The Most Massive Galaxies in the Universe
Optical images of six quasars - massive galaxies whose light is dominated by their central black hole engines and, in some cases, by huge bursts of star formation as well. New studies of distant massive quasars find both processes can proceed simultaneously. The quasars shown here (not from the study) are the bright star-like objects; the center and right-hand images reveal disrupted material from a galaxy-galaxy collision/merger.
The most massive galaxies in the universe (as far as astronomers know) contain about five hundred billion solar-masses of material; our Milky Way galaxy, for comparison, has a total mass of roughly about one hundred billion solar masses. Typically about 80% of the mass in a galaxy is in the form of stars and most of the rest is gas. (In addition, galaxies are inferred to have halos of dark matter of unknown composition containing much more matter, perhaps ten times more.)
Astronomers trying to understand how galaxies of all kinds form are keenly interested in the giants because they appear to exist in the early universe. The cosmos is about thirteen billion years old, and the Milky Way and other galaxies in our neighborhood no doubt benefited from having all that time to grow larger. But some of the most massive galaxies have been found back when the universe was only a few billion years old: how did they get to be so big so fast?
Part of the answer to this question can be found by looking at the production of new stars in a galaxy, as well as the growth of its central massive black hole. Both of these processes can be studied in distant objects because they produce bright radiation in the infrared, with the latter process also producing strong radio and X-ray emission. CfA astronomer Belinda Wilkes and her three colleagues studied these phenomena in three massive galaxies whose light has been traveling towards us for about eleven billion years. They used the Herschel Space Telescope’s infrared sensors, combined with data from the Chandra X-Ray Observatory and ground-based radio results. They find their sources are making stars at a rate of nearly 800 per year, hundreds of times faster than does the Milky Way, and moreover they do so while copiously accreting material onto their nuclear black holes, something that had previously been deemed unlikely. The new paper shows that activity in both the nucleus and star forming regions of a galaxy can occur simultaneously, even in the early universe. The results suggests that although such dramatic combined activity highlights atypical, massive galaxies, physically it may be playing a fundamental role in their growth and development.