Day 1648

Monday, Tuesday, Wednesday.

1911.06333

The shape of relaxed galaxy clusters and the public release of a HST shape measurement code, pyRRG

Harvey, et al

We study the shape of eight dynamically relaxed galaxy clusters observed with the Hubble Space Telescope and Chandra X-Ray Observatory. Using strong and weak gravitational lensing, the shape of the Brightest Cluster Galaxy and the X-ray isophote we study the ellipticity of the cluster halo at four different radii. We find that the proxies probing the inner regions of the cluster are strongly coupled with the BCG shape correlated with both the shape predicted by strong gravitational lensing and the X-ray isophote. Conversely we find no such correlation between the shape as predicted by the weak lensing and the other three probes suggesting any coupling between the inner and outer regions is weak. We also present in this paper the public release of the HST weak lensing shape measurement code pyRRG, directly available from PyPi (https://pypi.org/project/pyRRG/). This python3.7 code, based on Rhodes et al. (2000) adopts an automated star-galaxy classifier based on a Random Forest and outputs scientifically useful products such as weak lensing catalogues suitable for the mass mapping algorithm Lenstool.

1911.06456

Is there really a `Hubble tension'?

Rameez, Sarkar

Errors in redshift measurements as small as $\Delta z \sim 0.0001$ can have significant impact on the value of inferred cosmological parameters such as $H_0$ (Davis et al. 2019). We have earlier reported that the redshifts of over 100 Type Ia supernovae (SNe Ia) which are in common between the SDSSII-SNLS3 Joint Lightcurve Analysis (JLA) catalogue (Betoule et al. 2014) and the subsequent Pantheon compilation (Scolnic et al. 2018) are discrepant - some by as much as $\Delta z \sim 0.1$ (Rameez 2019a). We study the impact of this on the inferred value of $H_0$ using calibrations of the supernova absolute magnitude via the 'local distance ladder' (Riess et al. 2016). For supernovae with $\Delta z > 0.025$, the JLA redshifts favour $H_0 \sim 72$~km~s$^{-1}$Mpc$^{-1}$, while the Pantheon redshifts favour $H_0 \sim 68$~km~s$^{-1}$Mpc$^{-1}$. For comparison the value inferred (assuming the flat $\Lambda$CDM model) from the Planck data on CMB anisotropies is $H_0 \sim 67.4 \pm 0.5$~km~s$^{-1}$Mpc$^{-1}$ (Aghanim et al. 2018). Thus the systematic uncertainties that apparently still plague the distance ladder measurement of $H_0$ undermine the significance of the discrepancy claimed by Riess et al. (2019).

Messenger 117-3-13

The [ESO] distributed peer review experiment

Nando

All large, ground- and space-based astronomical facilities serving wide communities face a similar problem: in many cases the number of applications they receive in response to each call exceeds 1000. This poses a serious challenge to running an effective selection process under the classic peer-review paradigm, in which the proposals are assigned to pre-allocated panels with fixed compositions. Although, in principle, one could increase the size of the time allocation committee, this creates logistic and financial problems which place a practical limit on its maximum size, making this solution unviable beyond a certain volume of applications. For this reason, alternative solutions must be sought. One of these is the so-called Distributed Peer Review (DPR) in which, by submitting a proposal, the Principal Investigators (PIs) agree both to act as reviewers and to have their proposal reviewed by their peers. In this article we report the results of a DPR experiment run by ESO in Period 103, in parallel with the regular review by the Observing Programmes Committee (OPC).

