Extragalactic Science, Cosmology and Galactic Archaeology with the Subaru Prime Focus Spectrograph (PFS)

TL;DR

The paper advocates for the Subaru Prime Focus Spectrograph (PFS) as a powerful, massively multiplexed 3-arm spectrograph that enables three complementary science programs: cosmology, Galactic archaeology (GA), and galaxy evolution, across a broad redshift range. It details a ~300-night Subaru Strategic Program designed to measure BAO and RSD with high precision using emission-line galaxies, map the Milky Way and M31 chemo-dynamics with a medium-resolution mode, and probe galaxy growth from z~1 to z~7 including reionization-era populations. The authors provide extensive simulations and survey designs, outlining target selections, exposure times, field choices, and realistic systematics and instrumental requirements, and they discuss synergy with HSC imaging and Gaia astrometry for enhanced cosmological and near-field studies. The work argues that PFS will fill a critical gap between galaxy surveys and Lyα forest measurements, offering a unique combination of volume, wavelength coverage, and multiplexing to deliver transformative insights into dark energy, dark matter halos, and the early universe. If realized, PFS with its planned capabilities and collaborations could become a cornerstone facility for LSST-Euclid-TMT era cosmology and near-field cosmology.” wrap in $…$ if mathematical expressions appear; here kept narrative with minimal equations.

Abstract

The Subaru Prime Focus Spectrograph (PFS) is a massively-multiplexed fiber-fed optical and near-infrared 3-arm spectrograph (N_fiber=2400, 380<lambda<1260nm, 1.3 degree diameter FoV), offering unique opportunities in survey astronomy. Here we summarize the science case feasible for a survey of Subaru 300 nights. We describe plans to constrain the nature of dark energy via a survey of emission line galaxies spanning a comoving volume of 9.3 (Gpc/h)^3 in the redshift range 0.8<z<2.4. In each of 6 redshift bins, the cosmological distances will be measured to 3% precision via BAO, and redshift-space distortions will be used to constrain structure growth to 6% precision. In the GA program, radial velocities and chemical abundances of stars in the Milky Way and M31 will be used to infer the past assembly histories of spiral galaxies and the structure of their dark matter halos. Data will be secured for 10^6 stars in the Galactic thick-disk, halo and tidal streams as faint as V~22, including stars with V < 20 to complement the goals of the Gaia mission. A medium-resolution mode with R = 5000 to be implemented in the red arm will allow the measurement of multiple alpha-element abundances and more precise velocities for Galactic stars, elucidating the detailed chemo-dynamical structure and evolution of each of the main stellar components of the Milky Way Galaxy and of its dwarf spheroidal galaxies. For the extragalactic program, our simulations suggest the wide avelength range will be powerful in probing the galaxy population and its clustering over a wide redshift range. We propose to conduct a color-selected survey of 1<z<2 galaxies and AGN over 16 deg^2 to J~23.4, yielding a fair sample of galaxies with stellar masses above ~10^{10}Ms at z~2. A two-tiered survey of higher redshift LBGs and LAEs will quantify the properties of early systems close to the reionization epoch.

Figures (35)

• Figure 1: A brief overview of the baseline design of PFS instruments, which consist of components Wide Field Corrector, Field Rotator, Prime Focus Unit, and Fiber Positioner. A Fiber Connector relays light to four identical fixed-format 3-arm twin-dichroic all-Schmidt Spectrographs providing continuous wavelength coverage from 380nm to 1.26$\mu$m.
• Figure 2: Expected signal-to-noise ($S/N$) ratio for measuring the [ O ii] emission line as a function of redshift; the blue, green and red curves show the results for the PFS blue, red and IR arms in Table \ref{['tab:pfs_spec']}, respectively, for an total emission line flux of $5\times 10^{-17}$ erg/cm$^2$/s. To properly account for the uncertainties, we assumed the instrumentation parameters of the current baseline design listed in Table \ref{['tab:pfs_spec']}, an observation at the edge of the focal plane, and included the sky emission/absorption and the Galactic dust extinction of $E(B-V)=0.05$ and 26 degrees for the zenith angle of the telescope. This computation assumes 15 min total exposure (split into two exposures; $450{\rm sec}\times 2$), $\sigma_v=70$ km/s for the velocity dispersion (the intrinsic line width), and 0.8$"$ for the seeing FWHM. We also accounted for the finite galaxy size relative to the seeing profile and the fiber size, assuming an exponential profile with scale radius 0.3$"$ for the emission-line region (about 3.5 kpc$/h$ for a galaxy at $z=1$). Note that $S/N$ is estimated by the root-sum-square of the spectral pixels (i.e. it is a matched filter combining both doublet members). The current design allows a significant detection of [ O ii] emission line over a wide range of redshift, up to $z\simeq 2.4$ with near-equal sensitivities of the red and NIR arms.
• Figure 3: Left panel: The distribution of objects in the COSMOS Mock Catalog in the color-magnitude diagram. Right: The fraction of objects in each cell that are $z>0.7$ ELGs with [ O ii] doublets detectable at $\ge 8.5\sigma$ in PFS in $2\times 7.5$ min exposures. The third dimension ($r-i$) is not shown on this 2D plot, but allows us to select lower or higher redshift galaxies within the PFS survey range.
• Figure 4: The sensitivity of PFS (green curve; 8.5$\sigma$, $2\times 450$s exposures, dark time) to the [ O ii] doublet at $r_{\rm eff}=0.3"$ and 1:1 line ratio, versus the selected targets (red points). Note that the redshift $z=2.4$ corresponds to the long wavelength end of the NIR arm. Most targets will yield successful redshifts, but some are lost within the atmospheric emission or absorption lines, a few are at $z\ge2.5$, and there is a small number of faint blue nearby objects (lower-left corner) for which we cannot detect [ O ii]. Note that the line ratio ($F_{3726}:F_{3729}$) and effective radius are re-computed for each galaxy in the COSMOS Mock Catalog, and hence the sensitivity curve drawn does not correspond to an exact boundary between detections and non-detections.
• Figure 5: The distribution of successful redshifts ([ O ii] detected at $S/N>8.5$) for the proposed PFS cosmology survey, including breakdown into the two visits. The jagged features in the curves reflect the effect of sampling variance of large-scale structures in the COSMOS field due to the finite survey area (the mock is based on the data of 1.24 square degrees). We refer to the two visits as "Visit A" and "Visit B" (see text for details), respectively, where we preferentially select brighter targets with $g<23.9$ in Visit A in order to have some flexibility between dark/grey nights.