Big Three Dragons: A [N II] 122 $\mu$m Constraint and New Dust-continuum Detection of A $z = 7.15$ Bright Lyman Break Galaxy with ALMA

TL;DR

This study reports ALMA observations of a high-redshift galaxy, providing new dust continuum detection and constraints on nebular parameters, metallicity, and ionization conditions during the epoch of reionization.

Contribution

First detection of dust continuum at 120 μm and constraints on nebular properties of a z=7.15 galaxy using [NII], [OIII], and [CII] lines with Cloudy modeling.

Findings

Dust continuum detected with 19σ significance.

Upper limit on [NII] 122 μm line luminosity.

Nebular parameters suggest low metallicity and high ionization conditions.

Abstract

We present new Atacama Large Millimeter/submillimeter Array Band 7 observational results of a Lyman break galaxy at $ z=7.15 $, B14-65666 ("Big Three Dragons"), which is an object detected in [OIII] 88 $\rm{\mu m}$, [CII] 158 $\rm{\mu m}$, and dust-continuum emission during the epoch of reionization. Our targets are the [NII] 122 $\rm{\mu m}$ fine-structure emission line and underlying 120 $\rm{\mu m}$ dust continuum. The dust continuum is detected with a $ \sim $19$ \sigma $ significance. From far-infrared spectral energy distribution sampled at 90, 120, and 160 $\rm{\mu m}$, we obtaine a best-fit dust temperature of $ 40 $ K ($ 79 $ K) and an infrared luminosity of $ \log_{10}(L_{\rm IR}/{\rm L}_\odot)=11.6$ ($12.1$) at the emissivity index $ \beta = 2.0 $ (1.0). The [NII] 122 $\rm{\mu m}$ line is not detected. The 3$ \sigma $ upper limit of the [NII] luminosity is $ 8.1 \times 10^7\ {\rm L}_\odot$. From the [NII], [OIII], and [CII] line luminosities, we use the Cloudy photoionization code to estimate nebular parameters as functions of metallicity. If the metallicity of the galaxy is high ($ Z > 0.4\ {\rm Z}_\odot$), the ionization parameter and hydrogen density are $ \log_{10} U \simeq -2.7\pm0.1$ and $ n_\text{H} \simeq 50$-$250\ {\rm cm}^{-3}$, respectively, which are comparable to those measured in low-redshift galaxies. The nitrogen-to-oxygen abundance ratio, $\rm{N/O}$, is constrained to be sub-solar. At $ Z < 0.4\ {\rm Z}_\odot$, the allowed $ U $ drastically increases as the assumed metallicity decreases. For high ionization parameters, the $\rm{N/O}$ constraint becomes weak. Finally, our Cloudy models predict the location of B14-65666 on the BPT diagram, thereby allowing a comparison with low-redshift galaxies.