Astronomers using data from the APOGEE survey at Apache Point Observatory have pinpointed a 'fossil galaxy' concealed deep within the Milky Way.
Galaxies are constantly on the move in the universe, often colliding and merging. Around 10 billion years ago, a small galaxy brushed against the young Milky Way and was absorbed. Astronomers have now identified its stellar remnants. The findings are detailed in the Monthly Notices of the Royal Astronomical Society.
The team named this ancient galaxy Heracles, drawing from Greek mythology where the hero gained immortality amid the Milky Way's birth. Its remnants form about one-third of the Milky Way's spherical halo, underscoring the collision's profound impact on our galaxy's history.
Despite comprising such a significant portion of the halo, these stars evaded detection for so long because they lie buried in the Milky Way's dense central 'heart'.
'To detect a fossil galaxy like this, we analyzed the detailed chemical compositions and motions of tens of thousands of stars,' explains Ricardo Schiavon from Liverpool John Moores University (LJMU). 'This is particularly challenging for stars at the galaxy's center, obscured by interstellar dust clouds.'
The researchers leveraged APOGEE, a spectrograph that penetrates this dust. Over the past decade, it has profiled around 500,000 stars across the Milky Way, including several thousand in the core.
Among these, a few hundred stars exhibited distinct chemical signatures and velocities, indicating an extragalactic origin. Detailed analysis revealed their precise origins and history.
The Milky Way has absorbed other dwarf galaxies in its past. Recent Gaia mission data points to one such merger just 500 million years ago.
Looking ahead, the Milky Way will encounter the larger Andromeda galaxy in about 4.5 billion years. Whether it becomes prey remains to be seen.
