HCN (1−0) opacity of outflowing gas in Arp 220W

Abstract

Context. We present our findings for the HCN/H13CN 1−0 line ratio in the molecular outflow of Arp 220 west based on high-resolution ALMA data. Aims. Molecular gas masses in the outflowing gas of galaxies driven by active galactic nuclei or starbursts are important parameters for understanding the feedback of these latter two phenomena and star-formation quenching. The conversion factor of line luminosities to masses is related to the optical depth of the molecular lines. Methods. Using H13CN 1–0, the isotopic line of HCN 1−0, to obtain the line ratio of HCN/H13CN 1−0 is an ideal way to derive the optical depth of HCN 1−0 in outflowing gas. Results. With the nondetection of H13CN 1−0 in the outflowing gas, a 3σ lower limit of HCN/H13CN 1−0 line ratio is obtained, which is at least three times higher than that found in the whole of the whole system of Arp 220. The high HCN/H13CN 1−0 line ratio indicates low opacity of HCN 1−0 in the outflowing gas, even though the upper limit of HCN 1−0 opacity obtained here is still not good enough to draw any robust conclusions if HCN 1−0 is optically thin. A lower conversion factor of HCN 1−0 luminosity to dense gas mass in the outflowing gas should be used than that used for the host galaxy of Arp 220.

Arp 220: A Post-starburst Galaxy with Little Current Star Formation outside of Its Nuclear Disks

“…There is clear evidence for an embedded AGN in the western nucleus of Arp 220 (e.g Lockhart et al 2015;Rangwala et al 2015;Barcos-Muñoz et al 2018). A number of observations, including the detection of Fe Kα emission at 6.3 keV, support this conclusion (e.g., Yoast-Hull et al 2017;Wang et al 2021). Varenius et al (2016) further suggested that the alignment of the 150 MHz continuum morphology above and below the nuclear disk and the similarity to the 150 MHz observations of M82 are evidence for an outflow in the eastern nucleus, with the southern side being the closer (approaching) part consistent with CO (6-5) observations presented in Rangwala et al (2011).…”

Circumnuclear dense gas disk fuelling the active galactic nucleus in the nearby radio galaxy NGC 4261

“…We obtain L ′ HCN = 6.03 × 10 6 K km s −1 pc 2 . The luminosity-to-mass conversion factor, α HCN , can vary from 0.24 to over 20 M ⊙ (K km s −1 pc 2 ) −1 (Barcos- Muñoz et al 2018;Evans et al 2020;Jones et al 2021), depending on the gas density and the line opacity (Jones et al 2021;Wang et al 2021). If we consider the standard extragalactic conversion factor α HCN of 10 M ⊙ (K km s −1 pc 2 ) −1 (Gao & Solomon 2004), M dg is calculated to be 6.03 × 10 7 M ⊙ .…”