Celestial MUSYC: cosmic ABCs keep astronomers spellbound

Natural History, Sept, 2006 by Charles Liu

Acronyms are everywhere, alphabetically infecting daily life, IMHO (in my humble opinion). So perhaps we astronomers can be forgiven for sliding down the slippery slope of cryptic capital letters: MHD (magnetohydrodynamics), SETI (search for extraterrestrial intelligence), LGM (little green men).

Overall, though, we're pretty good at keeping things unacronymed, giving fairly simple names to cosmic phenomena. A reddish star that's really big, for instance, is called a red giant; a whitish star that's really small is a white dwarf. But it's back to the ABCs when we name our scientific projects, from the strictly functional (VLT for Very Large Telescope) to the embarrassingly cute (LALA for Large Area Lyman-Alpha survey).

Lest you think I'm blamelessly poking fun at my colleagues, I freely admit my own recent foray into acronymic vanity. A couple of years ago, I was invited to join a major international scientific collaboration that would examine four widely separated patches of sky, each a little larger than the size of the full moon. The survey would make images at many wavelengths of electromagnetic radiation (infrared, all the colors of visible light, ultraviolet, and X-ray) with the goal of examining how populations of galaxies changed with the passage of cosmic time.

As work progressed on this ambitious cosmic survey, we realized we needed a public name for our project. Members of the team ruminated on this question for days, weeks, even months. Who knows how much highly trained scientific brainpower was siphoned away from studying the mysteries of creation for the creation of acronyms? Finally, after dozens of candidates were proposed, everyone agreed that the acronym for our survey would acknowledge the two primary institutions in the collaboration: Yale University, and the University of Chile, in Santiago. One question remained. Would it be the Multiwavelength Survey by Chile-Yale (MUSCY), or the Multiwavelength Survey by Yale-Chile (MUSYC)? In the end, though a MUSCY survey might smell good enough, we agreed that MUSYC would sound much more harmonious.

Its whimsical name notwithstanding, MUSYC has moved forward rapidly. And perhaps not surprisingly, the objects we have studied so far are generally referred to by their acronyms: AGN, DRGs, and LAEs. AGN, active galactic nuclei, are supermassive black holes that act as gravitational dynamos at the hearts of galaxies, converting the potential energy of infalling matter into powerful outflowing jets and electromagnetic radiation. DRGs, distant red galaxies, are, well, galaxies that are distant (between 8 and 12 billion lightyears from Earth) and red, emitting so much more red light than blue light that it sometimes looks as if their stars are older than the universe itself [see "Seeing Red," by Charles Liu, March 2005].

LAEs--Lyman-alpha emitting galaxies or Lyman-alpha emitters--are particularly intriguing as markers of cosmic evolution, because they have no obvious counterparts in the local universe (AGN and DRGs at cosmic distances--many billions of light-years away--have similar counterparts closer by). LAEs get their name from the American physicist Theodore Lyman, who in the early twentieth century discovered that hydrogen gas emits ultraviolet light. LAEs are small, young, and bursting with new stars. According to a study led by Eric Gawiser, a postdoctoral fellow at Yale and one of MUSYC's principal investigators, they appear to be the primitive progenitors of modern, mainstream galaxies such as our own Milky Way.

When an atom is struck by another atom or a sufficiently powerful photon, the sudden influx of energy can tear one or more of the atom's outermost electrons away from the rest of the atom. The remaining atom, now ionized, with a net positive charge, readily recombines with other, free-flying electrons nearby. The new electrons, however, can fall only to certain discrete "levels" opened up by loss of the earlier electrons: levels dictated by the atom's quantum mechanical structure. If it helps, think of the structure as a big pachinko machine--those vertical pegboardlike games in which little steel bails fail from top to bottom--and imagine each recombining electron as one of the steel balls dropping downward through the machine.

Every time the electron falls from one atomic energy level to a lower one, the atom releases energy in the form of electromagnetic radiation--that is, light--at a specific wavelength or color. So within every cloud of ionized gas, electrons combining with ions are like bails at the top of atomic pachinko machines, in which the electrons cascade downward until they reach their lowest energy state. And the more likely a particular bounce, the more light of that color emerges from the cloud.

For hydrogen atoms, the most likely intra-atomic step downward is the one that takes the electron from its second-lowest energy state to its lowest, or "ground," state. The light emitted by this bounce, whose wavelength was first measured by Lyman, is the brightest spectroscopic feature emitted by any cloud of ionized hydrogen gas. Because most of the atoms in the universe, by far, are hydrogen, you would think that "Lyman-alpha" radiation would be easy to detect. It would be worth detecting, too, because Lyman-alpha radiation is a good measure of how much ionized hydrogen there is in any particular place in the universe.