Giant gas cloud boomeranging back into Milky Way

January 28, 2016
The trajectory of the Smith Cloud coming into the Milky Way galaxy. Credit: NASA and ESA

Since astronomers discovered the Smith Cloud, a giant gas cloud plummeting toward the Milky Way, they have been unable to determine its composition, which would hold clues as to its origin. University of Notre Dame astrophysicist Nicolas Lehner and his collaborators have now determined that the cloud contains elements similar to our sun, which means the cloud originated in the Milky Way's outer edges and not in intergalactic space as some have speculated.

The Smith Cloud, discovered in the 1960s, is the only high-velocity cloud in the galaxy for which its orbit is well-determined, thanks in particular to studies with radio telescopes like the Green Bank Telescope (GBT). The starless gas cloud is traveling at nearly 700,000 miles per hour and is expected to crash into the Milky Way disk in 30 million years. If it were visible, the Smith Cloud would have an apparent size of about 30 times the diameter of the moon from tip to tail.

Astronomers long thought that the Smith Cloud might be some starless galaxy or gas falling into the Milky Way from . If that were the case, the cloud composition would be mainly hydrogen and helium, not the heavier elements made by stars.

The team used Hubble to determine for the first time the amount of heavier elements relative to hydrogen in the Smith Cloud. Using Hubble's Cosmic Origins Spectrograph, the researchers observed the ultraviolet light from the bright cores of three active galaxies that reside billions of light-years beyond the cloud. The Smith Cloud absorbs some of its light in very small wavelength range, and by measuring the dip in brightness of these galaxies behind the cloud, the chemical makeup of the cloud can be estimated.

How researchers used the Hubble Space Telescope to view three distant galaxies through the Smith Cloud, a technique that helped them determine the makeup of the cloud. Credit: NASA

The researchers looked specifically for absorption from the sulfur element, which is a good gauge of how many reside in the cloud. "By measuring sulfur, you can learn how enriched in sulfur atoms the cloud is compared to the Sun," said team leader Andrew Fox of the Space Telescope Science Institute in Baltimore. The team then compared Hubble's sulfur measurements to hydrogen measurements made by the GBT.

The astronomers found that the Smith Cloud is as rich in sulfur as the Milky Way's outer disk, a region about 40,000 light-years from the galaxy's center and about 15,000 light-years farther out than our sun and solar system are. This means that it was polluted by material from stars. This would not happen if it were pristine hydrogen from outside the galaxy. Instead, the cloud appears to have had an intimate relationship with the Milky Way, but was somehow ejected from the outer Milky Way disk about 70 million years ago and is now boomeranging back onto its disk.

Astronomers believe the Smith Cloud, has enough gas to generate two million suns when it eventually hits the Milky Way disk. "We have found several massive gas clouds in the Milky Way halo that may serve as future fuel for star formation in its disk, but, for most of them, their origins remain a mystery. The Smith Cloud is certainly one of the best examples that shows that recycled gas is an important mechanism in the evolution of galaxies," said Lehner.

The study, titled "On the Metallicity and Origin of the Smith High-velocity Cloud," was published this month in the Astrophysical Journal Letters. Fox, Lehner and co-author Jay Lockman of the National Radio Astronomy Observatory discussed the discovery during the Space Telescope Science Institute Hubble Hangout on Thursday (Jan. 28).

The location of the Smith Cloud from data provided by F. Lockman. Credit: NASA

Explore further: Failed dwarf galaxy survives galactic collision thanks to full dark-matter jacket

More information: Andrew J. Fox et al. ON THE METALLICITY AND ORIGIN OF THE SMITH HIGH-VELOCITY CLOUD, The Astrophysical Journal (2015). DOI: 10.3847/2041-8205/816/1/L11

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2 / 5 (12) Jan 28, 2016
Re: "The Smith Cloud, discovered in the 1960s, is the only high-velocity cloud in the galaxy for which its orbit is well-determined, thanks in particular to studies with radio telescopes like the Green Bank Telescope (GBT)."

A question which must always be asked any time the topic of redshifts observed for interstellar hydrogen come up is whether or not the redshift corresponds to a known critical ionization velocity (CIV).

See paper at

"The numerical values of the HI linewidth families are similar to the values of Bands of CIVs for the most abundant atomic species; 34 km/s for helium, 13 km/s for C, N & O, and about 6 km/s for heavy metals such as Si, Al, and Fe [1], [4]."

In other words, these redshifts may not be indicative of velocities at all -- but rather of ongoing ionization, a plasma physics process. Such ionization-based redshifts may instead result from ROTATION due to field-aligned currents.
1.5 / 5 (8) Jan 28, 2016
The Smith Cloud is yet another vindication of the dialectical view of the energetics, the violent ejection, gravitational relaxation and the dynamics of galaxy formation and their inter-conversion; proposed by the Ambartsumian-Arp-Malek hypothesis : http://redshift.v...2MAL.pdf

The Smith Cloud was ejected from the Milky Way and is being drawn back by its gravitation. The morphology of the galaxies is therefore, mediated by the dialectical process of dispersion/ejection/deformation initiated by the catastrophic events of chance-accumulated matter/antimatter annihilation and the regularizing effects of gravitational attraction.
1 / 5 (1) Jan 28, 2016
If it was conjectured that this gas was ejected from a collapsing bh in the former Magellenic galaxy how would one prove it?
4 / 5 (8) Jan 28, 2016
Please clarify your point. How do your arguments affect the conclusion of the work, high metallicity ?
When you talk about redshifts do you mean the average redshift of the HI spectra, from which the velocity of the cloud is derived? The article does not mention red shift, so clarification is needed.
As I understand it is the line width, not the line shift, that is related to CIVs. The speed of the entire cloud is reflected in the line shift. This articles speaks only of the speed of the cloud, not of the velocity distribution within the cloud.

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