Friday.
1405.6721
Inferring the gravitational potential of the Milky Way with a few precisely measured stars
Price-Whelan, Hogg, Johnston, Hendel
The dark matter halo of the MW is expected to be triaxial and filled with substructure. It is hoped that streams of shells of stars produced by tidal disruption of stellar systems will provide precise measures of the gravitational potential to test these predictions. Develop a method for inferring the Galactic potential with tidal streams based on the idea that the stream stars were once close in phase space. Method can flexibly adapt to any form of the Galactic potential: it works in phase-space rather than action-space and hence relies neither on the ability to derive actions nor on the integrability of the potential. Model is probabilistic, with a likelihood function and priors on the parameters. The method can properly account for finite observational uncertainties and missing data dimensions. Test method on synthetic datasets generated from N-body simulations of satellite disruption in a static, multi-component MW including a triaxial DM halo with observational uncertainties chosen to mimic current and near-future surveys of various stars. Find that with just four well-measured stream stars, can infer properties of a triaxial potential with precision of order 5-7%. Without proper motions, obtain 15 % constraints on potential parameters and precisions sarong 25% for recovering missing phase-space coordinates. These results are encouraging for the eventual goal of using flexible, time-dependent potential models combined with larger data sets to unravel the detailed shape of the DM distribution around the MW.
1405.7029
The CIB-lensing bispectrum: impact on primordial non-Gaussianity and detectability for the Planck mission
Curto et al
Find that CIB-lensing bispectrum has considerable strength and that it can be detected with high significance with the Planck high-frequency maps. Also present forecasts of the contamination on different shapes of the primordial non-Gaussianity fnl parameter from the bispectrum and by the point sources bispectrum in the Planck high-res CMB anisotropy maps. The local, equilateral and orthogonal shapes and considered for 'raw' single-frequency (i.t., without applying any component separation technique) and foreground-reduced Planck temperature maps. The CIB-lensing correlation seems to mainly affect orthogonal shapes of bispectrum, while point sources mainly equilateral shapes. However, the results indicate that these contaminants do not induce any relevant bias on Planck fnl estimates when FG-reduced maps are considered (using SEVEM for component separation). The component separation technique is, in fact, able to partially clean the extragalactic sources contamination and the bias is reduced for all the shapes. Further developed single- and multiple- frequency estimators based on the Komatsu, Spergel & Wandelt (2005) formalism that can be implemented to efficiently detect this signal.
1405.7047
New limits on gamma-ray emission from galaxy clusters
Griffin, Dai, Kochanek
Galaxy clusters are predicted to produce gamma-rays through cosmic ray interactions and/or DM annihilation, potentially detectable by the Fermi-LAT. Present a new, independent stacking analysis of Fermi-Lat photon count maps using the 79 richest nearby clusters (z<0.12) from the 2MASS cluster catalog. Obtain the lowest limit on the photon flux to date, 1.1e-11 ph/s/cm^2 (95% CL) per cluster in the 0.8-100 GeV band, which corresponds to a luminosity limit of 1.7e44 ph/s. Also constrain the emission limits in a range of narrower energy bands. Scaling to recent CR acceleration and gamma-ray emission models, find that CRs represent a negligible contribution toe the intra-cluster energy density and gas pressure. Furthermore, either accretion shocks must have lower Mach numbers than usually assumed (2-4) or significantly less than 50% of the baryon mass has been processed through such shocks, and thus, the majority of baryons should be assembled through minor mergers or through cold accretion.
1405.7351
Toward and understanding of foreground emission in the BICEP2 region
Flauger, Hill, Spergel
BICEP2 has reported the detection of a degree-scale B-mode polarization patter in the CMB and has interpreted the measurement as evidence for primordial gravitational waves. Examine to what extent a combination of Galactic FGs and lensed E-modes could be responsible for the signal. Reanalyze the BICEP2 results and show that the 100x150 GHz and 150x150 GHz data are consistent with a cosmology with r=0.2 and negligible FGs, but also with a cosmology with r=0 and a significant dust polarization signal. Give independent estimates of the dust polarization signal in the BICEP2 region using four different approaches. While these approaches are consistent with each other, the expected amplitude of the dust polarization power spectrum remains uncertain about a factor of 3. The lower end of the prediction leaves room for a primordial contribution, but at the higher nd the dust in combination with the standard CMB lensing signal could account for the BICPE2 observations, without requiring the existence of primordial gravitational waves. By measuring the cross-correlations between the pre-Planck templates used in the BICEP2 analysis and between different versions of a data-based template, emphasize that cross-correlations between models are very sensitive to noise in the polarization angles and that measured cross-correlations are likely underestimates of the contribution of FGs to the map. These results suggest that BICEP1 and BICEP2 data alone cannot distinguish between FGs and a primordial gravitational wave signal, and that future Keck Array observations at 100 GHz and Planck observations at higher frequencies will be crucial to determine whether the signal is of primordial origin.
