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

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 ($α_{\rm asy}$), 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 $α_{\rm asy}$ 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.

Figures (12)

• Figure 1: (a) Overview of the surroundings of filament sample F30 from Ge2022. The background is the Hi-GAL-based $\rm N(H_2)$ map derived by Marsh2017, with contour levels ranging from 4 $\sigma_{\rm N(H_2)}$ to 10 $\sigma_{\rm N(H_2)}$ ($\sigma_{\rm N(H_2)} = 47.52\times 10^{20}\ \rm cm^{-2}$). The distance to the filament is indicated in the upper-right corner, and a 5 pc scale bar is shown in the lower-right corner of the image. The MST spine of the filament is represented by red crosses connected by black line segments, with the ID for each segment labeled around them. The boxes perpendicular to the filament spine frame the areas used to extract radial profiles for each segment. Their shapes imitate the "projection" regions in DS9, with the solid line representing the direction of extracting radial profiles. The lengths of the boxes are calculated in Section \ref{['subsec:radprofmethod']}. The cyan circles correspond to nearby Hii regions, adopted from the WISE catalog Anderson2014, with their distances in kpc labeled nearby, if available. (b) $\rm N(H_2)$ and $\rm T_d$ radial profiles of F30. The $\rm N(H_2)$ profile is shown in the upper panel, and the $\rm T_d$ profile in the lower panel. The blue vertical dotted line represents the start of the sampling region as half of the filament width. Regions shaded red and purple correspond to radial profiles extracted from the red and purple sampling regions in (a). Only these parts are taken into subsequent calculations and identification of asymmetry in the LSF.
• Figure 2: (a) Distribution of $n$, the ratio between the maximum asymmetry-recoverable sampling length and the Gaussian-smoothed FWHM of RCW120 filaments, and $\rm FWHM_{RCW120}$ at different distances. Each scattered point and error bar of $n$ corresponds to the mean and standard deviation among 1000 $n$ values, derived from mock radial profiles of RCW120 filaments at that distance. (b) Distribution of the sampling length, equal to $n\times \rm FWHM_{RCW120}$, versus LSF distance. The increasing trend of the curve highlights the importance of varying sampling lengths for LSFs at different distances.
• Figure 3: Distribution of the $\alpha_{\rm asy, N}$ and $\alpha_{\rm asy, T}$ for each segment in the spine of LSF F30 (same as Figure \ref{['fig1_F30']}), with the segment IDs labeled around them. The gray dashed line at a value of 1 represents the symmetric case. Segments highlighted in red or blue have $\alpha_{rm asy}$ that deviate from 1 within their uncertainty range and are thus identified as having asymmetric radial profiles. The values of $f_{\rm asy, N}$ and $f_{\rm asy, T}$ for this filament are displayed in the upper left or upper right corner of the panels. This filament has $f_{\rm asy, N} = 0.73$ and $f_{\rm asy, T} = 0.81$, meaning that 73% in length of its spine has asymmetric $\rm N(H_2)$ radial profile, while 81% in length of its spine has asymmetric $\rm T_d$ radial profile.
• Figure 4: (a) Distribution of the weighted average asymmetric degree $|1-\alpha_{\rm asy}|$ in the $\rm N(H_2)$ and $\rm T_d$ radial profiles of the 35 filaments, weighted by the segment length. The upper panel corresponds to the $\rm N(H_2)$ case, the lower to the $\rm T_d$ case. The x-axis shows the filaments ordered by increasing $|1-\alpha_{\rm asy, N}|$. The shape of the data points indicates the parental catalog of the filament, and color represents the morphology of the filament: L (red), C (orange), S (cyan), H (purple), and X (blue), as defined in Section \ref{['subsec:filsample']}. Additional gray dashed horizontal line at 0 represents the symmetric scenarios. (b) Violin plots of $|1-\alpha_{\rm asy}|$ for the five LSF morphology classes. The upper panel shows the $\rm N(H_2)$ case, the lower panel shows the $\rm T_d$ case. The violins have their corresponding morphology types labeled in the x-axis, and their colors are in the same definition as (a). The white point in each violin represents the median, and the thick black line spans the 25th and 75th percentiles. Additional gray dashed horizontal line at 0 marks symmetric scenarios.
• Figure 5: (a) Estimated $f_{\rm asy}$ based on the $\rm N(H_2)$ and $\rm T_d$ radial profiles of all 35 LSFs. The upper panel corresponds to the $\rm N(H_2)$ case, and the lower panel to the $\rm T_d$ case. The x-axis arranges the filaments by increasing $f_{\rm asy, N}$. Data point shapes and colors follow the same definitions as in Figure \ref{['fig4_alpha']}. Dashed gray dashed horizontal lines at 0 and 1 represent fully symmetric and fully asymmetric surroundings, respectively. (b) Violin plots of $f_{\rm asy}$ for the five LSF morphology classes, with the $\rm N(H_2)$ case in the upper panel, and the $\rm T_d$ case in the lower panel. Morphology labels and color conventions are the same as in Figure \ref{['fig4_alpha']}. Dashed gray horizontal lines at 0 and 1 represent fully symmetric and asymmetric surroundings, respectively.