It is not often that astronomers can see large changes in the brightness of a galaxy within a few years. Most galaxies change in brightness (and other properties) over millions or billions of years. So, when images of the distant 10-billion-light-year galaxy J0218-0036 showed that it dimmed to a twentieth of its previous brightness in just 20 years, observers were shocked. What would make it do that? That is not “normal” for AGN.
Astronomers from the Chiba Institute of Technology, the University of Potsdam (Germany), the University of Toyama, the Instituto de Astrofísica de Canarias (Spain), the National Astronomical Observatory of Japan, and Ritsumeikan University observed this eclipse when they compared images from the Sloan Digital Sky Survey with images and data taken by the Suprime-Caruba Hyper Suprime Suprime-Telescope. They found a decrease in the brightness of the central active galactic nucleus (AGN) of the group of stars in the period from the 2000s to 2023. That is not a normal period for AGN dimming. These objects usually vary in brightness by only 30 percent over long periods of time. That rapid rate raises many questions about what is happening in J0218-0036.
Understanding AGN and How They Change
A working star bug is exactly what the name implies: something at the heart of a galaxy. It is usually a supermassive black hole packed with enough mass to make anywhere from a hundred thousand to several billion times the mass of the Sun. The strong gravity of a black hole is enough to absorb enough material in the form of gas and dust to feed itself for a very long time. As the material spirals into the black hole in the accretion disk, it heats up due to friction. That raises the temperature and causes the material to emit more energy. That action causes the AGN to emit radiation across a wide range of wavelengths. Considering the endless supply of material for a black hole to feed on, the radiation changes little as it pulls in various gases and dust from its surroundings. If the accretion disk provides less material because the gas flow decreases, then the radiation it provides will decrease and affect the AGN brightness.
Hubble Space Telescope view of the galaxy Markarian 509 with its bright AGN at its heart. In addition to J0218-0036, other galaxies have these objects, too. Credit: NASA, ESA, J. Kriss (STScI) and J. de Plaa (SRON)
The brightness of J0218-0036 that astronomers observed included the brightness of the AGN region and the brightness of the entire galaxy. Long-term observations of the galaxy have given astronomers the opportunity to separate the AGN light from the light of the galaxy. They found that the activity of a supermassive black hole, especially J0218-0036, can change the brightness in a short period of time. It’s interesting, since conventional wisdom has suggested that AGN luminosity varies very slowly, perhaps over tens of thousands of years.
According to Tomoki Morokuma, who led the observations and subsequent studies of this AGN work, this discovery changes everything. “It is interesting that the active core can change its brightness in a very short period of time, and that this extinction appears to be caused by a large change in the accretion rate of the supermassive black hole,” said Morokuma. “Using data from extensive surveys, such as those from Hyper Suprime-Cam, we hope to find more objects like this and learn how the activity of super-massive holes closes and restarts.”
So, Is J0218-0036’s AGN Running Out of Fuel?
The team’s findings raise many questions about what causes the rapid dimming of this AGN galaxy. In this case, it seems that the activity of the central black hole is greatly reduced. The surprising explanation is that the supermassive black hole is running out of fuel. The research team compared observations of the galaxy’s core at infrared and optical wavelengths. They compared their findings with models from the AGN study. The team observed that the speed at which the black hole consumes gas has dropped to one-fiftieth of the original state. That happened very quickly, over a period of 7 years. It’s a big drop and points to a black hole that is rapidly losing food.
Bright images of the galaxy J0218−0036 (redshift 1.8; distance 10 billion light years), shown by yellow arrows. The image on the left is taken by the Sloan Digital Sky Survey (SDSS), and the image on the right is the Hyper Suprime-Cam (HSC) on the Subaru Telescope. Comparing the brightness of these two images shows that the galaxy lost a lot between the beginning in 2002. The HSC image shows it as it was in 2018. (Credit: SDSS, HSC-SSP/NAOJ)
While this is a good explanation, scientists need to rule out other possibilities, such as dust clouds blocking light from the AGN. However, remember that the light from AGN is seen across most of the electromagnetic spectrum. A cloud of dust couldn’t stop it all. So, that means that the changes come from inside the AGN and the accretion disk feeds the black hole.
So the question remains: what causes a supermassive black hole to lose its food source? It is known that these black holes can affect their galaxies, for example, by extinguishing the formation of stars as they eject nearby objects. But, what would cut off the material that feeds the accretion disk around the black hole? That’s what astronomers will focus on in the future as they conduct more multiwavelength surveys of J0218-0036 and other AGN in the Universe.
For more information
A Galaxy Far Far Away Has Completed 20 Times in Twenty Years
A Probable Mass-Closing Event in a Strong Galactic Nucleus at z ~ 1.8
#Black #Hole #Runs #Gas #Rapidly #Shrinking #Galaxy