Astronomers find vast ionized hydrogen cloud in ‘Whirlpool Galaxy’ using ultra-sensitive Arizona telescope

The M-51, or Whirlpool, Galaxy is one of the most studied
galaxies in the universe, but Case Western Reserve
University researchers have found a never-before seen cloud
of hydrogen gas associated with the galaxy. Credit: Case
Western Reserve University

Astronomers have been keenly peering into M51, or the
Whirlpool Galaxy, since the 1800s, its signature spiral
structure informing the earliest debates over the nature of
galaxies and the Cosmos at large.


But no one—not with the naked eye or with increasingly powerful
modern telescopes—has ever seen what Case Western Reserve
University astronomers first observed using a refurbished
75-year-old in the mountains of
southwest Arizona.

“I literally looked at the image and said, ‘What in the world
is that?'” said Case Western Reserve astronomy professor Chris
Mihos.

What it was turned out to be a massive cloud of ionized
hydrogen gas spewed from a nearby galaxy and then essentially
“cooked” by radiation from the galaxy’s central black hole.

Mihos and a trio of collaborators— led by then-graduate student
Aaron Watkins, and including Case Western Reserve Observatory
Manager Paul Harding and University of Wisconsin astronomer
Matthew Bershady—wrote about the discovery this month in the
journal The Astrophysical Journal Letters.

The discovery of the giant gas cloud, first observed by Watkins
in 2015 and announced by Mihos on Twitter in April, potentially
provides astronomers around the world with an unexpected “front
row seat” to view the behavior of a black hole and associated
galaxy as it consumes and “recycles” hydrogen gas.

“We know of a few clouds like this in distant , but not in one so close to us,” Mihos
said. “This gives astronomers a great opportunity to study up
close how gas is ejected from galaxies and how black holes can
influence large regions of space around those galaxies.”

A ‘wonderful telescope’

But how did Case Western Reserve scientists find something
others had missed?

In part, because they looked in the right spot with the right
equipment—and then enlisted the help of a colleague to confirm
it with additional data.

Case Western Reserve’s Burrell Schmidt telescope at the Warner
& Swasey Observatory is one among more than two dozen research
telescopes at Kitt Peak National Observatory, including the
National Optical Astronomy Observatory and National Solar
Observatory, pointed at the dark skies 60 miles southwest of
Tucson.

Although smaller and older than most telescopes on Kitt Peak,
the Case Western Reserve telescope is also constructed in a way
to provide a wide field of view, while also keeping out
unwanted stray light.

That allows astronomers to see things others don’t: diffuse
patches of light that are “over 100 times fainter than the
blackest night sky you can imagine,” Mihos said.

“What our telescope really does well is measure that very
diffuse, low surface brightness light emitted by gas or stars
around a galaxy,” Mihos said. “It’s a wonderful telescope, and
it has allowed us to make world-class advances in studying the
faint outskirts of galaxies.”

Mihos said Watkins, now doing his post-doctoral work in
Finland, had originally been imaging the Whirlpool to map the
faint streamers of starlight torn off by the collision between
the galaxies. Thinking there might also be gas in those
streamers, the team fitted the telescope with a special filter
to see hot, ionized hydrogen gas, which gives off a specific
wavelength of light.

“Finding stars is relatively straightforward, but gas doesn’t
shine at all wavelengths,” Mihos said. “That’s one of several
reasons why no one had ever seen this before— earlier studies
using these kind of hydrogen filters to look for ionized gas
couldn’t detect emission this faint and over such a wide area
around the Whirlpool to see what Aaron found.

But there was still one thing to double check: “Our worry was
really what I would call ‘the dirty windshield analogy’- what
if we were actually seeing a diffuse cloud of gas right in
front of us in our galaxy and it wasn’t really part of M51?”
Mihos said. “We could figure that out if we know how fast it
was moving—does it move slow like clouds in the Milky Way, or
much faster like those in M51?”

The CWRU astronomers teamed up with University of Wisconsin
astronomer Matthew Bershady to use the nearby WIYN Observatory
to confirm the cloud’s association with M51. The WIYN 3.5-meter
telescope was equipped with an instrument capable of taking a
detailed spectrum of the cloud to measure its speed.

“We needed to know whether that cloud was moving at ‘Milky Way
speed’ or ‘M-51 speed,'” Mihos said. “Once Aaron and Matt had
taken the spectrum of the cloud, they were able to tell how
fast it was moving away from us, and we immediately knew it was
part of M51, not something in our own back yard.”

The making of stars

The discovery’s role in more clearly understanding how galaxies
eject and “recycle” their gas and stars,” Mihos said, will be
determined in the coming years as more researchers dig into
information that had been there all along—even if unseen until
now.

“We need a lot more data to tell this tale fully, for sure, and
this is one of the most well-studied galaxies out there,” he
said. “So, we have a chance to better understand what is
happening in the Whirlpool, which informs and affects how we
understand everything else about how it has evolved over time.”

Explore further:

Nearby dwarf galaxy and possible protogalaxy discovered

More information: Aaron E. Watkins et al. Discovery of a
Vast Ionized Gas Cloud in the M51 System, The Astrophysical
Journal
(2018). DOI: 10.3847/2041-8213/aabba1


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