## We’re on a Collision Course with a Gas Cloud

A giant cloud of mostly hydrogen gas with enough material to make over a million suns is heading towards our Milky Way at a speed of 45 miles per second. Called the Smith Cloud (after Gail Bieger-Smith who discovered it in 1963), this 9,800 × 3,300 ly high velocity cloud (HVC) is about 40,000 ly distant and is expected to slam into our Milky Way galaxy in about 27 million years, causing the birth of many new stars a quarter-way round the galaxy from us.

The Smith Cloud is located in the constellation Aquila, and has an apparent diameter around 11° across its long axis. It is only visible using radio telescopes (spin-flip transition of neutral atomic hydrogen), or by detecting hydrogen absorption lines Doppler shifted and superimposed upon the spectra of more distant stars that are shining through the cloud.

The origin of the Smith Cloud is unknown. It may have originated within the Milky Way galaxy itself, or it may be extragalactic. The upcoming collision may not be the first time the Smith Cloud has encountered the disc of the Milky Way. It may be embedded in a large halo of dark matter which would have kept the cloud from being completely disrupted during any past encounters.

The Smith Cloud is a great example of an object that would never have been discovered were it not for radio astronomy. Felix J. Lockman, who has published extensively on the Smith Cloud, has created Radio Astronomy: Observing the Invisible Universe for The Great Courses. Dr. Lockman’s engaging lecture style, his clear explanations, and thorough knowledge of the subject matter makes this the perfect introduction to the subject. Highly recommended!

Incidentally, Jay Lockman discovered a region in Ursa Major that is relatively free of neutral hydrogen gas and dust, thus affording a clearer view into the distant universe. It is named, appropriately, the Lockman Hole.

References

Alig, C. et al. “Simulating the Impact of the Smith Cloud.” The Astrophysical Journal 869 (2018): 1-6.
arXiv:1901.01639 [astro-ph.GA]

Hu, Y. et al. “Magnetic field morphology in interstellar clouds with the velocity gradient technique.” Nature Astronomy (2019): 1-7.
arXiv:2002.09948 [astro-ph.GA]

Lockman, F.. “Accretion Onto the Milky Way: The Smith Cloud.” Proceedings of the International Astronomical Union 11 (2015): 9 – 12.
arXiv:1511.05423 [astro-ph.GA]

As the expanding universe cooled, the first neutral1 hydrogen atoms formed about 380,000 years after the Big Bang (ABB), and most of the hydrogen in the universe remained neutral until the first stars began forming at least 65 million years ABB.

The period of time from 380,000 to 65 million years or so ABB is referred to as the “dark ages” since at the beginning of this period the cosmic background radiation from the Big Bang had redshifted from visible light to infrared so the universe was truly dark (in visible light) until the first stars began to form at the end of this period.

All the while, neutral hydrogen atoms occasionally undergo a “spin-flip” transition where the electron transitions from the higher-energy hyperfine level of the ground state to the lower-energy hyperfine level, and a microwave photon of wavelength 21.1061140542 cm and frequency 1420.4057517667 MHz is emitted.

Throughout the dark ages, the 21 cm emission line was being emitted by the abundant neutral hydrogen throughout the universe, but as the universe continued to expand the amount of cosmological redshift between the time of emission and the present day has been constantly changing. The longer ago the 21 cm emission occurred, the greater the redshift to longer wavelengths. We thus have a great way to map the universe during this entire epoch by looking at the “spectrum” of redshifts of this particular spectral line.

380,000 and 65 million years ABB correspond to a cosmological redshift (z) of 1,081 and 40, respectively. We can calculate what the observed wavelength and frequency of the 21 cm line would be for the beginning and end of the dark ages.

$\lambda _{obs} = (z+1)\cdot \lambda_{emit}$

The observed wavelength (λobs) for the 21 cm line (λemit) at redshift (z) of 1,081 using the above equation gives us 22,836.8 cm or 228.4 meters.

$\nu = \frac{c}{\lambda }$

That gives us a frequency (ν) of 1.3 MHz (using the equation above), where the speed of light c = 299,792,458 meters per second.

So a 21 cm line emitted 380,000 years ABB will be observed to have a wavelength of 228.4 m and a frequency of 1.3 MHz.

Using the same equations, we find that a 21 cm line emitted 65 Myr ABB will be observed to have a wavelength of 8.7 m and a frequency of 34.7 MHz.

We thus will be quite interested in taking a detailed look at radio waves in the entire frequency range 1.3 – 34.7 MHz, with corresponding wavelengths from 228.4 m down to 8.7 m.2

The interference from the Earth’s ionosphere and the ever-increasing cacophony of humanity’s radio transmissions makes observing these faint radio signals all but impossible from anywhere on or near the Earth. Radio astronomers and observational cosmologists are planning to locate radio telescopes on the far side of the Moon—both on the surface and in orbit above it—where the entire mass of the Moon will effectively block all terrestrial radio interference. There we will finally hear the radio whispers of matter before the first stars formed.

1 By “neutral” we mean hydrogen atoms where the electron has not been ionized and resides in the ground state—not an excited state.

2 Incidentally, the 2.7 K cosmic microwave background radiation which is the “afterglow” of the Big Bang itself at the beginning of the dark ages (380,000 years ABB), peaks at a frequency between 160 and 280 GHz and a wavelength around 1 – 2 mm. So this is a much higher frequency and shorter wavelength than the redshifted 21 cm emissions we are proposing to observe here.

