Spitzer's mid-infrared view on an outer Galaxy Infrared Dark Cloud candidate toward NGC 7538

TL;DR

This paper reports the first detection of an Infrared Dark Cloud in the outer Galaxy using Spitzer mid-infrared observations, confirming that such star-forming regions exist throughout the entire Galactic Plane.

Contribution

It presents the first identification of an IRDC in the outer Galaxy, expanding the known distribution of early star formation regions beyond the inner Galaxy.

Findings

First IRDC detected in the outer Galaxy.

IRDC G111 is a massive, cold molecular clump.

Detection confirms IRDCs are present throughout the Galactic Plane.

Abstract

Infrared Dark Clouds (IRDCs) represent the earliest observed stages of clustered star formation, characterized by large column densities of cold and dense molecular material observed in silhouette against a bright background of mid-IR emission. Up to now, IRDCs were predominantly known toward the inner Galaxy where background infrared emission levels are high. We present Spitzer observations with the Infrared Camera Array toward object G111.80+0.58 (G111) in the outer Galactic Plane, located at a distance of ~3 kpc from us and ~10 kpc from the Galactic center. Earlier results show that G111 is a massive, cold molecular clump very similar to IRDCs. The mid-IR Spitzer observations unambiguously detect object G111 in absorption. We have identified for the first time an IRDC in the outer Galaxy, which confirms the suggestion that cluster-forming clumps are present throughout the Galactic Plane. However, against a low mid-IR back ground such as the outer Galaxy it takes some effort to find them.