Breathing Fire: Hot Dust in the Big Three Dragons at z = 7.15

Abstract

We present new Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 (${\lambda }_{\mathrm{obs}}=0.45\mathrm{mm}$) and 4 (${\lambda }_{\mathrm{obs}}=2.2\mathrm{mm}$) observations towards the Big Three Dragons, a pair of merging Lyman-break galaxies (LBGs) at $z=7.15$. The system was previously detected in ALMA Bands 6, 7 and 8 (${\lambda }_{\mathrm{obs}}=0.73-1.32\mathrm{mm}$), which, combined with our new observations, allows us to robustly constrain its dust temperature and obscured SFR. The unresolved Band 4 observations yield a $3.5\sigma$ detection, and the $0.4^{\prime \prime }$ Band 9 observations detect the Eastern and Western LBGs at $3.7$ and $3.5\sigma$, respectively. Through modified blackbody fitting, we infer a global dust temperature of $T_d=78_{-23}^{+35}\mathrm{K}$ for the system, which implies a high IR luminosity of $\log (L_{\mathrm{IR}}/L_{\odot })=12.32_{-0.41}^{+0.43}$. This makes the Big Three Dragons one of the most IR-luminous systems known at $z>7$, with a total ${\mathrm{SFR}}_{\mathrm{UV}+\mathrm{IR}}=267_{-153}^{+418}{M}_{\odot }{\mathrm{yr}}^{-1}$ that is almost completely obscured ($f_{\mathrm{obs}}=0.94_{-0.09}^{+0.04}$). Using resolved ALMA observations in Bands 6, 8 and 9, we confirm both LBGs have hot dust temperatures ($T_d\approx 67-84\mathrm{K}$) and correspondingly high obscured fractions ($f_{\mathrm{obs}}\approx 0.88-0.95$). We find the Western LBG to fall $\sim 1\mathrm{dex}$ above the canonical IRX-${\beta }_{\mathrm{UV}}$ relation, suggesting patchy dust obscuration. The compact Eastern component, on the other hand, is consistent with a Calzetti- or SMC-like dust screen within the uncertainties. Together with the similarly hot dust temperature recently reported for the $z=8.31$ galaxy MACS0416-Y1, our results suggest a non-negligible fraction of star formation at the bright end of the UV luminosity function is highly dust-obscured, even at $z\gtrsim 7$.

Submitted to MNRAS. Main text: 14 pages, 9 figures, 3 tables. Appendices: 2 pages, 4 figures, 2 tables