Quantifying Spectroscopic Flux Variations Between JWST NIRISS and NIRSpec: Slit Losses in Emission Line Measurements of z$\sim$1-3 Galaxies

TL;DR

This study quantifies how flux measurements differ between JWST NIRISS slitless spectroscopy and NIRSpec MOS for intermediate-redshift galaxies, emphasizing morphology-driven slit-loss effects. By analyzing 12 Abell 2744 galaxies with emission lines detected in both instruments and applying precise line-fitting plus Monte Carlo analyses on emission-line maps with resolution matching, the authors reveal a clear dichotomy: compact sources yield consistent fluxes between NIRISS and NIRSpec, while extended sources show significant NIRSpec under-recovery due to slit losses. They also find that equivalent widths are less sensitive to aperture effects than absolute fluxes, though line ratios such as Hα/[O III] can vary by up to ~0.3 dex across methods, impacting metallicity and SFR inferences. The work provides a practical framework for cross-instrument flux comparisons in JWST data and highlights the necessity of cross-calibration when combining slitless and slit-based spectroscopy, particularly for extended galaxies.

Abstract

We analyze JWST NIRISS and NIRSpec spectroscopic observations in the Abell 2744 galaxy cluster field. From approximately 120 candidates, we identify 12 objects with at least a prominent emission lines among \Oii, \Hb, \Oiiia, \Oiiib, and \Ha that are spectroscopically confirmed by both instruments. Our key findings reveal systematic differences between the two spectrographs based on source morphology and shutter aperture placement. Compact objects show comparable or higher integrated flux in NIRSpec relative to NIRISS (within 1$σ$ uncertainties), while extended sources consistently display higher flux in NIRISS measurements. This pattern reflects NIRSpec's optimal coverage for compact objects while potentially undersampling extended sources. Quantitative analysis demonstrates that NIRSpec recovers at least $63\%$ of NIRISS-measured flux when the slit covers $>15\%$ of the source or when $R_e<1$kpc. For lower coverage or larger effective radii, the recovered flux varies from $24\%$ to $63\%$. When studying the \Ha/\Oiiib emission line ratio, we observe that measurements from these different spectrographs can vary by up to $\sim$0.3 dex, with significant implications for metallicity and star formation rate characterizations for individual galaxies. These results highlight the importance of considering instrumental effects when combining multi-instrument spectroscopic data and demonstrate that source morphology critically influences flux recovery between slit-based and slitless spectroscopic modes in JWST observations.

Figures (8)

• Figure 1: NIRISS imaging cutouts are presented using a drizzled stack of the F115W, F150W, and F200W filters, processed to better highlight the light associated with each galaxy candidate. Solid white contours represent the NIRISS segmentation maps, rectangles show the NIRSpec slits assigned to the target (solid magenta and dash-dot white). The ID and redshift $z$ refer to measurements from NIRISS, using the catalog presented in Watson_2025 and a 0.6 arcsec scale bar is included. The "A" parameter, shown in the bottom right corner, represents the fractional area of the segmentation map that is encompassed by the slit. Objects are classified as either "compact" or "extended" based on the classification criteria presented in Sec.\ref{['sec: data selection']}.
• Figure 2: Example Analysis: Spectroscopically Confirmed Candidate NIRISS ID 2744 at z=1.86. Top left: NIRISS image cutout (0.6 arcsec scale bar shown) with the NIRISS segmentation map (solid white contour) and NIRSpec slits assigned to the target (solid magenta and dash-dot white). Top right: Combined spectral data showing the original NIRSpec spectrum (gray), the NIRISS spectrum (blue), and the NIRSpec spectrum after convolution to match the NIRISS resolution (red), with identified emission lines marked. Bottom panels: Detailed emission line fits for all the line matched between the two instruments. The shaded red regions in each panel indicate an example of the continuum estimation regions computed for a single iteration of the MC sampling.
• Figure 3: Probability density distributions of $\text{flux}_{\text{ratio}}$ (top row) and $\text{EW}_{\text{ratio}}$ (bottom row) for the emission lines of the source presented in Fig.\ref{['fig: spectra plots']}. The distributions are shown in logarithmic scale for visualization purposes and represent the marginalized probability densities obtained by varying the continuum estimation regions. In red the 50$^{\rm{th}}$ and from left to right in black 16$^{\rm{th}}$ and 84$^{\rm{th}}$ percentiles.
• Figure 4: All panels show results for the observed sources from three different JWST programs, color-coded by subsample (compact or extended). Error bars indicate 1$\sigma$ uncertainties calculated using the methodology described in Sec.\ref{['subsec: precise fitting']}. In these panels, we see that measurements for compact sources from NIRISS and NIRSpec are in good agreement. However, for more extended objects (shown on the left side of panel c), the NIRSpec measurements are systematically lower, as expected.
• Figure 5: Same format as Fig.\ref{['fig: combined_flux_ratio']}, but showing the logarithmic $\text{EW}_{\text{ratio}}$ as a function of various quantities. Unlike the dichotomy seen in $\text{flux}_{\text{ratio}}$, both compact and extended sources display similar scatter in $\text{EW}_{\text{ratio}}$ with no strong trends.