Crater II: A Ghostly Galaxy Losing Its Grip
A. Katherina Vivas and collaborators explore whether the faint Milky Way satellite known as Crater II (Cra II) is currently being torn apart by the Galaxy’s gravity. Discovered only in 2016, Cra II is unusually large and dim, with stars spread out across a vast area but moving at surprisingly low speeds. Such properties make it one of the “coldest” and most diffuse galaxies known. Previous studies had already hinted that Cra II might be in the process of tidal disruption, essentially being stretched out and losing stars into space, but this work provides new and deeper observations to confirm that picture.
Background: The Mystery of Crater II
Cra II stands out because it defies expectations for small galaxies orbiting the Milky Way. Its stars show a velocity dispersion of only about 2.7 km/s, far lower than expected for a system of its size. Some scientists have proposed that this can only be explained if Cra II has lost most of its mass through repeated gravitational encounters with the Milky Way. Others have even suggested that its strange behavior could test alternative theories of gravity, such as Modified Newtonian Dynamics. Using previous observations and Gaia data, astronomers estimate that Cra II’s orbit takes it very close to the Milky Way every few billion years, close enough for strong tidal forces to strip away its stars. Its stellar populations tell a similar story: two ancient bursts of star formation occurred about 10–12 billion years ago, and the younger population appears more concentrated in the center, consistent with the idea that the outer stars have been pulled away over time.
Observations and Methods
To test whether Cra II is truly disintegrating, Vivas and her team used the Dark Energy Camera (DECam) on the 4 m Blanco Telescope in Chile. They surveyed thirteen fields around the galaxy, extending up to 13 times its half-light radius in the farthest direction, nearly ten degrees across the sky. They focused on identifying RR Lyrae variable stars, which are excellent “standard candles” for measuring distance, as well as Anomalous Cepheids (AC), a rarer type of variable star. In total, they detected 97 periodic variable stars, 46 of which appear to be genuine Cra II members (37 RR Lyrae and 7 AC). The team used these stars to map Cra II’s structure, tracing how far the galaxy’s outer regions extend and how the distances of its stars change across the sky.
Results: Evidence for Tidal Tails
The distribution of the RR Lyrae stars clearly reveals that Cra II is not a compact, spherical system. Instead, the stars stretch into two long “tails” extending to the northeast and southwest, features typical of galaxies being disrupted by gravity. The authors measured a striking gradient in distance: stars on one side are about 45 kiloparsecs (∼147,000 light-years) farther away than those on the opposite side. The northeast tail is closer to Earth, while the southwest tail lies farther away. An over-density of RR Lyrae stars was also found about 3.25 degrees from the center of Cra II, possibly representing a clump or stream segment. The analysis suggests that more than half of the galaxy’s total population now lies outside its main body, meaning Cra II has already lost much of its mass to tidal stripping.
The Role of Anomalous Cepheids
Vivas and her team also examined 7 Anomalous Cepheids, brighter and more massive variable stars that can indicate younger or binary stellar populations. Using data from Gaia DR3 and published period–luminosity relationships, the authors compared their brightness and periods with expectations for stars at Cra II’s distance. Most of these ACs are consistent with being genuine members of Cra II or its tails, though one outlier (V178) likely belongs to the Milky Way’s foreground. Interestingly, most ACs appear to be pulsating in the first overtone mode rather than the fundamental one, suggesting a diverse evolutionary history. The presence of ACs in the tidal tails implies that not only old stars but also intermediate-age stars are being carried away.
Conclusions: A Galaxy in the Process of Dissolving
The findings leave little doubt: Crater II is currently being disrupted. The newly discovered variable stars trace tidal tails extending nearly 10° across the sky, with one side reaching distances of up to 140 kiloparsecs. The distance gradient, the elongated stellar distribution, and the presence of both RR Lyrae and Anomalous Cepheids all confirm that Cra II is in the late stages of tidal dissolution. Vivas and collaborators note that their deeper imaging allowed them to detect fainter stars and better define the galaxy’s structure than previous surveys. Looking ahead, they point out that the upcoming Vera Rubin Observatory’s LSST will be able to map similar faint stellar streams throughout the Milky Way’s halo, offering new insights into how galaxies like Cra II are slowly torn apart by gravity.
Source: Vivas
