NIRCam WFSS Deep Galaxy Observations
This example science program provides a walk-through of developing a JWST observing program using the NIRCam wide field slitless spectroscopy (WFSS) mode, including descriptions on how to apply the ETC and APT tools.
The science case we describe here is based on the Exploring the End of Cosmic Reionization survey (GTO program #1243) designed to explore the evolution of the intergalactic medium and of the circumgalactic environment at the tail end of reionization. It aims to:
- Measure the correlation between HI Lyman alpha opacity and galaxy overdensity.
- Identify the host systems of metal absorption systems at z > 5 in the quasar spectra to investigate the chemical enrichment and the ionization state of the gas in and around young galaxies.
- Characterize the nature of the quasar host galaxies and the surrounding large scale environment, and to measure their central black hole masses.
For the purpose of this use case, the survey will perform a census of emission line galaxies at 5.3 < z < 7.0 with [OIII]4959, 5007 Å + Hβ 4861 Å and at 3.7 < z < 5.1 with Hα 6564 Å. The survey is aimed to reach an imaging depth of m ~ 26.5 ABmag at 3.5μm and to detect an emission line with a flux of 10-17 erg/cm2/s at the 10-sigma level. The fields will be selected around known z > 6 quasars from the literature and are therefore pre-defined.
Steps for creating observations
Step 1 - Choose the instrument to use for the science case (NIRISS, NIRCam, or both), based on the wavelength coverage
NIRCam wide field slitless spectroscopy (WFSS) is chosen to obtain R ~ 1,600 slitless observations (~10Å/pixel) between the wavelengths of 2.5 and 5 μm. The targeted emission lines of the high redshift sources will be in this wavelength range. NIRISS WFSS observes at lower wavelengths (0.8–2.2 µm). Also note this project does not have an a-priori list of targets within each field and is therefore better suited to WFSS observations rather than NIRSpec multi-object spectroscopy.
NIRCam WFSS observations will yield the following data:
- Long wavelength grism data obtained with one or more filters between 2.5–5.0 µm
- Short wavelength imaging data obtained simultaneously on the same field with one or more filters between 0.6–2.4 µm
- Long wavelength direct image (one pointing) obtained after the grism observations for reference and calibration
- Long wavelength out of field imaging (2 pointings) obtained on a slightly wider field covering all the objects that contribute spectra to the grism data
We discuss the choices made for obtaining these data for this observing program below.
Step 2- Choose the blocking filters that cover the wavelengths of interest
This program uses the wide filter F356W to cover a sufficiently broad wavelength range (roughly 3.1–4.1 µm) to achieve the science goals. We accept that a wide filter yields a smaller full spectrum field of view. Among the wide filters, F356W yields the largest full spectrum field of view, as it is closest to 4 µm. To support these WFSS data, direct imaging and out of field imaging is also obtained in F356W.
While the WFSS data are being obtained in NIRCam's long wavelength channel, imaging is obtained simultaneously in NIRCam's short wavelength channel. This program uses the F115W and F200W filters. The combination of F115W, F200W, and F356W imaging is intended to constrain the stellar masses and star formation rates of these 3 < z < 7 galaxies, similar to the BzK diagnostic for z ~ 2 galaxies.
Step 3 - Check the direct image and grism (line and continuum) sensitivities in the WFSS mode(s) of interest
Main article: NIRCam Sensitivity
Based on the plots and tables for NIRCam WFSS sensitivity, we find an emission line at 3.5µm with flux <4 x 10-18 erg/cm2/s can be detected at 10-sigma in 10,000 s. The emission lines targeted here are brighter than that: 10-17 erg/cm2/s. A rough approximation is that a flux limit scales with 1/sqrt(exposure time). Thus, we might expect this 2.5 times brighter target to yield a 10-sigma detection in roughly 10,000 s / sqrt(2.5) ~ 6,300 s ~ 1.75 hours. Thus, we are confident we can design a observing program to yield 10-sigma detections with reasonable exposure times after consulting the ETC (see below).
Step 4 - Choose one or both of the orthogonal grisms
The use of both grisms (GRISMR and GRISMC) can be useful to disperse light in different directions and disentangle spectra of overlapping sources. However, the 2 grisms yield different full spectrum fields of view, and an even smaller area of overlap between the two (the "optimal field field of view"). For this program, the choice is made to use one grism only. GRISMR is preferred over GRISMC, as the GRISMC field of view may be affected by the coronagraph masks and substrate. Furthermore, GRISMR disperses light in different directions (flipped 180°) in each module, which may be made to overlap by using a mosaic (see Step 6 below).
Step 5 - Decide on dither pattern
Main articles: NIRCam Dithers and Mosaics
This program uses 3 INTRAMODULEX primary dithers and 4 standard subpixel dithers, for a total of 12 dithered exposures. The primary dithers are required to fill the detector gaps in the short wavelength images. INTRAMODULEX is slightly more compact than INTRAMODULE, preserving area observed at all dither positions. The subpixel dithers optimally improve the spatial sampling resulting in the combined images, while also further mitigating the effects of bad pixels.
Step 6 - Decide whether mosaicking is required to cover the target field for the science program
This program uses a 2 × 2 mosaic to cover a wider area (roughly 5' × 3') around a central target quasar. The mosaic pattern includes some overlap allowing every tile to include the z > 6 quasar so that it is observed at full depth.
Importantly, the mosaic also produces areas of overlap with complete spectra dispersed in opposite directions. This will enable spectra of neighboring sources to be disentangled. For the remaining areas with only a single dispersion direction, the survey will rely on detections of multiple emission lines (Hβ + [OIII]4959,5007) to unambiguously identify individual galaxies.
Step 7 - Decide the readout pattern to use
Above, we roughly estimated 6,300 s required for the long wavelength grism observations. We split this exposure time into 2 observations each ~3,000 s. The first is obtained simultaneously with F115W imaging, and the second is obtained simultaneously with F200W imaging. This observing time is further divided among 12 dither positions, yielding roughly ~250 s per exposure. Consulting NIRCam Imaging Sensitivity, we find the recommended readout pattern is SHALLOW4. We test our rough assumptions using the ETC (below). Note the exposure specifications (readout pattern and number of groups) will be the same for both the WFSS and imaging data obtained simultaneously.
Step 8 - Use the Exposure Time Calculator (ETC) to determine the exposure parameters for the direct images and for the dispersed images from the grisms
To determine the exposure parameters for this observation using the Exposure Time Calculator (ETC), please see the article Step-by-Step ETC Guide for NIRCam WFSS Deep Galaxy Observations.
Step 9 - Fill out the Astronomers Proposal Tool (APT)
For details filling out the Astronomer's Proposal Tool (APT) for this example science program, please see the article Step-by-Step APT Guide for NIRCam WFSS Deep Galaxy Observations.
The Exploring the End of Cosmic Reionization survey