The SAMI Galaxy Survey: Revising the Fraction of Slow Rotators in IFS Galaxy Surveys

TL;DR

This paper develops aperture correction methods for stellar kinematic measurements in galaxy surveys, revealing that the fraction of slow rotators increases with stellar mass and correcting for aperture bias is crucial for accurate galaxy dynamics studies.

Contribution

The study introduces a linear aperture correction technique for $V/\sigma$ and $\lambda_{R}$, improving the accuracy of slow rotator fraction estimates in galaxy surveys.

Findings

Aperture corrections for $V/\sigma$ and $\lambda_{R}$ are effective when extrapolating profiles.

The fraction of slow rotators increases with stellar mass, reaching about 36% for galaxies with $\log M_{*}/M_{\odot}>$ 11.

Aperture effects significantly bias kinematic classifications if uncorrected.

Abstract

The fraction of galaxies supported by internal rotation compared to galaxies stabilized by internal pressure provides a strong constraint on galaxy formation models. In integral field spectroscopy surveys, this fraction is biased because survey instruments typically only trace the inner parts of the most massive galaxies. We present aperture corrections for the two most widely used stellar kinematic quantities $V/\sigma$ and $\lambda_{R}$. Our demonstration involves integral field data from the SAMI Galaxy Survey and the ATLAS$^{\rm{3D}}$ Survey. We find a tight relation for both $V/\sigma$ and $\lambda_{R}$ when measured in different apertures that can be used as a linear transformation as a function of radius, i.e., a first-order aperture correction. We find that $V/\sigma$ and $\lambda_{R}$ radial growth curves are well approximated by second order polynomials. By only fitting the inner profile (0.5$R_{\rm{e}}$), we successfully recover the profile out to one $R_{\rm{e}}$ if a constraint between the linear and quadratic parameter in the fit is applied. However, the aperture corrections for $V/\sigma$ and $\lambda_{R}$ derived by extrapolating the profiles perform as well as applying a first-order correction. With our aperture-corrected $\lambda_{R}$ measurements, we find that the fraction of slow rotating galaxies increases with stellar mass. For galaxies with $\log M_{*}/M_{\odot}>$ 11, the fraction of slow rotators is $35.9\pm4.3$ percent, but is underestimated if galaxies without coverage beyond one $R_{\rm{e}}$ are not included in the sample ($24.2\pm5.3$ percent). With measurements out to the largest aperture radius the slow rotator fraction is similar as compared to using aperture corrected values ($38.3\pm4.4$ percent). Thus, aperture effects can significantly bias stellar kinematic IFS studies, but this bias can now be removed with the method outlined here.