We compare 237 Lyman-α (Lyα) spectra of the MUSE-Wide survey to a suite of radiative transfer simulations consisting of a central luminous source within a concentric, moving shell of neutral gas, and ...dust. This six parameter shell-model has been used numerously in previous studies, however, on significantly smaller data-sets. We find that the shell-model can reproduce the observed spectral shape very well – better than the also common “Gaussian-minus-Gaussian” model which we also fitted to the dataset. Specifically, we find that ~ 94% of the fits possess a goodness-of-fit value of p(χ2) > 0.1. The large number of spectra allows us to robustly characterize the shell-model parameter range, and consequently, the spectral shapes typical for realistic spectra. We find that the vast majority of the Lyα spectral shapes require an outflow and only ~ 5% are well-fitted through an inflowing shell. In addition, we find ~ 46% of the spectra to be consistent with a neutral hydrogen column density < 1017 cm-2 – suggestive of a non-negligible fraction of continuum leakers in the MUSE-Wide sample. Furthermore, we correlate the spectral against the Lyα halo properties against each other but do not find any strong correlation.
ABSTRACT
The escape of ionizing Lyman continuum (LyC) photons requires the existence of low-
N
H
i
sightlines, which also promote escape of Ly
α
. We use a suite of 2500 Ly
α
Monte-Carlo radiative ...transfer simulations through models of dusty, clumpy interstellar (“multiphase”) media from Gronke & Dijkstra, and compare the escape fractions of Ly
α
(
) and LyC radiation (
). We find that
and
are correlated: galaxies with a low
consistently have a low
, while galaxies with a high
exhibit a large dispersion in
. We argue that there is increasing observational evidence that Ly
α
escapes more easily from UV-faint galaxies. The correlation between
and
then implies that UV-faint galaxies contribute more to the ionizing background than implied by the faint-end slope of the UV luminosity function. In multiphase gases, the ionizing escape fraction is most strongly affected by the cloud covering factor,
f
cl
, which implies that
is closely connected to the observed Ly
α
spectral line shape. Specifically, LyC-emitting galaxies typically having narrower, more symmetric line profiles. This prediction is qualitatively similar to that for “shell models.”
ABSTRACT Both absorption and emission-line studies show that cold gas around galaxies is commonly outflowing at speeds of several hundred km s−1. This observational fact poses a severe challenge to ...our theoretical models of galaxy evolution since most feedback mechanisms (e.g. supernovae feedback) accelerate hot gas, and the time-scale it takes to accelerate a blob of cold gas via a hot wind is much larger than the time it takes to destroy the blob. We revisit this long-standing problem using three-dimensional hydrodynamical simulations with radiative cooling. Our results confirm previous findings that cooling is often not efficient enough to prevent the destruction of cold gas. However, we also identify regions of parameter space where the cooling efficiency of the mixed, ‘warm’ gas is sufficiently large to contribute new comoving cold gas, which can significantly exceed the original cold gas mass. This happens whenever, tcool, mix/tcc < 1, where tcool, mix is the cooling time of the mixed warm gas and tcc is the cloud-crushing time. This criterion is always satisfied for a large enough cloud. Cooling ‘focuses’ stripped material on to the tail where mixing takes place and new cold gas forms. A sufficiently large simulation domain is crucial to capturing this behaviour.
ABSTRACT
The existence of fast moving, cold gas ubiquitously observed in galactic winds is theoretically puzzling, since the destruction time of cold gas is much smaller than its acceleration time. ...In previous work, we showed that cold gas can accelerate to wind speeds and grow in mass if the radiative cooling time of mixed gas is shorter than the cloud destruction time. Here, we study this process in much more detail, and find remarkably robust cloud acceleration and growth in a wide variety of scenarios. Radiative cooling, rather than the Kelvin–Helmholtz instability, enables self-sustaining entrainment of hot gas on to the cloud via cooling-induced pressure gradients. Indeed, growth peaks when the cloud is almost co-moving. The entrainment velocity is of order the cold gas sound speed, and growth is accompanied by cloud pulsations. Growth is also robust to the background wind and initial cloud geometry. In an adiabatic Chevalier-Clegg type wind, for instance, the mass growth rate is constant. Although growth rates are similar with magnetic fields, cloud morphology changes dramatically, with low density, magnetically supported filaments, which have a small mass fraction but dominate by volume. This could bias absorption line observations. Cloud growth from entraining and cooling hot gas can potentially account for the cold gas content of the circumgalactic medium (CGM). It can also fuel star formation in the disc as cold gas recycled in a galactic fountain accretes and cools halo gas. We speculate that galaxy-scale simulations should converge in cold gas mass once cloud column densities of N ∼ 1018 cm−2 are resolved.
ABSTRACT
Identifying and characterizing reionized bubbles enables us to track both their size distribution, which depends on the primary ionizing sources, and the relationship between reionization ...and galaxy evolution. We demonstrate that spectrally resolved z ≳ 6 Lyman-alpha (Lyα) emission can constrain properties of reionized regions. Specifically, the distance from a source to a neutral region sets the minimum observable Lyα velocity offset from systemic. Detection of flux on the blue side of the Lyα resonance implies the source resides in a large, sufficiently ionized region that photons can escape without significant resonant absorption, and thus constrains both the sizes of and the residual neutral fractions within ionized bubbles. We estimate the extent of the region around galaxies which is optically thin to blue Lyα photons, analogous to quasar proximity zones, as a function of the source’s ionizing photon output and surrounding gas density. This optically thin region is typically ≲ 0.3 pMpc in radius (allowing transmission of flux ≳ −250 km s−1), ≲ 20 per cent of the distance to the neutral region. In a proof-of-concept, we demonstrate the z ≈ 6.6 galaxy COLA1 – with a blue Lyα peak – likely resides in an ionized region >0.7 pMpc, with residual neutral fraction <10−5.5. To ionize its own proximity zone we infer COLA1 has a high ionizing photon escape fraction (fesc > 0.50), relatively steep UV slope (β < −1.79), and low line-of-sight gas density (∼0.5 times the cosmic mean), suggesting it is a rare, underdense line-of-sight.
