Resolving stellar populations, star formation, and ISM conditions with JWST in a large spiral galaxy at z $\sim$ 2

Eleonora Parlanti, Giulia Tozzi, Natascha M. Förster Schreiber, Claudia Pulsoni, Letizia Scaloni, Stavros Pastras, Pascal Oesch, Capucine Barfety, Francesco Belfiore, Jianhang Chen, Giovanni Cresci, Ric Davies, Frank Eisenhauer, Juan M. Espejo Salcedo, Reinhard Genzel, Rodrigo Herrera-Camus, Jean-Baptiste Jolly. Lilian L. Lee, Minju M. Lee, Daizhong Liu, Dieter Lutz, Filippo Mannucci, Giovanni Mazzolari, Thorsten Naab, Amit Nestor Shachar, Sedona H. Price, Alvio Renzini, T. Taro Shimizu, Amiel Sternberg, Martina Scialpi, Eckhard Sturm, Linda J. Tacconi, Hannah Übler, Stijn Wuyts

First listed 2025-10-10 | Last updated 2025-10-14

Abstract

Cosmic noon represents the prime epoch of galaxy assembly, and a sweet spot for observations with the James Webb Telescope (JWST) and ground-based near-IR integral-field unit (IFU) spectrographs. This work analyses JWST NIRSpec Micro Shutter Array (MSA), NIRCam Wide Field Slitless Spectroscopy (WFSS) of K20-ID7, a large spiral, star-forming (SF) galaxy at z=2.2, with evidence for radial gas inflows. By exploiting the synergy with ground-based IFU ERIS observations, we conduct a comprehensive and resolved study of the interstellar medium (ISM) and stellar properties, from rest optical to near-IR, via emission-line diagnostics, resolved spectral energy distribution (SED) fitting of high-resolution imaging, and Pa$β$ line detection in NIRCam WFSS data. Our analysis reveals massive ($M_{\star}\simeq$(0.67-3.5)$\times$10$^{9}$ $M_{\odot}$) SF clumps with star formation rates (SFRs) ~3-24 $M_{\odot}$/yr, and quite low dust attenuation ($A_V\simeq$0.4), electron density ($n_{e}$<300 cm$^{-3}$), and ionisation (log(U)$\simeq -3.0$). The central bulge turns out to be modestly massive ($M_{\star}$=(7$\pm$3)$\times$10$^{9}$ M$_{\odot}$), heavily obscured ($A_V$=6.43$\pm$0.55), and likely to have formed most of its stellar mass in the past (SFR=82$\pm$42 $M_{\odot}$/yr over the last 100 Myr), yet still forming stars at a lower rate (SFR=12$\pm$8 M$_{\odot}$/yr over the last 10 Myr). We infer a metallicity 12+log(O/H)~8.54 and an apparent enhancement of the N/O abundance (log(N/O)$\simeq -1.0$) in all distinct galaxy regions, a likely consequence of dilution effects due to radial inflows of metal-poor gas. We measure a sub-solar sulfur abundance (log(S/O)$\simeq$-1.9). Finally, the radial stellar age profile reveals older stellar populations in the inner galaxy regions compared to the outskirts, pointing to an inside-out growth of K20-ID7.

Short digest

JWST/NIRSpec MSA and NIRCam WFSS, combined with VLT/ERIS, resolve the ISM and stellar populations of the z=2.224 grand‑design spiral K20-ID7 from rest‑optical to near‑IR. They find massive clumps with M⋆=(0.67–3.5)×10^9 M⊙ and SFR≈3–24 M⊙/yr with low AV≈0.4, ne<300 cm⁻³, and log U≈−3; the compact bulge is modestly massive, heavily obscured (AV=6.43±0.55), and shows declining recent SFR (82±42 over 100 Myr to 12±8 M⊙/yr over 10 Myr). Gas abundances are fairly uniform with 12+log(O/H)≈8.54 and elevated N/O (log N/O≈−1.0) plus sub‑solar S/O (log S/O≈−1.9), consistent with dilution by radial inflows of metal‑poor gas. Paβ mapping and a radial age gradient indicate inside‑out growth in an unperturbed spiral disk.

Key figures to inspect

• Figure 1: Inspect the F435W–F444W morphology to verify the regular two‑armed spiral and the compact, red F444W‑bright bulge whose prominence aligns with the high AV and older central ages inferred from SED fitting.
• Figure 2: Compare PRISM/CLEAR spectra from the seven MSA shutters to see region‑to‑region changes in continuum shape and key rest‑optical lines (e.g., Hβ, [O III], Hα, [N II], [S II]) used to derive ne, ionisation parameter, and abundances; note which shutters sample clumps versus bulge.
• Figure 3: Use the spatially resolved Paβ in WFSS to locate star‑forming clumps and quantify SFR; the double‑peaked 1D profile traces spatial offsets plus a velocity gradient along the dispersion, offering a cross‑check on kpc‑scale gas motions.
• Figure 4: Read the pixel‑scale maps (M⋆, SFR10 Myr/100 Myr, AV, age, χ²) to connect heavily obscured, older central stellar populations with younger, lower‑AV clumps in the arms, and to visualize the inside‑out growth signature; the χ²≈1 map supports the robustness of the SED solutions.