Measuring Galaxy Velocity Dispersions with Hectospec

TL;DR

This paper presents a robust method using ULySS for measuring galaxy velocity dispersions with Hectospec, demonstrating accuracy comparable to SDSS/BOSS and confirming no significant redshift systematics up to z~0.6.

Contribution

The paper introduces a new technique for measuring galaxy velocity dispersions with Hectospec, validated against SDSS/BOSS data, and assesses measurement errors and systematic effects.

Findings

Systematic differences with SDSS/BOSS are <7% for dispersions 100-300 km/s.

Achieves 30 km/s velocity dispersion errors in one hour for r=21 galaxies.

No significant redshift-dependent systematics up to z~0.6.

Abstract

We describe a robust technique based on the ULySS IDL code for measuring velocity dispersions of galaxies observed with the MMT's fiber-fed spectrograph, Hectospec. This procedure is applicable to all Hectospec spectra having a signal-to-noise >5 and weak emission lines. We estimate the internal error in the Hectospec velocity dispersion measurements by comparing duplicate measurements of 171 galaxies. For a sample of 984 galaxies with a median z=0.10, we compare velocity dispersions measured by Hectospec through a 1.5 arcsec diameter optical fiber with those measured by the Sloan Digital Sky Survey (SDSS) and Baryon Oscillation Spectral Survey (BOSS) through 3 arcsec and 2 arcsec diameter optical fibers, respectively. The systematic differences between the Hectospec and the SDSS/BOSS measurements are <7% for velocity dispersions between 100 and 300 km/s, the differences are no larger than the differences among the three BOSS velocity dispersion reductions. We analyze the scatter about the fundamental plane and find no significant redshift dependent systematics in our velocity dispersion measurements to z~0.6. This analysis also confirms our estimation of the measurement errors. In one hour in good conditions, we demonstrate that we achieve 30 km/s velocity dispersion errors for galaxies with an SDSS r fiber magnitude of 21.