Unknown structure in galaxy revealed by high contrast imaging

As a result of achieving high imaging dynamic range, a team of
astronomers in Japan has discovered for the first time a faint radio
emission covering a giant galaxy with an energetic black hole at its
center. The radio emission is released from gas created directly by
the central black hole. The team expects to understand how a black
hole interacts with its host galaxy by applying the same technique
to other quasars (https://bit.ly/3Mnh6mC).
3C273, which lies at a distance of 2.4 billion light-years from
Earth, is a quasar. A quasar is the nucleus of a galaxy believed to
house a massive black hole at its center, which swallows its
surrounding material, giving off enormous radiation. Contrary to
its bland name, 3C273 is the first quasar ever discovered, the
brightest, and the best studied. It is one of the most frequently
observed sources with telescopes because it can be used as a
standard of position in the sky: in other words, 3C273 is a radio
lighthouse.
When you see a car's headlight, the dazzling brightness makes it
challenging to see the darker surroundings. The same thing happens
to telescopes when you observe bright objects. Dynamic range is
the contrast between the most brilliant and darkest tones in an
image. You need a high dynamic range to reveal both the bright and
dark parts in a telescope's single shot. ALMA can regularly attain
imaging dynamic ranges up to around 100, but commercially available
digital cameras would typically have a dynamic range of several
thousands. Radio telescopes aren't very good at seeing objects with
significant contrast.
3C273 has been known for decades as the most famous quasar, but
knowledge has been concentrated on its bright central nuclei, where
most radio waves come from. However, much less has been known
about its host galaxy itself because the combination of the faint and
diffuse galaxy with the 3C273 nucleus required such high dynamic
ranges to detect. The research team used a technique called self
calibration to reduce the leakage of radio waves from 3C273 to the
galaxy, which used 3C273 itself to correct for the effects of Earth's
atmospheric fluctuations on the telescope system. They reached an
imaging dynamic range of 85000, an ALMA record for extragalactic
objects.
As a result of achieving high imaging dynamic range, the team
discovered the faint radio emission extending for tens of thousands
of light-years over the host galaxy of 3C273. Radio emission around
quasars typically suggests synchrotron emission, which comes from
highly energetic events like bursts of star formation or ultra-fast
jets emanating from the central nucleus. A synchrotron jet exists
in 3C273 as well, seen in the lower right of the images. An essential
characteristic of synchrotron emission is its brightness changes with
frequency, but the faint radio emission discovered by the team had
constant brightness irrespective of the radio frequency. After
considering alternative mechanisms, the team found that this faint
and extended radio emission came from hydrogen gas in the galaxy
energized directly by the 3C273 nucleus. This is the first time that
radio waves from such a mechanism are found to extend for tens
of thousands of light-years in the host galaxy of a quasar.
Astronomers had overlooked this phenomenon for decades in this
iconic cosmic lighthouse.
So why is this discovery so important? It has been a big mystery in
galactic astronomy whether the energy from a quasar nucleus can
be strong enough to deprive the galaxy's ability to form stars. The
faint radio emission may help to solve it. Hydrogen gas is an
essential ingredient in creating stars, but if such an intense light
shines on it that the gas is disassembled (ionized), no stars can be
born. To study whether this process is happening around quasars,
astronomers have used optical light emitted by ionized gas. The
problem working with optical light is that cosmic dust absorbs the
light along the way to the telescope, so it is difficult to know how
much light the gas gives off.
Moreover, the mechanism responsible for giving off optical light is
complex, forcing astronomers to make a lot of assumptions. The
radio waves discovered in this study come from the same gas due
to simple processes and are not absorbed by dust. Using radio waves
makes measuring ionized gas created by 3C273's nucleus much
easier. In this study, the astronomers found that at least 7% of the
light from 3C273 was absorbed by gas in the host galaxy, creating
ionized gas amounting to 10–100 billion times the sun's mass.
However, 3C273 had a lot of gas just before the formation of
stars, so as a whole, it didn't look like star formation was strongly
suppressed by the nucleus.
"This discovery provides a new avenue to studying problems
previously tackled using observations by optical light," says Shinya
Komugi, an associate professor at Kogakuin University and lead
author of the study published in The Astrophysical Journal.
"By applying the same technique to other quasars, we expect to
understand how a galaxy evolves through its interaction with the
central nucleus."