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Probing interstellar turbulence in spiral galaxies using HI power spectrum analysis

By Prasun Dutta, Ayesha Begum, Somnath Bharadwaj and Jayaram N. Chengalur

Abstract

We estimate the \HI intensity fluctuation power spectrum for a sample of 18 spiral galaxies chosen from THINGS. Our analysis spans a large range of length-scales from $\sim 300 {\rm pc}$ to $\sim 16 {\rm kpc}$ across the entire galaxy sample. We find that the power spectrum of each galaxy can be well fitted by a power law $P_{\rm HI}(U) = A U^{\alpha}$, with an index $\alpha$ that varies from galaxy to galaxy. For some of the galaxies the scale-invariant power-law power spectrum extends to length-scales that are comparable to the size of the galaxy's disk. The distribution of $\alpha$ is strongly peaked with 50% of the values in the range $\alpha=-1.9$ to 1.5, and a mean and standard deviation of -1.3 and 0.5 respectively. We find no significant correlation between $\alpha$ and the star formation rate, dynamical mass, \HI mass or velocity dispersion of the galaxies. Several earlier studies that have measured the power spectrum within our Galaxy on length-scales that are considerably smaller than $500 {\rm pc}$ have found a power-law power spectrum with $\alpha$ in the range $\approx -2.8$ to -2.5. We propose a picture where we interpret the values in the range $\approx -2.8$ to -2.5 as arising from three dimensional (3D) turbulence in the Interstellar Medium (ISM) on length-scales smaller than the galaxy's scale-height, and we interpret the values in the range $\approx -1.9$ to -1.5 measured in this paper as arising from two-dimensional ISM turbulence in the plane of the galaxy's disk. It however still remains a difficulty to explain the small galaxy to galaxy variations in the values of $\alpha$ measured here.Comment: 20 pages, 9 figures, 3 tables. accepted for publication in New Astronom

Topics: Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics
Year: 2012
DOI identifier: 10.1016/j.newast.2012.08.008
OAI identifier: oai:arXiv.org:1208.5386

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