FIRAS Instrument Research Articles

On the radiative and thermodynamic properties of the cosmic radiations using COBE FIRAS instrument data: I. Cosmic microwave background radiation

Using the explicit form of the functions to describe the monopole and dipole spectra of the Cosmic Microwave Background (CMB) radiation, the exact expressions for the temperature dependences of the radiative and thermodynamic functions, such as the total radiation power per unit area, total energy density, number density of photons, Helmholtz free energy density, entropy density, heat capacity at constant volume, and pressure in the finite range of frequencies v1≤v≤v2 are obtained. Since the dependence of temperature upon the redshift z is known, the obtained expressions can be simply presented in z representation. Utilizing experimental data for the monopole and dipole spectra measured by the COBE FIRAS instrument in the 60–600 GHz frequency interval at the temperature T=2.72548 K, the values of the radiative and thermodynamic functions, as well as the radiation density constant a and the Stefan-Boltzmann constant σ are calculated. In the case of the dipole spectrum, the constants a and σ, and the radiative and thermodynamic properties of the CMB radiation are obtained using the mean amplitude Tamp=3.358 mK. It is shown that the Doppler shift leads to a renormalization of the radiation density constant a, the Stefan-Boltzmann constant σ, and the corresponding constants for the thermodynamic functions. The expressions for new astrophysical parameters, such as the entropy density/Boltzmann constant, and number density of CMB photons are obtained. The radiative and thermodynamic properties of the Cosmic Microwave Background radiation for the monopole and dipole spectra at redshift z≈1089 are calculated.

Constraining resonant photon-axion conversions in the early universe

The presence of a primordial magnetic fieldwould have induced resonant conversionsbetween photons and axion-like particles (ALPs)during the thermal history of the Universe. Theseconversions would have distorted the blackbody spectrum of the cosmic microwave background (CMB).In this context, we derive bounds on the photon-ALPresonant conversions using the high precision CMBspectral data collected by the FIRAS instrument on board of the Cosmic Background Explorer.We obtain upper limits on the product of the photon-ALP coupling constant g times the magnetic field strength B down to gB ≲ 10−13 GeV−1 nGfor ALP masses below the eV scale.

Neutrinos and cosmology: A lifetime relationship

We consider the example of neutrino decays to illustrate the profound relation between laboratory neutrino physics and cosmology. Two case studies are presented: In the first one, we show how the high precision cosmic microwave background spectral data collected by the FIRAS instrument on board of COBE, when combined with Lab data, have greatly changed bounds on the radiative neutrino lifetime. In the second case, we speculate on the consequence for neutrino physics of the cosmological detection of neutrino masses even as small as ∼0.06 eV, the lower limit guaranteed by neutrino oscillation experiments. We show that a detection at that level would improve by many orders of magnitude the existing limits on neutrino lifetime, and as a consequence on some models of neutrino secret interactions.

A cosmological bound on radiative neutrino lifetime

Neutrino oscillation experiments and direct bounds on absolute masses constrain neutrino mass differences to fall into the microwave energy range, for most of the allowed parameter space. As a consequence of these recent phenomenological advances, older constraints on radiative neutrino decays based on diffuse background radiations and assuming strongly hierarchical masses in the eV range are now outdated. We thus derive new bounds on the radiative neutrino lifetime using the high precision cosmic microwave background spectral data collected by the FIRAS instrument on board of COBE. The lower bound on neutrino lifetime is between a few ×1019 s and ∼ 5 × 1020 s, depending on the neutrino mass ordering and on the absolute neutrino mass scale. However, due to phase space limitations, the upper bound on the effective magnetic moment mediating the decay is not better than ∼10-8 μB. We also comment about possible improvements of these limits, by means of recent diffuse infrared photon background data.

