A black hole is born from the collapse of a supermassive star – but what is a supermassive black hole born from? A pale galaxy seemed to hide the answer at its core – but it only added more mystery to the genesis of supermassive holes (or SMBH, its acronym in English).
The galaxy in question is NGC 404, about 10 million light years away, in the Andromeda constellation. From Earth, it appears obscured by the brightness of the star Mirach, which earned NGC 404 the nickname “Phantom of Mirach”.
There are two hypotheses for the genesis of SMBHs. In one, they would be formed through a process called “direct collapse” right after the Big Bang. They would be born monstrously massive and with a minimum size.
The other hypothesis holds that they would be the remnants of a massive collapsing star, and therefore relatively small (100 times the mass of our Sun), growing as they devour everything around them.
Cardiff University astrophysicist Timothy Davis and his team started looking for the smallest SMBHs they could find, using the Atacama Large Millimeter / Submilimeter Array (ALMA), a telescope located in the Chilean Andes. And that’s how they found what they were looking for, within the “Phantom of Mirach”.
Secret kept by “Phantom”
For a long time, NGC 404 was thought to be dead; what was the surprise when, under the ultraviolet light, she showed the presence of young stars in a never seen dust ring? What interested astronomers, however, was the SMBH inside.
“The SMBH at the center of NGC 404 is currently active and swallowing gas, so some of the more extreme models based on this hypothesis and which would only produce very large SMBHs cannot be true,” Davis told Live Science.
The supermassive black hole inside this lenticular galaxy is the smallest mass ever found: it weighs less than 1/1 million the mass of our Sun – which, by Davis and his team’s calculations, means it was half that weight when he was born.
Davis’ team measured the speed of the carbon monoxide present in the system as it is drawn towards the SMBH. According to Davis, “it’s like water flowing down a drain: the gas travels faster and faster as it approaches the black hole.” weighs the SMBH.
For the astronomer, small supermassive black holes may not have had time to consume large amounts of matter since birth. However, even this does not clarify the question of how they formed: SMBHs generally have billions of times the mass of our Sun, compared to that of heavy stars that form common black holes when they collapse.
“If there is a minimum mass for a supermassive black hole, we haven’t found it yet. Today, we know two main ways in which supermassive black holes are born, and none of them explains how this SMBH was formed, “Cardiff University researcher told Live Science website.