[Paper Review] On the observability and identification of Population III galaxies with JWST

James Trussler, Christopher J. Conselice+7 more|arXiv (Cornell University)|Nov 3, 2022

Astronomy and Astrophysical Research4 citations

TL;DR

This paper evaluates the detectability and identification of high-redshift Population III galaxies with the James Webb Space Telescope (JWST), finding that deep NIRCam and MIRI photometry using F444W–F560W and F560W–F770W colour cuts offers the most robust selection method. The definitive identifier is high equivalent width He ii λ1640 emission, requiring ultra-deep NIRSpec spectroscopy (10–250 h) for 5σ detection, though moderate lensing or higher galaxy mass can reduce integration time.

ABSTRACT

We utilise theoretical models of Population III stellar+nebular spectra to investigate the prospects of observing and accurately identifying Population III galaxies with JWST using both deep imaging and spectroscopy. We investigate a series of different colour cuts, finding that a combination of NIRCam and MIRI photometry through the F444W-F560W, F560W-F770W colours offers the most robust identifier of potential $z=8$ Pop III candidates. We calculate that NIRCam will have to reach $\sim$28.5-30.0 AB mag depths (1-20 h), and MIRI F560W must reach $\sim$27.5-29.0 AB mag depths (10-100 h) to achieve $5σ$ continuum detections of $M_* = 10^6~\mathrm{M}_\odot$ Pop III galaxies at $z=8$. We also discuss the prospects of identifying Pop III candidates through slitless and NIRSpec spectroscopic surveys that target Ly$α$, H$β$ and/or He II $λ1640$. We find small differences in the H$β$ rest-frame equivalent width (EW) between Pop III and non-Pop III galaxies, rendering this diagnostic likely impractical. Instead, we find that the detection of high EW He II $λ1640$ emission will serve as the definitive Pop III identifier, requiring (ultra-)deep integrations (10-250 h) with NIRSpec/G140M for $M_*=10^6~\mathrm{M}_\odot$ Pop III galaxies at $z=8$. However, MIRI F770W detections of Pop III galaxies will require substantial gravitational lensing ($μ=10$) and/or fortuitous imaging of exceptionally massive ($M_* = 10^7~\mathrm{M}_\odot$) Pop III galaxies. Thus, NIRCam medium-band imaging surveys that can search for high EW He II $λ1640$ emitters in photometry may perhaps be a viable alternative for finding Pop III candidates.

Motivation & Objective

• To assess the feasibility of detecting and identifying Population III galaxies at z ≈ 8 using JWST’s imaging and spectroscopic capabilities.
• To determine optimal photometric colour cuts for selecting high-redshift Pop III candidates from NIRCam and MIRI data.
• To evaluate the sensitivity requirements for continuum and emission-line detection of Pop III galaxies with JWST instruments.
• To identify the most reliable spectroscopic diagnostic for definitively distinguishing Pop III galaxies from non-Pop III counterparts.
• To explore the role of gravitational lensing and intrinsic galaxy mass in reducing required integration times for detection.

Proposed method

• Utilized theoretical models of Population III stellar and nebular spectra to simulate broadband photometry and emission-line features at z ≈ 8.
• Calculated required AB magnitude depths for 5σ continuum detection of M* = 10⁶ M☉ Pop III galaxies using NIRCam and MIRI filters.
• Evaluated photometric selection strategies using medium-band filters centered on He ii λ1640 to identify high equivalent width emitters.
• Simulated spectroscopic observations with NIRSpec/G140M and G395M to assess detectability of He ii λ1640 and Hβ lines at 5σ significance.
• Assessed metallicity constraints from Hβ detection and [O iii] λ5007 non-detection in NIRSpec G395M data.
• Incorporated effects of gravitational lensing (μ = 2–3, 10) and varying Pop III galaxy stellar masses (2–3×10⁶ M☉, up to 10⁷ M☉) on detectability.

Experimental results

Research questions

• RQ1 Which photometric colour cuts in NIRCam and MIRI bands provide the most robust identification of z ≈ 8 Population III galaxy candidates?
• RQ2 What integration depths (in AB magnitudes) are required for 5σ continuum detection of M* = 10⁶ M☉ Pop III galaxies with NIRCam and MIRI at z ≈ 8?
• RQ3 Can Hβ or He ii λ1640 emission lines be used to definitively distinguish Pop III galaxies from non-Pop III galaxies in JWST spectroscopy?
• RQ4 How deep must NIRSpec spectroscopic integrations be to achieve 5σ detection of He ii λ1640 emission in Pop III galaxies at z ≈ 8?
• RQ5 To what extent can gravitational lensing or higher intrinsic galaxy mass reduce the required integration time for Pop III detection?

Key findings

• The F444W–F560W and F560W–F770W colour cuts in NIRCam and MIRI photometry provide the most robust method for identifying z ≈ 8 Pop III galaxy candidates.
• NIRCam requires depth of ∼28.5–30.0 AB mag (1–20 h) and MIRI F560W requires ∼27.5–29.0 AB mag (10–100 h) for 5σ continuum detection of M* = 10⁶ M☉ Pop III galaxies at z ≈ 8.
• He ii λ1640 emission with high equivalent width is the most reliable spectroscopic identifier of Pop III galaxies, requiring 10–250 h of integration with NIRSpec/G140M for 5σ detection.
• Hβ line equivalent width differences between Pop III and non-Pop III galaxies are too small to serve as a practical diagnostic for identification.
• With moderate lensing (μ = 2–3) or higher galaxy mass (2–3×10⁶ M☉), He ii λ1640 detection becomes feasible within medium-sized JWST Guaranteed Time Observing programs.
• MIRI F770W detection of Pop III galaxies requires either μ ≈ 10 lensing or extremely massive galaxies (M* = 10⁷ M☉), making it highly challenging without strong lensing.