1405.6205
Systematic errors in the measurement of neutrino masses due to baryonic feedback processes: prospects for stage IV lensing surveys
Natarajan, Zentner, Battaglia, Trac
Examine the importance of baryonic feedback effects on the matter PS on small scales, and the implications for the precise measurement of neutrino masses through gravitational WL. Planned large galaxy surveys such as LSST and Euclid are expected to measure the sum of neutrino masses to extremely high precision, sufficient to detect non-zero neutrino masses even in the minimal mass normal hierarchy. Show that WL of galaxies while being a very good probe of neutrino masses, is extremely sensitive to baryonic feedback processes. Use publicly available results from the OWLS simulation project to investigate the effects of AGN feedback, the nature of the stellar IMF, and gas cooling rates, on the measured WL shear PS. Using the Fisher matrix formalism and priors from CMB+BAO data, show that when one does not account for feedback, the measured neutrino mass may be substantially larger or smaller than the true mass, depending on the dominant feedback mechanism, with the mass error |\Delta m_nu| often exceeding the mass m_nu itself. Also consider gravitational lensing of the CMB and show that it is not sensitive to baryonic feedback on scales l<2000, although CMB experiments that aim for sensitivities sigma(m_nu)< 0.02 eV will need to include baryonic effects in modeling the CMB lensing potential. A combination of CMB lensing and galaxy lensing can help break the degeneracy between neutrino masses and baryonic feedback processes. Conclude that future large galaxy lensing surveys such as LSST and Euclid can only measure neutrino masses accurately if the matter PS can be measured to similar accuracy.
1405.7367
The formation of CDM haloes II: collapse time and tides
Borzyszkowski, Ludlow, Porciani
Use two cosmological simulations of structure formation in the standard LCDM to study the evolutionary histories of DM halos and characterize the Lagrangian (protohalo) regions from which they form. Focus on analysis on haloes identified at z_id=0 and show that the classic ellipsoidal collapse model developed by Bond&Meyers [are theirs not based on sims?] systematically overestimates their collapse time. If one imposes that halo collapse takes place at z_id, B&M model requires starting from a significantly lower linear density contrast than what is measured in the simulations at the Lagrangian locations of halo formation. Attempting to explain this discrepancy, test two key assumptions of the model. First, show that the tides felt by collapsing haloes due to the surrounding LSS evolve non-linearly. Although this effect becomes increasingly important for low-mass haloes that reside in high-density regions, accounting for it in the ellipsoidal collapse model only marginally improves the agreement with N-body simulations. Second, track the time evolution of the physical volume occupied by forming haloes and show that, after turnaround, it generally stabilizes at a well defined time, z_c>z_id. Contrary to the basic assumption of extended PS theory based on the excursion-set approach, these results suggest that most haloes have already assembled their final mass into a coherent structure prior to z_id, and that the 'collapse redshift', z_c, increases with decreasing halo mass. Discuss the implications of this result for understanding the origin of the mass-dependence and scatter in the linear threshold for halo formation. Finally, show that, when tuned for collapse at z_c, a modified version of the ellipsoidal collapse model that also accounts for the triaxial nature of protohaloes predicts their linear density contrast in an unbiased way.
1405.7369
Carbon-enhanced metal-poor stars: relics from the dark ages
Cooke, Madau
Use detailed nucleosynthesis calculations and a realistic prescription for the environment of the first stars to explore the first episodes of chemical enrichment that occurred during the dark ages. Based on these calculations, propose a novel explanation for the increased prevalence of carbon-enhanced metal-poor (CEMP) stars with decreasing Fe abundance: the observed chemistry for the most metal-poor Galactic halo stars is the result of an intimate link between the explosions of the first stars and their host mini halos' ability to retain its gas. Specifically, high energy SNe produce a near solar ratio of C/Fe, but are effective in evacuating the gas form their host mini halo, thereby suppressing the formation of a second generation of stars. On the other hand, mini haloes that host low energy SNe are able to retain their gas and form a send stellar generation but, as a result, the second stars are borne with a super solar ratio of C/Fe. The models are able to accurately reproduce the observed distributions of [C/Fe] and [Fe/H], as well as the fraction of CEMP stars relative to non-CEMP stars as a function of [Fe/H] without any free parameters. Propose that the present lack of chemical evidence for very massive stars (>140 Msun), that ended their life as a highly energetic pair-instability SN, does not imply that such stars were rare or did not exist; the chemical products of these very massive first stars may have escaped from their host mini halo, and were never incorporated into subsequent generations of stars. Finally, the models suggest that the most Fe-poor stars currently known may have seen the enrichment from a small multiple of metal-free stars, and need not have been exclusively enriched by a solitary first star. These calculations support the idea that some of the surviving dwarf satellite galaxies of the MW are relics of the first galaxies.
