Figure 1: The deep image of Messier 77 taken with the Hyper Suprime-Cam (HSC) mounted at the Subaru Telescope. The picture is created by adding the color information from the Sloan Digital Sky Survey (Note 1) to the monochromatic image acquired by the HSC. (Credit: NAOJ/SDSS/David Hogg/Michael Blanton. Image Processing: Ichi Tanaka)
The galaxy Messier 77 (M77) is famous for its super-active nucleus
that releases enormous energy across the electromagnetic spectrum,
ranging from x-ray to radio wavelengths. Yet, despite its highly active
core, the galaxy looks like any normal quiet spiral. There's no visual
sign of what is causing its central region to radiate so extensively. It
has long been a mystery why only the center of M77 is so active.
Astronomers suspect a long-ago event involving a sinking black hole,
which could have kicked the core into high gear.
To test their ideas about why the central region of M77 beams massive
amounts of radiation, a team of researchers at the National
Astronomical Observatory of Japan and the Open University of Japan used
the Subaru Telescope to study M77. The unprecedented deep image of the
galaxy reveals evidence of a hidden minor merger billions of years ago.
The discovery gives crucial evidence for the minor merger origin of
active galactic nuclei.
The Mystery of Seyfert Galaxies
The galaxy Messier 77 (NGC 1068) is famous for harboring an active
nucleus at its core that releases an enormous amount of energy. The
existence of such active galaxies in the nearby universe was first noted
by the American astronomer Carl Seyfert more than 70 years ago.
Nowadays they are called the Seyfert galaxies (Note 2).
Astronomers think that the source of such powerful activity is the
gravitational energy released from superheated matter falling onto a
supermassive black hole (SMBH) that resides in the center of the host
galaxy. The estimated mass of such a SMBH for M77 is about 10 million
times that of the Sun.
It takes a massive amount of gas dumped on the galaxy's central black
hole to create such strong energies. That may sound like an easy task,
but it's actually very difficult. The gas in the galactic disk will
circulate faster and faster as it spirals into the vicinity of the SMBH.
Then, at some point the "centrifugal force" balances with the
gravitational pull of the SMBH. That actually prevents the gas from
falling into the center. The situation is similar to water draining out
of a bathtub. Due to the centrifugal force, the rapidly rotating water
will not drain out rapidly. So, how can the angular momentum be removed
from the gas circling near an active galactic nucleus? Finding the
answer to that question is one of the big challenges for researchers
today.
A Prediction Posed 18 Years Ago
In 1999, Professor Yoshiaki Taniguchi (currently at the Open
University of Japan), the team leader of the current Subaru study,
published a paper about the driving mechanism of the active nucleus of
Seyfert galaxies such as M 77. He pointed out that a past event – a
"minor merger" where the host galaxy ate up its "satellite" galaxy (a
small low-mass galaxy orbiting it) – would be the key to activating the
Seyfert nucleus (Note 3).
Usually, a minor merger event simply breaks up a low-mass satellite
galaxy. The resulting debris is absorbed into the disk of the more
massive host galaxy before it approaches the center. Therefore, it was
not considered as the main driver of the nuclear activity. "However, the
situation could be totally different if the satellite galaxy has a
(smaller) SMBH in its center (Note 4),"
Professor Taniguchi suggests, "because the black hole can never be
broken apart. If it exists, it should eventually sink into the center of
the host galaxy."
The sinking SMBH from the satellite galaxy would eventually create a
disturbance in the rotating gas disk around the main galaxy's SMBH.
Then, the disturbed gas would eventually rush into the central SMBH
while releasing enormous gravitational energy. "This must be the main
ignition mechanism of the active Seyfert nuclei," Taniguchi argued. "The
idea can naturally explain the mystery about the morphology of the
Seyfert galaxies," said Professor Taniguchi, pointing out the advantage
of the model of normal-looking galaxies also being very active at their
cores. (Note 5).
