Extracting local velocity from cosmic dipole using simulations

TL;DR

This paper presents a new simulation-based method to extract cosmic velocity from galaxy dipole measurements, accounting for individual source spectra, and applies it to quasar data, showing results slightly different from traditional methods.

Contribution

Introduces a simulation approach that uses individual source spectral indices to improve velocity extraction from cosmic dipole data, avoiding reliance on cumulative count fits.

Findings

Results differ from standard methods by about one sigma.

Method applied to CatWISE2020 quasar data.

Demonstrates the importance of source-specific spectral information.

Abstract

Our velocity with respect to the cosmic frame of rest leads to a dipole in the number count distribution of galaxies. The dipole depends on the source spectrum, which is usually assumed to be a power law, $S(\nu) \propto \nu^{-\alpha}$ and on the flux dependence of the number density of sources. The latter is also generally assumed to be a power law, parametrised with exponent $x$. The velocity can be extracted from the observed dipole once the two parameters $x$ and $\alpha$ are known. The standard procedure uses the mean value of $\alpha$ across the entire sample, and the parameter $x$ is inferred by fitting the cumulative number count, $\frac{dN}{d\Omega}(>S_*) \propto S_*^{-x}$, near the flux limit $S_*$ of the survey. Here, we introduce a simulation procedure to extract the velocity which directly uses the $\alpha$ values of each source rather than their mean and does not rely on the functional form of the cumulative number count near the flux limit. We apply this to the quasar sample in CatWISE2020 data and find that the final results differ from the standard procedure by approximately one sigma.