There has recently been a dramatic renewal of interest in hadron spectroscopy and charm physics. This renaissance has been driven in part by the discovery of a plethora of charmonium-like XYZ states at BESIII and B factories, and the observation of an intriguing proton-antiproton threshold enhancement and the possibly related X(1835) meson state at BESIII, as well as the threshold measurements of charm mesons and charm baryons. We present a detailed survey of the important topics in tau-charm physics and hadron physics that can be further explored at BESIII during the remaining operation period of BEPCII. This survey will help in the optimization of the data-taking plan over the coming years, and provides physics motivation for the possible upgrade of BEPCII to higher luminosity.

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We report the detection of Ne VIII in a z abs = 0.59961 absorber towards the QSO PG1407+265 (z em = 0.94). Besides Ne VIII, absorption from H I Lyman series lines (H I λ1025 -λ915), several other low (C II, N II, O II and S II), intermediate (C III, N III, N IV, O III, S IV and S V) and high (S VI, O VI and Ne VIII) ionization metal lines are detected. Disparity in the absorption line kinematics between different ions implies that the absorbing gas comprises of multiple ionization phases. The low and the intermediate ions (except S V) trace a compact (∼ 410 pc), metal-rich (Z ∼ Z ⊙ ) and over-dense (log n H ∼ −2.6) photoionized region that sustained star-formation for a prolonged period. The high ions, Ne VIII and O VI, can be explained as arising in a low density (−5.3 log n H −5.0), metal-rich (Z Z ⊙ ) and diffuse (∼ 180 kpc) photoionized gas. The S V, S VI and C IV (detected in the FOS spectrum) require an intermediate photoionization phase with −4.2 < log n H < −3.5. Alternatively, a pure collisional ionization model, as used to explain the previous known Ne VIII absorbers, with 5.65 < log T < 5.72, can reproduce the S VI, O VI and Ne VIII column densities simultaneously in a single phase. However, even such models require an intermediate phase to reproduce any observable S V and/or C IV. Therefore, we conclude that when multiple phases are present, the presence of Ne VIII is not necessarily an unambiguous indication of collisionally ionized hot gas.

Using VLT/X-shooter we searched for emission line galaxies associated to four damped Lyman-α systems (DLAs) and one sub-DLA at 2.73 z 3.25 towards QSO J2358+0149. We detect [O III] emission from a "low-cool" DLA at z abs = 2.9791 (having log N(H I) = 21.69 ± 0.10, [Zn/H] = −1.83 ± 0.18) at an impact parameter of, ρ ∼ 12 kpc. The associated galaxy is compact with a dynamical mass of (1 − 6) × 10 9 M ⊙ , very high excitation Hβ] both greater than 10), 12+[O/H] 8.5 and moderate star formation rate (SFR 2 M ⊙ yr −1 ). Such properties are typically seen in the low-z extreme blue compact dwarf galaxies. The kinematics of the gas is inconsistent with that of an extended disk and the gas is part of either a large scale wind or cold accretion. We detect Lyα emission from the z abs = 3.2477 DLA (having log N(H I)=21.12±0.10 and [Zn/H]=−0.97 ± 0.13). The Lyα emission is redshifted with respect to the metal absorption lines by 320 km s −1 , consistent with the location of the red hump expected in radiative transport models. We derive SFR∼0.2-1.7 M ⊙ yr −1 and Lyα escape fraction of 10 per cent. No other emission line is detected from this system. Because the DLA has a small velocity separation from the quasar (∼500 km s −1 ) and the DLA emission is located within a small projected distance (ρ < 5 kpc), we also explore the possibility that the Lyα emission is being induced by the QSO itself. QSO induced Lyα fluorescence is possible if the DLA is within a physical separation of 340 kpc to the QSO. Detection of stellar continuum light and/or the oxygen emission lines would disfavor this possibility. We do not detect any emission line from the remaining three systems.

Ne viii absorbers seen in QSO spectra are useful tracers of warm ionized gas, when collisional ionization is the dominant ionization process. While photoionization by the ultraviolet background (UVB) is a viable option, it tends to predict large line-of-sight thickness for the absorbing gas. Here, we study the implications of the recently updated UVB at low-z to understand the ionization mechanisms of intervening Ne viii absorbers. With the updated UVB, one typically needs higher density and metallicity to reproduce the observed ionic column densities under photoionization. Both reduce the inferred line-of-sight thicknesses of the absorbers. We find a critical density of 5 × 10 −5 cm −3 above which the observed N (Ne viii)/N (O vi) can be reproduced by pure collisional processes. If the gas is of near solar metallicity (as measured for the low ions) then the cooling timescales will be small (<10 8 yrs). Therefore, a continuous injection of heat is required in order to enhance the detectability of the collisionally ionized gas. Using photoionization models we find that in almost all Ne viii systems the inferred low ion metallicity is near solar or supersolar. If we assume the Ne viii phase to have similar metallicities then photoionization can reproduce the observed N (Ne viii)/N (O vi) without the line-of-sight thickness being unreasonably large and avoids cooling issues related to the collisional ionization at these metallicities. However the indication of broad Lyα absorption in a couple of systems, if true, suggests that the Ne viii phase is distinct from the low ion phase having much lower metallicity.