Probing highly obscured, self-absorbed galaxy nuclei with vibrationally excited HCN

Abstract

We present high resolution (0. ′′ 4) IRAM PdBI and ALMA mm and submm observations of the (ultra) luminous infrared galaxies ((U)LIRGs) IRAS17208-0014, Arp220, IC860 and Zw049.057 that reveal intense line emission from vibrationally excited (ν 2 =1) J=3-2 and 4-3 HCN. The emission is emerging from buried, compact (r <17-70 pc) nuclei that have very high implied mid-infrared surface brightness >5 × 10 13 L ⊙ kpc −2 . These nuclei are likely powered by accreting supermassive black holes (SMBHs) and/or hot (>200 K) extreme starbursts. Vibrational, ν 2 =1, lines of HCN are excited by intense 14 µm mid-infrared emission and are excellent probes of the dynamics, masses, and physical conditions of (U)LIRG nuclei when H 2 column densities exceed 10 24 cm −2 . It is clear that these lines open up a new interesting avenue to gain access to the most obscured AGNs and starbursts. Vibrationally excited HCN acts as a proxy for the absorbed mid-infrared emission from the embedded nuclei, which allows for reconstruction of the intrinsic, hotter dust SED. In contrast, we show strong evidence that the ground vibrational state (ν=0), J=3-2 and 4-3 rotational lines of HCN and HCO + fail to probe the highly enshrouded, compact nuclear regions owing to strong self-and continuum absorption. The HCN and HCO + line profiles are double-peaked because of the absorption and show evidence of non-circular motions -possibly in the form of in-or outflows. Detections of vibrationally excited HCN in external galaxies are so far limited to ULIRGs and early-type spiral LIRGs, and we discuss possible causes for this. We tentatively suggest that the peak of vibrationally excited HCN emission is connected to a rapid stage of nuclear growth, before the phase of strong feedback.

The unbearable opaqueness of Arp220

“…In this case, we observed a significant offset of 0.045 of the spectral feature toward the northwest (PA ∼ 120 • ) with respect to the continuum, in good agreement with the plane of rotation measured with HCN (PA ∼ 45 • Scoville et al 2015), but not with the gradient observed with CO in the inner region (Sakamoto et al 1999;Rangwala et al 2015). Again, when peering through the opaque mid-IR dust, the HCN v 2 = 1 f line might be probing the kinematics of a hot dusty circumnuclear disk, as has been proposed by Aalto et al (2015b), with a radius of ∼18 pc.…”

“…Vibrational HCN levels are efficiently excited by 14 µm continuum photons provided that T B > 100 K (Aalto et al 2015b). Our data show that the peak of the continuum emission matches the position at which vibrationally excited emission is found, which supports the idea that the emission is excited through continuum photons.…”

“…This feature is better identified in the narrower spectral lines observed in the LIRGs IC860 and Zw49-57 presented by Aalto et al (2015b). However, this has to be confirmed with other transitions of this molecule for a proper identification in Arp 220.…”

“…Based on the discussion on recent detections of vibrationally excited HCN in extragalactic objects, collisional excitation of HCN up to the vibrational levels has been discarded (Sakamoto et al 2010;Imanishi & Nakanishi 2013;Aalto et al 2015b). Vibrational HCN levels are efficiently excited by 14 µm continuum photons provided that T B > 100 K (Aalto et al 2015b).…”

“…However, a new obstacle was found in the form of line absorption features that erase the molecular emission from the very central regions. This absorption does not only affect the CO emission (Rangwala et al 2015), but also dense gas tracers such as HCN, HCO + , and CS (Scoville et al 2015;Aalto et al 2015b), as well as their rarer isotopic substitutions (Tunnard et al 2015), which were so far always regarded as a solution for optical thickness affecting the main isotopologs.…”