By Simon Birrer and Ethan O. Nadler
Two powerful probes of dark matter on small scales—strong gravitational lensing and ultra-faint dwarf galaxies—join their Wonder Twin powers to "Activate!" world-leading constraints on the nature of the dark matter particle in a study that recently appeared in The Astrophysical Journal. With this study, which was led by KIPAC researchers (including the authors), we were able to show that combining these probes provides a framework to detect "dark halos," clumps of dark matter that have no associated visible light, using the vast amounts of deeper data promised by upcoming surveys.
Most of the matter in the Universe is dark. Cold dark matter (CDM), which accounts for roughly 25% of the total matter and energy in the Universe, provides the backbone for structure and galaxy formation, shaping the cosmos we see today. Nonetheless, the particle nature of CDM is a key mystery in cosmology and particle physics, and KIPAC scientists are tackling the question of its properties in myriad ways.
CDM cosmology makes a crucial, unverified prediction: a huge number of self-gravitating dark matter clumps, called "halos," should exist that are too small to form galaxies. These dark, tiny halos are an important testing ground for dark matter models. In particular, the number of these small halos that formed in the early Universe and survive until today is sensitive to the microscopic properties of the dark matter particle.
For example, in one popular class of theories known as warm dark matter (WDM), dark matter particles move quickly enough to prevent the formation of small halos. A type of neutrino called sterile neutrinos has been hypothesized as a possible WDM candidate. Other promising dark matter candidates motivated by particle physics, including ultra-light axions, would also impact the abundance of small halos. The constraints placed on halo formation by such dark matter candidates as warm dark matter and axions were discussed in this April, 2018 research highlight by one of us (Nadler), and were more recently the subject of this August, 2020 News Release from Fermilab.
Using these tiny halos to learn about dark matter properties has a catch: because there is very little luminous matter in these systems, we can’t see them easily, if at all. Instead, cosmologists and astrophysicists have carefully crafted techniques to measure the subtle gravitational influences these halos have on visible matter and radiation.
Hunting for Tiny Halos
Many independent observations teach us that galaxies live in dark matter halos, and that more massive halos generally host brighter galaxies. Turning this logic around implies that fainter galaxies live in smaller dark matter halos. The most straightforward way to hunt for small halos is simply to search for these tiny galaxies, but detecting such ultra-faint dwarf galaxies is extremely challenging; the only such galaxies we can currently detect orbit as satellites of our Milky Way galaxy.
Our census of the faintest galaxies has dramatically improved in recent years thanks to the Dark Energy Survey (DES), with KIPAC scientists playing key roles in both the discovery and theoretical interpretation of these systems. By comparing the abundance of dwarf galaxies detected by DES and other modern surveys to detailed simulations, one author (Nadler), KIPAC Director Risa Wechsler, and others at KIPAC and in the DES collaboration have set precise constraints on the minimum halo mass for galaxy formation, along with a variety of dark matter properties.
It is especially helpful to combine this technique with complementary methods to detect the presence of tiny halos that do not rely on whether they are associated with visible matter. A technique called "strong gravitational lensing" (covered in a previous Oct 2016 KIPAC Research Highlight; see also this January 2020 NASA News Release) provides just this tool.
Stepping back a moment to review: gravity bends spacetime itself, and with it the path light takes through the cosmos. One special case is when a massive object, such as a galaxy, bends the light of a distant object such that light reaches us along multiple different paths, effectively creating multiple images of the same background object. The light’s path is perturbed by the presence of tiny halos along the line of sight, regardless of whether these halos are completely dark or harbor ordinary matter. This technique to probe dark matter goes by the name "strong gravitational lensing."
Indeed, the strength and occurrence rates of such perturbations of light paths enable us to determine the presence and abundance of dark matter halos in our Universe. This technique was used in the original strong lensing analysis, developed by one of us (Birrer), to place constraints on warm dark matter.
Combining Strong Lensing and Dwarf Galaxies
In a recent paper, the authors and Wechsler teamed up with other lensing experts, including Daniel Gilman, to combine dark matter constraints from strong lensing and dwarf galaxies. This required developing a unified modeling framework capable of describing the population of small halos that host very faint galaxies near the Milky Way as well as the halos near strongly lensed galaxies billions of light years away.
The results are clear: these two probes, which yield results that are consistent with the generally accepted CDM paradigm when analyzed individually, strengthen constraints on dark matter properties when combined. For example, this work provides the most stringent constraints to date on the mass of warm dark matter particles, requiring that the particles must weigh more than 10 kiloelectronvolts/c2 (or about 2% of the electron mass) to avoid wiping out too many small halos. In other words, lighter WDM particles would not produce enough structure to simultaneously explain strong lensing measurements and Milky Way satellite galaxy observations. This is the strongest constraint on the mass of warm dark matter particles set by any technique to date, and rules out large regions of parameter space for popular WDM models like sterile neutrinos.
Most excitingly, by probing different aspects of the halo populations associated with the Milky Way and strong lenses, the new probe combination provides complementary information relative to the dark matter constraints set individually. Such a complementary viewpoint on the problem enables us to extract more information from the datasets, while each individual probe is forced to make assumptions on aspects not directly available to it. Milky Way satellites probe both the numbers of and three-dimensional distribution of nearby dark matter halos, while strong lenses probe the total mass in dark matter near the lensed galaxy in a two-dimensional projection on the sky. Combining these techniques helps differentiate models with fewer small halos from models with fewer halos overall.
For the first time, these results demonstrate that the benefits of combining independent cosmological probes—which is a technique very familiar to scientists working on, for example, the cosmic microwave background and galaxy surveys—are equally important for studies of the smallest structures in the Universe.
The Future is Bright Dark
"This new study is exciting not only because it shows how two great techniques can be even more powerful together," says KIPAC director Risa Wechsler, "but also because it points the way forward to the potential of future observations to really shed light on the nature of dark matter by understanding its impact ton the smallest objects." Next-generation observational facilities, including the Vera C. Rubin Observatory, are expected to discover hundreds of dwarf galaxies—both those nearby, those orbiting as satellites of the Milky Way, and out to several million light years—as well as thousands of strongly lensed systems. Indeed, current forecasts suggest that with ten years of Rubin Observatory data and follow-up observations, astronomers should either obtain strong evidence for the existence of completely dark halos around the Milky Way, or for an indication that the CDM paradigm does not agree with the data (based on measurements of objects with masses of about one million to one hundred million solar masses).
Our new framework and constraints are particularly exciting in anticipation of the data coming online over the next decade. Combining complementary observational probes will maximize the chance to uncover the microphysical nature of dark matter. And that is something astrophysicists have been very keen on tracking down for a very long time!
