Spatially Resolved HCN Absorption Features in the Circumnuclear Region of NGC 1052

Abstract

We present the first VLBI detection of HCN molecular absorption in the nearby active galactic nucleus NGC 1052. Utilizing the 1 mas resolution achieved by the Korean VLBI Network, we have spatially resolved the HCN absorption against a double-sided nuclear jet structure. Two velocity features of HCN absorption are detected significantly at the radial velocity of 1656 and 1719 km s −1 , redshifted by 149 and 212 km s −1 with respect to the systemic velocity of the galaxy. The column density of the HCN molecule is estimated to be 10 15 -10 16 cm −2 , assuming an excitation temperature of 100-230 K. The absorption features show high optical depth localized on the receding jet side, where the free-free absorption occurred due to the circumnuclear torus. The size of the foreground absorbing molecular gas is estimated to be on approximately one-parsec scales, which agrees well with the approximate size of the circumnuclear torus. HCN absorbing gas is likely to be several clumps smaller than 0.1 pc inside the circumnuclear torus. The redshifted velocities of the HCN absorption features imply that HCN absorbing gas traces ongoing infall motion inside the circumnuclear torus onto the central engine.

A Broad HCO+ Absorption Line Associated with the Circumnuclear Torus of NGC 1052

“…The flux densities of the central component within 1.6 mas (the major axis of the synthesized beam), the eastern jet, and the western jet are determined to be 414, 92, and 93 mJy, by summing the CLEAN components. The continuum image is consistent with the previous KVN map at 89 GHz taken in 2015 (Sawada-Satoh et al 2016).…”

“…During a time gap of 27 months between the two observations of HCN and HCO + , the background receding jet component could move 0.18 pc outward, almost one beam size. Adopting an abundance ratio HCO + relative to H 2 of (2-3) ×10 −9 in Galactic diffuse molecular gas (Liszt & Lucas 2000Liszt et al 2010), the column density of molecular hydrogen (H 2 ) is derived to be N H 2 ∼ 10 24 -10 25 cm −2 , which is consistent with N H 2 estimated from the HCN absorption spectrum (Sawada-Satoh et al 2016).…”

The Two-sided Jet Structures of NGC 1052 at Scales from 300 to 4 × 107 Schwarzschild Radii

“…For 3C 84, non-detection of receding jet at low frequencies cannot be explained entirely by the Doppler debeaming effect, and therefore, free-free absorption due to the ionized gas is inferred (Walker et al 2000). For NGC 1052, Kameno et al (2001) found the presence of a torus-like ionized absorber around the nucleus in addition to molecular gas at outer radii with very high column densities (Sawada-Satoh et al 2016. It has been argued that these distributions suggest a transition from spherical radial accretion flow to rotational accretion orbits (see also, Kameno et al 2003Kameno et al , 2005Sawada-Satoh et al 2008).…”

Properties of the Obscuring Torus in NGC 1052 from Multi-epoch Broadband X-ray Spectroscopy

“…Very high opacity (τ 1000) has been observed within about 2 mas ( 0.2 pc) from the estimated position of the SMBH, obscuring the receding jet (Sawada-Satoh et al 2008). Multiple molecular species have been observed as masers or absorbers at comparable angular scale (Claussen et al 1998;Vermeulen et al 2003;Impellizzeri et al 2008), leading to estimates of N H in the CT regime in at least some parts of the obscuring torus (Sawada-Satoh et al 2016, 2019.…”