Galaxies. Группа авторов

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Galaxies - Группа авторов


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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).

      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.

      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.

Photos depict four relatively isolated grand-design spirals.

      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.

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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