MIDIS: The identification of deep MIRI-red sources as candidates for extreme Balmer-break and line emitting galaxies at high-z

We investigate the detection and nature of 5.6μm MIRI-red sources in the MIRI Deep Imaging Survey (MIDIS), covering 2.4arcmin$^2$ in the Hubble Ultra Deep Field. MIDIS is the deepest JWST/MIRI survey to date, probing faint limits and enabling studies of rare high-redshift galaxy populations. We define MIRI-red sources as those detected at 5$σ$ significance in MIRI/F560W with red colors: $m_{\rm F444W} - m_{\rm F560W} \ge 0.5$. Using an empirical methodology, we estimate the purity and completeness of MIRI detections and find that a 5-sigma detection at 28.75 mag has a purity of 92% and completeness of 54%. We identify seven MIRI-red galaxy candidates, including an F115W dropout consistent with a high-redshift galaxy candidate. We explore possible physical origins for the MIRI-red population, including active galactic nuclei, dust-obscured galaxies, extreme emission-line galaxies, evolved stellar populations, and Little Red Dots (LRDs). Given the proximity of the F444W and F560W filters and the depth of MIDIS, MIRI-red galaxies are consistent with emission-line galaxies with $EW_0(Hα) \ge 750$ Å or $EW_0(Hβ+ [OIII]) \ge 600$ Å, or high-redshift Balmer breaks of at least 1.6. We also discuss an extreme MIRI-red galaxy undetected in F444W, a potential MIRI-only source, for which we derive $EW_0(Hα) \sim 6000$ Å and $EW_0(Hβ+ [OIII]) \sim 4000$ Å, or high-$z$ LRD analogs with Balmer breaks of 6.3. Finally, we find fewer MIRI-red detections than expected from extrapolations of the H$α$ or H$β$+[OIII] line luminosity functions, consistent with previous deep searches, while the absence of $z>10$ LRD candidates agrees with theoretical expectations for the MIDIS volume.

💡 Research Summary

The paper presents the first ultra‑deep JWST/MIRI imaging of the Hubble Ultra Deep Field (HUDF) carried out by the MIRI Deep Imaging Survey (MIDIS). Using 50 hours of exposure in the F560W (5.6 µm) filter, the authors reach a 5σ point‑source depth of AB 28.75 mag (≈11.5 nJy) over a 2.4 arcmin² region that overlaps the deepest NIRCam, HST, MUSE, and ALMA coverage. They define “MIRI‑red” sources as objects detected at ≥5σ in F560W with a red colour, m_F444W − m_F560W ≥ 0.5 mag.

A careful assessment of the noise properties reveals that the standard JWST pipeline overestimates the signal‑to‑noise ratio by a factor of ~3. By inserting artificial sources and measuring recovery rates, the authors empirically determine that a 5σ detection at 28.75 mag has a purity of 92 % and a completeness of 54 %. This calibration allows a statistically robust selection of MIRI‑red candidates.

Seven candidates satisfy the colour cut. Six are detected in both F444W and F560W; one is detected only in F560W (a “MIRI‑only” source). Spectral energy distribution (SED) fitting, together with existing NIRSpec/PRISM and MUSE spectroscopy, is used to explore the physical origin of the red colour. Because the two filters are close in wavelength (Δλ ≈ 0.16 µm), a strong emission line falling in F560W can produce the observed colour without requiring an intrinsically red continuum. The authors find that the colour can be reproduced if the rest‑frame Hα equivalent width (EW₀) exceeds ~750 Å or the combined Hβ+