Astronomers have used the wide-field view of the Dark Energy Camera to confirm that supermassive-black-hole-powered quasars in the early universe were packed into dense neighborhoods. However, it seems these cosmic beasts weren't exactly the best neighbors.
The team behind this research found quasars are "noisy neighbors" blasting out radiation that can cut off star formation, thus "killing" galaxies that live in their close cosmic neighborhoods. As a result, the closest companion galaxies around some quasars fail to grow and are thus too small and dim to see.
The team says these results about the "urban density" of quasars and their companion galaxies could also explain why some prior studies about the early universe's density have shown galaxies and quasars tightly packed together while others have indicated a lack of companion galaxies around quasars.
To conduct their study, the researchers turned to the quasar VIK 2348–3054, located around 12.8 light-years from Earth. The distance to this quasar is very well-defined thanks to the Atacama Large Millimeter Array (ALMA).
With its target selected, the Dark Energy Camera, or DECam, mounted on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory in Chile, allowed the team to conduct the largest on-sky area search ever around an early-universe quasar. While DECam's three-square-degree field of view provided an expansive overview of the cosmic neighborhood of VIK 2348–3054, its narrowband filter was the perfect addition to allow the team to hone in on the quasar's surrounding companion galaxies.
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"This quasar study really was the perfect storm," team leader Trystan Lambert, a postdoc researcher at the University of Western Australia node of the International Center for Radio Astronomy Research (ICRAR), said in a statement. "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."
Early quasars had well-stocked larders
Quasars are among the brightest sources of light in the known universe, often outshining the combined light of every star in the galaxies surrounding them. The engine driving these emissions are central supermassive black holes with masses millions of times that of the sun.
Like any engine, these cosmic monsters need fuel. For quasars, this takes the form of gas and dust swirling around the respective black holes, called an "accretion disk," that gradually feeds the voids. The tremendous gravitational influence of the black hole causes a huge amount of friction in the accretion disk, superheating this material and creating plasma and intense electromagnetic radiation that forms the quasar's emissions.
Black holes are messy eaters, though. Some of the material is channeled by powerful magnetic fields to their poles, where it is accelerated to near-light speeds and blasted out as collimated jets of plasma. Bright electromagnetic emissions also accompany these jets.
To facilitate their powerful emissions and to allow their supermassive black holes to grow to tremendous sizes in the relatively early universe, quasars therefore must be surrounded by an abundance of material to feed upon.
The necessarily high rate of feeding has led many astronomers to propose that quasars must sit in some of the densest regions of the universe where lots of gas is available. Confusingly, however, observations haven't always supported that idea.
To investigate this, Lambert and colleagues counted companion galaxies around VIK J2348-3054 by measuring a specific emission called Lyman-alpha radiation. This is a signature of a form of hydrogen that has had its electrons stripped by high temperatures. Electrons and hydrogen nuclei then recombine, with the previously ionized hydrogen atoms grabbing back some electrons. This is a typical indicator of star formation, and thus indicates younger and smaller galaxies birthing stellar bodies.
Helpfully, Lyman-alpha radiation is a good determiner of redshift values, the change in light frequency we detect when a light source moves away from our vantage point in the universe. That means it serves as a good way to determine distances to these small, young galaxies. These measurements can then be used to build a three-dimensional model of the region around a quasar.
Doing this for quasar VIK J2348-3054, the team found 38 companion galaxies, out as far as 60 million light-years, indicating a dense region of space. To the surprise of Lambert and colleagues, they also found a complete absence of companion galaxies within a distance of 15 million light-years of the quasar.
That could explain why previous research investigating quasar environments has delivered conflicting density results. That's because research that indicated empty space around quasars may have focused on the immediate regions around these supermassive black holes. Those regions would have been populated with the undetectable, star-formation-quenched galaxies. Conversely, research that showed crowded regions of space around quasars looked at the larger picture but didn't zoom in on the immediate vicinity around quasars. DECam provided a clearer picture because it facilitated the only study yet that included large-area-to-small-area data.
"DECam's extremely wide view is necessary for studying quasar neighborhoods thoroughly. You really have to open up to a larger area," Lambert said. "This suggests a reasonable explanation as to why previous observations are in conflict with one another."
The researchers suspect they know the reason for the apparent dearth of companion galaxies in close proximity to this quasar. They suggest it could be the result of intense radiation from the quasar that is stymying the formation of stars and, thus, killing the growth of galaxies in close proximity. That means those galaxies are probably there, but are just too small and dim to see.
"Some quasars are not quiet neighbors," Lambert concluded. "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."
The team's research appears in the journal Astronomy & Astrophysics.
