Current Research Interests . . .
| I am currently interested in the Lyman-alpha emission in star-forming galaxies. This emission line is the most efficient tool to detect high-redshift galaxies and is widely used to investigate the star-formation history of the distant universe since Lyman-alpha becomes the strongest emission line in the optical-NIR window at redshifts z > 2.1. It is also used to study clustering properties, and the imprint of the IGM on the Lya strength and profile shape allows us, in theory, to put constraints on the final stage of the reionisation epoch.
However, the resonant nature of Lya photons makes high-z observations subject to large uncertainties. Dust, ISM morphology as well as neutral gas kinematics and column density may regulate the Lya escape fraction. The order of importance of these parameters is nevertheless still unclear. Without a rigorous understanding of the escape processes and regulators, all interpretations based on the Lyman-alpha line alone remain questionable and could be at fault. |
 Observations Through Ages: From local galaxies to reionisation era. |
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Studies of the Lyman-alpha line in nearby star-forming galaxies have been undertaken ever since the space-ultraviolet has become available. Attempts have been made to model the resulting complex Lyman-alpha line profiles, whether in emission or in absorption, for an evolving star forming event in a gas rich galaxy. Yet, narrow band imaging is needed to study Lya emission morphology and the outer diffuse emission.
Thanks to a high spatial resolution of the Advaced Camera for Survey onboard the Hubble Space Telescope, we can study the Lya variations at a very small spatial scale. Combined with ground based (ESO) observations to map the dust content and the neutral gas kinematics, it is possible to disentangle the different and complex Lya escape processes and infer implications for cosmological studies. In this way, Nearby starburst galaxies can be used as "templates" to calibrate high-z observations and investigate the evolution of galaxy properties and population with redshift.
| FUV 1500 A | Halpha | Lyman-alpha |
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| HST Imaging of Haro 11: Among the three main condensations observed in the UV and Halpha images, only one shows Lyman-alpha in emission, whereas absorption is found in the remaining knots. This absence of correlation is symptomatic of the resonant scattering mechanism of Lyman-alpha photons that also lead to the low surface brightness halo overall the galaxy. | ||
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Besides this observational work, I am interested in modeling and understanding the variety of Lyman-alpha profiles observed both at low- and high-z. They range from a pure emission to damped absorption or P-Cygni type. In collaboration with the Geneva group (Daniel Schaerer & Anne Verhamme), we use a Monte Carlo technique whithin a 3D radiation transfer code (3DLya) to simulate the journey of a Lya photon from its emission to its destruction or escape from the diffusion medium. The emission region is characterised by two parameters: The Equivalent Width (EW) and the Full Width at Half Maxmimum (FWHM) of the emission line. For the diffusion region (neutral gas), the code can deal with arbitrary ISM geometry, velocity fields, HI column density and dust attenuation.
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| Left: An exemple of line profile fitting with the galaxy FDF4691. The profile is fitted with a dust-free and quasi-static shell. Right: Grid of predicted Lyman-alpha profiles (in blue) compared to FDF2384 observed spectrum (in black) illustrating the constraints on the fit parameters. The column density is varying along the y axis and the dust attenuation follows the x axis. (Verhamme et al. 2008) | |
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Theoritical and observational evidences have been found indicating that galaxy-scale superwinds are ubiquitous in young starbursts and the role of the ISM kinematics in the Lya visibility, as stated above, has been discussed many years ago. Using a simple model consisting of an expanding shell of neutral gas surrounding the emission region we are able to reproduce and fit the variety of Lya profile shapes. We can constrain many parameters of the gas (such as the HI column density, the expansion velocity or the dust content ...) and the stellar population (emission strength, star-formation histoty ...). This model has successfully been applied to z~3 Lyman Break Galaxies (LBGs) fitting their Lya spectra and is currently applied to our local sample which has much more constraints on the different galaxy parameters.
