), peak brightness temperatures of similar to 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 approximate to 0.5 and hence inclinations greater than or similar to 60 degrees. The outermost elements include the bipolar outflow from Arp 220W. We estimate 1 mm dust opacity of tau(d, 1 mm )approximate to 2.2, 1.2, and less than or similar to 0.4, respectively, for NGC 4418, Arp 220W, and Arp 220E. The first two correspond to log(N-H/cm(2)) similar to 26 for conventional dust-opacity laws, and hence the nuclei are highly Compton thick.