Modelling of clumpy photon dominated regions

Observations of the interstellar medium (ISM) reveal its fractal structure over a large range of scales. The quantitative description shows that the ISM consists mainly of surfaces, so that the stellar UV-radiation can deeply penetrate into the ISM and in consequence dominates its physical and chemical conditions. Thus, the bulk of the ISM can be identified as photon dominated regions (PDRs). A simple model with the proper fractal characteristics is that of an ensemble of spherical clumps with a power-law mass spectrum and a power-law mass-size relation. Hence, we model the FIR line emission of the molecular gas by the integrated emission of such an ensemble, calculating the emission of individual clumps with the KOSMA-τ PDR code. This clumpy PDR-model, with observationally constrained parameters, is fitted to the CO, [C i], and [O i] emission along the Galactic plane as observed by the FIRAS instrument abord the COBE satellite. The FIR line intensity distribution with Galactic longitude is reproduced within factors of about 2. The predicted [C ii] 158 μm line intensity and FIR continuum emission also coincide with the observations.

Detection and Characterization of Cold Interstellar Dust and Polycyclic Aromatic Hydrocarbon Emission, fromCOBEObservations

Using data obtained by the DIRBE instrument on the COBE spacecraft, we present the mean 3.5-240 μm spectrum of high-latitude dust. Combined with a spectrum obtained by the FIRAS instrument, these data represent the most comprehensive wavelength coverage of dust in the diffuse interstellar medium, spanning the 3.5-1000 μm wavelength regime. At wavelengths shorter than ~60 μm the spectrum shows an excess of emission over that expected from dust heated by the local interstellar radiation field and radiating at an equilibrium temperature. The DIRBE data thus extend the observations of this excess, first detected by the IRAS satellite at 25 and 12 μm, to shorter wavelengths. The excess emission arises from very small dust particles undergoing temperature fluctuations. However, the 3.5-4.9 μm intensity ratio cannot be reproduced by very small silicate or graphite grains. The DIRBE data strongly suggest that the 3.5-12 μm emission is produced by carriers of the ubiquitous 3.3, 6.2, 7.7, 8.6, and 11.3 μm solid state emission features that have been detected in a wide variety of astrophysical objects. The carriers of these features have been widely identified with polycyclic aromatic hydrocarbons (PAHs). Our dust model consists of a mixture of PAH molecules and bare astronomical silicate and graphite grains with optical properties given by Draine & Lee. We obtain a very good fit to the DIRBE spectrum, deriving the size distribution, abundances relative to the total hydrogen column density, and relative contribution of each dust component to the observed IR emission. At wavelengths above 140 μm the model is dominated by emission from T ≈ 17-20 K graphite and 15-18 K silicate grains. The model provides a good fit to the FIRAS spectrum in the 140-500 μm wavelength regime but leaves an excess Galactic emission component at 500-1000 μm. The nature of this component is still unresolved. We find that (C/H) is equal to (7.3 ± 2.2) × 10-5 for PAHs and equal to (2.5 ± 0.8) × 10-4 for graphite grains, requiring about 20% of the cosmic abundance of carbon to be locked up in PAHs, and about 70% in graphite grains [we adopt (C/H)☉ = 3.6 × 10-4]. The model also requires all of the available magnesium, silicon, and iron to be locked up in silicates. The power emitted by PAHs is 1.6 × 10-31 W per H atom, by graphite grains 3.0 × 10-31 W per H atom, and by silicates 1.4 × 10-31 W per H atom, adding up to a total infrared intensity of 6.0 × 10-31 W per H atom, or ~2 L☉ M. The [C II] 158 μm line emission detected by the FIRAS provides important information on the gas phase abundance of carbon in the diffuse ISM. The 158 μm line arises predominantly from the cold neutral medium (CNM) and shows that for typical CNM densities and temperatures C+/H = (0.5-1.0) × 10-4, which is ~14%-28% of the cosmic carbon abundance. The remaining carbon abundance in the CNM, which must be locked up in dust, is about equal to that required to provide the observed IR emission, consistent with notion that most (75%) of this emission arises from the neutral component of the diffuse ISM. The model provides a good fit to the general interstellar extinction curve. However, at UV wavelengths it predicts a larger extinction. The excess extinction may be the result of the UV properties adopted for the PAHs. If real, the excess UV extinction may be accounted for by changes in the relative abundances of PAHs and carriers of the 2200 A extinction bump.

