Half a century after first getting a bead on quasars, astronomers still lack a basic understanding of how the most luminous objects in the universe work, a prominent researcher says.
Scientists first measured the distance to a quasar — an incredibly bright galactic core powered by a supermassive black hole — 50 years ago this Saturday, March 16, finding that it lay billions of light-years away.
The discovery was a seminal one in astronomy, opening the distant, ancient universe to observation and study. But in the decades since, researchers have shed little light on the powerful engine that drives quasars, says astrophysicist Robert Antonucci of the University of California, Santa Barbara.
"We have found thousands of quasars in the past 50 years, but we still don't have good physical models for how they radiate their prodigious energy," Antonucci writes in the current issue of the journal Nature, which was published online on March 13. "Without predictive theories, we have nothing. Our best hope for understanding quasars is that extraterrestrials might drop in and explain them to us." [Most Powerful Quasar Discovered (Video)]
The brightest objects in the universe
Quasars radiate energy broadly across the electromagnetic spectrum, but they take their name from their radio emissions. Astronomers dubbed them"quasi-stellar radio sources" because the signals appeared to be coming from one place, like a star. The shortened version of the moniker, "quasar," stuck.
Many quasars blast out twin jets of particles that travel at nearly the speed of light, which in turn create enormous, radio-emitting "lobes" near the quasars, Antonucci writes.
Scientists think quasars and other types of active galactic nuclei mark a particular stage in the lifetime of galaxies, one at which their central black holes, which can be more massive than 10 billion suns, are gobbling up lots of gas, dust and other matter.
"This trait was more common in the past, so there are fewer quasars today," Antonucci writes. "Now starved of fuel, black holes linger in galaxies, including our Milky Way."
Astronomers have been documenting ever-more distant quasars, pushing back closer and closer to the Big Bang that created our universe 13.7 billion years ago. But a fundamental understanding of quasars remains elusive, Antonucci says.
"The theory of radio sources has not changed significantly in the past 30 years," he writes. "Basic questions remain: do the jets and lobes comprise electrons and protons or electron-positron pairs? Do the protons carry a lot of energy, as cosmic rays do? Is the energy divided evenly between electric and magnetic fields? Without answers to these [questions], we can set only lower limits on how much energy the jets and lobes hold."
It doesn't help that astrophysicists continue to investigate quasars using models developed for much smaller black holes, Antonuccci says.
"These models simply don't match the observations without lots of special pleading," he writes. "The properties of small accretion disks that are inferred to exist around stellar-mass black holes cannot be scaled up to explain the spectra of much more luminous quasars."
But a better understanding of quasars is achievable, Antonucci adds, urging his colleagues to work on developing advanced computational models of black-hole systems. And more sensitive X-ray telescopes could make a difference, too.
"The important thing is that the X-rays come from so extremely close to the black hole, much closer than the optical light," Antonucci told SPACE.com via email. "So it offers a hope of giving us a picture of the 'central engine' of the black hole, where the gravitational potential energy is actually produced. That's where the money is."
Antonucci voiced optimism that astronomers will unlock quasars' key mysteries, though he's not sure about the timeline.
"Eventually, I suppose we'll get it, although I may be in heaven by then, or else in the other place!" he told SPACE.com.
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