High Contrast Imaging Reveals Unknown Structure in Galaxy

Artist’s 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 is hidden in the “shadows”. They accomplished this by extending the dynamic range of the Atacama Large Millimeter/Submm Matrix (Alma), the largest astronomical project in existence, in order to detect faint radio emission.

This faint radio emission, which is of constant brightness regardless of radio frequency, extends for tens of thousands of light-years across the host galaxy of quasar 3C 273, a famous cosmic beacon. This discovery may help reveal the secrets of the evolution of galaxies and the formation of stars.

As a result of achieving high dynamic range imaging, a team of astronomers in Japan has detected for the first time a faint radio emission covering a giant galaxy with active energy. Black hole in its centre. The radio emission from gas is emitted directly from the central black hole. The team expects to understand how the black hole interacts with its host galaxy by applying the same technology to other quasars.

3C 273, which is located at a distance of 2.4 billion light-years from Earth, is a quasar. A quasar is the nucleus of a galaxy believed to harbor a massive mass Black hole In its center, which swallows the surrounding material, releasing enormous radiation. Contrary to its lighter name, 3C273 is the first quasar ever discovered, the brightest and the best studied. It is one of the most frequently observed sources with telescopes because it can be used as a benchmark for location in the sky: in other words, 3C273 is a radio beacon.

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 they observe bright objects. Dynamic range is the contrast between the brightest and darkest colors in an image. You need a high dynamic range to detect the bright and dark parts of a telescope in a single shot. ALMA can regularly have dynamic imaging ranges of about 100, but commercially available digital cameras will usually have a dynamic range of several thousand. Radio telescopes are not very good at seeing things with great contrast.

3C273 has been known for decades as the most famous quasar, but knowledge has centered on its bright central core, where most of its radio waves come from. However, little is known about its host galaxy itself because the combination of the faint, diffuse galaxy with the nucleus 3C273 requires such high dynamic ranges to be detected. The research team used a technique called self-calibration to reduce radio wave leakage from 3C273 to galaxy, which used 3C273 itself to correct the effects of fluctuations in Earth’s atmosphere on the telescope system. They reached an imaging dynamic range of 85,000, which is the ALMA record for extragalactic objects.

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

As a result of achieving the high dynamic range imaging, the team detected a faint radio emission spanning tens of thousands of light-years over the host galaxy 3C273. Radio emissions around quasars usually suggest synchrotron emission, which comes from highly energetic events such as star-forming bursts or ultrafast jets emanating from the central core. A synchrotron jet is also present in 3C273, seen in the lower right of the images.

The primary characteristic of a synchrotron emission is its brightness changes with frequency, but the faint radio emission the team detected had a constant brightness regardless of radio frequency. After considering alternative mechanisms, the team found that this faint, extended radio emission came from hydrogen gas in the galaxy that is directly activated by a 3C 273 nucleus. This is the first time radio waves from such a mechanism have spanned tens of thousands of light-years in the host galaxy. for Quasar. Astronomers have ignored this phenomenon for decades at this famous cosmic lighthouse.

So why is this discovery so important? It has been a great mystery in galactic astronomy whether the energy from a quasar core could be strong enough to deny a galaxy the ability to form stars. A faint radio emission may help resolve it. Hydrogen gas is an essential component of star formation, but if it is shone so brightly that the gas has been dissociated (ionized), no stars can be born. To study whether this process occurs around quasars, astronomers have used optical light emitted by ionized gas. The problem with working with optical light is that cosmic dust absorbs light all the way into the telescope, so it’s hard to know how much light the gas emits.

Moreover, the mechanism responsible for giving off 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. The use of radio waves makes it much easier to measure the ionized gas produced by the 3C273 core. In this study, astronomers found that at least 7% of the light from 3C 273 was absorbed by gas in the host galaxy, producing ionized gas with a mass 10-100 billion times the mass of the Sun. However, 3C 273 had a lot of gas just before star formation, so as a whole, star formation does not appear to have been strongly suppressed by the core.

“This discovery provides a new avenue for studying problems previously addressed using observations with optical light,” says Shinya Komoji, assistant 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 the galaxy evolves through its interaction with the central core.”

Reference: “Detection of Millimeter-Extended Emissions in the Host Galaxy of 3C273 and Its Effects on QSO Feedback via High Dynamic Range for ALMA Imaging” By Shinya Komoji, Yoshiki Toba, Yoshiki Matsuoka, Toshiki Saito, and Takuji Yamashita, 28 Apr 2022, Available here. Astrophysical Journal.
DOI: 10.3847 / 1538-4357 / ac616e

The team consists of Shinya Komugi (Kogakuin University) and Yoshiki Toba (National Astronomical Observatory of Japan). [NAOJ]), Yoshiki Matsuoka (Ehime University), Toshiki Saito (NAOJ), and Takuji Yamashita (NAOJ).

This search was supported by JSPS KAKENHI Grant Numbers JP20K04015, JP21K13968, and JP19K14759.