References

Ananthaswamy, Anil, “The View from the Far Side of the Moon”, Scientific American, April 2021, pp. 60-63

Burns, Jack O., et al., “Global 21-cm Cosmology from the Farside of the Moon”, https://arxiv.org/ftp/arxiv/papers/2103/2103.05085.pdf

Koopmans, Léon, et al., “Peering into the Dark (Ages) with Low-Frequency Space Interferometers”, https://arxiv.org/ftp/arxiv/papers/1908/1908.04296.pdf

Ned Wright’s Javascript Cosmology Calculator, http://www.astro.ucla.edu/~wright/CosmoCalc.html

## Radio Telescope in a Carpet

The lunar farside would be a splendid place to do radio astronomy. First, the cacophony of the Earth would be silenced by up to 2,160 miles of rock. Second, lacking an atmosphere, a radio telescope located on the lunar surface would be able to detect radio waves at frequencies that are absorbed or reflected back into space by the Earth’s ionosphere.

Radio waves below a frequency of 10 MHz (λ ≥ 30 m) cannot pass through the ionosphere to reach the Earth’s surface. The Earth’s atmosphere is variably opaque to radio waves in the frequency range of 10 MHz to 30 MHz (λ = 10 to 30 m), depending upon conditions. The Earth’s atmosphere is mostly transparent to frequencies between 30 MHz (10 m) and 22 GHz (1.4 cm).

Not surprisingly, electromagnetic radiation of a non-terrestrial origin having wavelengths longer than 10 meters has been little studied. If we look, we might discover new types of objects and phenomena.

The best part is the lunar radio telescope wouldn’t have to be a steerable parabolic dish, but instead could be a series of dipole antennas (simple metal rods or wires) imbedded into a plastic carpet that could easily be rolled out onto the lunar surface. This type of radio telescope is “steered” (pointed) electronically through phasing of the dipole elements.

Even though the ever-increasing number of lunar satellites should be communicating at wavelengths far shorter than 10 meters, care must be taken to minimize their impact (both communication and noise emissions) upon all lunar farside radio astronomy.

If you thought light pollution is bad (and it is!), radio pollution for radio astronomers is much worse.  Even years ago, terrestrial pollution of the radio spectrum tended to swamp faint celestial sources at many frequencies, and in 1958 the FCC established a 13,000 square mile rectangular region of West Virginia, Virginia, and Maryland as the National Radio Quiet Zone.  Two facilities within this protected region—whose natural topography helps to screen out many terrestrial radio emissions—are the Sugar Grove Station and the Green Bank Observatory near Green Bank, West Virginia.  The world’s largest fully-steerable radio telescope dish was built at Green Bank in 1956.  Though the original 300-ft. dish collapsed in 1988 due to a structural failure, it was rebuilt in 2000 as the Robert C. Byrd Green Bank Telescope, a leading facility for radio astronomy.

Counties wholly within the NRQZ, where many radio-emitting sources are regulated or banned outright, are Alleghany, Augusta, Bath, Highland, Nelson, and Rockbridge in Virginia, and Hardy, Pendleton, Pocahontas, Randolph, and Upshur in West Virginia.

The NRQZ isn’t the only radio quiet zone.  Here are some others:

• Arecibo Observatory, Puerto Rico
• Astronomy Geographic Advantage Act (AGAA), South Africa
• Atacama Large Millimeter Array (ALMA), Chile
• Five hundred meter Aperture Spherical Telescope (FAST), China
• Institute for Radio Astronomy in the Millimeter Range (IRAM), Spain
• Itapetinga Radio Observatory (IRO), Brazil
• Large Millimeter Telescope (LMT), Mexico
• Pushchino Radio Astronomy Observatory, Russia

The best place in the world to do radio astronomy is not on our world at all but instead on the far side of the Moon.  Radio telescopes deployed on the lunar farside could “listen” to the universe with absolutely no interference from Earth.  The solid body of the Moon (and its lack of an atmosphere) would completely block all radio signals and noise emanating from the Earth and Earth orbit.  And some radio telescopes could be quickly and easily deployed (think long-wire antennas rather than radio dishes).  Of course, the Moon itself will need to be designated as a radio quiet zone so that any lunar colonies, rovers, or satellites operate at frequencies and times that will not interfere with scientific work.  Maybe infrared or optical lasers would be a better way to communicate?

How would data from a lunar farside radio observatory be transmitted back to Earth?  One way would be to have a dedicated lunar satellite that receives data from the radio observatory while it is traveling over the lunar farside.  It would then re-transmit that data to Earth while it is traveling over the Earth-facing nearside.

Another (probably more expensive) approach would be to have a series of radio relay towers spaced at intervals from the radio observatory around to the lunar nearside where a transmitter could send the data back to Earth.

A third choice would be to locate the radio observatory in a libration zone along the border between the lunar nearside and farside.  At a libration zone radio observatory, data would be collected and stored until each time libration allows a direct line-of-sight to Earth.

The crater Daedalus, near the center of the lunar farside, has been suggested as the best location for a radio astronomy facility on the Moon (Pagana et al. 2006).

There is also a region above the farside lunar surface where radio emissions from Earth and Earth-orbiting satellites, would be blocked by the Moon, called the “Quiet Cone”, as illustrated in the diagram below.

The Earth-Moon L2 Lagrange point (EML2) is probably going to be within the lunar quiet cone.  Because L2 is an unstable Lagrange point, a radio telescope in the quiet cone would need to be in a halo orbit about EML2, and a tight one at that to avoid “seeing” any radio emissions from the highest Earth-orbiting satellites.

References
Antonietti, N.; Pagana, G.; Pluchino, S.; Maccone, C.
A proposed space mission around the Moon to measure the Moon Radio-Quiet Zone, 36th COSPAR Scientific Assembly. Held 16 – 23 July 2006, in Beijing, China.