Large Area Near Infrared Surveys
WISE All-Sky Map
Image Credit: NASA/JPL-Caltech/WISE Team
The cameras on most other space telescopes (HST, Spitzer, JWST) typically have FOV's of order a few to a few tens of square arc minutes in size, and therefore those telescopes are best suited to deep, narrow field imaging and spectroscopy. The largest area surveyed by HST covers a region of sky only of order 2 square degrees. The largest area extraGalactic survey conducted by Spitzer (to date) covered of order 65 square degrees. The largest area Galactic survey conducted by Spitzer - the GLIMPSE and follow-on programs - covered the entire Galactic plane, with a total area of order 1000 square degrees1. By contrast, the WFIRST dark energy and microlensing should obtain data for >= 10,000 square degrees with a combination of depth and PSF that should allow revolutionary discoveries in almost every field of astronomy.
In order to broaden the science return of WFIRST, the 2010 Decadal report recommended that six months of the mission be devoted to a Galactic plane survey, and that an additional year of the mission be devoted to a Guest Observer program. These programs will enhance the return from WFIRST either by sampling regions of the sky not covered by the dark energy and microlensing surveys or by obtaining additional epochs of coverage of those regions in order to go deeper, provide additional bandpasses, or provide second epoch data for variability or proper motion studies. A few examples of the types of science that could be pursued with the large area surveys that WFIRST will provide include:
Brown Dwarfs
Field brown dwarfs cooler than spectral type T almost certainly must exist, and may in fact be very numerous. However, they are difficult to detect and to identify because they are very faint and we can only predict their likely photometric and spectroscopic signatures. WFIRST will provide multiple means to conduct large area, unbiased surveys for these elusive objects. Second epoch images could be obtained of fields imaged by the dark energy surveys in order to search for faint, high proper motion objects. Regardless of the details, the spectra of very cool brown dwarfs should be dominated by deep absorption bands due to water, methane and possibly other species, which should allow unambiguous identification in the wide field surveys obtained with WFIRST's spectroscopic channel. As a complementary technique, deep, wide-field multi-epoch observations could be obtained of young, nearby open clusters and star-forming regions in order to determine their mass functions down to masses of order 5 to 10 times the mass of Jupiter.
Galactic Structure
Surveys of the Galactic plane are fundamentally constrained by crowding effects (confusion) and by extinction. With its small pixel size and very well characterized PSF, the comparatively wide FOV of its imaging camera, and filters in the near-IR, WFIRST has the potential to greatly improve our knowledge of the structure of the Galactic disk. With the planned six-month Galactic plane survey, the entire Galactic plane could be surveyed in three filters over a several degree latitude range, or multiple epochs of imaging could be obtained for a more constrained latitude range. This imaging could be used to construct a census of red-clump giants throughout the entire MW disk, providing a better determination of the extent and geometry of the Galactic bar and of the shape of the disk warp. Star-formation tracers could be used to trace distant spiral arms and to determine the edge of the star-forming disk.
The WFIRST SDT is currently considering the possibility of adding a longer wavelength filter to the imaging camera - for example, a filter similar to the standard K-band. Such a filter would significantly improve the science return of the Galactic plane survey for all topics requiring multiple bands because the ultimate depth of such a survey is limited by the depth of the most extinguished (shortest wavelength) band.
Active Galaxies and First Light
The lack of deep, near-IR spectroscopy over a large area of sky has been an obstacle limiting any major progress in our understanding of galaxy formation and evolution since early cosmic time. The proposed slitless grism spectroscopy over many thousands of square degrees by WFIRST will offer the unprecedented opportunity to revolutionize our understanding of the fundamental physical properties of high-redshift galaxies. Similar to the Sloan Digital Sky Survey, near-IR spectra of millions of galaxies over many thousands of square degrees will allow us to address a wide variety of topics, ranging from characterization of star formation and AGN activities, to detecting the most luminous primordial galaxies at the epoch of reionization, to understanding of large scale structures and clustering properties of galaxies at z > 1.
Fluctuations in deep Spitzer images have been attributed to first-light galaxies containing Population III stars during reionization. Alternate studies show that a reasonable fraction (>50%) of the unresolved fluctuations is arising from faint, unresolved sources at z ~ 2. WFIRST through its deep, wide-field imaging will detect the contribution of z ~ 2 galaxies to the IRB fluctuations precisely and in addition, allow a measurement of the power spectrum of the residual emission. This will definitively yield the contribution of first light sources, which are individually too faint to detect even by JWST, to the IRB fluctuations.
