Day 681

Wednesday.

1406.4130
Constraining the galaxy's dark halo with RAVE stars
Piffl et al

Use the kinematics of ~200k giant stars that lie within ~1.5kpc of the plane to measure the vertical profile of mass density near the Sun.  Find that the dark mass constrained within the isodensity surface of the dark halo that passes through the Sun (6pm0.9e10Msun) and the surface density within 0.9 kpc of the plane (69pm10Msun/pc^2) are almost independent of the (oblate) halo's axis ratio q.  If the halo is spherical, 45% of the radial force on the Sun is provided by baryons, and only 4.3 % of the Galaxy's mass is baryonic.  If the halo is flattened, the baryons contribute even less strongly to the local radial force and to the Galaxy's mass.  The dark-matter density at the location of the Sun is 0.0126q^-0.89 Msun/pc^3=0.48 q^-0.89 GeV/cm^3.  When combined with other literature results, find hints for a mildly oblate dark halo with q~0.8.  Value for the dark mass within the solar radius is larger than that predicted by cosmological DM-only simulations but in good agreement with simulations once the effects of baryonic infall are taken into account.  The mass models consist of 3 double-exponential discs, an oblate bulge and a NFW DM halo, and model the dynamics of the RAVE stars in the corresponding gravitational fields by finding distribution functions f(J) that depend on three action integrals.  Statistical errors are completely swamped by systematic uncertainties, the most important of which is the distance to the stars in the photometric and spectroscopic samples.  Systematics other than the flattening of the dark halo yield overall uncertainties ~10%.

1406.4135
The bispectrum in the effective field theory of large scale structure
Baldauf, Mercolli, Mirbabayi, Pajer

Study the bispectrum in the EFT of LSS, consistently accounting for the effects of short-scale dynamics.  Begin by proving that, as long as the theory is perturbative, it can be formulated to arbitrary order using only operations that are local in time.  Then derive all the new operators required to cancel the UV-divergences and obtain a physically meaningful prediction for the one-loop bispectrum.  In addition to new, subleading stochastic noises and the viscosity term needed for the one-loop power spectrum, find 3 new effective operators.  The three new parameters can be constrained by comparing with N-body simulations.  The best fit is precisely what is suggested by the structure of UV-divergences, hence justifying a formula for the EFTofLSS bispectrum whose only fitting parameter is already fixed by the PS.  This result predicts the bispectrum of N-body simulations up to k~0.22 h/Mpc at z=0, an improvement by nearly a factor of two as compared to one-loop standard perturbation theory.

1406.4140
Beyond the linear-order relativistic effect in galaxy clustering: second-order gauge-invariant formalism
Yoo, Zaldarriaga

Present the second-order general relativistic description of the observed galaxy number density in a cosmological framework.  The observed galaxy number density is affected by the volume and the source effects, both of which arise due to the mismatch between physical and observationally inferred quantitates such as the redshift, the angular position, the volume, and the luminosity of the observed galaxies.  These effects are computed to the second order in metric perturbations without choosing a gauge condition or adopting any restrictions on vector and tensor perturbations, extending the previous linear-order calculations.  Paying particular attention to the second-order gauge transformation, explicitly isolate unphysical gauge modes and construct second-order gauge-invariant variables.  Moreover, by constructing second-order tetrads in the observer's rest frame, clarify the relate between the physical and parameterized photon wave vectors.  The second-order relativistic description will provide an essential tool for going beyond the PS in the era of precision measurements of galaxy clustering.  Discuss potential applications and extensions of the second-order relativistic description of galaxy clustering.

1406.4143
The one-loop matter bispectrum in the effective field theory of large scale structures
Angulo, Foreman, Schmittfull, Senatore

It is crucial to reliably predict the LSS survey observables.  The EFTofLSS provides a manifestly convergent perturbative scheme to compute the clustering of DM in the weakly NL regime in an expansion in k/k_NL, where k is the wavenumber and k_NL is the wavenumber associated to the NL scale.  It has been recently shown that the EFTofLSS matches to 1% level the DM PS at z=0 up to k~0.3 h/Mpc and k~0.6 h/Mpc at one and two loops respectively, using only one counter term that is fit to data.  Similar realists have been obtained for the momentum power spectrum at one loop.  This is a remarkable improvement with respect to former analytical techniques.  Study the prediction for the equal-time DM bispectrum at one loop.  Find that at this order, it is sufficient to consider the same counter term that was measured in the PS.  Without any remaining free parameter, and in a cosmology for which k_NL is smaller than in the previously considered cases, find that the prediction from the EFTofLSS agrees well with N-body sims up to k~0.3 h/Mpc, given the accuracy of the measurements, which is of order a few% at the highest k's of interest.  While the fit is very good on average up to k~0.3 h/Mpc, the fit performs slightly worse on equilateral configurations, failing approximately at k~0.23 h/Mpc, in agreement with expectations that for a given maximum k, equilateral triangles are the most nonlinear.