Is multiphase gas cloudy or misty? Gronke, Max; Oh, S Peng
Monthly notices of the Royal Astronomical Society. Letters,
05/2020, Volume:
494, Issue:
1
Journal Article
Peer reviewed
Open access
ABSTRACT
Cold T ∼ 104 K gas morphology could span a spectrum ranging from large discrete clouds to a fine ‘mist’ in a hot medium. This has myriad implications, including dynamics and survival, ...radiative transfer, and resolution requirements for cosmological simulations. Here, we use 3D hydrodynamic simulations to study the pressure-driven fragmentation of cooling gas. This is a complex, multistage process, with an initial Rayleigh–Taylor unstable contraction phase that seeds perturbations, followed by a rapid, violent expansion leading to the dispersion of small cold gas ‘droplets’ in the vicinity of the gas cloud. Finally, due to turbulent motions, and cooling, these droplets may coagulate. Our results show that a gas cloud ‘shatters’ if it is sufficiently perturbed out of pressure balance (δP/P ∼ 1) and has a large final overdensity χf ≳ 300, with only a weak dependence on the cloud size. Otherwise, the droplets reassemble back into larger pieces. We discuss our results in the context of thermal instability and clouds embedded in a shock-heated environment.
Abstract
Several “giant” Ly
α
nebulae with an extent ≳300 kpc and observed Ly
α
luminosity of ≳10
44
erg s
−1
cm
−2
arcsec
−2
have recently been detected, and it has been speculated that their ...presence hints at a substantial cold gas reservoir in small cool clumps not resolved in modern hydrodynamical simulations. We use the
Illustris
simulation to predict the Ly
α
emission emerging from large halos (
M
> 10
11.5
M
⊙
) at
z
∼ 2 and thus test this model. We consider both active galactic nucleus (AGN) and star driven ionization, and compare the simulated surface brightness maps, profiles, and Ly
α
spectra to a model where most gas is clumped below the simulation resolution scale. We find that with
Illustris,
no additional clumping is necessary to explain the extents, luminosities, and surface brightness profiles of the “giant Ly
α
nebulae” observed. Furthermore, the maximal extents of the objects show a wide spread for a given luminosity and do not correlate significantly with any halo properties. We also show how the detected size depends strongly on the employed surface brightness cutoff, and predict that further examples of such objects will be found in the near future.
ABSTRACT
Radiative mixing layers arise wherever multiphase gas, shear, and radiative cooling are present. Simulations show that in steady state, thermal advection from the hot phase balances ...radiative cooling. However, many features are puzzling. For instance, hot gas entrainment appears to be numerically converged despite the scale-free, fractal structure of such fronts being unresolved. Additionally, the hot gas heat flux has a characteristic velocity vin ≈ cs, cold(tcool/tsc, cold)−1/4 whose strength and scaling are not intuitive. We revisit these issues in 1D and 3D hydrodynamic simulations. We find that over-cooling only happens if numerical diffusion dominates thermal transport; convergence is still possible even when the Field length is unresolved. A deeper physical understanding of radiative fronts can be obtained by exploiting parallels between mixing layers and turbulent combustion, which has well-developed theory and abundant experimental data. A key parameter is the Damköhler number Da = τturb/tcool, the ratio of the outer eddy turnover time to the cooling time. Once Da > 1, the front fragments into a multiphase medium. Just as for scalar mixing, the eddy turnover time sets the mixing rate, independent of small scale diffusion. For this reason, thermal conduction often has limited impact. We show that vin and the effective emissivity can be understood in detail by adapting combustion theory scalings. Mean density and temperature profiles can also be reproduced remarkably well by mixing length theory. These results have implications for the structure and survival of cold gas in many settings, and resolution requirements for large scale galaxy simulations.
Abstract
We studied Lyman-
α
(Ly
α
) escape in a statistical sample of 43 Green Peas with
HST
/COS Ly
α
spectra. Green Peas are nearby star-forming galaxies with strong O
iii
λ
5007 emission lines. ...Our sample is four times larger than the previous sample and covers a much more complete range of Green Pea properties. We found that about two-thirds of Green Peas are strong Ly
α
line emitters with rest-frame Ly
α
equivalent width
. The Ly
α
profiles of Green Peas are diverse. The Ly
α
escape fraction, defined as the ratio of observed Ly
α
flux to intrinsic Ly
α
flux, shows anti-correlations with a few Ly
α
kinematic features—both the blue peak and red peak velocities, the peak separations, and the FWHM of the red portion of the Ly
α
profile. Using properties measured from Sloan Digital Sky Survey optical spectra, we found many correlations—the Ly
α
escape fraction generally increases at lower dust reddening, lower metallicity, lower stellar mass, and higher O
iii
/O
ii
ratio. We fit their Ly
α
profiles with the H
i
shell radiative transfer model and found that the Ly
α
escape fraction is anti-correlated with the best-fit
N
H
i
. Finally, we fit an empirical linear relation to predict
from the dust extinction and Ly
α
red peak velocity. The standard deviation of this relation is about 0.3 dex. This relation can be used to isolate the effect of intergalactic medium (IGM) scatterings from Ly
α
escape and to probe the IGM optical depth along the line of sight of each
Ly
α
emission-line galaxy in the
James Webb Space Telescope
era.
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