Australasian Science Magazine

Figure 1. Two galaxies in the process of merging. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

By Michael Cowley

Contrary to popular belief, new work from an Australian-led study suggests that supermassive black holes may not be starving galaxies like the Milky Way to death.

Located in the centre of most massive galaxies, supermassive black holes are objects of extreme density that can be billions of times more massive than our own Sun. When surrounding gas and dust falls into these monster black holes, vast amounts of energy is released, evidence of which is observed on scales far beyond the galaxy itself. Astronomers have long suspected that this energy may stop galaxies from forming new stars.

Star formation is a crucial driver of galaxy evolution and requires copious amounts of cold gas and dust. If this material is heated, blown away or consumed by supermassive black holes, it could potentially spell doom for embryonic stars and their entire host galaxy. While these dire predictions are the consensus among many astronomers, they stem from studies that have typically been limited to observations of a small number of nearby galaxies.

Expanding the Search

To address this shortcoming, my colleagues and I released a photo album of more than 70,000 galaxies accompanied by one of the most detailed galaxy studies ever compiled (http://zfourge.tamu.edu/). Our photo album spans a period of 12 billion years, which represents more than 90% of the age of the universe. We used the 6.5-metre Magellan Baade Telescope in Chile to snap these images over 45 nights, but also combined them with data from numerous other telescopes, including the Hubble, Chandra, Spitzer and Herschel space observatories, the Keck telescope in Hawaii, and the Very Large Array in New Mexico.

With enough data to investigate how galaxies evolve through most of cosmic time, we then set out to search for supermassive black holes. Unfortunately, black holes do not emit light, which instead gets caught inside an immense gravitational field. While black holes cannot directly be seen, they can be indirectly detected thanks to the energy released when they feast on surrounding gas and dust. As this material swirls into a black hole, frictional forces cause it to heat and emit unique radiation. By looking for this radiation, we were able to catalogue hundreds of distant galaxies that host supermassive black holes.

When we examined these galaxies, we found that they preferred crowded regions of the universe where frequent galaxy collisions occur, swirling up lots of material to provide black holes with copious amounts of gas and dust to quench their appetite. While galaxy collisions are present in our local universe (Fig. 1), they were more prevalent billions of years ago when the first galaxies were starting to form.

Therefore, the observation of distant galaxies represents a critical step towards our understanding of how supermassive black holes may impact their host galaxies through cosmic time.

What About the Milky Way?

Astronomers have long suspected that our galaxy, the Milky Way, hosts its own supermassive black hole, so it should come as no surprise that numerous telescopes have been pointed towards its central bulge about25,000 light-years from Earth. Unfortunately, compared with the distant and energetic supermassive black holes we observed, the Milky Way’s black hole is considered relatively passive, with only the occasional flicker of light thought to be the result of small blobs of material randomly being consumed.

Indeed, the Milky Way’s supermassive black hole was only discovered due to its influence on nearby objects. Specifically, astronomers performed observations of stars in the central bulge and found they were moving similarly to how the Earth orbits the Sun. The smoking gun came by measuring various properties of their orbit, which revealed that the central (and invisible) source was millions of times more massive than our Sun.

While the Milky Way’s supermassive black hole is currently dormant, the question remains if it once was or is yet to become a harbinger of doom.

Opening the Family Album

To further probe the Milky Way and its supermassive black hole, we turned to our album of galaxies and plucked out its progenitors: galaxies that are growing up to look like the Milky Way. By examining these sources, we mapped out the evolutionary path that our galaxy has taken over the past 12 billion years.

One of the exciting discoveries we made was that 10 billion years ago, the Milky Way was likely churning out newborn stars 30 times faster than the present. Our Sun, was late to the party, arising five billion years later at a time when the rate of star formation had dropped to a mere trickle. Was the Milky Way’s supermassive black hole to blame for this downturn in star formation?

In our most recent paper, published in the Monthly Notices of the Royal Astronomical Society (https://goo.gl/aK8Y7r), we attempted to answer this question by measuring the energy from both stars and supermassive black holes in Milky Way progenitors. If supermassive black holes do negatively impact a galaxy’s ability to form new stars, we expected to find a correlation between the two processes, perhaps one where higher levels of black hole energy correspond with suppressed levels of star formation energy.

However, after the challenging process of distinguishing which light came from which object, we crunched the numbers and found a disconnect. There was no evidence of a correlation at all. Contrary to studies of local galaxies, our findings cast doubts over the idea that the suppression of star formation in galaxies, including the Milky Way, has predominantly been driven by energetic outflows from its supermassive black hole.

What Does the Future Hold?

Our galaxy is on a collision course with its largest neighbour, Andromeda. This event, which is not expected for a few billion years or so, will see both galaxies and their supermassive black holes coalesce to form one megagalaxy. While the Milky Way and Andromeda are both spiral galaxies, with most stars concentrated in a disk-like structure, the megagalaxy will be more elliptical in shape and much smoother in appearance.

Towards the centre of the megagalaxy, the two supermassive black holes will also begin to merge. During this time, the merging process will generate strong gravitational waves that will radiate outwards until the merger is complete.

Astronomers have used simulations to show that this entire chaotic process, which is expected to last hundreds of millions of years, will see lots of gas, dust and even stars thrown about, potentially providing the new galaxy’s supermassive black hole with ample food to spark it to life. If the results of our study are correct, this event will likely have little to no impact on the merged galaxy’s ability to produce new stars.

Unfortunately, while we will not be alive to witness the event, nor take responsibility for our predictions, we remain hopeful for our galaxy’s future.