Structure Generation by Irradiation: What can GLIMPSE teach us about the ISM structure?

Abstract

Diffuse emission in the mid-infrared shows a wealth of structure, that lends itself to high-resolution structure analysis of the interstellar gas. A large part of the emission comes from polycyclic aromatic hydrocarbons, excited by nearby ultra-violet sources. Can the observed diffuse emission structure be interpreted as column density structure? We discuss this question with the help of a set of model molecular clouds bathed in the radiation field of a nearby O-star. The correlation strength between column density and ``observed'' flux density strongly depends on the absolute volume density range in the region. Shadowing and irradiation effects may completely alter the appearance of an object. Irradiation introduces additional small-scale structure and it can generate structures resembling shells around HII-regions in objects that do not possess any shell-like structures whatsoever. Nevertheless, structural information about the underlying interstellar medium can be retrieved. In the more diffuse regime ($n(HI)\lesssim 100$cm${}^{-3}$), flux density maps may be used to trace the 3D density structure of the cloud via density gradients. Thus, while caution definitely is in order, mid-infrared surveys such as GLIMPSE will provide quantitative insight into the turbulent structure of the interstellar medium.

19 pages, 20 figures, submitted to ApJ

References in corpus (14)

• HEALPix -- a Framework for High Resolution Discretization, and Fast Analysis of Data Distributed on the Sphere
• Interstellar Dust Grains
• Infrared Emission from Interstellar Dust. II. The Diffuse Interstellar Medium
• GLIMPSE: I. A SIRTF Legacy Project to Map the Inner Galaxy
• Gravitational Collapse in Turbulent Molecular Clouds. I. Gasdynamical Turbulence
• On Ultrasmall Silicate Grains in the Diffuse Interstellar Medium
• Gravitational Collapse in Turbulent Molecular Clouds. II. Magnetohydrodynamical Turbulence
• Magnetic Field Diagnostics Based on Far-Infrared Polarimetry: Tests Using Numerical Simulations
• Cloudshine: New Light on Dark Clouds
• A GLIMPSE of Star Formation in the Giant H II Region RCW 49
• Estimating Column Density in Molecular Clouds with FIR and Sub-mm Emission Maps
• High Resolution Mapping of Interstellar Clouds by Near--IR Scattering
• Dust heating by the interstellar radiation field in models of turbulent molecular clouds
• Cloud Dispersal in Turbulent Flows

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• Spitzer-IRAC GLIMPSE of high mass protostellar objects. I Infrared point sources and nebulae
• Gravity or turbulence V: Star forming regions undergoing violent relaxation
• The Extended Emission of Ultracompact HII Regions: An Overview and New Observations