SHARP -- A spectrograph proposal to fully exploit ELT capabilities and look beyond JWST

The story of how the universe came to be is written in the light of distant galaxies, but for decades, that story has been difficult to read clearly. Astronomers have long relied on powerful space telescopes to peer back in time, capturing the faint glow of stars that formed billions of years ago. However, even the most advanced space observatories face a fundamental limit: the atmosphere of our own planet blurs the view of ground-based telescopes, while the size of space telescopes limits how much detail they can resolve. To understand the true nature of the cosmos, scientists need to see not just that a galaxy exists, but how its internal parts move, how its stars are born, and how its chemical makeup changes from its center to its edges. This requires a level of sharpness and sensitivity that has remained just out of reach, leaving gaps in our understanding of how the first stars ignited and how massive galaxies grew so quickly in the early universe.

A new proposal called SHARP aims to bridge this gap by designing a specialized instrument for the upcoming Extremely Large Telescope, a ground-based observatory currently under construction in Chile with a mirror nearly forty meters wide. The paper outlines a concept for a near-infrared spectrograph, a device that splits light into its component colors to reveal the chemical fingerprints and motions of celestial objects. Unlike previous instruments that could only look at one object at a time or lacked the resolution to see fine details, SHARP is designed to work in tandem with a sophisticated adaptive optics system. This system acts like a dynamic lens that corrects for the twinkling of the atmosphere in real time, allowing the telescope to achieve a clarity that rivals or exceeds that of the James Webb Space Telescope across a much wider area of the sky. The proposal details how this instrument would be built to simultaneously observe dozens of galaxies or map the internal structure of a single one with unprecedented precision.

The core of the SHARP concept is a dual-mode design, meaning it can switch between two different ways of observing depending on the scientific question at hand. The first mode, called NEXUS, is a multi-object spectrograph designed to catch the faintest signals from the distant universe. It uses a configurable slit system, which can be thought of as a set of adjustable windows that can be opened to let light from specific targets enter the instrument while blocking the rest. This system allows astronomers to place up to thirty separate slits across the sky, each aligned perfectly with a different galaxy. A unique feature of this design is the ability to rotate the orientation of each slit independently, ensuring that the light from a galaxy is captured along its most informative axis, whether that is its long edge or its center. This capability allows researchers to measure the rotation and mass of many galaxies at once, providing a statistical view of how these systems evolve over cosmic time.

The second mode, known as VESPER, is designed for studying brighter, closer objects with extreme detail. Instead of using slits, this unit employs an image slicer, a device that takes a small patch of the sky and cuts it into thin strips, rearranging them into a long line that can be analyzed by a camera. This allows the instrument to create a detailed map of a single galaxy, breaking it down into hundreds of tiny pixels to measure how the age of stars and the abundance of heavy elements change from the core to the outskirts. By combining these two modes, the instrument can tackle the most pressing questions in astrophysics, from the formation of the first stars to the mysterious processes that shut down star formation in massive galaxies.

One of the primary goals of SHARP is to solve the mystery of how massive galaxies formed so early in the universe's history. Observations have revealed galaxies that are surprisingly old and massive when the universe was less than a billion years old, challenging current theories that suggest such structures should take much longer to assemble. To understand this, astronomers need to resolve the physical conditions inside these galaxies, specifically the giant clouds of gas where stars are born. These clouds are typically about two hundred parsecs across, a size that requires an angular resolution of roughly thirty-five thousandths of an arcsecond to see clearly from Earth. The paper demonstrates that SHARP, supported by the adaptive optics system, can achieve this resolution, allowing scientists to study the building blocks of galaxies rather than just their blurry outlines. This level of detail is essential for determining whether these ancient galaxies formed their stars in a single, rapid burst or through the gradual merging of smaller systems.

Another critical objective is the search for Population III stars, the theoretical first generation of stars that formed from pure hydrogen and helium, containing no heavier elements. These stars are believed to have been extremely massive and short-lived, making them incredibly difficult to detect. The paper calculates that finding the specific signature of these stars would require detecting a faint emission line from helium in the near-infrared spectrum. While current instruments struggle to reach the necessary sensitivity for such a faint signal, the proposed design of SHARP suggests that with a ten-hour exposure, it could reach the limit required to either detect these primordial stars or place strict upper limits on their existence. This would provide a definitive test for models of the early universe and the chemical enrichment of the cosmos.

The proposal also addresses the need to study the environment around young stars and the formation of planetary systems in dusty regions of our own galaxy. In these crowded areas, the ability to separate individual stars is crucial, as is the capacity to observe multiple targets simultaneously. The paper highlights that existing instruments often lack the combination of high resolution and the ability to observe many objects at once, forcing astronomers to choose between seeing a single object clearly or surveying a wide area. SHARP is designed to overcome this trade-off, offering the versatility to observe clusters of young stars or the centers of globular clusters with the same clarity that space telescopes provide, but with the added benefit of a much larger field of view.

To ensure the instrument can deliver on these promises, the authors have developed a detailed optical design that avoids complex curved mirrors, relying instead on a series of flat mirrors and prisms to guide the light. This approach simplifies the construction and maintenance of the instrument while maintaining high performance. The design includes a cryogenic system to keep the detectors cold, reducing thermal noise that could obscure faint signals. The paper provides a comprehensive overview of how the light travels from the telescope, through the adaptive optics correction, and into the two distinct observation modes. It also includes a specific example of how SHARP would observe a quiescent galaxy at a redshift of 2.66, a target that is currently difficult to study in detail. The simulation shows how the instrument could map the gas inflow and stellar populations of this galaxy, revealing why it has stopped forming stars despite having a large reservoir of gas.

The authors emphasize that while the James Webb Space Telescope has revolutionized our view of the early universe, it has limitations in angular resolution and field of view that prevent it from resolving the internal structures of distant galaxies. SHARP is not intended to replace space-based observatories but to complement them by providing the ground-based resolution needed to see the fine details that space telescopes cannot. The paper concludes that the proposed instrument is technically feasible and scientifically essential for the next generation of astronomical discovery. By fully exploiting the capabilities of the Extremely Large Telescope, SHARP would allow astronomers to move beyond simply counting galaxies to understanding the physical processes that drive their evolution, offering a clearer picture of how the universe assembled itself from the first moments of time to the present day.