Key Project 10

Key Project 10

Active Galactic Nuclei and High Redshift Quasars

Spectra of the nine newly-discovered high-redshift (5.7 ≤ z ≤ 6) Pan-STARRS1 quasars. For each quasar, the blue line shows radiative flux (a measure of light) as a function of wavelength (color) in angstroms. At these extreme distances, the Lyman-α emission line (whose rest wavelegth is 1216 Å in the ultraviolet) appears redshifted to the near-infrared between 8000 and 8500 Å.

A quasar—quasi-stellar object (QSO)—is an extremely luminous object found at such large cosmic distances that it appears point-like in the sky, similar to stars. In reality, quasars are supermassive black holes fed by accretion of matter at the centers of faraway, young galaxies. Today, observations suggest that all major galaxies, including the Milky Way, contain black holes as massive as several billion Suns in their centers, though not all black holes are actively accreting. Accreting supermassive black holes, which power active galactic nuclei (AGNs), can exhibit different characteristics depending on their inclination with respect to the line of sight.

Pan-STARRS1 Key Project 10 is dedicated to the study of AGNs and quasars at large cosmic distances, which translates into studying the early Universe, in particular early structure evolution and black hole formation less than a billion years after the Big Bang. They can further be used to probe the elemental composition (e.g., neutral hydrogen fraction and the evolution of metal abundances) of the early Universe and the end of cosmic reionization. The extensive sky coverage, depth, and filter set of the Pan-STARRS1 survey are critical for the detection of high-redshift quasars and AGNs. Moreover, Pan-STARRS1 provides proper motions, which serve to discriminate between very distant and nearby objects of similar color.

With Key Project 10, researchers hope to answer a number of outstanding scientific questions such as:

  • How does a quasar host galaxy evolve with distance and as a function of stellar mass?
  • How does the balance between radiative cooling and AGN heating vary with distance, mass, and environment?
  • When did the first black holes begin to accrete?
  • What are the properties of typical quasars in typical dark-matter halos in the early Universe?


Selection of quasars from Pan-STARRS1 dataset. The black lines on both panels indicate how a quasar's light shifts in color as a function of redshift z—an indication of cosmic distance given the overall expansion of the Universe. The redshift of a quasar is determined by running its light through a set of broadband filters. Depending on its redshift, a quasar may or may not show through a particular filter (the relevant Pan-STARRS1 filter set shown on the right panel).

Already several extremely distant quasars have been discovered thanks to the strengths of the Pan-STARRS1 dataset, increasing the number of published high-redshift (z > 5.7) quasars by more than 10%. Of particular interest is the variety of spectral features among these quasars—some have very bright emission lines while other have almost no detectable emission lines.

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