As today is obviously the day to be reporting on black hole research (the Monday before Black Friday will hereby be called “Black Hole Monday“) there’s a shocking report recently published by astrophysicists at the University of Arizona in Tucson: The Milky Way’s baby black holes are missing.
It might sound like a bizarre singularity kindergarten kidnapping case (I’m looking at you Andromeda), but Feryal Özel’s Arizona team are in doubt as to whether the baby black holes existed in the first place.
A stellar-mass black hole is theorized to be created after a supernova. Supernovae are triggered when stars over eight-times the mass of our sun reach the end of their lives after catastrophically running out of fuel feeding stellar core fusion.
Immediately after supernova, if the remnant core has a mass of less than three suns, one would expect a blob of degenerate neutron matter to be spinning in the explosion’s wake. This is a neutron star.
However, if the supernova remnant exceeds three solar masses, conventional physics suggests a black hole should be left behind.
But there’s a problem. No stellar-mass black holes weighing between 2-5 solar masses have ever been observed, putting a serious question mark over the lower-mass black hole theory.
What’s going on? Are low-mass black holes vanishing? Or is there some black hole formation process we’re not familiar with?
Özel’s team studied 16 binary systems in our galaxy known to contain a black hole and a stellar partner. None of them had a black hole of 2-5 solar masses living there, even after observational uncertainties were considered. Their findings will be published in the Astrophysical Journal.
Although I’d argue that 16 black holes is hardly a conclusive statistic to base a Milky Way-wide survey on, there is a possible mechanism at work that prevents the smallest stellar-mass black holes from being created post-supernova.
Chris Fryer of the Los Alamos National Laboratory in New Mexico told New Scientist that the most powerful supernovae are usually triggered by lower-mass stars. The larger stars — the ones that create black holes — are usually of lower energy than the supernovae that produce neutron stars.
The high-mass stars’ low-energy explosions will expel less material, meaning that when a small black hole is born, the material hangs around for longer and gets swallowed up by the baby black hole, quickly bulking it up.
This means that the lowest mass black holes may be very rare, short-lived or non-existent, creating a conspicuous gap between the most massive neutron stars and the least massive stellar black holes.
In other words, call off the search for the Milky Way’s missing baby black holes, they may never have existed.
Image credit: Aurore Simonnet/Sonoma State University/NASA
Publication: The Black Hole Mass Distribution in the Galaxy, Feryal Ozel et al., 2010. arXiv:1006.2834v2 [astro-ph.GA]