1608.04736
Blending bias impacts the host halo masses derived from a cross-correlation analysis of bright sub-millimetre galaxies
Cowley, Lacey, Baugh, Cole, et al
Placing bright sub-millimetre galaxies (SMGs) within the broader context of galaxy formation and evolution requires accurate measurements of their clustering, which can constrain the masses of their host DM halos. Recent work has shown that the clustering measurements of these galaxies may be affected by a 'blending bias,' which results in the angular correlation function of the sources extracted from single-dish imaging surveys being boosted relative to that of the underlying galaxies. This is due to confusion introduced by the coarse angular resolution of the single-dish telescope and could lead to the inferred halo masses being significantly overestimated. Investigate the extent to which this bias affects the measurement of the correlation function of SMBs when it is derived via a cross-correlation with a more abundant galaxy population. Find that the blending bias is essentially the same as in the auto-correlation case and conclude that the best way to reduce its effects is to calculate the angular correlation function using SMGs in narrow redshift bins. Blending bias causes the inferred host halo masses of the SMBs to be overestimated by a factor of ~6 when a redshift interval of delta z = 3 is used. However, this reduces to a factor of ~2 for delta z = 0.5. The broadening of photo-z probability distributions with increasing redshift can therefore impart a mild halo 'downsizing' effect onto the inferred host halo masses, though this trend is not as strong as seen in recent observational studies.
1608.06494
Evolution and statistics of non-sphericity of dark matter haloes from cosmological N-body simulation
Suto, Kitayama, Nishimichi, Sasaki, Suto
Revisit the non-sphericity of cluster-mass scale halos from cosmo N-body sim on the basis of triaxial modeling. In order to understand the difference between the sim results and the conventional ellipsoidal collapse model (EC), first consider the evolution of individual simulated halos. The major difference between EC and the simulation becomes appreciable after the turn-around epoch. Moreover, it is sensitive to the individual evolution history of each halo. Despite such strong dependence on individual halos, the resulting non sphericity of halos exhibits weak but robust mass dependence in a statistical fashion; massive halos are more spherical up to the turn-around, but gradually become less spherical by z=0. This is clearly inconsistent with the EC prediction; massive halos are usually more spherical. In addition, at z=0, inner regions of he halos are less spherical than outer regions, i.e., the density distribution inside the halos is highly inhomogeneous and therefore not self-similar. Since most of previous fitting formulae for the PDF of axis ratio of triaxial ellipsoids have been constructed under the self-similarity assumption, they are not accurate. Computing the PDF of projected axis ratio a1/a2 directly from the sim data without the self-similarity assumption finds that it is very sensitive to this assumption. The latter needs to be carefully taken into account in direct comparison with observations, and therefore provide and empirical fitting formulas for the PDF of a1/a2. The preliminary analysis suggests that the derived PDF of a1/a2 roughly agrees with the current WL observations. More importantly, the present results will be useful in future exploration of the non-sphericity of clusters in X-ray and optical observations.
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