First infrared-based implications for the dust attenuation and star formation of typical Lyα emitters

By stacking publicly available deep Spitzer/MIPS 24 μm and Herschel/PACS images for 213 $z\simeq 2.18$ $Ly\alpha$ Emitters (LAEs) in GOODS-South, we obtain a strong upper limit to the IR luminosity of typical LAEs and discuss their attenuation curve for the first time. The $3\sigma$ upper limit $L_{TIR}^{3\sigma }$ $=1.1\times {10}^{10}{L}_{\odot }$, determined from the MIPS data providing the lowest limit, gives $IRX\equiv L_{TIR}/L_{UV}⩽2.2$. Here we assume that the local calibration between the 8 μm emission and the dust SED shape and metallicity applies at high redshifts and that our LAEs have low metallicities as suggested by previous studies. The inferred escape fractions of $Ly\alpha$, 16–37%, and UV continuum, $⩾44\%$, are higher than the cosmic averages at the same epoch. The SMC attenuation curve is consistent with the IRX and the UV slope $\beta =-{1.4}_{-0.2}^{+0.2}$ of our stacked LAE, while the Meurer's relation (Calzetti curve) predicts a 3.8 times higher IRX; we also discuss the validity of PACS-based $L_{TIR}^{3\sigma }$ allowing the Meurer's relation. SED fitting using the Calzetti curve also gives a ∼10 times higher SFR than from the $L_{TIR}^{3\sigma }$ and $L_{UV}$. With $M_{\star }={6.3}_{-2.0}^{+0.8}\times {10}^8{M}_{\odot }$, our LAEs lie on a lower-mass extrapolation of the star formation main sequence at $z\sim 2$, suggesting that the majority of $z\sim 2$ LAEs are mildly star forming with relatively old ages of ∼200 Myr. The faint $L_{TIR}^{3\sigma }$ implies that LAEs contribute little to the faint ($\gtrsim 100\mu Jy$) submillimeter number counts by ALMA.