Title: Revealing strong bias in common measures of galaxy properties using new inclination-independent structures,
Abstract: Accurate measurement of galaxy structures is a prerequisite for quantitative
investigation of galaxy properties or evolution. Yet, the impact of galaxy
inclination and dust on commonly used metrics of galaxy structure is poorly
quantified. We use infrared data sets to select inclination-independent samples
of disc and flattened elliptical galaxies. These samples show strong variation
in Sérsic index, concentration, and half-light radii with inclination. We
develop novel inclination-independent galaxy structures by collapsing the light
distribution in the near-infrared on to the major axis, yielding
inclination-independent `linear' measures of size and concentration. With these
new metrics we select a sample of Milky Way analogue galaxies with similar
stellar masses, star formation rates, sizes and concentrations. Optical
luminosities, light distributions, and spectral properties are all found to
vary strongly with inclination: When inclining to edge-on, $r$-band
luminosities dim by $>$1 magnitude, sizes decrease by a factor of 2,
`dust-corrected' estimates of star formation rate drop threefold, metallicities
decrease by 0.1 dex, and edge-on galaxies are half as likely to be classified
as star forming. These systematic effects should be accounted for in analyses
of galaxy properties.