MegaMorph: classifying galaxy morphology using multi-wavelength Sérsic profile fits

¹ Institut für Astr- und Teilchenphysik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria
e-mail: vika.marina@gmail.com
² Carnegie Mellon University in Qatar, Education City, PO Box 24866, Doha, Qatar
³ Institute for Astronomy, Astrophysics, Space Applications & Remote Sensing, National Observatory of Athens, P. Penteli 15236 Athens, Greece
⁴ Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced Study, University of Tokyo, 277-8582 Kashiwa, Japan
⁵ School of Physics and Astronomy, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK
⁶ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
⁷ University of Hertfordshire, Hatfield, Hertfordshire, AL10 9AB, UK
⁸ Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15213, USA

Received: 17 October 2014
Accepted: 18 February 2015

Abstract

Aims. This work investigates the potential of using the wavelength-dependence of galaxy structural parameters (Sérsic index, n, and effective radius, R_e) to separate galaxies into distinct types.

Methods. A sample of nearby galaxies with reliable visual morphologies is considered, for which we measure structural parameters by fitting multi-wavelength single-Sérsic models. Additionally, we use a set of artificially redshifted galaxies to test how these classifiers behave when the signal-to-noise ratio decreases.

Results. We show that the wavelength-dependence of n may be employed to separate visually-classified early- and late-type galaxies in a manner similar to the use of colour and n. Furthermore, we find that the wavelength variation of n can recover galaxies that are misclassified by these other morphological proxies. Roughly half of the spiral galaxies that contaminate an early-type sample selected using (u − r) versus n can be correctly identified as late-types by 𝒩, the ratio of n measured in two different bands. Using a set of artificially redshifted images, we show that this technique remains effective up to z ~ 0.1. Therefore, 𝒩 can be used to achieve purer samples of early-types and more complete samples of late-types than using a colour-n cut alone. We also study the suitability of ℛ, the ratio of R_e in two different bands, as a morphological classifier, but find that the average sizes of both early- and late-type galaxies do not change substantially over optical wavelengths.