Observations using the Department of Energy-fabricated Dark Energy Camera (DECam) on the U.S. National Science Foundation Víctor M. Blanco 4-meter Telescope confirm astronomers’ expectation that early-Universe quasars formed in regions of space densely populated with smaller companion galaxies. DECam’s exceptionally wide field of view and special filters played a crucial role in reaching this conclusion, and the observations reveal why previous studies seeking to characterize the density of early-Universe quasar neighbourhoods have yielded conflicting results. Credit: NOIRLab/ NSF/ AURA/ M Garlick/ J da Silva (Spaceengine)/ M Zamani. CC BY.
New finding with the expansive Dark Energy Camera offers a clear explanation to quasar ‘urban density’ conundrum.
Quasars are the most luminous objects in the Universe and are powered by material accreting onto supermassive black holes at the centres of galaxies.
Studies have shown that early-Universe quasars have black holes so massive that they must have been swallowing gas at very high rates, leading most astronomers to believe that these quasars formed in some of the densest environments in the Universe where gas was most available.
However, observational measurements seeking to confirm this conclusion have thus far yielded conflicting results.
Now, a new study using the Dark Energy Camera (DECam) points the way to both an explanation for these disparate observations and also a logical framework to connect observation with theory.
DECam was fabricated by the Department of Energy and is mounted on the US National Science Foundation Víctor M Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory in Chile, a programme of NSF NOIRLab.
The study was led by Trystan Lambert, who completed this work as a PhD student at Diego Portales University’s Institute of Astrophysical Studies in Chile and is now a postdoc at the University of Western Australia node at the International Centre for Radio Astronomy Research (ICRAR).
Utilising DECam’s massive field of view, the team conducted the largest on-sky area search ever around an early-Universe quasar in an effort to measure the density of its environment by counting the number of surrounding companion galaxies.
Serendipitous
For their investigation, the team needed a quasar with a well-defined distance. Luckily, quasar VIK 2348–3054 has a known distance, determined by previous observations with the Atacama Large Millimeter/ submillimeter Array (ALMA), and DECam’s three-square-degree field of view provided an expansive look at its cosmic neighbourhood.
Serendipitously, DECam is also equipped with a narrowband filter perfectly matched for detecting its companion galaxies.
Lambert said: “This quasar study really was the perfect storm. We had a quasar with a well-known distance, and DECam on the Blanco telescope offered the massive field of view and exact filter that we needed.”
DECam’s specialised filter allowed the team to count the number of companion galaxies around the quasar by detecting a very specific type of light they emit, known as Lyman-alpha radiation.
Lyman-alpha radiation is a specific energy signature of hydrogen, produced when it is ionised and then recombined during the process of star formation.
Lyman-alpha emitters are typically younger, smaller galaxies, and their Lyman-alpha emission can be used as a way to reliably measure their distances.
Distance measurements for multiple Lyman-alpha emitters can then be used to construct a 3D map of a quasar’s neighbourhood.
After systematically mapping the region of space around quasar VIK J2348-3054, Lambert and his team found 38 companion galaxies in the wider environment around the quasar – out to a distance of 60 million light-years – which is consistent with what is expected for quasars residing in dense regions.
However, they were surprised to find that within 15 million light-years of the quasar, there were no companions at all.
Deceptive
This finding illuminates the reality of past studies aimed at classifying early-Universe quasar environments and proposes a possible explanation for why they have turned out conflicting results.
No other survey of this kind has used a search area as large as the one provided by DECam, so to smaller-area searches a quasar’s environment can appear deceptively empty.
Lambert explained: “DECam’s extremely wide view is necessary for studying quasar neighbourhoods thoroughly. You really have to open up to a larger area.
“This suggests a reasonable explanation as to why previous observations are in conflict with one another.”
The team also suggests an explanation for the lack of companion galaxies in the immediate vicinity of the quasar.
They postulate that the intensity of the radiation from the quasar may be large enough to affect, or potentially stop, the formation of stars in these galaxies, making them invisible to our observations.
Lambert said: “Some quasars are not quiet neighbours. Stars in galaxies form from gas that is cold enough to collapse under its own gravity.
“Luminous quasars can potentially be so bright as to illuminate this gas in nearby galaxies and heat it up, preventing this collapse.”
Lambert’s team is currently following up with additional observations to obtain spectra and confirm star formation suppression. They also plan to observe other quasars to build a more robust sample size.
