Science Focus: Galaxies
 How do galaxies form and evolve?

Soon after the Big Bang, the Universe evolved from a hot primordial plasma, which we now observe as the cosmic microwave background (CMB), to a space filled with dark matter, radiation, and neutral gas. During recombination, the Universe entered a starless Dark Age. Over the next several hundred million years, gravity produced a web of cores and interconnecting filamentary structure. As the cores grew, portions collapsed to form the first stars and clusters of stars. Eventually, groups of star clusters surrounded by gas became the first galaxies. At roughly this time, the first supermassive black holes emerged to power quasars, probably as a result of stellar evolution and subsequent accretion. At the end of the Dark Ages, ultraviolet and X-ray radiation from forming stars and quasars ionized bubbles in the surrounding HI (neutral hydrogen). Growth and merging of bubbles marked the transition to a nearly fully reionized IGM. As the Universe evolved to z ~1, galaxies developed the basic Hubble types observed nearby and large-scale structure developed into a tapestry of dense clusters, voids, and extended sheets.

  Galaxies and Black Holes

This image of the nearby galaxy Messier 81 combines data from the Hubble Space Telescope, the Spitzer Space Telescope, and the Galaxy Evolution Explorer (GALEX) missions. Click here to find out more.


Section Photo