Licentiate Seminar: Lyman-alpha radiative transfer in star burst galaxies
by
Florent Duval(Dept of Astronomy, Stockholm University)
→
Europe/Stockholm
FB52
FB52
Description
This thesis focuses on the intrinsically brightest spectral signature of star-forming galaxies,
the Lyman α recombination line of the hydrogen atom (hereafter Lyα). Located at the wave-
length of λ = 1215.67 Å in the rest-frame far-ultraviolet spectra of starburst galaxies, the Lyα
line has become our most powerful emission-line window to discover and to study the young
and primordial star-forming galaxies located at very high-redshifts.
However, although intrinsically very strong, the Lyα line strength is also its weakness as
neutral hydrogen (HI) atom is very likely to absorb it. Due to the resonant nature of the Lyα line,
the transport of the Lyα photons in the interstellar medium (ISM) of star-forming galaxies is
affected by many physical properties (HI density, dust content, HI gas kinematics and structure
of the ISM), implying many consequences for the emergent features of the Lyα line (intensity,
equivalent width and line profile).
In this thesis, we carry out a complete numerical study of both the Lyα and the ultraviolet
continuum (UV) radiative transfer in a large range of dusty, moving, homogeneous and clumpy
ISMs. In particular, this study allows us to examine how the ISM clumpiness affects the visibil-
ity, the equivalent width EW(Lyα) and the shape of the Lyα line in starburst galaxies.
Our numerical simulations indicate that the ISM clumpiness has a strong impact on the
Lyα radiative transfer in galaxies. We find that the Lyα photons are, in general, still more
attenuated than the UV continuum photons although an increase of the ISM clumpiness renders
the ISM more transparent to Lyα and UV continuum photons. As a consequence, the observed
equivalent width of the Lyα line stays lower than the intrinsic one for nearly all clumpy ISM
configurations considered. Nevertheless, we have been able to isolate some extreme conditions
to the ISM where the Lyα photons can escape the ISM more easily than the UV continuum
photons, resulting in an enhancement of the observed EW(Lyα). In this scenario, commonly
called the ”Neufeld scenario”, the ISM must be almost static (outflows < 200 km/s), extremely
clumpy (an interclump medium HI density which is up to 107 times lower than that is inside
the clumps) and very dusty (E(B-V) > 0.30). Furthermore, when these conditions are fulfilled,
a particular Lyα line profile emerges, showing an emission at line center and little asymmetry.
In view of these particular conditions, it seems quite unlikely to find realistic conditions under
which EW(Lyα) could be artificially enhanced in clumpy ISMs.
Concerning the Lyα line profiles, we find that the shape of the Lyα line is not strongly
dependant on the ISM clumpiness of a galaxy. The Lyα line profile of distant star-forming
galaxies cannot therefore be used as a robust tool to study the geometry and the clumpiness of
the ISM of high-redshift star-forming galaxies.
