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NASA NTRS1994

Angular power spectrum of the microwave background anisotropy seen by the COBE differential microwave radiometer

The angular power spectrum estimator developed by Peebles (1973) and Hauser & Peebles (1973) has been modified and applied to the 2 yr maps produced by the Cosmic Background Explorer Satellite Differential Microwave Radiometer (COBE DMR)). The power spectrum of the real sky has been compared to the power spectra of a large number of simulated random skies produced with noise equal to the observed noise and primordial density fluctuation power spectra of power-law form, with P(k) proportional to k(exp n). Within the limited range of spatial scales covered by the COBE DMR, corresponding to spherical harmonic indices 3 less than or = l is less than or approximately = 30, the best-fitting value of the spectral index is n = 1.25(sup +0.39 sub -0.44) with the Harrisson-Zel'dovich value n = 1 approximately 0.5 sigma below the best fit. For 3 less than or = l less than or approximately = 19, the best fit is n = 1.46(sup +0.39 sub -0.44). Comparing the COBE DMR delta-T/T at small l to the delta-T/T at l approximately = 50 from degree scale anisotropy experiments gives a smaller range of acceptable spectral indices which includes n = 1.

Wright, E. L.↗
NASA NTRS1994

Correlated noise in the COBE DMR sky maps

The Cosmic Background Explorer Satellite Differential Radiometer (COBE DMR) sky maps contain low-level correlated noise. We obtain estimates of the amplitude and pattern of the correlated noise from three techniques: angular averages of the covariance matrix, Monte Carlo simulations of two-point correlation functions and direct analysis of the DMR maps. The results from the three methods are mutually consistent. The noise covariance matrix of a DMR sky maps is diagonal to an accuracy of better than 1%. For a given sky pixel, the dominant noise covariance occure with the ring of pixels at an angular separation of 60 deg due to the 60 deg separation of the DMR horns. The mean covariance at 60 deg is 0.45%((sup +0.18)(sub -0.14)) of the mean variance. Additionally, the variance in a given pixel is 0.7% greater than would be expected from a single beam experiment with the same noise properties. Autocorrelation functions suffer from a approximately 1.5 sigma positive bias at 60 deg while cross-correlations have no bias. Published COBE DMR results are not significantly affected by correlated noise.

Lineweaver, C. H.↗
NASA NTRS1994

Large-scale structure effects on the gravitational lens image positions and time delay

We compute the fluctuations in gravitational lens image positions and time delay caused by large-scale structure correlations. We show that these fluctuations can be expressed as a simple integral over the density power spectrum. Using the Cosmic Background Explorer (COBE) normalization we find that positions of objects at cosmological distances are expected to deviate from their true positions by few arcminutes. These deflections are not directly observable. The positions of the images relative to one another fluctuate by a few percent of the relative separation, implying that one does not expect multiple images to be produced by large-scale structure. Nevertheless, the fluctuations are larger than the observational errors on the positions and affect reconstructions of the lens potential. The time delay fluctuations have a geometrical and a gravitational contribution. Both are much larger than the expected time delay from the primary lens, but partially cancel each other. We find that large-scale structure weakly affects the time delay and time delay measurements can be used as a probe of the distance scale in the universe.

Seljak, Uros↗
NASA NTRS1994

On the determination of the cosmic infrared background radiation from the high-energy spectrum of extragalactic gamma-ray sources

