PREPRINT

# 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

## 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 $5\sigma$ continuum detections of 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$\alpha$, H$\beta$ and/or He II $\lambda 1640$. We find small differences in the H$\beta$ 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 $\lambda 1640$ emission will serve as the definitive Pop III identifier, requiring (ultra-)deep integrations (10-250 h) with NIRSpec/G140M for Pop III galaxies at $z=8$. With moderate ($\mu =$2-3) lensing and/or moderately massive (${M}_{\ast }=2$-) Pop III galaxies, such line detections can be achieved in medium-sized JWST GO programs. However, MIRI F770W detections of Pop III galaxies will require substantial gravitational lensing ($\mu =10$) and/or fortuitous imaging of exceptionally massive () Pop III galaxies. Thus, NIRCam medium-band imaging surveys that can search for high EW He II $\lambda 1640$ emitters in photometry may perhaps be a viable alternative for finding Pop III candidates.

## Preprint

Comment: 27 pages, 18 figures, 7 tables

Subject: Astrophysics - Astrophysics of Galaxies