1610.04226
Halo ellipticity of GAMA galaxy groups form KiDS weak lensing
van Uitert, et al
Constrain the average halo ellipticity of ~2600 galaxy groups from GAMA, using the WL signal measured from KiDS. To do os, quantify the azimuthal dependence of the stacked lensing signal around 7 different proxies for the orientation of the DM distribution, as it is a priori unknown which one traces the orientation best. On small scales, the major axis of the brightest group/cluster member (BCG) provides the best proxy, leading to a clear detection of an anisotropic signal. In order to relate that to a halo ellipticity, a model density profile must be adopted. Derive new expressions for the quadrupole moments of the shear field given an elliptical model surface mass density profile. Derive new expressions for the quadruple moments of the shear field given an elliptical model surface mass density profile. Modeling the signal with an NFW profile on scales <250 kpc, which roughly corresponds to half the virial radius, and assuming that the BCG is perfectly aligned with the DM, find an average halo ellipticity of e_h=0.38±0.12. This agrees well with results from cold-DM-only sims, which typically report values of e_h~0.3. On large scales, the lensing signal around the BCG does not trace the DM distribution well, and the distribution of group satellites provides a better proxy for the halo-s orientation instead, leading to a 3-4 sigma detection of a non-zero halo ellipticity at scales between 250 and 750 kpc. The results suggest that the distribution of stars enclosed within a certain radius forms a good proxy for the orientation of the DM within that radius, which has also been observed in hydrodynamical simulations.
1610.04231
ZOMG I: how the cosmic web inhibits halo growth and generates assembly bias
Borzyszkowski, Porciani, Romano-Diaz, Garaldi
The clustering of DM haloes with fixed mass depends on their formation history, an effect known as assembly bias. Investigate the origins of this phenomenon using zoom N-body sims. Follow the formation of 7 galaxy-sized halos selected using a definition of collapse time that generates strong assembly bias. Haloes at z=0 are classified according to the time in which the physical volume contains their final mass becomes stable. For 'stalled' haloes this happens at z~1.5 while for 'accreting' haloes this has not happened yet. The zoom simulations confirm that stalled haloes do not grow in mass while accreting haloes show a net inflow. The reason is that accreting haloes are located at the nodes of a network of thin filaments which feed them. Conversely, each stalled halo lies within a prominent filament that is thicker than the halo size. Infalling material from the surroundings becomes part of the filament while matter within it recess from the halo. Conclude that assembly bias originates from quenching halo growth due to tidal forces following the formation of non-linear structures in the cosmic web. Also the internal dynamics of the haloes change: the velocity anisotropy profile is biased towards radial (tangential) orbits in accreting (stalled) haloes Finally, extend the excursion-set theory to account for these effects. A simple criterion biased on the ellipticity of the linear tidal field combined with the spherical collapse model provides excellent predictions for both classes of haloes.
1610.04232
The the Jeans don't fit: how stellar feedback drives stellar kinematics and complicates dynamical modeling in low-mass galaxies
El-Badry, Wetzel, Geha, Quataert, Hopkins, et al
In low-mass galaxies, stellar feedback can drive gas outflows that generate non-equilibrium fluctuations in the gravitational potential. Using cosmo zoom-in baryonic sims from the Feedback in Realistic Environments (FIRE) project, investigate how these fluctuations affect stellar kinematics and the reliability of Jeans dynamical modeling in low-mass galaxies. Find that stellar velocity dispersion and anisotropy profiles fluctuate significantly over the course of galaxies starburst cycles. Predict an observable correlation between SFR and stellar kinematics: dwarf galaxies with higher recent SFRs should have systematically higher stellar velocity dispersions. This predictions provides an observational test of the role of stellar feedback in regulating both stellar and DM densities in dwarf galaxies. Find that Jeans modeling, which treats galaxies as visualized systems in dynamical equilibrium, overestimates a galaxy's dynamical mass during periods of post-starburst gas outflow and underestimates it during periods of net inflow. Short-timescale potential fluctuations lead to typical errors of ~20% in dynamical mass estimates, even if full 3d stellar kinematics -- including the orbital anisotropy -- are known exactly. When orbital anisotropy is not known a priori, typical mass errors arising from non-equilibrium fluctuations in the potential are larger than those arising form the mass-anisotropy degeneracy. However, Jeans modeling along cannot reliably constrain the orbital anisotropy, and problematically, it often favors anisotropy models that do not reflect the true profile. If galaxies completely lose their gas and cease forming stars, fluctuations in the potential subside, and Jeans modeling becomes much more reliable.
1610.04606
KiDS-450: testing extensions to the standard cosmological model
Joudaki, et al
Test extensions to the standard cosmo model with WL tomography using 450 deg2 of imaging data form KiDS. In theses extended cosmologies, which include massive neutrinos, nonzero curvature, evolving DE, modified gravity and running of the scalar spectral index, also examine the discordance between KiDS and CMB from Planck. The discordance between the two datasets is largely unaffected by a a more conservative treatment of the lensing systematics and the removal of angular scales most sensitive to nonlinear physics. The only extended cosmology that simultaneously alleviates the discordance with Planck and is at least moderately favored by the data includes evolving DE with a time-dependent equation of state (in the form of the w0-wa parameterization). In this model, the respective S8=sigma8 sqrt(Omega_m/0.3) constraints agree at the 1 sigma level, and there is 'substantial concordance' between the KiDS and Planck datasets when accounting for the full parameter space. Moreover, the Planck constraint on the Hubble constant is wider than in LCDM and in agreement with the Reiss+2016 direct measurement of H0. The DE model is moderately favored as compared to LCDM when combining the KiDS and Planck measurements, and remains moderately favored after including an informative prior on the Hubble constant. In both of these scenarios, the DE parameters are discrepant with a cosmological constant at the 3 sigma level. Moreover, KiDS constraints the sum of neutrino masses to 4.0 eV (95% CL), finds no preference for time or scale dependent modifications to the metric potentials, and is consistent with flatness and no running of the spectral index. The analysis code is publicly available at github/sjoudaki/kids450.
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