Revealing strong bias in common measures of galaxy properties using new inclination-independent structures

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 S'{e}rsic 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.