Histogram of oriented gradients: a technique for the study of molecular cloud formation
Journal article, 2019
We introduce the histogram of oriented gradients (HOG), a tool developed for machine vision that we propose as a new metric for the systematic characterization of spectral line observations of atomic and molecular gas and the study of molecular cloud formation models. In essence, the HOG technique takes as input extended spectral-line observations from two tracers and provides an estimate of their spatial correlation across velocity channels. We characterized HOG using synthetic observations of HI and (CO)-C-13(J = 1 -> 0) emission from numerical simulations of magnetohydrodynamic (MHD) turbulence leading to the formation of molecular gas after the collision of two atomic clouds. We found a significant spatial correlation between the two tracers in velocity channels where v(HI) approximate to v(13CO), almost independent of the orientation of the collision with respect to the line of sight. Subsequently, we used HOG to investigate the spatial correlation of the HI, from The HI/OH/recombination line survey of the inner Milky Way (THOR), and the (CO)-C-13(J = 1 -> 0) emission from the Galactic Ring Survey (GRS), toward the portion of the Galactic plane 33.degrees 75 <= l <= 35.degrees 25 and vertical bar b vertical bar <= 1.degrees 25. We found a significant spatial correlation between the two tracers in extended portions of the studied region. Although some of the regions with high spatial correlation are associated with HI self-absorption (HISA) features, suggesting that it is produced by the cold atomic gas, the correlation is not exclusive to this kind of region. The HOG results derived for the observational data indicate significant differences between individual regions: some show spatial correlation in channels around v(HI) approximate to v(13CO) while others present spatial correlations in velocity channels separated by a few kilometers per second. We associate these velocity offsets to the effect of feedback and to the presence of physical conditions that are not included in the atomic-cloud-collision simulations, such as more general magnetic field configurations, shear, and global gas infall.
radio lines: ISM
ISM: clouds
ISM: molecules
ISM: structure
galaxies: ISM
ISM: atoms
Author
J. D. Soler
Max Planck Society
H. Beuther
Max Planck Society
M. Rugel
Max Planck Society
Y. Wang
Max Planck Society
P. C. Clark
Cardiff University
S. C. O. Glover
Heidelberg University
P. F. Goldsmith
California Institute of Technology (Caltech)
M. Heyer
University of Massachusetts
L. D. Anderson
West Virginia University
A. Goodman
Harvard-Smithsonian Center for Astrophysics
Th. Henning
Max Planck Society
Jouni Kainulainen
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
R. S. Klessen
Heidelberg University
S. N. Longmore
Liverpool John Moores University
N. M. McClure-Griffiths
Australian National University
K. M. Menten
Max Planck Society
J. C. Mottram
Max Planck Society
J. Ott
National Radio Astronomy Observatory Socorro
S. E. Ragan
Cardiff University
R. J. Smith
University of Manchester
J. S. Urquhart
University Of Kent
F. Bigiel
Heidelberg University
University of Bonn
P. Hennebelle
Paris Diderot University
N. Roy
Indian Institute of Science
P. Schilke
University of Cologne
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 622 A166Subject Categories
Meteorology and Atmospheric Sciences
Astronomy, Astrophysics and Cosmology
Geophysics
DOI
10.1051/0004-6361/201834300