If galaxies did not exist we would have no difficulty in explaining the fact.
Galaxies are fundamental constituents of the universe. They are groups of approximately 105-1013 stars1 that are gravitationally bound and take part in the general expansion of the universe. Galaxies have diameters ranging from 10,000 to 200,000 light-years2 and they possess widely varying gas and interstellar dust contents. The distances between galaxies are typically 100-1000 times their diameters. They represent 108 overdensities above the mean stellar density of the local universe. In total, galaxy masses range from 106-1014 solar masses (M).
Their component stars vary from ~0.1 M brown dwarfs that do not undergo thermonuclear fusion to rapidly evolving stars of at least 50 M and possibly as massive as 100 M. The stars evolutionary state ranges from protostars undergoing contraction to begin thermonuclear reactions, to main sequence dwarf stars that fuse hydrogen to helium in their cores, through to red giant stars with expansive gaseous atmospheres. The stellar end products are white dwarfs, neutron stars and black holes. The specific evolutionary path of a star is governed by its mass. The most massive stars evolve over millions of years, whilst the lowest mass stars can evolve for billions of years.
At least 300,000 years after the Big Bang start of the universe, structures that would become galaxies began to condense out of primordial hydrogen and helium. Current observational and theoretical studies of the formation and evolution of large-scale structure (groups, clusters and superclusters of galaxies) suggest that Cold Dark Matter (CDM) is the predominant matter in the universe. Observations of individual galaxies suggest that CDM is the dominant matter in galaxies (see Section 2.6 for more information), making up more than 50% of a galaxies mass. CDM, a theoretical massive, slow moving particle (or particles) has not yet been detected. However, a CDM dominated universe would suggest that galaxies were built from the aggregation of smaller structures, in a sort of 'bottom-up' construction approach. In fact the closer galaxies are detected to the time of the Big Bang, the stronger this argument becomes. From about 300,000 years after the Big Bang, the attractive force of gravity was in control and dictated that the first galaxies formed within 1 billion years (Gyr). Gravity increased gas densities and temperatures until physical conditions allowed the first stars to form. Elements heavier in atomic weight than hydrogen and helium were then made by thermonuclear fusion inside the first (massive) stars, and expelled into the interstellar medium (ISM) by subsequent supernovae explosions. These explosions propagated shock waves through nearby interstellar gas causing gas densities to increase and new star formation to be initiated. Numerous cycles of star formation-supernovae explosions-star formation continued, utilising the increasingly heavy element enriched ISM.
Enough time, about 14 Gyr, has now elapsed since the beginning of our universe, to allow a stable system of planets rich in heavy elements to exist around a very average G dwarf star called the Sun. Our Sun is in the outer suburbs of a dense, disk of stars that has spiral arms of gas and dust3 where stars preferentially form. A lower density, large spheroidal halo of predominantly older stars surrounds the Galaxy4. Our Galaxy contains at least 100 billion stars, and it is regarded as an average sized spiral. From our vantage point our Galaxy and a vast variety of other galaxies are observed.
Figure 1.1: Optical image of the barred spiral galaxy NGC 1365 in the Fornax cluster of galaxies. Credit: SSRO/PROMPT and NOAO/AURA/NSF.
The appearance of galaxies vary due to their intrinsic properties and the differing lines of sight (with differing gas and dust contents) through our Galaxy along which they are observed. These three dimensional objects are also viewed projected on a two dimensional sky surface or celestial sphere. Hence their angle of inclination to our line of sight greatly influences how an observer perceives them.
Galaxies can be broadly grouped into Elliptical, Spiral, Lenticular and Irregular classes. Ellipticals range from circular shaped to highly elongated or oval-like congregations of predominantly old, evolved stars. Spirals are Catherine Wheel-like with 'fireworks' of young stars, dust and gas illuminating majestic spiral arms that can wrap around their nuclei many times. Stellar bars (elongated distributions of usually old stars) can be found in the centers of some spirals giving a spindle-like appearance (see the barred spiral NGC 1365 in Figure 1.1). The Lenticulars (or S0 types) can be regarded as a visual link between ellipticals and spirals. They are disk (like spirals) or lens-like though composed of predominantly old stellar populations lacking in dust and gas (like ellipticals). The Irregulars, as their name suggests, lack coherent spiral or elliptical shapes. They contain regions rich in dust and gas and have active star formation sites, yet are structurally quite amorphous.
1The exponent above 10 represents the number of zeros after 1 - e.g. 109 is 1,000,000,000 or 1 billion. If the exponent is negative, e.g. 10-2 then it is 1 divided by 102 or 1/100 = 0.01.
2A light-year is the distance that light travels at 2.9 x 105 km s-1, in a year. It is approximately 9.46 x 1012 km.
3Interstellar dust grains make up about 1% of the ISM and are formed in the envelopes of late-evolved stars like red giants or carbon stars. They are much smaller than the dust on Earth - ranging from a few molecules to a diameter of 0.1 mm. They begin as carbon or silicate grains, and then accumulate additional atoms (e.g. H, O, C, N) to form icy mantles of water ice, methane, carbon monoxide, and ammonia. All of this is encased in a sticky outer layer of molecules and simple organic compounds created through the interaction of the mantle with UV radiation. The grains are about the same size as the wavelength of blue light meaning that they absorb and scatter UV and blue light much more efficiently than red light.
4Throughout the atlas the Milky Way, our Galaxy will be denoted by an upper case G.