1807.08819
The Andromeda galaxy's most important merger about 2 billion years ago as M32's likely progenitor
D'Souza, Bell
Although the proximity of the Andromeda galaxy (M31) offers an opportunity to understand how mergers affect galaxies, uncertainty remains about M31's most important mergers. Previous studies focused individually on the giant stellar stream or the impact of M32 on M31's disk, thereby suggesting many substantial satellite interactions. Yet models of M31's disk heating and the similarity between the stellar populations of different tidal substructures in M31's outskirts both suggested a single large merger. M31's stellar halo (its outer low-surface-brightness regions) is built up from the tidal debris of satellites and provides information about its important mergers. Here, use cosmo models of galaxy formation to show that M31's massive and metal-rich stellar halo, containing intermediate-age stars, dramatically narrows the range of allowed interactions, requiring a single dominant merger with a large galaxy (with stellar mass about 2.5e10 Msun, the third largest member of the Local Group) about 2 Gyr ago. This single event explains many observations that were previously considered separately: M31's compact and metal-rich satellite M32 is likely to be the stripped core of the disrupted galaxy, its rotating inner stellar halo contains most of the merger debris, and the giant stellar stream is likely to have been thrown out during the merger. This interaction may explain M31's global burst of star formation about 2 Gyr about in which approximately a fifth of its stars were formed. Moreover, M31's disk and bulge were already in place, suggesting that mergers of this magnitude need not dramatically affect galaxy structure.
1807.09195
Constraints on cosmology and baryonic feedback with the deep lens survey using galaxy-galaxy and galaxy-mass power spectra
Soon, Jee, Tyson, Schmidt, Wittman, Choi
Present cosmological parameter measurements from DLS using galaxy-mass and galaxy-galaxy power spectra in the multipole range ell=250~2000. Measure gg power spectra from two lens bins centered at z~0.27 and 0.54 and galaxy-mass power spectra by cross-correlating the positions of galaxies in these two lens bins with galaxy shapes in 2 source bins centered at z~0.64 and 1.1. Marginalize over a baryonic feedback process using a single-parameter representation and a sum of neutrino masses, as well as photometric z and shear calibration systematic uncertainties. For a flat LCDM cosmology, determine S8==sigma_8 sqrt(Omega_m/0.3)=0.818+0.031-0.039, in good agreement with the previous DLS cosmic shear and the Planck CMB measurements. Without the baryonic feedback marginalization, S8 decreases by ~0.05 because the DM-only power spectrum lacks the suppression at the highest ell due to AGN feedback. Together with the Planck CMB measurement, constrain the baryonic feedback parameter to A_baryon=1.07+0.29-0.37, which suggests an interesting possibility that the actual AGN feedback might be stronger that the recipe used in the OWLS simulations. The interpretation is limited by the validity of this one-parameter representation of the baryonic feedback effect.
1807.09540
Beyond the traditional line-of-sight approach of cosmological angular statistics
Schöneberg, et al
Present a new efficient method to compute the angular power spectra of large-scale structure observables that circumvents the numerical integration over Bessel functions, expanding on a recently proposed algorithm based on FFTlog. This new approach has better convergence properties. The method is explicitly implemented in the CLASS code for the case of number count C_ell's (including redshift-space distortions, weak lensing, and all other relativistic corrections) and cosmic shear C_ell's. In both cases the approach speeds up the calculation of the exact C_ell's (without the Limber approximation) by a factor of order 400 at a fixed precision target of 0.1%.
1807.09645
Ultra-high-energy cosmic rays
Anchordoqui
In this report, review the important progress made in recent years towards understanding the experimental data on ultra-high-energy (E>~1e9 GeV) cosmic rays. Begin with a general survey of the available data, including a description of the energy spectrum, the nuclear composition, and the distribution of arrival directions. At this point, also give a synopsis of experimental techniques. After that, introduce the fundamentals of cosmic ray acceleration and energy loss during propagation, with a view of discussing the conjectured nearby sources. Next, survey the state of the art regarding the high- and ultra-high-energy cosmic neutrinos which should be produced in association with the observed cosmic rays. These neutrinos can constitute key messengers identifying currently unknown cosmic accelerators, possibly in the distant universe, because their propagation is not influenced by background photon or magnetic fields. Subsequently, summarize the collider data to ultra-high energies and the main EM processes that govern the longitudinal shower evolution. Armed with these two principal shower ingredients and motivation from the underlying physics, describe the different methods proposed to distinguish the primary particle species. In the end, explore how ultra-high-energy cosmic rays can be used as proves of beyond standard model physics models.
1807.09796
Galaxy two-point correlation function in general relativity
Scaccabarozzi, Yoo, Biern
Perform theoretical and numerical studies of the full relativistic two-point galaxy correlation function, considering the linear-order scalar and tensor perturbation contributions and the wide-angle effects. Using the gauge-invariant relativistic description of galaxy clustering and accounting for the contributions at the observer position, demonstrate that the complete theoretical expression is devoid of any long-mode contributions from scalar or tensor perturbations and it lacks the infrared divergences in agreement with the equivalence principle. Using the full gauge-invariant expression, numerically compute the galaxy 2pt correlation function and study the individual contributions in the conformal Newtonian gauge. Find that several terms at the observer position that are missing in the standard formalism dominate over the other relativistic contributions in the conformal Newtonian gauge. Compared to the standard theoretical predictions, the relativistic effects in galaxy clustering result in a few percent-level systematic errors beyond the scale of the baryonic acoustic oscillation. This theoretical and numerical studies provide a comprehensive understanding the relativistic effects in the galaxy 2pt correlation function.
1807.10163
Dark Energy Survey Year 1 results: measurement of the galaxy angular power spectrum
Camacho, et al
Use data from the Y1 observations of the DES collaboration to measure the galaxies angular power spectrum (APS), and search for its BAO feature using a template-fitting method. Test the methodology in a sample of 1800 DES Y1-like mock catalogs. The APS is measured with the pseudo-C_ell method, using pixelized maps constructed from the mock catalogs and the DES mask. The covariance matrix of the C_ell's in these tests are also obtained from the mock catalogs. Use templates to model the measured spectra and estimate template parameters firstly from the C_ell's of the mocks using two different methods, a maximum likelihood estimator and a MCMC, finding consistent results with a good reduced chi^2. Robustness tests are performed to estimate the impact of different choices of settings used in the analysis. After these tests on mocks, apply the method to a galaxy sample constructed from DES Y1 data specifically for LSS studies. This catalog comprises galaxies within an effective area of 1318 deg^2 and 0.6<z<1.0. Fit the observed spectra with the optimized templates, considering models with and without BAO features. Find that the DES Y1 data favors a model with BAO at the 2.6 sigma CL wit ha best-fit shift parameter of alpha=1.023±0.047. However, the goodness-of-fit is somewhat poor, with chi^2/dof = 1.49. Identify a possible cause of this issue and show that using a theoretical covariance matrix obtained from C_ell's that are better adjusted to data results in an improved value of chi^2/dof = 1.36, which is similar to the value obtained with the real-space analysis. The results correspond to a distance meausrement of D_A(z_eff=0.81)/r_d = 10.65±0.49, consistent with the main DES BAO findings. This is a companion paper to the main DES BAO article showing the details of the harmonic space analysis.
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