Interpretation of the COBE FIRAS CMBR spectrum

The cosmic microwave background spectrum measured by the FIRAS instrument on NASA's COBE is indistinguishable from a blackbody, implying stringent limits on energy release in the early universe later than the time t = 1 yr after the big bang. We compare the FIRAS data to previous precise measurements of the cosmic microwave background spectrum and find a reasonable agreement. We discuss the implications of the |y| < 2.5 x 10^-5^ and |μ| < 3.3 x 10^-4^ 95% confidence limits found by Mather et al. (1994) on many processes occurring after t = 1 yr, such as explosive structure formation, reionization, and dissipation of small- scale density perturbations. We place limits on models with dust plus Population III stars, or evolving populations of IR galaxies, by directly comparing the Mather et al. spectrum to the model predictions.

Morphology of the interstellar cooling lines detected by COBE

The FIRAS instrument on the COBE satellite has conducted an unbiased survey of the far-infrared emission from our Galaxy. The first results of this survey were reported by Wright et al. (1991). We report the results of new analyses of this spectral survey, which includes emission lines from 158 um C+, 122 um and 205 um N+, 370 um and 609 um C, and CO J=2-1 through 5-4. We report the morphological distribution along the galactic plane (b=0) of the spectral line emission, and the high galactic latitude intensities of the C+ and 205 um N+ emission. The high galactic latitude intensity cosecant of the 158 um fine structure transition from C+ is presented, and C+ is seen to decrease more rapidly than the far infrared intensity with increasing galactic latitude. C+ and H I emission are closely correlated with a C+ cooling rate of (2.65 +/- 0.15)x10^{-26} erg/s/H-atom. We conclude that this emission arises almost entirely from the Cold Neutral Medium. The high galactic latitude intensity of the 205 um fine structure transition from N+ arises entirely from the Warm Ionized Medium, and its cosecant is presented. We estimate the total ionizing photon rate in the Galaxy to be 3.5x10^{53} ionizing photons per second, based on the 205 um N+ transition.

Galactic fine-structure lines - Morphologies of the warm ionized interstellar medium

Far-infrared ionic fine-structure emission lines associated with the large-scale structure of the Galactic ionized medium are interpreted in terms of a heuristic model in which three distinct source morphologies are considered: ionization-bounded H II regions, an extended, low-density medium, and externally ionized surfaces of neutral clouds. The model reveals that the overall luminosity of the Galaxy in the fine-structure lines of both C II and N II is due, in comparable proportions, to diffuse medium and the boundaries between ionized and neutral regions. Limits are set on the luminosity of the diffuse component in the plane of the Galaxy. Some individual lines of sight of the COBE FIRAS instrument are dominated, in these spectral lines, by H II regions, even with its 7 deg field of view. Average densities of both H II regions and photodissociation regions are estimated on a Galactic scale.

Preliminary Observations of the Galaxy with a 7° Beam by the Cosmic Background Explorer (COBE)

The FIRAS instrument on COBE has mapped almost the entire sky with a 7° beam and a spectral resolution of about 1 cm−1 over the 1-100 cm−1 range. Maps showing the strengths of the three main components: dust continuum, [C II] and [N II] are presented. A mean spectrum of the galactic emission is found. These results are used to compare the Milky Way to other galaxies.

A preliminary measurement of the cosmic microwave background spectrum by the Cosmic Background Explorer (COBE) satellite

A preliminary spectrum is presented of the background radiation between 1 and 20/cm from regions near the north Galactic pole, as observed by the FIRAS instrument on the COBE satellite. The spectral resolution is 1/cm. The spectrum is well fitted by a blackbody with a temperature of 2.735 + or - 0.06 K, and the deviation from a blackbody is less than 1 percent of the peak intensity over the range 1-20/cm. These new data show no evidence for the submillimeter excess previously reported by Matsumoto et al. (1988) in the cosmic microwave background. Further analysis and additional data are expected to improve the sensitivity to deviations from a blackbody spectrum by an order of magnitude.