Project Overview - Objective 1
A histogram of galaxy luminosities down to absolute magnitudes as faint as -9 mag
Are there more faint dwarf galaxies in the Universe than big luminous galaxies, or is it the other way around? Determining the galaxy makeup of the Universe has been a major area of recent astronomical study. Galaxies possess a range of (intrinsic) brightnesses and careful observations of a fixed volume of space can enable one to measure the relative numbers of bright and faint galaxies. Technically, astronomers present this measurement as a "luminosity function". This is simply a histogram presenting the numbers of galaxies with different luminosities, where the luminosity of a galaxy is the stellar light output (within a specified wavelength range, such as through a red or blue light filter) from all the stars within that galaxy. Average galaxies like the Milky Way have luminosities approximately equal to L* = 1011 Lsun (= 1037 Watts), where Lsun is the luminosity of our Sun. The smallest galaxies detected in the Coma cluster Treasury Survey have luminosities equal to 10-5 L*, that is, 100,000 times fainter than the Milky Way.
Because of large telescopes and sensitive detectors, the difficulty in making this measurement is not so much in detecting small faint galaxies but in distinguishing them from the large number of more distant background galaxies (beyond the Coma cluster) which are intrinsically luminous but appear faint because they are far away. These two kinds of galaxies can be distinguished by their detailed morphologies: galaxies with low intrinsic luminosity have low surface brightness (intensity per unit area) and lack well-defined features like spiral structure and galactic bars. High-resolution images are therefore required to identify true low-luminosity galaxies within the Coma cluster. The Hubble Space Telescope is the best instrument to provide these images.
The figure below shows our measurement of the (optical) "luminosity function" for the total area of our survey. The horizontal-axis represents the absolute magnitude of a galaxy, which is a measure of its luminosity. In these units, the brighter the galaxy the more negative the absolute magnitude, so that a galaxy with an absolute magnitude of -22 mag is very luminous but a galaxy with an absolute magnitude of -10 mag is very faint (the Milky Way is about -20 mag). The vertical-axis represents the number of galaxies (per unit volume) having a particular absolute magnitude. The quantity "alpha" is the (logarithmic) slope of the luminosity function at faint magnitudes, and is a rough attempt to describe the shape of the "luminosity function" with a single number. If alpha=-1, then there are as many faint galaxies as there are large galaxies, but if alpha=-2 then there are very many more small faint galaxies as there are large galaxies. The data reveal that at the faintest magnitudes for which we can measure galaxies in the Coma cluster (the points at the right in the figure), the value of alpha is somewhere between -1 and -1.5. That is, within the Coma cluster, each successively faint magnitude interval contains more galaxies than the previous (brighter) magnitude interval.
These results are particularly interesting when compared to the theoretical mass function of dark matter halos. The dark matter mass function has a slope alpha=-2. Therefore there are (expected to be) very many small dark matter halos relative to the number of large halos. But the same is not true for galaxies in the Coma Cluster. Because galaxies reside within dark matter halos, these observations may be revealing that there are many small dark matter halos that do not contain stars and would therefore not be identified as galaxies. There is independent evidence that this is also the situation in the "Local Group" of galaxies which includes our Milky Way. It is known that galaxies in dense clusters like the Coma cluster were the first to form in the Universe, and so our results may therefore imply that many small, dark matter halos without galaxies existed early in the history of the Universe.
Image credit: Neil Trentham.