An international team of astronomers have released the first large-scale map of the ages of various components at the center of the Milky Way and have established that the core of our galaxy was assembled over 4 billion years ago. The study will be presented at the European Week of Astronomy and Space Science (EWASS) in Liverpool.
The Milky Way is a spiral galaxy that bulges in the center. This bulge is thousands of light-years across and contains a quarter of the total stars of our galaxy. The age of the bulge has puzzled astronomers for awhile. Observations that used photometry, the color and brightness of stars, found evidence that it was old, but a spectroscopic technique, where the light of stars is analyzed in more detail, showed a significant fraction of young stars.
In work to be submitted to the journal Astronomy & Astrophysics, the team used regular main-sequence stars that have exhausted fuels and are about to start burning helium as an age probe. Their findings suggest that a single age cannot be assumed for the core.
“This is the first time that the age distribution in the central parts of the Milky Way, its bulge, is determined from deep photometric observations that cover both a wide area as well as sufficient depth such that we can map the age-sensitive features: the main-sequence turnoff stars,” co-author and European Southern Observatory astronomer Dr Marina Rejkuba, who will present the results at EWASS, told IFLScience.
The center of the Milky has two components – a spherical distribution at the very core made of older stars and an elongated bar made of younger objects. The team estimate that the younger stars are about 7 billion years old. This is older than previous observations have suggested but significantly younger than the 11-billion-year-old stars in the spherical core. This might help us understand how the bar formed.
“The Milky Way bar has formed quite quickly and has not been growing significantly through the addition of newly formed stars since the past 7 billion years,” Dr Rejkuba told IFLScience. “Once the full map of the distribution of stellar ages is complete, the theorists who are building the numerical models of the Milky Way bulge formation will have new constraints for their models based on large statistics and wide area observations.”
The work is part of the PhD research of Francisco Surot Madrid, who co-led the study with Dr Elena Valenti. The team hope to expand their observations as well as gain more information about the stars, like their motion, to better understand how the bulge came to form.