Ultra-Faint Dwarf Galaxies: Not as Small as We Thought?

Paper Title: Extended Stellar Populations in Ultra-Faint Dwarf Galaxies

Authors: Elisa A. Tau, A. Katherina Vivas, Clara E. Martínez-Vázquez

First Author’s Institution: Departamento de Astronomía, Universidad de La Serena, La Serena, Chile

Status: published in The Astronomical Journal [open access]

The Runts of the Galaxy Litter

The first galaxies formed only a few hundred million years after the Big Bang. Galaxies start out small, then grow by pulling in the surrounding gas and merging with each other. Most galaxies today are about 100,000 light-years across (a pretty good size estimate for the size of the Milky Way), but some of the largest galaxies can be millions of light-years across. At the other end of the scale, we have dwarf galaxies, which are closer to 10,000 light-years across. As a result of their small size, dwarf galaxies have few stars and aren’t very luminous. Dwarf galaxies often orbit larger galaxies as satellites, similar to how moons orbit planets and planets orbit stars.

Dozens of dwarf galaxies have been discovered orbiting the Milky Way. You’ve likely already heard of the most famous of these – the Large and Small Magellanic Clouds (LMC/SMC), which are visible from the Southern Hemisphere. However, most of these dwarf galaxies are much smaller and dimmer, making them difficult to make out with the naked eye. Astronomers often identify dwarf galaxies by finding a collection of stars and gas that moves together with star-mapping surveys like Gaia. Ultra-faint dwarfs (UFDs) are particularly difficult to detect because they have so few stars; where a normal dwarf galaxy may have millions or billions of stars, UFDs usually only contain thousands or tens of thousands. This lack of stars is a consequence of most of their mass consisting of dark matter.

A topic of discussion among astronomers studying dwarf galaxies is how large the dwarfs can be. Stars located near the edges of dwarf galaxies would belong to a stellar halo, which contains older stars. Stellar halos around dwarf galaxies are incredibly difficult to detect because they contain so few stars, so it is not clear how often UFDs host stellar halos or what the halo properties are. One way to trace stars that may belong to stellar halos is with RR Lyrae (RRL) variable stars. In addition to being older stars, meaning we expect them to be more common in stellar halos, these stars are a type of “standard candle,” meaning they have a known luminosity or energy output. RRLs pulsate, and the variation in their luminosities is related to the period of their pulsations, so astronomers can infer an RRL’s luminosity by measuring changes in its brightness. Previous studies have attempted to look for stellar halos in UFDs with RRLs, but this method has only been possible with the data and telescopes available in the last few years. The authors of today’s paper are continuing the search for RRLs in the outer reaches of UFDs to probe the presence of extended stellar populations and stellar halos.

Finding Ultra-Faint Dwarfs

The authors compiled a list of known UFDs around the Milky Way, many of which were discovered by the Dark Energy Survey (DES). They also compiled a sample of RRLs that were detected by a series of stellar surveys, including Gaia, the Zwicky Transient Facility (ZTF) surveys, DES, and the first Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) survey.

But how do astronomers actually determine which stars belong to a UFD, especially when they contain so few stars? Astronomers use a couple of quantities to determine this. First, they are looking for a collection of stars that are in the same area of the sky and are all roughly the same distance from Earth. The authors measure their distances by using their luminosity, since it is related to the distance by the equation L=4πFd2, where L is the luminosity (total light emitted), F is the flux (total light detected at Earth), and d is the distance. Again, astronomers know what the luminosity of an RRL is because the luminosity is related to how quickly its light output varies. By comparing this known luminosity to how much light they actually detect, astronomers can estimate the total distance between us and the RRL.

The next main variable they use is proper motion, which is a measurement of how quickly an object moves across the sky. A group of stars with similar proper motions suggests the stars are likely moving together, which is necessary for the collection to be called a dwarf galaxy. Astronomers consider the angular distance between the RRL and the center of the dwarf galaxy, the distance, and the proper motion to determine that the RRL belongs to the dwarf galaxy and not the Milky Way.

