Overview

Exploring the complex mechanisms by which baryonic matter assembled in the earliest epochs to form the first galaxies, and those governing their evolution, relies on measuring and tracing some fundamental properties of galaxies over time. For an accurate reconstruction of the evolutionary path, these properties must be gathered for galaxies residing in diverse environments from the earliest epochs to the present. As we go back to ever more remote epochs, time becomes a constraint capable of discerning physical processes characterized by different timescales. Depending on the redshift of the source, the wavelength coverage provided by SHARP will probe the near-IR (z<1), optical (z~2-4), U-band (z~5-6) and UV (z~8-12) rest-frame lines (see Fig. 2), enabling a variety of different studies related to the aforementioned wavelength regimes. Emission and absorption lines will be probed by MORFEO-SHARP with (at least) twice the spectral resolution and sharper spatial scale than offered by JWST. 

Gravitational Lensing: the Early Universe Under the Microscope

Where did star formation take place in early galaxies? 

What are the extreme physical conditions that govern star formation in the early Universe ?

The field of view and the multiplex capability of the multi-IFU VESPER will provide opticl and UV spectral maps at 31 mas/pix of dozens of galaxies simultaneously with just 2-3 SHARP pointings. This capability makes SHARP the optimal spectrograph for follow-up studies of MORFEO-MICADO imaging and, in particular, for targeting lensed (multiple) images of galaxies at redshift between 2 and 13 located in the field of lensing clusters.

Figure 1 shows a few examples extracted from the Hubble Frontier Field galaxy cluster A2744 recently observed with JWST, in which an overdensity of galaxies has been secured at z=7.88, along with the confirmation of early galaxies at z~9.3 and 9.7 and several others at z>3-6, and proto-globular clusters at z=4. All of them show clumpy star formation addressable with integral field spectroscopy. With MORFEO-SHARP, the observed angular scale of a galaxy with magnification factor (μ) will be  √μ smaller, such that with μ=2(4) each spaxel probes 31mas/√μ = 22(15)mas. Even in a regime of modest amplification (μ<5), resolved studies on scales of <100 pc/pix for sources at z>2 will be possible, and of a few tens of parsec in case of larger magnifications.

This, in conjunction with MICADO imaging, will open to unprecedented and systematic study of star-forming modes at the lowest spatial scales at any cosmic epoch, from the star forming complexes to single star clusters and/or proto-glubulars, which are expected to be an important (if not dominant) site of star formation in the high-redshift Universe (see Fig. 1).

Star Formation in High-redshift and Primordial Galaxies

What are the conditions that drive the formation of massive galaxies ? 

What is their origin ?

According to our model (LCDM) of galaxy formation, there simply wasn’t enough time since the Big Bang to have formed galaxies so massive as those recently observed in the early Universe.  Deriving the Star Formation Rate (SFR) of high-redshift (z>3-4) and primordial galaxies holds utmost significance as it enables us to understand the physically unexplained extremely high star formation required to explain their presence. While integrated measurements can be conducted by JWST, spatially resolved spectroscopy required to constrain the physics behind such extreme processes exceeds the capabilities of JWST. MORFEO-SHARP would provide this information by sampling optical and UV continuum and emission lines on scales <150 pc (31 mas/pix) at z~5 (optical) and up to z~13-14 (UV), the scales of giant molecular clouds where massive star formation is expected to occur. 

What sources are responsible for re-ionization ?

Additionally, the knowledge of the SFR in the very early Universe is essential to determine the nature of the sources responsible for the reionization process, which remains elusive.  

Will we reveal the elusive primordial population of PopIII stars ?

It is worth mentioning the search for the first stars (PopIII) expected to be hosted in isolated pockets of pristine gas at very low-luminosity regimes. SHARP will probe the key transition HeII1640 and possible absence of metal lines in the ultraviolet up to z~14, just 300 million years after the Big Bang.

The Dark Side of Galaxies: Testing Galaxy Formation Models

How do galaxies grow ? 

What is their dark matter content ?

In the current paradigm of galaxy formation, dark matter (DM) drives the assembly of galaxies and the formation of the first stars. Pristine gas is drawn towards the center of DM haloes' potential wells, triggering collapse and sparking the ignition of stars generating, first, low-mass galaxies. Subsequent accretion of smaller satellite galaxies over cosmic time is believed to give rise to higher-mass and larger galaxies. This process is expected to increase the DM fraction and modify its distribution within galaxies. However, recent observations have shown the presence of massive quiescent galaxies at redshift z>3 (up to z~5). This suggest that also in situ star formation might be an efficient mechanism of mass accretion at high redshift. Therefore, quantifying the DM content, its spatial distribution, and its evolution over time is a fundamental test for the current model of galaxy formation and for the different scenarios of mass accretion.

