Research Focus

Our current understanding of the Universe depicts a gradual build-up of large dark matter halos from the merging of smaller entities. It is inside these halos that baryons assemble into stars and lead to the formation of the first galaxies. Once a galaxy forms there are a number of processes that help shape its structure. These processes can be broadly grouped into external and internal processes, and also in short and long timescales mechanisms. These processes include in-situ star formation that drives secular structural evolution through instabilities in the gaseous disk and ex-situ star formation that refers to stellar components that were formed elsewhere and later aggregated by the galaxy through galaxy harassment and merger events. During a galaxy lifetime, we expect that gas in the intergalactic medium will infall onto the disk due to their gravitational potential and eventually form new stars. If gas was able to cool efficiently, then all gas would be converted into stars. However, we observe that only $\sim10\%$ of the baryonic matter is in the form of stars. Thus, one or more mechanisms that prevents gas from cooling and renders star formation inefficient is necessary to explain observations. It is now recognized that stellar and active galactic nuclei feedback, in the form of winds or supernovae explosions or an active central super massive black hole, is likely the explanation for the suppression of star formation in galaxies.

Galaxy formation, and early stage evolution, is believed to be a turbulent process where gas inflows, strong winds and galaxy-galaxy interactions give rise to the intricate shapes we encounter in HST photometric observations of high redshift galaxies. The shape of galaxies is a simple, yet fundamental, property of galaxies. Despite not being a property directly correlated with physics, as do stellar masses, star-formation rates or metallicities, it is obviously a consequence of all physical processes at play during galaxy evolution. As such, it provides with clues and insights onto the dominant processes which took part in shaping galaxies. For these reasons, I have developed a unique expertise in quantifying the morphological properties of distant galaxies, using the best imaging and spectrophotometric observations to identify the key physical processes driving galaxy evolution. It is my goal to continue pushing the frontier of morphological studies to get further insight in the early phases of galaxy assembly.

Morphology of galaxies in their assembly phase

I have mainly done my research on the topic of galaxy morphology when I enrolled in a PhD fellowship to study galaxy morphology in the high redshift Universe. This project is included in a large international collaboration centered on the largest high redshift (at $2<z\lesssim6$) spectroscopic survey to date: the VIMOS Ultra Deep Survey (VUDS). In order to understand how to best tackle the issue of quantifying galaxy morphology at these high redshifts, where galaxies are tendentiously more and more irregular, I was given some liberty to explore several aspects linked to this particular area of research.

A new take on galaxy sizes

One first aspect was to devise a new way of measuring galaxy sizes by accounting for the fact that a majority of sources are irregular in nature at such epochs. At first, I used a classical parametric profile fitting method using GALFIT to derive galaxy sizes. I then measure the total pixel area covered by a galaxy above a given surface brightness threshold, which overcomes the difficulty of measuring sizes of galaxies with irregular shapes. I find that the sizes of galaxies (in a stellar mass selected sample) computed with the non-parametric approach span a large range but remain roughly constant on average with a median value $r_T^{100}\sim2.2$ kpc for galaxies with $2<z<4.5$. This is in stark contrast with the strong downward evolution of $r_e$ with increasing redshift, down to sizes of $<1$ kpc at $z\sim4.5$ (see figure on the side). I analyze the difference and find that parametric fitting of complex, asymmetric, multi-component galaxies is underestimating their sizes by virtue of locking the model to the brightest component. This new non-parametric measurement also allows for the definition of sizes at other flux levels (such as the half light radius or the 80\% light radius as the number of pixels that sum up to X\% of the total galaxy flux define to be above the isophote threshold). This extended size definition allows for an alternative computation of the concentration parameter as defined by e.g. Conselice (2003). With this new approach I find that galaxies present more concentrated light profiles as we move towards higher redshifts.

A characterization of high-redshift galactic clumps

By extending my non-parametric algorithm I was able to run a clump detection method that looks for clumps of all shapes within a surface brightness threshold. After applying this algorithm to the same sample used above I find that the population of galaxies with more than one clump is dominated by galaxies with two clumps, representing $\sim21-25$\% of the sample. The fraction of clumpy galaxies is in the range $\sim35-55\%$ over $2<z<6$ (see figure on the side, next page), increasing at higher redshifts, indicating that the fraction of irregular galaxies remains high up to the highest redshifts. The large and bright clumps (M${star}\sim10^9$ up to $\sim10^{10}$M$\odot$) are found to reside predominantly in galaxies with two clumps.

Smaller and lower luminosity clumps are found in galaxies with three clumps or more. I interpret these results as evidence for two different modes of clump formation working in parallel. The small low luminosity clumps are likely the result of disc fragmentation, with violent disc instabilities (VDI) forming several long-lived clumps {\it in-situ}, as suggested from simulations. A fraction of these clumps is also likely coming from minor mergers. The clumps in the dominating population of galaxies with two clumps are significantly more massive and have properties akin to those in merging pairs observed at similar redshifts; they appear as more massive than the most massive clumps observed in VDI numerical simulations. I infer from these properties that the bright and large clumps are most likely the result of major mergers bringing-in {\it ex-situ} matter. The diversity of clump properties suggests that the assembly of star-forming galaxies at z$\sim 2-6$ proceeds from several different dissipative processes including an important contribution from major and minor mergers.

