112 research outputs found
On the (non-)enhancement of the Lyman \alpha\ equivalent width by a multiphase interstellar medium
It has been suggested that radiative transfer effects may explain the
unusually high equivalent widths (EWs) of the Lya line, observed occasionally
from starburst galaxies, especially at high redshifts. If the dust is locked up
inside high-density clouds dispersed in an empty intercloud medium, the Lya
photons could scatter off of the surfaces of the clouds, effectively having
their journey confined to the dustless medium. The continuum radiation, on the
other hand, does not scatter, and would thus be subject to absorption inside
the clouds. This scenario is routinely invoked when Lya EWs higher than what is
expected theoretically are observed, although the ideal conditions under which
the results are derived usually are not considered. Here we systematically
examine the relevant physical parameters in this idealized framework, testing
whether any astrophysically realistic scenarios may lead to such an effect. It
is found that although clumpiness indeed facilitates the escape of Lya, it is
highly unlikely that any real interstellar media should result in a
preferential escape of Lya over continuum radiation. Other possible causes are
discussed, and it is concluded that the observed high EWs are more likely to be
caused by cooling radiation from cold accretion and/or anisotropic escape of
the Lya radiation.Comment: Submitted to ApJ. Comments welcom
Lyman-alpha line and continuum radiative transfer in a clumpy interstellar medium
Aims: We aim to study the effects of an inhomogeneous interstellar medium
(ISM) on the strength and the shape of the Lyman alpha (Lya) line in starburst
galaxies. Methods: Using our 3D Monte Carlo Lya radiation transfer code, we
study the radiative transfer of Lya, UV and optical continuum photons in
homogeneous and clumpy shells of neutral hydrogen and dust surrounding a
central source. Our simulations predict the Lya and continuum escape fraction,
the Lya equivalent width EW(Lya), the Lya line profile and their dependence on
the gas geometry and the main input physical parameters. Results: The ISM
clumpiness is found to have a strong impact on the Lya line radiative transfer,
entailing a strong dependence of the emergent features of the Lya line (escape
fraction, EW(Lya)) on the ISM morphology. Although a clumpy and dusty ISM
appears more transparent to radiation (both line and continuum) compared to an
equivalent homogeneous ISM of equal dust optical depth, we find that the Lya
photons are, in general, still more attenuated than UV continuum radiation. As
a consequence, the observed Lya equivalent width (EWobs(Lya)) is lower than the
intrinsic one (EWint(Lya)) for nearly all clumpy ISM configurations considered.
There are, however, special conditions under which Lya photons escape more
easily than the continuum, resulting in an enhanced EWobs(Lya). The requirement
for this to happen is that the ISM is almost static (galactic outflows < 200
km/s), extremely clumpy (with density contrasts >10^7 in HI between clumps and
the interclump medium), and very dusty (E(B-V) > 0.30). When these conditions
are fulfilled the emergent Lya line profile shows no velocity shift and little
asymmetry. Given the asymmetry and velocity shifts generally observed in
starburst galaxies with Lya emission, we conclude that clumping is unlikely to
significantly enhance their relative Lya/UV transmission.Comment: 20 pages, 22 figures. Accepted for publication in A&
Oral Kaposi Sarcoma in two patients living with HIV despite sustained viral suppression : new clues
Kaposi sarcoma (KS) etiologically linked to Kaposi sarcoma-associated herpesvirus (KSHV) is the most common HIV associated cancer despite the generalization of antiretroviral therapy. Head, neck, and especially oral cavity are common and specific sites f
The Lyman alpha Reference Sample: Extended Lyman alpha Halos Produced at Low Dust Content
We report on new imaging observations of the Lyman alpha emission line (Lya),
performed with the Hubble Space Telescope, that comprise the backbone of the
Lyman alpha Reference Sample (LARS). We present images of 14 starburst galaxies
at redshifts 0.028 < z < 0.18 in continuum-subtracted Lya, Halpha, and the far
ultraviolet continuum. We show that Lya is emitted on scales that
systematically exceed those of the massive stellar population and recombination
nebulae: as measured by the Petrosian 20 percent radius, RP20, Lya radii are
larger than those of Halpha by factors ranging from 1 to 3.6, with an average
of 2.4. The average ratio of Lya-to-FUV radii is 2.9. This suggests that much
of the Lya light is pushed to large radii by resonance scattering. Defining the
"Relative Petrosian Extension" of Lya compared to Halpha, \xi_ext = RP20_Lya /
RP20_Ha, we find \xi_ext to be uncorrelated with total Lya luminosity. However
\xi_ext is strongly correlated with quantities that scale with dust content, in
the sense that a low dust abundance is a necessary requirement (although not
the only one) in order to spread Lya photons throughout the interstellar medium
and drive a large extended Lya halo.Comment: Published in ApJ Letters ~~ 6 pages using emulateapj, 4 figures ~~
Higher-resolution, larger, nicer jpeg versions of Figures 1 and 2 can be
found here: http://xayes.org/pub/press_lars.htm
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