Large-scale filaments associated with Milky Way spiral arms

TL;DR

This paper introduces a new method using Herschel data to systematically identify the largest, coldest, and densest filaments in the Milky Way, revealing their properties and association with spiral arms.

Contribution

A novel approach for detecting large-scale Galactic filaments using Herschel data, expanding understanding of their characteristics and spatial distribution relative to spiral arms.

Findings

Identified 9 prominent large-scale filaments in the Galaxy.

Filaments measure 37-99 pc in length and are mostly within spiral arms.

Most filaments are cold and located near the Galactic mid-plane.

Abstract

The ubiquity of filamentary structure at various scales through out the Galaxy has triggered a renewed interest in their formation, evolution, and role in star formation. The largest filaments can reach up to Galactic scale as part of the spiral arm structure. However, such large scale filaments are hard to identify systematically due to limitations in identifying methodology (i.e., as extinction features). We present a new approach to directly search for the largest, coldest, and densest filaments in the Galaxy, making use of sensitive Herschel Hi-GAL data complemented by spectral line cubes. We present a sample of the 9 most prominent Herschel filaments, including 6 identified from a pilot search field plus 3 from outside the field. These filaments measure 37-99 pc long and 0.6-3.0 pc wide with masses (0.5-8.3)$\times10^4 \, M_\odot$, and beam-averaged ($28"$, or 0.4-0.7 pc) peak H$_2$ column densities of (1.7-9.3)$\times 10^{22} \, \rm{cm^{-2}}$. The bulk of the filaments are relatively cold (17-21 K), while some local clumps have a dust temperature up to 25-47 K. All the filaments are located within <~60 pc from the Galactic mid-plane. Comparing the filaments to a recent spiral arm model incorporating the latest parallax measurements, we find that 7/9 of them reside within arms, but most are close to arm edges. These filaments are comparable in length to the Galactic scale height and therefore are not simply part of a grander turbulent cascade.