Dense cloud cores revealed by CO in the low metallicity dwarf galaxy WLM

TL;DR

This study detects and characterizes dense CO clouds in the low-metallicity dwarf galaxy WLM, revealing that despite their small size, these clouds have properties similar to those in the Milky Way, informing star formation processes.

Contribution

First interferometric detection of CO clouds in a low-metallicity dwarf galaxy, showing they are small but dense, with implications for star cluster formation in such environments.

Findings

CO clouds in WLM are tiny but dense, similar to Milky Way clouds.

Star clusters in dwarf irregulars have comparable densities to those in spiral galaxies.

Low cloud masses imply star clusters will likely be low mass unless triggered by external compression.

Abstract

Understanding stellar birth requires observations of the clouds in which they form. These clouds are dense and self-gravitating, and in all existing observations, they are molecular with H_2 the dominant species and CO the best available tracer. When the abundances of carbon and oxygen are low compared to hydrogen, and the opacity from dust is also low, as in primeval galaxies and local dwarf irregular galaxies, CO forms slowly and is easily destroyed, so it is difficult for it to accumulate inside dense clouds. Here we report interferometric observations of CO clouds in the local group dwarf irregular galaxy Wolf-Lundmark-Melotte (WLM), which has a metallicity that is 13% of the solar value and 50% lower than the previous CO detection threshold. The clouds are tiny compared to the surrounding atomic and H_2 envelopes, but they have typical densities and column densities for CO clouds in the Milky Way. The normal CO density explains why star clusters forming in dwarf irregulars have similar densities to star clusters in giant spiral galaxies. The low cloud masses suggest that these clusters will also be low mass, unless some galaxy-scale compression occurs, such as an impact from a cosmic cloud or other galaxy. If the massive metal-poor globular clusters in the halo of the Milky Way formed in dwarf galaxies, as is commonly believed, then they were probably triggered by such an impact.