Morphological Parameters of a Spitzer Survey of Stellar Structure in Galaxies

January 2014 • 2014ApJ...781...12H

Authors • Holwerda, B. W. • Muñoz-Mateos, J. -C. • Comerón, S. • Meidt, S. • Sheth, K. • Laine, S. • Hinz, J. L. • Regan, M. W. • Gil de Paz, A. • Menéndez-Delmestre, K. • Seibert, M. • Kim, T. • Mizusawa, T. • Laurikainen, E. • Salo, H. • Laine, J. • Gadotti, D. A. • Zaritsky, D. • Erroz-Ferrer, S. • Ho, L. C. • Knapen, J. H. • Athanassoula, E. • Bosma, A. • Pirzkal, N.

Abstract • The morphology of galaxies can be quantified to some degree using a set of scale-invariant parameters. Concentration (C), asymmetry (A), smoothness (S), the Gini index (G), the relative contribution of the brightest pixels to the second-order moment of the flux (M ₂₀), ellipticity (E), and the Gini index of the second-order moment (G_M ) have all been applied to morphologically classify galaxies at various wavelengths. Here, we present a catalog of these parameters for the Spitzer Survey of stellar structure in Galaxies, a volume-limited, near-infrared (NIR) imaging survey of nearby galaxies using the 3.6 and 4.5 μm channels of the Infrared Array Camera on board the Spitzer Space Telescope. Our goal is to provide a reference catalog of NIR quantified morphology for high-redshift studies and galaxy evolution models with enough detail to resolve stellar mass morphology. We explore where normal, non-interacting galaxies—those typically found on the Hubble tuning fork—lie in this parameter space and show that there is a tight relation between concentration (C ₈₂) and M ₂₀ for normal galaxies. M ₂₀ can be used to classify galaxies into earlier and later types (i.e., to separate spirals from irregulars). Several criteria using these parameters exist to select systems with a disturbed morphology, i.e., those that appear to be undergoing a tidal interaction. We examine the applicability of these criteria to Spitzer NIR imaging. We find that four relations, based on the parameters A and S, G and M ₂₀, G_M , C, and M ₂₀, respectively, select outliers in morphological parameter space, but each selects different subsets of galaxies. Two criteria (G_M > 0.6, G > -0.115 × M ₂₀ + 0.384) seem most appropriate to identify possible mergers and the merger fraction in NIR surveys. We find no strong relation between lopsidedness and most of these morphological parameters, except for a weak dependence of lopsidedness on concentration and M ₂₀.

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Seppo Laine

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