We describe the design and performance of the Medium Resolution Spectrometer (MRS) for the JWST-MIRI instrument. The MRS incorporates four coaxial spectral channels in a compact opto-mechanical layout that generates spectral images over fields of view up to 7.7 x 7.7 arcseconds in extent and at spectral resolving powers ranging from 1,300 to 3,700. Each channel includes an all-reflective integral field unit (IFU): an 'image slicer' that reformats the input field for presentation to a grating spectrometer. Two 1024 x 1024 focal plane arrays record the output spectral images with an instantaneous spectral coverage of approximately one third of the full wavelength range of each channel. The full 5 to 28.5 µm spectrum is then obtained by making three exposures using gratings and pass-band-determining filters that are selected using just two three-position mechanisms. The expected on-orbit optical performance is presented, based on testing of the MIRI Flight Model and including spectral and spatial coverage and resolution. The point spread function of the reconstructed images is shown to be diffraction limited and the optical transmission is shown to be consistent with the design expectations.

Here we present the analysis of 3D spectroscopic data of three Blue Compact Galaxies (Mrk 324, Mrk 370, and III Zw 102). Each of the more than 22500 spectra obtained for each galaxy has been fitted by a single gaussian from which we have inferred the velocity dispersion (σ), the peak intensity (I peak ), and the central wavelength (λ c ). The analysis shows that the σ vs I peak diagrams look remarkably similar to those obtained for giant extragalactic H II regions. They all present a supersonic narrow horizontal band that extends across all the range of intensities and that result from the massive nuclear star-forming regions of every galaxy. The σ vs I peak diagrams present also several inclined bands of lower intensity and an even larger σ, arising from the large galactic volumes that surround the main central emitting knots. Here we also show that the σ vs λ c and λ c vs I peak diagrams, are powerful tools able to unveil the presence of high and low mass stellar clusters, and thus allow for the possibility of inferring the star formation activity of distant galaxies, even if these are not spatially resolved.

Disky bulges in spiral galaxies are commonly thought to form out of disk materials (mainly) via bar driven secular processes, they are structurally and dynamically distinct from 'classical bulges' built in violent merger events. We use high-resolution GTC/MEGARA integral-field unit spectroscopic observations of the Sa galaxy NGC 7025, obtained during the MEGARA commissioning run, together with detailed 1D and 2D decompositions of this galaxy's SDSS i-band data to investigate the formation of its disky (bulge) component which makes up ∼ 30% of the total galaxy light. With a Sérsic index n ∼ 1.80 ± 0.24, half-light radius R e ∼ 1.70 ± 0.43 kpc and stellar mass M * ∼ (4.34 ± 1.70) × 10 10 M ⊙ , this bulge dominates the galaxy light distribution in the inner R ∼ 15 ′′ (∼ 4.7 kpc). Measuring the spins (λ) and ellipticities (ǫ) enclosed within nine different circular apertures with radii R ≤ R e , we show that the bulge, which exhibits a spin track of an outwardly rising λ and ǫ, is a fast rotator for all the apertures considered. Our findings suggest that this inner disky component is a pseudo-bulge, consistent with the stellar and dust spiral patterns seen in the galaxy down to the innermost regions but in contrast to the classical bulge interpretation favored in the past. We propose that a secular process involving the tightly wound stellar spiral arms of NGC 7025 may drive gas and stars out of the disk into the inner regions of the galaxy, building up the massive pseudo-bulge.

We present our analysis of high-resolution (R ∼ 20 000) GTC/MEGARA integral-field unit spectroscopic observations, obtained during the commissioning run, in the inner region (12.5 arcsec × 11.3 arcsec) of the active galaxy NGC 7469, at spatial scales of 0.62 arcsec. We explore the kinematics, dynamics, ionisation mechanisms and oxygen abundances of the ionised gas, by modelling the Hα-[N II] emission lines at high signal-to-noise (> 15) with multiple Gaussian components. MEGARA observations reveal, for the first time for NGC 7469, the presence of a very thin (20 pc) ionised gas disc supported by rotation (V/σ = 4.3), embedded in a thicker (222 pc), dynamically hotter (V/σ = 1.3) one. These discs nearly co-rotate with similar peak-to-peak velocities (163 vs. 137 km s −1 ), but with different average velocity dispersion (38 ± 1 vs. 108 ± 4 km s −1 ). The kinematics of both discs could be possibly perturbed by star-forming regions. We interpret the morphology and the kinematics of a third (broader) component (σ > 250 km s −1 ) as suggestive of the presence of non-rotational turbulent motions possibly associated either to an outflow or to the lense. For the narrow component, the [N II]/Hα ratios point to the star-formation as the dominant mechanism of ionisation, being consistent with ionisation from shocks in the case of the intermediate component. All components have roughly solar metallicity. In the nuclear region of NGC 7469, at r ≤ 1.85 arcsec, a very broad (FWHM = 2590 km s −1 ) Hα component is contributing (41 per cent) to the global Hα-[N II] profile, being originated in the (unresolved) broad line region of the Seyfert 1.5 nucleus of NGC 7469.

MEGARA is the multi-object medium-resolution spectrograph for the GTC 10m telescope. MEGARA offers two observing modes, the LCB mode, a large central IFU; and a MOS mode composed by 92 robotic positioners carrying 7 fibers minibundles. Microlens are required to fit the GTC f/17 to the f/3 at the fiber entrance, where pupil image is oversized to have a fiber-to-fiber flux variation better than 10%. This tight requirement imposed manufacturing tolerances for the different components and required the development of a gluing station to provide a centering precision better than 5μm. We present the overview of the optical bundles, the gluing station and the final performance obtained during the integration and tests.