The Milky Way is slim, which is proved again by LAMOST “RR Lyae variables”. About 400 billion suns’ mass within 16,0000 light years.

The research team led by Gang Zhao at National Astronomical Observatories of Chinese Academy of Sciences (NAOC) constrained the circular velocity curve of the Milky Way, and ultimately, found an enclosed mass within 16,0000 light years of 400 billion suns’ worth, which favored the slim Milky Way in previous studies. The discovery, published in The Astrophysical Journal, is based on a sample of about one thousand halo RR Lyae variables, of which 70% are from LAMOST, an enormous spectroscopic survey of millions of stars in the Milky Way. Using LAMOST measurements of RR Lyae variables’ center-of-mass radial velocities in the Galactic stellar halo, the researchers inferred the amount of gravity required to keep the motion of these stars to determine the circular velocity curve and the mass. Most of the gravity comes from an extended invisibly mysterious dark matter.

“RR Lyae variables are bright enough to be seen in distant halo. However, they are sparse in the Milky Way. So, it is time-consuming to observe them spectroscopically. Thanks to millions of stellar spectra from LAMOST, it can provide spectra for two times more RR Lyae variables than SDSS”, said Gang Zhao.

In this project, the research team carefully constructed the largest sample of RR Lyae variables with spectra by crossing match RR Lyrae variables identified from Catalina Survey with LAMOST/SDSS spectra. Spectra from LAMOST and SDSS can provide radial velocities and metallicities, and the distances of RR Lyae variables can be determined from their period-luminosity-metallicity relations. In addition, all variables have pulsations, which lead to periodic mutations of radial velocities. Therefore, it is crucial and difficult for RR Lyae variables to correct the velocity effect of pulsations. The researchers used the exact observation time of the RR Lyae variables in LAMOST in combination with their periods and luminosities from Catalina Survey to correct the velocity effect of variables’ pulsations successfully.

The most recent previous studies of circular velocity curve and mass of the Milky Way used either “blue horizontal branch (BHB)” stars or “K type giant (KG)” stars. By contrast, RR Lyae variables have most precise distance estimations, which are helpful on mass constrain. The mass of the Milky Way is one of the most fundamental parameters for understanding how the Galaxy formed and solving missing satellites problem, but it is hard to measure because we live in the Milky Way. Restricted by the development of spectroscopic observation, the uncertainty of mass measurement is still large. Our Galaxy could be either fat (2 trillion suns’ mass in total) or slim (1 trillion suns’ mass in total). The measurements of circular velocity curve and mass distribution by Gang Zhao’s team support the slim Milky Way found by BHB and KG. This project shows great potential of LAMOST to observe more RR Lyae variables. The researchers expect more distant RR Lyae variables with spectra to explore the mass distribution in the Milky Way in future based on LAMOST or even large spectroscopic surveys.