In the Milky Way’s stellar halo, astronomers have found more than 200 distant variable stars known as RR Lyrae stars. Nearly half the distance to our neighboring galaxy, Andromeda, about 2.5 million light-years away, is traveled by the most distant of these stars, which is more than a million light-years from Earth.
The stars in the constellation RR Lyrae provide ideal “standard candles” for measuring galactic distances because of their distinctive pulsations and brightness. This new data allowed the researchers to determine the extreme boundaries of the Milky Way’s halo.
Raja GuhaThakurta, professor and chair of astronomy and astrophysics at UC Santa Cruz, said: “This study redefines what constitutes the outer boundaries of our galaxy. Our galaxy and Andromeda are both large; there is hardly any space between the two galaxies.”
“The stellar halo component of our galaxy is much larger than the disk, which is about 100,000 light-years across. Our solar system is located in one of the spiral arms of the disk. At the center of the disc is a central bulge and around it is the halo, which contains the oldest stars in the galaxy and stretches hundreds of thousands of light-years in all directions.”
“The halo is the hardest to study because the outer limits are so far away. The stars are sparse compared to the high stellar densities of the disc and bulge, but the halo is dominated by dark matter and contains most of the galaxy’s mass.”
Yuting Feng, a PhD student working with GuhaThakurta at UCSC, said: “Previous modeling studies had calculated that the stellar halo should extend to about 300 kiloparsecs or 1 million light-years from the galactic center. (Astronomers measure galactic distances in kiloparsecs; one kiloparsec is equal to 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 determine the distances. Our observations confirm the theoretical estimates of the halo’s size, 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 examine a cluster of galaxies far beyond the Milky Way, provided the basis for the conclusions. The study was not designed to look for RR Lyrae stars, so the researchers had to actively look for them in the data. The massive cluster of galaxies 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 is a by-product of that research.”
GuhaThakurta claims that the excellent quality of the NGVS data allowed the team to obtain the most reliable and accurate characterization of RR Lyrae at these distances. RR Lyrae are ancient stars with specific physical properties that cause them to expand and contract in a regularly repeating cycle.
GuhaThakurta said: “The way their brightness varies is like an EKG — they’re like the heartbeats of the galaxy — so the brightness goes up quickly and goes down slowly, and the cycle repeats perfectly with this very characteristic shape. Moreover, if you take their average measure 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 can look bright because it’s very bright or very close, and it can be hard to tell the difference. Astronomers can identify an RR Lyrae star by its characteristic pulsations and then use the observed brightness to calculate how far away it is. However, the procedures are not simple. 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 powerful tool to study the halo and test our current models of the size and mass of our galaxy.”
Researchers presented their findings in two lectures at the meeting of the American Astronomical Society in Seattle on Jan. 9 and 11.