A comprehensive morphological characterization of the star-forming population

In terms of the visual classification of high-$z$ star -forming galaxies I have defined 4 different main classes of star-forming galaxies based on the spatial distribution of the rest-frame UV light and find that 40-50\% of all star-forming galaxies in the VUDS sample with $\log_{10}(M_\star/M_\odot)>9.5$ are classified as irregular systems. I explore the physical properties of each sub-sample linked to a single morphological class and find that they all span wide ranges in stellar mass, star formation rates, ages and dust extinction values. I argue that there are several physical mechanisms at play that provide a variety of formation scenarios (or formation stages) for the same morphological class and that no single morphological population is univocally linked to a single formation scenario.

On a last topic I have shown how different observed spatial line emission profiles of Ly-$\alpha$ correlate with the physical properties of the host galaxy. I have defined 4 different main classes of star-forming galaxies based on the spatial properties of the Ly-$\alpha$ emission as compared to the UV continuum emission (see figure on the side). I find a total of 1022 galaxies with line emission of which 51\%, 21\% and 10\% have coincident, extended and offset line emission, respectively. I find that extended line emitters show the strongest equivalent widths. In terms of physical properties, extended line emitters distinguish themselves from the other classes by being the least massive, least star-forming, lower dust content and smaller UV continuum sizes. I conclude that this class of Ly-$\alpha$ emission is likely the population analog to LAEs found in narrow-band imaging surveys or deep integral field spectroscopy observations.

The overall result that I obtained during the course of my past research is that the large diversity in morphological properties I observe in star-forming galaxies at at $2<z<6$ are indicative of the variety of physical mechanisms that drive evolution in the early phase of galaxy assembly. The large range in galaxy sizes, from a few hundred to ten thousand parsecs, the high number of clumpy systems, with a significant population of systems with two bright clumps, and the non distinctive physical properties associated with several morphological classes all point to a complex picture of galaxy formation which goes beyond a simple scenario of cold gas accretion driving the evolution of all galaxies.

Undergraduate research

Rare populations in the local Universe}

In the local Universe, our understanding of galaxy morphology is vast, and we observe that there are several correlations between galaxy structure and color, star-formation history, black hole masses, merger activities and environment. However, there are still puzzling populations of galaxies that defy our models of galaxy formation and evolution. One of those cases is the existence of massive and red disk galaxies that potentially host a central black hole despite having no evidence of possessing any bulge. This class of objects was the starting point for the project I was involved in my Master’s project. It started with the quantification of bulge light in a sample of 7 bulgeless AGN candidates. To do so, I have used GALFIT to produce 2D synthetic models of galaxies with an exponential disk and an additional bulge with a free-Sérsic index profile. I have shown that 5 of the 7 candidates had light profiles consistent with a single exponential disk.

This class of massive and red galaxies with little or no bulge are of particular interest in terms of understanding their existence and to pinpoint where they evolved from. To further my investigation of this population, regardless of their AGN activity, I have studied a sample of 67 of these objects, selected from the New York Value Added Galaxy catalog (Blanton et al. 2005) along with a control sample of $39$ galaxies with the same masses and colors but with prominent bulges. I have applied the same method of bulge-to-disk decomposition to all 106 galaxies. The structural properties of these galaxies have then be correlated with their physical properties derived from spectra synthesis of stellar populations performed by a collaborator using starlight. I have shown that the use of 2D models (instead of fitting directly 1D surface brightness profiles) yields better results at separating galaxies with pseudo-bulges and pure disks when using single Sérsic models. This analysis shows that pure disk galaxies and pseudo-bulge galaxies occupy the same locus in terms of metallicity, ages and stellar mass in opposition to their counterparts. There is also an observed trend separating these three classes in terms of their position with respect to the Faber-Jackson relation. As expected, the less prominent the bulge the more deviated from the canonical relation they are. An analysis of diagnostic emission-line ratios through the classical BPT diagram reveals a systematic trend along the bulgeless $\rightarrow$ pseudo-bulge $\rightarrow$ bulgy sequence, further highlighting the diverse nature of these entities.

Moving towards intermediate redshifts

Alongside my master project, I started working on the analysis of the two-point correlation function of a sample of 31 915 galaxies in the redshift range $0.4<z<1.0$ based on the compiled catalog described in Bizzocchi et al. (2014) selected in four of the deepest, largest multi-wavelength data sets available (COSMOS, AEGIS, GEMS, and GOODS). I have setup the algorithm based on the Landy \& Szalay (1993) estimator for parallel computing the correlation function of this sample. Such computation was done for the entire sample and by dividing it into subsamples based on redshift, color, mass, luminosity, star-formation activity, and morphology. Preliminary results show that the correlation function is not affected by the redshift considered. The brightest galaxies are more correlated that their faint counterparts. The same happens for the most massive and redder galaxies when compared to lower mass/ bluer galaxies respectively. Quiescent galaxies are more correlated than star-forming galaxies, and bulgeless systems are less correlated than galaxies with prominent bulges. Due to lack of funding, this project was never carried out to its full extent, but there is a paper draft which is available upon request.