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Black holes are common. But actually counting them is no easy task. Even the brightest can be hard to see. Of course, “bright black holes” is a bit of an oxymoron. But when supermassive black holes at the center of a galaxy feed, the material falling into them heats up, giving off a bright glow across the electromagnetic spectrum.
Infrared light in particular is good to look for when black-hole hunting. Some feasting black holes are obscured by gas and dust, which absorbs much of their glow—but they still shine in the infrared.
It’s no surprise, then, that NASA’s infrared WISE spacecraft found a bunch. WISE launched in 2009 to survey the entire sky in the infrared. Now researchers have used WISE data to count the luminous black holes in a well-known corner of the sky called the COSMOS field. In just that tiny region, WISE found about 130 glowing black holes, or active galactic nuclei. The study will appear in the Astrophysical Journal. [Daniel Stern et al., Mid-Infrared Selection of AGN with the Wide-Field Infrared Survey Explorer. I. Characterizing WISE-Selected AGN in COSMOS]
Extrapolating from this small area, at least two million active black holes dot the sky, confirming that these extreme astrophysical objects are common indeed.
—John Matson
[The above text is a transcript of this podcast.]
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4 Comments
Add CommentActive supermassive black holes are probably just a small subset of the black hole family. With this assumption, the opening statement in the article seems to be a pretty big understatement, doesn't it?
Reply | Report Abuse | Link to thisI wonder when & how we will be able to detect medium & small balck holes? How much of the assumed dark matter could actually be smaller, dark black holes?
What does 'dot the sky' mean. There are only 2 million of these in the universe? That isn't a lot but super rare.
Reply | Report Abuse | Link to thisGood point about the estimate being based only on galaxies with visible relativistic expulsion jets. It could even be argued that nearly all galaxies have SMBHs (even though only a small portion are active), so the total number of SMBHs may be nearly the same as the total number of galaxies...
Reply | Report Abuse | Link to thisRegarding galactic dark matter, please see "Inappropriate Application of Kepler's Empirical Laws of Planetary Motion to Spiral Galaxies Created the Perceived Galaxy Rotation Problem - Thereby Establishing a Galactic Presence for the Elusive, Inferred Dark Matter",
http://fqxi.org/community/forum/topic/1419
Even if you dismiss that argument (and others) against the presence of galactic dark matter, in order for dark matter to explain the discrepancy between observed galactic rotation curves and those expected by applying Keplerian relations to spiral galaxies, the distribution of disk mass must be extended far beyond the visible galaxy. For black holes to provide the necessary mass distribution would require that their mass be nearly 10 times the total mass estimated from luminous matter, in a disperse configuration essentially identical to the dark matter halos envisioned for special WIMP dark matter. That would require some explanation for how all those black holes got so far away from the ordinary galactic matter, why they remain out there and how they avoid gravitationally interacting with ordinary matter, noticeably perturbing the orbits of ordinary objects within the visible disk periphery.
In any case, it would e important to understand why there's nearly always just enough dark matter (10x ordinary matter) precisely configured to produce the observed flat rotation curve of each visible galaxy...
For an example of the mass distribution required of galactic dark matter, please see the illustrations at "Serious Blow to Dark Matter Theories?",
http://www.eso.org/public/news/eso1217/
Astronomers can't have a direct detection of BHs. They sense the presence of some BHs by observing the stellar motion, emission of radiations or some plasma from the vicinity of BHs. All these observed phenomenon are correlated to the degree of "activeness" of the BH. If some BH is dormant, no radiations or plasma may be observed despite the BH being present. This may be more pronounced for small and medium sized dormant BHs. As such, a large nos. of BHs may not be enlisted within the census of astronomers but they may be casting their gravitational influence on adjoining stellar and galactic motion. Therefore, stellar and galactic motion may also be influenced by small and medium dormant BHs which remain undetected.
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