In a recent paper Stecker, De Jager, & Salamon have suggested using the observed approximately MeV to TeV spectra of extragalactic gamma-ray sources as probes of the local density of the cosmic infrared background radiation (CIBR) and have subsequently claimed a first possible measurement of the CIBR from the analysis of the gamma-ray spectrum of Mrk 421 (De Jager, Stecker, & Salamon). The CIBR from normal galaxies consists of two components: a stellar emission component (CIBRs), and a thermal dust emission component (CIBRd). Photons with energies in the approximately 0.1-2 TeV range interact primarily with the CIBRs, whereas interactions with CIBRd dominate the absorption of photons in the approximately 2-100 TeV energy range. SDS 92 and DSS94 considered only the interaction of the gamma-rays with the dust emission component of the CIBR. We present here an improved analysis of the absorption of extragalactic TeV gamma rays by the CIBR, taking the dual nature of its origin into account. Applying the analysis to the observed gamma-ray spectrum of Mrk 421, a BL Lac object at z = 0.031, we find agreement with DSS94 tentative evidence for absorption by the CINRs. Our analysis therefore limits the detection of the CIBR to the approximately 15-40 micron wavelength regime which, considering the uncertainties in the highest energy (greater than 4 TeV) data and ion the possibility of absorption inside the source, many turn out to be an upper limit on its energy density. At shorter wavelengths (lambda approximately = 1-15 microns), where the gamma-ray interactions are dominated by the CIBRs, our analysis definitely yields only an upper limit on the energy density of the CIBR. In contrast, DSS94 have claimed a possible first measurement of the CIBR over the entire 1-120 micron wavelength region. The upper limit on the CIBRs and tentative detection of the CIBRd are consistent with normal galaxies contributing most of the energy to the CIBR, and constrain the contribution of some exotic sources. With careful modeling of infrared foreground emissions, these constraints on the CIBR are above the values measurable by the DIRBE experiment on board the Cosmic Background Explorer (COBE) satellite.

Dwek, Eli↗
NASA NTRS1994

Compton scattering of the microwave background by quasar-blown bubbles

At least 10% of quasars drive rapid outflows from the central regions of their host galaxies. The mass and energy flow rates in these winds are difficult to measure, but their kinetic luminosities probably exceed 10(exp 45) ergs/s. This kind of outflow easily sunders the interstellar medium of the host and blows a bubble in the intergalactic medium. After the quasar shuts off, the hot bubble continues to shock intergalactic gas until its leading edge merges with the Hubble flow. The interior hot gas Compton scatters microwave background photons, potentially providing a way to detect these bubbles. Assuming that quasar kinetic luminosities scale with their blue luminosities, we integrate over the quasar luminosity function to find the total distortion (y) of the microwave background produced by the entire population of quasar wind bubbles. This calculation of y distortion is remarkably insensitive to the properties of the intergalactic medium (IGM), quasar lifetimes, and cosmological parameters. Current Cosmic Background Explorer (COBE) limits on y constrain the kinetic luminosities of quasars to be less than several times their bolometric radiative luminosities. Within this constraint, quasars can still expel enough kinetic luminosity to shock the entire IGM by z = 0, but cannot heat and ionize the IGM by z = 4 unless omega(sub IGM) much less than 10(exp -2).

Voit, G. Mark↗
NASA NTRS1994

Infrared observations of Comet Austin (1990 V) by the COBE/Diffuse Infrared Background Experiment

Comet Austin was observed by the Cosmic Background Explorer (COBE)/Diffuse Infrared Background Experiment (DIRBE) with broadband photometry at 1-240 micrometers during the comet's close passage by Earth in 1990 May. A 6 deg long (6 x 10(exp 6) km) dust tail was found at 12 and 25 micrometers, with detailed structure due to variations in particle properties and mass-loss rate. The spectrum of the central 42 x 42 sq arcmin pixel was found to agree with that of a graybody of temperature 309 +/- 5 K and optical depth 7.3 +/- 10(exp -8). Comparison with IUE and ground-based obervations indicates that particles of radius greater than 20 micrometers predominate by surface area. A mass-loss rate of 510 (+510/-205) kg/s and a total tail mass of 7 +/- 2 x 10(exp 10) kg was found for a model dust tail composed of Mie spheres with a differential particle mass distribution dn/d log m approx. m(exp -0.63) and 2:1 silicate:amorphous carbon composition by mass.

