A ground-breaking new survey of some 500 ancient stars within our Milky Way provides evidence that our galaxy likely formed in a more solitary environment than previously thought. Or at least that’s one of the survey’s conclusions put forth at the 241st meeting of the American Astronomical Society (AAS) earlier this month in Seattle.
This directly contradicts conventional theory which posits that our galaxy was built up over time by the merger of many smaller galaxies.
The survey also produced evidence that our galaxy’s first generation of stars were on average more massive than stars being produced in the Milky Way’s star-forming regions today.
The inner Galaxy is the oldest part of our Milky Way, so it is best for studying the very beginning of our galaxy’s history, study lead Madeline Lucey, an astrophysicist and graduate researcher at the University of Texas in Austin, told me.
Studying the chemical and dynamical properties of Carbon-Enhanced Metal-Poor (CEMP) stars in the Milky Way’s inner galaxy can shed light on their origins and how our galaxy evolved, Lucey noted in her AAS presentation.
But as she and colleagues also noted in a 2021 paper that appeared in the journal The Monthly Notices of the Royal Astronomical Society (MNRAS), the center of our galaxy is also one of the least understood components. Historically, it has been difficult to study. High levels of crowding, which makes it difficult to resolve individual stars, and of extinction, which makes it difficult to achieve high signal-to-noise ratio data, have prevented substantial studies of the galactic bulge until recently, the authors write.
It is thought that star formation in an environment with only hydrogen, helium and lithium is quite different from star formation today where there are more elements, says Lucey. Without the heavier elements, it is difficult to form smaller stars and therefore the first generation of stars would generally be more massive, she says. I find evidence that the generation of stars that came before these carbon-enhanced stars had a top-heavier initial mass function, says Lucey.
She wrote a dissertation focused on the chemical abundance analysis of some 500 metal-poor stars using a multi-object fiber-fed spectrograph, dubbed FLAMES, at the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile.
Depending on the brightness of the star, each was observed for between 3 and 10 hours in June and July of 2012, says Lucey. FLAMES has about 100 optical fibers and you can place each fiber-optic cable, so they measure the light from a single specific star, she says.
This allows us to be able to get data from some 100 individual stars at a time, says Lucey. As VLT time is extremely valuable, we likely wouldn’t have been able to observe this many stars without this efficient instrument, she says.
For one thing, it’s thought that bottom-up galaxy formation would create a classical bulge, a structure that is formed from small galaxies merging early in a galaxy’s formation, says Lucey. We do not see evidence of such a structure in the Milky Way, prompting the conclusion that our galaxy’s early formation may have been more isolated than previously thought, she says.
Despite gaps in our current theory of our galaxy’s formation and evolution, the fact that we now have the technology to pierce the gas and dust in our Milky Way’s central regions is testament to how far galactic astronomy has progressed in just the last decade.
There is even speculation that our own Milky Way may still harbor a few of the universe’s very first, long-sought-after so-called ‘Population III’ stars.
There is some work that suggests Population III stars could still exist in the inner galaxy today, says Lucey. Searches are underway for these stars, but it is difficult because it is a ‘needle in the haystack’ problem, she says.
“There are billions of stars in our galaxy, and it is impossible to observe them all,” said Lucey.