Power Spectrum Of Intensity Fluctuations Research Articles

Precision tests of CO and [CII] power spectra models against simulated intensity maps

Line intensity mapping (LIM) is an emerging technique with a unique potential to probe a wide range of scales and redshifts. Realizing the full potential of LIM, however, relies on accurate modeling of the signal. We introduce an extended halo model for the power spectrum of intensity fluctuations of CO rotational lines and [CII] fine transition line in real space, modeling nonlinearities in matter fluctuations and biasing relation between the line intensity fluctuations and the underlying dark matter distribution. We also compute the stochastic contributions beyond the Poisson approximation using the halo model framework. To establish the accuracy of the model, we create the first cosmological-scale simulations of CO and [CII] intensity maps, MithraLIMSims, at redshifts 0.5 ≤ z≤6, using halo catalogs from Hidden-Valley simulations, and painting halos according to mass-redshift-luminosity relations for each line. We show that at z=1 on scales kmax≲ 0.8 Mpc-11h, the model predictions of clustering power (with only two free parameters) are in agreement with the measured power spectrum at better than 5%. At higher redshift of z=4.5, this remarkable agreement extends to smaller scale of kmax≲ 2 Mpc-11h. Furthermore, we show that on large scales, the stochastic contributions to CO and CII power spectra are non-Poissonian, with amplitudes reproduced reasonably well by the halo model prescription. Lastly, we assess the performance of the theoretical model of the baryon acoustic oscillations (BAO) and show that hypothetical LIM surveys probing CO lines at z=1, that can be deployed within this decade, will be able to make a high significance measurement of the BAO. On a longer time scale, a space-based mission probing [CII] line can uniquely measure the BAO on a wide range of redshifts at an unprecedented precision.

Angular power spectrum of supernova remnants: effects of structure, geometry and diffuse foreground

Abstract The study of the intensity fluctuation power spectrum of individual supernova remnants (SNRs) can reveal the structures present at sub-pc scales, and also constrain the physical process that generates those structures. There are various effects, such as the remnant shell thickness, projection of a three-dimensional structure onto a two-dimensional observational plane, and the presence of diffuse “foreground” emission, which causes the observed power spectrum to deviate from the intrinsic power spectrum of the fluctuations. Here, we report results from a systematic study of these effects, using direct numerical simulations, in the measured power spectrum. For an input power-law power spectrum, independent of the power-law index, we see a break in the observed power law at a scale which depends on the shell thickness of a shell-type SNR, and the three-dimensional turbulence changes to two-dimensional turbulence beyond that scale. We also report how the estimated power spectrum is expected to deviate from the intrinsic SNR power spectrum in the presence of additional diffuse Galactic synchrotron emission (DGSE) around the remnant shell. For a reasonable choice of the parameters, if the intrinsic SNR power spectrum is shallower than the DGSE power spectrum, the SNR contribution dominates at small angular scales of the estimated power spectra. On the other hand, if the SNR power spectrum is relatively steeper, the original power spectra is recovered only over a small window of angular scales. This study shows how detailed modeling may be used to infer the true power spectrum from the observed SNR intensity fluctuations power spectrum, which in turn can be used to constrain the nature of the turbulence that gives rise to these small scale structures.

Turbulent power spectrum in warm and cold neutral medium using the Galactic H i21 cm emission

Small-scale fluctuations of different tracers of the interstellar the medium can be used to study the nature of turbulence in astrophysical scales. Of these, the `continuum' emission traces the fluctuations integrated along the line of sight whereas, the spectral line tracers give the information along different velocity channels as well. Recently, Miville-Desch\^enes et al. (2016) have measured the intensity fluctuation power spectrum of the continuum dust emission, and found a power law behaviour with a power law index of $-2.9 \pm 0.1$ for a region of our Galaxy. Here, we study the same region using high-velocity resolution 21-cm emission from the diffuse neutral medium, and estimate the power spectrum at different spectral channels. The measured 21-cm power spectrum also follows a power law, however, we see a significant variation in the power law index with velocity. The value of the power-law index estimated from the integrated map for different components are quite different which is indicative of the different nature of turbulence depending on temperature, density and ionization fraction. We also measure the power spectra after smoothing the 21 cm emission to velocity resolution ranging from $1.03$ to $13.39~{\rm km~s^{-1}}$, but the power spectrum remains unchanged within the error bar. This indicates that the observed fluctuations are dominantly due to density fluctuations, and we can only constrain the power-law index of velocity structure function of $0.0 \pm 1.1$ which is consistent with the predicted Kolmogorov turbulence $(\gamma=2/3)$ and also with a shock-dominated medium $(\gamma=1.0)$.

