A REVISED PARALLEL-SEQUENCE MORPHOLOGICAL CLASSIFICATION OF GALAXIES: STRUCTURE AND FORMATION OF S0 AND SPHEROIDAL GALAXIES

By J. Kormendy, R. Bender. 2011

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We update van den Bergh's parallel-sequence galaxy classification in which S0 galaxies form a sequence S0a–S0b–S0c that parallels the sequence Sa–Sb–Sc of spiral galaxies. The ratio B/T of bulge-to-total light defines the position of a galaxy in this tuning-fork diagram. Our classification makes one major improvement. We extend the S0a–S0b–S0c sequence to spheroidal (“Sph”) galaxies that are positioned in parallel to irregular galaxies in a similarly extended Sa–Sb–Sc–Im sequence. This provides a natural “home” for spheroidals, which previously were omitted from galaxy classification schemes or inappropriately combined with ellipticals. To motivate our juxtaposition of Sph and Im galaxies, we present photometry and bulge–disk decompositions of four rare, late-type S0s that bridge the gap between the more common S0b and Sph galaxies. NGC 4762 is an edge-on SB0bc galaxy with a very small classical-bulge-to-total ratio of B/T = 0.13 ± 0.02. NGC 4452 is an edge-on SB0 galaxy with an even tinier pseudobulge-to-total ratio of PB/T = 0.017 ± 0.004. It is therefore an SB0c. VCC 2048, whose published classification is S0, contains an edge-on disk, but its “bulge” plots in the structural parameter sequence of spheroidals. It is therefore a disky Sph. And NGC 4638 is similarly a “missing link” between S0s and Sphs—it has a tiny bulge and an edge-on disk embedded in an Sph halo. In the Appendix, we present photometry and bulge–disk decompositions of all Hubble Space Telescope Advanced Camera for Surveys Virgo Cluster Survey S0s that do not have published decompositions. We use these data to update the structural parameter correlations of Sph, S+Im, and E galaxies. We show that Sph galaxies of increasing luminosity form a continuous sequence with the disks (but not bulges) of S0c–S0b–S0a galaxies. Remarkably, the Sph–S0–disk sequence is almost identical to that of Im galaxies and spiral galaxy disks. We review published observations for galaxy transformation processes, particularly ram-pressure stripping of cold gas. We suggest that Sph galaxies are transformed, “red and dead” Scd–Im galaxies in the same way that many S0 galaxies are transformed, red and dead Sa–Sc spiral galaxies.

Subfields: Astronomy and Astrophysics, Ecology

View Paper: https://www.semanticscholar.org/paper/b87736688f18928059350c311ad24bca012efd42

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Citations in the corpus

Physical Models of Galaxy Formation in a Cosmological Framework (2014, background): “Similarly, the dwarf galaxies that are confusingly termed “dwarf spheroidals” and “dwarf ellipticals” obey very different Fundamental Plane relations than do classical bulges and ellipticals of all luminosities (Kormendy & Bender 2012, Kormendy et al. 2009).”
The current status of galaxy formation (2012, background): “This sequence seems to acts in parallel to the pseudobulges or bulges of S0 galaxies being generated via stripped/harassed or simply starved disk galaxies (Kormendy & Bender, 2012).”
A CLASSICAL MORPHOLOGICAL ANALYSIS OF GALAXIES IN THE SPITZER SURVEY OF STELLAR STRUCTURE IN GALAXIES (S4G) (2015, methodology): “These galaxies have been found by Kormendy & Bender (2012) to have a natural “home’ in a parallel sequence among extreme late-type galaxies.”
The ATLAS3D project – XX. Mass–size and mass–σ distributions of early-type galaxies: bulge fraction drives kinematics, mass-to-light ratio, molecular gas fraction and stellar initial mass function (2012, background): “…nras.oxfordjournals.org/ D ow nloaded from The ATLAS3D project – XX. Mass–size 1887 trends we observed in galaxy scaling relations (Paper I and Cappellari 2011a), an independent confirmation of this picture, including its extension to low-mass systems was also provided by Kormendy & Bender (2012).”
ANGULAR MOMENTUM AND GALAXY FORMATION REVISITED (2012, unknown): “Third, the extensions of our study to lower-mass (e.g., Kormendy & Bender 2012) as well as to higher-redshift galaxies (cf. Puech et al. 2007; Burkert et al. 2010), to freshly accreted material within galaxies (cf. Stewart et al. 2011), and to the orientations of the j⋆ vectors (e.g., Codis et al.…”
Structure and Kinematics of Early-Type Galaxies from Integral Field Spectroscopy (2016, methodology): “The fast rotators, like S0 galaxies (Laurikainen et al. 2010; Kormendy & Bender 2012), span the full range of bulge fraction as spiral galaxies (Krajnović et al. 2013a), and include some extremely flat disks.”
Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies (2013, background): “Also, Kormendy & Bender (2012) found some pseudobulges that are more compact than classical bulges of the same luminosity.”
The SLUGGS Survey: kinematics for over 2500 globular clusters in 12 early-type galaxies (2012, background): “The observational differences between these two scenarios are minimal and it also depends upon the galaxy environment (Kormendy & Bender 2012).”
The Evolution of Galaxy Structure Over Cosmic Time (2014, background): “Also shown is the extension of this sequence to dwarf spheroidal galaxies and irregular galaxies, both of which are lower mass systems (Kormendy & Bender 2012). galaxies.”
How do minor mergers promote inside-out growth of ellipticals, transforming the size, density profile and dark matter fraction? (2012, background): “…of high Sersic indices of present day large and massive ellipticals (Graham et al. 2001; Trujillo et al. 2004; Kormendy et al. 2009; Hoyos 2011; Kormendy & Bender 2012) some of which are most likely the descendants of compact ’red and dead’ flattened high-redshift galaxies with lower Sersic…”
Galaxy Zoo 2: detailed morphological classifications for 304,122 galaxies from the Sloan Digital Sky Survey (2013, result): “This is consistent with the “parallel-sequence” model of van den Bergh (1976) and later revised by several groups (including Cappellari et al. 2011; Laurikainen et al. 2011; Kormendy & Bender 2012).”
THE STAR FORMATION HISTORIES OF LOCAL GROUP DWARF GALAXIES. I. HUBBLE SPACE TELESCOPE/WIDE FIELD PLANETARY CAMERA 2 OBSERVATIONS (2014, background): “We have summarized the spatial locations and areal coverage of each field in Table 3, and 10 We include M32 in this category for convenience, even though it is a compact elliptical (e.g., Kormendy 1985; Kormendy & Bender 2012) list other observable properties in Table 4.”
Dependence of galaxy quenching on halo mass and distance from its centre (2012, background): “Pure ellipticals tend to be the most massive of intrinsically red galaxies and reside in the centres of large clusters (Kormendy & Bender 2012).”