On the observability and identification of Population III galaxies with JWST

James A. A. Trussler, Christopher J. Conselice, Nathan J. Adams, Roberto Maiolino, Kimihiko Nakajima, Erik Zackrisson, Leonardo Ferreira

Submitted on 3 November 2022


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 28.5-30.0 AB mag depths (1-20 h), and MIRI F560W must reach 27.5-29.0 AB mag depths (10-100 h) to achieve 5σ continuum detections of M=106 M Pop III galaxies at z=8. We also discuss the prospects of identifying Pop III candidates through slitless and NIRSpec spectroscopic surveys that target Lyα, Hβ and/or He II λ1640. We find small differences in the Hβ rest-frame equivalent width (EW) between Pop III and non-Pop III galaxies, rendering this diagnostic likely impractical. 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=106 M Pop III galaxies at z=8. With moderate (μ=2-3) lensing and/or moderately massive (M=2-3×106 M) Pop III galaxies, such line detections can be achieved in medium-sized JWST GO programs. However, MIRI F770W detections of Pop III galaxies will require substantial gravitational lensing (μ=10) and/or fortuitous imaging of exceptionally massive (M=107 M) 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.


Comment: 27 pages, 18 figures, 7 tables

Subject: Astrophysics - Astrophysics of Galaxies