Tools for Identifying Spurious Luminosity Offsets in Tully-Fisher Studies: Application at Low Redshift and Implications for High Redshift

Abstract

Studies of high-redshift galaxies usually interpret offsets from the Tully-Fisher (TF) relation as luminosity evolution. However, apparent luminosity offsets may actually reflect anomalous velocity widths. Rotation curve anomalies such as strong asymmetries or radial truncation are probably common in high-z samples, as a result of frequent galaxy interactions and in some cases low signal-to-noise ratio data, although low physical resolution may mask these anomalies. In this paper we analyze well-resolved, one-dimensional optical emission-line rotation curves from two low-z samples: the Close Pairs Survey, which contains a high frequency of interacting galaxies, and the Nearby Field Galaxy Survey (NFGS), which represents the general galaxy population. Unlike most low-z TF samples, but in the spirit of many high-z samples, these surveys reflect the natural diversity of emission-line galaxy morphologies, including peculiar, interacting, and early-type galaxies. We adopt objective, quantitative criteria to reject galaxies with severe kinematic anomalies, and we use a statistical velocity width measure that is insensitive to minor kinematic distortions. Severely anomalous galaxies are roughly twice as frequent in the Close Pairs Survey as in the NFGS, and these galaxies' TF offsets collectively resemble the "differential luminosity evolution" claimed in some high-z studies, with larger offsets at lower luminosities. With the anomalous galaxies rejected, however, the TF relations for the Close Pairs Survey and the NFGS are quite similar. Furthermore, the two surveys follow very similar relations between color and TF residuals. The Close Pairs Survey color–TF residual relation extends to bluer colors and brighter TF residuals. Strong outliers from this relation are virtually always kinematically anomalous. As a result, the color–TF residual relation can serve as a powerful tool for separating reliable luminosity offsets from offsets associated with kinematic anomalies. This tool may prove especially useful at high z, where direct detection of kinematic distortions is not always feasible. Although we cannot reliably measure luminosity evolution for galaxies with kinematic anomalies, the TF offsets associated with these anomalies may offer a sensitive probe of evolution in the frequency and intensity of mergers and interactions on different mass scales. We perform a preliminary reanalysis of high-z TF data from the FORS Deep Field and find (1) overall luminosity evolution of ∼0.3 mag; (2) strong slope evolution driven by kinematically anomalous galaxies, which show TF offsets of up to ∼2 mag at low luminosities; and (3) an additional zero-point offset of ∼0.2 mag linked to kinematically anomalous galaxies.