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 Universe, galaxies 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
galaxy clusters 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 emitted.
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, quasars
are sometimes used as a proxy. When a large volume of gas falls towards
the super massive black hole 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 protoclusters.
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 early Universe 12 billion years
ago.
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 vicinity.
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 distant
Universe 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."
These results were published on January 1, 2018 in the HSC special issue of the Publications of the Astronomical Society of Japan (Toshikawa et al. 2018, "GOLDRUSH. III. A Systematic Search of Protoclusters at z~4 Based on the >100 deg2 Area", PASJ, 70, S12; Uchiyama et al. 2018, "Luminous Quasars Do Not Live in the Most Overdense Regions of Galaxies at z~4", PASJ, 70, S32; Onoue et al. 2018, "Enhancement of Galaxy Overdensity around Quasar Pairs at z less than 3.6 based on the Hyper Suprime-Cam Subaru Strategic Program Survey",
PASJ, 70, S31). These projects are supported by Grants-In-Aid
JP15H03645, JP15K17617, and JP15J02115.
