ACTIVE GALACTIC NUCLEI

Galaxy Messier 77 Hiding A Supermassive Black Hole

The European Southern Observatory’s Very Large Telescope Interferometer (ESO’s VLTI) observed a cloud of cosmic dust at the centre of the galaxy Messier 77 that is hiding a supermassive black hole. These new observations confirmed predictions made around three decades ago and are now giving new insight into “active galactic nuclei” or AGNs. Observations showing a roughly dough-nut-shaped cloud of cosmic dust and gas shrouding a huge black hole at the heart of a galaxy similar in size to our Milky Way are providing scientists with new clarity about the universe's most energetic objects.

At the center of some galaxies, strong energy sources called active galactic nuclei (AGN) can be found that are powered by supermassive black holes which feed on large amounts of cosmic dust and gas. This process released huge amounts of energy that often outshine the stars in the galaxy. There are different types of AGNs. Some release bursts of radio waves, some shine brightly and some, like the Messier 77 AGN, are more subdued. Some 50 years ago, astronomers formulated a theory that the common factor between all AGNs is that they are powered by supermassive black holes. This theory is known as the Unified Model theory. The theory also states that the visual differences between AGNs stem from the position of the black holes relative to them as seen from Earth. If the AGN obscures the black hole from the viewpoint of Earth, then the black hole will be entirely hidden, as is the case with Messier 77.

Messier 77, also called NGC 1068 or the Squid Galaxy, is located 47 million light-years – the distance light travels in a year, 9.5 trillion kilometers (5.9 trillion miles) – from Earth in the constellation Cetus. Its supermassive black hole has a mass roughly 10 million times greater than our sun. The observations, using the European Southern Observatory's (ESO) Very Large Telescope in Chile's Atacama Desert, provided strong support for what is called the "unified model" of active galactic nuclei. This model holds that all active galactic nuclei are basically the same but that some appear from the vantage point of Earth to have different properties. Some look intensely bright because the position of their ring-like cloud does not obscure the gas plummeting into the black hole from our viewing angle. Others look dark because the cloud blocks our view of what is truly happening. Messier 77's active galactic nucleus is one of the dark ones, but the new observations indicate that it actually possesses the same qualities as the bright ones.

When galaxies get close enough to each other, tidal forces can tug entire star systems out of place, distorting the shapes of the interacting pair in sometimes dramatic fashion. When galaxies with active galactic nuclei interact, the result can be spectacular, as in this Hubble Space Telescope view of Arp 282, made up of Seyfert galaxy NGC 169 (bottom) and IC 1559 (top). Hidden in the cores of both galaxies are supermassive black holes, actively feasting on surrounding stars, gas and dust. Delicate streams of matter can be seen visibly connecting the two galaxies in a dramatic, 3D-like demonstration of titanic tidal interactions.

A period of AGN activity (corresponding to a rapid increase in supermassive black hole mass) will lead to a galaxy being quenched, irrespective of whether this burst of energy was recent, or a long time in the past. Not only that, but this activity is more important than any other galaxy properties in stopping the formation of new stars in a galaxy. This is a testament to the power of AGN: regardless of what a galaxy is experiencing now, a powerful AGN in its past can impact its star formation rate for billions of years to come. In the future, studies on the entire histories of AGN will hopefully shed even more light on the dramatic impact that they have on their host galaxies.