37 research outputs found
Turbulence comes in bursts in stably stratified flows
There is a clear distinction between simple laminar and complex turbulent
fluids. But in some cases, as for the nocturnal planetary boundary layer, a
stable and well-ordered flow can develop intense and sporadic bursts of
turbulent activity which disappear slowly in time. This phenomenon is
ill-understood and poorly modeled; and yet, it is central to our understanding
of weather and climate dynamics. We present here a simple model which shows
that in stably stratified turbulence, the stronger bursts can occur when the
flow is expected to be more stable. The bursts are generated by a rapid
non-linear amplification of energy stored in waves, and are associated with
energetic interchanges between vertical velocity and temperature (or density)
fluctuations. Direct numerical simulations on grids of 2048^3 points confirm
this somewhat paradoxical result of measurably stronger events for more stable
flows, displayed not only in the temperature and vertical velocity derivatives,
but also in the amplitude of the fields themselves
Helicity dynamics in stratified turbulence in the absence of forcing
A numerical study of decaying stably-stratified flows is performed.
Relatively high stratification and moderate Reynolds numbers are considered,
and a particular emphasis is placed on the role of helicity (velocity-vorticity
correlations). The problem is tackled by integrating the Boussinesq equations
in a periodic cubical domain using different initial conditions: a non-helical
Taylor-Green (TG) flow, a fully helical Beltrami (ABC) flow, and random flows
with a tunable helicity. We show that for stratified ABC flows helicity
undergoes a substantially slower decay than for unstratified ABC flows. This
fact is likely associated to the combined effect of stratification and large
scale coherent structures. Indeed, when the latter are missing, as in random
flows, helicity is rapidly destroyed by the onset of gravitational waves. A
type of large-scale dissipative "cyclostrophic" balance can be invoked to
explain this behavior. When helicity survives in the system it strongly affects
the temporal energy decay and the energy distribution among Fourier modes. We
discover in fact that the decay rate of energy for stratified helical flows is
much slower than for stratified non-helical flows and can be considered with a
phenomenological model in a way similar to what is done for unstratified
rotating flows. We also show that helicity, when strong, has a measurable
effect on the Fourier spectra, in particular at scales larger than the buoyancy
scale for which it displays a rather flat scaling associated with vertical
shear
A New Measurement of the Temperature Density Relation of the IGM From Voigt Profile Fitting
We decompose the Lyman-{\alpha} (Ly{\alpha}) forest of an extensive sample of
74 high signal-to-noise ratio and high-resolution quasar spectra into a
collection of Voigt profiles. Absorbers located near caustics in the peculiar
velocity field have the smallest Doppler parameters, resulting in a low-
cutoff in the - set by the thermal state of intergalactic
medium (IGM). We fit this cutoff as a function of redshift over the range
, which allows us to measure the evolution of the IGM
temperature-density () relation parameters
and . We calibrate our measurements against Ly forest
simulations, using 21 different thermal models of the IGM at each redshift,
also allowing for different values of the IGM pressure smoothing scale. We
adopt a forward-modeling approach and self-consistently apply the same
algorithms to both data and simulations, propagating both statistical and
modeling uncertainties via Monte Carlo. The redshift evolution of shows a
suggestive peak at , while our evolution of is consistent with
and disfavors inverted temperature-density relations. Our
measured evolution of and are generally in good agreement with
previous determinations in the literature. Both the peak in the evolution of
at , as well as the high temperatures K
that we observe at , strongly suggest that a significant episode
of heating occurred after the end of HI reionization, which was most likely the
cosmic reionization of HeII.Comment: Accepted for publication in ApJ, 23 pages, 26 figures, machine
readable tables available onlin
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A new measurement of the intergalactic temperature at z ∼ 2.55–2.95
We present two measurements of the temperature–density relationship (TDR) of the intergalactic medium (IGM) in the redshift range 2.55 < z < 2.95 using a sample of 13 high-quality quasar spectra and high resolution numerical simulations of the IGM. Our approach is based on fitting the neutral hydrogen column density
NHI
and the Doppler parameter b of the absorption lines in the Lyα forest. The first measurement is obtained using a novel Bayesian scheme that takes into account the statistical correlations between the parameters characterizing the lower cut-off of the
b--NHI
distribution and the power-law parameters T0 and γ describing the TDR. This approach yields T0/103 K = 15.6 ± 4.4 and γ = 1.45 ± 0.17 independent of the assumed pressure smoothing of the small-scale density field. In order to explore the information contained in the overall
b--NHI