Lastly, we need to discuss a quantity called the half-light radius, which the authors denote by Rh. The half-light radius is the radius that contains half the light emitted by a galaxy. Since galaxies don’t have clear edges, the half-light radius provides a useful scale for measuring galaxy sizes. Note that the full radius of a galaxy isn’t 2Rh since galaxies are brighter in the center and dimmer near the edges. The authors are looking for RRLs that are several half-light radii from the centers of their UFDs, which is where they believe the stellar halos would exist.

Living on the Edge

The authors identify over 100 RRLs in UFDs from the surveys mentioned above and find that nearly half of the UFDs contain at least one RRL. Some examples of the RRLs and their locations in the dwarfs are shown in Figure 1.

Plots showing the positions of RR Lyrae stars in four of the ultra-faint dwarfs used in this study (Bootes 1, upper left; Bootes 3, upper right; Grus 2, lower left; and Ursa Major 1, lower right). Different symbols correspond to the different surveys used for the positions of the stars. The contours represent different radial measurements in units of the half-light radius.
Figure 1: A sample of UFDs and their associated RRLs. Symbols correspond to the catalog that the star’s position comes from, and the contours represent different radii. Note that some of the stars for the Bootes dwarf galaxies may actually belong to the Sagittarius Stream, which is a part of the Milky Way. (Figure 3 from today’s paper.)

These UFDs all contain at least one RRL at a distance of over 4Rh, which is what the authors consider to be the boundary between the main body of the UFD and the stellar halo surrounding it. Since RRLs exist here, we suspect other stars do too, indicating that these UFDs do contain stars that have a large spatial extension from the main body of the dwarf galaxy and may belong to a stellar halo.

Histograms of the distances between RR Lyrae stars and the centers of their host galaxies, in units of half-light radii. The blue histogram represents all RR Lyrae stars. The orange histogram represents a cleaned sample of stars where the authors removed stars from dwarf galaxies where they were concerned about contamination (for example, removing stars in the Bootes dwarf galaxies due to contamination from the Sagittarius Stream). The left panel shows distances up to 60 half-light radii. The right panel is a zoom-in of the left panel, showing distances up to 15 half-light radii.
Figure 2: Histograms of RRLs and their distances from the center of their UFD in units of Rh. Blue represents all RRLs, and orange represents a cleaned sample that has removed stars from UFDs where the authors are concerned about contamination (e.g., stars in the Bootes dwarf galaxies due to contamination from the Sagittarius Stream). The right panel is a zoom-in of the left panel, focusing on distances of up to 15Rh. (Figure 9 from today’s paper.)

A more comprehensive showcase of the RRLs is highlighted in Figure 2. While most RRLs (and, therefore, most stars) are expected to be within 2Rh of the center, several have been found well beyond this distance. This again highlights that RRLs can be located beyond the main body of the UFDs in what are likely stellar halos.

The authors note that there are opportunities for further studies in this area. One thing that remains unclear is why only some UFDs are observed to host RRLs, and why some don’t contain RRLs at distances that would suggest the existence of a stellar halo. The authors propose that gravitational interactions with the Milky Way may tidally disrupt the dwarfs, changing their shape and potentially leading to extended stellar populations, so dwarfs that have close encounters with the Milky Way may be more likely to host a stellar halo. In addition to further observational surveys, cosmological simulations of dwarf galaxy formation may help shed light on how and why extended stellar populations develop.

Astrobite edited by Alexandra Masegian

Featured image credit: Sergey E. Koposov, Vasily Belokurov, Gabriel Torrealba, N. Wyn Evans. “Beasts of the Southern Wild: Discovery of Nine Ultra Faint Satellites in the Vicinity of the Magellanic Clouds” (Figure 15, left panel)

About Brandon Pries

I am a graduate student in physics at Georgia Institute of Technology (Georgia Tech). I do research in computational astrophysics with John Wise, using machine learning to study the formation and evolution of supermassive black holes in the early universe. I've also done extensive research with the IceCube Collaboration as an undergraduate at Michigan State University, studying applications of neural networks to event reconstructions and searching for signals of neutrinos from dark matter annihilation.

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