Lisse, C. M.↗
NASA NTRS1994

Measurement of the cosmic microwave background spectrum by the COBE FIRAS instrument

The cosmic microwave background radiation (CMBR) has a blackbody spectrum within 3.4 x 10(exp -8) ergs/sq cm/s/sr cm over the frequency range from 2 to 20/cm (5-0.5 mm). These measurements, derived from the Far-Infrared Absolute Spectrophotomer (FIRAS) instrument on the Cosmic Background Explorer (COBE) satellite, imply stringent limits on energy release in the early universe after t approximately 1 year and redshift z approximately 3 x 10(exp 6). The deviations are less than 0.30% of the peak brightness, with an rms value of 0.01%, and the dimensionless cosmological distortion parameters are limited to the absolute value of y is less than 2.5 x 10(exp -5) and the absolute value of mu is less than 3.3 x 10(exp -4) (95% confidence level). The temperature of the CMBR is 2.726 +/- 0.010 K (95% confidence level systematic).

Mather, J. C.↗
NASA NTRS1994

Microwave background fluctuations due to the Sunyaev-Zel'dovich effects in pancakes

We calculate distortions in the microwave background radiation from the Sunyaev-Zel'dovich effect, produced by hot gas in large (approximately 100 Mpc) pancakes. The large-scale distribution of the pancakes is taken to be that of a Voronoi foam. Fluctuations for this scenario are estimated to be on the order of delta T/T is approximately 10(exp -5). Using computer simulations, we produce several 32 deg x 32 deg images with 0.25 deg resolution. These images show characteristic linear features produced when a pancake is viewed nearly edge-on. By calculating the two-point and the degenerate three-point correlation functions, we are able to statistically detect such non-Gaussian features even in the presence of a relatively large amount of Gaussian noise. The degenerate three-point correlation function is found to be particularly useful since it is insensitive to correlated Gaussian noise. We also smooth our data over a 7 deg Full Width at Half Maximum (FWHM) Gaussian window to simulate the Cosmic Background Explorer Satellite (COBE) observations. We find that under such low-resolution conditions, the features are highly suppressed.

Subbarao, M. U.↗
NASA NTRS1994

Amplitude of primeval fluctuations from cosmological mass density reconstructions

We use the POTENT reconstruction of the mass density field in the nearby universe to estimate the amplitude of the density fluctuation power spectrum for various cosmological models. We find that sigma(sub 8) Omega(sub m sup 0.6) = 1.3(sub -0.3 sup +0.4), almost independently of the power spectrum. This value agrees well with the Cosmic Background Explorer (COBE) normalization for the standard cold dark matter model, while alternative models predict an excessive amplitude compared with COBE. Flat, low Omega(sub m) models and tilted models with spectral index n less than 0.8 are particularly discordant.

Seljak, Uros↗
NASA NTRS1994

The photoelectric heating mechanism for very small graphitic grains and polycyclic aromatic hydrocarbons

We have theoretically modeled the gas heating associated with the photoelectric ejection of electrons from a size distribution of interstellar carbon grains which extends into the molecular domain. We have considered a wide range of physical conditions for the interstellar gas (1 less than G(sub 0) less than 10(exp 5), with G(sub 0) being the intensity of the incident far-UV field in units of the Habing interstellar radiation field; 2.5 x 10( exp -3) less than n(sub e) less than 75/cu cm, with n(sub e) being the electron density; 10 less than T less than 10,000 K, with T being the gas temperature). The results show that about half of the heating is due to grains less than 1500 C atoms (less than 15 A). The other half originates in somewhat larger grains (1500-4.5 x 10(exp 5) C atoms; 15 less than 100 A). While grains larger than this do absorb about half of the available far-UV photons, they do not contribute appreciably to the gas heating. This strong dependence of gas heating on size results from the decrease in yield and from the increased grain charge (hence larger Coulomb losses) with increasing grain size. We have determined the net photoelectric heating rate and evaluated a simple analytical expression for the heating efficiency, dependent only on G(sub 0), T, and n(sub e). This expression is accurate to 3% over the whole parameter range and is valid up to gas temperatures of 10(exp 4) K, at which point the dominant gas-dust heat exchange mechanism becomes the recombination of electrons with grains rather than photoelectric ejection. The calculated heating efficiency for neutral grains is in good agreement with that derived from observations of the diffuse interstellar clouds. Our results also agree well with the Far Infrared Absolute Spectrometer (FIRAS) observations on the Cosmic Background Explorer Satellite. Finally, our photoelectric heating efficiency is compared to previous studies.