Probing the Theory Using the Subaru Telescope
Recent advances in observational technique allow the detection of the
extremely faint structure around galaxies, such as loops or debris that
are likely made by dynamical interactions with satellite galaxies.. The
outermost parts of galaxies are often considered as relatively "quiet"
with a longer dynamical timescale than anywhere inside. Simulations show
that the faint signature of a past minor merger can remain several
billion years after the event. "Such a signature can be a key test for
our minor merger hypothesis for Seyfert galaxies. Now it is time to
revisit M77," said Taniguchi.
The team's choice to look for 'the past case' was, of course, the
Subaru Telescope and its powerful imaging camera, Hyper Suprime-Cam. The
observing proposal was accepted and executed on Christmas night 2016.
"The data was just amazing," said Dr. Ichi Tanaka, the primary
investigator of the project. "Luckily, we could also retrieve the other
data that was taken in the past and just released from the Subaru
Telescope's data archive. Thus, the combined data we got finally is
unprecedentedly deep."
Figure 2: (Left) The newly-discovered,
extremely diffuse structures around M77. The innermost color part of the
picture shows the bright part of the galaxy (from SDSS: see the center
of Figure 1).
The middle part in red-brown is the contrast-enhanced expression of the
faint one-arm structure (labeled as "Banana") to the right, as well as
the ripple structure (labeled as "Ripple") to the left. All the
fore/background objects unrelated to M77 are removed during the process.
The outermost monochrome part shows the faint ultra-diffuse structures
in yellow circles (labelled as "UDO-SE", "UDO-NE", "UDO-SW"). A deep
look at them indicates the latter two ("UDO-NE", "UDO-SW") constitute a
part of the large loop-like structure. (Credit: NAOJ)
(Right) Artist's impression of M77. The illustration in the right is created and copyrighted by Mr. Akihiro Ikeshita. (Credit: Akihiro Ikeshita
Subaru's great photon-collecting power and the superb performance of
the Hyper Suprime-Cam were crucial in the discovery of the extremely
faint structures in M77. Their discovery reveals the normal-looking
galaxy's hidden violent past.. "Though people may sometimes make a lie,
galaxies never do. The important thing is to listen to their small
voices to understand the galaxies," said Professor Taniguchi.
The team will expand its study to more Seyfert galaxies using the
Subaru Telescope. Dr. Masafumi Yagi, who leads the next phase of the
project said, "We will discover more and more evidences of the satellite
merger around Seyfert host galaxies. We expect that the project can
provide a critical piece for the unified picture for the triggering
mechanism for active galactic nuclei."
The result is going to be published in the Volume 69 Issue 6 of the
Publications of the Astronomical Society of Japan (I. Tanaka, M.Yagi
& Y. Taniguchi 2017, "Morphological evidence for a past minor merger in the Seyfert galaxy NGC 1068").
The research is financially supported by the Basic Research A grant
JP16H02166 by the Grant-in-Aid for Scientific Research progrram.
Notes
Note1: The color image by the Sloan Digital Sky Survey used for Figure 1 is under the copyright of David W. Hogg and Michael R. Blanton.
Note 2: Seyfert galaxies are actually a subclass of the active
galactic nuclei. There are even more powerful active galactic nuclei
called quasar in the universe. Usually quasars are found much farther
away than M77.
Note 3: Satellite galaxies are common for large galaxies. For
example, there are two bright satellite galaxies called Large and Small
Magellanic Clouds associated with our Milky Way. The Andromeda galaxy,
our nearest neighbor, also has two bright satellites called Messier 32 and NGC 205.
Note 4: Astronomers believe that most galaxies have an SMBH in
their central regions, with its mass mysteriously scaled to the mass of
the host galaxy. It is also known that some satellite galaxies also
have smaller SMBH. For example, Messier 32 (satellite of the Andromeda
galaxy) is likely to have a SMBH much heavier than a million times the
mass of our Sun. It is however not easy to directly prove the existence
of the SMBH for satellite galaxies due to its light weight.
Note 5: Y. Taniguchi 1999, ApJ, 524, 65, for the reference.
The research team:
- Ichi Tanaka: Subaru Telescope, National Astronomical Observatory of Japan
- Masafumi Yagi: National Astronomical Observatory of Japan
- Yoshiaki Taniguchi: The Open University of Japan
