Ancient Quasar Shines Brightly, But All the Galaxy’s Stars Are Missing

by Nancy Atkinson on October 23, 2012

Hubble Space Telescope image of J1148+5251. Credit: NASA/ESA/M. Mechtley, R. Windhorst, Arizona State University

Quasars have been the best and most easily observed beacons for astronomers to probe the distant Universe, and one of the most distant and brightest quasars is providing a bit of a surprise. Astronomers studying a distant galaxy, dubbed J1148+5251 and which contains a bright quasar, are seeing only the quasar and not the host galaxy itself. It has been thought that the quasar has been feeding on a handful of stars every year in order to bulk up to its size of three billion solar masses over just a few hundred million years. But where are all the stars?

Likely, the quasar hasn’t gone on a feeding frenzy and eaten everything in sight! But it might be eating on the sly. Near infrared views with the Hubble Space Telescope’s Wide Field Camera 3 are only providing hints of what might be taking place: the galaxy is so enshrouded with dust that none of the starlight can be seen; only the bright, blaring quasar shines through. Just how many stars this quasar is eating is now uncertain, as the carnage is taking place undercover.

While most early galaxies contain hardly any dust — the early universe was dust-free until the first generation of stars started making dust through nuclear fusion – previous submillimeter observations showed this galaxy harbors large amounts of dust, so that is somewhat of a mystery, too.

So how could this all be happening?

Artist’s impression of one of the most distant, oldest, brightest quasars ever seen is hidden behind dust. The dust is also hiding the view of the underlying galaxy of stars that the quasar is presumably embedded in. (Credit: NASA/ESA/G.Bacon, STScI)

“If you want to hide the stars with dust, you need to make lots of short-lived massive stars earlier on that lose their mass at the end of their lifetime. You need to do this very quickly, so supernovae and other stellar mass-loss channels can fill the environment with dust very quickly,” said Rogier Windhorst of Arizona State University (ASU), Tempe, Ariz.
“You also have to be forming them throughout the galaxy to spread the dust throughout the galaxy,” added Matt Mechtley, also of ASU.

This quasar was first identified in the Sloan Digital Sky Survey (SDSS) and the follow-up submillimeter observations showed significant dust but not how and where it was distributed.

Windhorst and his team used Hubble to very carefully subtract light from the quasar image and look for the glow of surrounding stars. They did this by looking at the glow of a reference star in the sky near the quasar and using it as a template to remove the quasar light from the image. Once the quasar was removed, no significant underlying starlight was detected. The underlying galaxy’s stars could have been easily detected, had they been present and relatively unobscured by dust in at least some locations.

“It is remarkable that Hubble didn’t find any of the underlying galaxy,” said Windhorst. “The underlying galaxy is everywhere much fainter than expected, and therefore must be in a very dusty environment throughout. It’s one of the most rip-roaring forest fires in the universe. It’s creating so much smoke that you’re not seeing any starlight, anywhere. The forest fire is complete, not a tree is spared.”

Because we don’t see the stars, we can rule out that the galaxy that hosts this quasar is a normal galaxy,” said Mechtley. “It’s among the dustiest galaxies in the universe, and the dust is so widely distributed that not even a single clump of stars is peeking through. We’re very close to a plausible detection, in the sense that if we had gone a factor of two deeper we might have detected some light from its young stars, even in such a dusty galaxy.”

This result was published in the Sept. 10 issue of the Astrophysical Journal Letters in the team’s paper.

The only way to get to the bottom of this mystery, Windhorst said, is to wait for the James Webb Space Telescope to launch and come online.

“The Webb telescope is designed to make a definitive detection of this,” he said. “ We will get solid detections of the stars with Webb’s better sensitivity to longer wavelengths of light, which will better probe the dusty regions in these young galaxies.”

The Webb telescope will also have the infrared sensitivity to peer all the way back to 200 million years after the Big Bang. If galaxies started forming stars at this early epoch, Webb is designed and being built to detect them.

So only then will the true nature – and potential carnage – of this system be revealed.

Read the team’s paper.
Source: NASA


Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

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