The Haunting of Boötes’s Backyard

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Title: A Ghost in Boötes: The Least Luminous Disrupted Dwarf Galaxy
Authors: Vedant Chandra et al.
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian
Status: Accepted to ApJ

Some Stellar Snacks

When you think of a galaxy, your mind might conjure up images of grandiose spirals with beautiful dusty arms or the beautiful JWST image of Stephan’s Quintet: five galaxies sweeping out gas and dust while engaging in a complex dance. And when it comes to the Milky Way’s neighborhood, it may seem that Andromeda and its companions Messier 32 and Triangulum are the only other residents of our Local Group of galaxies. But besides these spectacular shapes, our universe also has other galactic guests lurking right in our backyard, hidden in not-so-plain sight.

These sneaky structures are some of our faintest neighbors, discovered to be haunting the periphery of galaxies in the late 1930s. Dubbed “dwarf galaxies,” they’re much smaller than galaxies like our Milky Way and are only home to about a billion stars, hundreds of times fewer stars than typical galaxies. The Small Magellanic Cloud — a fuzzy patch floating in the Southern Hemisphere sky that can be seen with the naked eye — is a prime example of a nearby dwarf galaxy. Our Local Group has dozens of known dwarf galaxies, usually orbiting near the biggest galaxies like ours, but not all dwarf galaxies are as fortunate — because dwarf galaxies are smaller and less massive, an interaction with a much larger galaxy can be fatal. Tidal forces from the interaction can rip the smaller galaxy apart, stretching it out and scattering its stars about the outskirts of the larger galaxy.

These galactic remnants can be seen as stellar streams in the halos surrounding dominant galaxies, and they can be vital clues in learning about the formation and evolution of present-day galaxies as well as understanding the distribution of their dark matter. Our own Milky Way harbors many detected streams, including one made by the aforementioned Magellanic Clouds, but astronomers are always on the hunt for more signs of our galaxy’s hungry past. The ghost-busting authors of today’s article uncovered another stellar stream haunting our galaxy — the faintest one so far.

One might think that because these streams are so close, they’d be easy to find, but unfortunately that’s not the case — the first stellar streams were only found in the early 1970s, half a century after the discovery of the dwarf galaxies they can originate from! Though stellar streams can be found within the Milky Way, most of them are extremely faint. In fact, they can be so faint that astronomers hunting for streams in the outskirts of our galaxy have a hard time deciding whether individual stars are part of a larger gravitationally bound structure or just part of the Milky Way foreground. This detection method relies on measuring how bright a given patch of sky is and determining if there’s a significant overdensity of stars that could be a disrupted dwarf galaxy. However, relying on brightness alone makes this method increasingly unreliable for fainter and fainter streams.

Putting the “Boo” in Boötes

Today’s authors used a different method to hunt for low-luminosity dwarf galaxies: determining the motions of stars and their chemical compositions. The team used data from the H3 Spectroscopic Survey, a collection of hundreds of thousands of stars in the Milky Way’s stellar halo that were analyzed with a spectrograph. This is an instrument that splits the light from these stars into different wavelengths so astronomers can tell what kind of elements are present and the velocity at which they’re moving. If a group of stars has the same velocity and very similar chemical composition, it’s likely that the stars were once part of a dwarf galaxy or globular cluster that was disrupted by the Milky Way.

Out of six groups of co-moving stars the authors found, one group in the constellation Boötes did not look like it belonged to any known structures. The authors identified two stars that were moving through space at the same velocity and were both pretty metal poor. To determine whether these twin stars are part of a stellar stream, the authors used the newly released Gaia Data Release 3 catalog to look for more stars around this pair that might also be moving with the same proper motion, along with additional searches through the Sloan Extension for Galactic Understanding and Exploration (SEGUE) and Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) surveys. They also narrowed down the search by selecting only stars that lay close to a specific isochrone (a line indicating a stellar population of the same age) on a color–magnitude diagram, which relates the color of stars to how bright they are. The team found an overdense population of such stars in the Gaia catalog, and they plotted and smoothed out the positions of these stars. Overlaying the member stars that exist in the H3 survey and one extra star they found in LAMOST, the team uncovered the extended structure that you see in Figure 1, aptly named “Specter” due to its ghostly nature found through spectroscopy.

plot of the stellar density in the area near the Specter stellar stream

Figure 1: Plot showing the smoothed density of stars in the area around Specter that were filtered by their proper motions and position on a color–magnitude diagram. The red dashed line shows Specter’s calculated trajectory, the blue dashed rectangle defines the selection of stars that make up Specter, the dotted orange lines are areas that are background stars, the red stars are the stars the authors picked out as being part of Specter from the H3 survey, the green diamond is the one star they found in LAMOST that they believe is part of Specter, and the grey dot-dashed line on the left shows where the galactic latitude of 35° is located. [Chandra et al. 2022]

A Fatal Attraction

Using the stars they identified, the authors modeled the structure of the stream and also analyzed its chemical composition, in part to determine whether the stream was created by a disrupted dwarf galaxy or large globular cluster. By looking at the ratios of number densities between different elements, they inferred that Specter’s progenitor object was probably forming stars for an extended period of time — when massive stars went supernova, the remnants from those explosions would enrich the gas around them. This means there were multiple generations of stars living within Specter’s progenitor object. They also fit the stream’s color–magnitude diagram with different isochrones to determine its age, and they found that the stream is most likely pretty old — more than 12 billion years old. Because Specter is also wide and has a large spread of metallicities, the authors conclude that it is a disrupted dwarf galaxy and not a globular cluster. In fact, it’s wider than other intact dwarf galaxies of similar luminosity, letting the authors infer that it might have gotten stretched out as it orbited around the uneven gravity well of the Milky Way.

So why has finding Specter been so difficult? Like a stereotypical ghost, it’s pretty much invisible to ordinary detection methods since it has very few stars and an extremely low surface brightness — in fact, the authors classify it as the faintest dwarf galaxy stellar stream ever discovered! And given its serendipitous detection, the authors worked out that the probability of finding other “ghosts” at similar distances to Specter is about 3%. This suggests the possibility that there might be up to 20–50 undetected streams between 33,000 and 65,000 light-years from the Sun, meaning there’s a good chance that many other Specter-like ghosts haunt our Milky Way. Specter’s discovery is the spark that can hopefully ignite the search for other extremely low-luminosity disrupted dwarf galaxies and their progenitors to teach us more about how galaxies can form on extremely small mass scales. The authors hope that in the near future, spectroscopy and spectroscopic surveys can be the key to uncovering some of our faintest neighbors that lurk around our galactic community, waiting to be seen.

Original astrobite edited by Aldo Panfichi.

About the author, Katya Gozman:

Hi! I’m a third-year PhD candidate at the University of Michigan. I’m originally from the Northwest suburbs of Chicago and did my undergrad at the University of Chicago. There, my research primarily focused on gravitational lensing and galaxies while also dabbling in machine learning and neural networks. Nowadays I’m working on galaxy mergers and stellar halos, currently studying the spiral galaxy M94. I love doing astronomy outreach and frequently volunteer with a STEAM education non-profit in Wisconsin called Geneva Lake Astrophysics and STEAM, as well as work at our on-campus observatory and planetarium.