We present the performance of the upGREAT heterodyne array receivers on the SOFIA telescope after several years of operations. This instrument is a multi-pixel high resolution (R 10 7 ) spectrometer for the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA). The receivers use 7-pixel subarrays configured in a hexagonal layout around a central pixel. The low frequency array receiver (LFA) has 2x7 pixels (dual polarization), and presently covers the 1.83-2.06 THz frequency range, which allows to observe the [CII] and [OI] lines at 158 µm and 145 µm wavelengths. The high frequency array (HFA) covers the [OI] line at 63 µm and is equipped with one polarization at the moment (7 pixels, which can be upgraded in the near future with a second polarization array). The 4.7 THz array has successfully flown using two separate quantum-cascade laser local oscillators from two different groups. NASA completed the development, integration and testing of a dual-channel closed-cycle cryocooler system, with two independently operable He compressors, aboard SOFIA in early 2017 and since then, both arrays can be operated in parallel using a frequency separating dichroic mirror. This configuration is now the prime GREAT configuration and has been added to SOFIA's instrument suite since observing cycle 6.

We present a [C ii] 158 µm map of the entire M51 (including M51b) grand-design spiral galaxy observed with the FIFI-LS instrument on SOFIA. We compare the [C ii] emission with the total far-infrared (TIR) intensity and star formation rate (SFR) surface density maps (derived using Hα and 24µm emission) to study the relationship between [C ii] and the star formation activity in a variety of environments within M51 on scales of 16 corresponding to ∼660 pc. We find that [C ii] and the SFR surface density are well correlated in the central, spiral arm, and inter-arm regions. The correlation is in good agreement with that found for a larger sample of nearby galaxies at kpc scales. We find that the SFR, and [C ii] and TIR luminosities in M51 are dominated by the Pineda, J.L. et al.extended emission in M51's disk. The companion galaxy M51b, however, shows a deficit of [C ii] emission compared with the TIR emission and SFR surface density, with [C ii] emission detected only in the S-W part of this galaxy. The [C ii] deficit is associated with an enhanced dust temperature in this galaxy. We interpret the faint [C ii] emission in M51b to be a result of suppressed star formation in this galaxy, while the bright midand far-infrared emission, which drive the TIR and SFR values, are powered by other mechanisms. A similar but less pronounced effect is seen at the location of the black hole in M51's center. The observed [C ii] deficit in M51b suggests that this galaxy is a valuable laboratory to study the origin of the apparent [C ii] deficit observed in ultra-luminous galaxies.

Context. The observed proportionality between the centripetal contribution of the dynamically insignificant HI gas in the discs of spiral galaxies and the dominant contribution of dark matter (DM) -the "Bosma effect" -has been repeatedly mentioned in the literature but largely ignored. Since this phenomenology, if statistically significant, tells us something about the relationship between the visible baryonic and invisible DM, it is important to re-examine the reality of this effect using formal tests and more modern data. Aims. We have re-examined the evidence for the Bosma effect, either by scaling the contribution of the HI gas alone or by using both the observed stellar disc and HI gas as proxies. Methods. We have calculated Bosma effect models for 17 galaxies in The HI Nearby Galaxy Survey data set. The results are compared with two models for exotic cold DM: internally consistent cosmological Navarro-Frenk-White (NFW) models with constrained compactness parameters, and "universal rotation curve" (URC) models using fully unconstrained Burkert density profiles. Results. Fits to spiral galaxy rotation curves computed using just HI scaling are inadequate, despite the clear proportionality seen in the outer discs. The poor performance is obviously related to the prominent decrease in the HI surface density in regions of high stellar surface density, where HI has been converted into molecules and stars. The Bosma models that partially correct for this physical effect using the stellar discs as additional proxies are statistically nearly as good as the URC models and clearly better than the NFW ones. Conclusions. We confirm the correlation between the centripetal effects of DM and that of the interstellar medium of spiral galaxies. The efficacy of "maximal disc" models is explained as the natural consequence of "classic" Bosma models which include the stellar disc as a proxy in regions of reduced atomic gas. The perception that the Bosma effect could be due to the near-equality of the HI surface density and the projected mass density of a cold DM halo is incorrect, both theoretically and empirically. The standard explanation -that the effect reflects a statistical correlation between the visible and exotic DM -seems highly unlikely, given that the geometric forms and hence centripetal signatures of spherical halo and disc components are so different. A literal interpretation of the Bosma effect as being due to the presence of significant amounts of disc DM requires a median visible baryon to disc DM ratio of about 40%.

We present the first complete, velocity-resolved [C ii] 158 μm image of the M51 grand-design spiral galaxy, observed with the upgraded German Receiver for Astronomy at Terahertz frequencies instrument on the Stratospheric Observatory for Infrared Astronomy. [C ii] is an important tracer of various phases of the interstellar medium (ISM), including ionized gas, neutral atomic, and diffuse molecular regions. We combine the [C ii] data with H i, CO, 24 μm dust continuum, FUV, and NIR K-band observations to study the evolution of the ISM across M51's spiral arms in both position–position and position–velocity space. Our data show strong velocity gradients in H i, 12CO, and [C ii] at the locations of the stellar arms (traced by K-band data) with a clear offset in position–velocity space between upstream molecular gas (traced by 12CO) and downstream star formation (traced by [C ii]). We compare the observed position–velocity maps across the spiral arms with synthetic observations from numerical simulations of galaxies with both dynamical and quasi-stationary steady spiral arms that predict both tangential and radial velocities at the location of the spiral arms. We find that our observations, based on the observed velocity gradients and associated offset between CO and [C ii], are consistent with the presence of shocks in the spiral arms in the inner parts of M51 and in the arm connecting the companion galaxy, M51b, in the outer parts of M51.