Scientists Recreate Spiral Arms Of Ancient Milky Way
March 4, 2013

Scientists Take The Milky Way Galaxy Back In Time

Brett Smith for — Your Universe Online

Using observations made possible by the European Space Agency´s INTEGRAL satellite, a pair of French scientists has been able to recreate how the Milky Way´s spiral arms looked millions of years ago.

According to their report in the Astrophysical Journal, the two researchers based their recreation on the observations of high mass X-ray binaries (HMXBs), pairs of massive stars that are surrounded with the remnants of supernovas.

These stellar systems contain either a neutron star or a black hole that pulls in solar matter from its massive companion in a process known as accretion. The consumption of solar hydrogen and helium occurs so quickly that one star explodes as a supernova within a few tens of millions of years, making the system relatively unstable when compared to the rest of the universe.

The INTEGRAL spacecraft has allowed for the rapid expansion of the list of known HMXBs and supergiant X-ray binaries in the galaxy. Out of a total of more than 200 HMXBs, 35 are supergiants.

Previous studies have shown that HMXBs are associated with the spiral structure of the galaxy and the two French scientists decided to carry out an analysis of their distribution and locations in the Milky Way.

First, the team decided to determine the distances of a sample set of HMXBs. By comparing their brightness with their light spectrum, the astrophysicists could tell their approximate temperature and energy output.

"By using this novel technique, we were able to find a strong correlation between the positions of HMXBs and star-forming complexes in the Milky Way," said Alexis Coleiro, from the Université Paris Diderot, France.

Their analysis showed HMXBs are clustered with star formation complexes, so-called galactic nurseries known for birthing stars from swaths of stellar material. These regions are typically 1000 light years across.

"As expected, the current distribution of high mass X-ray binaries is closely linked to the stellar nurseries where they were born, some tens of millions of years ago, because the HMXBs have not been in existence long enough to have migrated very far from their birthplaces," said Coleiro.

"We know that HMXBs are born in star forming complexes, which are usually located in the Galaxy's spiral arms," said Sylvain Chaty, Coleiro´s supervisor in the study. "Star formation in the spiral arms is triggered by density waves, regions of enhanced density where interstellar gas and dust are slowed down and compressed.

"We also know that HMXBs are short-lived, surviving for only a few tens of millions of years, so it ought to be possible to link them to their birthplaces,” he added. “As a result, we decided to investigate the relationship between the binary systems and the spiral structure of the Milky Way."

Based on knowledge of how density waves behave, the two scientists were able to calculate the possible HMXB locations in relation to the spiral arms at certain times in the past. They then compared these calculated positions with the current locations of 13 HMXBs.

"For the first time, we have accurately derived the distances and distribution of a large sample of high mass X-ray binaries in our Galaxy, bringing new constraints on their formation and evolution," said Coleiro.