Discovery never-before-seen structure in galaxy

Artist’s conception of a giant galaxy with a high-energy jet. Credit: ALMA (ESO/NAOJ/NRAO)

A team of astronomers in Japan has discovered for the first time a faint radio emission covering a giant galaxy with a Black Hole energy at its 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. His study was published in the magazine The Astrophysical Journal.

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Object 3C273, located 2.4 billion light-years from Earth, is a quasar. A quasar is the core of a galaxy believed to harbor a massive black hole at its center, which engulfs the surrounding material, emitting 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 observed sources with telescopes because it can be used as a position pattern in the sky: in other words, 3C273 is a radio beacon.

When we see a car headlight, the dazzling glow makes it difficult to see the darker surroundings. The same thing happens with telescopes when we look at bright objects. Dynamic range is the contrast between the brightest and darkest tones in an image. It takes a high dynamic range to reveal the light and dark parts of a single telescope image. The ALMA radio telescope in the Atacama Desert (Chile) can regularly achieve dynamic imaging ranges of 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 focused on its bright central cores, where most radio waves come from. However, much less is known about its own host galaxy, because the combination of the faint and diffuse galaxy with the core of 3C273 required very high dynamic ranges for detection. The research team used a technique called self-calibration to reduce leakage of radio waves from 3C273 into the galaxy, which used 3C273 itself to correct for the effects of Earth’s atmospheric fluctuations on the telescope system. They achieved an imaging dynamic range of 85,000, an ALMA record for extragalactic objects.

Left: quasar 3C273 observed by the Hubble Space Telescope (left). Excessive brightness results in radial light leaks created by the light scattered by the telescope. In the lower right corner is a high-energy jet released by the gas around the central black hole. Right: Radio image of 3C273 observed by ALMA, showing weak and extended radio emission (in blue-white color) around the nucleus. The bright central font was subtracted from the image. The same jet in the image on the left can be seen in orange. Credit: Komugi et al., Hubble, Nasa/ESA

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As a result of obtaining high dynamic range imaging, the team discovered faint radio emission that spans tens of thousands of light-years over 3C273’s host galaxy. Radio emission around quasars typically suggests synchrotron emission, which comes from highly energetic events such as bursts of star formation or ultrafast jets emanating from the central core. A synchrotron jet also exists on 3C273, seen in the lower right of the images. A key feature of synchrotron emission is that its brightness changes with frequency, but the faint radio emission discovered by the team had a constant brightness regardless of the radio frequency.

After considering alternative mechanisms, the team found that this weak, extended radio emission came from hydrogen gas in the galaxy energized directly by 3C273’s core. This is the first time that radio waves from such a mechanism have spanned tens of thousands of light-years into a quasar’s host galaxy. Astronomers have ignored this phenomenon for decades at this iconic cosmic beacon.

Why is this discovery so important? It has been a great mystery in galactic astronomy whether the energy of a quasar core can be strong enough to deprive the galaxy of its ability to form stars. Poor radio emission can help to solve it. Hydrogen gas is an essential ingredient in the creation of stars, but if such intense light shines on it that the gas is “dismantled” (ionized), no star can be born. To study whether this process is happening around quasars, astronomers 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.

New way

Furthermore, the mechanism responsible for emitting 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 from 3C273 was absorbed by the gas in the host galaxy, creating ionized gas worth 10 to 100 billion times the mass of the Sun. However, 3C273 had a lot of gas just before star formation, so as a whole it did not appear that star formation was strongly suppressed by the core.

“This discovery provides a new avenue to study problems previously addressed using light-optical observations,” said Shinya Komugi, associate professor at Kogakuin University (Japan) and lead author of the study. “By applying the same technique to other quasars, we hope to understand how a galaxy evolves through its interaction with the central core.”