Neutral hydrogen filaments in interstellar media: Are they physical?

TL;DR

This review examines the physical nature, formation, and properties of interstellar medium filaments, highlighting their significance in galaxy structure and star formation, while discussing ongoing debates about their definition and characteristics.

Contribution

It consolidates current observational and theoretical perspectives on ISM filaments, clarifying their formation mechanisms and physical properties amidst active scientific debate.

Findings

Filaments are widespread in the ISM with high aspect ratios.

They form through magnetized turbulence stretching and tearing.

Filaments are likely magnetically aligned and undergo heating and cooling phases.

Abstract

The trending term "filament" is extensively used in the interstellar medium (ISM) and the star formation community, and is believed to be one of the most important objects that gauge molecular cloud and star formation. However, the physical definition of these ubiquitous, elongated, high contrast features is poorly defined and still actively debated. Despite the absence of a unified consensus, filaments are believed to be involved in many important physical processes from galaxy structure formation to the emergence of protostellar objects. Therefore, understanding how filaments form, what constrains their growth, and their general physical properties, are extremely important for theorists and observers who study the dynamics of the ISM and consequent star formations. This review serves as a collection of the community's views and develops the concept of "filaments" in the context of the ISM and star-forming clouds. Observationally, filaments are seen across the entire sky and often carry an aspect ratio of the order of hundreds. In the context of the ISM, filaments are believed to form by stretching and tearing from magnetized ISM turbulence. ISM filaments are subjected to heating and cooling phases, and are likely to be magnetically aligned. Cold clouds are formed inside ISM due to turbulence instability. This review updates the understanding of ISM filaments in the community.