Stars hiding in dark matter halos made early galaxies look big

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.

NASA/JPL-Caltech/A. Kashlinsky (GSFC)

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.

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