The JWST-NIRCam View of Sagittarius C. III. The Extinction Curve

TL;DR

We derive the infrared extinction curve toward the Galactic centre using JWST-NIRCam observations of Sagittarius C across $1-5\,\mu\mathrm{m}$, leveraging the reddening vector of red clump stars in colour--magnitude diagrams. Two RC-slope methods (with a fixed reference filter and with consecutive filters) yield consistent extinction ratios relative to $A_{\mathrm{F162M}}$, including $A_{\mathrm{F115W}}/A_{\mathrm{F162M}} = 1.84 \pm 0.03$ and $A_{\mathrm{F212N}}/A_{\mathrm{F162M}} = 0.607 \pm 0.014$, among others. The inferred extinction index shows $\alpha_{\lambda-\mathrm{F162M}} \approx 2$ for $\lambda \sim 1-2.5\,\mu$m and $\approx 1.4$ for $\lambda \sim 2.5-5\,\mu$m, with the curve flattening at longer wavelengths and a CO ice absorption feature near $4.7\,\mu$m. The JWST-derived curve agrees with prior GC extinction curves (e.g., Nishiyama 2009, Fritz 2011) and with some non-GC curves (Indebetouw 2005), supporting a largely line-of-sight invariant infrared extinction law in the inner Galaxy within uncertainties, and showcasing JWST’s capability for precise extinction studies in crowded, highly extinct regions.

Abstract

Determining the infrared extinction curve towards the Galactic centre is crucial for accurately correcting observed data and deriving the underlying stellar populations. However, extinction curves reported in the literature often show discrepancies. We aim to derive the infrared extinction curve towards the Galactic centre based on JWST-NIRCam data for the first time, using observations of the Sagittarius C region in the 1-5 $μ$m range. We determined extinction ratios using two different methods, both based on measuring the reddening vector using the slope of red clump stars, whose intrinsic properties are well known, in observed colour-magnitude diagrams. The extinction curve derived in this work is in good agreement with previous results in the literature. We obtained the following extinction ratios relative to F162M: $A_\mathrm{F115W} : A_\mathrm{F162M} : A_\mathrm{F182M} : A_\mathrm{F212N} : A_\mathrm{F360M} : A_\mathrm{F405N} : A_\mathrm{F470N} : A_\mathrm{F480M} = 1.84 \pm 0.03 : 1.00 : 0.789 \pm 0.005 : 0.607 \pm 0.014 : 0.306 \pm 0.011 : 0.248 \pm 0.017 : 0.240 \pm 0.019 : 0.21 \pm 0.03$. Besides, we found different values of the extinction index for the short- ($λ\sim 1-2.5\,μ$m, $α\sim 2$) and long-wavelength ($λ\sim 2.5-5\,μ$m, $α\sim 1.4$) regimes, with the extinction curve flattening at longer wavelengths. Comparison with extinction curves derived both inside and outside the Galactic centre suggests that the infrared extinction curve does not significantly vary in the central regions, and shows no significant evidence for variations between different lines of sight beyond the inner Galaxy within the uncertainties.

Figures (11)

• Figure 1: False-colour red-green-blue image of the Sgr C region created from Stage 3 mosaics. The red, green, and blue channels correspond to the F480M, F405N, and F162M filters, respectively. The dashed white line outlines the region overlapping with the GALACTICNUCLEUS survey (see Sect. \ref{['GNS']}).
• Figure 2: Top: Uncertainties returned by StarFinder. Bottom: Uncertainties estimated from multiple detections of the same stars.
• Figure 3: F480M versus F162M-F480M CMD. The purple box highlights the RC feature, and the black arrow is the reddening vector for $A_\mathrm{F480M} = 0.3$ mag. The two purple arrows indicate the bright (top) and faint (bottom) RC features. The inset shows a zoomed-in view of the RC region, where the two features are identified using the GMM approach described in Sect. \ref{['subsec:methods']}. The cyan points correspond to the positions of the RC features in each bin, and the cyan lines show the best-fitting linear regressions. The uncertainties, although included, are smaller than the marker size and therefore not visible. The purple box in the CMD is also included for reference.
• Figure 4: Extinction curve derived using the reference (blue, first method) and the consecutive filter (orange, second method) methods with their uncertainties. The x-axis shows the effective wavelength for each filter. The extinction curve from the first method has been spline-interpolated (see Sect. \ref{['subsec:methods']}). In both cases, the plotted extinction ratios correspond to the mean values obtained from the bright and faint RC features. In most cases, the uncertainties are smaller than the marker size and thus not visible.
• Figure 5: Comparison between the transmission curves of the HAWK-I filters H and $\mathrm{K_s}$ and the JWST filters F162M and F212N.