paper

The Missing Massive Satellites of the Milky Way

arXiv:1203.4097 · doi:10.1111/j.1365-2966.2012.21357.x

Abstract

Recent studies suggest that only three of the twelve brightest satellites of the Milky Way (MW) inhabit dark matter halos with maximum circular velocity, V_max, exceeding 30km/s. This is in apparent contradiction with the LCDM simulations of the Aquarius Project, which suggest that MW-sized halos should have at least 8 subhalos with V_max>30km/s. The absence of luminous satellites in such massive subhalos is thus puzzling and may present a challenge to the LCDM paradigm. We note, however, that the number of massive subhalos depends sensitively on the (poorly-known) virial mass of the Milky Way, and that their scarcity makes estimates of their abundance from a small simulation set like Aquarius uncertain. We use the Millennium Simulation series and the invariance of the scaled subhalo velocity function (i.e., the number of subhalos as a function of ν, the ratio of subhalo V_max to host halo virial velocity, V_200) to secure improved estimates of the abundance of rare massive subsystems. In the range 0.1<ν<0.5, N_sub(>ν) is approximately Poisson-distributed about an average given by <N_sub>=10.2x(ν/0.15)^(-3.11). This is slightly lower than in Aquarius halos, but consistent with recent results from the Phoenix Project. The probability that a LCDM halo has 3 or fewer subhalos with V_max above some threshold value, V_th, is then straightforward to compute. It decreases steeply both with decreasing V_th and with increasing halo mass. For V_th=30km/s, ~40% of M_halo=10^12 M_sun halos pass the test; fewer than 5% do so for M_halo>= 2x10^12 M_sun; and the probability effectively vanishes for M_halo>= 3x 10^12 M_sun. Rather than a failure of LCDM, the absence of massive subhalos might simply indicate that the Milky Way is less massive than is commonly thought.

5 figures, 8 pages. Accepted for publication in MNRAS. The number of massive satellites within a bigger radius r100 and in a WMAP7 cosmology model is considered now

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