Research

Today, 13.8 billion years after the Big Bang, we see that galaxies in the local universe do not look all the same, but they show a diversity in their morphology, i.e., in the way they appear. Hubble was the first to classify galaxies based on their morphology, with spiral galaxies (like our own Milky Way) at one end, and elliptical galaxies at the other end.

A SPIRAL GALAXY is a disk of gas and stars rotating around the center of the galaxy, and formation of new stars is still happening in these galaxies. Because new stars are still forming in spiral galaxies, they tend to look blue, as the young, massive, very luminous, short-lived stars are blue. ELLIPTICAL GALAXIES are very large, massive spheroids of stars orbiting on randomly oriented orbits around the center. Elliptical galaxies are not forming stars anymore, and ceased forming stars a long time ago. The stars left in these galaxies are old and red (since the blue massive stars died a long time ago). Understanding how these different types of galaxies formed, or, more precisely, understanding the physical processes responsible for shaping the galaxies into their present forms, is one of the major unsolved problems in astrophysics and represents the ultimate goal of my research activity. And one of the most controversial questions regarding the formation of galaxies is WHEN and HOW the most massive galaxies that we see today in the universe (which are the elliptical galaxies) have built their stellar content and ended up with their spheroidal morphologies. To answer these questions from an observational perspective, my research has mostly focused on the study of distant galaxies in the early young universe, right at the time when the formation and assembly of these galaxies is most active.

Thanks to a series of very successful cosmology experiments in the past few decades, we now have a precise understanding of the properties of the universe we happen to live in. In the standard model of cosmology, the universe is expanding and it all began 13.8 billion years ago in an event we call the Big Bang. The content of the universe today is dominated by dark energy (~68%) and dark matter (~27%), with only 5% contributed by ordinary matter (i.e., the matter we and stars are made of). Dark Matter (DM) is a type of matter that we cannot see directly (hence dark), but whose existence is inferred from its gravitational effects on the visible matter. Dark energy is instead a form of energy that permeates all of space and tends to accelerate the expansion of the universe. Many aspects of the nature of dark energy remain topics of speculation, but the experiments tell us that dark energy started to be an important component of the universe only when the universe was roughly half of its current age, whereas before that time the content of the universe was dominated by dark matter.

Because the speed of light is finite, it takes time for light to travel from one point to another. For example, when we look at the Sun, the light that we see left the Sun about 8 minutes before, because it takes that long for light to travel from the Sun to Earth. Similarly, the light we receive now from the “nearby” Andromeda galaxy was emitted 2.5 million years ago, as Andromeda is so far away that this is the time it takes light to travel all the way to Earth. We therefore cannot see how Andromeda looks today, in the present, but we only see it as it was 2.5 million years ago. Therefore, the farther we look in distance, the further back we look in time, and we can use this to our advantage. We can actually see into the past by looking at more and more distant objects, and this allows us to study how galaxies formed and evolved. In place of light-years, astrophysicists prefer to use the redshift, indicated by the letter “z”. Because the universe is expanding, the spectra of galaxies are shifted to longer wavelengths, i.e., redshifted. It can be shown that the ratio between the size of the universe today and the size of the universe at a certain time in the past is given by the quantity (1+z). Once the cosmological model is fixed, there is a direct relation between redshift and quantities such as distance, look-back time, age of the universe at a specific redshift, etc. Specifically, z=0 means today, and z=0.75 corresponds to when the universe was half of its current age, i.e., 6.9 billion years old. The larger the redshift, the farther back in time we see.

The earliest time from which we can directly observe light is when the universe was only 380,000 years old, at a redshift z~1100, because at earlier times light was trapped by the free electrons and could not travel freely. At z~1100, the universe cooled enough for the electrons to combine with protons and helium nuclei to form neutral hydrogen and helium. From this time on, light was free to travel and “cool” with the expanding universe, so that today we can observe it redshifted all the way into the microwave part of the electromagnetic spectrum. We call this fossil light received from a very young universe the cosmic microwave background radiation (CMBR). The CMBR is very important because, among other things, it allows us to infer the distribution of matter (specifically dark matter, given its dominance at early times) when the universe was only 380,000 years old. It shows us that matter was not distributed perfectly uniformly, but that some regions had slightly enhanced matter content compared to others. The regions with slightly enhanced matter content represent the seeds from which galaxies formed, given enough time, and evolved into the galaxies we see today in the local universe, at z=0.

Understanding how galaxies form and evolve means understanding how the tiny differences in the distribution of matter inferred from the CMBR grew and evolved into the galaxies we see today. The working hypothesis is that galaxies form under the influence of gravity, and galaxy formation can be seen as a two-step process. First, the gravity of DM causes the tiny seeds in the matter distribution to grow bigger with time. As they grow more massive, the gravitational attraction becomes stronger, making it easier for these structures to attract additional matter. As time proceeds, the DM distribution becomes clumpier and the universe grows lumpier, with DM structures forming and growing. As the DM structures grow, they pull in also the gas, made of hydrogen and helium, which is the primary ingredient for the formation of stars, and hence for the formation of the stellar content of galaxies. Whereas the DM halos’ formation and evolution with time are relatively easy to simulate (since it is regulated only by the force of gravity, which is well understood), the formation of the stellar content inside these DM structures involves many physical processes that are much more complicated and quite poorly understood from a theoretical perspective. These physical processes include how gas cools and collapses to form stars, the process of star formation itself, merging of galaxies, feedback from star formation (i.e., the intense radiation from massive stars and supernova explosions heating or ejecting the gas, preventing it from forming further stars), and from active galactic nuclei (AGNs; accreting super-massive black holes whose energy release can affect the evolution of the galaxy itself). The stellar mass of a present-day massive galaxy was assembled by a combination of star formation and mergers. But any successful theory of galaxy formation and evolution has to predict the properties of the galaxies we observe, both at z=0 and as a function of cosmic time, galaxy type, and environment.

My research activity in the past decade has focused on understanding how galaxies formed after the Big Bang, and how their properties (e.g., the stellar mass, the level of star formation activity, the morphology and structural parameters, the level of activity of the hosted super-massive black hole, etc.) have changed as a function of cosmic time. Since we cannot follow the same galaxy evolving in time, we need to connect the galaxies we observe at a certain redshift (i.e. a certain snapshot in time) to those we observe at a smaller redshift (i.e., at a later time in cosmic history) in order to infer how the properties of galaxies have actually changed and what physical mechanisms are responsible for these changes. The better we understand the galaxy properties at a certain time and the more finely in time we can probe the cosmic history, the easier it becomes to connect galaxies’ populations seen at different snapshots in time, linking progenitors and descendants across cosmic time. Ultimately, my research aims at understanding what galaxy population seen at one epoch will evolve into at a later epoch, and what are the physical processes responsible for the inferred changes in the galaxies’ properties. In order to do this, I have adopted two different but complementary approaches. The first approach consists of statistical studies of the galaxy populations at different cosmic times; the second approach consists of detailed studies of individual galaxies to robustly derive their properties.

Below you can find a list of papers I led or co-led with my research group and collaborators. If you want to know more about the individual works, please click on the title of the paper. The published articles can be downloaded by clicking on the authors’ names: