Starving black hole returns galaxy to the shadows

Sep 16, 2016

For the second time since it was first observed in 1974, a nearby galaxy has mysteriously changed its look – making it one of only very few known cases, and the most dramatic one by far.

Galaxy Mrk 1018, with a supermassive black hole at its centre, had been on the radar of astronomers since the mid 80s when they first noticed that it had brightened so much within five years that it had to be re-classified.

Around 20 of such ‘changing look’ galaxies are known, but there are only two or three others like Mrk 1018 that have changed their look twice.

Originally classified as a dim “Type 2” galaxy, Mrk 1018 became a “Type 1” galaxy and remained bright for three decades until suddenly changing back and returning to the shadows in a process that started a couple of years ago.

This second transition was discovered as part of the “Close Active Galactic Nuclei Reference Survey”, or CARS for short, that used a powerful spectrograph on the Very Large Telescope in Chile.

Recognising the very intriguing behaviour of Mrk 1018, the astronomers were also awarded time to observe the galaxy with other prestigious telescopes at different wavelengths, such as the Chandra X-ray Observatory and Hubble Space Telescope.

“It is very exciting to look at your data and see something you weren’t expecting.” says CAASTRO PhD student Rebecca McElroy at the University of Sydney who made the discovery.

“We have been able to rule out a couple of scenarios but data are still flooding in, so the team is really keen on finding out more about the physics that drive the behaviour of this unusual galaxy”.

Several telescopes have been keeping an eye on Mrk 1018 to test the most plausible scenario – that the central black hole might be starving, drawing in less matter and thus decreasing the brightness of its emission.
One cause of its starvation could just be the black hole running out of fuel and ramping down.

Another cause is somewhat more spectacular though: a second supermassive black hole might have fallen into the centre of the galaxy, disrupting the normal feeding processes of the original galactic black hole.

The CARS team will be using the Very Long Baseline Array – an array of radio telescopes in New Mexico (US) – to get a high resolution picture of the core of Mrk 1018 and search for a potential second black hole.

“We are very lucky to have caught the transition in time to continue observing it with several telescopes, in many different ways with a variety of instruments available for our follow-up observations.” says the CARS principle investigator Dr Bernd Husemann at the European Southern Observatory in Germany.

Since the initial discovery of Mrk 1018’s dimming in 2015, the team has been able to confirm that the galaxy’s brightness is further decreasing.

This research is being published in two companion papers, both as letters in the journal Astronomy & Astrophysics, this week.

CAASTRO is a collaboration of The University of Sydney, The Australian National University, The University of Melbourne, Swinburne University of Technology, The University of Queensland, The University of Western Australia and Curtin University, the latter two participating together as the International Centre for Radio Astronomy Research (ICRAR). CAASTRO is funded under the Australian Research Council (ARC) Centre of Excellence program, with additional funding from the seven participating universities and from the NSW State Government’s Science Leveraging Fund.

Publication details:

McElroy et al. (A&A Letters 2016): “The Close AGN Reference Survey: Mrk 1018’s return to the shadows after 30 years as a Seyfert 1” and Husemann et al. (A&A Letters 2016): “The Close AGN Reference Survey (CARS): What is causing Mrk 1018’s return to the shadows after 30 years?

Contact details:

Rebecca McElroy
CAASTRO, University of Sydney
Mobile: +61 421 882 513 | Twitter: @re_mcelroy

Dr Bernd Husemann
European Southern Observatory, Germany
Phone: +49 89 3200 6750

Dr Wiebke Ebeling
CAASTRO, media contact
Phone: +61 8 9266 9174 | Mobile: +61 423 933 444