These simulations demonstrate the usefulness of WFIRST for grism observations of high-redshift galaxies,
specifically in mapping the distribution of emission-line galaxies, allowing the measurement of cosmic expansion
history through the use of baryon acoustic oscillations.
The telescope, instrument, and other parameters assumed for these simulations are
given in the accompanying table.
Generating the Sky Scene
Input lists of galaxies with different fluxes, sizes, sky coordinates, and
spectral energy distributions are created together with emission lines. These
lists are generated both by converting grism observations taken by the Hubble
Space Telescope WFC3 camera and also by generating galaxies using
emission line luminosity functions. These lists are then processed through
applying the predicted pixel scale, throughput, wavelength resolution,
and spectral sensitivity of the WFIRST grism. The same fields can be simulated at several
different suggested roll angles, allowing for the examination of the
potential issues of contamination and spectral overlap in the WFIRST grism data.
As input one needs the
WFIRST effective area and sensitivity
as a function of wavelength and filter.
Simulated images from the WFIRST Wide-Field Instrument. The first image shows the direct image (F110W) of a randomized
list of objects taken from real detections in the HST/WFC3 images of the WISP (WFC3 Infrared Spectroscopic Parallel;
Atek et al. 2010
survey. The second image shows the simulated grism image of the same field. The predicted image distortion, wavelength dispersion, and alignment relative to the direct image have been applied. The image is roughly 25% of a single full 4088x4088 detector and contains approximately 1000 galaxies down to H(AB)=25.
Telescope and Instrument Parameters Used By Grism Simulations
Mirror diameter (m)
Plate scale (arc-sec/pix)
Dispersion (Å /pix)
10.4-11.4 (position dependent)
Background flux (e‐/s/pix)
Exposure time (sec)
347 (1 exposure)
2100-2500 (combined exposures)
Work is underway at IPAC/Caltech to create a more advanced simulation pipeline with the ability
to generate random objects with realistic morphologies, luminosity functions, and redshift distribution.
STScI is working on alternate analysis tools based on linear and forward modeling and on other inputs to the sky scenes,
based on existing HST grism programs that cover a cross-section of the different GO science ideas.
NASA/Goddard Space Flight Center is producing a semi-automated emission line measurement tool. Below we give some
examples of the ongoing simulations.
Realistic Sky Simulations
We present a simulation done for the SCA1 detector intended to be a realistic depiction of the sky. It
includes emission line objects inserted based on observed Hα luminosity functions (from
Colbert et al. (2013)
and known distributions
of equivalent width, stars based on stellar luminosity functions and a model (thin/thick disk+halo) of stellar
density (the galactic stellar density model is based on model of
Juric et al. 2008
, mostly using the parameters
of Chang et al. 2010
, and the stellar luminosity
function comes from Just et al. 2015
background galaxies based on measured near-infrared number counts. All the significant galaxy
parameters – major axis, axial a/b ratio, J-H color – have been chosen using distributions derived from
HST WFC3 observations.
Each image is roughly half of a full WFIRST detector, 7.5 × 3.75 arc-minutes in size, and contain about 3,300
individual objects, although most are too faint to produce a detectable spectrum in the grism images. The
simulated exposure times are 400s for the direct image and 2100s for the grism images. The grism integration
time was chosen to represent a combination of six shorter 350s integration images, as presently proposed for
individual exposures in the High Latitude Survey (HLS). Similarly, the noise is based on an assumed typical
near-infrared background for the HLS. The Galactic coordinates used for this simulation and its star count
model were latitude=-49 and longitude=149 (R.A. = 2h, Dec. = +10.0 deg). The star images are generated from a PSF
created using STSci's
The galaxies are simple elliptical gaussian models.
The purpose of this set of simulations is to test the effects of crowding and contamination, in particular as
a function of the grism dispersion. The top image is a simulated H-band direct image, but the next two images
are the simulated grisms assuming 10.8 Å/pix (middle) and 8 Å/pix (bottom) dispersion. Both use the same
spectral coverage: 1 to 1.9 microns.
