Recent Talks

Molecular hydrogen (H2) is a fundamental component of galaxies, being the most abundant element in molecular clouds, where stars form, and an important source of radiative cooling at low temperature. With the advent of the ALMA telescope, a large amount of data about the distribution of H2 in galaxies has become available. However, the large majority of numerical simulations on galactic and cosmological scales still lacks the ability to directly follow the formation and dissociation of H2, and must rely on pre-calibrated sub-grid models to compare the results with observations. I will present a new model to self-consistently track the evolution of H2, including gas and dust shielding, H2 self-shielding, star formation (SF), supernova feedback, and extragalactic and local stellar radiation. I will discuss the results of a suite of hydrodynamic simulations of an isolated gas-rich galaxy at z=3, showing that the model can naturally reproduce the observed correlation between SF and H2 surface densities, without assuming any a priori dependence of SF on the H2 abundance. I will also present a study of the kinematics and dynamics of molecular gas in high-redshift quasars (z=6), where we investigate whether a central accreting black hole (BH) can significantly affect the H2 distribution in the host galaxy and generate molecular outflows.

Lecture 3 by George

More than 40 years ago, Skumanich (1972) showed how rotation and magnetic activity decreased with the age of a solar-like star. While this result was based on the study of young cluster stars, later observations  of other clusters, still younger than the Sun, agreed with this “gyrochronology” relationship.

With the high-quality photometric data collected by the Kepler mission, we have the opportunity to test and study the evolution of stellar dynamics to older field stars. While for clusters, the determination of stellar ages is eased by the fact that the stars were born from the same molecular cloud, it gets trickier and less precise for field stars. This is where asteroseismology plays an important role by providing more precise ages than any other classical methods.

In this talk I will mostly focus on asteroseismic targets from solar-like stars to red giants where we could measure surface rotation, core rotation, and magnetic activity. I will show how the photometric data of Kepler is providing key information in the understanding of angular momentum transport in stars and of magnetic activity at different evolutionary stages of a star like the Sun.

The most metal-poor stars in the Galaxy are relics from the first generations of star formation, and their properties can reveal key information about the formation and evolution of the Milky Way. However, only a small number of these extremely rare stars are currently known, due to the difficulty in finding them amongst the overwhelmingly more abundant stars of higher metallicity. In this talk, I will present the Pristine survey, a narrow-band photometric survey in the wavelength region around the Ca H&K absorption lines designed to efficiently search for extremely metal-poor (EMP) stars. In the first three years of the survey, we have covered ~2,500 square degrees of sky in the Northern hemisphere using the CFHT on Mauna Kea in Hawaii, as well as a sizeable spectroscopic follow-up sample using mostly the INT and WHT in La Palma. With this data, we have demonstrated success rates of 70% for finding stars with [Fe/H] < -2.5, and 22% for stars with [Fe/H] < -3.0. This represents a significant improvement upon previous searches for EMP stars, which have reported success rates of 3-4%. With this efficiency, the Pristine survey is poised to make a significant contribution to constraining the metal-poor tail of the metallicity distribution function, as well as increasing the number of known ultra metal-poor (UMP) stars in the literature. In addition, I will discuss how the Pristine survey is being used to characterise the faint dwarf galaxy population, and analyse substructure in the Galactic Halo.

We employ a Bayesian method to infer stellar parameters from the PARSEC  v1.2S library of stellar evolution models and test the accuracy of these theoretical predictions. Detached eclipsing binaries are ideal for testing. We employ a compilation of 165 detached eclipsing binary systems of our galaxy and the Magellanic clouds with reliable metallicities and measurements for the mass and radius to 2 per cent precision for most of them. We complement the analysis with 107 stars that are closer than 300 pc, for which we adopted solar metallicity. The applied Bayesian analysis relies on a prior for the initial mass function and flat priors for age and metallicity, and it takes on input the effective temperature, radius, and metallicity, and their uncertainties, returning theoretical predictions for other stellar parameters of the binaries. Our research is mainly based on the comparison of dynamical masses with the theoretical predictions for the selected binary systems. We determine the precision of the models. Also, we derive distances for the binaries, which are compared with trigonometric parallaxes whenever possible. We discuss the effects of evolution and the challenges associated with the determination of theoretical stellar ages.

Black holes are a fundamental ingredient in our current understanding of galaxy formation. In the absence of their feedback, state-of-the-art numerical simulations fail to match the observed properties of massive galaxies. Effectively, within a Lambda Cold Dark Matter Universe, black holes reconcile cosmology and galaxy formation theories by regulating baryonic processes. However, despite of this widely-accepted and fundamental role, evidence of black hole regulated star formation remains elusive. I will present our observational efforts to characterize and understand the interplay between black hole activity and star formation, based on detailed stellar populations analyses. Our observations show that black hole and stellar population properties are tightly related, calling for a rich and complex observational framework where star formation, black holes and chemical enrichment evolve coupled in time.

Only once in a generation is there the opportunity to reveal the basic properties of a new galaxy type for the first time. With the advent of more sensitive instruments in the current large telescopes, an entirely new universe is being revealed, as it had never been seen before. And it is a challenging one, a low-luminosity universe that is populated by a myriad of new galaxies that are classified into new fancy families: the ultra-compact dwarfs (UCDs), the compact ellipticals (cEs) and the ultra-diffuse galaxies (UDGs). 

Despite some attempts to characterize and understand such galaxies, a recurrent topic prevails: what are they really? Are they intrinsic objects, i.e. were they formed as we see them now?; or were they initially other types of galaxies that have later evolved due to external interactions, which shaped them into what we see now? In the case of cEs, we have been lucky enough to catch some of them 'in the act', being stripped by a larger galaxy. However, at the same time, some of them have been found to be completely isolated and with no signs of interaction. In this talk, I will discuss the different pathways for cE formation and the expectations from their super massive black holes. I will also show how a similarly detailed study for all the faint families together can provide crucial clues for the galaxy evolution paradigm.

The Vlasov-Poisson system is the mean field limit of the classical N-body problem as N goes to infinity. First considered in plasma physics, it has been also used as an approach to describe self-gravitating systems. We will review the basic properties of this model, examine its catalog of stationary solutions and discuss its usefulness to describe galaxies.

We describe how a simple class of out of equilibrium, rotating and asymmetrical mass distributions evolve under their self-gravity to produce a quasi-planar spiral structure surrounding a virialized core, qualitatively resembling a spiral galaxy. The spiral structure is transient, but can survive tens of dynamical times, and further reproduces qualitatively noted features of spiral galaxies as the predominance of trailing two-armed spirals and large pitch angles. As our models are highly idealized, a detailed comparison with observations is not appropriate, but generic features of the velocity distributions can be identified to be potential observational signatures of such a mechanism. Indeed, the mechanism leads generically to a characteristic transition from predominantly rotational motion, in a region outside the core, to radial ballistic motion in the outermost parts. Such radial motions are excluded in our Galaxy up to 15 kpc, but could be detected at larger scales in the future by GAIA. We explore the apparent motions seen by external observers of the velocity distributions of our toy galaxies, and find that it is difficult to distinguish them from those of a rotating disc with sub-dominant radial motions at levels typically inferred from observations. These simple models illustrate the possibility that the observed apparent motions of spiral galaxies might be explained by non-trivial non-stationary mass and velocity distributions without invoking a dark matter halo or modification of Newtonian gravity. In this scenario the observed phenomenological relation between the centripetal and gravitational acceleration of the visible baryonic mass could have a simple explanation.