1406.4149
The inside-out growth of the most massive galaxies at 0.3<z<0.9
Bai, Yee, ... Gladders, et al

Study the surface brightness profiles of a sample of BCGs with 0.3<z<0.9.  The BCGs are selected from the RCS and X-ray cluster survey.  The surface brightness profiles of the BCGs are measured using HST ACS images, and most of them can be all modeled by a Sersic profile with index ~6 and half-light radius ~30 kpc.  Although the single Sersic model fits the profiles well, argue that the systematics in the sky background measurement and the coupling between the model parameters make the comparison of the model parameters ambiguous.  Direct comparison of the BCG profiles, on the other hand, has revealed an inside-out growth for these most massive galaxies: as the mass of a BCG increases, the central mass density of the galaxy increase slowly (rho_1kpc ~ M*^0.39), while the slope of the outer profile grows continuously shallower (alpha~M*^-2.5).  Such a fashion of growth continues down to the less massive ETGs, without apparent distinction between BCGs and non-BCGs.  For the very massive ETGs and BCGs, the slope of the Kormendy relation starts to trace the slope of the surface brightness profiles and becomes insensitive to subtle profile evolution.  These results are generally consistent with dry mergers being the major driver of the mass growth for BCGs and massive ETGs.  Also find strong correlations between the richness of clusters and the properties of BCGs: the more massive the clusters are, the more massive the BCGs (M*_bcg ~ M^0.6_clusters) and the shallower their surface brightness profiles.  After taking into account the bias in the cluster samples, find the masses of the BCGs have grown by at least a factor of 1.5 from z=0.5 to z=0, in contrast to the previous findings of no evolution.  Such an evolution validates the expectation from the LCDM model.

1406.4159
Halo abundances within the cosmic web
Alonso, Eardley, Peacock

Investigate the dependence of the MF of DM haloes on their environment within the cosmic web of LSS.  A dependence of the halo MF on large-scale mean density is a standard element of cosmo theory, allowing mass-dependent biasing to be understood via the peak-background split.  On the assumption of a Gaussian density field, this analysis can be extended to ask how the MF depends on the geometrical environment: clusters, filaments, sheets and voids, as classified via the tidal tensors (the Hessian matrix of the gravitational potential).  In linear theory, the problem can be solved exactly, and the result is attractively simple: the conditional MF has no explicit dependence on the local tidal field, and is a function only of the local density on the filtering scale used to define the tidal tensor.  There is nevertheless a strong implicit predicted dependence on geometrical environment, because the local density couples statistically to the derivatives of the potential.  Compute the predictions of this model and study the limits of their validity by comparing them to results deduced empirically from N-body simulations.  For sufficiently large filtering sizes, the agreement is good, but there are deviations from the Gaussian prediction at high nonlinearities.  Discuss how to obtain improved predictions in the regime, using the `effective-universe` approach.

1406.4357
Galaxy filaments are pearl necklaces
Tempel, Kpper, Saar, Bussov, Pelt

The characteristic length of galaxy (pearl) placement in a filament (necklace) is 7 Mpc/h, although they are not spaced uniformly.

1406.4369
Why are the magnetic field directions measured by Voyager 1 on both sides of the heliopause so similar?
Grygorczuk, Czechowski, Grzedzielski

The solar wind carves in the interstellar plasma a cavity bounded by a surface, called the heliopause (HP), that separates the plasma and B-field of solar origin from the interstellar ones.  It is now generally accepted that in August 2012 Voyager 1 (V1) crossed that boundary.  Unexpectedly, the B-fields on both its sides, although theoretically independent of each other, were found to be similar in direction.  This delayed the identification of the boundary as the HP and led to many alternative explanations.  Show that the V1 observations can be readily explained and, after the Interstellar Boundary Explorer (IBEX) discovery of the ribbon, could even have been predicted.  The explanation relies on the fact that V1 and the undisturbed interstellar field directions (which is assumed to be given by the IBEX ribbon center) share the same heliolatitude (~34.5 deg) and are not far separated in longitude (~27 degrees).  Result confirmed that V1 has indeed crossed the HP and offers the first independent confirmation that the IBEX ribbon center is in fact the direction of the undisturbed interstellar magnetic field.  For V2 predict that the difference between the inner and outer magnetic field directions at the HP will be significantly larger that the one observed by V1 (~30 degs instead of 20 degs), and that the outer field direction will be close to the ribbon center.

1406.4407
Photometric redshift analysis in the Dark Energy Survey Science Verification data
Sánchez, et al

Present results form a study of photo-z performance of DES, using the early data from a Science Verification (SV) period of observations in late 2012 and early 2013 that provided science-quality images for almost 200 sq deg at the nominal depth of the survey.  Assess the photo-z performance using about 15k galaxies with spectra-z available from other surveys.  These galaxies are used, in different configurations, as a calibration sample, and photo-z's are obtained and studied using most of the existing photo-z codes.  A weighting method in a multi-dimensional color-magnitude space is applied to the spectra sample in order to evaluate the photo-z performance with sets that mimic the full DES photo sample, which is on average significantly deeper that the calibration sample due to the limited depth of spectra surveys.  Empirical photo-z methods using, for instance, ANN or Random Forests, yield the best performance in the tests, achieving core photo-z resolutions sigma_68~0.08.  Moreover, the results from most of the codes, including template fitting methods, comfortably meet the DES requirements on photo-z performance, therefore, providing an excellent precedent for future DES data sets.