INvestigating Stellar Populations In RElics (INSPIRE)

Investigate Stellar Populations In Relics (INSPIRE) is a revolutionary way to understand the early assembly of the most massive galaxies in the Universe, giant ellipticals. This is very important since this family of objects contains more than half of the stars and the majority of the chemical elements in our Universe.

Massive ellipticals are believed to form through a two-phase process. At the early times of the Universe's life, an intense, short-lived star formation episode creates compact and massive objects ("red nuggets"). Then, during a second, more time-extended phase, red nuggets merge with other galaxies or gas falls into them and forms new stars.

This causes a dramatic growth in size and transforms red nuggets into the massive, giant elliptical galaxies we observe today. Unfortunately, in local ellipticals this secondary "accreted" material contaminates the "pristine" component that encodes the information about the first phases of the mass assembly. This irremediably prevents us from investigating on the very early stages of structure formation in the Universe.

Luckily, some red nuggets do not coalesce with any other structure in their lifetime and so continue on their isolated path over cosmic time without increasing in size or, importantly, without changing their stellar content. These lonely red nuggets are up to 6 times smaller in size and 100 times denser than local ellipticals. They are frozen in time and are therefore called "relics" of the ancient Universe; studying them is akin to an archaeologist learning from dinosaur fossils.

Relics contain the oldest stars and hence have the potential to unlock our understanding of the very early formation of structures within our Universe.

But how many relics exist at each epoch? How could they passively evolve through cosmic time without experiencing any interaction with other systems? And in which kind of environment are they preferentially found?

These questions are at the core of my successful research proposal, which uses the forefront facilities of the European Southern Observatory.

From high-quality spectra already available on the largest sample of ultra-compact massive galaxies, I will measure their ages to confirm them as relics as old as the Universe. I will fully characterise the stars in these systems, to reveal how relics originally formed and how they have changed since, if at all.

Since relics are the "seeds" of local giant ellipticals, my investigation will revolutionise our understanding of the processes driving the initial assembly of the most massive galaxies in the present-day Universe.

Matching the INSPIRE data to data from publicly available wide field sky surveys, I will be able study in detail the environment in which relics are preferentially found. This is crucial to understanding how the densest and most compact objects in the Universe form and how they could evolve into local ellipticals.

I will also compare my observational results with computer simulations of the entire Universe. The number of relics at each epoch and their characteristics predicted by simulations depend on the ingredients that they use to reproduce the size growth of galaxies (e.g. the number of interactions with other objects that a given galaxy undergoes).

Finally, thanks to new imaging and spectroscopy data with the highest spatial resolution reachable from the ground, which will be delivered next year, I will be able to resolve the internal structure of a sub-sample of relics. I will trace the spatial distribution of the stars within the relics and whether they rotate in a disk-like structure, as predicted by some theoretical models.

I will build a dynamical model of relics' light profiles, in order to indirectly constrain the characteristics of the dark matter halos in which the densest objects in the Universe are embedded.