Extragalactic background light measurements and applications.

This review covers the measurements related to the extragalactic background light intensity from γ-rays to radio in the electromagnetic spectrum over 20 decades in wavelength. The cosmic microwave background (CMB) remains the best measured spectrum with an accuracy better than 1%. The measurements related to the cosmic optical background (COB), centred at 1 μm, are impacted by the large zodiacal light associated with interplanetary dust in the inner Solar System. The best measurements of COB come from an indirect technique involving γ-ray spectra of bright blazars with an absorption feature resulting from pair-production off of COB photons. The cosmic infrared background (CIB) peaking at around 100 μm established an energetically important background with an intensity comparable to the optical background. This discovery paved the way for large aperture far-infrared and sub-millimetre observations resulting in the discovery of dusty, starbursting galaxies. Their role in galaxy formation and evolution remains an active area of research in modern-day astrophysics. The extreme UV (EUV) background remains mostly unexplored and will be a challenge to measure due to the high Galactic background and absorption of extragalactic photons by the intergalactic medium at these EUV/soft X-ray energies. We also summarize our understanding of the spatial anisotropies and angular power spectra of intensity fluctuations. We motivate a precise direct measurement of the COB between 0.1 and 5 μm using a small aperture telescope observing either from the outer Solar System, at distances of 5 AU or more, or out of the ecliptic plane. Other future applications include improving our understanding of the background at TeV energies and spectral distortions of CMB and CIB.

Comparison of Solar Wind Speeds Using Wavelet Transform and Fourier Analysis in IPS Data

The power spectra of intensity fluctuations in interplanetary scintillation (IPS) observations can be used to estimate solar-wind speeds in the inner heliosphere. We obtain and then compare IPS spectra from both wavelet and Fourier analyses for 12 time series of the radio source 3C48; these observations were carried out at Japan’s Solar-Terrestrial Environment Laboratory (STEL) facility, at 327 MHz. We show that wavelet and Fourier analyses yield very similar power spectra. Thus, when fitting a model to spectra to determine solar-wind speeds, both yield comparable results. Although spectra from wavelet and Fourier closely match each other for solar-wind speed purposes, those from the wavelet analysis are slightly cleaner, which is reflected in an apparent level of intensity fluctuations that is enhanced, being ≈ 13 % higher. This is potentially useful for records that show a low signal-to-noise ratio.

Effect of turbulent velocity on the H i intensity fluctuation power spectrum from spiral galaxies

We use numerical simulations to investigate effect of turbulent velocity on the power spectrum of \HI intensity from external galaxies when (a) all emission is considered, (b) emission with velocity range smaller than the turbulent velocity dispersion is considered. We found that for case (a) the intensity fluctuation depends directly only on the power spectrum of the column density, whereas for case (b) it depends only on the turbulent velocity fluctuation. We discuss the implications of this result in real observations of \HI fluctuations.

Theoretical and observational constraints on the H i intensity power spectrum

Mapping of the neutral hydrogen (HI) 21-cm intensity fluctuations across redshifts promises a novel and powerful probe of cosmology. The neutral hydrogen gas mass density, $\Omega_{\rm HI}$ and bias parameter, $b_{\rm HI}$ are key astrophysical inputs to the HI intensity fluctuation power spectrum. We compile the latest theoretical and observational constraints on $\Omega_{\rm HI}$ and $b_{\rm HI}$ at various redshifts in the post-reionization universe. Constraints are incorporated from galaxy surveys, HI intensity mapping experiments, damped Lyman-$\alpha$ system observations, theoretical prescriptions for assigning HI to dark matter halos, and the results of numerical simulations. Using a minimum variance interpolation scheme, we obtain the predicted uncertainties on the HI intensity fluctuation power spectrum across redshifts 0-3.5 for three different confidence scenarios. We provide a convenient tabular form for the interpolated values of $\Omega_{\rm HI}$, $b_{\rm HI}$ and the HI power spectrum amplitude and their uncertainties. We discuss the consequences for the measurement of the power spectrum by current and future intensity mapping experiments.

