Doppler magnification in flux-limited galaxy number counts with finite redshift bin width

TL;DR

This paper analyzes how Doppler magnification affects flux-limited galaxy counts within finite redshift bins, deriving correction formulas for extreme cases and testing their validity through simulations, which show the importance of bin width in calculations.

Contribution

It derives analytical correction formulas for Doppler magnification in finite redshift bins and demonstrates their limitations through simulations, highlighting the need for careful bin width consideration.

Findings

Correction formulas differ for narrow and broad redshift bins.

Simulation shows perturbation varies smoothly with bin width.

Approximate formulas are valid only for small bin widths.

Abstract

In this paper, I investigated the Doppler magnification effect in the flux-limited galaxy number counts with finite redshift bin width. In contradiction with our intuition, the number counts correction formula for different redshift bin are different. For the redshift window as delta function and constant function, the correction formulas have been derived analytically in this paper. These two windows correspond to two extreme cases(i.e. extremely narrow bin and extremely broad bin). An simulation have been implemented to test these formulas. The simulation results indicate the perturbation changes smoothly from one extreme case to another while extending the redshift bin width from half bin width $0.05$ to $0.4$. As a result, Doppler magnification caused number counts perturbation of finite redshift bin width can NOT be compute via redshift integration of the perturbation formula derived with delta function redshift window. It is only a good approximation when the bin width is small. These results are important for galaxy redshift survey kinematic dipole estimation, and meaningful for the relativistic galaxy number counts angular power spectrum estimation, such as CLASSgal and CAMB source.