Bakes, E. L. O.↗
NASA NTRS1994

Bias and high-order galaxy correlation functions in the APM galaxy survey

On large scales, the higher order moments of the mass distribution, S(sub J) = bar-zeta(sub J)/bar-zeta(sup J-1)(sub 2), e.g., the skewness S(sub 3) and kurtosis S(sub 4), can be predicted using nonlinear perturbation theory. Comparison of these predictions with moments of the observed galaxy distribution probes the bias between galaxies and mass. Applying this method to models with initially Gaussian fluctuations and power spectra P(k) similar to that of galaxies in the Automatic Plate Measuring (APM) survey, we find that the predicted higher order moments S(sub J)(R) are in good agreement with those directly inferred from the APM survey in the absence of bias. We use this result to place limits on the linear and nonlinear bias parameters. Models in which the extra power observed on large scales (with respect to the standard cold dark matter (CDM) model) is produced by scale-dependent bias match the APM higher order amplitudes only if nonlinear bias (rather than nonlinear gravity) generates the observed higher order moments. When normalized to Cosmic Background Explorer Differential Microwave Radiometer (COBE DMR), these models are siginificantly ruled out by the S(sub 3) observations. The cold plus hot dark matter model normalized to COBE can reproduce the APM higher order correlations if one introduces nonlinear bias terms, while the low-density CDM model with a cosmological constant does not require any bias to fit the large-scale amplitudes.

Gaztanaga, Enrique↗
NASA NTRS1994

Velocity correlations of galaxy clusters

We determine the velocity correlation function, pairwise peculiar velocity difference, and rms pairwise peculiar velocity dispersion of rich clusters of galaxies, as a function of pair separation, for three cosmological models: Omega = 1 and Omega = 0.3 cold dark matter (CDM), and Omega = 0.3 primeval baryonic isocurvature (PBI) models (all flat and Cosmic Background Explorer (COBE)-normalized). We find that close cluster pairs, with separation r is less than or equal to 10/h Mpc, exhibit strong attractive peculiar velocities in all models; the cluster pairwise velocities depend sensitively on the model. The mean pairwise attractive velocity of clusters on 5/h Mpc scale ranges from approximately 1700 km/s for Omega = 1 CDM to approximately 1000 km/s for PBI to approximately 700 km/s for Omega = 0.3 CDM. The small-scale pairwise velocities depend also on cluster mass: richer, more massive clusters exhibit stronger attractive velocities than less massive clusters. On large scales, from approximately 20 to 200/h Mpc, the cluster peculiar velocities are increasingly dominated by bulk and random motions; they are independent of cluster mass. The cluster velocity correlation function is negative on small scales for Omega = 1 and Omega = 0.3 CDM, indicating strong pairwise motion relative to bulk motion on small scales; PBI exhibits relatively larger bulk motions. The cluster velocity correlation function is positive on very large scales, from r approximately 10/h Mpc to r approximately 200/h Mpc, for all models. These positive correlations, which decrease monotonically with scale, indicate significant bulk motions of clusters up to approximately 200/h Mpc. The strong dependence of the cluster velocity functions on models, especially at small separations, makes them useful tools in constraining cosmological models when compared with observations.

