From AAS NOVA : “Seeking the Origins of Galactic Stellar Streams”



31 March 2021
Susanna Kohler

Like NGC 5907, the edge-on galaxy shown here, our own Milky Way hosts faint streams of stars that loop around it. But where did those streams originate? Credit:R. Jay GaBany]

The Milky Way is enwreathed in long streams of stars that hold clues to everything from our galaxy’s history to the nature of dark matter. New research has now identified the likely origins of some of these subtle ribbons.

Streams Across the Sky

The orbital energy vs. angular momentum of the stars in 23 of the Milky Way’s stellar streams (colored and labeled data), as compared to field stars (black data). Credit:Bonaca et al. 2021.

Stellar streams are associations of stars that are grouped into elongated filaments arcing around a host galaxy. These filaments are thought to be produced when a stream progenitor — like a globular cluster or a satellite dwarf galaxy — is disrupted by its host galaxy’s tidal forces. Stars are drawn out from the progenitor into a tidal stream that then orbits the host galaxy; the progenitor itself may remain connected to the stream, orbit separately, or disrupt entirely.

We’ve observed stellar streams in other galaxies (like NGC 5907, shown above), but we needn’t look that far away — our own Milky Way is host to more than 60 catalogued streams. Of these thin trails, only a handful have been connected to a known progenitor, like a surviving globular cluster. The rest have unknown origins, leaving a number of open questions that only now, with current observations, have answers within reach.

In a recent study led by Ana Bonaca (Center for Astrophysics | Harvard & Smithsonian), a team of scientists has leveraged the incredible precision of the Gaia space observatory to hunt for the origins of 23 cold stellar streams in the Milky Way halo.

Locations in orbital phase space of the 23 stellar streams, labeled by whether they have a dwarf galaxy progenitor (pentagon) or globular cluster progenitor (star). Only one stream, Svöl, falls into the region associated with possible in situ formation (rather than having been brought in via a dwarf galaxy). Credit: Bonaca et al. 2021.

A Disrupted Home

Bonaca and collaborators make use of improved proper motions provided in Gaia’s Early Data Release 3 for stars in these 23 streams. By analyzing the energies and 3D angular momenta of these streams, and by examining how the streams are distributed in physical space, the authors are able to identify the probable progenitors for most of the streams.

According to the authors’ results, only 1 of the streams plausibly originated from a globular cluster that was born in the Milky Way. The vast majority instead originated from dwarf galaxies that the Milky Way has accreted. Some of the streams were produced from the dwarf galaxies themselves; others were likely formed from disrupted globular clusters that orbited those dwarf galaxies.

Several of the 23 streams have similar properties, suggesting that many originated from the same progenitors. The authors identify original host dwarf galaxy candidates for 20 of the streams, and they point to 6 specific globular clusters as the origin of 8 of the streams.

Illuminating Dark Matter

Sky map showing the 6 globular clusters (crosses) that the authors associate with 8 stellar streams (circles). Credit: Bonaca et al. 2021.

What can we do with this information? Understanding the origin of these stellar streams allows us to better trace their paths, how long they’ve been orbiting, and what other gravitational interactions they may have had over time. These details are valuable not just for understanding galaxy evolution, but also for mapping out the big-picture distribution of dark matter in our galaxy and studying the small-scale structure of dark matter in the streams’ host galaxies.

Further expansion of Bonaca and collaborators’ work to the other stellar streams orbiting the Milky Way will rely on continued high-quality proper motion measurements of these faint and distant sources. Look for more results as future Gaia data is released!


“Orbital Clustering Identifies the Origins of Galactic Stellar Streams,” Ana Bonaca et al 2021 ApJL 909 L26.

See the full article here .


Please help promote STEM in your local schools.

Stem Education Coalition


AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.