260 research outputs found
Resonant line transfer in a fog: Using Lyman-alpha to probe tiny structures in atomic gas
Motivated by observational and theoretical work which both suggest very small
scale (pc) structure in the circum-galactic medium of galaxies
and in other environments, we study Lyman- (Ly) radiative
transfer in an extremely clumpy medium with many "clouds" of neutral gas along
the line of sight. While previous studies have typically considered radiative
transfer through sightlines intercepting clumps, we explore the
limit of a very large number of clumps per sightline (up to ). Our main finding is that, for covering factors greater than some
critical threshold, a multiphase medium behaves similar to a homogeneous medium
in terms of the emergent Ly spectrum. The value of this threshold
depends on both the clump column density and on the movement of the clumps. We
estimate this threshold analytically and compare our findings to radiative
transfer simulations with a range of covering factors, clump column densities,
radii, and motions. Our results suggest that (i) the success in fitting
observed Ly spectra using homogeneous "shell models" (and the
corresponding failure of multiphase models) hints towards the presence of very
small-scale structure in neutral gas, in agreement within a number of other
observations; and (ii) the recurrent problems of reproducing realistic line
profiles from hydrodynamical simulations may be due to their inability to
resolve small-scale structure, which causes simulations to underestimate the
effective covering factor of neutral gas clouds.Comment: 18 pages, 21 figures; submitted to A&A; animations available at
http://bit.ly/a-in-a-fo
Cooling driven coagulation
Astrophysical gases such as the interstellar-, circumgalactic- or
intracluster-medium are commonly multiphase, which poses the question of the
structure of these systems. While there are many known processes leading to
fragmentation of cold gas embedded in a (turbulent) hot medium, in this work,
we focus on the reverse process: coagulation. This is often seen in wind-tunnel
and shearing layer simulations, where cold gas fragments spontaneously
coalesce. Using 2D and 3D hydrodynamical simulations, we find that sufficiently
large (), perturbed cold gas clouds develop
overstable sound waves which ensure cold gas mass growth over an extended
period of time (). This mass growth efficiently accelerates
hot gas which in turn can entrain cold droplets, leading to coagulation. The
attractive inverse square force between cold gas droplets has interesting
parallels with gravity; the `monopole' is surface area rather than mass. We
develop a simple analytic model which reproduces our numerical findings.Comment: 13 pages, 11 figures; submitted to MNRA
Can Galaxy Evolution Mimic Cosmic Reionization?
Lyα emitting galaxies are powerful tools to probe the late stages of cosmic reionization. The observed sudden drop in Lyα fraction at z > 6 is often interpreted as a sign of reionization, since the intergalactic medium (IGM) is more neutral and opaque to Lyα photons. Crucially, this interpretation of the observations is only valid under the assumption that galaxies themselves experience a minimal evolution at these epochs. By modeling Lyα radiative transfer effects in and around galaxies, we examine whether a change in the galactic properties can reproduce the observed drop in the Lyα fraction. We find that an increase in the galactic neutral hydrogen content or a reduction in the outflow velocity toward higher redshift both lead to a lower Lyα escape fraction, and can thus mimic an increasing neutral fraction of the IGM. We furthermore find that this change in galactic properties leads to systematically different Lyα spectra which can be used to differentiate the two competing effects. Using the CANDELSz7 survey measurements which indicate slightly broader lines at z ∼ 6, we find that the scenario of a mere increase in the galactic column density toward higher z is highly unlikely. We also show that a decrease in outflow velocity is not ruled out by existing data but leads to more prominent blue peaks at z > 6. Our results caution using Lyα observations to estimate the IGM neutral fraction without accounting for the potential change in the galactic properties, e.g., by mapping out the evolution of Lyα spectral characteristics
The Lyman Alpha Reference Sample. VIII. Characterizing Lyman-Alpha Scattering in Nearby Galaxies
We examine the dust geometry and Ly{\alpha} scattering in the galaxies of the
Lyman Alpha Reference Sample (LARS), a set of 14 nearby (0.02 < < 0.2)
Ly{\alpha} emitting and starbursting systems with Hubble Space Telescope
Ly{\alpha}, H{\alpha}, and H{\beta} imaging. We find that the global dust
properties determined by line ratios are consistent with other studies, with
some of the LARS galaxies exhibiting clumpy dust media while others of them
show significantly lower Ly{\alpha} emission compared to their Balmer
decrement. With the LARS imaging, we present Ly{\alpha}/H{\alpha} and
H{\alpha}/H{\beta} maps with spatial resolutions as low as 40 pc, and
use these data to show that in most galaxies, the dust geometry is best modeled
by three distinct regions: a central core where dust acts as a screen, an
annulus where dust is distributed in clumps, and an outer envelope where
Ly{\alpha} photons only scatter. We show that the dust that affects the escape
of Ly{\alpha} is more restricted to the galaxies' central regions, while the
larger Ly{\alpha} halos are generated by scattering at large radii. We present
an empirical modeling technique to quantify how much Ly{\alpha} scatters in the
halo, and find that this "characteristic" scattering distance correlates with
the measured size of the Ly{\alpha} halo. We note that there exists a slight
anti-correlation between the scattering distance of Ly{\alpha} and global dust
properties.Comment: 32 pages, 51 figures, accepted to Ap
High-resolution spectroscopy of a young, low-metallicity optically-thin L=0.02L* star-forming galaxy at z=3.12
We present VLT/X-Shooter and MUSE spectroscopy of an faint F814W=28.60+/-0.33
(Muv=-17.0), low mass (~<10^7 Msun) and compact (Reff=62pc) freshly
star-forming galaxy at z=3.1169 magnified (16x) by the Hubble Frontier Fields
galaxy cluster Abell S1063. Gravitational lensing allows for a significant jump
toward low-luminosity regimes, in moderately high resolution spectroscopy
(R=lambda/dlambda ~ 3000-7400). We measured CIV1548,1550, HeII1640,
OIII]1661,1666, CIII]1907,1909, Hbeta, [OIII]4959,5007, emission lines with
FWHM< 50 km/s and (de-lensed) fluxes spanning the interval 1.0x10^-19 -
2.0x10^-18 erg/s/cm2 at S/N=4-30. The double peaked Lya emission with
Delta_v(red-blue) = 280(+/-7)km/s and de-lensed fluxes
2.4_(blue)|8.5_(red)x10^-18 erg/s/cm2 (S/N=38_(blue)|110_(red)) indicate a low
column density of neutral hydrogen gas consistent with a highly ionized
interstellar medium as also inferred from the large [OIII]5007/[OII]3727>10
ratio. We detect CIV1548,1550 resonant doublet in emission, each component with
FWHM ~< 45 km/s, and redshifted by +51(+/-10)km/s relative to the systemic
redshift. We interpret this as nebular emission tracing an expanding
optically-thin interstellar medium. Both CIV1548,1550 and HeII1640 suggest the
presence of hot and massive stars (with a possible faint AGN). The ultraviolet
slope is remarkably blue, beta =-2.95 +/- 0.20 (F_lambda=lambda^beta),
consistent with a dust-free and young ~<20 Myr galaxy. Line ratios suggest an
oxygen abundance 12+log(O/H)<7.8. We are witnessing an early episode of
star-formation in which a relatively low NHI and negligible dust attenuation
might favor a leakage of ionizing radiation. This galaxy currently represents a
unique low-luminosity reference object for future studies of the reionization
epoch with JWST.Comment: 7 pages, 4 figures and 1 table; ApJL, accepted for publicatio
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