By Andrew Fox, ESA-AURA Astronomer at STScI
Like a superpower that imports gas to satisfy its energy needs, the Milky Way depends on fresh gas supplies to fuel its star formation. Our galaxy converts gas to stars at a rate of about one or two solar masses per year, and has enough gas reserves to continue forming stars at this rate for about two billion years. But two billion years is not that long compared to the life of a Galaxy, so without replenishment of its gas supplies, the star formation would eventually cease.
Fortunately for the Milky Way, there is a reservoir of gas to import, seen in the form of the so-called high-velocity clouds (HVCs), a population of gaseous objects that orbit in the halo of the Galaxy. Although not all HVCs are inflowing, they provide an average inflow rate of about one solar mass of gas per year, close to the star-formation rate. HVCs allow us to study the process of Galactic accretion, the delivery of gas to the Galactic disk where star formation takes place.
One of the best-characterized HVCs is the Smith Cloud, named after Gail Bieger (née Smith), who discovered it via its radio emission in her PhD thesis in 1963. We know how far away the Smith Cloud is (13 kpc), how fast it is moving (300 km/s), how much gas it contains (two million solar masses), and even what its orbit looks like: it came out of the Galactic disk about 70 million years ago, and is due to impact the disk about 27 million years from now. It has a comet-like appearance (see Figure 1) and is fragmenting as it interacts with the surrounding gas in the Galactic halo.
Figure 1: The Smith Cloud, as observed in neutral hydrogen 21 cm emission from the Green Bank Telescope, superimposed on an optical image of the Milky Way. The Cloud is ~3×1 kpc in size and is moving toward the Galactic disk. This image is color-coded by the intensity of 21 cm emission. Credit: Saxton/Lockman/Levay/NRAO/AUI/NSF/STScI.
Despite our solid understanding of the Smith Cloud, one key property was (until recently) unknown: its chemical composition. This is a vital clue to its origin, since a cloud containing low levels of heavy elements (in the jargon, low metallicity) likely originates outside the Galaxy, perhaps from the intergalactic medium or from a dwarf galaxy. On the other hand, a cloud originating in the Galaxy should have much higher metallicity, close to the levels measured in the Sun, because heavy elements are forged in the cores of massive stars, so their presence indicates prior star formation has taken place.
We used the Cosmic Origins Spectrograph on the Hubble Space Telescope to determine the Smith Cloud’s metallicity and therefore constrain its origin. By using the technique of spectroscopy, the ultraviolet light from three background active galactic nuclei (AGN) was split into different wavelengths, and the absorption-line signatures caused by sulfur atoms in the Smith Cloud were measured. We also used radio observations from the Green Bank Telescope in West Virginia to measure how much neutral hydrogen exists in each direction. Combining the sulfur and hydrogen measurements in the three sightlines allowed us to measure the metallicity of the Smith Cloud, and we found it to be one-half of the solar value. This experiment is illustrated in Figure 2.
Figure 2: Illustration of our experiment to determine the Smith Cloud’s metallicity (courtesy Ann Feild/STScI). Ultraviolet and radio observations of three lines-of-sight through the Cloud toward background active galactic nuclei (AGN) allow us to measure the abundance of sulfur atoms.
One-half solar is too high a metallicity for the Cloud to represent a dwarf-galaxy neighbor of the Milky Way (and besides, the Cloud contains no stars). Yet it is too low to have originated in the disk of the Galaxy near the Sun, where we expect solar metallicity. However, one-half solar matches the abundances in the outer disk of the Milky Way. Indeed, if we trace back the orbit of the Smith Cloud to where it last crossed the disk, we find this occurred at about 13 kpc out from the Galactic Center (for comparison, the Sun is at about 8.5 kpc). In other words, both the metallicity and orbit of the Smith Cloud are consistent with it being flung out of the outer disk of the Milky Way, about 70 million years ago. This scenario is illustrated in Figure 3.
Figure 3: The orbit of the Smith Cloud. The Cloud is being shaped by gravitational and gas-pressure forces. The Cloud’s kinematics and metallicity suggest an origin in the outer disk about 70 million years ago. 30 million years from now, the cloud is expected to return to the disk. Courtesy Ann Feild.
These results confirm that the Smith Cloud is made of Galactic material. But they do not explain how the Cloud was launched into its orbit. Could it have been blown out of the outer disk by a cluster of supernovae? Supernovae are known to drive winds of gas and dust out of galaxies, and this would naturally explain the high velocity of the Cloud. However, the Cloud is much more massive than those observed in the vicinity of known Galactic supernovae explosions, so it would have had to be an unusually energetic supernova event.
A more exotic possibility invokes regions of dark matter, known as mini-halos, which are thought to be constantly bombarding the disk of the Milky Way. If one of these mini-halos were to accumulate Galactic gas during its passage through the disk, it could create an object like the Smith Cloud: a blob of Galactic gas held together by the gravity of unseen dark matter. Far-fetched as this may sound, mini-halos are predicted by theoretical work on galaxy formation, with simulations expecting that tens or hundreds of such mini-halos should exist for every large galaxy like the Milky Way. This creates a fascinating picture of the disk of the Galaxy, where gas can be transported from one location to another by catching a ride on a dark mini-halo. More research will explore this possibility, particularly by predicting what the observational signatures of these mini-halos should be, and seeing whether the observed morphology of high-velocity clouds can be reproduced.
For more reading, check out:
Fox et al. 2016, ApJL, 816, L11 (reporting the above results)
Galyardt & Shelton 2016, ApJL, 816, L18 (discussing dark-matter mini-halos)
Lockman et al. 2008, ApJL, 679, L21 (background on the Smith Cloud)