A team of European Southern Observatory (ESO) astronomers looking to better understand the formation history of our Milky Way Galaxy has started creating the first age-map of the Galaxy’s bulge. Their current results show that a period of star formation lasting around 4 billion years created the complex structure in the bulge.
The infrared map constructed from the VVV survey of the inner Milky Way showing the three areas studied. Image credit: ESO / M. Rejkuba / F. Surot Madrid / E. Valenti.
The Milky Way is a spiral galaxy with a bulge at the center, thousands of light years in diameter, that contains about a quarter of the total mass of stars.
Previous studies have shown that the bulge hosts two components: a population of metal-poor stars that have a spherical distribution, and a population of metal-rich stars that form an elongated bar with a ‘waist,’ like an X or a bi-lobed peanut. However, analyses of the ages of the stars to date have produced conflicting results.
Now, ESO astronomer Marina Rejkuba and colleagues have analyzed the color, brightness and spectral information on chemistry of individual stars to produce the age-map of the Milky Way’s bulge.
The researchers have used simulated and observed data for millions of stars from the VISTA Variables in the Via Lactea (VVV) infrared survey of the inner Milky Way and compared them with measurements of the metal content of around 6,000 stars across the inner bulge from a spectroscopic survey carried out on ESO’s Very Large Telescope.
“We analyzed the color and brightness of stars to find those that have just reached the point of exhausting their hydrogen fuel-burning in the core, which is a sensitive age indicator,” Dr. Rejkuba said.
“Our findings were not consistent with a purely old Milky Way bulge, but require star formation lasting around 4 billion years and starting around 11 billion years ago.”
“The youngest stars that we see are at least 7 billion years old, which is older than some previous studies had suggested.”
One of the three areas studied, showing a superposition of almost 3 million stars belonging to different Milky Way structures along the line of sight: the Milky Way disk, as well as the stars belonging to the bulge region. Image credit: ESO / M. Rejkuba / F. Surot Madrid / E. Valenti.
The team’s results are based on the analysis of three areas of the VVV infrared map, which, combined, make up the largest area studied so far in the Milky Way bulge. In all three areas, the findings on the age range of the stars are consistent.
“Previous studies have told us that the metal-rich stars in the bar are likely to be the youngest stars,” said team member Dr. Francisco Surot Madrid.
“Whilst we can’t disentangle which star belongs to the bar/peanut or the spheroid component in the data we are using, our results tell us that the bar was already formed about 7 billion years ago and there were no large amounts of gas inflowing and forming stars along the bar after that.”
“The final map will show us the star formation rate as a function of both age and metallicity for the stars across the bulge,” said team member Dr. Elena Valenti.
“This will be an important ingredient in telling the complete story of the formation of the Milky Way bulge.”
The astronomers presented their results today at the European Week of Astronomy and Space Science (EWASS) in Liverpool, UK.
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Marina Rejkuba et al. VVV constrains the age distribution of the Milky Way bulge. EWASS 2018, abstract # 1118
