120 research outputs found
On the Doublet Flux Ratio of Mg II Resonance Lines in and Around Galaxies
Observations of metallic doublet emission lines, particularly Mg II 2796,
2803, provide crucial information for understanding galaxies and their
circumgalactic medium. This study explores the effects of resonant scattering
on the Mg II doublet lines and the stellar continuum in spherical and
cylindrical geometries. Our findings show that under certain circumstances,
resonance scattering can cause an increase in the doublet flux ratio and the
escaping flux of the lines beyond what are expected in optically thin spherical
media. As expected, the doublet ratio is consistently lower than the intrinsic
ratio when the scattering medium is spherically symmetric and dusty. However,
if the scattering medium has a disk shape, such as face-on disk galaxies, and
is viewed face-on, the doublet ratio is predicted to be higher than two. These
results may provide a valuable insight regarding the complexity of the shape
and orientation of distant, spatially-unresolved galaxies. The importance of
the continuum-pumped emission lines and expanding media is discussed to
understand various observational aspects, including doublet flux ratios, which
can be lower than 1.5 or higher than two, as well as symmetric or asymmetric
line profiles. It is also discussed that the diffuse warm neutral medium would
be an essential source of Mg II emission lines.Comment: submitted to the AAS journal (on 16-Sep-2023
Radiative Transfer Model of Dust Attenuation Curves in Clumpy, Galactic Environments
The attenuation of starlight by dust in galactic environments is investigated
through models of radiative transfer in a spherical, clumpy ISM. Extinction
properties for MW, LMC, and SMC dust types are considered. It is illustrated
that the attenuation curves are primarily determined by the wavelength
dependence of absorption rather than by the underlying extinction
(absorption+scattering) curve. Attenuation curves consistent with the "Calzetti
attenuation curve" are found by assuming the silicate-carbonaceous dust model
for the MW, but with the 2175A absorption bump suppressed or absent. The
discrepancy between our results and previous work that claimed the SMC-type
dust to be the most likely origin of the Calzetti curve is ascribed to the
difference in adopted albedos; this study uses the theoretically calculated
albedos whereas the previous ones adopted empirically derived albedos from
observations of reflection nebulae. It is also found that the model attenuation
curves calculated with the MW dust are well represented by a modified Calzetti
curve with a varying slope and UV bump strength. The strong correlation between
the slope and UV bump strength, with steeper curves having stronger bumps, as
found in star-forming galaxies at 0.5<z<2.0, is well reproduced by our models
if the abundance of the UV bump carriers or PAHs is assumed to be 30% or 40% of
that of the MW-dust. The trend is explained by radiative transfer effects which
lead to shallower attenuation curves with weaker UV bumps as the ISM is more
clumpy and dustier. We also argue that at least some of the IUE local starburst
galaxies may have a UV bump feature in their attenuation curves, albeit much
weaker than that of the MW extinction curve.Comment: 28 pages, 30 figures, submited to ApJ
Dust Scattering In Turbulent Media: Correlation Between The Scattered Light and Dust Column Density
Radiative transfer models in a spherical, turbulent interstellar medium (ISM)
in which the photon source is situated at the center are calculated to
investigate the correlation between the scattered light and the dust column
density. The medium is modeled using fractional Brownian motion structures that
are appropriate for turbulent ISM. The correlation plot between the scattered
light and optical depth shows substantial scatter and deviation from simple
proportionality. It was also found that the overall density contrast is
smoothed out in scattered light. In other words, there is an enhancement of the
dust-scattered flux in low-density regions, while the scattered flux is
suppressed in high-density regions. The correlation becomes less significant as
the scattering becomes closer to be isotropic and the medium becomes more
turbulent. Therefore, the scattered light observed in near-infrared wavelengths
would show much weaker correlation than the observations in optical and
ultraviolet wavelengths. We also find that the correlation plot between
scattered lights at two different wavelengths shows a tighter correlation than
that of the scattered light versus the optical depth.Comment: 6 pages, 5 figure, accepted for publication in the ApJ Letter
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