Cen, Renyue↗
NASA NTRS1994

Large-angle cosmic microwave background anisotropies in an open universe

If the universe is open, scales larger than the curvature scale may be probed by observation of large-angle fluctuations in the cosmic microwave background (CMB). We consider primordial adiabatic perturbations and discuss power spectra that are power laws in volume, wavelength, and eigenvalue of the Laplace operator. Such spectra may have arisen if, for example, the universe underwent a period of `frustated' inflation. The resulting large-angle anisotropies of the CMB are computed. The amplitude generally increases as Omega is decreased but decreases as h is increased. Interestingly enough, for all three Ansaetze, anisotropies on angular scales larger than the curvature scale are suppressed relative to the anisotropies on scales smaller than the curvature scale, but cosmic variance makes discrimination between various models difficult. Models with 0.2 approximately less than Omega h approximately less than 0.3 appear compatible with CMB fluctuations detected by Cosmic Background Explorer Satellite (COBE) and the Tenerife experiment and with the amplitude and spectrum of fluctuations of galaxy counts in the APM, CfA, and 1.2 Jy IRAS surveys. COBE normalization for these models yields sigma(sub 8) approximately = 0.5 - 0.7. Models with smaller values of Omega h when normalized to COBE require bias factors in excess of 2 to be compatible with the observed galaxy counts on the 8/h Mpc scale. Requiring that the age of the universe exceed 10 Gyr implies that Omega approximately greater than 0.25, while requiring that from the last-scattering term in the Sachs-Wolfe formula, large-angle anisotropies come primarily from the decay of potential fluctuations at z approximately less than 1/Omega. Thus, if the universe is open, COBE has been detecting temperature fluctuations produced at moderate redshift rather than at z approximately 1300.

Kamionkowski, Marc↗
NASA NTRS1994

Analysis of small-scale microwave background radiation anisotropy in the presence of foreground contamination

Many of the current round of experiments searching for anisotropies in the microwave background radiation (MBR) are confronting the problem of how to disentangle the cosmic signal from contamination due to Galactic and intergalactic foreground sources. Here we show how commonly used likelihood function techniques can be generalized to account for foreground. Specifically we set some restrictions on the spectrum of foreground contamination but allow the amplitude to vary arbitrarily. The likelihood function thus generalized gives reasonable limits on the MBR anisotropy which, in some cases, are not much less restrictive than what one would get from more detailed modeling of the foreground. Furthermore, the likelihood function is exactly the same as one would obtain by simply projecting out foreground contamination and looking at the reduced data set. We apply this generalized analysis to the recent medium-angle data sets of ACME-HEMT (Gaier et al. 1992; Schuster et al. 1993) and MAX (Meinhold et al. 1993; Gunderson et al. 1993). The resulting analysis constrains the one free parameter in the standard cold dark matter theory to be Q(sub rms-ps) = 18(sub -5 sup +8) microKelvin. This best fit value, although in striking agreement with the normalization from Cosmic Background Explorer (COBE), is not a very good fit, with an overall chi-squared/degrees of freedom = 208/168. We also argue against three commonly used methods of dealing with foreground: (1) ignoring it completely; (2) subtracting off a best-fit foreground and treating the residuals as if uncontaminated; and (3) culling data which appears to be contaminated by foreground.

Dodelson, Scott↗
NASA NTRS1994

Cosmic background radiation anisotropy in an open inflation, cold dark matter cosmogony

We compute the cosmic background radiation anisotropy, produced by energy-density fluctuations generated during an early epoch of inflation, in an open cosmological model based on the cold dark matter scenario. At Omega(sub 0) is approximately 0.3-0.4, the Cosmic Background Explorer (COBE) normalized open model appears to be consistent with most observations.

Kamionkowski, Marc↗
NASA NTRS1995

Near- and far-infrared observations of interplanetary dust bands from the COBE diffuse infrared background experiment

Data from the Diffuse Infrared Background Experiment (DIRBE) instrument aboard the Cosmic Background Explorer Satellite (COBE) spacecraft have been used to examine the near and far infrared signatures of the interplanetary dust (IPD) bands. Images of the dust band pairs at ecliptic latitudes of +/- 1.4 deg and +/- 10 deg have been produced at DIRBE wavelengths from 1.25 to 100 micrometers. The observations at the shorter wavelengths provide the first evidence of scattered sunlight from particles responsible for the dust bands. It is found that the grains in the bands and those in the smooth IPD cloud have similar spectral energy distributions, suggesting similar compositions and possibly a common origin. The scattering albedos from 1.25 to 3.5 micrometers for the grains in the dust bands and those in the IPD cloud are 0.22 and 0.29, respectively. The 10 deg band pair is cooler (185 +/- 10 K) than the smooth interplanetary dust cloud (259 +/- 10 K). From both parallactic and thermal analyses, the implied location of the grains responsible for the peak brightness of the 10 deg band pair is 2.1 +/- 0.1 AU the Sun A parallactic distance of 1.4 +/- 0.2 AU is found for the peak of the 1.4 deg band pair.