The contribution of star-forming galaxies to fluctuations in the cosmic background light

Star-forming galaxies which are too faint to be detected individually produce intensity fluctuations in the cosmic background light. This contribution needs to be taken into account as a foreground when using the primordial signal to constrain cosmological parameters. The extragalactic fluctuations are also interesting in their own right as they depend on the star formation history of the Universe and the way in which this connects with the formation of cosmic structure. We present a new framework which allows us to predict the occupation of dark matter haloes by star-forming galaxies and uses this information, in conjunction with an N-body simulation of structure formation, to predict the power spectrum of intensity fluctuations in the infrared background. We compute the emission from galaxies at far-infrared, millimetre and radio wavelengths. Our method gives accurate predictions for the clustering of galaxies for both the one-halo and two-halo terms. We illustrate our new framework using a previously published model which reproduces the number counts and redshift distribution of galaxies selected by their emission at m. Without adjusting any of the model parameters, the predictions show encouraging agreement at high frequencies and on small angular scales with recent estimates of the extragalactic fluctuations in the background made from early data analysed by the Planck Collaboration. There are, however, substantial discrepancies between the model predictions and observations on large angular scales and at low frequencies, which illustrates the usefulness of the intensity fluctuations as a constraint on galaxy formation models.

Probing interstellar turbulence in spiral galaxies using H i power spectrum analysis

We estimate the H i intensity fluctuation power spectrum for a sample of 18 spiral galaxies chosen from THINGS. Our analysis spans a large range of length-scales from ∼300pc to ∼16kpc across the entire galaxy sample. We find that the power spectrum of each galaxy can be well fitted by a power law PHI(U)=AUα, with an index α that varies from galaxy to galaxy. For some of the galaxies the scale-invariant power-law power spectrum extends to length-scales that are comparable to the size of the galaxy’s disk. The distribution of α is strongly peaked with 50% of the values in the range α=-1.9 to 1.5, and a mean and standard deviation of −1.3 and 0.5 respectively. We find no significant correlation between α and the star formation rate, dynamical mass, H i mass or velocity dispersion of the galaxies.Several earlier studies that have measured the power spectrum within our galaxy on length-scales that are considerably smaller than 500pc have found a power-law power spectrum with α in the range ≈-2.8 to −2.5. We propose a picture where we interpret the values in the range ≈-2.8 to −2.5 as arising from three dimensional (3D) turbulence in the interstellar medium (ISM) on length-scales smaller than the galaxy’s scale-height, and we interpret the values in the range ≈-1.9 to −1.5 measured in this paper as arising from two-dimensional ISM turbulence in the plane of the galaxy’s disk. It however still remains a difficulty to explain the small galaxy to galaxy variations in the values of α measured here.

Relation between the contrast in time integrated dynamic speckle patterns an the power spectral density of their temporal intensity fluctuations

Scattering fluid flux can be quantified with coherent light, either from the contrast of speckle patterns, or from the moments of the power spectrum of intensity fluctuations. We present a theory connecting these approaches for the general case of mixed static-dynamic patterns of boiling speckles without prior assumptions regarding the particle dynamics. An expression is derived and tested relating the speckle contrast to the intensity power spectrum. Our theory demonstrates that in speckle contrast the concentration of moving particles dominates over the contribution of speed to the particle flux. Our theory provides a basis for comparison of both approaches when used for studying tissue perfusion.

Angular Anisotropies in the Cosmic Gamma-Ray Background as a Probe of Its Origin

Notwithstanding the advent of the Gamma-ray Large Area Space Telescope, theoretical models predict that a significant fraction of the cosmic γ-ray background (CGB), at a level of 20% of the currently measured value, will remain unresolved. The angular power spectrum of intensity fluctuations of the CGB contains information on its origin. We show that probing the latter on scales from a few tens of arcminutes to several degrees, together with complementary GLAST observations of γ-ray emission from galaxy clusters and the blazar luminosity function, can discriminate between a background that originates from unresolved blazars or cosmic rays accelerated at structure formation shocks.

