Double or nothing—astronomers rethink quasar environment

Galaxy distribution and close-ups of some protoclusters
revealed by HSC. Higher- and lower-density regions are
represented by redder and bluer colors, respectively. In
the close-ups, white circles indicate the positions of
distant galaxies. The red regions are expected to evolve
into galaxy clusters. From the close-ups, we can see
various morphologies of the overdense regions: some have
another neighboring overdense region, or are elongated like
a filament, while there are also isolated overdense
regions. Credit: NAOJ

Using Hyper Suprime-Cam (HSC) mounted on the Subaru
Telescope, astronomers have identified nearly 200
“protoclusters,” the progenitors of galaxy clusters, in the
early universe, about 12 billion years ago, about ten times
more than previously known. They also found that quasars
don’t tend to reside in protoclusters; but if there is one
quasar in a protocluster, there is likely a second nearby.
This result raises doubts about the relation between
protoclusters and quasars.

In the , are not distributed uniformly. There are
some places, known as clusters, where dozens or hundreds of
galaxies are found close together. Other galaxies are isolated.
To determine how and why clusters formed, it is critical to
investigate not only mature as seen in the present universe
but also observe protoclusters, galaxy clusters in the process
of forming.

Because the speed of light is finite, observing distant objects
allows us to look back in time. For example, the light from an
object 1 billion light-years away was actually emitted 1
billion years ago and has spent the time since then traveling
through space to reach us. By observing this light, astronomers
can see an image of how the universe looked when that light was

Even when observing the distant (early) universe, protoclusters
are rare and difficult to discover. Only about 20 were
previously known. Because distant protoclusters are difficult
to observe directly, are sometimes used as a
proxy. When a large volume of gas falls towards the super
in the center of a
galaxy, it collides with other gas and is heated to extreme
temperatures. This hot gas shines brightly and is known as a
quasar. The thought was that when many galaxies are close
together, a merger, two galaxies colliding and melding
together, would create instabilities and cause gas to fall into
the super massive black hole in one of the galaxies, creating a
quasar. However, this relationship was not confirmed
observationally due to the rarity of both quasars and

In order to understand protoclusters in the distant universe a
larger observational sample was needed. A team including
astronomers from the National Astronomical Observatory of
Japan, the University of Tokyo, the Graduate University for
Advanced Studies, and other institutes is now conducting an
unprecedented wide-field systematic survey of protoclusters
using the Subaru Telescope’s very wide-field camera, Hyper
Suprime-Cam (HSC). By analyzing the data from this survey, the
team has already identified nearly 200 regions where galaxies
are gathering together to form protoclusters in the 12 billion years ago.

Stars indicate quasars and bright (faint) galaxies at the
same epoch are shown as circles (dots). The galaxy
overdensity with respect to the average density is shown by
the contour. The pair members are associated with high
density regions of galaxies. Credit: NAOJ

The team also addressed the relationship between protoclusters
and quasars. The team sampled 151 luminous quasars at the same
epoch as the HSC protoclusters and to their surprise found that
most of those quasars are not close to the overdense regions of
galaxies. In fact, their most luminous quasars even avoid the
densest regions of galaxies. These results suggest that quasars
are not a good proxy for protoclusters and more importantly,
mechanisms other than galactic mergers may be needed to explain
quasar activity. Furthermore, since they did not find many
galaxies near the brightest quasars, that could mean that hard
radiation from a quasar suppresses galaxy formation in its

On the other hand, the team found two “pairs” of quasars
residing in protoclusters. Quasars are rare and pairs of them
are even rarer. The fact that both pairs were associated with
protoclusters suggests that quasar activity is perhaps
synchronous in protocluster environments. “We have succeeded in
discovering a number of protoclusters in the for the first time and have
witnessed the diversity of the quasar environments thanks to
our wide-and-deep observations with HSC,” says the team’s
leader Nobunari Kashikawa (NAOJ).

“HSC observations have enabled us to systematically study
protoclusters for the first time.” says Jun Toshikawa, lead
author of the a paper reporting the discovery of the HSC
protoclusters, “The HSC protoclusters will steadily increase as
the survey proceeds. Thousands of protoclusters located 12
billion light-years away will be found by the time the
observations finish. With those new observations we will
clarify the growth history of protoclusters.”

Explore further:

Subaru telescope discovers the most distant protocluster of

More information: Jun Toshikawa et al. GOLDRUSH. III. A
systematic search for protoclusters at z ∼ 4 based on the
>100 deg2 area, Publications of the Astronomical Society
of Japan
(2017). DOI:

Hisakazu Uchiyama et al. Luminous quasars do not live in the
most overdense regions of galaxies at z ∼ 4, Publications of
the Astronomical Society of Japan
(2017). DOI: 10.1093/pasj/psx112

Masafusa Onoue et al. Enhancement of galaxy overdensity around
quasar pairs at z Publications of the Astronomical Society of
Japan (2017). DOI:

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