First Observation of CO$_2$ Emission and foreground absorption Toward the Galactic Center with JWST

Technical Deep Dive

Overview

This study presents the first observation of gas-phase CO₂ absorption and emission toward the Galactic Center using the James Webb Space Telescope (JWST). In addition to CO₂, the authors also detect absorption from gas-phase C₂H₂ and HCN, as well as CO₂ ice, across two observed fields.

Methodology

• The authors used JWST's mid-infrared integral field spectrograph MIRI/MRS to observe two fields in the Galactic Center region: one in the Circumnuclear Disk (CND) and one in the central cavity (CC) of the CND.
• They fit the observed CO₂ absorption features using a local thermodynamic equilibrium (LTE) model to derive the temperature, radial velocity, and column density of the CO₂ gas.
• The authors also analyzed the spatial variation of the CO₂ column density and detected a point-like CO₂ emission source coincident with the known M-giant star IRS 11SW.
• In addition, they estimated the column densities of C₂H₂ and HCN from their absorption features.
• The authors also measured the column density of solid CO₂ ice across the fields.

Results

• The CO₂ gas has a temperature between 20-50 K and a column density of around 2×10¹⁵ cm⁻².
• The C₂H₂ gas has a similar column density to CO₂ but a higher temperature of ~100 K, while HCN has a lower temperature of ~10 K.
• The CO₂ ice column density corresponds to an ice-to-gas ratio of ~90.
• The CO₂ absorption likely originates from one or more extended foreground molecular cloud(s) at distances between 0.15 and 4 kpc from Earth.
• The authors also detected a point-like CO₂ emission source coincident with the M-giant star IRS 11SW, which may be associated with a symbiotic binary system.

Interpretation

• The authors conclude that the observed absorption features are most likely produced by one or more foreground molecular clouds, based on the radial velocities consistent with zero and the similar column densities between the two observed fields.
• The clumpy nature of the clouds is suggested by the spatial variations in CO₂ column density up to a factor of 2.
• The CO₂ emission associated with IRS 11SW is likely produced by the circumstellar environment of this M-giant star, potentially related to a compact companion.

Limitations & Uncertainties

• The authors note that the temperature uncertainties are not straightforward to calculate due to the correlation between temperature and line normalization.
• The HCN column density uncertainties are larger due to the complex baseline in that spectral region.
• The authors also performed non-LTE modeling to assess the potential impact of radiative pumping, but found it to be negligible for the conditions likely present in the foreground clouds.

What Comes Next

• Future observations of other Galactic Center environments and IR-bright regions could further improve our understanding of the distribution and physical conditions of gas-phase and solid-state CO₂.
• Investigating the relationship between the gas-phase and ice-phase CO₂ across a larger sample of sightlines may provide insights into gas-grain interactions within molecular clouds.