A quick follow-up to yesterday’s post. The idea of a stream of debris or even large objects like comets or asteroids from another star continues to resonate with me. The odds on identifying such a stream in terms of origin seem stupendous, but the benefits of doing so would be obvious. I notice that another kind of stellar stream is in the news, one involving not debris but entire stars. The Icarus stream is a grouping of stars that seem to have been tidally disrupted by the Milky Way, probably from an earlier encounter between the parent galaxy and a dwarf galaxy.

Digging a bit, I learned that we can carry the idea of stellar streams back to the work of Donald Lynden-Bell, who in 1995 proposed the stream concept to explain the long structure or filament of stars evidently tidally stripped from the Sagittarius Dwarf Spheroidal Galaxy, the latter being a satellite galaxy of the Milky Way. The Sgr dSph, as it is known, actually contains four globular clusters within it. It travels a polar orbit around the Milky Way at a distance of some 50,000 light years from galactic center, and has evidently passed through the plane of the galaxy at least several times.

Image: The Sagittarius dwarf galaxy, a small satellite of the Milky Way that is leaving a stream of stars behind as an effect of our Galaxy’s gravitational tug, is visible as an elongated feature below the Galactic center and pointing in the downwards direction in the all-sky map of the density of stars observed by ESA’s Gaia mission between July 2014 to May 2016. Scientists analyzing data from Gaia’s second release have shown our Milky Way galaxy is still enduring the effects of a near collision that set millions of stars moving like ripples on a pond. The close encounter likely took place sometime in the past 300–900 million years, and the culprit could be the Sagittarius dwarf galaxy. Credit: ESA/Gaia/DPAC,CC BY-SA 3.0 IGO.

Stellar streams, like the debris streams we considered yesterday, are the result of tidal interactions that are now well studied, resulting in the identification of further structures like the Icarus stream (the Gaia observatory is frequently relied upon for the relevant data). Such stretched out streams of stars contain clues about the Milky Way’s gravitational potential and the distribution of mass, which accounts for the continuing interest in the formations. The Icarus stream apparently came from a dwarf galaxy with a stellar mass of about one billion solar masses in a low-inclination orbit.

We now have a new paper from a team of scientists led by Paola Re Fiorentin (Observatory of Turin), again using Gaia data along with results from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and the Galactic Archaeology with HERMES (GALAH). 81 members of the Icarus stream have been identified, upon which the group has managed to conduct chemical analysis and investigate the dynamics of the stream, learning that it is at least 12 billion years old. Everything about this work supports the notion of the disrupted galaxy remnant scenario.

But we’re early in the game when it comes to these probes of galactic interactions and what they tell us about the earliest days of the Milky Way. The paper adds this:

…even though we cannot exclude the contamination of unevolved low metallicity in situ stars, we argue that the Icarus stream is consistent with debris of a low-inclination prograde dwarf galaxy with a stellar mass ∼ 10⁹M_⊙. In any case, we remark that this scenario assumes the existence of a protogalactic disk formed in situ at ages > 12 Gyr (cf. Xiang & Rix 2022). A similar scenario was suggested by Carter et al. (2021) that proposed the accretion of low-α stars from a co-rotating dwarf galaxy onto a primordial high-α disk.

Image: The Milky Way and its halo of stars, many of which have been cannabalised via collisions with other galaxies (Image credit: ESA/Gaia/DPAC, T Donlon et al. 2024; Background Milky Way and Magellanic Clouds: Stefan Payne-Wardenaar).

In the quotation above, the term low-α stars refers to stars low in the elements formed through the fusion of helium nuclei in what are known as alpha-capture processes. These would include oxygen, magnesium, sulfur, silicon, etc. Low-α stars are considered to have formed in regions where star formation was relatively slow. Studying this variable can offer insights into star formation histories. What all this means is that the Icarus stream seems consistent with the scenario of galaxy disruption.

But the paper adds:

However, the origin of metal-poor stars with disk kinematics is currently a matter of lively debate in the astronomical community. We think that such controversial interpretations could derive from the different selection criteria that may generate kinematically or chemically biased samples.

Such studies probe the formation history of the galaxy, and are in their early stages. The Icarus stream does appear to have formed outside the Milky Way, which would be consistent with everything we’re learning about the formation of stellar streams. All of which points to the complex interactions that move material among the stars, but reinforces the difficulty of deducing the origin of material from any single star.

The Lynden-Bell paper on the Sagittarius stream is “Ghostly streams from the formation of the Galaxy’s halo,” Monthly Notices of the Royal Astronomical Society, Volume 275, Issue 2 (July 1995), pp. 429–442 (abstract). The Paola Re Fiorentin paper is “Icarus revisited: An Ancient, Metal-poor Accreted Stellar Stream in the Disk of the Milky Way,” accepted at The Astrophysical Journal and available as a preprint.