<scp>gofher</scp> – which side is closer? Automatically disambiguating the tilt of disc galaxies by measuring differential reddening

ABSTRACT As viewed from the Earth, most spiral galaxies are tilted – i.e. somewhere between edge-on and face-on – giving them an elliptical appearance. Although we can estimate the tilt by ‘de-projecting’ the ellipse back to a circle, the sign of the tilt remains ambiguous: we cannot easily determine which side of the tilted disc is closer to the Earth. In this paper, we introduce an automated method for determining which side of the disc is closer by measuring differential reddening. In particular, the side of the bulge that is behind the nearer side of the disc will suffer more dust-induced reddening than the side of the bulge that is not disc-obscured. While close manual inspection of the bulge by eye can sometimes reveal which side of the galaxy is more red, automating this process is fraught with pitfalls and potential selection effects, such as exactly how and where to split the galaxy in half. We present our method and test it against the recent ‘spin parity’ paper, which carefully determined the near side of over 500 galaxies. Our method achieves 95 per cent agreement with the confident human classifications, and 73 per cent agreement in the less confident cases.