[Paper Review] Detection of HCN, HCO+ and HNC in the Mrk231 molecular outflow - Dense molecular gas in the AGN wind

S. Aalto, S. García‐Burillo+6 more|Chalmers Publication Library (Chalmers University of Technology)|Nov 29, 2011

Astrophysics and Star Formation Studies39 references98 citations

TL;DR

This study presents the first high-resolution detection of HCN, HCO⁺, and HNC 1–0 emission in a large-scale galactic outflow from the ULIRG Mrk 231, revealing broad line wings at ±750 km s⁻¹ and spatially extended emission (>700 pc) in a north-south direction. The enhanced HCN emission relative to CO, high line ratios, and dense gas diagnostics (n > 10⁴ cm⁻³, X(HCN) > 10⁻⁸) indicate shock-compressed, clumpy molecular gas, with alternative excitation by mid-IR radiation being less likely due to the outflow's extended nature.

ABSTRACT

We detect luminous emission from HCN, HCO+ and HNC 1--0 in the QSO ULIRG Mrk~231 with the IRAM Plateau de Bure Interferometer at 1."55 by 1."28 resolution. All three lines show broad line wings - which are particularly prominent for HCN. Velocities are found to be similar (750 km/s) to those found for CO 1-0. This is the first time bright HCN, HCO+ and HNC emission has been detected in a large-scale galactic outflow. We find that both the blue- and red-shifted line wings are spatially extended by at least 0."75 (700 pc) in a north-south direction. The line wings are brighter (relative to the line center intensity) in HCN than in CO 1-0 and line ratios suggest that the molecular outflow consists of dense (n>10E4 cmE-3) and clumpy gas with a high HCN abundance X(HCN)>10E-8. These properties are consistent with the molecular gas being compressed and fragmented by shocks in the outflow. Alternatively, HCN is instead pumped by mid-IR continuum, but we propose that this effect is not strong for the spatially extended outflowing gas. In addition, we find that the rotation of the main disk, in east-west direction, is also evident in the HCN, HCO+ and HNC line emission. An unexpectedly bright HC3N 10-9 line is detected inside the central 400 pc of Mrk231. This HC3N emission may emerge from a shielded, dust-enshrouded region within the inner 40-50 pc where the gas is heated to high temperatures (200 - 300 K) by the AGN.

Motivation & Objective

• To investigate the physical and chemical properties of dense molecular gas in large-scale outflows driven by active galactic nuclei (AGNs), particularly in ULIRGs like Mrk 231.
• To determine whether the observed molecular outflow is powered by AGN feedback or starburst activity, by analyzing high-excitation molecular lines.
• To assess the role of shocks and mid-IR radiation in exciting HCN, HCO⁺, and HNC in outflowing gas, especially given the high luminosity and extreme conditions in Mrk 231.
• To identify the origin of unexpectedly bright HC₃N 10–9 emission in the central 400 pc, which may trace a shielded, warm, dusty region near the AGN.
• To constrain the mass outflow rate and gas density in the outflow using high-resolution line emission and line ratio diagnostics.

Proposed method

• High-resolution (1.″55 × 1.″28) interferometric observations of HCN, HCO⁺, HNC 1–0, and HC₃N 10–9 transitions were conducted using the IRAM Plateau de Bure Interferometer.
• Spectral line imaging was used to map the spatial distribution and kinematics of emission, identifying blue- and redshifted wings indicative of outflowing gas.
• Line ratios (e.g., HCN/HCO⁺, HCN/HNC) and luminosity comparisons with CO 1–0 were used to infer gas density (n > 10⁴ cm⁻³) and molecular abundances.
• Excitation analysis and comparison with shock and radiative transfer models were applied to assess the dominant excitation mechanism—shocks versus mid-IR pumping.
• The spatial extent of line wings (>0.″75, or >700 pc) was measured to assess the dynamical range and energy budget of the outflow.
• The HC₃N 10–9 line was analyzed for spatial localization and temperature constraints, suggesting emission from a shielded, warm (200–300 K) region within 40–50 pc of the nucleus.

Experimental results

Research questions

• RQ1 What is the physical origin of the broad HCN, HCO⁺, and HNC 1–0 line wings observed in Mrk 231’s molecular outflow?
• RQ2 Is the enhanced HCN emission in the outflow wings due to shock-induced chemistry or mid-IR pumping from the AGN?
• RQ3 What is the density and abundance of molecular gas in the outflow, and how do they compare to those in the quiescent disk?
• RQ4 Where is the bright HC₃N 10–9 emission localized, and what does its presence imply about the physical conditions in the innermost region of Mrk 231?
• RQ5 How do the observed line ratios and kinematics constrain the mass outflow rate and the driving mechanism of the outflow?

Key findings

• HCN, HCO⁺, and HNC 1–0 emission were detected in the molecular outflow of Mrk 231 for the first time, with broad line wings extending to ±750 km s⁻¹.
• The line wings are spatially extended by at least 0.″75 (700 pc) in a north-south direction, indicating a large-scale, collimated outflow.
• HCN 1–0 emission is enhanced by factors of 2–5 in the line wings relative to CO 1–0, indicating a high fraction of dense gas (n > 10⁴ cm⁻³).
• The HCN/HCO⁺ line peak ratio reaches 2.2 in the line wings, significantly higher than the 1.3 ratio at the line center, supporting elevated HCN abundance (X(HCN) > 10⁻⁸).
• The HCN/HNC line ratio increases from 1.8 at the line center to 2.8 in the wings, consistent with enhanced HCN production in outflowing, shocked gas.
• An unexpectedly bright HC₃N 10–9 line was detected within the central 400 pc, likely originating from a shielded, dust-enshrouded region heated to 200–300 K by the AGN, possibly representing the final stage of obscured accretion.