Journal Article

Extraction of homogeneous turbulent motions in the ionized interstellar medium: application to the NGC 595 nebula

Dominic Lagrois and Gilles Joncas

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 413, issue 2, pages 721-740
Published in print May 2011 | ISSN: 0035-8711
Published online April 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.18134.x
Extraction of homogeneous turbulent motions in the ionized interstellar medium: application to the NGC 595 nebula

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Relatively unexplored in recent years, turbulence in the ionized interstellar medium could allow us to explain peculiar gas motions observed in H ii regions. We present an improved method that allows the extraction of quasi-homogeneous turbulent motions from bidimensional radial velocity maps. This paper is an application to the NGC 595 nebula, a giant H ii region in M33. The use of the Zurflueh filter is re-instated in order to separate ordered motions from turbulent fluctuations in astronomical observations. A statistical approach, based on the application of the autocorrelation and second-order structure functions, is used on the filtered centroid measurements in Hα, [O iii] and [S ii]. Following the work of Brunt & Mac Low, an investigation of the density inhomogeneities in NGC 595 suggests that turbulence in the large nebula could be at least partially supersonic. Compressible shock waves related to stellar winds of massive stars are presented as the main source for stirring up optical gases into high-amplitude turbulent velocity fluctuations. Unfortunately, the transition from a 2D to 3D geometry, required in order to compare observations to existing turbulence models, could not be appropriately accomplished. This is attributed to an important scattering characterizing the supersonic regime in the theoretical results. Using particular conclusions reached in our ongoing work on standard H ii regions in M33, the kinematical disorder induced by turbulent velocities and observed on the plane of the sky is translated into a mean broadening effect along the line of sight for each investigated ion. This confirms that the O++ sphere is largely governed by supersonic turbulence while the same argument remains inconclusive in Hα and [S ii]. The amplitude of the turbulent motions (velocities) seems to decrease with distance from the central star cluster. This provides a reasonable explanation for a well-defined signature in non-thermal linewidths already identified in our first paper on NGC 595.

Keywords: turbulence; techniques: interferometric; H ii regions; ISM: individual objects: NGC 595; ISM: kinematics and dynamics

Journal Article.  14903 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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