Bright Galaxy Clusters in the Era of Peak Star Formation
An infrared image taken by the IRAC camera of the cluster of galaxies HS1549. The contours show the intensities at two far-infrared wavelengths and delineate the massive, star-forming regions in the galaxy cluster. The 3 arcminute circle around the cluster core corresponds to a dimension of about five million light-years. Astronomers have found that within this region nearly ten thousand solar-masses of new stars are forming every year.
Galaxies actively engaged in making stars produce many hot massive stars that emit copious amounts of uv radiation. The neutral hydrogen gas in these galaxies (or in the intervening intergalactic medium) absorbs nearly all the uv light that is shorter than 912 Angstroms, the characteristic wavelength of the hydrogen transition. Observers of these starburst galaxies thus see a sudden drop in their spectrum at this wavelength, called the Lyman-break. (For reference, visible blue light lies at a much longer wavelength range, around 4000 Angstroms.) Since galaxies in the distant universe are expanding away from us, as their apparent spectrum is shifted to the red, their Lyman break is shifted to visible wavelengths where optical instruments can detect it.
Massive galaxy clusters in the local universe, and their massive members, must have begun forming stars in the early universe to have grown so large today. Astronomers do in fact see significantly enhanced star-formation activity in distant proto-galaxy clusters, identified in part with searches for their Lyman break signatures. CfA astronomer Mark Gurwell was a member of an international team that used the Submillimeter Array (SMA) and the SCUBA-2 submillimeter camera on the James Clerk Maxwell Telescope to study two of the largest known dense galaxy clusters from an epoch about two and one-half billion years after the big bang. The SMA was able to resolve the spatially blended galaxies. All together the team found fifty-six individual bright galaxies in very densely packed regions only about five million light-years in diameter (for comparison, the closest galaxy to our Milky Way, Andromeda, is two and one-half million light-years away).
The scientists used the infrared and submillimeter brightness of these clusters to estimate their star formation activity and find fantastic rates -- about ten thousand and three thousand solar-masses per year of new stars, respectively (the Milky Way is making about one new star per year). The results are consistent with the idea that during this epoch the universe was making stars at a considerable higher rate than today. The work also helps to refine our understanding of the distribution of dark matter (these galaxies form in dark matter halos) by refining simulations of galaxy structure formation.
"Two Sub-Millimetre Bright Protoclusters Bounding the Epoch of Peak Star-Formation Activity," Kevin M. Lacaille, Scott C. Chapman, Ian Smail, C. C. Steidel, A. W. Blain, J. Geach, A. Golob, M. Gurwell, R. J. Ivison, N. Reddy, and M. Sawicki, MNRAS 488, 1790, 2019.