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$σ$ 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.

Figures (13)

• Figure 1: RGB image of the MIDIS field constructed with trilogyCoe2012_Trilogy, combining F560W within the MIDIS footprint and F444W outside of it. Red solid and blue dashed contours indicate MIDIS Deep and Shallow regions, respectively. Yellow and brown contours show Band 3 (3 mm) and Band 6 (1 mm) ALMA observations ASPECS1_Walter2016GonzalezLopez2019_ALP1. Magenta squares mark MIRI-red galaxy candidates and their IDs.
• Figure 2: Illustration of the mask applied to MIDIS/F560W bright sources ($<23.5$,mag). The MIDIS Deep and MIDIS Shallow contour regions are overlaid. After masking the bright galaxies, the remaining area corresponds to 91.3% of the total MIDIS Deep area.
• Figure 3: Background noise level as a function of aperture diameter of MIDIS Deep. The black markers indicate the background noise level derived from the scatter in empty background apertures. The red markers denote the average error derived from the sum in quadrature of the pixels in the associated variance map to the MIDIS observations. The shaded red markers denote the average error derived after rescaling the variance maps. The background noise levels ($\sigma_{\rm bkg}$) are used as the 1$\sigma$ error for photometric measurements in given aperture sizes, as the average errors ($\sigma_{\rm var}$) are underestimated.
• Figure 4: Purity and completeness of the MIDIS Deep survey (excluding detection brighter than 23.5 mag) as a function of MIRI 5.6 $\mu$m magnitude and flux. At a $5\sigma$ detection threshold, the survey reaches 92% purity and 54% completeness.
• Figure 5: Eazy SED fit of the MIRI-red galaxy candidates sample including the deep F770W and F1000W data from MIDIS-red. In Appendix \ref{['sec:AppendixD_SED']}, we show the same figure without MIDIS-red data. Interestingly, without the MIDIS-red data, MIRI-red candidate with ID=205 shows H$\alpha$ emission line in F560W instead of F770W, as it is not constrained without the MIDIS-red observations. In Appendix \ref{['sec:AppendixD_SED']} are the photometric redshift probability distribution corresponding to all the fits.