A nearby galaxy, NGC 1448, which was discovered to have an actively accreting supermassive black hole that has been hiding behind extremely thick clouds of gas and dust recently (artist illustration of an obscured black hole is shown in the inset image).
Mass chart for dead stars and black holes, which form some of the most compact objects in the universe. Pix by nasa/jpl — Caltech.

A BLACK hole is one of the most mysterious and fascinating objects in the universe. It is an incredibly massive object, which has been squeezed into a very tiny volume.

As a result, gravitational force becomes so strong that anything, which falls within its vicinity, will stand no chance of escaping. This is why black holes are technically invisible, as even light cannot escape their gravitational pull.

Due to this, we cannot observe black holes directly. Scientists often infer their presence, and study them by investigating the surrounding matter, such as stars and gas. As matter falls and accelerates into a black hole, it heats up and emits X-ray radiation. This infalling matter forms some of the brightest objects in the universe.

There are few different kinds of black holes: stellar-mass, supermassive and intermediate mass. They are categorised into these different classifications on the basis of their mass. One of the techniques that scientists use to measure the mass of a black hole is by studying the motion of matter that is orbiting the black hole.

Stellar-mass black holes are those with mass ranging between five and 100 times the mass of the Sun (Msun ~ 2 x 1,030 kg). These black holes are the end product of the death of massive stars when they collapse due to their own gravity. Smaller stars die to form white dwarfs or neutron stars, depending on their masses. The sun will die to become a white dwarf. This process is sometimes observed as a hypernova explosion or gamma-ray burst. They are often observed as binary systems, when they accrete material from their companion stars. The falling of the material onto the black hole releases a huge amount of gravitational potential energy, causing matter to heat up to several hundred million degrees in temperatures and emit X-rays. We can, therefore, detect the black hole using X-ray telescopes, and observe its companion star using optical telescopes.

Recently, scientists have provided the first observation of gravitational waves that are caused by the merger of two stellar-mass black holes. This observation is the first direct detection of such events.

Supermassive black holes, as they are called, are the largest kind of black holes with mass of at least a million times more than the sun. They can be found at the centre of most massive galaxies in the universe, including the Milky Way. The supermassive black hole in our galaxy is called Sagittarius A*, and it is about four million times more massive than the sun.

A supermassive black hole, which is actively accreting materials surrounding it, is called an “active galactic nucleus” (AGN). All supermassive black holes are thought to undergo this phase of rapid accretion at some point in their lifeand, therefore, at some point, are AGNs. This is when the bulk of the growth of supermassive black holes is thought to occur. The rapid accretion of material onto the black hole generates luminous light across the electromagnetic spectrum, from radio to gamma rays. In some cases, the light emitted is so luminous that it can outshine the light from the galaxy that it resides. A galaxy that hosts an AGN is called an active galaxy. Our galaxy is not an active galaxy as our supermassive black hole is not active.

The origin of supermassive black holes is still an open question. Scientists have proposed several theories, which include a lower mass black hole at the centre of galaxy that has grown into a supermassive black hole by accreting matter or merging with other black holes. Other theories suggest that it is formed from the collapse of a dense cluster of stars, or the collapse of a massive cloud of gas at the early stage of the formation of the galaxy.

On Jan 7, my colleague and I presented the discovery of two hidden active supermassive black holes in nearby galaxies at the 229th American Astronomical Society Meeting. I led the research on one of the black holes discovered, which is located in a galaxy called NGC 1448, located about 38 million light years away from us (one light year is approximately 9.5 trillion kilometre). Due to the vast size of our universe, astronomers consider this distance very nearby, and therefore the fact that the active black hole was only discovered very recently is quite shocking.

There has been an evidence for the presence of an active supermassive black hole at the centre of NGC 1448. So my team and I followed up this study, and observed the galaxy using several ground- and space-based telescopes at different wavelengths (optical, mid infrared, soft X-rays and hard X-rays). Our data provide the first direct views of this monster, and therefore confirm this previous study. Not only that, our data also shows that this supermassive black hole is covered by thick layers of gas and dust, hiding it from view. However, we managed to view the black hole, mainly using Nasa’s hard X-ray energy telescope, called (Nuclear Spectroscopic Telescope Array (NuSTAR).

NuSTAR has the capabilities to detect high-energy X-ray lights coming from the black hole that is able to penetrate through the thick layers of gas and dust. This is similar to how doctors use X-rays to look into the structures of our bone.

The final category of black hole is intermediate mass black holes, with mass in between the mass of stellar-mass black holes and supermassive black holes (10^2 — 10^5 Msun). The existence of this class of black hole remains hypothetical. Scientists have found several candidates, which include some ultraluminous X-ray sources, and low luminosity AGNs in nearby galaxies. However, this is yet to be proven. Several postulated scenarios on how they could be formed include the merging of stellar-mass black holes or collision of massive stars.

Till this day, scientists are still trying to fully understand black holes. With the advent of new high technology telescopes, such as NuSTAR, we are able to make new discoveries and learn new things about them every day.

NUR ADLYKA ANNUAR is a postgraduate astrophysics student at Durham University, United Kingdom. She recently presented the discovery of an active supermassive black hole very close to the Milky Way at the American Astronomical Society meeting in Grapevine, Texas.

Email her at adlyka@gmail.com.

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