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The shape of the low-mass (faint) end of the galaxy stellar mass function (SMF) or ultraviolet luminosity function (UVLF) at $$z \gtrsim 6$$ is an open question for understanding which galaxies primarily drove cosmic reionization. Resolved photometry of Local Group low-mass galaxies allows us to reconstruct their star formation histories, stellar masses, and UV luminosities at early times, and this fossil record provides a powerful ‘near-far’ technique for studying the reionization-era SMF/UVLF, probing orders of magnitude lower in mass than direct HST/JWST observations. Using 882 low-mass ($$M_{\rm star}\lesssim 10^{9}\, \rm {M_\odot }$$) galaxies across 11 Milky Way (MW)- and Local Group-analogue environments from the FIRE-2 cosmological baryonic zoom-in simulations, we characterize their progenitors at $$z=6\!-\!9$$, the mergers/disruption of those progenitors over time, and how well their present-day fossil record traces the high-redshift SMF. A present-day galaxy with $$M_{\rm star}\sim 10^5\, \rm {M_\odot }$$ ($$\sim 10^9\, \rm {M_\odot }$$) had $$\approx 1$$ ($$\approx 30$$) progenitors at $$z\approx 7$$, and its main progenitor comprised $$\approx 100~{{\ \rm per\ cent}}$$ ($$\approx 10~{{\ \rm per\ cent}}$$) of the total stellar mass of all its progenitors at $$z\approx 7$$. We show that although only $$\sim 15~{{\ \rm per\ cent}}$$ of the early population of low-mass galaxies survives to present day, the fossil record of surviving Local Group galaxies accurately traces the low-mass slope of the SMF at $$z \sim 6 \!-\! 9$$. We find no obvious mass dependence to the mergers and accretion, and show that applying this reconstruction technique to just low-mass galaxies at $z = 0$ and not the MW/M31 hosts correctly recovers the slope of the SMF down to $$M_{\rm star} \sim 10^{4.5}\, \rm {{\rm M}_{\odot }}$$ at $$z \gtrsim 6$$. Thus, we validate the ‘near-far’ approach as an unbiased tool for probing low-mass reionization-era galaxies.
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This content will become publicly available on January 22, 2027
The first GLIMPSE of the faint galaxy population at Cosmic Dawn with JWST : The evolution of the ultraviolet luminosity function across z ∼ 9−15
ABSTRACT Using ultra-deep James Webb Space Telescope (JWST)/NIRCam imaging from the GLIMPSE survey, enhanced by gravitational lensing from the Abell S1063 cluster, we investigate the faintest galaxies ever observed at redshifts $$z\sim 9{\text{-}}15$$. We identify 105 galaxy candidates spanning absolute ultraviolet (UV) magnitudes $$M_{\mathrm{UV}}$$\sim -18$$ to $-13$, about 3 mag fainter, on average, than prior JWST studies. We place strong constraints on the ultra-faint end of the UV luminosity function (UVLF), finding minimal evolution in the faint-end slope, from $$\alpha =-2.01\pm 0.20$$ at $z=9$ to $$\alpha =-2.10\pm 0.19$$ at $z=13$, in contrast to the rapid evolution observed at $$z\sim 0{\text{-}}9$$. Integrating the UVLF down to $$M_{\mathrm{UV}}$$=-16$, we find the cosmic star formation rate density, $$\rho _{\rm SFR}$$, evolves as $$\propto (1+z)^{-2.94^{+0.06}_{-0.10}}$$, over $$z=9{\text{-}}13$$, which is significantly shallower than most theoretical predictions. Extending the integration limit to $$M_{\mathrm{UV}}$$=-13$ reveals galaxies fainter than $$M_{\mathrm{UV}}$$=-16$ contribute over 50 per cent of the total cosmic star formation rate density at $$z\sim 12$$. This excess may indicate enhanced star formation efficiency during the earliest phases of galaxy formation. Alternatively, it could arise from bursty star formation histories; minimal dust attenuation; or an evolving initial mass function. However, existing models incorporating these effects fail to fully reproduce the observed redshift evolution of $$\rho _{\rm SFR}$$. We note that low-redshift contamination and cosmic variance may affect our results, as the limited survey volume may not be representative of the broader galaxy population. Similar observations and spectroscopic confirmation are required to validate these findings.
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- Award ID(s):
- 2408637
- PAR ID:
- 10709574
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- MNRAS
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 546
- Issue:
- 2
- ISSN:
- 0035-8711
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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