How do galaxies form?

摘要

The current theory of galaxy formation holds that galaxies were assembled through the chaotic hierarchical merging of massive haloes of dark matter, in which star-forming matter was later embedded. One would therefore expect the properties of individual galaxies to be determined by numerous independent factors, such as star-forming history, merger history, mass, angular momentum, size and environment. It is thus surprising that galaxies seem to form an (almost) one-parameter family in which their mass is the dominant factor, as an investigation by Disney et al.l (on page 1082 of this issue) suggests. New observations often provide a fresh perspective on the Universe, thus opening another window through which to view ancient problems. Disney et al.1 provide a catalogue of roughly 200 galaxies - from giant spirals to extreme dwarfs - that were selected from a large, blind sky survey for neutral hydrogen (Hi) emission using the hydrogen spectral line at a wavelength of 21 cm. For each galaxy in the catalogue, they measured a number of quantities: the total hydrogen mass; the width of the hydrogen spectral line; the redshift; the inclination with respect to the line of sight; two radii, containing 50% and 90% of the light, respectively; and the optical luminosity in four different colour bands.