Hubble Investigates Ghosts of Quasars Past

A galaxy-size blob of gas discovered eight years ago by a Dutch schoolteacher has galvanized the study of the spectral remains of once-bright quasars.

Hanny's Voorwerp

Hanny's Voorwerp, seen here in green south of spiral galaxy IC 2497, was a deep blue in the Galaxy Zoo image that van Arkel saw. The exact hue depends on image processing, but the color is unique. Nearly 200 Galaxy Zoo volunteers found 19 more of these so-called Voorwerpjes.
NASA / ESA / W. Keel

In 2007, a fuzzy blue blob earned a Dutch schoolteacher worldwide fame. As she sifted through image after image in the citizen-science project Galaxy Zoo, Hanny van Arkel spotted a smudge just south of the galaxy IC 2497. She had already clicked on to the next image, having classified the galaxy as an anti-clockwise spiral, when she thought to herself, “Wait, what was that?”

The smudge she discovered turned out to be a galaxy-size reservoir of gas 16,000 light-years across. Intense radiation had stripped its oxygen atoms of their outer electrons, imbuing the gas with its distinctive hue (deep blue in the original image that Hanny saw, bright green in the image to the right due to a different image processing).

The neighboring galaxy isn’t bright enough to have stripped the electrons — at least not in the present day. What van Arkel had spotted was the echo of a recent past, when the supermassive black hole at IC 2497’s center had powered a potent beacon of light, called a quasar.

Roughly 30,000 years ago, the quasar’s intense radiation had traveled from the galaxy’s core to the nearby cloud to blast the electrons off the cloud’s atoms. Sometime between then and now, the quasar faded away, leaving a normal-looking spiral galaxy with a dormant black hole at its center. The echo of light in the glowing cloud remains because the stripped electrons take a long time to find a new home in the sparse intergalactic environment.

Soon known as Hanny’s Voorwerp (Dutch for “Hanny’s Object”), the find galvanized a follow-up search for more so-called Voorwerpjes. Bill Keel (University of Alabama) led the charge of nearly 200 Galaxy Zoo volunteers as they hunted through Hubble Space Telescope images, searching for blobs with that distinctive color and offset by more than 33,000 light-years from the nearest galaxy.

Out of almost a million galaxy images, only 19 made the cut. Of these, eight clouds were echoing light from quasars that had faded from their former glory. These ghosts may share the untold story of how and why quasars turn off.

Light echoes

These eight light echoes jumped out of Galaxy Zoo data thanks to the distinctive green hue of the ionized gas, offset from the nearest galaxy by a minimum of 33,000 light-years.
NASA / ESA / W. Keel

Light Echoes of a Messy Past

To learn that story, Keel’s team first needed to understand where these glowing clouds come from. The authors found that the (mostly elliptical) host galaxies of all eight faded quasars show signs of recent or ongoing mergers. Some exhibit tidal tails left over from a cannibalized galaxy; others contain dusty disks warped by galactic encounters.

NGC 5972 and the ghost of quasars past

Hubble found green wisps of ionized gas extending out to 160,000 light-years around galaxy NGC 5972.
NASA / ESA / W. Keel

The lit-up reservoirs outside the galaxies are probably the gaseous remains of these mergers. For one thing, the glowing gas clouds are roughly in the same spot as the tidal debris from the collisions. For another, spectra show that the gas clouds are rotating, motion that’s likely leftover from the now-cannibalized galaxy.

Surprisingly, none of these mergers appears to have triggered much, if any, star formation, despite large reservoirs of gas. In the original Hanny’s Voorwerp, a galaxy-scale wind from the spiral IC 2497 generated new stars in the nearest part of the glowing gas cloud — a scenario in keeping with general ideas of galaxy evolution. So Keel and his colleagues expected to find similar star formation triggered in these galaxy/cloud pairs.

But instead they found that galaxy-scale outflows, such as supernova-driven winds or winds/jets from the central black hole, don’t seem to play a large role in any of these objects. And there aren’t many stars forming near the tidal debris or in the host galaxies themselves.

Did the once-bright quasars quench any existing star formation, as the authors speculate? Or could we be witnessing these mergers early on, before activity has had a chance to ignite? "It is absolutely possible that . . . either star formation, AGN activity, or both will be triggered later on, after the gas settles more," says Freeke van de Voort (University of California, Berkeley).

But, she cautions, it could be that a gas-rich spiral galaxy merging into a gas-poor elliptical simply doesn't trigger star formation. Ultimately, simulations would help clarify the nature of these mergers.

In the meantime, Keel is working on assembling another sample of faded quasars, this time in non-interacting galaxies, to help understand the mechanics of turning off quasar radiation without the complicating influence of mergers. Coauthor Christopher Lintott (Oxford University) also plans to turn attention from the host galaxies to the history of the faded quasars themselves, studying how and why the beacons faded.

Reference:

William Keel et al. "HST Imaging of Fading AGN Candidates I: Host-Galaxy Properties and Origin of the Extended Gas," accepted for publication in Astronomical Journal.


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