Starving black hole returns brilliant galaxy to the shadows
Many galaxies are found to have an extremely bright core powered by a supermassive black hole. These cores make "active galaxies" some of the brightest objects in the Universe. They are thought to shine so brightly because hot material is glowing fiercely as it falls into the black hole, a process known as accretion. This brilliant light can vary hugely between different active galaxies, so astronomers classify them into several types based on the properties of the light they emit .
The discovery of Markarian 1018's fickle nature was a chance by-product of the Close AGN Reference Survey (CARS) , a collaborative project between ESO and other organisations to gather information on 40 nearby galaxies with active cores. Routine observations of Markarian 1018 with the Multi-Unit Spectroscopic Explorer (MUSE) installed on ESO's Very Large Telescope revealed the surprising change in the light output of the galaxy.
"We were stunned to see such a rare and dramatic change in Markarian 1018," said Rebecca McElroy, lead author of the discovery paper and a PhD student at the University of Sydney and the ARC Centre of Excellence for All Sky Astrophysics (CAASTRO).
The chance observation of the galaxy so soon after it began to fade was an unexpected opportunity to learn what makes these galaxies tick, as Bernd Husemann, CARS project leader and lead author of one of two papers associated with the discovery, explained: "We were lucky that we detected the event just 3-4 years after the decline started so we could begin monitoring campaigns to study details of the accretion physics of active galaxies that cannot be studied otherwise."
The research team made the most of this opportunity, making it their first priority to pinpoint the process causing Markarian 1018's brightness to change so wildly. This could have been caused by any one of a number of astrophysical events, but they could rule out the black hole pulling in and consuming a single star  and cast doubt on the possibility of obscuration by intervening gas . But the true mechanism responsible for Markarian 1018's surprising variation remained a mystery after the first round of observations.
However, the team were able to gather extra data after they were awarded observing time to use the NASA/ESA Hubble Space Telescope, and NASA's Chandra X-ray Observatory. With the new data from this suite of instruments they were able to solve the mystery -- the black hole was slowly fading because it was being starved of accretion material.
"It's possible that this starvation is because the inflow of fuel is being disrupted," said Rebecca McElroy. "An intriguing possibility is that this could be due to interactions with a second supermassive black hole." Such a black hole binary system is a distinct possibility in Markarian 1018, as the galaxy is the product of a major merger of two galaxies -- each of which likely contained a supermassive black hole in its centre.
Research continues into the mechanisms at work in active galaxies such as Markarian 1018 that change their appearance. "The team had to work fast to determine what was causing Markarian 1018's return to the shadows," comments Bernd Husemann. "Ongoing monitoring campaigns with ESO telescopes and other facilities will allow us to explore the exciting world of starving black holes and changing active galaxies in more detail."
 The brightest of the active galaxies are quasars, where the brilliant nucleus outshines the rest of the galaxy. Another, less extreme, class are known as Seyfert galaxies. Originally a method was developed that used brightness and the emission spectrum -- the plot of the strength of radiation emitted at different wavelength -- to distinguish between just two types of Seyfert galaxies, Type 1 and Type 2, but extra classifications such as Type 1.9 Seyferts have since been introduced.
 Such a tidal disruption event occurs when a star strays too close to a supermassive black hole and is torn apart by the extreme gravitational tidal force. This results in a sharp rise in the brightness of the central region that slowly declines over a period of years. The observed brightness variations of Markarian 1018 were found not to match the profile of such an event.
 Gas obscuration can affect the classification of an active galaxy by blocking the line of sight, drifting in front of the galaxy's bright core like fog in front of a car's headlights, and dimming the light passing through. This also affects the spectrum of galaxy, perhaps changing its classification.
This research was presented in two papers entitled “Mrk 1018 returns to the shadows after 30 years as a Seyfert 1”, and “What is causing Mrk 1018’s return to the shadows after 30 years?”, both to appear as Letters in the journal Astronomy & Astrophysics.
Source: European Southern Observatory - ESO [September 16, 2016]