Galactic Archaeology with the Subaru `\=Onohi`ula Prime Focus Spectrograph Strategic Program

TL;DR

This paper outlines a large-scale Galactic Archaeology survey using Subaru's Prime Focus Spectrograph to study galaxy structure, evolution, and dark matter properties in the Local Group through extensive spectroscopic observations.

Contribution

It presents a novel, comprehensive survey strategy leveraging PFS to analyze dwarf galaxy density profiles, galaxy assembly histories, and Milky Way accretion events.

Findings

Modeling of velocity and abundance distributions for six dwarf galaxies.

Observation of 30,000 stars in M31's halo and outer disk.

Velocity and metallicity measurements for tens of thousands of stars.

Abstract

The recently commissioned Subaru `\=Onohi`ula Prime Focus Spectrograph (PFS) will obtain spectra from nearly 2,400 fibers that cover 1.24 square degrees. The 360 night Subaru Strategic Program for PFS is dedicating approximately one-third of its allocation (130 nights) to study the structure and evolution of galaxies in the Local Group. This Galactic Archaeological survey has three pillars. (1) We will determine whether the mass density profiles of dwarf galaxies are consistent with cusps, as expected for cold dark matter, or cores, as expected from alternative dark matter theories or baryonic feedback. We will deduce the density profiles as a function of radius from modeling of the full line-of-sight velocity and abundance distributions for six dwarf galaxies. Our total sample will consist of 18,000 member stars to beyond the nominal tidal radius of each system. (2) From measurements of the [alpha/Fe] abundance ratio, we will learn the difference in assembly history of the two most massive galaxies in the Local Group: M31 and the Milky Way. We will observe 30,000 member stars over 45 square degrees of M31's halo and outer disk. (3) We will uncover how the most fragile (outer) part of the Milky Way responded to accretion events both in the distant past (such as Gaia-Sausage Enceladus) and in more recent history (such as the Sagittarius dwarf spheroidal galaxy). To support this study, PFS will provide velocities and metallicities--from which, in combination with photometry, we will deduce ages--for tens of thousands of main-sequence stars out to a Galactocentric distance of ~30 kpc.