ALMA INVESTIGATION OF VIBRATIONALLY EXCITED HCN/HCO⁺/HNC EMISSION LINES IN THE AGN-HOSTING ULTRALUMINOUS INFRARED GALAXY IRAS 20551−4250

Abstract

We present the results of ALMA Cycle 2 observations of the ultraluminous infrared galaxy IRAS 20551−4250 at HCN/HCO + /HNC J = 3-2 lines at both vibrational ground (v = 0) and vibrationally excited (v 2 = 1) levels. This galaxy contains a luminous buried active galactic nucleus (AGN), in addition to starburst activity, and our ALMA Cycle 0 data revealed a tentatively detected vibrationally excited HCN v 2 = 1f J = 4-3 emission line. In our ALMA Cycle 2 data, the HCN/HCO + /HNC J = 3-2 emission lines at v = 0 are clearly detected. The HCN and HNC v 2 = 1f J = 3-2 emission lines are also detected, but the HCO + v 2 = 1f J = 3-2 emission line is not. Given the high energy level of v 2 = 1 and the resulting difficulty of collisional excitation, we compared these results with those of the calculation of infrared radiative pumping, using the available infrared 5-35 μm spectrum. We found that all of the observational results were reproduced if the HCN abundance was significantly higher than that of HCO + and HNC. The flux ratio and excitation temperature between v 2 = 1f and v = 0, after correction for possible line opacity, suggests that infrared radiative pumping affects rotational ( J-level) excitation at v = 0 at least for HCN and HNC. The HCN-to-HCO + v = 0 flux ratio is higher than those of starburst-dominated regions, and will increase even more when the derived high HCN opacity is corrected. The enhanced HCN-to-HCO + flux ratio in this AGN-hosting galaxy can be explained by the high HCN-to-HCO + abundance ratio and sufficient HCN excitation at up to J = 4, rather than the significantly higher efficiency of infrared radiative pumping for HCN than HCO + .

ALMA HCN AND HCO+ J = 3 − 2 OBSERVATIONS OF OPTICAL SEYFERT AND LUMINOUS INFRARED GALAXIES: CONFIRMATION OF ELEVATED HCN-TO-HCO+ FLUX RATIOS IN AGNS

“…Here, the possible difference in population between HCN and HCO + at rotational J-levels within v=0 and v 2 =1 is not considered. As discussed in Imanishi et al (2016a), the B v2=0−1,vib values are comparable within 10% between HCN and HCO + . The F ν(IR) values at 12 µm and 14 µm for the observed galaxies are derived from their Spitzer IRS low-resolution spectra (Brandl et al 2006;Imanishi et al 2007b;Wu et al 2009).…”

“…These ratios are several factors higher than that in the Galactic active (>10 L ⊙ /M ⊙ ) and luminous (>10 7 L ⊙ ) star-forming region, W49A (<1.2 × 10 −9 ) (Nagy et al 2015;Imanishi et al 2016a). A luminous buried AGN is a plausible origin, although the possibility of a very compact extreme starburst cannot be completely ruled out (Aalto et al 2015b).…”

“…As the frequencies of HCN v 2 =1f and HCO + v=0 are very close to each other at J=3-2 and J=4-3, we can only clearly separate these lines for galaxies with small molecular line widths, such as NGC 4418 (Sakamoto et al 2010), IRAS 20551−4250 (Imanishi & Nakanishi 2013b;Imanishi et al 2016a), and IC 860 (Aalto et al 2015b). For the majority of the other galaxies, these lines are blended.…”

“…Thus, HCN vibrational excitation, through the absorption of infrared 14 µm photons, is expected to occur more efficiently in an AGN than in a starburst. In fact, the HCN v 2 =1f emission lines at J=3-2 and/or J=4-3 have recently been detected in gas/dust-rich LIRGs, which most likely, or plausibly, contain luminous AGNs, i.e., NGC 4418 (Sakamoto et al 2010;Costagliola et al 2015), IRAS 20551−4250 (Imanishi & Nakanishi 2013bImanishi et al 2016a), Mrk 231 (Aalto et al 2015a), and a few further LIRGs (Aalto et al 2015b;Martin et al 2016), demonstrating that this infrared radiative pumping mechanism actually works in some AGNs.…”

“…Finally, in ALMA band 6, in addition to the HCN J=3-2 and HCO + J=3-2 lines, vibrationally excited v 2 =1, l=1f (hereafter v 2 =1f) emission lines of HCN (ν rest = 267.20 GHz) and HCO + (ν rest = 268.69 GHz) can also be simultaneously covered in one shot, with the 5 GHz-wide correlator unit. These v 2 =1f emission lines can be used to investigate how infrared radiative pumping works in observed galaxies and affects rotational excitation at v=0 (Sakamoto et al 2010;Imanishi & Nakanishi 2013b;Aalto et al 2015a,b;Imanishi et al 2016a;Martin et al 2016). The simultaneous observations of all of these lines make their flux ratios reliable, with the effect of possible systematic uncertainties being minimized.…”