Mapping CO Ice in a Star-Forming Filament in the 3 kpc Arm with JWST

Abstract

CO gas emission is a fundamental tool for measuring column density, but in cold, dark clouds, much of the CO is locked away in ice. We present JWST results from observations of a star forming filament (G0.342+0.024) that that appears to be associated with the 3 kpc arm. This filament is backlit by the Galactic Center, which has allowed us to construct a high-resolution extinction map (mean separation between stars of ~1" outside the filament, ~2" in the filament). ALMA Band 3 data reveals embedded star formation within the cloud. Using the CO ice feature covered by the F466N band, we map the CO ice column density of the filament. By combining the extinction map, CO ice column density map, and archival CO observations, we examine the efficacy of standard CO X-factor measurements of mass in star forming gas. We find that 50-88% of the CO is locked away in ice at large column densities ($N_{\rm \rm H_2} \gtrsim 10^{22} \rm ~cm^{-2}, 200 \rm ~M_{\odot} \rm ~pc^{-2}$) in the filament. The primary sources of uncertainty in this estimate are due to uncertainty in the ice composition and lab measurements of ice opacities. This shows that systematic corrections are needed for mass measurements in the Milky Way and nearby galaxies at high column densities.

Figures (19)

• Figure 1: Overview figure using Spitzer I4, I3, and I1 Churchwell2009. The bottom panel shows the context of the Galactic Center Central Molecular Zone. The top left panel zooms in on the dust ridge region, labeling each of the dust ridge clouds lis1999, and showing the field of view of the JWST NIRCam field of view (the jagged, zig-zag field). The top right panel shows a zoom in of the Filament.
• Figure 2: The process of "destreaking" the _cal images. Left is the starting _cal image. Middle is the image after 1/f noise reduction. Right is the residual, or difference between the two images. The process is not perfect, as seen by the difference in the columns/amplifiers in the middle image.
• Figure 3: Three color image of the region observed with JWST in parallel. Red is F466N, blue is F405N, and green is their combination. Several IRDCs are visible in this image. Appendix Figure \ref{['fig:cropped']} is a version of this figure with each of the distinct IRDCs labeled.
• Figure 4: The left panel shows the locations of 37344 stars detected towards the filament with the color map showing the color changes due to dust extinction in [F187N]-[F405N]. The region used for close-in analysis of the filament is over-plotted. The top right panel shows how all three colors [F182M]-[F212N], [F212N]-[F410M], and [F187N]-[F405N] are reddened by dust extinction. The bottom right panel shows how [F405N] - [F466N] differs from the other colors, becoming bluer with increasing extinction past a certain point, due to the inclusion of [F466N], which is affected by CO ice in the band. The black vectors in the color-color diagrams show the direction of reddening from dust extinction, and it shows how much a star at their origin would be reddened or moved in color-color space by $\mathrm{A_V}=20$ of extinction using the CT06 extinction law. The black marker in the bottom left of each CCD represents the typical errorbars for each point, each on the order of 0.05mag.
• Figure 5: Extinction map of the filament created using $\mathrm{A_V}$ measured with [F182M]-[F410M]. Stars behind the filament that were detected in F410M but not F182M were given a flat $A_V=85$, then a constant value of 22mag was subtracted from the whole map to remove foreground extinction.