Unknown structure in the galaxy revealed by high contrast images

Unknown structure in the galaxy revealed by high contrast images

Unknown structure in the galaxy revealed by high contrast images

The artist’s impression of a giant galaxy with a high energy beam. Credit: ALMA (ESO / NAOJ / NRAO)

As a result of achieving a highly dynamic imaging range, a team of astronomers in Japan have for the first time discovered a weak radio radiation covering a giant galaxy with an energetic black hole in the 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 technology to other quasars.

3C273, located 2.4 billion light-years from Earth, is a quasar. A quasar is the core of a galaxy that is believed to house a massive black hole in its center, which swallows its surrounding material and emits enormous amounts of radiation. Contrary to its bland name, the 3C273 is the first quasar ever discovered, the brightest and best studied. It is one of the most observed sources with telescopes because it can be used as a standard for position in the sky: in other words, 3C273 is a radio beacon.

When you see a car’s headlights, the dazzling brightness makes it challenging to see the darker surroundings. The same thing happens with telescopes when you observe bright objects. Dynamic range is the contrast between the brightest and darkest tones in an image. You need one high dynamic range to reveal both the light and dark parts of a single shot of a telescope. ALMA can regularly achieve image dynamic ranges up to about 100, but commercially available digital cameras would typically have a dynamic range of several thousand. Radio telescopes are not 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 cores, where most radio waves come from. However, much less has been known about the host galaxy itself because the combination of the weak and diffuse galaxy with the core 3C273 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 the Earth’s atmospheric fluctuations on the telescope system. They reached an imaging dynamic range of 85,000, an ALMA record for extragalactic objects.

Unknown structure in the galaxy revealed by high contrast images

Quasar 3C273 observed by Hubble Space Telescope (HST) (left). The excessive brightness results in radial leaks of light created by light scattered by the telescope. At the bottom right is a high-energy jet that is released by the gas around the central black hole. | Radio image of 3C273 observed by ALMA, showing the faint and increased radio emission (in blue and white) around the core (right). The bright central source has been subtracted from the image. The same beam as the image on the left can be seen in orange. Credit: Komugi et al., NASA / ESA Hubble Space Telescope

As a result of achieving high imaging dynamic rangethe team discovered the fainting radio emissions extends for tens of thousands of light years across the host galaxy 3C273. Radio emission around quasars usually indicates synchrotron emission, which comes from very energetic events such as eruptions of star formation or ultra-fast jets coming from the central core. A synchrotron beam is also found in 3C273, which can be seen in the lower right of the pictures. An essential feature of synchrotron emission is that its brightness changes with frequency, but the weak radio emission detected by the team had constant brightness regardless of radio frequency. After considering alternative mechanisms, the team found that this weak and extended radio radiation came from hydrogen in the galaxy driven directly by the core 3C273. This is the first time that radio waves from such a mechanism have been shown to extend over tens of thousands of light-years in a quasi-host galaxy. Astronomers had overlooked this phenomenon for decades in this iconic cosmic lighthouse.

So why is this discovery so important? It has been a great mystery in galactic astronomy whether the energy from a quasar nucleus can be strong enough to deprive the galaxy of its ability to form stars. The weak radio radiation can help solve it. Hydrogen is an essential ingredient for creating stars, but if such an intense light shines on it that the gas is dismantled (ionized), no stars can be born. Astronomers have used astronomers to study whether this process takes place around quasars optical light emitted by ionized gas. The problem with working with optical light is that cosmic dust absorbs light along the path to the telescope, so it is difficult to know how much light the gas emits.

In addition, the mechanism responsible for emitting optical light is complex, forcing astronomers to make many assumptions. The radio waves detected in this study come from the same gas due to simple processes and are not absorbed by dust. Using radio waves makes it much easier to measure ionized gas created by the core of the 3C273. In this study, astronomers found that at least 7% of the light from 3C273 was absorbed by gas in host galaxy, creates ionized gas that amounts to 10-100 billion times the mass of the sun. However, 3C273 had a lot of gas just before the stars formed, so as a whole it did not look like the star formation was strongly suppressed by the nucleus.

“This discovery provides a new way to study problems previously addressed with optical light observations,” said Shinya Komugi, associate professor at Kogakuin University and lead author of the study, published in The Astrophysical Journal. “By applying the same technology to other quasars, we expect to understand how a galaxy evolves through its interaction with the central core.”

Astronomers discover a new radio source of unknown origin

More information:
Shinya Komugi et al, Detection of extended millimeter emission in the host galaxy for 3C 273 and its implications for QSO feedback via ALMA imaging with high dynamic range, The Astrophysical Journal (2022). DOI: 10.3847 / 1538-4357 / ac616e

Provided by Alma Observatory

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