2011.01820
Optical to NIR magnitude measurements of the Starlink LEO Darksat satellite and effectiveness of the darkening treatment
Tregloan-Reed, et al
Four observations of Starlink's LEO communication satellites, Darksat and STARLINK-1113 were conducted on two nights with two telescopes. The Chakana 0.6\,m, telescope at the Ckoirama observatory (Chile) observed both satellites on 2020-03-05 (UTC) and 2020-03-07 (UTC) using a Sloan {\it r'} and Sloan {\it i'} filter, respectively. The ESO Visible and Infrared Survey Telescope for Astronomy (VISTA) 4.0\,m telescope with the VISTA InfraRed CAMera (VIRCAM) observed both satellites on 2020/03/05 (UTC) and 2020/03/07 (UTC) in the NIR {\it J}-band and NIR {\it Ks}-band, respectively. The calibration, image processing, and analysis of the Darksat images give r ~5.6\,mag, i ~5.0\,mag, J ~4.2\,mag, and Ks ~4.0\,mag when scaled to a range of 550\,km (airmass $=1$) and corrected for the solar incidence and observer phase angles. In comparison the STARLINK-1113 images give r ~4.9\,mag, i ~4.4\,mag, J ~3.8\,mag, and Ks ~3.6\,mag when corrected for range, solar incidence and observer phase angles. The data and results presented in this work, show that the special darkening coating used by Starlink for Darksat has darkened the Sloan {\it r'} magnitude by 50\,\%, Sloan {\it i'} magnitude by 42\,\%, NIR J magnitude by 32\,\%, NIR Ks magnitude by 28\,\%. The results show that both satellites increase in reflective brightness with increasing wavelength and that the effectiveness of the darkening treatment reduces at longer wavelengths. This shows that the mitigation strategies being developed by Starlink and other LEO satellite operators This work highlights the continued importance of obtaining multi-wavelength observations of many different LEO satellites in order to characterise their reflective properties, to aid the community in developing impact simulations and develop mitigation tools.
2011.01836
Two-point statistics without bins: a continuous-function generalization of the correlation function estimator for large-scale structure
Storey-Fisher, Hogg
The two-point correlation function (2pcf) is the key statistic in structure formation; it measures the clustering of galaxies or other density field tracers. Estimators of the 2pcf, including the standard Landy-Szalay (LS) estimator, evaluate the 2pcf in hard-edged separation bins, which is scientifically inappropriate and results in a poor trade-off between bias and variance. We present a new 2pcf estimator, the Continuous-Function Estimator, which generalizes LS to a continuous representation and obviates binning in separation or any other pair property. Our estimator, inspired by the mathematics of least-squares fitting, replaces binned pair counts with projections onto basis functions; it outputs the best linear combination of basis functions to describe the 2pcf. The choice of basis can take into account the expected form of the 2pcf, as well as its dependence on pair properties other than separation. We show that the Continuous-Function Estimator with a cubic-spline basis better represents the shape of the 2pcf compared to LS. We also estimate directly the baryon acoustic scale, using a small number of physically-motivated basis functions. Critically, this leads to a reduction in the number of mock catalogs required for covariance estimation, which is currently the limiting step in many 2pcf analyses. We discuss further applications of the Continuous-Function Estimator, including determination of the dependence of clustering on galaxy properties and searches for potential inhomogeneities or anisotropies in large-scale structure.
2011.01945
Reconciling galaxy cluster shapes, measured by theorists vs observers
Harvey, et al
If properly calibrated, the shapes of galaxy clusters can be used to investigate many physical processes: from feedback and quenching of star formation, to the nature of dark matter. Theorists frequently measure shapes using moments of inertia of simulated particles'. We instead create mock (optical, X-ray, strong- and weak-lensing) observations of the twenty-two most massive ($\sim10^{14.7}\,M_\odot$) relaxed clusters in the BAHAMAS simulations. We find that observable measures of shape are rounder. Even when moments of inertia are projected into 2D and evaluated at matched radius, they overestimate ellipticity by 56\% (compared to observable strong lensing) and 430\% (compared to observable weak lensing). Therefore, we propose matchable quantities and test them using observations of eight relaxed clusters from the {\emph Hubble Space Telescope} and {\emph Chandra X-Ray Observatory}. We also release our HST data reduction and lensing analysis software to the community. In real clusters, the ellipticity and orientation angle at all radii are strongly correlated. In simulated clusters, the ellipticity of inner ($<r_{\mathrm{vir}}/20$) regions becomes decoupled: for example with greater misalignment of the central cluster galaxy. This may indicate overly efficient implementation of feedback from active galactic nuclei. Future exploitation of cluster shapes as a function of radii will require better understanding of core baryonic processes. Exploitation of shapes on any scale will require calibration on simulations extended all the way to mock observations.
2011.02377
On the halo-mass and radial scale dependence of the lensing is low effect
Lange, et al
The canonical $\Lambda$CDM cosmological model makes precise predictions for the clustering and lensing properties of galaxies. It has been shown that the lensing amplitude of galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS) is lower than expected given their clustering properties. We present new measurements and modelling of galaxies in the BOSS LOWZ sample. We focus on the radial and stellar mass dependence of the lensing amplitude mis-match. We find an amplitude mis-match of around $35\%$ when assuming $\Lambda$CDM with Planck Cosmological Microwave Background (CMB) constraints. This offset is independent of halo mass and radial scale in the range $M_{\rm halo}\sim 10^{13.3} - 10^{13.9} h^{-1} M_\odot$ and $r=0.1 - 60 \, h^{-1} \mathrm{Mpc}$ ($0.05\ h/{\rm Mpc} \lesssim k \lesssim 20 \ h/{\rm Mpc}$). The observation that the offset is both mass and scale independent places important constraints on the degree to which astrophysical processes (baryonic effects, assembly bias) can fully explain the effect. This scale independence also suggests that the "lensing is low" effect on small and large radial scales probably have the same physical origin. Resolutions based on new physics require a nearly uniform suppression, relative to $\Lambda$CDM predictions, of the amplitude of matter fluctuations on these scales. The possible causes of this are tightly constrained by measurements of the CMB and of the low-redshift expansion history.
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