Day 53

Saturday.  Went to Yufuin hot springs with Nozomi-san.  It was great!  I want to go there again.




1107.5046
Are halo and galaxy formation histories correlated?
Tinker, Wetzel, Conroy


* Assembly bias: properties of DM haloes (including mass growth) correlates with large-scale environment, at fixed mass.


Does assembly bias manifest itself in galaxy properties?  Use group-finders on SDSS DR7 to determine centrals and satellites; use 4000A break as a measure of SFH (*how?*), examine correlation between quenched fraction f_q, and large-scale environment, rho.  There is a positive, monotonic relationship between f_q and rho, at all galaxy magnitudes.  Use group catalog to decompose correlation into the contribution from central and stellite galaxies.  Observed correlation primarily due to variations of the halo-mass function as a function of the environment rho (causes larger fraction of satellites galaxies at high rho).  Low mass central galaxies (M < 1e11Msun/h^2) show no correlation between f_q and rho, which is inconsistent with the strong assembly bias of DM halos seen in this mass regime, if galaxy formation is correlated with with halo growth (demonstrated by N-body simulation).  Also, mean stellar age of quenched central galaxies is independent of rho at fixed M, while the formation time of low mass halos vary significantly (but does it correlate with rho?).  Conclude: processes that halt the star formation of low mass central galaxies not correlated to the formation histories of their host halos; old galaxies do not reside preferentially in old halos.


* how is rho defined?


1107.5056
The final parsec problem: aligning a binary with an external accretion disc
Nixon, King, Pringle


Consider a binary system with an external accretion disc with a misaligned angular momentum (e.g., galaxy merger with SMBH centers, formation of stellar mass binaries in star clusters).  Calculate gravitational torque between binary and disc.  Their angular momenta J_b and J_d stably counteralign if their initial orientation is sufficiently retrograde (cos(theta) < -J_d/2J_b) on a short time compared with the mass gain time of the accretors.  J_b remains unchanged in this process.  Counteralignment can promote rapid merger of supermassive black hole binaries; formation of coplanar but retrograde planets around stars in binary systems.


1106.4404
Scale dependent bias from primordial non-Gaussianity with trispectrum
Gong, Yokoyama


Study scale-dependent bias of the halo power spectrum arising from primordial non-Gaussianity.  Present analytic result of the halo bias including up to the trispectrum contributions.  Find scale-dependent bias opens a new possibility of probing the relation between the non-linearity parameters f_nl and t_nl.