Galactic Large-scale Filaments Resident in Asymmetric Environments: Clues from Cross-filament Profiles of Density and Temperature

TL;DR

This study analyzes the density and temperature profiles of 35 large-scale Galactic filaments, revealing their asymmetric environments and the influence of nearby HII regions on their structure and evolution.

Contribution

It provides the first comprehensive statistical analysis of radial profiles of large-scale filaments, highlighting environmental asymmetries and the impact of HII regions.

Findings

Galactic filaments reside in asymmetric surroundings.

Asymmetry in density and temperature profiles varies independently.

HII regions influence filament profiles but do not solely trigger their formation.

Abstract

Large-scale filaments ubiquitously exist in the Galactic interstellar medium, and their radial profiles offer insights into their formation mechanisms. We present a statistical analysis of molecular hydrogen column density ($\rm N(H_2)$) and dust temperature ($\rm T_d$) radial profiles for 35 Galactic large-scale filaments. We divided their spines into 315 segments, extracted the radial profiles of each segment using $\rm N(H_2)$ and $\rm T_d$ maps derived from $Herschel$ Hi-GAL data, and estimated the asymmetry degree within the radial profiles ($\alpha_{\rm asy}$), as well as the length proportion of segments with asymmetric profiles across the entire filament ($f_{\rm asy}$). We found that Galactic large-scale filaments reside in surroundings distinctly asymmetric and varied in $\rm N(H_2)$, and mild asymmetric yet stable in $\rm T_d$. Different filament morphology types do not show significant differences in $\alpha_{\rm asy}$ or $f_{\rm asy}$. A bent filament shape does not necessarily correspond to an asymmetric radial profile, whereas a straight filament shape may be associated with a symmetric profile. Segments with asymmetric surroundings in $\rm N(H_2)$ may not simultaneously appear asymmetric in $\rm T_d$, and vice versa. We found three filaments with 4-44% of their spine show asymmetric $\rm N(H_2)$ and $\rm T_d$ radial profiles in inverse trends, likely caused by nearby HII region. HII regions of similar scale to large filaments can induce asymmetric radial profiles within them, indicating their influence on filament evolution. However, they are unlikely to independently trigger the formation of an entire Galactic large-scale filament, in contrast to their role in small-scale filament formation.