Extended Gas in Active Galaxie
Formation of Extragalactic Streamers of Ionized Gas
NASA's Hubble Space Telescope has photographed a set of wispy, goblin-green objects that are the ephemeral ghosts of quasars that flickered to life and then faded.
The glowing structures have looping, helical, and braided shapes.
"They don't fit a single pattern," said Bill Keel of the University of
Alabama, Tuscaloosa, who initiated the Hubble survey. Keel believes the
features offer insights into the puzzling behavior of galaxies with
energetic cores.
The ethereal wisps outside the host galaxy are believed to have been
illuminated by powerful ultraviolet radiation from a supermassive black
hole at the core of the host galaxy. The most active of these galaxy
cores are called quasars, where infalling material is heated to a point
where a brilliant searchlight shines into deep space. The beam is
produced by a disk of glowing, superheated gas encircling the black
hole.
"However, the quasars are not bright enough now to account for what
we're seeing; this is a record of something that happened in the past,"
Keel said. "The glowing filaments are telling us that the quasars were
once emitting more energy, or they are changing very rapidly, which
they were not supposed to do."
Keel said that one possible explanation is that pairs of co-orbiting
black holes are powering the quasars, and this could change their
brightness, like using the dimmer switch on a chandelier.
The quasar beam caused the once invisible filaments in deep space to
glow through a process called photoionization. Oxygen atoms in the
filaments absorb light from the quasar and slowly re-emit it over many
thousands of years. Other elements detected in the filaments are
hydrogen, helium, nitrogen, sulfur, and neon.
"The heavy elements occur
in modest amounts, adding to the case that the gas originated in the
outskirts of the galaxies rather than being blasted out from the
nucleus," Keel said.
The green filaments are believed to be long tails of gas pulled apart
like taffy under gravitational forces resulting from a merger of two
galaxies. Rather than being blasted out of the quasar's black hole,
these immense structures, tens of thousands of light-years long, are
slowly orbiting their host galaxy long after the merger was completed.
"We see these twisting dust lanes connecting to the gas, and there's a
mathematical model for how that material wraps around in the galaxy,"
Keel said. Potentially, you can say we're seeing it 1.5 billion years
after a smaller gas-rich galaxy fell into a bigger galaxy."
The ghostly green structures are so far outside the galaxy that they
may not light up until tens of thousands of years after the quasar
outburst, and would likewise fade only tens of thousands of years after
the quasar itself does. That's the amount of time it would take for
the quasar light to reach them.
Not coincidentally, galaxy mergers would also trigger the birth of a
quasar by pouring material into the central supermassive black hole.
The first "green goblin" type of object was found in 2007 by Dutch
schoolteacher Hanny van Arkel. She discovered the ghostly structure in
the online Galaxy Zoo project. The project has enlisted the public to
help classify more than a million galaxies catalogued in the Sloan
Digital Sky Survey (SDSS), and moved on to add galaxies seen in Hubble
images probing the distant universe. The bizarre feature was dubbed
Hanny's Voorwerp, Dutch for Hanny's object.
Because his follow-up Hubble images of Hanny's Voorwerp were so
intriguing, Keel started a deliberate hunt for more bizarre objects
like it. They would share the rare and striking color signature of
Hanny's Voorwerp on the SDSS images.
Keel had 200 people volunteer specifically to look at over 15,000
galaxies hosting quasars. Each candidate had to have at least 10 views
that collectively reveal weirdly colored clouds.
Keel's team took the galaxies that looked the most promising and
further studied them by dividing their light into its component colors
through a process called spectroscopy. In follow-up observations from
Kitt Peak National Observatory and the Lick Observatory, his team found
20 galaxies that had gas that was ionized by radiation from a quasar,
rather than from the energy of star formation. And, the clouds extended
more than 30,000 light-years outside the host galaxies.
Eight of the newly discovered clouds were more energetic than would
be expected given the amount of radiation coming from the host quasar,
even when observed in infrared light by NASA's Wide-field Infrared
Survey Explorer (WISE) space telescope. The host quasars were as little
as one-tenth the brightness needed to provide enough energy to
photoionize the gas. Keel said that presumably the brightness changes
are governed by the rate at which material is falling onto the central
black hole.
Keel speculated that this quasar variability might be explained if
there are two massive black holes circling each other in the host
galaxy's center. This could conceivably happen after two galaxies
merged. A pair of black holes whirling about each other could disrupt
the steady flow of infalling gas. This would cause abrupt spikes in the
accretion rate and trigger blasts of radiation.
When our Milky Way galaxy merges with the Andromeda galaxy (M31) in
about 4 billion years, the black holes in each galaxy could wind up
orbiting each other. So in the far future, our galactic system could
have its own version of Hanny's Voorwerp encircling it.
Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu
Georgia Bladon
ESA/Hubble, Garching, Germany
011-44-7816291261
gbladon@partner.eso.org
William Keel
University of Alabama, Tuscaloosa, Ala.
205-348-1641
Source: HubbleSite
