Insignificant in visible light, halo stars are big in the infrared background.

When the known galaxies and stars in the right panel are subtracted, what remains is the cosmic infrared background shown at left. Astronomers have determined much of this haze is from stars in the dark matter halos of early galaxies. The search for the earliest galaxies in the Universe is ongoing.

Since these galaxies are far removed from us in time, they are very faint and very red-shifted, making it hard to determine how many there were and where they were distributed. To find these galaxies, some astronomers are looking at the Universe’s infrared background across large patches of the sky. Fluctuations in the temperature of this infrared background are likely indicators of the first galaxies, which heated and ionized much of the gas in the Universe.

A new observation using data from the Spitzer infrared space telescope has found the expected signature of distant, faint galaxies. However, the magnitude of the fluctuations was surprisingly high: these early galaxies appeared bigger and brighter than expected from theory and observations at other wavelengths. In a new Nature paper, Asantha Cooray and colleagues suggest that much of this infrared radiation came from stars in the galactic halos, which were thought to be mostly dark matter. Typical galaxies such as the Milky Way have two basic parts: the luminous portion (which is what we usually think of as the galaxy), and a dark matter halo that envelops it and contains most of the mass. Even though most of a galaxy’s stars are in the luminous portion, the halo does contain a substantial number of stars, although they’re at a much lower density. Recent studies have shown that halo stars contribute more to the total light profile of a galaxy than we previously thought.

The mass of the dark matter halos is thought to have been instrumental in drawing atoms into the first galaxies, a process that left its mark on the early Universe. The first stable atoms formed around 400,000 years after the Big Bang. As electrons joined with protons, they emitted light we now see as the cosmic microwave background. When the first stars and galaxies formed, however, their intense radiation stripped electrons from atoms again, an event known as reionization. According to theory, that is: while the ionized gas has been seen, the stars that drove it are distant and hard to observe. However, the earliest stars and galaxies should contribute to the total infrared glow of the Universe, known as the cosmic near-infrared background (CNIB).

(“Nearinfrared” refers to wavelengths closest to visible light in the electromagnetic spectrum; in this case, the study was in the 1 to 5 micron range.) Much of the haze in the CNIB is from the Milky Way and known galaxies, but a significant portion is not associated with any obvious sources.

Astronomers have postulated it must originate in either to dwarf galaxies (which are too small to be seen at significant distances) or faint galaxies from the early Universe. Until the current study, no survey of the CNIB had sufficient resolution to distinguish between small (but relatively close) galaxies and the first to form in the Universe. The researchers decisively determined the overabundance of infrared emission was originating from an area that is too large to be dwarf galaxies.