Day 1377

Monday.

1803.00689
Measuring the microlensing parallax from various space observatories
Bachelet, Hinse, Street

A few observational methods allow the measurement of the mass and distance of the lens-star for a micro lensing event.  A first estimate can be obtained by measuring the microlensing parallax effect produced by either the motion of the Earth (annual parallax) or the contemporaneous observation of the lensing event from two (or more) observatories (space or terrestrial parallax) sufficiently separated from each other.  Further developing ideas originally outlined by Gould (2013) and Mogavero & Beaulieu (2016), review the possibility of measuring systematically the microlensing parallax using a telescope based on the Moon surface and other space-based observing platforms including the upcoming WFIRST space-telescope.  First generalize the Fisher matrix formulation and present results demonstrating the advantage for each observing scenario.  Conclude by outlining the limitation of the Fisher matrix analysis when submitted to a practical data modeling process.  By considering a lunar-based parallax observation, find that parameter correlations introduce a significant loss in detection efficiency of the probed lunar parallax effect.


1803.00728
Accurate determination of halo velocity bias in simulations and its cosmological implications
Chen, Zhang, Zheng, Yu, Jing

A long standing issue in peculiar velocity cosmology is whether the halo/galaxy velocity bias b_v=1 at large scale.  The resolution of this important issue must resort to high precision cosmological simulations.  However, this is hampered by another long standing "sampling artifact" problem in volume weighted velocity measurement.  Circumvent this problem with a hybrid approach.  First measure statistics free of sampling artifact, then link them to volume weighted statistics in theory, and finally solve for the velocity bias.  b_v determined by this method is not only free of sampling artifact, but also free of cosmic variance.  Apply this method to a LCDM N-body sim of 3072^3 simulation particles and 1200 Mpc/h box size.  For the first time, determine the halo velocity bias of various mass and redshift to 0.1%-1% accuracy.  Major findings are as follows.  (1) b_v != 1 at k>0.1 h/Mpc.  The deviation from unity increases with k.  Depending on the halo mass and redshift, it may reach O(0.01) at k=0.2 h/Mpc and O(0.05) at k~0.3 h/Mpc.  The discovered b_v != 1 has statistically significant impact on structure growth rate measurement by spectroscopic redshift surveys, including DESI, Euclid and SKA.  (2) Both the sign and the amplitude of b_v-1 depend on mass and redshift.  These results disagree with the peak model prediction on porto-halos in that b_v has much weaker deviation from unity, varies with redshift, and can be bigger than unity.  (3) Most of the mass and redshift dependencies can be compressed into a single dependence on the halo density bias.  Biased on this finding, provide an approximate two-parameter fitting formula.