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

Abstract

CO$_2$ is an important, stable, and abundant molecule in the Universe, but it is very difficult to detect because it has no observable pure rotational transitions. The unique sensitivity and resolution of the James Webb Space Telescope (JWST) provide a fresh way to investigate it. CO$_2$ is typically found in the solid phase (ice) on grain mantles in dense molecular clouds, but is less commonly detected in the gas phase (compared to common molecules such as CO and H$_2$O) and has mostly been found in protostellar and proto-planetary environments. Here, we report and characterize the first observations of gas-phase CO$_2$ absorption toward two IR-bright regions of the Galactic Center, thanks to the high sensitivity of JWST. Using an LTE model we find a CO$_2$ gas excitation temperature between 20 and 50~K, a column density around 2$\times$10$^{15}$~cm$^{-2}$ and a radial velocity consistent with 0. We also report: 1) simultaneous detections of C$_2$H$_2$ and HCN absorption bands (near 13.7 and 14.0 $μ$m, respectively), with column densitiy ratios of 1:3 and 3:2 with respect to gas-phase CO$_2$, and 2) CO$_2$ ice absorption with a ice-to-gas ratio of 90, consistent with previous findings. We conclude that the absorbing medium is likely in the foreground, most likely from one or more somewhat clumpy cloud(s), located between 0.15 and 4~kpc away from Earth. Additionally, we detected point-like CO$_2$ emission likely associated with a Galactic Center star (IRS~11SW), which is also spatially coincident with a previously reported X-ray source, raising the possibility that the system is a symbiotic binary.

Figures (13)

• Figure 1: The observation fields overlaid upon a JVLA 2cm continuum map of the Galactic Center, from Morris+17. The CC and CND pointings are represented by the cyan and red rectangles, respectively. The location of Sgr A* is marked by the magenta cross.
• Figure 2: Integrated spectra over each of the two fields showing the detection of the CO$_2$ absorption feature. The spectrum extracted from the Circumnuclear Disk (CND) field is shown in blue and that from the Central Cavity (CC) pointing is shown in green, with their corresponding scales on the right and the left, respectively. The dark gray, dashed lines show the baseline model (see Section \ref{['sec:analysis']} for details). The vertical light gray lines indicate the wavelengths of modeled transitions. The acetylene and hydrogen cyanide features are at shorter wavelengths (not shown -- see Figure \ref{['fig:hcn_and_acetylene']}), while part of the broad solid CO$_2$ feature can be seen to the right of this Figure (see Figure \ref{['fig:co2_ice_spectrum']}).
• Figure 3: The top panel shows CO$_2$ absorption in the baseline-subtracted spectrum (solid line) along with the LTE model fit (dashed line), and the bottom panel shows model residuals for both pointings. Color and scale correspondences same as Figure \ref{['fig:integrated_spectrum']}; the red dashed lines represent the best-fit LTE models. The two shaded columns in the bottom panel mark deviations from the model fit caused by the inherent noise in the data (close to 14.8 $\mu$m) and the inaccurate baseline tracing due to absorption from CO$_2$ ice (close to 15.15 $\mu$m).
• Figure 4: C$_2$H$_2$ and HCN absorption in the baseline-subtracted spectra, along with the LTE model fit (red dashed lines)(top panel), and model residuals for both pointings (bottom panel). The Q-branch transitions of C$_2$H$_2$ are around 13.7 $\mu$m, while those for HCN are around 14.05 $\mu$m. The orange vertical lines represent modeled C$_2$H$_2$ transitions, while the cyan vertical lines represent modeled HCN transitions. The shaded column in the bottom panel contains the region with absorption features not accounted for by the LTE model of the two molecules.
• Figure 5: Column density maps and selected spectra of CO$_2$ absorption from the CND pointing (left column) and the CC pointing (right column). The upper spectra are from the solid magenta apertures and the lower spectra are from the dashed magenta apertures. Gray solid lines represent the baseline fits and the red dashed lines represent the best LTE model fit.