Galaxy Parameter Simulations
The grism simulations are also being used to test ranges of specific galaxy parameters. In this simulation we
test a range of Hα emission line fluxes (5×10-17
at different redshifts (z=0.55 – 1.9), while keeping the equivalent width constant (100 Å, rest wavelength).
The galaxy size (major axis) is varied using known size distributions for emission line galaxies derived from HST WFC3
observations, but the same simple gaussian elliptical shape and orientation is used for all simulated objects. For these galaxy
parameter tests, we lay the emission line galaxies out in a non-overlapping grid to prevent
the issue of contamination from confusing the analysis. We ran these simulations at two different potential
dispersions that are being considered (8 and 10.8 Å/pix) and then measured the emission lines for the new
simulated fields. Below we present some of the resulting analysis: line-centroiding accuracy (in terms of
measured redshift) and emission line recovery as a function of signal-to-noise for each of the two dispersions.
Top: This is the direct image, with all the emission line galaxies laid out in a grid intended to
avoid spectral overlap. Each image is roughly 7.5x3.75 arc-minutes
in size (half the size of a full WFIRST detector). The full image contains roughly 550 emission
line galaxies. There are no "background" or non-emission line galaxies included. The simulated exposure
time is 400 seconds. Middle: The dispersed grism image at the nominal 10.8 Å/pixel wavelength
resolution, running from roughly 1 to 1.9 microns. The exposure time is 2100s, intended to approximate
the typical depth of the High Latitude Survey after the combination of multiple shorter individual exposures.
The main emission line visible is Hα, although a weaker set of [OIII]
lines (fixed at half the flux of Hα) can also be seen. Bottom: The exact same set of emission line galaxies
at the exact same locations on the detector as above, but done for the 8 Å/pixel wavelength resolution.
Below are two sets of analysis plots made from the nominal (top) and high dispersion (below) simulations.
Each set presents the line centroiding accuracy in terms of
redshift ([zmeasured-zinput]/[1+zinput]) and the recovery percentage as
functions of Hα line flux. The empty circles are all the simulated Hα emission lines, while the
red, green and blue points are the same emission lines after S/N = 3, 5 and 7 cuts, respectively.
The dotted lines represent the redshift accuracy goal of 0.1% that has been discussed for the main redshift
survey. Recovery percentage is the number of objects at each Hα line flux after the signal-to-noise cut
has been made, divided by the total number or input emission line objects at that flux. While similar
to a completeness, the recovery percentage only takes into account the signal-to-noise of the line and does
not (at this point) include the chance the emission line might be missed altogether, due to contamination
or automated line finding failure. Nevertheless, it gives a good indication of the general depth the survey
can reach. We note that the sky background assumed is for a typical HLS field, so some HLS fields will reach
fainter emission lines than presented here.
aXeSIM was adopted from HST/WFC3 spectral simulations to WFIRST/WFI. The simulation package uses
the same components as used by aXe for the extraction of slitless spectra and thus aims at spectrophotometric
integrity - useful for observation design but essential for the quantitative assessment of slitless data.
Work is underway at IPAC and STScI to add more advanced instrumental and observational effects, such as intrapixel
capacitance, charge diffusion, cosmic rays, jitter, and others to aXeSIM, and to include more flexibility in aXe/aXeSIM,
required by the large field of view and complex focal plane of WFIRST.
For more information about aXe see
Kümmel et al. (2009).
Output From the Simulations
The output from aXeSIM simulations will always consist of a simulated 2D slitless dispersed image with spectra
of the simulated objects. For grism images, the simulations can include several dispersed orders. aXeSIM will
also produce the direct image associated with the slitless image. Sky background and random noise (readout and
photon noise from background and objects) can be added to the output images.