Day 649

Wednesday.

1405.1035
Probing the distance and morphology of the Large Magellanic Cloud with RR Lyrae stars
Klein, Cenko, Miller, Norman, Bloom

Present Bayesian analysis of the distances to 15k LMC RR Lyrae stars using V- and I-band light curves from OGLE, in combination with new z-band observations from the DECam.  Median individual RR Lyrae distance statistical error is 1.89 kpc (fractional distance error of 3.76%).  Present 3d contour plots of the number density of LMC RR Lyrae stars and measure a distance to the core LMC RR Lyrae center of 50.25pm0.05(stat)pm0.46(sys) kpc, equivalently mu_LMC=18.506pm0.002(stat)pm0.02(sys).  This finding is statistically consistent with and 4x more precise than the canonical value determined by a recent meta-analysis of 233 separate LMC distance determinations.  Also measure a maximum tilt angle of 11.84degpm0.80deg at a position angle of 62deg, and report highly precise constraints on the V, I and z RR Lyrae period-magnitude relations.  The full dataset of observed mean-flux magnitudes, derived color excess E(V-I) values, and fitted distances for the 15k RR Lyrae stars produced through this work is made available through the publications' associated online data.

1405.1040
Near-field limits on the role of faint galaxies in cosmic reionization
Boylan-Kolchin, Bullock, Garrison-Kimmel

Reionizing the Universe with galaxies appears to require significant SF in low-mass halos at early times, while local dwarf galaxy counts tell us that SF has been minimal in small halos around us today.  Using simple models in the ELVIS simulation suite, show that reionization scenarios requiring appreciable SF in haloes with M_vir~1e8 Msun at z=8 are in serious tension with galaxy counts in the LG.  This tension originates from the seemingly inescapable conclusion that 30-60 haloes with Mvir>1e8Msun at z=8 will survive to be distinct bound satellites of the MW at z=0.  Reionization models requiring SF in such haloes will produce dozens of bound galaxies in the MW's virial volume today (and 100-200 throughout the LG), each with >1e5 Msun of old stars (>13 Gyr).  This exceeds the stellar mass function of classical MW satellites today, even without allowing for the (significant) post-reionization SF observed in these galaxies.  One possible implication of these findings is that SF became sharply inefficient in halos smaller than ~1e9Msun at early times, implying that the high-z luminosity function must break at magnitudes brighter than is often assumed (at M_UV~-14).  Results suggest that JWST (and possibly even HST 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 SF in M_vir~1e8 Msun haloes during the epoch of reionization.