Determination of the mass function of extra-galactic GMCs via NIR color maps

Testing the method in a disk-like geometry

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany e-mail: jkainula@astro.helsinki.fi
² Observatory, PO Box 14, 00014 University of Helsinki, Helsinki, Finland
³ Calar Alto Observatory, Centro Astronomico Hispano, Alemán, C/q Jesús Durbán Remón 2-2, 04004 Almeria, Spain 


Received: 7 December 2006
Accepted: 6 March 2007

Abstract


The giant molecular clouds (GMCs) of external galaxies can be mapped with sub-arcsecond resolution using multiband observations in the near-infrared. However, the interpretation of the observed reddening and attenuation of light, and their transformation into physical quantities, is greatly hampered by the effects arising from the unknown geometry and the scattering of light by dust particles. We examine the relation between the observed near-infrared reddening and the column density of the dust clouds. In this paper we particularly assess the feasibility of deriving the mass function of GMCs from near-infrared color excess data. We perform Monte Carlo radiative transfer simulations with 3D models of stellar radiation and clumpy dust distributions. We include the scattered light in the models and calculate near-infrared color maps from the simulated data. The color maps are compared with the true line-of-sight density distributions of the models. We extract clumps from the color maps and compare the observed mass function to the true mass function. For the physical configuration chosen in this study, essentially a face-on geometry, the observed mass function is a non-trivial function of the true mass function with a large number of parameters affecting its exact form. The dynamical range of the observed mass function is confined to ~10^3.5 10^5.5 regardless of the dynamical range of the true mass function. The color maps are more sensitive in detecting the high-mass end of the mass function, and on average the masses of clouds are underestimated by a factor of ∼10 depending on the parameters describing the dust distribution. A significant fraction of clouds is expected to remain undetected at all masses. The simulations show that the cloud mass function derived from JHK color excess data using simple foreground screening geometry cannot be regarded as a one-to-one tracer of the underlying mass function.