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Could Intermediate-Mass Black Holes Lurking in Galactic Halos Account for Dark Matter? Probably Not

02/19/2017 04:24PM

Could Intermediate-Mass Black Holes Lurking in Galactic Halos Account for Dark Matter?  Probably Not
The nature of dark matter has long been questioned, but the recent discovery of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) has renewed interest in the possibility that dark matter could actually consist of primordial black holes in the mass range of 10 to 1000 solar masses.

Could dark matter be made of these intermediate-mass black holes that formed at the beginning of the Universe? A recent study takes a new look at this question and concludes... Probably not.

According to this model, the extreme density of matter present during the Universe's early expansion led to the formation of a large number of intermediate-mass black holes. Many of these black holes now hide in the halos of galaxies, constituting the mass that we've measured dynamically but remains unseen.

The two LIGO gravitational wave detectors, jointly operated by Caltech and MIT, are located in Hanford, Washington and Livingston, Louisiana. To date these detectors have caught two robust signals produced by two black holes coalescing into a single black hole (September 14, 2014 and December 26, 2015).

LIGO's first gravitational-wave detection revealed the merger of two black holes that were both tens of solar masses in size. If primordial black holes are indeed a major constituent of dark matter, then LIGO's detection is consistent with what we would expect to find -- occasional mergers of the intermediate-mass black holes that formed in the early universe and now lurk in galactic halos.

But there's a catch, however. If there truly were a large number of intermediate-mass primordial black holes hiding in galactic halos, they wouldn't go completely unnoticed. We would see signs of their presence in the gravitational micro-lensing of background quasars. Unseen primordial black holes in a foreground galaxy would cause an image of a background quasar to briefly brighten, which would provide us with clear evidence of such black holes despite our inability to detect them directly.

A team of scientists led by Evencio Mediavilla (Institute of Astrophysics of the Canaries, University of La Laguna) has now used our observations of quasar micro-lensing to place constraints on the amount of dark matter that could be made up of intermediate-mass primordial black holes.

Mediavilla and collaborators used simulations to estimate the effects of a distribution of masses on light from distant quasars, and they then compared their results to micro-lensing magnification measurements from 24 gravitationally lensed quasars. In this way, they were able to determine both the abundance and masses of possible objects causing the quasar micro-lensing effects we see.

The researchers found that the observations constrain the mass of the possible micro-lensing objects to be between 0.05 and 0.45 solar masses -- too small and not at all the intermediate-mass sized black holes that have been postulated. What's more, they find that the lensing objects make up about 20 percent of the total matter, which is barely more than expected for normal stellar matter. This suggests that normal stars are doing the majority of the quasar micro-lensing, not a large population of intermediate-mass black holes.

What does this mean for primordial black holes as dark matter? Black holes in the range of 10 to 200 stellar masses are unlikely to account for much (if any) dark matter, Mediavilla and collaborators conclude, which means that LIGO's detection of gravitational waves likely came from two black holes that collapsed from stars, not primordial black holes.

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