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ABSTRACT 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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Accelerated by dark matter: a high-redshift pathway to efficient galaxy-scale star formation
ABSTRACT In the local Universe, star formation is typically inefficient both globally and when considered as the fraction of gas converted into stars per local free-fall time. An important exception to this inefficiency is regions of high gravitational accelerations g, or equivalently surface densities $$\Sigma = g/(\pi \, G)$$, where stellar feedback is insufficient to overcome the self-gravity of dense gas clouds. In this paper, I explore whether dark matter can play an analogous role in providing the requisite accelerations on the scale of entire galaxies in the early cosmos. The key insight is that characteristic accelerations in dark matter haloes scale as $(1+z)^2$ at fixed halo mass. I show this is sufficient to make dark matter the source of intense accelerations that might induce efficient star formation on galactic scales at cosmic dawn in sufficiently massive haloes. The mass characterizing this regime scales as $$(1+z)^{-6}$$ and corresponds to a relatively constant comoving number density of $$n(>\!M_{\rm {vir}}) \approx 10^{-4}\, {\rm Mpc}^{-3}$$ at $$z \gtrsim 8$$. For somewhat rarer haloes, this model predicts stellar masses of $$M_{\star }\sim 10^{9}\, {\rm M}_{\odot }$$ can form in regions that end up with sizes $$\mathcal {O}(100\, {\rm pc})$$ over $$40\, {\rm Myr}$$ time-scales at $$z\approx 12-14$$; these numbers compare well to measurements for some of the brightest galaxies at that epoch from JWST observations. Dark matter and standard cosmological evolution may therefore be crucial for explaining the surprisingly high levels of star formation in the early Universe revealed by JWST.
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- PAR ID:
- 10580972
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 538
- Issue:
- 4
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 3210-3218
- Size(s):
- p. 3210-3218
- Sponsoring Org:
- National Science Foundation
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