1603.02679
The Local Group: the ultimate deep field
Boylan-Kolchin, Weisz, Bullock, Cooper
Near-field cosmology - using detailed observations of the LG and its environs to study wide-ranging questions in galaxy formation an DM physics - has become a mature and rich field over the past decade. There are lingering concerns, however, that the relatively small size of the present-day LG (~2 Mpc diameter) imposes insurmountable sample-variance uncertainties, limiting its broader utility. Consider the evolution of the LG with time and show that it reaches 3'~7 co-moving Mpc in linear size (a volume of ~350 Mpc3) at z=7. The LG is a representative portion of the Universe at early cosmic epochs according to multiple metrics. In a sense, the LG is therefore the ultimate deep field: its stellar fossil record traces the cosmic evolution for galaxies with 1e3<M*(z=0)/Msun<1e9 (reaching m_1500>38 at z~7) over a region that, in terms of size, is comparable to or larger than the HUDF for the entire history of the Universe. It is highly complementary to the HDUF, as it probes much fainter galaxies but does not contain the intrinsically rarer, brighter sources that tare detectable in the HUDF. Archaeological studies in the LG also provide the ability to trace the evolution of individual galaxies across time as opposed to evaluating statistical connections between temporally distinct populations. In the JWST era, resolved stellar populations will probe regions larger than the HUDF and any deep JWST fields, further enhancing the value of near-field cosmology.
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