Blazar Evolution With Cosmic Archaeology

Apr 25, 2013

Peering into the past, KIPAC scientists use Fermi gamma-ray telescope observations to unlock the story of blazar evolution over the history of the Universe.

The density of FSRQ type blazars in the Universe as a function of time. The present time is at the far left of the graph and higher redshift means farther back in time. The density is shown as number per cubic megaparsec, which is a large unit of volume.

One of the many places high energy physics and astrophysics meet is in active galactic nuclei (AGN), where in distant galaxies the interaction of a huge quantity of infalling matter with a monstrous spinning supermassive black hole at the center results in enormous jets of particles and radiation. When the alignment of the AGN happens to be such that a jet is pointed right at us, we observe what we call a blazar, an AGN object with significant emission in gamma rays, the highest energy photons of light.
Data from the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope, which was designed and constructed at SLAC before being launched into orbit three and a half years ago, has revolutionized the study of blazars, among many other cosmic phenomena. The latest Fermi-LAT contribution to blazar science comes from a team led by KIPAC postdoc Marco Ajello, graduate student Michael Shaw, and professor Roger Romani, along with KIPAC postdoc Luigi Costamonte and several colleagues from the California Institute of Technology. The team used LAT observations of the gamma rays from blazars in combination with optical telescope observations of the redshifts - changes in the frequency of emitted light that tells us how cosmically distant each object is. In order to ensure complete redshift information, they restricted their analysis to one of the two categories of blazars, called Flat Spectrum Radio Quasars (FSRQs). FSRQs are characterized by broad emission lines, meaning that the optical light emitted from substances in the accretion disk of matter around the supermassive black hole is bright and the matter is moving fast.
Like AGN phenomena generally, blazars present many puzzles. Because we can see them when they are relatively far away, and it takes light a long time to reach us from a great distance, looking at distant AGN is seeing them as they were in the past. In this way, they are great candidates for cosmic archaeology, where scientists try to piece together the changes in their characteristics over time, analogous to what biologists and geologists do with fossils. Ajello and colleagues have done just that, using the gamma-ray and redshift information to piece together the evolution of FSRQs over the history of the Universe.
Such an analysis requires care and skill because the data is incomplete and biased, as only ever brighter blazars are actually seen at increasing distances, and because the Fermi-LAT misses many objects due to their emission spectrum. The KIPAC-based team has used statistical maximum-likelihood techniques to derive the best-fit behavior of FSRQ-type blazars over time from the Fermi-LAT gamma-ray data. They find that going back in time in the Universe there were many more FSRQs than now, until one goes back far enough at which point there start to be fewer and fewer. It seems that the density of FSRQs ramped up with the evolution of the Universe, peaked, and came down for the present.
The team finds a familiar pattern for the inherent luminosity of FSRQs over time, with average brightness decreasing from the past to the present.  Combining the average luminosity and average source density, the total gamma-ray output of FSRQs peaked around a redshift of 1.5, when the Universe was less than half as old as it is now. These evolutionary patterns for FSRQs are largely in agreement with those found previously for various AGN types, and the similarities and subtle differences with them will help piece together the story of the evolution of AGNs and the enigmatic supermassive black holes and accretion disks of matter that power them.
This work is presented in a paper submitted to the Astrophysical Journal and available from astro-ph at arXiv:1110.3787.
Science contact:
Marco Ajello