Hubble Space Telescope Probes Archeology of Milky Way's Bulge

Many disc galaxies, including our own Milky Way Galaxy, have a central concentration of stars called a ‘bulge.’ For decades, astronomers had a simple view of Milky Way’s bulge as a quiescent place composed of extremely old stars. Now, a new analysis of about 10,000 Sun-like stars reveals that our Galaxy’s bulge is a dynamic environment of variously aged stars zipping around at different speeds. This conclusion is based on nine years’ worth of archival data from the NASA/ESA Hubble Space Telescope.

This composite image shows the central region of our Milky Way Galaxy. The Galactic center is located within the bright white region to the right of and just below the middle of the image. Image credit: NASA / ESA / SSC / CXC / STScI.

A team of astronomers led by Dr. Will Clarkson from the University of Michigan-Dearborn found that the motions of bulge stars are different, depending on a star’s chemical composition.

Stars richer in elements heavier than hydrogen and helium have less disordered motions, but are orbiting around the Milky Way’s center faster than older stars that are deficient in heavier elements.

“There are many theories describing the formation of our Galaxy and its bulge,” said Dr. Annalisa Calamida, from the Space Telescope Science Institute.

“Some say the bulge formed when the Galaxy first formed about 13 billion years ago. In this case, all bulge stars should be old and share a similar motion.”

“But others think the bulge formed later in the Galaxy’s lifetime, slowly evolving after the first generations of stars were born. In this scenario, some of the stars in the bulge might be younger, with their chemical composition enriched in heavier elements expelled from the death of previous generations of stars, and they should show a different motion compared to the older stars.”

“The stars in our study are showing characteristics of both models. Therefore, this analysis can help us in understanding the bulge’s origin.”

The researchers divided the stars by their chemical compositions and then compared the motions of each group.

They determined the stars’ chemical content by studying their colors and divided them in two main groups according to their heavy-element (iron) abundance.

The chemically enriched stars are moving twice as fast as the other population.

“By analyzing nine years’ worth of data in the archive and improving our analysis techniques, we have made a clear, robust detection of the differences in the motion for chemically deficient and chemically enriched Sun-like stars,” Dr. Clarkson said.

“We hope to continue our analysis, which will allow us to make a 3D chart of the rich chemical and dynamical complexity of the populations in the bulge.”

The astronomers studied Sun-like stars because they are so abundant and easily within Hubble’s reach. Previous observations looked at brighter, aging red giant stars, which are not as plentiful because they represent a brief episode in a star’s lifetime.

“Hubble gave us a narrow, pencil-beam view of the Galaxy’s core, but we are seeing thousands more stars than those spotted in earlier studies,” Dr. Calamida said.

“We next plan to extend our analysis to do additional observations along different sight-lines, which will allow us to make a 3D probe of the rich complexity of the populations in the bulge,” Dr. Clarkson added.

The team presented its results January 11 at the 231st Meeting of the American Astronomical Society in Washington, D.C.

_____

William I. Clarkson et al. 2018. Chemically-dissected Rotation Curves of the Galactic Bulge from Hubble Space Telescope Proper Motions on the Main Sequence. 231st AAS Meeting, abstract # 411.04

Let's block ads!(Why?)

Read Again Hubble Space Telescope Probes Archeology of Milky Way's Bulge : http://ift.tt/2DsyhS4

Let's block ads! (Why?)