The Milky Way’s galactic center as seen from the Atacama Desert in Chile. Credit: G. Hüdepohl/ESO

A stream of at least 150 ancient variable stars has been confirmed to extend some 130,000 light years beyond our own galaxy’s stellar halo --- on the fringes of the Intergalactic Medium, where aside from hot gas and dark matter, space-time becomes as sparse as the deep Sahara.

The confirmation, based on analysis of 10 billion year-old dying RR Lyrae stars in the Gemini constellation, was done by an international team of astronomers who report their findings in The Astrophysical Journal. In the paper, the team argues for a “statistically significant” cache of stars extending some 326,000 light years and spanning a good portion of our galaxy and beyond.

If looking at the Milky Way “edge on” this “Gemini stream” would appear to pop out in roughly perpendicular fashion in relation to the North Galactic halo, before looping over the top of our galaxy in a relatively narrow stream.

Such stellar streams are believed to be the last remnants of dwarf galaxies destroyed by gravitational interactions with our own burgeoning galaxy. At least a dozen such dwarf galaxies are thought to have once surrounded our own Milky Way.

“We found about 150 RR Lyrae stars in this Gemini stream, the Milky Way’s most distant galactic stream found to date,” said Andrew Drake, a Caltech astronomer and the paper’s lead author. But based on stellar evolution this is only the tip of a stellar iceberg, Drake notes that the stream likely contains at least 150,000 stars in total.

By analyzing measurements of 1207 RR Lyrae stars taken by the Catalina Mount Lemmon Telescope in Tucson, the team was able to observe dramatic brightness variations in these stars, taking place over periods as short as half a day. Because RR Lyrae stars have the same average brightness, astronomers use them as standard candle distance indicators.

Upon knowing the RR Lyrae stars’ distances, Drake and colleagues were able to determine their position across the sky and noted that the stream almost followed a line that extended farther and farther out.

It’s still uncertain exactly where this Gemini stream comes from or where it goes, says Drake. He notes it could be part of the Sagittarius stream, a remnant of the Sagittarius dwarf galaxy, long assumed to be one of the building blocks of our own Milky Way.

Or the Gemini stream could be a remnant of another as yet undetected dwarf galaxy still in the process of being torn asunder by gravitational tidal interactions within our own Milky Way.

Based on models of galaxy formation, for years theorists have expected to find a lot more of these dwarf galaxies around our own Milky Way. That is, if galaxies formed in part from accretion of other small galaxies.

The central disk of our Milky Way galaxy with the Gemini stellar stream highlighted in the top right of the image. Credit: Andrew Drake/Axel Mellinger

The next step, Drake says, is to definitively determine whether this newly-detected stream was part of the Sagittarius dwarf galaxy or whether it came from a wholly-undetected dwarf galaxy. If the latter, there should be evidence for it in the outer regions of the galactic disk.

“If this is a stellar stream from an undiscovered dwarf galaxy," said Drake, "that would boost the [theory] that our Milky Way has accreted lots of smaller galaxies.”

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