1405.7382
The Sloan digital sky survey stripe 82 imaging data: depth-optimized co-adds over 300 deg^2 in five filters
Jiang, Fan, ... Strauss, et al
Release SDSS Stripe82 coadd images, from 70-90 repeated scans, in each of the ugriz. Optimized for depth. Input single-epoch frames processed, and weighted based on seeing, sky transparency, and BG noise before co-addition. The resultant products are co-added science images and their associated weight images that record relative weights at individual pixels. The depths of the co-adds, measured as the 5 sigma detection limits of the aperture (3.2 arc sec diameter) magnitudes for point sources, are roughly 23.9, 25.1, 24.6, 24.1, and 22.8 AB magnitudes in the five bands, respectively. They are 1.9-2.2 mag deeper than the best SDSS single-epoch data. The co-added images have good image quality, with an average PSF FWHM of ~1 arc sec in the r, i, and z bands. Also release object catalogs that were made with SExtractor. These co-added products have many potential uses for studies of galaxies, quasars, and Galactic structure. Further present and release NIR J-band images that cover ~90 deg^2 of Stripe 82. These images were obtained using the NEWFIRM camera on the NOAO 4m Mayall telescope, and have a depth of about 20-20.5 Vega magnitudes (also 5 sigma detection limits for point sources).
1405.7385
Can the 21 cm signal probe Population III and II star formation?
Yajima, Khochfar
Using varying models of the SFR of Pop III and III stars at z>6, derive the expected redshift history of the global 21 cm signal from the IGM. To recover the observed Thomson scattering optical depth of the CMB requires SFRs of 1e-3 Msun/yr/Mpc^3 at z~15 from Pop III stars, or 1e-1 Msun/yr/Mpc^3 at z~7 from Pop II stars which is higher than that indicated by recent observations of galaxies. In the case the SFR is dominated by Pop III stars, the IGM quickly heats above the CMB at z>~12 due to heating from SNe. In addition, Lya photons from haloes hosting Pop III stars couple the spin temperature to that of the gas, resulting in a deep absorption signal in the early universe. If the SFR is dominated by Pop II stars, the IGM slowly heats and exceeds the CMB temperature at z~10. At z<~10, all the tested models are degenerate and show positive 21 cm signals and similar mean differential brightness temperatures due to the dominance of Pop II stars. However, the larger and varying fraction of Pop III stars at z>~10 with next generation radio telescopes such as the SKA will be able to investigate the contribution from Pop III and Pop II stars to the global SFR.
1405.7423
Accounting for baryonic effects in cosmic shear tomography: determining a minimal set of nuisance parameters using PCA
Eifler, Krause, Dodelson, Zentner, Hearin, Gnedin
Systematic uncertainties that have been subdominant in past LSS surveys are likely to exceed statistical uncertainties of current and future LSS data sets, potentially limiting the extraction of cosmological information. Present a general framework (PCA marginalization) to consistently incorporate systematic effects into a likelihood analysis. This technique naturally accounts for degeneracies between nuisance parameters and can substantially reduce the dimension of the parameter space that needs to be sampled. As a practical application, apply PCA marginalization to account for baryonic physics as an uncertainty in cosmic shear tomography. Specifically, use CosmoLike to run simulated likelihood analyses on 3 independent sets of numerical simulations, each covering a wide range of baryonic scenarios differing in cooling, SF, and feedback mechanisms. Simulate a Stage III (DES) and Stage IV (LSST/Euclid) survey and find a substantial bias in cosmological constraints if baryonic physics is not accounted for. Then show that PCA marginalization (employing at most 3 to 4 nuisance parameters) removes this bias. The study demonstrates that it is possible to obtain robust, precise constraints on the DE EoS even in the presence of large levels of systematic uncertainty in astrophysical processes. Conclude that the PCA marginalization technique is a powerful, general tool for addressing many of the challenges facing the precision cosmology program.
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