SERENADE III: Insight into the Origin of the High Dust Temperature and High [O III]/[C II] Ratio at z 6

We present an analysis of ALMA high-frequency observations of nine bright Lyman-break galaxies at 5.8<z spec<8.3. Our sample consists of five galaxies at z about6 newly observed in Band-9 and/or 10, allowing us to better constrain the dust temperature (T dust) in a statistical sample of z 6 galaxies. Our measurements of the dust temperature at z about6-9 suggest most of the star-forming galaxies show T dust about40, K on average, whereas three galaxies show significantly higher T dust (60, K). We find a potential negative correlation between T dust and gas-phase metallicity with T dust proportional to Z-0.50 plus or minus0.19, implying decreased dust shielding and efficient dust heating in low-metallicity environments. Given the systematic offset of T dust between z about0 and z about6-9 at fixed metallicity, we find T dust is well described by sSFR as well as Z with T dust=-0.25+0.03-0.04 times(Z-0.57 times sSFR)+4.88+0.47-0.43 across z about0 to z about6-9. Simultaneously with the dust continuum, these observations cover the [OI]63 μ m emission line for five galaxies, which traces dense neutral gas. We find a lower [CII]/[OI] ratio in z 6 galaxies than in local samples, suggesting that [CII] becomes fainter due to collisional de-excitation at high gas density. Combining the [OI], [OIII], and [CII] lines and cloudy modeling, our results imply U ion about-2 and n H,[cm-3] about2.5 in z 6 galaxies, which is about3-10 times higher U ion and about2-3 times higher n H than the local samples. The combination of these enhanced U ion and n H naturally explains the high [OIII]/[CII] ratio at z 6.