Faint satellite galaxies of the Milky Way are being discovered that are dimmer than some of the Milky Way's star clusters. This finding poses a fundamental question: what are galaxies?
Of his 1938 discovery of two tiny satellite galaxies accompanying our own Milky Way, the astronomer Harlow Shapley wrote1: “Presumably the gamut of galaxies had already been run. All forms had long been fully described. There were spirals, spheroidals, irregulars, with many variations on the spiral theme. The newly found organizations in [the dwarf galaxies] Sculptor and Fornax did not seem essential in order to fill in a natural sequence; they were not logically necessary. On the contrary, they introduced some doubt into the picture we had sketched — they suggested that we may be farther than we think from understanding the world of galaxies.”
Seven decades later, these words still ring true. We now know that 'dwarf spheroidals' of the type discovered by Shapley are in fact the most common type of galaxy in the Universe. And, in a paper to be published in The Astrophysical Journal, Belokurov et al.2 report the discovery, in data from the Sloan Digital Sky Survey, of four more dwarf spheroidal satellites of the Milky Way system. One of these, the Coma dwarf galaxy, is the faintest galaxy ever observed, and is two orders of magnitude fainter than the brightest clusters of old stars known within the Milky Way. So where exactly does one draw the line between a bright star cluster and a faint galaxy?
The obvious distinguishing criterion, size, is not always reliable: although galaxies are usually larger than star clusters, tidal forces can strip off the large envelopes of galaxies, leaving behind only a compact rump. So-called dark matter, on the other hand, does provide a reliable way of telling a galaxy from a star cluster. All galaxies are thought to be embedded in a halo of this type of matter, which emits no light and so cannot be observed directly; no dark matter has ever been detected in a star cluster.
Because they have no dark matter contributing to their overall mass, star clusters that formed from fragments of disintegrating tidal arms are expected to have small ratios of mass to light emitted. This implies that they have a lower spread of internal velocities than galaxies, as this spread is intimately connected to a body's mass. Individual stars in star clusters also all formed at roughly the same time, and therefore have similar metal abundances, whereas stars in galaxies, which require a significantly longer time to assemble, can have a large range of metallicities. But applying these criteria to faint, distant clusters is difficult, as even the largest telescopes require a great deal of observing time to measure metallicities and internal velocities. The metallicity-dispersion criterion is also likely to be useless for the least luminous dwarfs, as these are expected to contain only very metal-poor stars.
Of the five objects discovered altogether by Belokurov et al.2, three (dubbed Canes Venatici II, Hercules and Leo IV) have standardized radii of more than 100 parsecs (1 parsec is 3.26 light years). They are therefore large enough to be definitely regarded as faint galaxies. But the smallest of the objects, Segue 1, has a radius of only around 30 parsecs, placing it at the upper bound of the size range of known Galactic star clusters. The fifth object, the Coma dwarf, is of an intermediate size between the largest known star cluster and the smallest known galaxy. Only velocity-spread measurements will be able to tell if this object truly is a galaxy containing dark matter, or a huge star cluster that does not.
If we add the objects found by Belokurov et al.2, and the star clusters Omega Centauri and NGC 2419, which might be the stripped cores of dwarf spheroidals, the Milky Way has 21 known companions within a distance of around 500 kiloparsecs, equivalent to about 1.6 million light years. The three most luminous of these — the Large Magellanic Cloud, the Small Magellanic Cloud and an object known as NGC 6822 — might actually be, or at least have started their lives as, independent members of the Local Group of galaxies. This group is centred on the Milky Way and M31, the great Andromeda spiral. By coincidence, the number of companions presently known within 500 kiloparsecs of Andromeda's nucleus is also 21; but some of these too might be free-ranging members of the Local Group, rather than satellites of their central galaxy.
A curious fact, first noted by Jaan Einasto and colleagues3 in 1974, is that most of the close satellites of M31 and the Galaxy are elliptical or spheroidal, whereas many of their more distant companions have irregular forms. Unfortunately, some of the newly discovered objects2 have such low star counts, and the subtraction of light from background sources is therefore so uncertain, that it is difficult to be sure whether they are irregular or dwarf spheroidal galaxies.
Another interesting facet that emerges from these latest discoveries of satellite galaxies is that the radial distances of the known companions of M31 and the Milky Way from the nucleus of their parent galaxy seem to be roughly similarly distributed. For distances of less than 150 kiloparsecs, the number N of companions within a radius R is reasonably well represented by a logarithmic correlation, logN=−0.24+0.80logR. Perhaps because of the incompleteness of the data, above 150 kiloparsecs the cumulative number drops below that expected from this relation. Even so, half of the known companions of these two galaxies are situated more than 90 kiloparsecs from their host. At that distance, gravity from an unseen source must be acting to keep the companion galaxies in tow, attesting to the enormous size of the haloes of dark matter that must surround the Milky Way and M31.
Numerical simulations4,5 have predicted the existence of a swarm of smaller, denser dark-mass haloes embedded within the general dark halo of the Milky Way. The gravity of each of these mini-dark-haloes should have attracted a clump of ordinary, visible matter. Thus, these predictions seemed to conflict with the observation that our Galaxy is surrounded by only a handful of companions. But the faintness of Belokurov and colleagues' new discoveries2 indicates that they might be just the first of a vast population of ultra-faint dwarf galaxies surrounding the Milky Way system that is yet to be discovered.