Day 1345

Monday.

1712.00058
The dynamics of stellar disks in live dark-matter halo
Fujii, et al

Recent developments in computer hardware and software enables researchers to simulate the self-gravitating evolution of galaxies at a resolution comparable to the actual number of stars.  Present the results of a series of such simulations.  Perform N-body sims of disk galaxies at with 100 and 500 million particles over a wide range of initial conditions.  The calculations include a live bulge, disk, and DM halo, each of which is represented by self-gravitating particles in the N-body code.  The simulations are performed using the gravitational N-body tree-code Bonsai running on the Piz Daint supercomputer.  Find that the time scale over which the bar forms increase exponentially with decreasing disk-mass fraction.  The effective criterion for bar formation is obtained in the simulations for a disk-to-halo mass-fractions >~0.25.  These results can be explained with the swing-amplification theory.  The conditions for the formation of m=2 spirals is consistent with that for the formation of the bar, which also is an m=2 phenomenon.  Further argue that the 2-armed structures in grand-design spiral galaxies is a transitional phenomenon, and that these galaxies evolve to barred galaxies on a dynamical timescale.  The resulting barred galaxies have rich morphology, which is also present in the Hubble sequence.  Explain the sequence of spiral-galaxies in the Hubble diagram by the bulge-to-disk mass fraction, and the sequence of barred-spiral galaxies is a consequence of secular evolution.


1712.00094
Strong orientation dependence of surface mass density profiles of dark haloes at large scales
Osato, et al

Study the dependence of surface mass density profiles, which can be directly measured by weak gravitational lensing, on the orientation of haloes with respect to the line-of-sight direction, using a suite of N-body simulations.  Find that, when major axes of haloes are aligned with the line-of-sight direction, SMD profiles have higher amplitudes than those averaged over all halo orientations, over all scales from 0.1 to 100 Mpc/h studied.  While the orientation dependence at small scales is ascribed to the halo triaxiality, the results indicate even stronger orientation dependence in the so-called 2-halo regime, up to 100 Mpc/h.  The orientation dependence for the 2-halo term is well approximated by a constant shift of the amplitude and therefore a shift in the halo bias parameter value.  The halo bias from the 2-halo term can be overestimated or underestimated by up to ~30% depending on the viewing angle, which translates into the bias in estimated halo masses by up to a factor of 2 from halo bias measurements.  The orientation dependence at large scales originates from the anisotropic halo-matter correlation function, which has an elliptical shape with the axis ratio of ~0.55 up to 100 Mpc/h.  Discuss potential impacts of halo orientation bias on other observables such as optically selected cluster samples and a clustering analysis of large-scale structure tracers such as quasars.