Improving NASA/IPAC Extragalactic Database Redshift Calculations

TL;DR

This paper identifies a systematic error in NED's redshift calculations caused by a low-redshift approximation, discusses its implications, and describes updates to correct this error for more accurate astrophysical measurements.

Contribution

It reveals a previously unrecognized systematic error in NED's redshift data and details the software updates implemented to correct it for improved accuracy.

Findings

Systematic redshift errors up to 0.001 at z=1 identified

Software updates implemented to correct the systematic error

Potential impact on cosmological measurements and Hubble tension

Abstract

The NASA/IPAC Extragalactic Database (NED) is an impressive tool for finding near-exhaustive information on millions of astrophysical objects. Here, we outline a small systematic error that occurs in NED because a low-redshift approximation is used when making the correction from redshifts in the heliocentric frame to the cosmic microwave background (CMB) rest frame. It means that historically NED systematically misreported the values of CMB-frame redshifts by up to $\sim10^{-3}z$ (about 0.001 at redshift of 1). This is a systematic error, and therefore the impact on applications requiring precise redshifts has the potential to be significant -- for example, a systematic redshift error of $\sim10^{-4}$ at low redshift could resolve the Hubble tension. We have consulted with the NED team and they are updating the software to remove this systematic error so these corrections are accurate at all redshifts. Here, we explain the changes and how they impact the redshift values NED currently reports.