科研之友
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摘要
Reionizing the Universe with galaxies appears to require significant star formation in low-mass haloes at early times, while local dwarf galaxy counts tell us that star formation has been minimal in small haloes around us today. Using simple models and the ELVIS simulation suite, we show that reionization scenarios requiring appreciable star formation in haloes with M-vir approximate to 10(8) M-circle dot at z = 8 are in serious tension with galaxy counts in the Local Group. This tension originates from the seemingly inescapable conclusion that 30-60 haloes with M-vir > 10(8) M-circle dot at z = 8 will survive to be distinct bound satellites of the Milky Way at z = 0. Reionization models requiring star formation in such haloes will produce dozens of bound galaxies in the Milky Way's virial volume today (and 100-200 throughout the Local Group), each with greater than or similar to 10(5)M(circle dot) of old stars (greater than or similar to 13 Gyr). This exceeds the stellar mass function of classical Milky Way satellites today, even without allowing for the (significant) post-reionization star formation observed in these galaxies. One possible implication of these findings is that star formation became sharply inefficient in haloes smaller than similar to 10(9) M-circle dot at early times, implying that the high-z luminosity function must break at magnitudes brighter than is often assumed (at M-UV similar or equal to -14). Our results suggest that the James Webb Space Telescope (and possibly even the Hubble Space Telescope with the Frontier Fields) may realistically detect the faintest galaxies that drive reionization. It remains to be seen how these results can be reconciled with the most sophisticated simulations of early galaxy formation at present, which predict substantial star formation in M-vir similar to 10(8) M-circle dot haloes during the epoch of reionization.
- 出版日期2014-9-1
