The Next Generation Virgo Cluster Survey (NGVS). XL. The Morphological Classification of Virgo Cluster Galaxies

Abstract

We present a study of morphologies, based on deep $u^{*}g^{\prime}i^{\prime}z^{\prime}$ imaging of the Virgo Cluster from the Next Generation Virgo Cluster Survey (NGVS), for 3689 Virgo cluster members spanning a mass range of $\sim$$10^{11}M_{\odot}$ to $\sim$$10^5~M_{\odot}$. Our analysis introduces a new, two-component visual classification scheme developed to capture the morphological diversity of galaxies over more than six orders of magnitude in stellar mass. Our morphological classifications use two parameters to describe the global structure and star formation activity of each galaxy. Structural sub-codes denote features such as spiral arms, bars, disks, shells, streams, while star formation sub-codes indicate the form and location of the current star formation activity (e.g., in cores, clumps, filaments, etc). These visual classifications rely on deep $g^\prime$-band images, supplemented by $u^{*}g^{\prime}i^{\prime}$ color images, as well as unsharp-masked images for a subset of objects. We compare our classifications to previous results for bright member galaxies that used more established schemes, finding good agreement. We also measure quantitative classification statistics (e.g., CASGM$_{20}$) for a subset of the brighter galaxies, and present catalogs for some galaxy types of special interest, including structurally compact galaxies, ultra-diffuse galaxies, candidate ultra-compact dwarf transition objects, as well as candidate post-merger systems. These morphological classifications may be useful as a training set in the application of machine learning tools to the next generation of wide-field imaging surveys.

Figures (17)

• Figure 1: A schematic illustration of the NGVS morphological classification scheme for a selection of high-mass (M$_{*} \ge$ 10$^{9}$M$_{\odot}$) galaxies. The six possible structural codes are arranged roughly from early to late types (E $\rightarrow$ S $\rightarrow$ I), including "transitional" categories (ES, SI, EI). The three rows correspond to the three possible star formation codes: Quiescent (Q), Intermediate (I) and Active (A). Galaxies are labeled by their NGVS identification number --- or VCC number if the galaxy was previously cataloged in the VCC Binggeli1985 --- morphology and stellar mass. Empty entries in this matrix, denoted by an 'X', are either galaxies that do not exist in nature, or plausibly exist but are not found in the NGVS, as explained in the text.
• Figure 2: Same as the previous figure, except for the case of low-mass (M$_{*} \leq$ 10$^{9}$M$_{\odot}$) galaxies. The classification categories in terms of structural and star formation properties are applicable across the full range of stellar mass probed by the NGVS.
• Figure 3: As in Figs. \ref{['fig:NGVS_HighMass_Morphologies']} and \ref{['fig:NGVS_LowMass_Morphologies']}, but highlighting transitional NGVS structural codes ( ES, SI, and EI). Thumbnails are annotated (top to bottom) with the VCC identifier and VCC type from Binggeli1985, the compact NGVS code, and the stellar mass.
• Figure 4: Comparison of our morphological classifications to those from the Virgo Cluster Catalog of Binggeli1985. The pie charts show the breakdown of our NGVS classifications for each of six major morphological categories from the VCC: Es; S0s; dEs/dE,Ns; dS0s/dS0,Ns; spirals (Sa -- Sm); and irregulars (Im).
• Figure 5: Fractional confusion matrix comparing VCC and NGVS structural classifications. Each row corresponds to a VCC morphological class, and the colour shading indicates the fraction of galaxies assigned to each NGVS structural category.