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

Abstract

We investigate the detection and nature of 5.6~μm MIRI-red sources in the MIRI Deep Imaging Survey (MIDIS), covering 2.4~arcmin${}^2$ 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$\ddot{I}ƒ$ significance in MIRI/F560W with red colors: $m_{F444W}-m_{F560W}\ge 0.5$. 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 $E{W}_0(H\hat{I}\pm )\ge 750$ à or $E{W}_0(H{\hat{I}}^2+[OIII])\ge 600$ à , 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 $E{W}_0(H\hat{I}\pm )\sim 6000$ à and $E{W}_0(H{\hat{I}}^2+[OIII])\sim 4000$ à , 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$\hat{I}\pm$ or H${\hat{I}}^2$+[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.

19 pages, 23 figures, submitted to A&A