1809.00180
The distances to Novae as seen by Gaia
Schaefer
The Gaia spacecraft has just released a large set of parallaxes, including 41 novae for which the fractional error is <30%. Use these to evaluate the accuracy and bias of the many prior methods for getting nova-distances. The best of the prior methods is the geometrical parallaxes from HST for just four novae, although the real error bars are 3x larger than stated. The canonical method for prior nova-distnces has been the expansion parallaxes from the novel shells, but this method is found to have real 1-sigma uncertainty of 0.95 mag in the distance modulus, and the prior quoted error bars are on average 3.6x worse than advertised. The many variations on the maximum-magnitude-rate-of-decline (MMRD) relation are all found to be poor, too poor to be usable, and even to be non-applicable for 5-out-of-7 samples of nova, so the MMRD should no longer be used. The prior method of using various measures of the extinction from the interstellar medium have been notoriously bad, but now a new version by Ozdonmez and coworkers has improved this to an unbiased method with 1-sigma uncertainty of 1.14 mag in the distance modulus. For the future, recommend in order (1) using the Gaia parallax, () using the catalog of Ozdonmez, (3) using M_max=-7.0±1.4 mag as an empirical method of poor accuracy, and (4) if none of these methods is available, then to not use the nova for purposes where a distance is needed.
1809.00282
Constraining dark energy with stacked concave lenses
Dong, Zhang, et al
Low density regions are less affected by the nonlinear structure formation and baryonic physics. They are ideal places for probing the nature of dark energy, a possible explanation for the cosmic acceleration. Unlike void lensing, which require identifications of individual voids, study the stacked lensing signals around the low-density-positions (LDP), defined as places that are devoid of foreground bright galaxies in projection. The method allows a direct comparison with numerical results by drawing correspondence between the bright galaxies with haloes. It leads to lensing signals that are significant enough for differentiating several dark energy models. In this work, use the CFHTLenS catalogue to define LDPs, as well as measuring their background lensing signals. Consider several different definitions of the foreground bright galaxies (redshift range & magnitude cut). Regarding the cosmological model, run six simulations: the first set of simulations have the same initial conditions, with w_de=-1,-0.5,-0.8,-1.2; the second set of simulations include a slightly different LCDM model and a w(z) model from 2017NatAs...1..627Z. The lensing results indicate that the models with w_de=-0.5,-0.8 are not favored and the other four models all achieve comparable agreement with the data.
1809.00523
ELUCID. IV: Cosmic variance of galaxy distribution in the local Universe
Chen, Mo, et al
Halo merger trees are constructed from ELUCID, a constrained N-body simulation in SDSS volume. These merger trees are used to populate DM haloes with galaxies according to an empirical model of galaxy formation. Mock catalogs in the SDSS sky coverage are constructed, which can be used to study the spatial distribution of galaxies in the low-z Universe. These mock catalogs are used to quantify the cosmic variance in the galaxy stellar mass function (GSMF) measured from the SDSS survey. The GSMF estimated from the SDSS magnitude-limited sample can be underestimated significantly by the presence of the under-dense region at z<0.03, so that the low-mass end of the function can be underestimated significantly. Several existing methods designed to deal with the effects of the cosmic variance in the estimate of GSMF are tested, and none is found to be able to fully account for the cosmic variance. Propose a method based on the conditional stellar mass functions in DM haloes, which can provide an unbiased estimate of the global GSMF. The application of the method to the SDSS data shows that the GSMF has a significant upturn at M*<1ee9.5 M_sun/h, which has been missed in many earlier measurements of the local GSMF.
1809.01057
The effect of microlensing on the observed X-ray energy spectra of gravitationally lensed quasars
Krawczynski, et al
The Chandra observations of several gravitationally lensed quasars show evidence for flux and spectral variability of the X-ray emission that is uncorrelated between images and is thought to result from the microlensing by stars in the lensing galaxy. Report here on the most detailed modeling of such systems to date, including simulations of the emission of the Fe K-alpha fluorescent radiation from the accretion disk with a GRic ray tracing code, the use of realistic microlensing magnification maps derived from inverse ray shooting calculation, and the simulations of the line detection biases. Use lensing and black hole parameters appropriate for the quadruply lensed quasar RX J1131-1231, and compare the simulated results with the observational results. The simulations cannot fully reproduce the distribution of the detected line energies indicating that some of the assumptions underlying the simulations are not correct, or that the simulations are missing some important physics. Conclude by discussing several possible explanations.
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