SHARP's core requirements are primarily motivated by the desire to understand and reconstruct how baryonic matter assembled at early times to form the first stars, galaxies and structures, and how these evolved over cosmic time. The complex physics underlying these phases requires us to homogeneously trace the properties of galaxies from the earliest epochs to the present. These properties, such as the kinematics and chemical abundance of the gas and stellar component, the star formation rate and the age of the stellar population, can be derived by mapping the atomic emission and absorption lines within galaxies over the entire cosmic time. This constrains a well-defined combination of wavelength range, angular and spectral resolution:

• wavelength range extending to 2.45 μm, the near-IR limit still efficiently reachable from the ground;

• angular resolution suited to spatially resolve spectral properties over angular scales <40 mas;

• spectral resolutions in the range R~300-6000 for extended sources, and R>15000 for point sources;

• multiple spectra at a time;

The justification of the main requirements as emerged from the Science Cases is summarized below.

Why λlim~2..45 μm

• Most diagnostic spectral features successfully applied to low redshift galaxies fall at optical rest-frame wavelengths (0.35-0.65 μm). Similar ground-based measurements in galaxies at earlier epochs, e.g. when the Universe was 20% of the present age or younger (z>2.5) and the cosmic star formation rate density was almost ten times higher than now, require deep observations extending up to the limit where sky transmission is still high and sky background can be still efficiently removed, i.e. to  λlim~2.45 μm. Figure 1 shows an example of the required wavelength coverage needed to study high-redshift galaxies.

• The probability to detect the first stars in the Universe, namely PopIII stars whose marker is supposed to be HeII[1640] line, depends also on the redshift at which HeII can be detected. Given the extremely short lifetime expected for PopIII stars, the higher the redshift the higher the probability to detect them. Maximizing this probability involves detecting HII[1640] at z>10, in the first 450 million lifetime of the Universe, which requires observations above λ=1.8 μm.

Fig. 2 shows the observed wavelength of some of the main spectral features as a function of the redshift of the observed galaxy.  The blue line marks λ=1.8 μm, the wavelength limit of the new generation ESO's spectrographs MOONS, MOSAIC and ANDES.

Why Angular Resolution (AO correction)

• Giant molecular clouds are the largest fuel reservoir (up to millions of solar masses of molecular gas) for star formation. They are the place where massive star formation occurs. Their size can reach  ~150-200 pc. Therefore, to map star formation, bulk motions and the presence of multiple stellar populations within galaxies over most of the cosmic time, an angular resolution suited to resolve spatial scales comparable to the size of the giant molecular clouds is needed. An angular resolution of about 30-35 mas ensures this sampling over the entire cosmic time (see Fig. 3).

• Dark matter (DM) is believed a basic component of the Universe. It drives the formation and the mass assembly of galaxies in models of galaxy formation. Its measure is not trivial. A way to estimate the DM fraction in galaxies is to compare the total galaxy mass derived from kinematics tracers sensitive to the potential well (i.e. to the dark + baryonic mass) with the stellar mass. An appropriate sampling of the emission line wavelength along the major axis of a galaxy is necessary to determine the rotation component. This translates into an angular resolution ~30 mas for the very high redshift galaxies seen by JWST, whose size is much less than 1 kpc.

Why Multiplexing capabilities (and Multi-conjugate AO)

• All the above measurements should be possible for several galaxies at once, allowing us to efficiently study clusters, protoclusters and overdensities of galaxies. This maximizes the efficiency of the telescope. Therefore, multiplexing capabilities coupled with a large area uniformly corrected for atmospheric turbulence are needed.