FEAST: a NIRSpec/MOS survey of emerging young star clusters in NGC 628

Abstract

JWST can pierce through dusty molecular clouds to study the early stages of star formation, where young star clusters are actively driving stellar feedback and still emerging from their natal cloud. We present a first look of the JWST/NIRSpec multiplex spectroscopy observations acquired by the Feedback in Emerging extrAgalactic Star clusTers (FEAST) program for the nearby spiral galaxy NGC628. We showcase JWST's ability to resolve the spectral properties of emerging young star clusters (eYSCs) and their immediate interstellar medium (ISM) by focusing on a bright star-forming complex ($0.5\times0.5~\mathrm{kpc}^2$) in the northern spiral arm as a science proof-of-concept. The eYSC spectra are rich in ionized gas (from HII regions), as well as warm H$_2$ and polycyclic aromatic hydrocarbon (PAH) emission from photodissociation regions (PDRs), consistent with young star formation. $\mathrm{Pa}α$ equivalent widths and H/He ionizing photon fluxes both indicate the presence of hot, young massive stars (O8.5V-O8V), consistent with photometry SED estimates. The ionized gas is highly correlated with H$_2$ and PAH emission, suggesting that the PDR morphology evolves as clusters emerge from their natal cloud. We find a photoionization-dominated regime from independent line diagnostics, with little contribution from Supernovae-driven shocks, highlighting the importance of pre-Supernovae feedback when massive stars are present. This pilot study showcases how JWST's multiplex spectroscopy mode can disentangle the mechanisms present in the youngest stages of star formation for the first time outside the Local Group.

Figures (15)

• Figure 1: Top: Composite image of NGC 628 with the 3.3 $\mu$m PAH feature (NIRCam/F335M) in red, Br$\alpha$ (NIRCam/F405N) in green, and the stellar continuum (NIRCam/F444W) in blue. The circles denote the positions of the emerging young star clusters (blue for eYSC-I and pink for eYSC-II) from the FEAST catalog pedrini_2025ApJ...992...96P. The crosses display the locations of all MOS slits targeting a source (white) or background (orange). The region of interest, which the present paper focuses on, is highlighted by the white box, and further zoomed into in the bottom 3 subplots. Bottom: The zoomed-in subplots showcase the NIRCam/F335M in the background, and the slit footprints of the NIRSpec/MOS observations for this region (with slit IDs), across the 3 different MSA configurations (left, middle, right). The locations of eYSC-Is, eYSC-IIs and unclassified sources (see main text) within the spectroscopic slit observations are shown as blue triangles, pink triangles and white circles, respectively. The gray rectangles above and below each slit represent the additional shutters opened for the three-nod dithering pattern.
• Figure 2: Normalized distributions of age (left), mass (middle), and extinction (right) for all FEAST eYSCs in NGC 628 (dashed line) against the MSA targets (filled), from the CIGALE SED fits pedrini_2025ApJ...992...96P. The MSA targets have properties which are representative of the full galaxy eYSC population.
• Figure 3: Example of the methodology adopted for the extraction of source spectra. Top: 2D spectrum of the G235M grating for slit 8 (see Figure \ref{['fig:fullngc628']}). The corresponding wavelengths (in $\mu$m) to the dispersion pixels are shown in the top x-axis. The Pa$\alpha$ line at $1.87~\mu$m is highlighted. Bottom: Median cross-dispersion profile around the Pa$\alpha$ line for slit 8 (taken within the gray region in the top subplot, see main text). The dashed lines display the position of the cataloged eYSCs and "unclassified" sources in this slit. The pink shaded regions denote the extraction aperture (i.e., range of cross-dispersion pixels) chosen for the relevant sources.
• Figure 4: Spectra of sources within the region presented in this work (see Figure \ref{['fig:fullngc628']}), ordered by decreasing $3.3~\mu\mathrm{m}~\mathrm{PAH}$ emission strength from top to bottom. The presented fluxes have been corrected for extinction, and wavelengths are in the rest-frame. The spectra are color-coded as: blue for eYSC-I, pink for eYSC-II, purple for blended eYSC-I+II, and black for unclassified sources. The spectra are offset by an arbitrary amount to avoid overlaps. The key emission lines and features used in the present analysis are highlighted by the dashed lines, and labeled.
• Figure 6: Spectrum of source 100310 (top spectrum in Figure \ref{['fig:source_spectra_all']}) subdivided into 3 subplots to better illustrate the richness of emission lines and absorption features detected in the FEAST MOS observations. The spectrum shown in black is not continuum-subtracted, but is corrected for extinction and redshift. Identified lines are shown by the black dashed lines, with H recombination lines highlighted in blue, H$_2$ lines in orange, the PAH components in red, the CO$_2$ ice absorption feature (at 4.27 $\mu$m) in dark purple, and gas-phase CO transitions in the $R$ and $P$ branches in green (see text).