distribution rather than only the lower cut-off, we obtain a second measurement based on a similar Bayesian analysis of the median Doppler parameter for separate column-density ranges of the absorbers. In this case, we obtain T0/103 K = 14.6 ± 3.7 and γ = 1.37 ± 0.17 in good agreement with the first measurement. Our Bayesian analysis reveals strong anticorrelations between the inferred T0 and γ for both methods as well as an anticorrelation of the inferred T0 and the pressure smoothing length for the second method, suggesting that the measurement accuracy can in the latter case be substantially increased if independent constraints on the smoothing are obtained. Our results are in good agreement with other recent measurements of the thermal state of the IGM probing similar (over-)density ranges.MH acknowledges support by ERC ADVANCED GRANT 320596 ‘The Emergence of Structure during the epoch of Reionization’. GDB was supported by the National Science Foundation through grant AST-1615814. JSB acknowledges the support of a Royal Society University Research Fellowship. MTM thanks the Australian Research Council for Discovery Project grant DP130100568. This work made use of the DiRAC High Performance Computing System (HPCS) and the COSMOS shared memory service at the University of Cambridge. These are operated on behalf of the STFC DiRAC HPC facility. This equipment is funded by BIS National E-infrastructure capital grant ST/J005673/1 and STFC grants ST/H008586/1, ST/K00333X/1
Probing the thermal state of the intergalactic medium at z > 5 with the transmission spikes in high-resolution Ly α forest spectra
We compare a sample of five high-resolution, high S/N Ly forest
spectra of bright QSOs aimed at spectrally resolving the
last remaining transmission spikes at with those obtained from mock
absorption spectra from the Sherwood and Sherwood-Relics suites of
hydrodynamical simulations of the intergalactic medium (IGM). We use a profile
fitting procedure for the inverted transmitted flux, , similar to the
widely used Voigt profile fitting of the transmitted flux at lower
redshifts, to characterise the transmission spikes that probe predominately
underdense regions of the IGM. We are able to reproduce the width and height
distributions of the transmission spikes, both with optically thin simulations
of the post-reionization Universe using a homogeneous UV background and full
radiative transfer simulations of a late reionization model. We find that the
width of the fitted components of the simulated transmission spikes is very
sensitive to the instantaneous temperature of the reionized IGM. The internal
structures of the spikes are more prominant in low temeperature models of the
IGM. The width distribution of the observed transmission spikes, which require
high spectral resolution ( 8 km/s) to be resolved, is reproduced for
optically thin simulations with a temperature at mean density of K at . This is weakly
dependent on the slope of the temperature-density relation, which is favoured
to be moderately steeper than isothermal. In the inhomogeneous, late
reionization, full radiative transfer simulations where islands of neutral
hydrogen persist to , the width distribution of the observed
transmission spikes is consistent with the range of caused by spatial
fluctuations in the temperature-density relation
Metals in the z ~ 3 intergalactic medium: Results from an ultra-high signal-to-noise ratio UVES quasar spectrum
In this work, we investigate the abundance and distribution of metals in the
intergalactic medium (IGM) at through the
analysis of an ultra-high signal-to-noise ratio UVES spectrum of the quasar
HE0940-1050. In the CIV forest, our deep spectrum is sensitive at
to lines with column density down to and in 60
per cent of the considered redshift range down to . In our sample,
all HI lines with show an associated CIV absorption.
In the range , 43 per cent of HI lines has an
associated CIV absorption. At , the detection rates
drop to per cent, possibly due to our sensitivity limits and not to an
actual variation of the gas abundance properties. In the range , we observe a fraction of HI lines with detected CIV a factor of 2
larger than the fraction of HI lines lying in the circum-galactic medium (CGM)
of relatively bright Lyman-break galaxies hosted by dark matter haloes with
M. The comparison of our results with
the output of a grid of photoionization models and of two cosmological
simulations implies that the volume filling factor of the IGM gas enriched to a
metallicity should be of the order of
percent. In conclusion, our results favour a scenario in which metals are found
also outside the CGM of bright star-forming galaxies, possibly due to pollution
by lower mass objects and/or to an early enrichment by the first sources.MV is supported by the ERC Starting Grant ‘cosmoIGM’ and PD51 INDARK grant. TSK acknowledges funding support from the ERC Starting Grant ‘cosmoIGM’, through grant GA-257670. MH was supported by the ERC Advanced Grant 320596 ‘The Emergence of Structure during the epoch of Reionisation’. PB is supported by the INAF PRIN-2014 grant ‘Windy black holes combing galaxy evolution’. Parts of this research were conducted by the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project number CE110001020.This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw216
Exploring the thermal state of the low-density intergalactic medium at = 3 with an ultrahigh signal-to-noise QSO spectrum
