Doctor in Astrophysics
Post-Doc at UCL

ESTELLE BAYET

RESEARCH THEMES:

How stars are forming in the galaxies ?


My research in astrophysics aims to answer these two fundamental questions by looking at star-forming regions in external galaxies.

To understand the formation and the evolution of the galaxies in the Universe, it is essetial to study in partcular one of the fundamental component of a galaxy: the stars. Indeed, these stars are directly responsible of the past, present and future of the galaxy they belong to. They are the "engine" regulating the History of the galaxies. Better describing the formation of stars, and this, in various types of galaxy is thus vital, especially for better understanding the galaxies themselves.

In this vast topic of astrophysical research, I am more especially interesting on the determination of the physical conditions (temperature, gas density, etc) under which the stars are forming in galaxies. Depending on the morphological type of the galaxy (irregular, starburst, spiral normal, merger, ... - cf images to the right), we expect that the conditions of star formation might vary from a source to another. However we can wonder if differences actually might occur since, after all, the stars are all forming from the same material called interstellar medium, present in every galaxies. This also includes the formation of our own star, the Sun, formed in our own Galaxy , the Milky Way!

The interstellar medium is known as the second fundamental component of a galaxy, after considering its stellar content. It is as essential for the galaxy but may be less directly influent than the stars in the determination of the galaxy History. However it is important in the determination of the properties of the galaxy. The interstellar medium contains itself two components: the gas and the dust. I am studying mainly the interstellar gas even if I have recently focussed some of my research also on dust.

In a galaxy, the gas can be found under various phases. It is observed in its ionised phase as well as in its molecular phase, the first one being the faintest part of the gas while the second one being the densest. It is inside extreme contractions of gas called molecular clouds (cf. images on the right) that stars are actually forming. These clouds taken all togeteher form what we call a star-forming region, objects I am in fact studying in the Universe.

The properties of these regions are obtained via the detection of emissions, each providing a complementary source of informations to the other. The molecular gas in contraction is getting warmer during this process, but for being able to get even denser and form stars, it needs to evacuate its energy. It is basically doing it by emetting radiations. These are these radiation that I am catching and which allow me to derive the physical conditions of the observed gas. These emissions are actually from the molecules contained in the gas. They appear at various wavelengths and need various instrument to detect them. On average, it has been defined a wavelength domain where a lot of molecular emission occur: this is the submillimeter-millimeter domain. I am focussing my work on such domain.

For detecting the molecualr emissions coming from various galaxies, I am using various observatory such as the Caltech Submillimeter Observatory (CSO), the James Clerk Maxwell Telescope (JCMT), or the 'IRAM-30m, etc (cf images on the right). Each telescope contains a specific sample of instruments (receivers and spectrometers) and I often need to use one preferentially to the others, depending on the molecule I want to observe. To get time on professional observatory, it is good to remind that a file (called "proposal") has to be created. The proposal presents both the scientific goals the team wants to reach with the proposed observations as well as a technical part justifying the faisability of such observations. It is then examined by a jury of astrophysists and astronomers who will judge of the relevancy of the project and award to it a certain amount of time (e.g. few hours or few nights). The richness of observing in the submillimeter--millimeter domain is that technically speaking it is possible to observe 24h a day, being not disturbed neither by the Sun nor the atmosphere since transparent at such wavelentghs.

Once detected and with the help of models containing physical and chemical laws, the molecular emission gives me direct informations on the nature and the physical conditions of the studied gas. Some molecules exist only under specific conditions of temperature and density (e.g. HCOOCH3, CH3OH) whereas some others are seen everywhere in the galaxy (H2). Thus, some molecules are characteristics either of a very dense (~10^5-7cm-3) and warm (>70K) molecular gassuch as CS, HCN or HNC, either of a more extended (~10^3cm-3) and less warm (<50K) molecular gas such as CO. During my thesis I am investigated especially the gas emitting in the CO lines and the C lines. In my Post-doc in UCL, I am rather determining the properties of the densest part of the molecular gas as traced by the CS line emission. Starting from the extended gas to the densest gas, in various types of galaxies, I am like that getting closer and closer to the stars in formation, understanding and estimating better how they form.

The models I am using and that I am also developing for some of them, are mainly Large Velocity Gradient (LVG) and chemical (Chem_mod) models; and Photo-Dominated Regions (PDR) codes, the latest offering the possibility to be or not time dependent (see UCL_PDR and Meudon_PDR). I am interesting on the modelisation of extragalactic star-forming regions which implies to modify the models parameters (metallicity, gas-to-dust mass ratio, FUV radiation field, etc) I am using since these models have been firstly dedicated to reproduce rather Galactic sources, with a priori different properties than external star-forming regions. From these models part of my work consists in formulating some observational predictions on the detectability of some molecule for particular environments. From these models, I am also deriving the average physical properties which are able to reproduce my observations the best (see tables et figures on the right).

In parallel, as previously mentioned, I am studying more recently the dust emission maintaining always my approach on a large sample of galaxy type. My research focusses on the comparison between the molecular gas emission and the dust emission, each providing an interesting piece of informations on the environment they are coming from. More precisely, I am using already published infrared satelites ISO and Spitzer results (emission maps) for tracing the dust emission. My idea is to see if any systematic differences appear from a galaxy to another in the properties of their star-forming regions.

To summarize, as a researcher in Astrophysics in the submm/mm extragalactic domain, my current work and future projects show a real desire for confronting observational and modelling approaches by maintaining a pluri-disciplinarity of the treatments both astrophysics and astrochemical for better understanding how stars are forming in galaxies.