1911.07832
Angular correlation function estimators accounting for contamination from probabilistic distance meausrements
Awan, Gawiser

With the advent of surveys containing millions to billions of galaxies, it is imperative to develop analysis techniques that utilize the available statistical power. In galaxy clustering, even small sample contamination arising from distance uncertainties can lead to large artifacts, which the standard estimator does not account for. We first introduce a formalism, termed decontamination, that corrects for sample contamination by utilizing the observed cross-correlations in the contaminated samples; this corrects any correlation function estimator for contamination. Using this formalism, we present a new estimator that uses the standard estimator to measure correlation functions in the contaminated samples but then corrects for contamination. We also introduce a weighted estimator that assigns each galaxy a weight in each redshift bin based on its probability of being in that bin. We demonstrate that these estimators effectively recover the true correlation functions and their covariance matrices. Our estimators can correct for sample contamination caused by misclassification between object types as well as photometric redshifts; they should be particularly helpful for studies of galaxy evolution and baryonic acoustic oscillations, where forward-modeling the clustering signal using the contaminated redshift distribution is undesirable.

1911.07855

The impact of our local environment of cosmological statistics

Hall

We conduct a thorough investigation into the possibility that residing in an overdense region of the Universe may induce bias in measurements of the large-scale structure. We compute the conditional correlation function and angular power spectrum of density and lensing fluctuations while holding the local spherically-averaged density fixed and show that for Gaussian fields this has no effect on the angular power at $l>0$. We identify a range of scales where a perturbative approach allows analytic progress to be made, and we compute leading-order conditional power spectra using an Edgeworth expansion and second-order perturbation theory. We find no evidence for any significant bias to cosmological power spectra from our local density contrast. We show that when smoothed over a large region around the observer, conditioning on the local density typically affects density power spectra by less than a percent at cosmological distances, below cosmic variance. We find that while typical corrections to the lensing angular power spectrum can be at the 10% level on the largest angular scales and for source redshifts $z_s \lesssim 0.1$, for the typical redshifts targeted by upcoming wide imaging surveys the corrections are sub-percent and negligible, in contrast to previous claims in the literature. Using an estimate of the local spherically-averaged density from a composite galaxy redshift catalogue we find that the corrections from conditioning on our own local density are below cosmic variance and subdominant to other non-linear effects. We discuss the potential implications of our results for cosmology and point out that a measurement of the local density contrast may be used as a consistency test of cosmological models.

1911.07869

Maximum cosmological information from type-Ia supernova observations

Yoo

Type-Ia supernova observations yield estimates of the luminosity distance, which includes not only the background luminosity distance, but also the fluctuation due to inhomogeneities in the Universe. In particular, the spatial correlation of the host galaxies is a dominant source of the fluctuation in the luminosity distance measurements. Utilizing the recent theoretical framework that accurately quantifies the information contents accounting for the three-dimensional correlation of the observables on the past-light cone, we compute the maximum cosmological information obtainable from idealized supernova surveys as a function of maximum redshift $z_m$. Here we consider two cosmological parameters $\Omega_m$ and $w_0$ and show that these parameters can be constrained at maximum 1% levels in an idealized survey with $z_m=1$. We discuss how these fundamental limits set by cosmic variance can be overcome.

1911.08275

Corrfunc: blazing fast correlation functions with AVX512F SIMD intrinsics

SInha, Garrison

Correlation functions are widely used in extra-galactic astrophysics to extract insights into how galaxies occupy dark matter halos and in cosmology to place stringent constraints on cosmological parameters. A correlation function fundamentally requires computing pair-wise separations between two sets of points and then computing a histogram of the separations. Corrfunc is an existing open-source, high-performance software package for efficiently computing a multitude of correlation functions. In this paper, we will discuss the SIMD AVX512F kernels within Corrfunc, capable of processing 16 floats or 8 doubles at a time. The latest manually implemented Corrfunc AVX512F kernels show a speedup of up to $\sim 4\times$ relative to compiler-generated code for double-precision calculations. The AVX512F kernels show $\sim 1.6\times$ speedup relative to the AVX kernels and compare favorably to a theoretical maximum of $2\times$. In addition, by pruning pairs with too large of a minimum possible separation, we achieve a $\sim 5-10\%$ speedup across all the SIMD kernels. Such speedups highlight the importance of programming explicitly with SIMD vector intrinsics for complex calculations that can not be efficiently vectorized by compilers. Corrfunc is publicly available at https://github.com/manodeep/Corrfunc/.