The unbearable opaqueness of Arp 220

S. Martín; S. Aalto; K. Sakamoto; E. González-Alfonso; S. Muller; C. Henkel; S. García-Burillo; R. Aladro; F. Costagliola; N. Harada; M. Krips; J. Martín-Pintado; S. Mühle; P. van der Werf; S. Viti

doi:10.1051/0004-6361/201528064

paper

The unbearable opaqueness of Arp 220

arXiv:1603.01291 · doi:10.1051/0004-6361/201528064

Abstract

We explore the potential of imaging vibrationally excited molecular emission at high angular resolution to better understand the morphology and physical structure of the dense gas in Arp~220 and to gain insight into the nature of the nuclear powering sources. Vibrationally excited emission of HCN is detected in both nuclei with a very high ratio relative to the total $L_{F I R}$ , higher than in any other observed galaxy and well above what is observed in Galactic hot cores. HCN $v_{2} = 1 f$ is observed to be marginally resolved in $\sim 60 \times 50$ ~pc regions inside the dusty $\sim 100$ ~pc sized nuclear cores. Its emission is centered on our derived individual nuclear velocities based on HCO $^{+}$ emission ( $V_{W N} = 5342 \pm 4$ and $V_{E N} = 5454 \pm 8$ ~\kms, for the western and eastern nucleus, respectively). With virial masses within $r \sim 25 - 30$ ~pc based on the HCN~ $v_{2} = 1 f$ line widths, we estimate gas surface densities (gas fraction $f_{g} = 0.1$ ) of $3 \pm 0.3 \times 1 0^{4} M_{⊙} p c^{- 2}$ (WN) and $1.1 \pm 0.1 \times 1 0^{4} M_{⊙} p c^{- 2}$ (EN). The $4 - 3/3 - 2$ flux density ratio could be consistent with optically thick emission, which would further constrain the size of the emitting region to $> 15$ ~pc (EN) and $> 22$ ~pc (WN). The absorption systems that may hide up to $70%$ of the HCN and HCO $^{+}$ emission are found at velocities of $- 50$ ~\kms~(EN) and $6$ , $- 140$ , and $- 575$ ~\kms (WN) relative to velocities of the nuclei. Blueshifted absorptions are the evidence of outflowing motions from both nuclei. The bright vibrational emission implies the existence of a hot dust region radiatively pumping these transitions. We find evidence of a strong temperature gradient that would be responsible for both the HCN $v_{2}$ pumping and the absorbed profiles from the vibrational ground state as a result of both continuum and self-absorption by cooler foreground gas.

arXiv comment14 pages accepted for publication in A&A

The unbearable opaqueness of Arp 220

Abstract

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