In the stellar halo of the Milky Way, astronomers have found more than 200 far-off variable stars known as RR Lyrae stars. Nearly half of the distance to our neighboring galaxy, Andromeda, located approximately 2.5 million light-years away, is covered by the most distant of these stars, which is more than a million light-years from Earth.
The stars in the RR Lyrae constellation offer ideal “standard candles” for gauging galactic distances because of their distinctive pulsations and brightness. The researchers were able to determine the Milky Way’s halo’s outermost limits thanks to these new data.
Raja GuhaThakurta, professor and chair of astronomy and astrophysics at UC Santa Cruz, said, “This study is redefining what constitutes the outer limits of our galaxy. Our galaxy and Andromeda are both big; there’s hardly any space between the two galaxies.”
“The stellar halo component of our galaxy is much bigger than the disk, which is about 100,000 light-years across. Our solar system resides in one of the spiral arms of the disk. In the middle of the disk is a central bulge, and surrounding it is the halo, which contains the oldest stars in the galaxy and extends for hundreds of thousands of light years in every direction.”
“The halo is the hardest part to study because the outer limits are so far away. The stars are sparse compared to the high stellar densities of the disk and the bulge, but the halo is dominated by dark matter and contains most of the galaxy’s mass.”
Yuting Feng, a doctoral student working with GuhaThakurta at UCSC, said, “Previous modeling studies had calculated that the stellar halo should extend to around 300 kiloparsecs or 1 million light years from the galactic center. (Astronomers measure galactic distances in kiloparsecs; one kiloparsec equals 3,260 light-years.) The 208 RR Lyrae stars detected by Feng and his colleagues ranged from about 20 to 320 kiloparsecs.”
“We could use these variable stars as reliable tracers to pin down the distances. Our observations confirm the theoretical estimates of the size of the halo, so that’s an important result.”
Data from the Next Generation Virgo Cluster Survey (NGVS), a project using the Canada-France-Hawaii Telescope (CFHT) to explore a cluster of galaxies far outside the Milky Way, formed the basis for the conclusions. The survey was not meant to look for RR Lyrae stars, so the researchers had to hunt for them in the data actively. The enormous galaxy cluster known as the Virgo Cluster is home to the massive elliptical galaxy M87.
Feng explained, “To get a deep exposure of M87 and the galaxies around it, the telescope also captured the foreground stars in the same field, so the data we used are a by-product of that survey.”
GuhaThakurta claims that the excellent quality of the NGVS data enabled the team to obtain the most reliable and precise characterization of RR Lyrae at these distances. RR Lyrae is old stars with specific physical properties that cause them to expand and contract in a regularly repeating cycle.
GuhaThakurta said, “The way their brightness varies looks like an EKG—they’re like the heartbeats of the galaxy—so the brightness goes up quickly and comes down slowly, and the cycle repeats perfectly with this very characteristic shape. In addition, if you measure their average brightness, it is the same from star to star. This combination is fantastic for studying the structure of the galaxy.”
“The sky is full of stars, some brighter than others, but a star may look bright because it is very luminous or very close, and it can be hard to tell the difference. Astronomers can identify an RR Lyrae star from its characteristic pulsations, then use its observed brightness to calculate how far away it is. The procedures are not simple, however. More distant objects, such as quasars, can masquerade as RR Lyrae stars.”
Feng said, “Only astronomers know how painful it is to get reliable tracers of these distances. This robust sample of distant RR Lyrae stars gives us a potent tool for studying the halo and testing our current models of the size and mass of our galaxy.”
Researchers presented their findings in two talks at the American Astronomical Society meeting in Seattle on January 9 and 11.