1807.01316
Constraints on early star formation from the 21-cm global signal
Madau
The tentative detection by the EDGES experiment of a global 21-cm absorption trough centered at z~17 opens up the opportunity to study the birth of the first luminous sources, the intensity of radiation backgrounds at cosmic down, the thermal and ionization history of the young IGM. Focus on the astrophysical implications of the Lyman-alpha photon field needed to couple the spin temperature to the kinetic temperature of the gas at these early epochs. Under the basic assumption that the 21-cm signal is activated by extremely metal-poor stellar systems, show that the EDGES results are consistent with an extrapolation of the declining galaxy UV luminosity density measured at 4<z<9 by deep HST observations. A substantially enhanced SFR density or new exotic sources of UV photons are not required at the z of the EDGES signal . The amount of ionizing radiation produced by the same stellar systems that induce Lyman-alpha coupling is significant, of order 0.5 Lyman-continuum photons per H-atom per 100 Myr. To keep hydrogen largely neutral and delay the reionization process consistently with recent Planck CMB results, mean escape fractions of f_esc<20% are required at z>15.
1807.01318
The gravitational mass of Proxima Centauri measured with SPHERE from a microlensing event
Zurlo, et al
Proxima Centauri, our closest stellar neighbor, is a low-mass M5 dwarf orbiting in a triple system. An Earth-mass planet with an 11 day period has been discovered around this star. The star's mass has been estimated only indirectly using a mass-luminosity relation, meaning that large uncertainties affect the knowledge of its properties. To refine the mass estimate, an independent method has been proposed: gravitational microlensing. By taking advantage of the close passage of Proxima Cen in front of two background stars, it is possible to measure the astrometric shift caused by the microlensing effect due to these close encounters and estimate the gravitational mass of the lens (Proxima Cen). Microlening events occurred in 2014 and 2016 with impact parameters, the closest approach of Proxima Can to the background star, of 1"6±0"1 and 0"5±0"1, respectively. Accurate measurements of the positions of the background stars during the last two years have been obtained with HST/WFC3, and with VLT/SPHERE from the ground. The SPHERE campaign started on March 2015, and continued for more than 2 years, covering 9 epochs. The parameters of Proxima Centauris motion on the sky, along with the pixel scale, true North, and centering of the instrument detector were readjusted for each epoch using the background stars visible in the IRDIS field of view. The experiment has been successful and the astrometric shift caused by the microlensing effect has been measured for the second event in 2016. Used this measurement to derive a mass of 0.150+0.062-0.015 (an error of ~40%) Msun for Proxima Centauri acting as lens. This is the first and the only currently possible measurement of the gravitational mass of Proxima Centauri.\
1806.01689
Testing emergent gravity with mass densities of galaxy clusters
Halenka, Miller
Use a sample of 23 galaxy clusters to test the predictions of an emergent gravity (EG) as alternative to dark matter. The sample has both weak lensing inferred total mass profiles as well as X-ray inferred baryonic gas mass profiles. Using nominal assumptions about the WL and X-ray mass profiles, find that the EG predictions (based on no DM) are acceptable fits only near the viral radius. In the cores and in the outskirts, the mass profile shape differences allow to rule out EG at >5 sigma. However, when accounting for systematic uncertainties in the observed profiles, find good agreement for the EG predictions. For instance, if the WL total mass profiles are shallow in the core and the X-ray gas density profiles are steep in the outskirts, EG can predict the observed DM profile from 0.3<=r<=1 R_200, where R_200 is the radius which encloses 200 times the critical density of the Universe. The required X-ray and lensing shapes are within the current observational systematics-limited errors on cluster profiles. Also show that EG itself allows flexibility in its predictions, which can allow for good agreement between the observations and the predictions. Conclude that the EG as an alternative to DM on the cluster scale cannot be formally ruled out, and that better constraints are required on the WL and gas mass profile shapes in the region 0.3<=r<= 1 R_200.
1806.01711
Einstein's Universe: cosmological structure formation in numerical relativity
Macpherson, Price, Lasky
Perform large-scale cosmo sims that solve Einstein's equations directly via numerical relativity. Starting with initial conditions sampled from the CMB, track the emergence of a cosmic web without the need for a background cosmology. Measure the back reaction of large-scale structure on the evolution of averaged quantities in a matter-dominated universe. Find negligible global back reactions in the simulations, with cosmo parameters Omeag_m=1.005, Omega_R=-1.2e-8, and Omega_Q+Omega_L=-2.9e-9. Sampling smaller scales, above the homogeneity scale of the Universe (100-180/h Mpc), find 2-3% variations in mean spatial curvature and back reaction.
1806.02059
Testing the universality of free fall by tracing a pulsar in a stellar triple system
Archibald, et al
Einstein's theory of gravity, GR, has passed stringent tests in laboratories, elsewhere in the Solar System, and in pulsar binaries. Nevertheless it is known to be incompatible with quantum mechanics and must differ from the true behavior of matter in strong fields and at small spatial scales. A key aspect of GR to test is the strong equivalence principle (SEP), which states that all freely falling objects, regardless of how strong their gravity, experience the same acceleration in the same gravitaional field. Essentially all alternatives to GR violate this principle at some level. Previous direct tests of the SEP are limited by the weak gravity of the bodies in the Earth-Moon-Sun system or by the weak gravitational pull of the Galaxy on pulsar-white dwarf binaries. PSR J0337+1715 is a hierarchical stellar triple system, where the inner binary consists of a millisecond radio pulsar in a 1.6 day orbit with a white dwarf. This inner binary is in a 327-day orbit with another WD. In this system, the pulsar and the inner companion fall toward the outer comparnion with an acceleration about 1e8 times grater than that produced by falling in the Galactic potential, and the pulsar's gravitational binding energy is roughly 10% of its mass. Report that in spite of the pulsar's strong gravity, the accelerations experienced by it and the inner WD differ by a fraction of no more than 2.6e-6 (95% CL). Can roughly compare this to other SEP tests by using the strong-field Nordtvedt parameter \hat\eta_N. The limit on \hat\eta_N is a factor of 10 smaller than that obtained from (weak-field) Solar-System SEP tests and a factor of almost a 1000 smaller than that obtained from other strong-field SEP tests.
No comments:
Post a Comment