NIRSpec MOS Deep Extragalactic Survey
This example science program provides a walkthrough of creating a JWST deep extragalactic survey program of multi-object spectroscopy (MOS) using NIRSpec MOS.
This article walks users through a science use case to plan deep NIRSpec Micro-Shutter Assembly (MSA) observations of faint high redshift galaxies in a pencil-beam survey field. The goal is to obtain medium resolution spectroscopy for a sample of ~200 galaxies with redshift (z) in the range 2 < z < 6. The full suite of optical emission lines, from Hα to [OII] will be observed, along with the ultraviolet [CIII], CIII] 1907, 1909 Å doublet for a sub-sample of galaxies at 5 < z < 6. We will also obtain NIRCam parallel imaging to help us select targets for future follow-up spectroscopy at these same redshifts.
Galaxies are selected which have a spectroscopic or photometric redshift which places their Hα and/or [CIII], CIII] emission in the NIRSpec medium resolution gratings. Figure 1 shows an example galaxy field from a Hubble image with slits placed over three galaxies of interest to illustrate this science use case.
The science goals of this program are to obtain measurements of star-formation rates, dust content, ionization state, and spectroscopic redshifts for a sample of ~200 galaxies at 2 < z < 6. This will help us understand the physical states of galaxies in the very high redshift universe. It will significantly improve our understanding of how galaxies evolve in their early turbulent phases, and will place constrains on models for galaxy formation and evolution. Specifically, for galaxies at redshifts 2 < z < 5, we will use the Hα emission line in the optical at 6563 Å and additional optical lines that fall into the wavelength ranges of our observed spectra. At redshifts 5 < z < 6 we have access to both Hα and the coveted [CIII], CIII] doublet. This diagnostic will allow for a better determination of the galaxies’ metallicities and ionization states. We do not need for the doublet to be resolved for our science goals. Note that we are only observing the brightest sources at 5 < z < 6, where the relatively low equivalent CIII] emission is most likely to be detected.
We aim to design observations for an extragalactic multi-object spectroscopy (MOS) survey to meet our science goals by surveying emission lines from a statistical sample of galaxies. We choose the GOODS-S field for these observations, using the catalog from Momcheva et al. (2016).
Step 1 - Determine the range of feasible MOS Aperture Position Angles (APA) for the area of interest
We use the JWST General Target Visibility Tool (GTVT) to find reasonable APAs. For the GOODS-S field which has a central pointing of RA = 03:32:33.00, DEC = -27:48:47.0, we find that APAs between 30° and 210° will be available from August 2021 to February 2022. For the case shown here, we test an APA of 60°.
We recommend that users test several angles using the steps that follow, to determine whether multiplexing (science) results depend on the assigned orientation. In general, specifying a tightly constrained orientation is discouraged, as it may decrease scheduling efficiency.
Step 2 - Obtain or create a source catalog of sufficient astrometric accuracy
Existing Hubble Space Telescope (HST) imaging within the GOODS-S field has relative astrometric accuracy better than 20 mas, allowing precise target acquisition and placement of objects in the 0.2” by 0.5” MSA shutters.
Step 3 - Decide on a target acquisition strategy
Main article: NIRSpec MSA Target Acquisition
We choose the MSA Target Acquisition (MSATA) strategy for this program since this is the only way to reliably place the targets in the MSA shutters.
Step 4 - Optional NIRCam Pre-Imaging
Main article: NIRSpec MOS Operations - Pre-Imaging Using NIRCam
This step can be skipped since the field has existing HST imaging which achieves the required relative astrometry accuracy.
Step 5 - Pick desired wavelength setting(s) for MOS observations
Main article: NIRSpec Dispersers and Filters
We wish to observe the Hα emission line in galaxies at redshifts 2 < z < 5, requiring grating/filter combinations of G140M/F100LP, G235M/F170LP, G395M/F290LP. For the higher redshift galaxies at 5 < z < 6 where we hope to detect [CIII] and CIII], we require the G140M/F100LP disperser/filter combination.
Step 6 - Determine which dither strategy to use
We choose to nod, taking an exposure in each shutter of our three-shutter slitlet. Since nod positions can serve as dithers, we choose not to add any additional dithers. This choice increases efficiency in two ways:
- We do not have to reconfigure the MSA, saving 90 seconds of overhead.
- We do not suffer multiplexing losses, due to the requirement that our sources be observable in two configurations, rather than one.
The down-side to this choice is that our spectra will have a wavelength gap, due to NIRSpec’s chip gap. We accept this compromise. We note that adding a 20” Fixed Dither to cover the chip gap would still result in half the exposure time, and a non-uniform signal-to-noise, over certain wavelengths. For users concerned about wavelength coverage, the NIRSpec team has provided the MSA Spectral Visualization Tool to quantify the wavelengths that are missed for each slit in a given configuration.
Step 7 - Learn about recommended practices for dithering, background subtraction, MSA leakage and general MOS recommendations
Step 8 - Determine the required exposure and detector parameters for your MOS observations
We chose the NIRSpec IRS2 Detector Readout Mode, which reduces correlated noise and is recommended for long exposures. The IRS2 readout mode has two patterns: NRSIRS2 and NRSIRS2RAPID. The former reduces data volume by averaging five frames, whereas the latter does not average any frames. The NIRSpec Detector Recommended Strategies article provides more guidance on choosing a readout mode and pattern. We choose NRSIRS2, as data volume from the added NIRCam parallels can be large. Since our exposures are long, we anticipate that frame-averaging will have an insignificant impact on our measurements.
Steps 9 - 17 - Create NIRSpec MOS Observations in the MSA Planning Tool (MPT) and Astronomers' Proposal Tool (APT)
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