1507.07554
Deriving star formation histories from photometry using energy balance spectral energy distribution modelling
Smith, Hayward
Panchromatic SED fitting is a critical tool for determining the physical properties of distant galaxies, such as their stellar mass and SFR. One widely used method is the publicly available MAGPHYS code. Build on the previous analysis (Hayward & Smith 2015) by presenting some modifications which enable MAGPHYS to automatically estimate galaxy SFHs, including uncertainties, based on UV to FIR photometry. Use state-of-the-art synthetic photometry derived by performing 3d dust radiative transfer on hydro sims of isolated disc and merging galaxies to test how well the modified MAGPHYS is able to recover SFHs under idealized conditions, where the true SFH is known. Find that while the SFH of the model with the best fit to the synthetic photometry is a poor representation of the true SFH (showing large variations with the line-of-sight to the galaxy and spurious bursts of SF), median-likelihood SFHs generated by marginalizing over the default MAGPHYS libraries produce robust estimates of the smoothly-varying isolated disk simulation SFHs. This preference for the median-likelihood SFH is quantitatively underlined by the estimates of chi2_SFH (analogous to the chi2 goodness-of-it estimator) and Delta M/M (the integrated absolute mass discrepancy between the model and true SFH) that strongly prefer the median-likelihood SFHs over those that best fit the UV-to-far-IR photometry. In contrast, unable to derive a good estimate of the SFH for the merger simulations (either best-fit or median-likelihood) despite being able to obtain a reasonable fit to the simulated photometry, likely because the analytic SFHs with bursts superposed in the standard MAGPHYS library are insufficiently general/realistic.
1507.07560
Probing the isotropy of cosmic acceleration traced by Type Ia supernovae
Javanmardi, Porciani, Kroupa, Pflamm-Altenburg
Present a method to test the isotropy of the magnitude-redshift relation of SNe Ia and single out the most discrepant direction (in terms of the S/N ratio) with respect to the all-sky data. The technique accounts for possible directional variations of the correction for SNe Ia and yields all-sky maps of the best-fit cosmological parameters with arbitrary angular resolution. To show its potential, apply the method to the recent Union2.1 compilation, building maps with 3 different angular resolution. Use a MC method to estimate the statistical significance with which can reject the null hypothesis that the magnitude-redshift relation is isotropic based on the properties of the observed most discrepant directions. Find that, based on pure S/N arguments, the null hypothesis cannot be rejected at any meaningful CL. However, if one considers that the strongest deviations in the Union 2.1 sample closely align with the dipole temperature anisotropy of the CMB, find that the null hypothesis should be rejected at the 95-99% CL, slightly depending on the angular resolution of the study. If this result is not due to a statistical fluke, it might either indicate that the SN data have not been cleaned from all possible systematics or even point towards new physics. Finally discuss future perspectives in the field for achieving larger and more uniform data sets that will vastly improve the quality of the results and optimally exploit the method.
1507.07642
Extreme value statistics of CMB lensing deflection angles
Merkel, Schaefer
The smaller the angular scales on which the anisotropies of the CMB are probed, the more important their distortion due to GL becomes. Investigate the maxima and minima of the CMB lensing deflection field using general extreme value statistics. Since general extreme value statistics applies to uncorrelated data in first place, consider appropriately low-pass filtered defection maps. Besides the suppression of correlations filtering is required for another reason: The lensing field itself is not directly observable but needs to be (statistically) reconstructed from the lensed CMB by means of a quadratic estimator. This reconstruction, though, is noise dominated and therefore requires smoothing, too. In idealized Gaussian realization as well as in realistically reconstructed data, find that both maxima and minima of the deflection angle components follow consistently a general extreme value distribution of Weibull-type. However, its shape, location and scale parameters vary significantly between different realizations. The statistics' potential power to constrain cosmological models appears therefore rather limited.
1507.07843
Intrinsic alignments of galaxies in the Horizon-AGN cosmological hydrodynamical simulation
Chisari, ... Miller, ... Benabed, et al
The IA of galaxies are recognised as a contaminant to WL measurements. In this work, study the alignment of galaxy shape and spins at z~0.5 in Hirozon-AGN, an adaptive-mesh-refinement hydro cosmo sim box of 100 Mpc/h a side with AGN feedback implementation. Find that spheroidal galaxies in the simulation show a tendency to be aligned radially towards over-densities in the DM density field and other spheroidal. This trend is in agreement with observations, but the amplitude of the signal depends strongly on how shapes are measured and how galaxies are selected in the sim. Disc galaxies show a tendency to be oriented tangentially around spheroidals in 3d. While this signal seems suppressed in projection, this does not guarantee that disc alignments can be safely ignored in future WL surveys. The shape alignments of luminous galaxies in Horizon-AGN are in agreement with observations and other simulation works, but find less alignment for lower luminosity populations. Also characterize the systematics of galaxy shapes in the simulation and how that they can be safely neglected when measuring the correlation of the density field and galaxy ellipticities.
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