Wide-Field InfrarRed Survey Telescope-Astrophysics Focused Telescope Assets WFIRST-AFTA 2015 Report

Abstract

This report describes the 2014 study by the Science Definition Team (SDT) of the Wide-Field Infrared Survey Telescope (WFIRST) mission. It is a space observatory that will address the most compelling scientific problems in dark energy, exoplanets and general astrophysics using a 2.4-m telescope with a wide-field infrared instrument and an optical coronagraph. The Astro2010 Decadal Survey recommended a Wide Field Infrared Survey Telescope as its top priority for a new large space mission. As conceived by the decadal survey, WFIRST would carry out a dark energy science program, a microlensing program to determine the demographics of exoplanets, and a general observing program utilizing its ultra wide field. In October 2012, NASA chartered a Science Definition Team (SDT) to produce, in collaboration with the WFIRST Study Office at GSFC and the Program Office at JPL, a Design Reference Mission (DRM) for an implementation of WFIRST using one of the 2.4-m, Hubble-quality telescope assemblies recently made available to NASA. This DRM builds on the work of the earlier WFIRST SDT, reported by Green et al. (2012) and the previous WFIRST-2.4 DRM, reported by Spergel et. (2013). The 2.4-m primary mirror enables a mission with greater sensitivity and higher angular resolution than the 1.3-m and 1.1-m designs considered previously, increasing both the science return of the primary surveys and the capabilities of WFIRST as a Guest Observer facility. The addition of an on-axis coronagraphic instrument to the baseline design enables imaging and spectroscopic studies of planets around nearby stars.

Figures (145)

• Figure 1: -1: Field of view comparison, to scale, of the WFIRST-AFTA wide field instrument with wide field instruments on the Hubble and James Webb Space Telescopes. Each square is a$\mathbf{4 k} \boldsymbol{\times} \mathbf{4 k ~ H g C d T e ~ s e n s o r ~ a r r a y . ~ T h e ~ f i e l d ~ o f ~ v i e w ~}$ is 0.28 degrees ${ }^{\mathbf{2}}$. The pixels are mapped to 0.11 arcseconds on the sky.
• Figure 2: -1: A high-level view of the WFIRST-AFTA dark energy program. The supernova (SN) survey will measure the cosmic expansion history through precise spectrophotometric measurements of more than 2700 supernovae out to redshift$\mathrm{z}=1.7$. The high-latitude survey (HLS) will measure redshifts of 18 million emission-line galaxies and shapes (in multiple filters) of 380 million galaxies. The former allow measurements of "standard ruler" distances through characteristic scales imprinted in the galaxy clustering pattern, while the latter allow measurements of matter clustering through the "cosmic shear" produced by weak gravitational lensing and through the abundance of galaxy clusters with masses calibrated by weak lensing. As indicated by crossing arrows, weak lensing measurements also constrain distances, while the galaxy redshift survey provides an alternative measure of structure growth through the distortion of redshift-space clustering induced by galaxy motions. Boxes in the middle layer list the forecast aggregate precision of these measurements in different ranges of redshift. These high-precision measurements of multiple cosmological observables spanning most of the history of the universe lead to stringent tests of theories for the origin of cosmic acceleration, through constraints on the dark energy equation-of-state parameter w(z), on deviations $\boldsymbol{\Delta} \mathbf{G}(\mathbf{z})$ from the growth of structure predicted by General Relativity, or on deviations between the gravitational potentials that govern relativistic particles (and thus weak lensing) and non-relativistic tracers (and thus galaxy motions).
• Figure 3: -1: A high-level view of the WFIRST-AFTA dark energy program. The supernova (SN) survey will measure the cosmic expansion history through precise spectrophotometric measurements of more than 2700 supernovae out to redshift$\mathrm{z}=1.7$. The high-latitude survey (HLS) will measure redshifts of 18 million emission-line galaxies and shapes (in multiple filters) of 380 million galaxies. The former allow measurements of "standard ruler" distances through characteristic scales imprinted in the galaxy clustering pattern, while the latter allow measurements of matter clustering through the "cosmic shear" produced by weak gravitational lensing and through the abundance of galaxy clusters with masses calibrated by weak lensing. As indicated by crossing arrows, weak lensing measurements also constrain distances, while the galaxy redshift survey provides an alternative measure of structure growth through the distortion of redshift-space clustering induced by galaxy motions. Boxes in the middle layer list the forecast aggregate precision of these measurements in different ranges of redshift. These high-precision measurements of multiple cosmological observables spanning most of the history of the universe lead to stringent tests of theories for the origin of cosmic acceleration, through constraints on the dark energy equation-of-state parameter w(z), on deviations $\boldsymbol{\Delta} \mathbf{G}(\mathbf{z})$ from the growth of structure predicted by General Relativity, or on deviations between the gravitational potentials that govern relativistic particles (and thus weak lensing) and non-relativistic tracers (and thus galaxy motions).
• Figure 4: -2: The survey time per unit area per filter, including overheads, for imaging surveys. The gray squares indicate the baseline HLS survey depth. The right hand axis marked in deg$^{2}$ covered per filter per year of observing time. The upper axis shows the effective weak lensing source density in H-band as function of depth.
• Figure 5: -3: The survey time per unit area for the grism spectroscopy required to observe a certain number density of Ha emitters at$7 \sigma$, at redshift $\mathrm{z}=1.5$. The corresponding flux limit for an $\mathrm{r}_{\text{eff }}=0.3$ arcsec source is marked on the upper axis. The gray square indicates the baseline strategy. The right axis is marked in number of deg $^{2}$ covered per year of observing time.