Rest-frame ultraviolet-to-optical spectral characteristics of extremely metal-poor and metal-free galaxies

TL;DR

The paper develops a comprehensive rest-frame UV-to-optical spectral model for EMP and metal-free galaxies, with emphasis on accurate nebular line and continuum emission. By coupling stellar SEDs across a broad metallicity grid to CLOUDY-based nebular calculations and including dust and LyC escape effects, it generates machine-readable tables of 119 emission lines and analyzes equivalent widths and broadband colors to identify primordial candidates. Key diagnostics include very low [O III] / Hβ ratios ($<0.1$), strong Balmer-line EWs, and Balmer/Lyman jumps, with practical selection criteria for HST and JWST observations; the study also assesses the detectability of these signatures with JWST/NIRSpec at $z∼6$–8. The work provides actionable guidance for preselecting EMP/Pop III galaxies from deep-field surveys and for confirming their nature spectroscopically, significantly informing strategies to probe the first generation of galaxies. The findings have direct implications for planning JWST spectroscopic campaigns and interpreting high-$z$ galaxy spectra in the era of next-generation telescopes.

Abstract

Finding the first generation of galaxies in the early Universe is the greatest step forward for understanding galaxy formation and evolution. For strategic survey of such galaxies and interpretation of the obtained data, this paper presents an ultraviolet-to-optical spectral model of galaxies with a great care of the nebular emission. In particular, we present a machine-readable table of intensities of 119 nebular emission lines from Ly$α$ to the rest-frame 1 $μ$m as a function of metallicity from zero to the Solar one. Based on the spectral model, we present criteria of equivalent widths of Ly$α$, He {\sc ii} $\lambda1640$, H$α$, H$β$, [O {\sc iii}] $\lambda5007$ to select extremely metal-poor and metal-free galaxies although these criteria have uncertainty caused by the Lyman continuum escape fraction and the star formation duration. We also present criteria of broad-band colours which will be useful to select candidates for spectroscopic follow-up from drop-out galaxies. We propose the line intensity ratio of [O {\sc iii}] $\lambda5007$ to H$β$ $<0.1$ as the most robust criterion for $<1/1000$ of the Solar metallicity. This ratio of a galaxy with a few $M_\odot$ yr$^{-1}$ at $z\sim8$ is detectable by spectroscopy with the James Webb Space Telescope within a reasonable exposure time.

Figures (14)

• Figure 1: Emissivities relative to H$\beta$ of six strongest metal emission lines as a function of metallicity. The circles are results calculated with the code cloudy 08.00 (Ferland 1998) in this work: no dust model (filled) and dusty model (open). The squares are empirical results by Anders & Fritze-v. Alvensleben (2003). The triangles are the observations of I Zw 18 (northwest and southeast components) measured by Izotov et al. (1999). The diamonds are observations of a $z=2.3$ galaxy measured by Erb et al. (2010). The dashed lines are empirical results by Nagao et al. (2006) and updated by Maiolino et al. (2008).
• Figure 2: H$\beta$ luminosity per unit star formation rate as a function of metallicity. The diamonds, triangles, and squares are the cases of the LyC escape fraction $f_{\rm esc}=0$ and the constant star formation duration of 10, 100, and 500 Myr. The crosses are the case of $f_{\rm esc}=0.5$ and 100 Myr. The small triangles are the same case but obtained by the $f_{\rm esc}$ scaling of equation (3). The arrows at the left edge of the panel are the metal-free cases. The LyC absorption in nebulae is neglected (i.e. $f_{\rm dust}=0$).
• Figure 3: Rest-frame model spectra of (a) Pop III ($Z=0$) galaxies and (b) moderate sub-solar metallicity ($Z=0.004$: $\log_{10}(Z/Z_\odot)=-0.7$) galaxies. In each panel, the lines correspond to 10, 100, and 500 Myr of constant star formation (bottom to top). LyC escape fractions $f_{\rm esc}=0.5$ and 0 are assumed for the solid and the dotted lines, respectively. The emission line width is assumed to be 300 km s$^{-1}$ as an example. No attenuation by dust and IGM is not included.
• Figure 4: Rest-frame spectra of luminosity density ratio of nebular emission to stellar emission: (a) Pop III and (b) moderate sub-solar metallicity ($Z=0.004$: $\log_{10}(Z/Z_\odot)=-0.7$). In each panel, the solid lines correspond to the LyC escape fraction $f_{\rm esc}=0.5$ and the dotted lines correspond to $f_{\rm esc}=0$. For each type of the lines, the top to bottom correspond to 10, 100, and 500 Myr of constant star formation. The dashed horizontal lines indicate the ratio to be unity (i.e. equal contribution from stellar and nebular emissions).
• Figure 5: Luminosity density ratio of nebular to stellar emissions at 3640 Å, very close to the Balmer limit (3646 Å), as a function of the duration of star formation. There are 7 metallicity cases: $\log_{10}(Z/Z_\odot)=\infty$ (plus), $-5.3$ (asterisk), $-3.3$ (circle), $-1.7$ (diamond), $-0.7$ (triangle), $-0.4$ (square), and 0.0 (cross). No escape of the LyC is assumed ($f_{\rm esc}=0$).