A JWST NIRCam/MIRI view of the W51A high-mass star-forming region

TL;DR

We present the first JWST NIRCam/MIRI imaging of the W51A region at sub-arcsecond resolution, enabling detailed morphology of dust filaments, PAH-rich zones, and HII regions around the protoclusters W51-E and W51-IRS2. Using five NIRCam and five MIRI filters, we separately map ionized gas, warm dust, and PAHs, revealing converging dust filaments feeding W51-E and a cleared interior around W51-IRS2, consistent with feedback suppressing infall in that protocluster. Cross-matching with ALMA shows only $\sim\text{$\sim11\%$}$ of ALMA sources have JWST counterparts, implying many objects remain deeply embedded; a prominent [Fe II] and H$_2$ knot near W51-IRS2 traces a powerful protostellar jet impacting dense ISM, while multi-wavelength context links this knot to an outflow from W51north. Overall, the study provides a complementary long-wavelength view of a 4 × 8 pc star-forming complex and establishes a foundation for future SED fitting, kinematic analyses, and extended JWST–ALMA studies in W51A.

Abstract

We present observations of the W51A region, including the massive protoclusters W51-E and W51-IRS2, with JWST in 10 NIRCam and 5 MIRI filters. In this work, we highlight the most novel features apparent in these images and compare them with other multi-wavelength images. The broad view of the NIRCam/MIRI images of the W51A region shows that areas dominated by warm dust and ionized gas are distinct from those dominated by PAHs. The high angular resolution of the JWST images resolves dust filaments in high contrast, revealing geometrically converging features feeding W51-E and a cavity around W51-IRS2. This picture adds support to the hypothesis that feedback from W51-IRS2 is suppressing further gas infall onto the protocluster, while by contrast, gas is still accreting onto W51-E. Comparing the NIRCam and MIRI images to ALMA data, we find 24 sources detected by both JWST and ALMA, accounting for only $\sim10\%$ of the ALMA sources; the rest are too embedded or too cool to be detected by JWST. A knot of [Fe II] and H$_2$ emission north of W51-IRS2, previously detected in ground-based images, reveals peculiarly bright and compact peaks detected in all JWST bands. The knot is likely the most energetic example of a protostellar jet driven by a massive star impacting dense interstellar medium. The new images provide a complementary view to the previous long-wavelength perspective on this 4 x 8 pc area of one of the most active star-forming regions in our Galaxy, revealing new mysteries to be further explored.

Figures (9)

• Figure 1: An overview of W51A region. The composite image is produced by combining NIRCam F360M (blue), F410M (green), and MIRI F560W (red). The north and east directions in ICRS coordinates are marked as arrows at the upper left corner.
• Figure 2: W51A MIRI overview. The image shows a composite of F770W (blue), F1280W (green), and F2100W (red). The pixels saturated in any filters are filled with white color.
• Figure 3: W51A NIRCam overview showing a F405N (red)/F335M (green)/F187N (blue) composite image. The positions of the cutouts for the zoomed-in images displayed in Fig. \ref{['fig:higlights_cutouts']} are marked.
• Figure 4: Multiple color images of W51A region in a large-scale view. The images of F405N-F410M (top), F480M-F360M (middle), and F335M/F480M (bottom) are displayed to represent ionized gas, warm dust, and PAH abundance, respectively. The cyan contour marks the flux density of F405N-F410M image at 250 MJy/sr.
• Figure 5: The cutout images of the highlighted boxes in Fig. \ref{['fig:highlight1']}. a: A dust filament around W51-E. b: W51-IRS2 protocluster. c: Cometary objects around W51-IRS2. d: W51-E protocluster. e: A bar at the edge of IRS1 Hii region. f: W51 IRS1 Hii region shell structure. g: W51b1 Hii region. h: W51b2 Hii region and YSOs. i: W51e7 Hii region. j: W51c1 Hii region. k and l: Newly discovered Hii regions.