A galaxy classification grid that better recognises early-type galaxy morphology

Abstract

A modified galaxy classification scheme for local galaxies is presented. It builds upon the Aitken-Jeans nebula sequence, by expanding the Jeans-Hubble tuning fork diagram, which itself contained key ingredients from Curtis and Reynolds. The two-dimensional grid of galaxy morphological types presented here, with elements from de Vaucouleurs' three-dimensional classification volume, has an increased emphasis on the often overlooked bars and continua of disc sizes in early-type galaxies - features not fully captured by past tuning forks, tridents, or combs. The grid encompasses nuclear discs in elliptical (E) galaxies, intermediate-scale discs in ellicular (ES) galaxies, and large-scale discs in lenticular (S0) galaxies, while the E4-E7 class is made redundant given that these galaxies are lenticular galaxies. Today, these disc structures continue to be neglected, or surprise researchers, perhaps partly due to our indoctrination to galaxy morphology through the tuning fork diagram. To better understand the past and present classification schemes - whose origins reside in solar/planetary formation models - a holistic overview is given. This provides due credit to some of the lesser known pioneers, presents some rationale for the grid, and reveals the incremental nature of, and some of the lesser known connections in, the field of galaxy morphology.

11 pages (incl. 2 figures) plus an extensive reference list

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Cited by in corpus (26)

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• SDSS IV MaNGA: Visual Morphological and Statistical Characterization of the DR15 sample
• Appreciating mergers for understanding the non-linear $M_{\mathrm{bh}}$-$M_{\ast ,\mathrm{spheroid}}$ and $M_{\mathrm{bh}}$-$M_{\ast ,\mathrm{galaxy}}$ relations, updated herein, and the implications for the (reduced) role of AGN feedback
• Defining the (Black Hole)-Spheroid Connection with the Discovery of Morphology-Dependent Substructure in the $M_{\mathrm{BH}}$--${\mathrm{n}}_{\mathrm{sph}}$ and $M_{\mathrm{BH}}$--${\mathrm{R}}_{\mathrm{e},\mathrm{sph}}$ Diagrams: New Tests for Advanced Theories and Realistic Simulations
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