Sometimes in astronomy you find something rather massive that’s been hiding in plain sight. Well, or at least hiding from one type of telescope but plainly visible in another. In this case, several quasars have been discovered in an infrared survey that may represent an previously unseen phase of galaxy evolution.
A quasar is a bright point source on the sky that is powered by an active supermassive black hole in the center of a distant galaxy. These quasars usually shine brightly in optical and ultraviolet light as stray material, such as gas and dust, wanders too close to the black hole and violently spirals in towards its demise. Distant quasars appear redshifted due to the expansion of the universe, so they are also detected with infrared light.
These particular quasars, in a paper by Manda Banerji and collaborators, are very, very infrared. So much so that expansion of the universe does not account for the color. Instead, these are enshrouded in thick layers of dust which absorb some of the optical and infrared light from around the supermassive black hole and “reprocess” it, emitting infrared light. How does such a system come about?
We are starting to put together pieces of the story of the life of a typical galaxy over cosmic time. When we say “galaxy evolution” we really mean the aging and changing of a single galaxy (and any it merges with) over time. We know that many galaxies went through a violent starburst period early in their history, creating stars at a rate from ten to one thousand times as fast as our Milky Way Galaxy does today. We also know that the supermassive black holes in the centers of all galaxies feed off the gas and dust that can accompany a starburst or merger event. But the story isn’t complete with the data that we have on hand.
Since these quasars are SO dusty, it is likely that these are a stage of evolution after a major starburst. The most massive stars explode as supernovae and leave debris, including dust, littered all over the galaxy. This material can then feed the central black hole, making it brighter. Eventually, the combination of the supernovae and bright quasar blow massive outflows of gas and dust out of the galaxy, revealing the bright central quasar.
These galaxies are at some in-between stage where all the dust isn’t gone, but “reddens” the quasar light. This hypothesis is bolstered by the discovery from the quasar spectra that the material is moving rapidly and in all directions, signs of a massive outflow.
All this information comes from tiny points of light like the red dot in the image above. Well, okay, it’s a little more complicated than that. The spectra of the quasar is used for a lot of this work, that is, the light is spread out by wavelength not unlike a rainbow. Only, this is an infrared rainbow and it helps astronomers piece together the types of gas present in the galaxy, where outflows are present, and that so much dust is present. Particularly, the astronomers compared the data to models of quasars to find a description that best fit what they were seeing.
From just a tiny point of light we can learn so much about the universe’s history, and thus, our own history as well. I think that is pretty amazing stuff.
Image Credit: Infrared colour image of ULASJ1234+0907. From Banerji et al. 2012, MNRAS. Created using data from UKIDSS and WISE.
This work is publishing in the Monthly Notices of the Royal Astronomical Society, and a pre-print can be found at arXiv.org.