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

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.

Galaxy And Mass Assembly (GAMA): the wavelength dependence of galaxy structure versus redshift and luminosity

“…In both volume-limited samples we see that the median location of the high-n galaxies lies very close to the median point of the elliptical sample, whilst the median of the low-n galaxies lies close to that of the Sab-Scd galaxies. Vika et al (2015) see identical behaviour for a sample of very low-redshift galaxies, and show that this is robust against artificial redshifting, and investigate the use of N and R as morphological classifiers.…”

Galaxy And Mass Assembly (GAMA): understanding the wavelength dependence of galaxy structure with bulge-disc decompositions

“…We see that at N 0.9 ( 2) we expect 50% (80%) of our population to be disc-dominated. We can therefore use N to determine how likely it is that a given galaxy has a prominent disc, although selecting galaxies in combination with the Sérsic index in a single band would be most effective (see Vika et al 2015).…”

“…There is, however, a large change in Re with wavelength for high-n galaxies, which shows that elliptical galaxies contain a radial progression of different stellar populations, possibly resulting from multiple minor merging events throughout the galaxy's lifetime. Vika et al (2015) found that by combining N with the colour information of the galaxy we can separate elliptical galaxies from S0s more reliably than other photometric classification methods.…”

“…Kormendy & Bender 1996, and references therein), or disc-like structures (Emsellem et al 2007; Krajnović et al 2011; Emsellem et al 2011;Cappellari et al 2011), and can exhibit a large range of disc-to-total (D/T) flux ratios (Krajnovic et al 2013), with D/T ∼ 0.4 typical. This calls into question the tradition of classifying galaxies by their Sérsic index or morphology (Vika et al 2015).…”

“…V14 and Vika et al (2015) suggest that inferences about a galaxy's bulge-disc nature can be made from single-Sérsic fits. In this paper we use a large sample of low-redshift galaxies to study whether there is a connection between bulgedisc properties and single-Sérsic results.…”