Scintillation Caustics in Planetary Occultation Light Curves

We revisit the GSC 5249-01240 light curve obtained during its occultation by Saturn's north polar region. In addition to refractive scintillations, the power spectrum of intensity fluctuations shows an enhancement of power between refractive and diffractive regimes. We identify this excess power as due to high-amplitude spikes in the light curve and suggest that these spikes are due to caustics associated with ray-crossing situations. The flux variation in individual spikes follows the expected caustic behavior including diffraction fringes, which we have observed for the first time in a planetary occultation light curve. The presence of caustics in scintillation light curves requires an inner scale cutoff to the power spectrum of underlying density fluctuations associated with turbulence. Another possibility is the presence of gravity waves in the atmosphere. While occultation light curves previously showed the existence of refractive scintillations, a combination of small projected stellar size and a low relative velocity during the event have allowed us to identify caustics in this occultation. This has led us to reexamine previous data sets, in which we have also found likely examples of caustics.

Use of diffusion wave spectroscopy in diagnostics of blood

The method of diffusion-wave spectroscopy based on the analysis of fragments of temporal autocorrelation functions or power spectra of intensity fluctuations of scattered coherent laser radiation is considered as applied to noninvasive in vitro diagnostics of blood. The results of experiments on noninvasive diagnostics of blood in vitro are interpreted within the framework of the theory of diffusion wave spectroscopy. The proposed method can be recommended for laboratory and clinical investigations of formed blood elements.

Power spectra of VHF intensity scintillations from F2- and E-region ionospheric irregularities

An experiment is described for the routine study of scintillations and ionospheric irregularities at high-latitudes using NNSS satellites with additional coordinated observations by means of the EISCAT ionospheric radar facility. Early results, obtained during the development phase of the experiment, are presented of the power spectra of intensity fluctuations at 150 MHz observed at the equatorwards edge of the high-latitude irregularity zone. The spectra of 165 samples of night-time scintillation recorded during October 1982 to May 1983 show a spectral index with a mean value of −3.58 and a steepening of the spectral slope with increasing S 4. Some examples of scintillation arising from irregularities at E-layer height show spectral indices of magnitude generally smaller than for F-region cases. A few spectra have been found with a clear break in spectral slope at around 10 Hz, suggesting two regimes for irregularities of different scale sizes.

Theory of refractive scattering in scintillation phenomena

For a plane wave incident upon a thin plane one-dimensional phase-changing screen that has an outer scale L o , an inner scale L i and an intervening inverse power-law spectrum (spectral index p), calculations are made of the spatial power spectrum of intensity fluctuations in a reception plane parallel to the screen. The outer scale is taken to be large compared with the Fresnel scale F, and results are obtained for a range of values of the spectral index and of the mean square fluctuation of phase (ΔΦ) 2 . For large values of (ΔΦ) 2 , lens action occurs in the screen, producing focal action in the reception plane. A lens scale F [2 (ΔΦ) 2 ] 1 4 is defined, as well as a focal scale l that varies from L o [(ΔΦ) 2] −1 2 at p = 5 to L o[2 (ΔΦ) 2 ] −1 at p = 2. When refractive scattering dominates, l −1 is the upper roll-off angular spatial frequency for intensity fluctuations in the reception plane whether this is the focal plane or not. The lower roll-off angular spatial frequency occurs at a scale such that its geometric mean with the focal scale is equal to the Fresnel scale. Refractive scattering dominates even for weak scattering if p ~ 5 or p > 5. For lower values of p, diffractive scattering dominates unless the lens scale exceeds the focal scale. Refractive scattering dominates when the focal scale is small compared with the lens scale. Saturation of the intensity spectrum occurs when the focal scale is small compared with the Fresnel scale. The scintillation index tends to unity as (ΔΦ) 2 → ∞ for all p. The turbulence parameter approximation is satisfactory for a range of spectral indices near 2, but not for p ~ 3 and p > 3.

A survey of clear-air propagation effects relevant to optical communications

The theory and observations of the optical propagation effects of the clear turbulent atmosphere are reviewed, with particular attention to those characteristics most important to the designer of an optical communication system. Among the phenomena considered are the variance, probability distribution, spatial covariance, aperture smoothing, and temporal power spectrum of intensity fluctuations, and similar quantities for phase fluctuations and angle of arrival.