High-contrast image reveals unknown structure in galaxy

(ORDO NEWS) — As a result of achieving a high dynamic range of imaging, a team of astronomers from Japan for the first time detected weak radio emission engulfing a giant galaxy with an energetic black hole at the center. The radio emission comes from gas created directly by the central black hole.

3C273, 2.4 billion light-years from Earth, is a quasar. A quasar is the core of a galaxy, at the center of which is believed to be a massive black hole that absorbs the surrounding matter and emits huge radiation.

Contrary to its modest name, 3C273 is the first quasar ever discovered, the brightest and most studied. It is one of the most commonly observed sources with telescopes because it can be used as a standard for sky position: in other words, 3C273 is a radio beacon.

When you see the headlights of a car, the blinding brightness makes it difficult 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 high dynamic range so that both bright and dark details can be seen in the same telescope image. ALMA can regularly achieve an image dynamic range of up to about 100, but commercially available digital cameras typically have a dynamic range of several thousand. Radio telescopes are not very good at seeing objects with significant contrast.

Quasar 3C273 has been known for decades as the most famous quasar, but all knowledge has been focused on its bright central core, where most of the radio waves come from.

However, much less was known about the host galaxy itself, since the combination of a dim and diffuse galaxy with the core of 3C273 requires a very high dynamic range for detection.

To reduce the leakage of radio waves from 3C273 into the galaxy, the research team used a technique called self-calibration. They achieved an image dynamic range of 85,000, which is an ALMA record for an extragalactic object.

As a result of achieving a high dynamic range image, the team detected a faint radio emission extending tens of thousands of light-years above the host galaxy 3C273.

The radio emission around quasars is usually indicative of synchrotron radiation, which comes from high-energy events such as bursts of star formation or ultrafast jets emanating from the central core. The synchrotron jet also exists in 3C273 and is visible in the lower right corner of the image.

An important characteristic of synchrotron radiation is that its brightness varies with frequency, but the weak radio emission detected by the team has a constant brightness regardless of the frequency of the radio emission.

After considering alternative mechanisms, the team concluded that that this weak and extended radio emission comes from hydrogen gas in the galaxy, receiving energy directly from the nucleus of 3C273.

This is the first time that radio waves from such a mechanism have traveled tens of thousands of light-years in a quasar’s host galaxy. Astronomers have not noticed such a phenomenon in this legendary space beacon for decades.

Why is this discovery so important? It remains a big mystery in galactic astronomy whether the energy of a quasar’s core can be strong enough to deprive a galaxy of its ability to form stars. Weak radio emission can help solve this problem.

Hydrogen gas is a necessary ingredient to create stars, but if such intense light shines on it that the gas is disassembled (ionized), stars cannot be born.

To study whether this process occurs around quasars, astronomers used optical light emitted by ionized gas. The problem with optical light is that cosmic dust absorbs light on its way to the telescope, so it’s hard to tell how much light the gas is emitting.

In addition, the mechanism responsible for the emission of optical light is complex, leading astronomers to make many assumptions. The radio waves found in this study come from the same gas through simple processes and are not absorbed by the dust.

The use of radio waves greatly simplifies the measurement of the ionized gas created by the 3C273 nucleus. In this study, astronomers found that at least 7% of the light from 3C273 is absorbed by gas in the host galaxy, creating ionized gas 10 to 100 billion times the mass of the sun.

However, 3C273 had a lot of gas just before star formation, so it does not appear that star formation was strongly suppressed by the core in general.

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