At low densities, the standard ionization history of the intergalactic medium (IGM) predicts a decreasing temperature of the IGM with decreasing density once hydrogen (and helium) reionization is complete. Heating the high-redshift, low-density IGM above the temperature expected from photoheating is difficult, and previous claims of high/rising temperatures in lowdensity regions of the Universe based on the probability density function (PDF) of the opacity in Ly forest data at 2 < < 4 have been met with considerable scepticism, particularly since they appear to be in tension with other constraints on the temperature–density relation (TDR). We utilize here an ultrahigh signal-to-noise spectrum of the Quasi-stellar object HE0940-1050 and a novel technique to study the low opacity part of the PDF. We show that there is indeed evidence (at 90 per cent confidence level) that a significant volume fraction of the underdense regions at ~ 3 has temperatures as high or higher than those at densities comparable to the mean and above. We further demonstrate that this conclusion is nevertheless consistent with measurements of a slope of the TDR in overdense regions that imply a decreasing temperature with decreasing density, as expected if photoheating of ionized hydrogen is the dominant heating process. We briefly discuss implications of our findings for the need to invoke either spatial temperature fluctuations, as expected during helium reionization, or additional processes that heat a significant volume fraction of the low-density IGM.We thank Volker Springel for making GADGET-3 available. This work made use of the Distributed Research utilising Advanced Computing High Performance Computing System (HPCS) and the COSMOlogy Supercomputer shared memory service at the University of Cambridge. These are operated on behalf of the Science and Technology Facilities Council (STFC) DiRAC HPC facility. This equipment is funded by Department for Business, Innovation and Skills National E-infrastructure capital grant ST/J005673/1 and STFC grants ST/H008586/1, ST/K00333X/1. We acknowledge Partnership for Advanced Computing in Europe for awarding us access to the Curie supercomputer, based in France at the Tres Grand Centre de Calcul (TGCC), through the 8th regular call. Support by the European Research Council Advanced Grant 320596 ‘The Emergence of structure during the epoch of reionization’ is gratefully acknowledged. ET is supported by the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project number CE110001020. AR thanks Joseph F. Hennawi and the ENIGMA group at the Max Planck institute for Astronomy for helpful comments and discussion. MV and TSK acknowledges funding support to the European Research Council Starting Grant ‘Cosmology with the IGM’ through grant GA-257670. PB is supported by the Istituto Nazionale di Astrofisica PRIN-2014 grant ”Windy black holes combing galaxy evolution”
Metals in the z ∼ 3 intergalactic medium: results from an ultra-high signal-to-noise ratio UVES quasar spectrum
In this work, we investigate the abundance and distribution of metals in the intergalactic medium (IGM) at 〈z〉 ≃ 2.8 through the analysis of an ultra-high signal-to-noise ratio UVES spectrum of the quasar HE0940-1050. In the C IV forest, our deep spectrum is sensitive at 3σ to lines with column density down to log NCIV ≃ 11.4 and in 60 per cent of the considered redshift range down to ≃11.1. In our sample, all H I lines with log NHI ≥ 14.8 show an associated C IV absorption. In the range 14.0 ≤ log NHI < 14.8, 43 per cent of H I lines has an associated C IV absorption. At log NHI < 14.0, the detection rates drop to <10 per cent, possibly due to our sensitivity limits and not to an actual variation of the gas abundance properties. In the range log NHI ≥ 14, we observe a fraction of H I lines with detected C IV a factor of 2 larger than the fraction of H I lines lying in the circumgalactic medium (CGM) of relatively bright Lyman-break galaxies hosted by dark matter haloes with 〈M〉 ∼ 1012 M⊙. The comparison of our results with the output of a grid of photoionization models and of two cosmological simulations implies that the volume filling factor of the IGM gas enriched to a metallicity logZ/Z⊙≳−3 logZ/Z⊙≳−3 should be of the order of ∼10-13 per cent. In conclusion, our results favour a scenario in which metals are found also outside the CGM of bright star-forming galaxies, possibly due to pollution by lower mass objects and/or to an early enrichment by the first sources
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Research data supporting 'Measurement of the Small-Scale Structure of the Intergalactic Medium Using Close Quasar Pairs'
These data includes observations conducted with the MagE spectrograph at 'Las Campanas' observatory, in April 2008 and March 2009. They were obtained targeting quasar pairs separated by small angles in the sky and are not publicly available in other archives. These quasar pair spectra are part of a larger dataset used to measure the transverse coherence in the absorption due to the effect of thermal pressure in the intergalactic medium of the ancient universe.
The repository contains both science and calibration frames, as marked in the header of each files. More information about the targets and the observing run can be found in Prochaska, Hennawi et al,2013 ApJ, 776, 13