The anatomy of the Orion B giant molecular cloud: A local template for studies of nearby galaxies

¹ IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
e-mail: pety@iram.fr
² LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, École normale supérieure, 75005 Paris, France
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138, USA
⁴ Joint ALMA Observatory (JAO), Alonso de Cordova 3107 Vitacura, Santiago de Chile, Chile
⁵ Univ. Grenoble Alpes, IRAM, 38000 Grenoble, France
⁶ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, 22903, USA
⁷ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 92190 Meudon, France
⁸ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁹ ICMM, Consejo Superior de Investigaciones Cientificas (CSIC), 28049 Madrid, Spain
¹⁰ IRAM, Avenida Divina Pastora, 7, Núcleo Central, 18012 Granada, España

Received: 7 October 2016
Accepted: 8 November 2016

Abstract

Context. Molecular lines and line ratios are commonly used to infer properties of extra-galactic star forming regions. The new generation of millimeter receivers almost turns every observation into a line survey. Full exploitation of this technical advancement in extra-galactic study requires detailed bench-marking of available line diagnostics.

Aims. We aim to develop the Orion B giant molecular cloud (GMC) as a local template for interpreting extra-galactic molecular line observations.

Methods. We use the wide-band receiver at the IRAM-30 m to spatially and spectrally resolve the Orion B GMC. The observations cover almost 1 square degree at 26′′ resolution with a bandwidth of 32 GHz from 84 to 116 GHz in only two tunings. Among the mapped spectral lines are the ¹²CO, ¹³CO, C¹⁸O, C¹⁷O, HCN, HNC, ¹²CN, C₂H, HCO⁺, N₂H⁺(1−0), and ¹²CS, ³²SO, SiO, c - C₃H₂, CH₃OH (2−1) transitions.

Results. We introduce the molecular anatomy of the Orion B GMC, including relationships between line intensities and gas column density or far-UV radiation fields, and correlations between selected line and line ratios. We also obtain a dust-traced gas mass that is less than approximately one third the CO-traced mass, using the standard X_CO conversion factor. The presence of over-luminous CO can be traced back to the dependence of the CO intensity on UV illumination. As a matter of fact, while most lines show some dependence on the UV radiation field, CN and C₂H are the most sensitive. Moreover, dense cloud cores are almost exclusively traced by N₂H⁺. Other traditional high-density tracers, such as HCN(1−0), are also easily detected in extended translucent regions at a typical density of ~500 H₂ cm^-3. In general, we find no straightforward relationship between line critical density and the fraction of the line luminosity coming from dense gas regions.

Conclusions. Our initial findings demonstrate that the relationships between line (ratio) intensities and environment in GMCs are more complicated than often assumed. Sensitivity (i.e., the molecular column density), excitation, and, above all, chemistry contribute to the observed line intensity distributions, and they must be considered together when developing the next generation of extra-galactic molecular line diagnostics of mass, density, temperature, and radiation field.