Histogram of oriented gradients: a technique for the study of molecular cloud formation
Artikel i vetenskaplig tidskrift, 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


J. D. Soler


H. Beuther


M. Rugel


Y. Wang


P. C. Clark

Cardiff University

S. C. O. Glover

Universität Heidelberg

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


Jouni Kainulainen

Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik

R. S. Klessen

Universität Heidelberg

S. N. Longmore

Liverpool John Moores University

N. M. McClure-Griffiths

Australian National University

K. M. Menten


J. C. Mottram


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

Universität Heidelberg

Universität Bonn

P. Hennebelle

Université Paris Diderot

N. Roy

Indian Institute of Science

P. Schilke

Universität zu Köln

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 622 A166


Meteorologi och atmosfärforskning

Astronomi, astrofysik och kosmologi




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