Histogram of oriented gradients: a technique for the study of molecular cloud formation

Abstract

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 characterize HOG using synthetic observations of Hi and 13 CO (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 find a significant spatial correlation between the two tracers in velocity channels where v HI ≈ v 13CO , almost independent of the orientation of the collision with respect to the line of sight. Subsequently, we use HOG to investigate the spatial correlation of the Hi, from The Hi/OH/Recombination line survey of the inner Milky Way (THOR), and the 13 CO (J = 1 → 0) emission from the Galactic Ring Survey (GRS), toward the portion of the Galactic plane 33. • 75 ≤ l ≤ 35. • 25 and |b| ≤ 1. • 25. We find 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 ≈ 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.

The history of dynamics and stellar feedback revealed by the H I filamentary structure in the disk of the Milky Way

“…Second, although there is a morphological correlation of the HI and the 13 CO, as quantified in Soler et al (2019), the filamentary structure in the HI emission is not necessarily related to that of the 13 CO. In general, the much larger filling factor of the HI makes it unlikely that most of its structure is related to that of the less-filling molecular gas.…”

Atomic and molecular gas properties during cloud formation

“…In Appendix B we briefly outline the basic principles involved in this method. A comprehensive description is given by Soler et al (2019).…”

“…The Histogram of Oriented Gradients (HOG; Soler et al 2019) is based on the assumption that the appearance and shape of an object can be characterized by the representation of its local intensity gradients. One simple application is deriving the spatial correlation between two images.…”

Intensity Gradients Technique: Synergy with Velocity Gradients and Polarization Studies

“…They therefore erroneously assumed that in thin velocity channels all intensity fluctuations are due to density enhancements. Soler et al (2019) and used different approaches in the analysis of intensity distributions in thin velocity channels 6 . The former does not use the procedure of sub-block averaging, which is disadvantageous and prevents Soler et al (2019) from reliably tracing magnetic field direction as it is demonstrated in ; Hu et al (2019a,b); González-Casanova & Lazarian (2019) To compare IGT and HRO, we follow the recipe used in Soler et al (2013), which calculates θ using a combination of the scalar and vector product of vectors:…”

“…VGT, in contrast, explores the pointwise statistics of the magnetic field and does not require any outside measurements. In addition, some of VGT ideas and approaches were successfully borrowed and implemented within HRO as it was evolving (see Soler et al 2019).…”