Spiesman, William J.↗
NASA NTRS1995

The COBE normalization for standard cold dark matter

The Cosmic Background Explorer Satellite (COBE) detection of microwave anisotropies provides the best way of fixing the amplitude of cosmological fluctuations on the largest scales. This normalization is usually given for an n = 1 spectrum, including only the anisotropy caused by the Sachs-Wolfe effect. This is certainly not a good approximation for a model containing any reasonable amount of baryonic matter. In fact, even tilted Sachs-Wolfe spectra are not a good fit to models like cold dark matter (CDM). Here, we normalize standard CDM (sCDM) to the two-year COBE data and quote the best amplitude in terms of the conventionally used measures of power. We also give normalizations for some specific variants of this standard model, and we indicate how the normalization depends on the assumed values on n, Omega(sub B) and H(sub 0). For sCDM we find the mean value of Q = 19.9 +/- 1.5 micro-K, corresponding to sigma(sub 8) = 1.34 +/- 0.10, with the normalization at large scales being B = (8.16 +/- 1.04) x 10(exp 5)(Mpc/h)(exp 4), and other numbers given in the table. The measured rms temperature fluctuation smoothed on 10 deg is a little low relative to this normalization. This is mainly due to the low quadrupole in the data: when the quadrupole is removed, the measured value of sigma(10 deg) is quite consistent with the best-fitting the mean value of Q. The use of the mean value of Q should be preferred over sigma(10 deg), when its value can be determined for a particular theory, since it makes full use of the data.

Bunn, Emory F.↗
NASA NTRS1995

Massive black holes and light-element nucleosynthesis in a baryonic universe

We reexamine the model proposed by Gnedin & Ostriker (1992) in which Jeans mass black holes (M(sub BH) approximately = 10(exp 6) solar mass) form shortly after decoupling. There is no nonbaryonic dark matter in this model, but we examine the possibility that Omega(sub b) is considerably larger than given by normal nucleosynthesis. Here we allow for the fact that much of the high baryon-to-photon ratio material will collapse leaving the universe of remaining material with light-element abundances more in accord with the residual baryonic density (approximately = 10(exp -2)) than with Omega(sub 0) and the initial baryonic density (approximately = 10(exp -1)). We find that no reasonable model can be made with random-phase density fluctuations, if the power on scales smaller than 10(exp 6) solar mass is as large as expected. However, phase-correlated models of the type that might occur in connection with topological singularities can be made with Omega(sub b) h(exp 2) = 0.013 +/- 0.001, 0.15 approximately less than Omega(sub 0) approximately less than 0.4, which are either flat (Omega(sub lambda) = 1 - Omega(sub 0)) or open (Omega(sub lambda) = 0) and which satisfy all the observational constraints which we apply, including the large baryon-to-total mass ratio found in the X-ray clusters. The remnant baryon density is thus close to that obtained in the standard picture (Omega(sub b) h(exp 2) = 0.0125 +/- 0.0025; Walker et al. 1991). The spectral index implied for fluctuations in the baryonic isocurvature scenario, -1 less than m less than 0, is in the range expected by other arguments based on large-scale structure and microwave fluctuation constraints. The dark matter in this picture is in the form of massive black holes. Accretion onto them at early epochs releases high-energy photons which significantly heat and reionize the universe. But photodissociation does not materially change light-element abundances. A typical model gives bar-y approximately = 1 x 10(exp -5), n(sub e)/n(sub H)(z = 30) approximately = 0.1, and a diffuse gamma-ray background at 100 keV near the Cosmic Background Explorer Satellite (COBE) limit of the order of 10% of that observed which originates from high-redshift quasars. Reionization in this model occurs at redshift 600 and reaches (H II/H(sub tot) approximately = 0.1-0.2.

Gnedin, Nickolay Y.↗