1805.06799
Radio galaxy shape measurement wit Hamiltonian Monte Carlo in the visibility domain
Rivi, Lochner, Balan, Harrison, Abdalla
Radio WL, while a highly promising complementary probe to optical WL, will require incredible precision in the measurement of galaxy shape parameters. In this paper, extend the Bayesian Inference for Radio Observations model fitting approach to measure galaxy shapes directly from visibility data of radio continuum surveys, instead of from image data. Apply a Hamiltonian Monte Carlo (NMC) technique for sampling the posterior, which is more efficient than the standard MCMC method when dealing with a large dimensional parameter space. Adopting the exponential profile for galaxy model fitting allows analytical calculation of the likelihood gradient required by HMC, allowing a faster and more accurate sampling. The method is tested on SKA1-MID simulated observations at 1.4 GHz of a field containing up to 1000 star-forming galaxies. It is also applied to a simulated observation of the WL precursor survey SuperCLASS. In both cases obtain reliable measurements of the galaxies' ellipticity and size for all sources with SNR >=10, and also find relationships between the convergence properties of the HMC technique and some source parameters. Direct shape measurement in the visibility domain achieves high accuracy at the expected source number densities of the current and next SKA precursor continuum surveys. The proposed method can be easily extended for the fitting of other galaxy and scientific parameters, as well as simultaneously marginalizing over systematic and instrumental effects.
1805.06804
Does GW170817 falsify MOND?
Sanders
The gravitational-wave event GW170817 and the near-simultaneous corresponding gamma-ray burst (GRB 170817A) falsify modified gravity theories in which the gravitational geometry differs non-conformally from physical geometry. Thus, the observations of this event definitively rule out theories, such as TeVeS, a suggested relativistic extension of Milgrom's modified Newtonian dynamics (MOND), that predict a significantly different Shapiro delay for EM and gravitational radiation. While not falsifying MOND per se, GW170817 severely constrains relativistic extension of MOND to theories that do not rely on additional matter-coupling fields but rather upon modified field equations for one universal gravitational and physical metric. Here, mention a simple preferred-frame theory as an example.
1805.06938
The impact of assembly bias on the halo occupation in hydrodynamical simulations
Artale, Zahavi, Contreras, Norberg
Investigate the variations in galaxy occupancy of the DM haloes with the LS environment and halo formation time, using two state-of-the-art hydro cosmo sims, EAGLE and Illustris. For both simulations, use 3 galaxy samples with a fixed number density ranked by stellar mass. For these samples, find that low-mass haloes in the most dense environments are more likely to host a central galaxy than those in the least dense environments. When splitting the halo population by formation time, these relations are stronger. Hence, at fixed low halo mass, early-formed haloes are more likely to host a central galaxy than late-formed haloes since they have had more time to assemble. The satellite occupation shows a reverse trend where early-formed haloes host fewer satellites due to having more time to merge with the central galaxy. Also analyse the stellar mass -- halo mass relation for central galaxies in terms of the LS environment and formation time of the haloes. Find that low mass haloes in the most dense environment host relatively more massive central galaxies. This trend is also found when splitting the halo population by age, with early-formed haloes hosting more massive galaxies. The results are in agreement with previous findings form SAMs, providing robust predictions for the occupancy variation signature in the HOD of galaxy formation models.
1805.06976
Cosmological cluster tension
Blanchard, Sakr, Ilic
The abundance of clusters is a classical cosmological probe sensitive to both the geometrical aspects and the growth rate of structures. The abundance of clusters of galaxies measured by Planck has been found to be in tension with the prediction of the LCDM models normalized to Planck CMB fluctuations power spectra. The same tension appears with X-ray cluster local abundance. Massive neutrinos and modified gravity are two possible solutions to fix this tension . Alternatively, others options include a bias in the selection procedure or in the mass calibration of clusters. Present a study, based on the recent work, updating the present situation on this topic and discuss the likelihood of the various options.
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