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Arxiv: Balmer Decrement Anomalies in Galaxies at z ~ 6 Found by JWST Observations: Density-Bounded Nebulae or Excited H I Clouds? Published: 3/29/2024 2:02:13 PM Updated: 3/29/2024 2:02:13 PM

Paper abstract: We investigate the physical origins of the Balmer decrement anomalies inGS-NDG-9422 (Cameron et al. 2023) and RXCJ2248-ID (Topping et al. 2024)galaxies at z~ 6 whose \mathrm{H}\alpha/\mathrm{H}\beta values aresignificantly smaller than 2.7, the latter of which also shows anomalous\mathrm{H}\gamma/\mathrm{H}\beta and \mathrm{H}\delta/\mathrm{H}\betavalues beyond the errors. Because the anomalous Balmer decrements are notreproduced under the Case B recombination, we explore the nebulae with theoptical depths smaller and larger than the Case B recombination by physicalmodeling. We find two cases quantitatively explaining the anomalies; 1)density-bounded nebulae that are opaque only up to around Ly\gamma-Ly8transitions and 2) ionization-bounded nebulae partly/fully surrounded byoptically-thick excited H{\sc i} clouds. The case of 1) produces more H\betaphotons via Ly\gamma absorption in the nebulae, requiring fine tuning inoptical depth values, while this case helps ionizing photon escape for cosmicreionization. The case of 2) needs the optically-thick excited H{\sc i} cloudswith N_2~eq 10^{12}-10^{13} \mathrm{cm^{-2}}, where N_2 is the columndensity of the hydrogen atom with the principal quantum number of n=2.Interestingly, the high N_2 values qualitatively agree with the recent claimsfor GS-NDG-9422 with the strong nebular continuum requiring a number of2s-state electrons and for RXCJ2248-ID with the dense ionized regions likelycoexisting with the optically-thick clouds. While the physical origin of theoptically-thick excited H{\sc i} clouds is unclear, these results may suggestgas clouds with excessive collisional excitation caused by an amount ofaccretion and supernovae in the high-z galaxies.