The Milky Way Tomography with Subaru Hyper Suprime-Cam: Halo substructures
Yoshihisa Suzuki
Graduate Student
Tohoku University
In the context of the current benchmark model, the so-called ΛCDM model, galaxies like the Milky Way (MW) are thought to have been formed through the repeated merging and accretion of smaller stellar systems. Simulations based on this model suggest that the traces of these assemblies are imprinted as spatially inhomogeneous substructures in the halo, a diffuse region surrounding the galaxy. However, these structures are challenging to observe due to their low surface brightness of 28-32 mag/arcsec².
In this study, we utilize photometric data obtained with the Hyper-Suprime Camera (HSC) mounted on the Subaru Telescope to explore the substructures in the MW halo. This dataset has the advantage of achieving depth and width, with a 5σ limiting magnitude of 26.5 and a footprint of 1200 square degrees. Using abundant faint main-sequence stars as tracers, we have developed a method to detect old, metal-poor stellar systems based on stellar evolution models. As a result, we have revealed the presence of substructures in the directions of Pisces and Boötes at a distance of 60 kpc from the Galactic center.
The former is known as Pisces overdensity using RR Lyrae and blue horizontal branch stars, but this study reveals that it has a more widely spread, stream-like structure. It has been proposed that it may originate from the Large Magellanic Cloud (LMC) or Gaia Enceladus/Sausage (GE/S), and our study suggests that its origin is at least a metal-poor system. The latter is a previously undiscovered substructure, and its origin may be debris from the LMC on its second passage around the MW.
This study reveals that the halo of the MW retains traces of the past orbits of galaxies such as the LMC and GE/S as unrelaxed spatial distributions. The detailed origin of substructures is expected to be revealed by the Subaru/Prime Focus Spectrograph, which is scheduled to begin operations next year.
Hosted by Prof. Kirby