In the stellar halo of the Milky Way, astronomers have discovered more than 200 distant variable stars referred to as RR Lyrae stars. The most distant of these stars are more than a million light years from Earth, nearly half the distance to our neighboring galaxy, Andromeda, which is more or less 2.5 million light years away.
The characteristic pulsations and brightness of RR Lyrae stars make them superb common candles for determining galactic distances. These new observations enabled the scientists to trace the outer limits of the Milky Way’s halo.
“This study is redefining what constitutes the outer limits of our universe,’ said Raja GuhaThakurta, UC Santa Cruz professor as well as chair of astronomy and astrophysics. “Since our galaxy as well as Andromeda are both so huge, there is hardly any space between the two galaxies.”
GuhaThakurta pointed out that the stellar halo element of our galaxy is much larger compared to the disk, which is approximately 100,000 light-years across. Our world is situated on the disk in one of its spiral arms. A main bulge lies in the center of the disk, and around it is the halo, which contains the oldest stars in the galaxy and stretches in most direction for hundreds of thousands of light years.
“The halo is the toughest part to learn, since the outer limits are far away,” stated GuhaThakurta. “The stars are extremely sparse when compared with the high stellar densities of the disk and the bulge, though the halo is dominated by dark matter and actually consists of most of the mass of the galaxy.
The new study was led by Yuting Feng, a doctoral student working with GuhaThakurta at the UCSC, and presented their results in two presentations at the Jan 9 as well as 11 meeting of the American Astronomical Society in Seattle.
Previous modeling studies had estimated the stellar halo should increase from the galactic center to more or less 300 kiloparsecs, or one million light years, according to Feng. (Astronomers measure galactic distances in kiloparsecs). (One kiloparsec is equal to 3,260 light years) The stars spotted by Feng as well as his colleagues, 208 RR Lyrae, ranged in distance from approximately 20 to 320 kiloparsecs.
“these variable stars could make use of them as reliable tracers to pin down the distances,” said Feng. “Our evaluation verify the theoretical estimations of the dimensions of the halo, so this is an important result.’
The conclusions are derived from information from the next Generation Virgo cluster Survey (NGVS), a plan that makes use of the Canada-France-Hawaii Telescope (CFHT) to examine a cluster of galaxies far beyond the Milky Way. The survey wasn’t intended to detect RR Lyrae stars, so the scientists needed to extract them from the dataset. The Virgo Cluster is a big Cluster of galaxies that consists of the giant Ellipsical galaxy M87.
In order to obtain a full exposure of M87 and the galaxies around it, the telescope also captured the foreground stars in the same area, so the data we used are kind of a by-product of that survey, “said Feng.
According to GuhaThakurta, the excellent quality of NGVS data allowed the team to obtain the most reliable and precise characterization of RR Lyrae in these distances. Lyrae are old stars which have very particular physical properties that cause them to expand and contract in a frequently repeating cycle.
“their brightness differs as an EKG, they are like the heartbeats of the galaxy, so the brightness goes up rapidly and comes down slowly, along with the cycle repeats well with this very characteristic shape,” said GuhaThakurta. “Furthermore, in case you measure their average brightness, it’s exactly the same from star to star. This particular combination is excellent for understanding the structure of the galaxy.”
There are many stars in the sky, many of which are brighter than others. But, a star may seem bright just because it’s really bright, or as it is really near, and it’s difficult to tell which one would be the brighter star. Astronomers are able to tell an RR Lyrae star from its distinctive pulsations as well as make use of its observed brightness to compute the distance it is. However, the procedures are not easy. Distant objects such as quasars can masquerade as RR Lyrae stars.
Mainly astronomers are aware of how painful it can be to get reliable tracers of these distances, Feng said. “This strong sample of distant RR Lyrae stars provides us with a very powerful tool for studying the halo and for evaluating our current models of the size and mass of our galaxy,” it stated.
This analysis is based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/IRFU, at the Canada-France-Hawaii Telescope (CFHT), which is managed by the National Research Council (NRC) of Canada, the Institut National des Sciences de l ‘Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii.