The Mid-Infrared Colors of the ISM and Extended Sources at the Galactic Center

TL;DR

This study uses Spitzer IRAC mid-infrared data to analyze the Galactic Center's ISM and sources, revealing a uniform PAH emission color and identifying regions with intense radiation or obscured star formation.

Contribution

It provides the first large-scale mid-infrared survey of the Galactic Center, characterizing ISM emission properties and the effects of radiation fields on PAH and dust emission.

Findings

The 8 to 5.8 um color of the ISM is highly uniform across the surveyed region.

Regions with redder emission indicate strong local radiation fields or obscured star formation.

PAH emission remains consistent unless affected by intense radiation or dust obscuration.

Abstract

A mid-infrared (3.6-8 um) survey of the Galactic Center has been carried out with the IRAC instrument on the Spitzer Space Telescope. This survey covers the central 2x1.4 degree (~280x200 pc) of the Galaxy. At 3.6 and 4.5 um the emission is dominated by stellar sources, the fainter ones merging into an unresolved background. At 5.8 and 8 um the stellar sources are fainter, and large-scale diffuse emission from the ISM of the Galaxy's central molecular zone becomes prominent. The survey reveals that the 8 to 5.8 um color of the ISM emission is highly uniform across the surveyed region. This uniform color is consistent with a flat extinction law and emission from polycyclic aromatic hydrocarbons (PAHs). Models indicate that this broadband color should not be expected to change if the incident radiation field heating the dust and PAHs is <10^4 times that of the solar neighborhood. The few regions with unusually red emission are areas where the PAHs are underabundant and the radiation field is locally strong enough to heat large dust grains to produce significant 8 um emission. These red regions include compact H II regions, Sgr B1, and wider regions around the Arches and Quintuplet Clusters. In these regions the radiation field is >10^4 times that of the solar neighborhood. Other regions of very red emission indicate cases where thick dust clouds obscure deeply embedded objects or very early stages of star formation.