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We present VLT/MUSE observations targeting the extended Lyman-α(Lyα) emission of five high-redshift (z ∼ 3-4) submillimeter galaxies (SMGs) with increasing quasi-stellar object (QSO) radiation: two SMGs; two SMGs that host a QSO; and one SMG that hosts a QSO with an SMG companion (QSO+SMG). These sources are predicted to be located in dark matter halos of comparable masses (average mass ofM_DM ∼ 10^12.2 M_⊙). We quantified the luminosity and extent of the Lyαemission, together with its kinematics, and examined four Lyαpowering mechanisms: photoionization from QSOs or star formation, shocks by galactic and/or QSO outflows, gravitational cooling radiation, and Lyαphoton resonant scattering. We find a variety of Lyαluminosities and extents, with the QSO+SMG system displaying the most extended and bright nebula, followed by the SMGs hosting a QSO, and finally the undetected circumgalactic medium of SMGs. This diversity implies that gravitational cooling is unlikely to be the main powering mechanism. We show that photoionization from the QSO and QSO outflows can contribute to power the emission for average densitiesn_H > 0.5 cm⁻³. Moreover, the observed Lyαluminosities scale with the QSO’s budget of Lyαphotons modulo the dust content in each galaxy, highlighting a possible contribution from resonant scattering of QSO radiation in powering the nebulae. We find larger Lyαlinewidths (FWHM ≳ 1200 km s⁻¹) than usually reported around radio-quiet systems, pointing to large-scale outflows. A statistical survey targeting similar high-redshift massive systems with known host properties is needed to confirm our findings. 

Abstract To understand the mechanism behind high-zLyαnebulae, we simulate the scattering of Lyαin a Hihalo about a central Lyαsource. For the first time, we consider both smooth and clumpy distributions of halo gas, as well as a range of outflow speeds, total Hicolumn densities, Hispatial concentrations, and central source galaxies (e.g., with Lyαline widths corresponding to those typical of active galactic nucleus or star-forming galaxies). We compute the spatial-frequency diffusion and the polarization of the Lyαphotons scattered by atomic hydrogen. Our scattering-only model reproduces the typical size of Lyαnebulae (∼100 kpc) at total column densitiesN_{H I}≥ 10²⁰cm⁻²and predicts a range of positive, flat, and negative polarization radial gradients. We also find two general classes of Lyαnebula morphologies: with and without bright cores. Cores are seen whenN_{H I}is low, i.e., when the central source is directly visible, and are associated with a polarization jump, a steep increase in the polarization radial profile just outside the halo center. Of all the parameters tested in our smooth or clumpy medium model,N_{H I}dominates the trends. The radial behaviors of the Lyαsurface brightness, spectral line shape, and polarization in the clumpy model with covering factorf_c≳ 5 approach those of the smooth model at the sameN_{H I}. A clumpy medium with highN_{H I}and lowf_c≲ 2 generates Lyαfeatures via scattering that the smooth model cannot: a bright core, symmetric line profile, and polarization jump.