A DEEP NARROWBAND IMAGING SEARCH FOR C iv AND He ii EMISSION FROM LyαBLOBS

We conduct a deep narrowband imaging survey of 13 Ly α blobs (LABs) located in the SSA22 proto-cluster at z ∼ 3.1 in the C iv and He ii emission lines in an effort to constrain the physical process powering the Ly α emission in LABs. Our observations probe down to unprecedented surface brightness (SB) limits of (2.1–3.4) × 10 −18 erg s −1 cm −2 arcsec −2 per 1 arcsec 2 aperture (5 σ ) for the He ii λ 1640 and C iv λ 1549 lines, respectively. We do not detect extended He ii and C iv emission in any of the LABs, placing strong upper limits on the He ii /Ly α and C iv /Ly α line ratios, of 0.11 and 0.16, for the brightest two LABs in the field. We conduct detailed photoionization modeling of the expected line ratios and find that, although our data constitute the deepest ever observations of these lines, they are still not deep enough to rule out a scenario where the Ly α emission is powered by the ionizing radiation from an obscured active galactic nucleus. Our models can accommodate He ii /Ly α and C iv /Ly α ratios as low as ≃0.05 and ≃0.07, respectively, implying that one needs to reach SB as low as (1–1.5) × 10 −18 erg s −1 cm −2 arcsec −2 (at 5 σ ) in order to rule out a photoionization scenario. These depths will be achievable with the new generation of image-slicing integral field units such as the Multi Unit Spectroscopic Explorer (MUSE) on VLT and the Keck Cosmic Web Imager (KCWI). We also model the expected He ii /Ly α and C iv /Ly α in a different scenario, where Ly α emission is powered by shocks generated in a large-scale superwind, but find that our observational constraints can only be met for shock velocities v s ≳ 250 km s −1 , which appear to be in conflict with recent observations of quiescent kinematics in LABs.