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

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.

10 pages, 6 fig

References in corpus (6)

• Iron line and diffuse hard X-ray emission from the starburst galaxy M82
• Detection of extragalactic H3O+
• Detection of CO+ in the nucleus of M82
• Near threshold rotational excitation of molecular ions by electron-impact
• CO+ in M 82: A Consequence of Irradiation by X-rays
• Upper limits to the water abundance in starburst galaxies

Cited by in corpus (16)

• Molecular line emission in NGC1068 imaged with ALMA: II. The chemistry of the dense molecular gas
• The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
• Lambda = 3 mm line survey of nearby active galaxies
• Winds, Clumps, and Interacting Cosmic Rays in M82
• Warm Molecular Hydrogen in the Galactic Wind of M82
• Electron Excitation of High Dipole Moment Molecules Reexamined
• Complex organic molecules in strongly UV-irradiated gas
• Spectral Line Survey toward Spiral Arm of M51 in the 3 mm and 2 mm Bands
• Gas phase Elemental abundances in Molecular cloudS (GEMS). IV. Observational results and statistical trends
• Molecular-Cloud-Scale Chemical Composition II: Mapping Spectral Line Survey toward W3(OH) in the 3 mm Band
• Starburst Energy Feedback Seen Through HCO${}^{+}$/HOC${}^{+}$ Emission in NGC 253 from ALCHEMI
• The chemical footprint of the star formation feedback in M 82 on scales of 100 pc
• An optical--near-IR study of a triplet of super star clusters in the starburst core of M82
• Properties of dense molecular gas along the major axis of M 82
• Toward a robust physical and chemical characterization of heterogeneous lines of sight: The case of the Horsehead nebula
• Multi-line Observations, Models, and Data Needed to Understand the Nature of UV-irradiated Interstellar Matter