NIRSpec MOS Deep Extragalactic Survey

Example Science Program #25

This example science program provides a walkthrough of creating a JWST deep extragalactic survey program of multi-object spectroscopy using NIRSpec MOS.

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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.

Figure 1. Zoom-in of Hubble image of a deep pencil beam field with slits placed over target galaxies

A zoom-in Hubble image of a deep pencil beam field is shown. Target galaxies have slits placed over them (red boxes). The three-shutter slitlet pattern used in this use case is shown.

This article is structured as follows:



Science goals

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. Figure 2 shows an example of observations from the nearby Universe, which we aim to measure at high-redshifts. The correlation between galaxy masses and gas-phase metallicities is an important diagnostic, as it provides constraints on models for feedback and outflows.

Figure 2. The mass-metallicity relation from Tremonti et al. 2004

The mass – metallicity relation at a redshift of ~0.1. Comparison of this relation to models, at all observable redshifts, shows evidence of feedback and outflows. One of the goals of this use case is to measure this relation at redshifts above two.

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.



Observing strategy

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. Existing HST imaging 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. Through experimentation with the Microshutter Assembly (MSA) planning tool (MPT), and following the procedures outlined in this article, we determined that four NIRSpec MOS pointings will be required to observe around 200 galaxies at 2 < z < 6.

Determining MSA pointings when parallel observations are needed

We want our NIRCam parallel observations to cover a semi-regular area on the sky (e.g., rectangle). This severely restricts the location of the MSA pointings. To obtain a NIRCam mosaic, MSA points must also have a well-defined mosaic. Therefore, in this use case, we will restrict the MSA pointings to specific regions on the sky.

Restricted MSA pointings are not the default mode in MPT. Generally, MPT finds an optimal set of pointings over the full field, maximizing the number of targets (or target weights) that will be observed. While this approach is advantageous for spectroscopy, it has the disadvantage that any associated NIRCam parallel observations will have non-contiguous area and/or a non-uniform depth.

We choose an area on the sky of 8 × 8 arcminutes, which corresponds to roughly 2 × 2 MSA footprints. In order to define a sample of galaxies in this area, we take the GOODS-S Catalog from Momcheva et al. (2016), and restrict it to a circular region around the coordinates of the center of our pointing RA 03:32:33.00, DEC -27:48:47.0. A 5.5 arcminute radius around this pointing gives a region that will circumscribe an 8 × 8 arcminute square. By using this circular region, which is somewhat larger than the area selected, we ensure that our catalog contains sources that will fill the MSA for any possible aperture position angle that is assigned. In total, this sub-region contains 27,964 objects. 

In order to make efficient use of the MOS, while also carrying out a cohesive science investigation, we define primary and filler candidates from our catalog. For primary candidates, we choose galaxies with photometric redshifts above 5, where NIRSpec can access both Hα and [CIII], CIII]. For filler candidates, we use galaxies with photometric redshifts at 2 < z < 5. In this use case, lower-redshift galaxies are well suited as filler targets, since they will typically be brighter than primary targets. While the MSA planning tool ensures that primary targets will be observed at all nods and dithers, filler targets may be observed in only a subset, resulting in reduced sensitivity. 

General Consideration: In the MPT, targets in the primary catalog will receive full exposure depth. Targets in the filler catalog may not. In other words, there is no guarantee that targets in the filler catalog will receive any more depth than a single integration. If the user wants to ensure that their filler targets receive full exposure depth, these objects should be added to the primary candidate set and assigned an appropriate weight.

We also provide step-by-step ETC calculations to support this example, and step-by-step APT instructions.



References

Curtis-Lake, E., et al. 2016, MNRAS, Vol. 457, issue 1, p. 440
Non-parameteric analysis of rest-frame UV sizes and morphological disturbance amongst L* galaxies at 4 < z < 8

Grogin, N.,et al. 2011, ApJS, Vol. 197, Issue 2, article id. 35
CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey

Momcheva, I. G., et al. 2016, ApJS, Vol. 225, Issue 2, article id. 27
The 3D-HST Survey: Hubble Space Telescope WFC3/G141 Grism Spectra, Redshifts, and Emission Line Measurements for ~100,000 Galaxies

Rigby, J. R., et al. 2015, ApJL, Vol. 814, Issue 1, article id. L6
C III] Emission in Star-forming Galaxies Near and Far

Tremonti, C. A., et al. 2004, ApJ, Vol. 613, Issue 2, 898
The Origin of the Mass-Metallicity Relation: Insights from 53,000 Star-forming Galaxies in the Sloan Digital Sky Survey




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