XID+PRIMA, II: Stepping Through Hyperspectral Imaging to Deblend PRIMAger Beyond the Extragalactic Confusion Limit

TL;DR

This paper addresses the FIR confusion limit in upcoming PRIMAger extragalactic imaging surveys by introducing XID+stepwise, a Bayesian deblending approach that propagates informative flux priors along PRIMAger's hyperspectral channels. By leveraging spectral information and Euclid-like positional priors, the method recovers fluxes down to well below the classical confusion limit, achieving flux accuracy within approximately $\pm20\%$ across 16 channels, with reductions up to factors of $\sim$1.3–7 in limiting flux depending on channel and prior set. The authors demonstrate that both Euclid Wide and Deep priors, as well as self-contained blind detections, enable robust flux measurements for IR-luminous galaxies at cosmic noon ($z\sim2$), including near-complete sampling of the FIR SED for ULIRGs. They also show that the approach is resilient to realistic systematics, such as cirrus contamination and beam imperfections, supporting PRIMAger's potential to perform deep extragalactic imaging surveys beyond classical confusion limits.

Abstract

The PRobe far-Infrared Mission for Astrophysics concept aims to map large areas with spectral coverage and sensitivities inaccessible to previous FIR space telescopes, covering 25-235um. We synthesise images representing a deep imaging survey, with realistic instrumental and confusion noise, reflecting the latest PRIMAger instrument specifications. We present a new Bayesian modelling approach XID+stepwise that exploits PRIMAger's hyperspectral imaging to derive self-consistent, informative flux priors by sequentially propagating constraints from short to long wavelengths. With Euclid-like prior source positions, this method recovers fluxes to within 20% to 0.2-0.7 mJy across 45-84 um, which correspond to factors of 1.3-3.4 fainter than the confusion limit. For the most confusion-dominated channels, accurate fluxes are measured to 0.9, 2.5, 7.6 and 14.8 mJy at 92, 126, 183 and 235 um, respectively, which are factors of 3-5 better than the confusion limit. Using a deeper Euclid-based prior catalogue and weak ancillary flux priors at 25 um yields further improvements, reaching up to a factor ~7 fainter than the confusion limit at 96 um. Additionally, we demonstrate that positional priors from blind source detection followed by deblending via XID+ enables PRIMAger to achieve sensitivity beyond the confusion limits using PRIMAger data alone. We show that IR-luminous galaxies at z~2 are robustly detected in a large fraction of the PRIMAger channels (>98% in 12 out of the 16 considered channels), providing dense sampling of the FIR SED even for sources several factors below the confusion limit. We explore the impact on our results for a range of systematic effects, including cirrus contamination, optical degradation, and calibration uncertainties. These findings indicate that confusion noise will not limit the key science from PRIMA extragalactic imaging surveys when employing XID+.

Figures (16)

• Figure 1: Filter transmission curves of all PHI channels, both narrow (top) and coadded (bottom). The filters are coloured to match their associated coadded channel.
• Figure 2: Cutouts of simulated observations including both instrumental and confusion noise for six representative PRIMAger channels. The transition between instrumental and confusion noise dominating is apparent. Each cutout covers an area of 4.24 x 4.24$^\prime$. The instrumental noise is added on a per pixel level representative of a deep ($\sim$1500 hr deg$^{-2}$) survey, and is discussed in \ref{['sec:observations_mapmaking']}.
• Figure 3: Cutouts of simulated observations in PPI1 in the presence of various cirrus levels. All cutouts share the same colour scale, are mean-subtracted, and span 34 x 34$^\prime$. The bounded area in the top left panel corresponds to the area shown in Figure \ref{['fig:noisy_cutouts']}.
• Figure 4: Distribution of SIDES sources which are above the point source sensitivity in at least 5 of the PRIMAger bands. Bottom: Stellar mass-redshift distribution of the PRIMAger detectable sources within SIDES relative to the Euclid Q1 95% stellar mass completeness limit from EuclidQ1 (solid blue curve), representing a Euclid Wide Field survey depth and a 0.8 dex deeper Euclid Deep Field Survey (solid pink curve). Top: The percentage of PRIMAger detectable sources in SIDES which are above the mass completeness limits for the respective Euclid survey depths.
• Figure 5: Percentage of sources within the Euclid prior catalogues which are above the instrument point source sensitivity at each of the PRIMAger channels for the considered survey. At the shortest wavelengths, less than 20% of the sources within the prior catalogues are detectable above the point source sensitivities and significantly contributing flux to the maps.