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 — quasar 3C 273. The radio emission is released from gas that is created directly by the central black hole. The result was obtained through high dynamic range imaging. The team expects to understand how a black hole interacts with its host galaxy by applying the same technique to other quasars.
3C 273, 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. Black holes swallow their surrounding gas, but the surrounding gas on the brink of being eaten by the black hole gives off an enormous amount of radiation. This makes quasars one of the brightest objects in the sky. Contrary to its bland name, quasar 3C 273 is the very first quasar ever discovered, the brightest, and the best studied. As such, 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, quasar 3C 273 is a radio lighthouse.
When you see the headlight of a car, the dazzling brightness makes it difficult to see the darker surroundings. The same thing happens to telescopes when you observe bright objects. The contrast between the brightest and darkest tones in an image is called dynamic range. To reveal both the bright and dark parts in a telescope’s single shot, you need high dynamic range. ALMA can regularly attain imaging dynamic ranges up to around 100, but commercially available digital cameras would normally have a dynamic range of several thousands. Radio telescopes aren’t very good at seeing objects with large contrast.
Quasar 3C 273 is the most famous quasar
Quasar 3C 273 has been known for decades as the most famous quasar, but knowledge has been concentrated on its bright central nuclei where most of the radio waves come from. Much less has been known about its host galaxy itself, however, because the combination of the faint and diffuse galaxy with quasar 3C 273’s 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 quasar 3C 273 to the galaxy image, which used quasar 3C 273 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.
![Quasar 3C273 observed by the Hubble Space Telescope (HST) (left) [1] [2] . The exceeding brightness results in radial leaks of light, created by light scattered by the telescope. At the lower right is a high-energy jet released by the gas around the central black hole. Radio image of quasar 3C273 observed by ALMA, showing the faint and extended radio emission (in blue-white color) around the nucleus (right).](https://vtwp-media.s3-accelerate.amazonaws.com/2022/06/komugi2022_image2_EN-668x317-1.jpg)
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 quasar 3C 273. Radio emission around quasars normally suggests that it is 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 quasar 3C 273 as well, seen in the lower right of the images. An important characteristic of synchrotron emission is that 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 that was energized directly by quasar 3C 273’s 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 to form stars, but if such a strong 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 gave 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 it that much easier to measure ionized gas created by quasar 3C 273’s nucleus. In this study, the astronomers found that at least 7 percent of the light from quasar 3C 273 was absorbed by gas in the host galaxy, creating ionized gas amounting to 10-100 billion times the mass of the sun. However, quasar 3C 273 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. Shinya Komugi, an associate professor at Kogakuin University and lead author of the study published in the Astrophysical Journal, said:
“This discovery provides a new avenue to studying problems previously tackled using observations by optical light. By applying the same technique to other quasars, we expect to understand how a galaxy evolves through its interaction with the central nucleus.”
Provided by Alma Observatory [Note: Materials may be edited for content and length.]
