Big Three Dragons: A [N II] 122 $\hat{I}¼$m Constraint and New Dust-continuum Detection of A $z=7.15$ Bright Lyman Break Galaxy with ALMA

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 $\hat{\mathrm{I}}¼\mathrm{m}$, [CII] 158 $\hat{\mathrm{I}}¼\mathrm{m}$, and dust-continuum emission during the epoch of reionization. Our targets are the [NII] 122 $\hat{\mathrm{I}}¼\mathrm{m}$ fine-structure emission line and underlying 120 $\hat{\mathrm{I}}¼\mathrm{m}$ dust continuum. The dust continuum is detected with a $\sim$19$\ddot{I}ƒ$ significance. From far-infrared spectral energy distribution sampled at 90, 120, and 160 $\hat{\mathrm{I}}¼\mathrm{m}$, we obtaine a best-fit dust temperature of $40$ K ($79$ K) and an infrared luminosity of ${\log }_{10}(L_{\mathrm{IR}}/{\mathrm{L}}_{\odot })=11.6$ ($12.1$) at the emissivity index ${\hat{I}}^2=2.0$ (1.0). The [NII] 122 $\hat{\mathrm{I}}¼\mathrm{m}$ line is not detected. The 3$\ddot{I}ƒ$ upper limit of the [NII] luminosity is $8.1\times 10^7{\mathrm{L}}_{\odot }$. 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 ($Z>0.4{\mathrm{Z}}_{\odot }$), the ionization parameter and hydrogen density are ${\log }_{10}U\simeq -2.7\pm 0.1$ and $n_{\text{H}}\simeq 50$-$250{\mathrm{cm}}^{-3}$, respectively, which are comparable to those measured in low-redshift galaxies. The nitrogen-to-oxygen abundance ratio, $N/O$, is constrained to be sub-solar. At $Z<0.4{\mathrm{Z}}_{\odot }$, the allowed $U$ drastically increases as the assumed metallicity decreases. For high ionization parameters, the $N/O$ 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.

17 pages, 13 figures, 2 tables. accepted for publication in ApJ