[Paper Review] On the chemistry and distribution of HOC+ in M82: More evidence for extensive PDRs

A. Fuente, S. García‐Burillo+9 more|ArXiv.org|Oct 5, 2008

Astrophysics and Star Formation StudiesPhysics and Astronomy32 references29 citations

TL;DR

This study investigates the chemistry and spatial distribution of HOC+ in the starburst galaxy M82 using high-resolution interferometric observations and improved PDR models. It confirms that HOC+ emission arises in dense, UV-irradiated photodissociation regions (PDRs) with n ≥ 10⁴ cm⁻³, primarily in small, dense clouds (A_V = 5 mag) accounting for 87% of the molecular gas mass, indicating that M82 is an old starburst where star formation has nearly exhausted the reservoir of molecular gas.

ABSTRACT

The molecular gas composition in the inner 1 kpc disk of the starburst galaxy M82 resembles that of Galactic Photon Dominated Regions (PDRs). In particular, large abundances of the reactive ions HOC+ and CO+ have been measured in the nucleus of this galaxy. To investigate the origin of the large abundances of reactive ions in M82, we have completed our previous 30m HOC+ J=1-0 observations with the higher excitation HCO+ and HOC+ J=4-3 and 3-2 rotational lines. In addition, we have obtained with the IRAM Plateau de Bure Interferometer (PdBI) a 4" resolution map of the HOC+ 1-0 emission, the first ever obtained in a Galactic or extragalactic source. Our HOC+ interferometric image shows that the emission of the HOC+ 1-0 line is mainly restricted to the nuclear disk, with the maxima towards the E. and W. molecular peaks. In addition, line excitation calculations imply that the HOC+ emission arises in dense gas. Therefore, the HOC+ emission is arising in the dense PDRs embedded in the M82 nuclear disk, rather than in the intercloud phase and/or wind. We have improved our previous chemical model of M82 by (i) using the new version of the Meudon PDR code, (ii) updating the chemical network, and (iii) considering two different types of clouds (with different thickness). Most molecular observations (HCO+, HOC+, CO+, CN, HCN, H3O+) are well explained assuming that ~ 87 % of the mass of the molecular gas is forming small clouds (Av=5 mag) while only ~ 13 % of the mass is in large molecular clouds (Av=50 mag). Such small number of large molecular clouds suggests that M82 is an old starburst, where star formation has almost exhausted the molecular gas reservoir.

Motivation & Objective

• To determine the origin of high abundances of reactive ions like HOC+ and CO+ in M82's nucleus.
• To distinguish between UV and X-ray dominance in driving molecular chemistry in the galaxy's intense interstellar medium.
• To refine PDR chemical models using new high-excitation line data and interferometric imaging.
• To assess the spatial distribution and physical conditions of HOC+ emission in the M82 nuclear disk.

Proposed method

• Conducted 30m and JCMT observations of HOC+ and HCO+ at J=3→2 and J=4→3 transitions to probe line excitation.
• Obtained a 4" resolution interferometric map of HOC+ 1→0 emission using the IRAM Plateau de Bure Interferometer.
• Applied the Meudon PDR code with updated chemical networks and two cloud types (A_V = 5 and 50 mag) irradiated by G₀ = 10⁴ Habing units.
• Perfomed excitation calculations to infer gas density (n ≥ 10⁴ cm⁻³) from line intensity ratios.
• Compared model predictions with observed line ratios (e.g., [CN]/[HCN], [CO+]/[HCO+]) to constrain cloud mass fractions.
• Used a two-component cloud model to reproduce observed molecular abundances across multiple species (HCO+, HOC+, CO+, CN, HCN, H₃O+).

Experimental results

Research questions

• RQ1 What physical conditions (density, UV field, cloud size) are required to reproduce the observed HOC+ and other ion abundances in M82?
• RQ2 Is the high abundance of HOC+ driven primarily by UV photons or X-rays from the starburst?
• RQ3 What fraction of the molecular gas mass is in small, dense clouds (A_V = 5 mag) versus large, extended clouds (A_V = 50 mag) in M82?
• RQ4 Can a PDR model with enhanced cosmic rays (ζ = 4×10⁻¹⁵ s⁻¹) and two cloud types reproduce the full set of observed molecular line ratios?
• RQ5 Does the spatial distribution of HOC+ emission support a PDR origin rather than emission from the intercloud medium or outflow?

Key findings

• The HOC+ 1→0 emission is confined to the nuclear disk, peaking toward the eastern and western molecular peaks, with a 4" resolution interferometric map confirming its origin in dense gas.
• Line excitation calculations show that HOC+ emission arises in dense gas with n ≥ 10⁴ cm⁻³, ruling out emission from low-density intercloud or wind phases.
• The HOC+ emission is best explained by UV-driven chemistry in dense PDRs, not X-ray ionization, as shown by the lack of correlation with diffuse X-ray emission.
• The best-fitting PDR model requires 87% of the molecular gas mass to be in small, dense clouds (A_V = 5 mag) and only 13% in large clouds (A_V = 50 mag), indicating an old starburst with minimal remaining gas reservoir.
• The model successfully reproduces observed line ratios: [CN]/[HCN] = 6, [HCO+]/[HOC+] = 44, and [CO+]/[HCO+] = 0.04, consistent with UV-dominated chemistry.
• The model predicts N(HOC+) = 2.5×10¹³ cm⁻² and N(CO+) = 1.5×10¹³ cm⁻², matching observed values within a factor of 2, confirming the robustness of the two-cloud, UV-irradiated PDR scenario.