Leveraging the extensive SHARP wavelength coverage up to 2.45μm, both VESPER the multi-IFU, and NEXUS the MOS offer a unique opportunity to derive velocity rotational curves for disk galaxies up to z~13 from UV emission lines of ionized gas. MORFEO-SHARP would provide a spatial sampling at ~100-150 pc at this redshift, with VESPER able to provide the gas velocity field inside the galaxy at the same spatial scale. Velocity curves for bulge-dominated galaxies can be measured from stellar absorption lines (e.g., H&K Ca lines) up to z~5. This would enable us to directly test the hypothesis of mass accretion through minor merging, where satellite accretion should deposit DM in the periphery of the galaxy. 

Stellar population gradients in high redshift galaxies

How do galaxies assemble their stellar mass ?

Which are the quenching mechanisms that actually occur ?

What is the time scale of quenching ? 

The properties of galaxies result from the interplay of some mechanisms that we suppose can occur: dissipative collapse of gas and stellar feedback; mergers, either of similar-mass progenitors (major) or minor mergers, i.e. accretion of satellites (as discussed above); feedback from the AGN; morphological quenching that can follow the formation of a massive and dense stellar spheroid and, perhaps, others unknown to us. When, where, and how these mechanisms take place determines spatial and temporal variations in the properties (e.g., metallicity and age) and dynamics of the stars that are formed and/or accreted. Stellar population gradients and velocity profiles are therefore strictly related to the different mechanisms through which a galaxy assembled its stellar mass and halt its star formation (e.g., inside-out; sharply or softly).  This kind of investigation requires spatially resolved spectroscopy at moderate resolution, in order to track the spatial variation of age- (e.g. D4000, Hdelta) and metallicity-sensitive (e.g., Mgb, iron lines) absorption features across the galaxies.  The advent of Integral field spectrographs allowed us to obtain these information for local galaxies, whose properties are averaged over ~13 Gyr of evolution. To identify the processes occurring in the formation, such observations must be made for galaxies at increasingly high redshift. The multi-IFU VESPER of SHARP would provide this information by sampling optical continuum and absorption features on scales <150 pc (31 mas/pix) up to z~5, when the Universe was ~1 Gyr old.  VESPER can efficiently carry out these observations for many galaxies at a time, exploring efficiently protoclusters and overdensity environments at very high redshit.

The appearance of the main scaling relations

What regulates metal enrichment in galaxies ?

What regulates the star formation in galaxies ?

What is the physical interplay between black holes and galaxies ?

Scaling relations (SRs) encompass a series of empirical correlations between various physical properties of galaxies, including mass, stellar age, stellar and gas metallicity, velocity dispersion, black-hole masses,  radii, and mean surface brightness, among others. These relationships are the result of the physical mechanisms that intervened in the formation and evolution of galaxies. Therefore, they represent a fundamental benchmark for any theoretical model of galaxy formation and evolution. Observations of galaxies at increasingly higher redshifts trace the evolution (if any) of these scaling relationships providing, on the one hand, constraints on the evolutionary processes and, on the other, on the physical conditions that determined them.

The main scaling relationships involving the stellar population and dynamical properties, such as stellar mass/velocity dispersion vs metallicity, the Fundamental Plane, etc., are already defined up to z~1.5 and will soon be extensively studied up to z~2.5 thanks to the incoming multi-object spectrographs (e.g. MOONS, MOSAIC) limited to λ = 1.8 μm. However, to study these scaling relations in the first 2.5 Gyr of cosmic time, when most of the stellar mass is forming and the SFR is growing, observations at wavelengths >1.8μm are necessary. The characteristics of SHARP-NEXUS coupled with the MCAO correction by MORFEO would allow us to probe this cosmic epoch defining the nature of the scaling relations. 

Fig. 5 - Some of the main scaling relations. From top-left: the Fundamental Plane (FP) of cluster elliptical galaxies that links a galaxy size (Re) to the surface brightness and the stellar velocity dispersion (credit: Saracco et al. 2020); the Black Hole mass vs stellar velocity dispersion of galaxies (credit: the American Astronomical Society 2013); the stellar metallicity vs mass relation (credit: John F Wu 2020); the Star Formation main sequence that links the star formation rate to the stellar mass of the galaxy (Rodighiero et al. 2011).