Longer cosmic ruler based on black holes

New method may improve extreme distance measurements

Astronomers have a new gadget in their cosmic toolbox that is capable of measuring distances to very faraway objects. The method uses active galactic nuclei, the bright, violent regions at the centers of galaxies, to gauge distances farther than existing cosmic yardsticks can reach.

Having such an odometer is crucial for understanding how space, time and matter behave at the scale of the whole universe, and could help solve mysteries such as the nature of the dark energy that is accelerating the expansion of the universe.

For four decades, astronomers have been trying to turn these luminous beacons into cosmic mile markers.  Now, scientists at the University of Copenhagen’s Dark Cosmology Centre and their collaborators think they’ve got it worked out. The brightness of an active nucleus is tightly related to the radius of a region of hot gases surrounding the central black hole. When scientists determine that radius, they can predict how intrinsically bright the nucleus should be — and compare that value to how bright it appears, which depends on distance. Astronomers call objects whose properties can be used to predict their actual brightness standard candles.

“It’s the radius-luminosity relationship that allows us to assume that these active galactic nuclei are standard candles,” says postdoctoral fellow Kelly Denney of the Dark Cosmology Centre, an author of the study, which will appear in the Astrophysical Journal.

The stable of standard candles already houses type 1a supernovas and Cepheid variable stars, which have predictable luminosities but are good only for measuring distances to objects present when the universe was nearly 4 billion years old. Active galactic nuclei would extend that capability to objects at distances corresponding to when the universe was only 1.5 billion years old.

“Right now we rely so much on supernovas, it would be really nice to have independent verification of cosmological parameters,” says astrophysicist Bradley Peterson of Ohio State University.  “I’m really excited about this result.”

A technique called reverberation mapping measures how long it takes photons being kicked out of the black hole’s immediate neighborhood to reappear after they’ve traversed the hot, gassy maelstrom surrounding the black hole. Because light travels at a constant speed, astronomers can determine the gassy region’s radius. Then, the luminosity of the active galactic nucleus can be calculated.

Until now, tightening the relationship between radius and luminosity has been tricky. Among other reasons, starlight from a host galaxy contaminated the brightness measurements of its active nucleus. But the team had in hand data from astrophysicist Misty Bentz of Georgia State University that corrected for the effects of surrounding starlight, in addition to Denney’s own precise measurements of radii.

“I think this paper is really clever,” Bentz says. “But it needs a few improvements before it can be comparable to supernovae and stuff like that.”

Denney says the team is planning on calibrating the method using observations of additional galaxies and — if given access to the Hubble Space Telescope — measurements produced by Cepheid variable stars.

Peterson says the method will probably take a decade to catch on within the astrophysics community. It will take that long to expand the dataset, add more distance calibrations, and reduce some of the noise in the data.

But the team is optimistic that active galactic nuclei will become an accepted distance measure, since they are more numerous and more easily observable than existing standard candles.  “It could end up being one very large rung on the distance ladder,” Bentz says. “It would put everything on the same scale, instead of using one thing to calibrate something else that’s used to calibrate something else.… There’s less potential for something to go wrong.”

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