This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library “MaStar”). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17).

A new multicamera soft x-ray detector system has been constructed for the pumped divertor phase of JET to provide improved spatial and temporal resolution. It is based on 12 compact pinhole cameras each of which uses a 35 element PIN photodiode array. The cameras are housed in a secondary vacuum and view the plasma through beryllium windows 250-μm-thick which absorb x rays with energies below ∼2 keV. The front-end electronics, located up to 10 m away from the detectors, sample data at 250 kHz, with a 100 kHz passband. The diodes are operated at zero bias voltage. The system ran successfully during the 1995 phase of JET operation with poloidal mode numbers up to m=5 being successfully reconstructed. Detector noise levels (<1 nA) were small compared to signal statistical noise. Some deterioration in diode performance was observed after exposure to an integrated neutron fluence of ∼1013 n. cm−2. Tests have shown that operating with 3 V reverse bias can offset much of the fall in performance from radiation damage. Using a similar 16 element PIN photodiode, a windowless camera has been developed for installation in the JET primary vacuum, to investigate fast XUV/x-ray events which develop at lower energies near the plasma edge or in the divertor. Finally, a radiation hardened system is being developed for D-T operation.

We present the second data release for the HI-MaNGA programme of H i follow-up observations for the SDSS-IV MaNGA survey. This release contains measurements for 3669 unique galaxies, combining 2108 Green Bank Telescope observations with an updated crossmatch of the MaNGA sample with the ALFALFA survey. We combine these data with MaNGA spectroscopic measurements to examine relationships between H i-to-stellar mass ratio (${\rm M_{H\, {\small I}}/{M_*}}$) and average ISM/star formation properties probed by optical emission lines. ${\rm M_{H\, {\small I}}/{M_*}}$ is very weakly correlated with the equivalent width of Hα, implying a loose connection between the instantaneous star formation rate and the HI reservoir, although the link between ${\rm M_{H\, {\small I}}/{M_*}}$ and star formation strengthens when averaged even over only moderate timescales (∼30 Myrs). Galaxies with elevated H i depletion times have enhanced [O i]/Hα and depressed Hα surface brightness, consistent with more H i residing in a diffuse and/or shock heated phase which is less capable of condensing into molecular clouds. Of all optical lines, ${\rm M_{H\, {\small I}}/{M_*}}$ correlates most strongly with oxygen equivalent width, EW(O), which is likely a result of the existing correlation between ${\rm M_{H\, {\small I}}/{M_*}}$ and gas-phase metallicity. Residuals in the ${\rm M_{H\, {\small I}}/{M_*}}$−EW(O) relation are again correlated with [O i]/Hα and Hα surface brightness, suggesting they are also driven by variations in the fraction of diffuse and/or shock-heated gas. We recover the strong anti-correlation between ${\rm M_{H\, {\small I}}/{M_*}}$ and gas-phase metallicity seen in previous studies. We also find a relationship between ${\rm M_{H\, {\small I}}/{M_*}}$ and [O i]6302/Hα, suggesting that higher fractions of diffuse and/or shock-heated gas are more prevalent in gas-rich galaxies.

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