Free Astronomy Magazine March-April 2016

SPACE CHRONICLES full moon. Astronomers long thought that the Smith Cloud might be a failed, starless galaxy, or gas falling into the Milky Way from intergalactic space. If either of these sce- narios proved true, the cloud would contain mainly hy- drogen and helium, not the heavier elements made by stars. But if it came from within the galaxy, it would contain more of the ele- ments found within our sun. The team used Hubble to measure the Smith Cloud's chemical composition for the first time, to determine where it came from. They observed the ultraviolet light from the bright cores of three active galaxies that re- side billions of light-years be- yond the cloud. Using Hubble's Cos- mic Origins Spectrograph, they mea- sured how this light filters through the cloud. In particular, they looked for sulfur in the cloud which can ab- sorb ultraviolet light. "By measuring sulfur, you can learn how enriched in sulfur atoms the cloud is compared to the sun," Fox explained. Sulfur is a good gauge of how many heavier T his com- posite image shows the size and lo- cation of the Smith Cloud on the sky. The cloud ap- pears in false-color, radio wave- lengths as observed by the Robert C. Byrd Green Bank Telescope in West Virginia. The visible-light image of the background star field shows the cloud's lo- cation in the direction of the summer constellation Aquila. The cloud is 15 degrees across in angular size — the width of an outstretched hand at arm's length. The appar- ent size of the full moon is added for comparison. [NASA, ESA, and Z. Levay (STScI)] elements reside in the cloud. 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 (about 15,000 light-years farther out than our sun and solar system). This means that the Smith Cloud was enriched by material from stars. This would not happen if it were pristine hydrogen from out- side the galaxy, or if it were the rem- nant of a failed galaxy devoid of stars. Instead, the cloud appears to have been ejected from within the Milky Way and is now boomerang- ing back. Though this settles the mystery of the Smith Cloud's origin, it raises new questions: how did the cloud get to where it is now? What calamitous event could have catapulted it from the Milky Way's disk, and how did it remain intact? Could it be a re- gion of dark mat- ter — an invisible form of matter — that passed through the disk and captured Milky Way gas? The answers may be found in future research. T he infalling Smith Cloud does not emit light at wavelengths that the Hubble Space Telescope is sensitive to. However, Hubble's Cos- mic Origins Spectrograph can mea- sure how the light from distant background objects is affected as it passes through the cloud. These measurements yield clues to the chemical composition of the cloud. By using these intergalactic foren- sics, Hubble astronomers trace the cloud's origin to the disk of our Milky Way. Combined ultraviolet and radio observations correlate to the cloud's infall velocities, provid- ing solid evidence that the spectral features link to the cloud's dynamics. [NASA, ESA, and A. Feild (STScI)] n