Improving NASA/IPAC Extragalactic Database Redshift Calculations

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 ∼10−3 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 ∼10−4 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.