We explore the design of a space mission called Project Lyman that has the goal of quantifying the ionization history of the universe from the present epoch to a redshift of z ~ 3. Observations from WMAP and SDSS show that before a redshift of z (Symbol not available. See manuscript.) 6 the first collapsed objects, possibly dwarf galaxies, emitted Lyman continuum (LyC) radiation shortward of 912 Å that reionized most of the universe. Theoretical estimates of the LyC escape fraction ( fesc ) required from these objects to complete reionization is fesc ~10%. How LyC escapes from galactic environments, whether it induces positive or negative feedback on the local and global collapse of structures, and the role played by clumping, molecules, metallicity and dust are major unanswered theoretical questions, requiring observational constraint. Numerous intervening Lyman limit systems frustrate the detection of LyC from high z objects. They thin below z ~ 3 where there are reportedly a few cases of apparently very high fesc. At low z there are only controversial detections and a handful of upper limits. A wide-field multi-object spectroscopic survey with moderate spectral and spatial resolution can quantify fesc within diverse spatially resolved galactic environments over redshifts with significant evolution in galaxy assemblage and quasar activity. It can also calibrate LyC escape against Lyα escape, providing an essential tool to JWST for probing the beginnings of reionization. We present calculations showing the evolution of the characteristic apparent magnitude of star-forming galaxy luminosity functions at 900 Å, as a function of redshift and assumed escape fraction. These calculations allow us to determine the required aperture for detecting LyC and conduct trade studies to guide technology choices and balance science return against mission cost. Finally we review our efforts to build a pathfinding dual order multi-object spectro/telescope with a (0.5°)2 field-of-view, using a GSFC microshutter array, and crossed delay-line micro-channel plate detector.
Shull et al. have asserted that the contribution of stars, relative to quasars, to the metagalactic
background radiation that ionizes most of the baryons in the universe
remains almost completely unknown at all epochs. The potential to
directly quantify this contribution at low redshift has recently become
possible with the identification by GALEX of large numbers of
sparsely distributed faint ultraviolet galaxies. Neither STIS nor
FUSE nor GALEX have the ability to efficiently survey these sparse
fields and directly measure the Lyman continuum radiation that may leak
into the low redshift (z < 0.4) intergalactic medium. We present
here a design for a new type of far ultraviolet spectrograph, one that
is more sensitive, covers wider fields, and can provide spectra and
images of a large number of objects simultaneously, called the
Far-ultraviolet Off Rowland-circle Telescope for Imaging and
Spectroscopy (FORTIS). We intend to use a sounding rocket flight to
validate the new instrument with a simple long-slit observation of the
starburst populations in the galaxy M83. If however, the long-slit
were replaced with microshutter array, this design could isolate the
chains of blue galaxies found by GALEX over an ~30' diameter
field-of-view and directly address the Lyman continuum problem in a
long duration orbital mission. Thus, our development of the sounding
rocket instrument is a pathfinder to a new wide field spectroscopic
technology for enabling the potential discovery of the long
hypothesized but elusive Lyman continuum radiation that is thought to leak from low redshift galaxies and contribute to the ionization of the universe.
The ACS solar blind channel (SBC) is a photon-counting MAMA detector capable of producing two-dimensional imaging in the UV at wavelengths 1150-1700 Angstroms, with a field of view (FOV) of 31" × 35". We describe the on-orbit performance of the ACS/SBC from an analysis of data obtained from the service mission observatory verification (SMOV) programs. Our summary includes assessment of the point-source image quality and point spread function (PSF) over the SBC FOV, the dark current measurements, the characteristics of the flat fields, fold analysis, throughput, and the UV sensitivity monitor to check for contamination. Where appropriate, a comparison with pre-launch calibration data will also be made.
The off-axis location of the Advanced Camera for Surveys causes strong geometric distortion in all detectors -- the Wide Field Camera (WFC), High Resolution Camera (HRC), and Solar Blind Camera (SBC). Dithered observations of rich star cluster fields are used to calibrate the distortion. We describe the observations obtained, the algorithms used to perform the calibrations and the accuracy achieved. We present our best current calibration of the geometric distortion of each of the detectors.
We present an overview of the ACS on-orbit performance based on the calibration observations taken during the first three months of ACS operations. The ACS meets or exceeds all of its important performance specifications. The WFC and HRC FWHM and 50% encircled energy diameters at 555 nm are 0.088" and 0.14", and 0.050" and 0.10". The average rms WFC and HRC read noises are 5.0 e- and 4.7 e-. The WFC and HRC average dark currents are ~ 7.5 and ~ 9.1 e-/pixel/hour at their operating temperatures of - 76°C and - 80°C. The SBC + HST throughput is 0.0476 and 0.0292 through the F125LP and F150LP filters. The lower than expected SBC operating temperature of 15 to 27°C gives a dark current of 0.038 e-/pix/hour. The SBC just misses its image specification with an observed 50% encircled energy diameter of 0.24" at 121.6 nm. The ACS HRC coronagraph provides a 6 to 16 direct reduction of a stellar PSF, and a ~1000 to ~9000 PSF-subtracted reduction, depending on the size of the coronagraphic spot and the wavelength. The ACS grism has a position dependent dispersion with an average value of 3.95 nm/pixel. The average resolution λ/Δλ for stellar sources is 65, 87, and 78 at wavelengths of 594 nm, 802 nm, and 978 nm.