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is directly related to one of Hubble’s classification criteria for spirals: that Sa galaxies have more prominent central concentrations than do Sc galaxies. The latest stages (Sd-Im) have virtually no central concentration and the lowest average surface brightnesses. Similarly, there is a relatively smooth variation in HI mass-to-blue light ratio across the spiral sequence, ranging from 0.08 at stage S0/a to 0.5 at stage Im, a factor of 6.5 change (Buta et al. 1994).
1.11. Other approaches to galaxy classification
The CVRHS system is only one approach to galaxy classification, but it has several advantages: (1) a high focus on features that are likely intimately connected to dynamics and evolution, such as bars, rings and spirals; (2) correlation with star formation history; and (3) the broadest perspective on galaxy morphology without being too unwieldy. However, in the era of large imaging surveys like the Sloan Digital Sky Survey (SDSS; Gunn et al. 1998, 2006; York et al. 2000), the number of classifiable images of galaxies available is literally in the millions. CVRHS classification by a single individual (for example, Buta 2019) will likely be impractical for samples much larger than 20,000 objects. Another approach to galaxy classification is needed.
Nair and Abraham (2010) provide classifications for 14,034 SDSS galaxies in the redshift range 0.01 < z < 0.1 in a system similar to the CVRHS. In this case, a single professional astronomer classified each galaxy twice to check for consistency. In the studies of Fukugita et al. (2007), Baillard et al. (2011) and Ann et al. (2015), 3–10 professional astronomers classified either the whole or part of samples of 2253, 4458 and 5836 SDSS galaxies, respectively. In each case, the final classification is an average of the multiple astronomers, usually after intercomparing results and taking into account “personal equations” of each classifier. One of the largest applications of the multiclassifier approach was made by Kartaltepe et al. (2016), who had the input of 65 professional astronomers.
In spite of these efforts, the number of galaxies that have been classified by experts is still relatively small. This led some professional astronomers to try “crowd-sourcing” galaxy classification by engaging with the general public and enlisting the help of “citizen scientists”. The first major crowd-sourcing effort is described by Lintott et al. (2008) and is called “Galaxy Zoo 1” (GZ1). In GZ1, a web interface was created to display SDSS color images of several hundreds of thousands of galaxies accompanied by a set of “buttons” for selecting rudimentary elements of morphology, such as whether a galaxy is disk-shaped or not. This was followed by “Galaxy Zoo 2” (Willett et al. 2013), which allowed classifiers to specify more detail, such as bars, rings and spiral arm character. The classifications in GZ2 were combined using a vote scheme, and an attempt was made to correct the classifications for biases due to resolution and distance. GZ2 provided classifications for nearly 300,000 SDSS galaxies based on about 80,000 participants. Willett et al. (2013) provide short abbreviations of these classifications that are compared with the CVRHS classifications of Buta (2019) and Buta et al. (2019) in Table 1.12. In these abbreviations, E galaxies are smooth while S galaxies are disk shaped with structure. The other codes are described in Appendix A of Willett et al. (2013). Four of the galaxies in Table 1.12 are shown in Figure 1.27.
1.12. Interpretations of morphology
We have seen that galaxy morphology includes a bewildering array of complex structures, and that the evolutionary path that any galaxy took to reach its current morphological state is not obvious. Nevertheless, some reasonable judgments can be made. This section summarizes some aspects of morphology that are well understood, and others that are still largely uncertain.
Galaxy formation: The best current theory of galaxy formation is the Λ Cold Dark Matter model (White and Rees 1978). In this model, galaxy clusters begin as tiny fluctuations in the temperature of the cosmic microwave background radiation that are expanded during the inflationary period to much larger sizes. The fluctuations become “seeds” of cold dark matter within which baryonic matter collects. The formation of collapsing proto-galactic clouds is enhanced by the extra gravity of the dark matter. Whether a galaxy becomes a disk galaxy or a non-disk galaxy depends on how much angular momentum the collapsing cloud has from tidally induced torques due to neighboring clouds (Ryden 1988) and on how effectively it converts its baryonic material into stars. If this conversion is largely complete within a billion or so years after collapse, then the object could become an elliptical galaxy. If instead the rate of conversion is slower, the proto-galactic cloud may have time to form a disk. Multiple mergers of small objects with this disk-shaped object could lead to the build up of what is generally known as a “classical bulge”. This formation scenario is not expected to form a bulge-less, pure disk galaxy, and it is also well-established that some ellipticals have likely formed from mergers of disk-shaped galaxies (Schweizer 1982).
Figure 1.27. Images of four relatively isolated grand-design spirals (Buta et al. 2019)
Although the “formative” phase of a galaxy never really ends (i.e. a galaxy may continue to accrete material long after the formation period largely ended, just as planets still accrete interplanetary debris even now), at some point the accretion rate slows down and galactic changes occur very much more slowly. This is the time when secular evolution takes over as the dominant mechanism of change (Kormendy and Kennicutt 2004). Secular evolution can be driven entirely by internal processes or by external processes. The idea is that the formative phase of galaxy evolution is fairly rapid compared to secular evolution. To interpret the morphology of some nearby galaxies, we should appeal to secular evolutionary processes.
The origin of S0 galaxies: Dressler (1980) describes the morphology-density relation in rich galaxy clusters, where the most common types of galaxies found are E and S0. This relation, which was first described by Hubble and Humason (1935), led to the general idea, originally proposed by Spitzer and Baade (1951), and elaborated upon by van den Bergh (1976), that S0 galaxies are former spirals that have been stripped of their interstellar gas and dust. The stripping shut down (“quenched”) star formation and essentially “killed” the spiral arms. In this scenario, as already noted in section 1.5, S0s cannot be transition stages between ellipticals and spirals, but must form a sequence of decreasing bulge to total luminosity ratio parallel to spirals. This idea is strongly supported by kinematic studies of early-type galaxies (Cappellari et al. 2011), photometric studies of dE and dS0 galaxies (Kormendy and Bender 2012) and by multicomponent analysis of a large sample of S0s (Laurikainen et al. 2011).
Table 1.1. Comparison of classifications
| Galaxy1 | CVRHS type2 | Willett et al. 2013 Type 3 |
| NGC 5057 | (RL)SA(l)0+ | E(r)r |
| NGC 6116 | SA(s)a | Sb2m |
| UGC 10258 | (R′)SA(s)ab | Sb2t |
| NGC 2649 | SA(rs)bc | Sc2t |
| CGCG 62-1 | SA(rs)bc | Sc2l |
| CGCG 91-20 | SA(s)bc | Sc2t |
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