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ABSTRACT The discovery of giant quasar Ly α nebulae at z > 2 has opened up the possibility to directly study in emission the Circumgalactic and Intergalactic Medium (CGM/IGM). However, the resonant nature of the Ly α line and its different emission mechanisms hamper the ability to constrain both the kinematics and physical properties of the CGM/IGM. Here, we present results of a pilot project aiming at the detection of CGM H α emission, a line which does not suffer from these limitations. To this end, we first used KCWI to detect Ly α emission around three bright quasars with 2.25 < z < 2.27, a range which is free from bright IR sky lines for H α, and then selected the most extended nebula for H α follow-up with MOSFIRE. Within the MOSFIRE slit, we detected H α emission extending up to 20 physical kpc with a total H α flux of FH α = (9.5 ± 0.9) × 10$$^{-18}~\mathrm{erg\, s^{-1}\, cm^{-2}}$$. Considering the Ly α flux in the same region, we found FLy α/FH α = 3.7 ± 0.3 consistent with that obtained for the Slug Nebula at z = 2.275 and with recombination radiation. This implies high densities or a very broad density distribution within the CGM of high-redshift quasars. Moreover, the H α line profile suggests the presence of multiple emitting components overlapping along our line of sight and relatively quiescent kinematics, which seems incompatible with either quasar outflows capable of escaping the potential well of the host halo or disc-like rotation in a massive halo (>1012 M⊙). 

ABSTRACT We study the gas kinematics of a sample of six isolated gas-rich low surface brightness galaxies, of the class called ultra-diffuse galaxies (UDGs). These galaxies have recently been shown to be outliers from the baryonic Tully–Fisher relation (BTFR), as they rotate much slower than expected given their baryonic mass, and to have a baryon fraction similar to the cosmological mean. By means of a 3D kinematic modelling fitting technique, we show that the H i in our UDGs is distributed in ‘thin’ regularly rotating discs and we determine their rotation velocity and gas velocity dispersion. We revisit the BTFR adding galaxies from other studies. We find a previously unknown trend between the deviation from the BTFR and the exponential disc scale length valid for dwarf galaxies with circular speeds ≲ 45 km s−1, with our UDGs being at the extreme end. Based on our findings, we suggest that the high baryon fractions of our UDGs may originate due to the fact that they have experienced weak stellar feedback, likely due to their low star formation rate surface densities, and as a result they did not eject significant amounts of gas out of their discs. At the same time, we find indications that our UDGs may have higher-than-average stellar specific angular momentum, which can explain their large optical scale lengths.