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. (arXiv:2109.06695v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Sakamoto_K/0/1/0/all/0/1">Kazushi Sakamoto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gonzalez_Alfonso_E/0/1/0/all/0/1">Eduardo Gonzalez-Alfonso</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Martin_S/0/1/0/all/0/1">Sergio Martin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wilner_D/0/1/0/all/0/1">David J. Wilner</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aalto_S/0/1/0/all/0/1">Susanne Aalto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Evans_A/0/1/0/all/0/1">Aaron S. Evans</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Harada_N/0/1/0/all/0/1">Nanase Harada</a>

We observed with ALMA three deeply buried nuclei in two galaxies, NGC 4418
and Arp 220, at $sim$0.2$”$ resolution over a total bandwidth of 67 GHz in
$f_{rm rest}$ = 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_{rm H_2}/{rm 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$0.1$”$-0.3$”$
(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 ~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 $tau_{rm
d,1mm} approx 2.2$, $1.2$, and $stackrel{<}{sim} 0.4$ respectively for NGC
4418, Arp 220W, and Arp 220E. The first two correspond to $log(N_{rm H}/{rm
cm}^{-2}) sim 26$ for conventional dust-opacity laws, and hence the nuclei are
highly Compton thick.

We observed with ALMA three deeply buried nuclei in two galaxies, NGC 4418
and Arp 220, at $sim$0.2$”$ resolution over a total bandwidth of 67 GHz in
$f_{rm rest}$ = 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_{rm H_2}/{rm 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$0.1$”$-0.3$”$
(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 ~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 $tau_{rm
d,1mm} approx 2.2$, $1.2$, and $stackrel{<}{sim} 0.4$ respectively for NGC
4418, Arp 220W, and Arp 220E. The first two correspond to $log(N_{rm H}/{rm
cm}^{-2}) sim 26$ for conventional dust-opacity laws, and hence the nuclei are
highly Compton thick.

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