The Extremely Buried Nucleus of IRAS 17208–0014 Observed at Submillimeter and Near-infrared Wavelengths

Abstract

The ultraluminous infrared galaxy IRAS 17208−0014 is a late-stage merger that hosts a buried active galactic nucleus (AGN). To investigate its nuclear structure, we performed high-spatial-resolution ( ∼ 0.″04 ∼ 32 pc) Atacama Large Millimeter/submillimeter Array (ALMA) observations in Band 9 (∼450 μm or ∼660 GHz), along with near-infrared AKARI spectroscopy in 2.5–5.0 μm. The Band 9 dust continuum peaks at the AGN location, and toward this position CO(J = 6 − 5) and CS(J = 14 − 13) are detected in absorption. Comparison with nonlocal thermal equilibrium calculations indicates that, within the central beam (r ∼ 20 pc), there exists a concentrated component that is dense (107 cm−3) and warm (>200 K) and has a large column density ( N H 2 > 10 23 cm − 2 ). The AKARI spectrum shows deep and broad CO rovibrational absorption at 4.67 μm. Its band profile is well reproduced with a similarly dense and large column but hotter (∼1000 K) gas. The region observed through absorption in the near-infrared is highly likely in the nuclear direction, as in the submillimeter, but with a narrower beam including a region closer to the nucleus. The central component is considered to possess a hot structure where vibrationally excited HCN emission originates. The most plausible heating source for the gas is X-rays from the AGN. The AKARI spectrum does not show other AGN signs in 2.5–4 μm, but this absence may be usual for AGNs buried in a hot mid-infrared core. Further, based on our ALMA observations, we relate the various nuclear structures of IRAS 17208−0014 that have been proposed in the literature.

Multiphase Gas Nature in the Sub-parsec Region of the Active Galactic Nuclei. I. Dynamical Structures of Dusty and Dust-free Outflow

“…Furthermore, our results imply that a hot dust (T d ∼ 1000 K) component in a central sub-parsec region can exist in the near-IR source, we expect to be observed by molecular CO absorption lines in ultra-luminous infrared galaxies (e.g., Baba et al 2018Baba et al , 2022Onishi et al 2021) and Seyfert galaxies (Uzuo et al 2021;Matsumoto et al 2022).…”

Warm Molecular Gas in the Central Parsecs of the Buried Nucleus of NGC 4418 Traced with the Fundamental CO Rovibrational Absorptions

“…Imanishi et al (2010) reported the AKARI CO absorption depth τ (CO) = 0.5. Here, τ(CO) is the apparent depth of the blended CO feature measured at the bottom of the blended R-branch feature 9 , and is often used as an indicator of the absorbing CO column density without careful analysis using LTE spectral modeling (Spoon et al 2004;Baba et al 2018Baba et al , 2022. We note, however, that τ(CO) is proportional to the CO column density only until individual CO absorption lines are saturated; once saturated, it increases with the line (velocity) width (see Baba et al 2018 for the demonstration).…”

“…The AKARI spectroscopy is particularly efficient for detecting the CO absorptions in low-redshift galaxies due to its wavelength coverage below 5 μm. Our sample comes mainly from Baba et al (2022), who compiled the AKARI CO measurements, and two others with other CO measurements. We specifically compare galaxies with either deep CO absorption, enhanced vibrationally excited HCN emission at submillimeter wavelengths, or both.…”

“…Onishi et al (2021) further studied this galaxy to examine the inflow and outflow of warm CO in detail, and Matsumoto et al (2022) used a theoretical model to demonstrate that such flow signatures can be observed. Baba et al (2018) studied a sample of nearby obscured AGNs with lowresolution AKARI and Spitzer spectra to systematically examine the properties of the hot gas in AGNs (see also Baba et al 2022). We will compare NGC 4418 with these galaxies in detail in Section 6.5.…”

Characterizing Compact 15–33 GHz Radio Continuum Sources in Local U/LIRGs

“…Although this galaxy is detected in the hard X-ray with NuSTAR and Chandra, it is inconclusive from analysis of the X-ray spectra whether a buried AGN is present et al 2013). It has been argued in several studies that the system likely hosts buried AGNs (e.g., Iwasawa et al 2011;Falstad et al 2021;Baba et al 2022). The brightest region on the west is located at the dynamical center of the molecular gas as revealed by ALMA (2018.1.00486.…”