Bulk flow in the combined 2MTF and 6dFGSv surveys

TL;DR

This paper combines galaxy surveys to measure the local Universe's bulk flow with high accuracy, introducing a new estimator that accounts for measurement errors and biases, confirming consistency with cosmological models.

Contribution

The paper presents a novel maximum likelihood estimator for bulk flow that improves accuracy by handling non-Gaussian errors and biases, validated on large-scale simulations.

Findings

Bulk flow measured as 288±24 km/s at 40 h$^{-1}$ Mpc scale.

Flow direction determined as (l,b)=(296±6°, 21±5°).

Results align with $\\Lambda$CDM predictions.

Abstract

We create a combined sample of 10,904 late and early-type galaxies from the 2MTF and 6dFGSv surveys in order to accurately measure bulk flow in the local Universe. Galaxies and groups of galaxies common between the two surveys are used to verify that the difference in zero-points is $<0.02$ dex. We introduce a maximum likelihood estimator ($\eta$MLE) for bulk flow measurements which allows for more accurate measurement in the presence non-Gaussian measurement errors. To calibrate out residual biases due to the subtle interaction of selection effects, Malmquist bias and anisotropic sky distribution, the estimator is tested on mock catalogues generated from 16 independent large-scale GiggleZ and SURFS simulations. The bulk flow of the local Universe using the combined data set, corresponding to a scale size of 40 h$^{-1}$ Mpc, is $288\pm24$ km s$^{-1}$ in the direction $(l,b)=(296\pm6^{\circ}, 21\pm5^{\circ})$. This is the most accurate bulk flow measurement to date, and the amplitude of the flow is consistent with the $\Lambda$CDM expectation for similar size scales.