Artistic impression of a giant galaxy with a high energy jet. Credit: ALMA (ESO / NAOJ / NRAO)
Astronomers have just discovered an unknown structure in a galaxy that had been hiding in the “shadows”. They did this by expanding the dynamic range of the Atacama Large Millimeter / Submillimeter Array (ALMA), the largest astronomical project in existence, to detect weak radio emissions.
This faint radio emission, which has a constant brightness regardless of radio frequency, extends for tens of thousands of light-years across the host galaxy of the 3C 273 quasar, an iconic cosmic beacon. This discovery may help unlock secrets of galaxy evolution and star formation.
As a result of achieving a high dynamic range image, a team of astronomers from Japan has discovered for the first time a faint radio emission covering a giant galaxy with an energetic black hole in the center. The radio emission is released from the gas created directly by the central black hole. The team hopes to understand how a black hole interacts with its host galaxy by applying the same technique to other quasars.
3C 273, located 2.4 billion light-years from Earth, is a quasar. A quasar is the core of a galaxy that is believed to harbor a massive black hole in its center, which swallows the surrounding material, emitting enormous radiation. Contrary to its soft name, 3C273 is the first quasar ever discovered, the brightest and most 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: that is, 3C273 is a radio beacon.
Bright Quasar 3C 273 The first quasar to be identified, 3C 273, was discovered by astronomer Allan Sandage in the early 1960s. Despite being located about 2.4 billion light-years away in a galaxy on giant elliptical in the constellation of Virgo, is the brightest optically quasar in the sky from Earth.
When you see the headlight of a car, the dazzling glow makes it difficult to see the darker surroundings. The same goes for telescopes when you look at bright objects. Dynamic range is the contrast between the brightest and darkest tones in an image. You need a high dynamic range to reveal both bright and dark parts in a single shot from a telescope. ALMA can regularly reach dynamic image ranges of about 100, but commercially available digital cameras would normally 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 focused on its bright central nuclei, where most radio waves come from. However, much less is known about its own host galaxy because the combination of the faint, fuzzy galaxy with the 3C273 core required such high dynamic ranges to detect them. The research team used a technique called autocalibration to reduce the leakage of radio waves from the 3C273 into the galaxy, which used the same 3C273 to correct for the effects of the Earth’s atmospheric fluctuations on the telescope’s system. They reached a dynamic image range of 85,000, an ALMA record for extragalactic objects.
Quasar 3C273 observed by the Hubble Space Telescope (HST) (left). Excessive brightness results in radial light leakage created by light scattered by the telescope. At the bottom right is a high-energy jet released by the gas around the central black hole. | 3C273 radio image observed by ALMA, showing faint and extended radio emission (blue-white) around the core (right). The bright central source has been subtracted from the image. The same jet 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 a high dynamic range image, the team discovered the faint radio emission that stretched for tens of thousands of light years over the host galaxy of 3C273. The emission of radio around quasars typically suggests the emission of synchrotron, which comes from high-energy events such as star-forming bursts or ultra-fast jets emanating from the central core. There is also a synchrotron beam at 3C273, seen at the bottom right of the images.
An essential feature of synchrotron emission is that its brightness changes with frequency, but the faint radio emission discovered by the equipment had a constant brightness regardless of the radio frequency. After considering alternative mechanisms, the team found that this faint, extended radio emission came from the hydrogen gas in the galaxy fed directly by the 3C 273 nucleus. This is the first time that radio waves from a ‘This type spread over tens of thousands of light years in a galaxy’s 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 of a quasar nucleus can be strong enough to deprive the galaxy of its ability to form stars. Weak radio broadcasting can help resolve this issue. Hydrogen gas is an essential ingredient for the creation of stars, but if a light shines so brightly that the gas is disassembled (ionized), no star can be born. To study whether this process is happening around quasars, astronomers have used optical light emitted by ionized gas. The problem with working with optical light is that cosmic dust absorbs light along the way to the telescope, so it’s hard to know how much light the gas emits.
In addition, the mechanism responsible for the emission of optical light is complex, forcing astronomers to make many assumptions. The radio waves discovered in this study come from the same gas due to simple processes and are not absorbed by dust. The use of radio waves makes it much easier to measure the ionized gas created by the 3C273 core. In this study, astronomers found that at least 7% of the light in 3C 273 was absorbed by the gas in the host galaxy, creating an ionized gas between 10 and 100 billion times the mass of the sun. However, 3C 273 had a lot of gas just before star formation, so overall it did not appear that star formation was strongly suppressed by the nucleus.
“This finding provides a new way to study problems addressed earlier through optical light observations,” says Shinya Komugi, an associate professor at Kogakuin University and lead author of the study published in the Astrophysical Journal. “Applying the same technique to other quasars, we hope to understand how a galaxy evolves through its interaction with the core.”
Reference: “Extended millimeter emission detection in the host galaxy of 3C273 and its implications for QSO feedback using high dynamic range ALMA images” by Shinya Komugi, Yoshiki Toba, Yoshiki Matsuoka, Toshiki Saito and Takuji Yamashita , April 28, 2022, The Astrophysical Journal .DOI: 10.3847 / 1538-4357 / ac616e
The team consists of Shinya Komugi (Kogakuin University), Yoshiki Toba (Japan National Astronomical Observatory). [NAOJ]), Yoshiki Matsuoka (Ehime University), Toshiki Saito (NAOJ) and Takuji Yamashita (NAOJ).
This research was supported by JSPS KAKENHI Grant Numbers JP20K04015, JP21K13968 and JP19K14759.