Geometrical tests of cosmological models. II. Calibration of rotational widths and disc scaling relations

TL;DR

This paper calibrates galaxy scaling relations involving size, brightness, and rotation velocity using nearby galaxy data, to improve geometrical tests of cosmological models at high redshift.

Contribution

It compares rotation indicators and derives precise scaling relations, including size-rotation velocity, for standard rods in cosmological tests.

Findings

Strong correlation between Halpha and [OII] rotation velocities.

Derived size-rotation velocity relation with minimal scatter.

Provided calibration expressions for galaxy properties and rotation speeds.

Abstract

This series of papers is dedicated to a new technique to select galaxies that can act as standard rods and standard candles in order to perform geometrical tests on large samples of high redshift galaxies to constrain different cosmological parameters. The goals of this paper are (1) to compare different rotation indicators in order to understand the relation between rotation velocities extracted from observations of the Halpha line and the [OII]3727 line, and (2) determine the scaling relations between physical size, surface brightness and magnitude of galaxies and their rotation velocity using the SFI++, a large catalog of nearby galaxies observed at I-band. A good correlation is observed between the rotation curve-derived velocities of the Halpha and [OII] observations, as well as between those calculated from velocity histograms, justifying the direct comparison of velocities measured from Halpha rotation curves in nearby galaxies and from [OII] line widths at higher redshifts. To provide calibration for the geometrical tests, we give expressions for the different scaling relations between properties of galaxies (size, surface brightness, magnitude) and their rotation speeds. Apart from the Tully-Fisher relation, we derive the size-rotation velocity and surface brightness-rotation velocity relations with unprecedentedly small scatters. We show how the best size-rotation velocity relation is derived when size is estimated not from disc scale lengths but from the isophotal diameter r23.5, once these have been corrected for inclination and extinction effects.