1508.02715
Comprehensive assessment of the Too-big-to-fail problem
Jiang, van den Bosch
"Too Big To Fail" problem: refers to the overabundance of massive, dense sub haloes predicted by CDM compared to the observed number of relatively luminous satellite galaxies of the MW or the LG. Use a SAM for the substructure of DM haloes to assess the TBTF problem. The model accurately reproduces the average sub halo mass and velocity functions, as well as their halo-to-halo variance, in N-body sims. Construct thousands of realizations of MW size host haloes, allowing investigation of the TBTF problem with unprecedented statistical power. Examine the dependence on host halo mass and cosmology, and explicitly demonstrate that a reliable assessment of TBTF requires large samples of hundreds of host haloes. Argue that previous statistics used to address TBTF suffer from the look-elsewhere effect and/or disregard certain aspects of the data on the MW satellite population. Devise a new statistic that is not hampered by these shortcomings, and using only data on the 9 known MW satellite galaxies with V_max>15 km/s, demonstrate that 1.4±3% of MW-size host halos have a sub halo population in statistical agreement with that of the MW. However, when using data on the MW satellite galaxies down to V_max=8km/s, this MW consistent fraction plummets to <5e-4 (at 68% CL). Hence, if it turns out that the inventory of MW satellite galaxies is complete down to 8 km/s, then the maximum circular velocities of MW satellites are utterly inconsistent with LCDM predictions, unless baryonic effects can drastically increase the spread in V_max values of satellite galaxies compared to that of their sub haloes.
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