Melanie Gendre
Researcher in Astrophysics (2006-2013)
Physics Teacher


Research interests

I have always been fascinated by galaxies. The most massive ones first formed by gravitational growth of density perturbations and expend from mergers. They evolved from giant dusty star-forming objects into red, quiet elliptical galaxies, and have complex structures and dark matter content.

For the past few years, I have been focusing my interest on Active Galactic Nuclei (AGN) by studying radio galaxies and their evolution. The basic principle behind AGN is the ignition of relativistic jets from the central black hole region. They are very powerful sources which are commonly classified based on e.g. morphology, spectral shape or luminosity. I have compiled a large comprehensive catalogue of radio galaxies, including morphological classification, spectral indices and redshifts, which I used to model the evolution of space densities of various AGN classes. Several aspects of AGNs need to be further examined: What are the mechanisms powering them? How do AGN form and evolve? What are the physical processes differentiating their various classes? Do galaxies undergo a single AGN phase or several?

Centaurus A at optical and radio wavelength.

AGNs are poorly understood in the context of galaxy evolution. The accepted hierarchical model of galaxy formation and evolution predicts that, in the local universe, large old galaxies should be the bluest, most star forming objects in the Universe. However, observations contradict this, showing that these giant galaxies are almost exclusively quiescent red ellipticals. Something must then have stopped star formation, and the popular hypothesis is AGN feedback, with the jets dissipating and heating the ISM. In what proportion does this feedback contribute to the quenching of star formation? Is it sufficient to completely stop star formation in galaxies?

Elliptical galaxy Spiral galaxy

Going backward in time, this leads to the question of AGN origins. What are the conditions required to trigger the ignition of jets in the nuclei? This requires a knowledge of the AGN progenitors, which are currently believed to be SubMillimeter Galaxies (SMG), very dusty objects seen at high redshifts, undergoing a massive star formation phase and visible only through the submm radiation from heated dust. How do massive systems form at such early times in the Universe? How is build-up of star, dust and gas related to the growth of central black holes in massive galaxies?

Another subject related to galaxy formation and evolution that I wish to eventually investigate is the distribution of dark matter, both on galactic and inter-galactic scales and in particular detection via gravitational lensing. The mapping of the unseen mass provides us with key observations for testing theories of galaxy formation, from the creation of the first clumps of matter to the evolution of galactic structures.

Gravitational lens (yellow-ish foreground cluster) and lensed galaxies (blue-ish elongated, ring-like features). HST image.

Finally, during my postoctoral research contract in Manchester, I had the occasion to work on the evolution of supernova remnants (SNR) in M82. SNRs are the results of the explosion of stars in a supernova, and are bounded by an expanding shock wave which emits at radio wavelengths. By looking at the evolution of SNRs, one can study both the characteristics of the parent supernova and the properties of the inter-stellar medium (the gas and dust between stars in a galaxy) in which the shell is expending.

M82, starburst galaxy, seen at radio wavelengths