Day 471

Tuesday.

1307.5107
How are the brightest group galaxies statistically special?
Shen, Yang, Mo, van den Bosch, More

Statistical properties of BGGs using SDSS DR7.  Test whether BGGs and other bright members of groups are consistent with an ordered population among the total population of group galaxies.  Find that the luminosity distributions of BGGs do not follow the predictions from the order statistics (OS).  The average luminosities of BGGs are systematically brighter than OS predictions.  On the other hand, by properly taking into account the brightening effect of the BGGs, the luminosity distributions of the second brightest galaxies are in excellent agreement with the expectations of OS.  The brightening of BGGs relative to the OS expectation is consistent with a scenario that the BGGs on average have over-grown about 20% masses relative to the other member galaxies.  The growth (Delta M) is not stochastic but correlated with the magnitude gap (G_1,2) between the brightest and the second brightest galaxy.  The growth (Delta M) is larger for the groups having more prominent BGGs and contributes on average about 30%  of the final G_1,2 of the groups of galaxies.

1307.5176
Constraining primordial black-hole bombs through spectral distortions of the cosmic microwave background
Pani, Loeb

Consider the imprint of superradian instabilities of nonevaporating primordial BHs (PBHs) on the spectrum of CMB.  In the radiation dominated era, PBHs are surrounded by a roughly homogeneous cosmic plasma which endows photons with an effective mass through the plasma frequency.  In this setting, spinning PBHs are unstable to a spontaneous spindown through the well-known "BH bomb" mechanism.  At linear level, the photon density is trapped by the effective photon mass and grows exponentially in time due to superradiance.  As the plasma density declines due to cosmic expansion, the associated energy around PBHs is released and dissipated in the CMB.  Evaluate the resulting spectral distortions of the CMB in 1e3 < z < 2e6.  Derive upper limits on the fraction of DM that can be associated with spinning PBHs in the mass range 1e-8 Msun < M < 0.2 Msun.  For maximally-spnning PBHs, limits are much tighter than those derived from microlensing or other methods.  Future data from the proposed PIXIE mission could improve limits by several orders of magnitude.