The unbearable opaqueness of Arp220

Abstract

Context. The origin of the enormous luminosities of the two opaque nuclei of Arp 220, the prototypical ultra-luminous infrared galaxy, remains a mystery because we lack observational tools to explore the innermost regions around the nuclei. Aims. 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. Methods. The Atacama Large Millimeter/submillimeter Array (ALMA) provided simultaneous observations of HCN, HCO + , and vibrationally excited HCN v 2 = 1 f emission. Their J = 4-3 and 3-2 transitions were observed at a matching resolution of ∼0.5 , which allows us to isolate the emission from the two nuclei. Results. The HCN and HCO + lines within the ground-vibrational state poorly describe the central ∼100 pc region around the nuclei because there are strong effects of cool absorbing gas in the foreground and severe line blending that is due to the prolific molecular emission of Arp 220. Vibrationally excited emission of HCN is detected in both nuclei with a very high ratio relative to the total L FIR , 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 ∼60 × 50 pc regions inside the dusty ∼100 pc sized nuclear cores. Its emission is centered on our derived individual nuclear velocities based on HCO + emission (V WN = 5342 ± 4 and V EN = 5454 ± 8 km s −1 , for the western and eastern nucleus, respectively). With virial masses within r ∼ 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 ± 0.3 × 10 4 M pc −2 (WN) and 1.1 ± 0.1 × 10 4 M pc −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 km s −1 (EN) and 6, −140, and −575 km s −1 (WN) relative to velocities of the nuclei. Blueshifted absorptions are the evidence of outflowing motions from both nuclei. Conclusions. Although vibrationally excited molecular transitions could also be affected by opacity, they may be our best tool to peer into the central few tens of parsecs around compact obscured nuclei like those of Arp 220. 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.

Super Hot Cores in NGC 253: witnessing the formation and early evolution of super star clusters

“…As a consequence of the high column densities, the extinction in star forming regions is very high, preventing the direct observation of the hot dust in the mid-IR. However, the HC 3 N rotational transitions from its ground and vibrationally excited states are emitted in the (sub)millimeter range which is basically unaffected by dust extinction (even in extremely obscured objects like the nuclei of the ultraluminous IR galaxy Arp 220, Barcos-Muñoz et al 2015;Martín et al 2016), allowing to probe deeply embedded sources. Therefore, measuring multiple rotational transitions from different vibrational states of HC 3 N provides a unique tool to infer their physical properties, their thermal and density structures, and the kinematics of the material heated by the protostars.…”

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

“…For the obscured AGN-hosting LIRGs with detectable HCN v 2 =1f emission lines, significant flux attenuation by line opacity of the HCN v=0 emission is indicated (Sakamoto et al 2010;Aalto et al 2015a,b;Imanishi et al 2016a;Martin et al 2016). For NGC 4418, IRAS 20551−4250, and Mrk 231, the line-opacity-corrected intrinsic HCN v 2 =1f to v=0 flux ratios are quantitatively estimated to be ∼0.01 (Sakamoto et al 2010;Aalto et al 2015a;Imanishi et al 2016a).…”

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

“…Very recently, the detection of the HCN v 2 =1f J = 3-2 emission lines in three LIRGs (IC 860, Zw 049.057, and Arp 220) and HCN v 2 = 1f J = 4-3 emission lines in two LIRGs (Arp 220W and IRAS 17208−0014) has been reported (Aalto et al 2015a;Martin et al 2016). This paper adds the detection of HCN v 2 = 1f J = 3-2 and HNC v 2 = 1f J = 3-2 emission lines in IRAS 20551−4250.…”

“…The information of flux attenuation for v = 0 emission is needed to derive the intrinsic ratio. These four sources are not included in our quantitative discussion about intrinsic flux ratios, but it is quite possible that these sources have similarly low intrinsic values to the above three LIRGs because large line opacities for the v = 0 emission are argued (Aalto et al 2015a;Martin et al 2016).…”