Of Dwarf Galaxies and Dark Matter
by Adam Hadhazy
The hunt for dark matter runs straight through the universe's tiniest galaxies
The Author
Some of the biggest clues to solving the mystery of dark matter might be found in some of the smallest cosmic places. Dark matter has confounded for decades as the "missing mass" that explains how spinning galaxies do not fling their outer stars off into space. Dark matter is apparently very substantial, outnumbering normal matter about six to one, yet unobservable. Most likely, dark matter is a kind of as-yet-undiscovered elementary particle that hardly ever interacts with normal matter, except through gravity. Yet extremely sensitive searches for these hypothetical dark matter particles have turned up nothing. That has left scientists to continue to try to gain insights into dark matter's nature by studying its apparent effects, which while writ large all over the universe, are arcane. Dark matter's fingerprints are all over the grandiose structure of the universe, comprised of great walls and filaments of galaxies, on down to individual clusters of galaxies. Yet one of the best places to study something so big is actually where things get small—in dwarf galaxies. Because these galaxies are small and have few stars, they are reckoned to be dark matter-dominated, and thus ideal laboratories for studying the enigmatic stuff. To wit, multiple studies published recently have taken this tack. Bit by bit, dark matter may just be coming into the light, and courtesy of the universe's most unassuming galactic entities.
Spying super-faint galaxies' imprint to probe dark matter
Researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University have synergistically combined a particular type of observation, involving a particular type of galaxy, to find new limits on the properties of dark matter. The particular observation involves so-called strong gravitational lensing, when the gravity from a massive foreground object (like a big galaxy) bends light streaming from a bright background object. The particular kind of galaxy is called an ultra-faint dwarf galaxy. Various theories of dark matter offer different predictions about a minimum mass threshold needed for these small, dim galaxies to form in the universe. The theories manifest in measuring how many of the lilliputian galaxies detectably exist. That's a tall order, though, because ultra-faint dwarfs can only be seen just outside our Milky Way before they get lost in the grander cosmic glare of larger galaxies. So what the KIPAC researchers did was to look for tiny brightness perturbations in the strongly warped light in gravitational lenses. Those perturbations could plausibly be caused by otherwise unseeable dwarf galaxies associated with the big lensing galaxy. The observations helpfully rule out some dark matter theories while giving a boost to others.
How star-starved, stretched-out dwarf galaxies end up in cosmic voids
Speaking of dwarf galaxies, not all are created equal. Some dwarf galaxies are paradoxically large, but still retain dwarf status in the sense that they contain very few stars, given their whopping size. Astronomers' name for these creatures of the cosmic night? Ultra-diffuse galaxies, or UDGs. To better understand the populations of galaxies inhabiting our universe, researchers at the Massachusetts Institute of Technology's Kavli Institute for Astrophysics and Space Research (MKI) ran simulations that produced a rare kind of UDG, called a quenched UDG. The "quenched" refers to the galaxy no longer producing new stars, having somehow been stripped of cool gas necessary for starmaking. In the simulations, these quenched UDGs did not appear in association with galaxy clusters as is often actually observed, but way out in the great empty voids of space between clusters. To figure out how quenched UDGs could form or end up in such hinterlands, the MKI researchers essentially ran their simulations backwards. The work showed that quenched UDGs can form in galaxy clusters, but with unusually high angular momentum. Translation: Nascent dwarf galaxies can whirl off from their birth cluster, stretching the galaxy out and explaining the diffuseness of its stellar concentrations. Subsequent gravitational interactions with the bigger galaxies in the cluster then could have finally chucked the UDG out into the void, peeling off its starmaking fuel in the process. The findings suggest astronomers should be able to find quenched UDGs out in the barren prairies of the cosmos.
Sorting out dark matter in dwarf galaxies
Mark Vogelsberger, an MKI astrophysicist who co-led the previously described quenched UDG study, is a coauthor of another recent study dealing, once again, with dwarf galaxies. The specific bugaboo explored in this other study is the cuspy halo problem. In cosmic simulations of galaxy formation, dwarf galaxies tend to have dark matter density that increases toward their cores; however, when observing dwarf galaxies and inferring their dark matter content by tracking the rotations of stars through the galaxy, dwarf galaxies do not display this dark matter architecture. The new study explores two solutions to this problem, both pf which could be at work. One possibility is that supernova explosions of massive stars fling normal matter around in the galaxies. That moving matter gravitationally pulls dark matter along with it, and over time smooths out dark matter's initially unequal distribution. The dark matter might also interact with itself (though still not with normal matter, other than through gravity), as some theories about dark matter's nature hold. Some of this self-interaction, coupled with supernova bursts, succeeds in producing dwarf galaxies that closely match observations, the study found, offering further insight into galaxy formation and dark matter's properties.
"Hycean" worlds expand notions of habitability
The ongoing search for habitable planets that could host alien life has logically sought out Earthly twins—worlds with our temperature conditions, oceans, atmospheric makeup, and so on. But not so fast, argue researchers at the Kavli Institute for Cosmology, Cambridge (KICC) at the University of Cambridge. In a new study, they argue for more inclusive considerations of potential habitability, and specifically for a type of world that would be ordinarily dismissed as not being remotely conducive to life. The planets in question could be bigger than Earth—so-called super-Earths—and be found way out of the "habitable zone," the orbital band around a star where temperatures are clemently terrestrial. These proposed planets would be covered in a global ocean and have thick hydrogen atmospheres. Serendipitously, such worlds are thought to exist in abundance. Their thick atmospheres could trap or block out heat, enough either way, to keep that global ocean from freezing or boiling away. Organisms could safely live in the water, which would protect them from the otherwise crushing atmospheric pressure. The KICC researchers, led by Nikku Madhusudhan, dub these worlds "Hycean" planets, combining "hydrogen" and "ocean." It's an outside-the-box concept that goes to show how extraterrestrial life could very well find a way, though in ways we do not readily expect.
Discovering exoplanets thanks to gravitational waves
Gravitational waves have been all the rage in astrophysics since their direct discovery by the MKI co-led Laser Interferometer Gravitational-wave Observatory (LIGO) project. Grav waves studied to date have all come from black holes and neutron stars. But future gravitational wave instruments placed in space will tune in on different sets of waves made by still other kinds of supremely dense, compact objects—namely, white dwarf stars. These leftovers from typical stars like our Sun can occur in binary systems. As the white dwarfs whirl about each other, they should produce detectable ripples in spacetime. Researchers at the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University have made the novel suggestion that exoplanets orbiting double white dwarfs could impart additional signatures upon the gravitational waves streaming from these systems. Intriguingly, because these kinds of gravitational waves can be sensed from well beyond the Milky Way and out into our local galactic group, it's possible that this theoretical planet-hunting method could turn up solar systems in whole other galaxies.