T. Kelsall

The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background. I. Limits and Detections

The Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer (COBE) spacecraft was designed primarily to conduct a systematic search for an isotropic cosmic infrared background (CIB) in 10 photometric bands from 1.25 to 240 μm. The results of that search are presented here. Conservative limits on the CIB are obtained from the minimum observed brightness in all-sky maps at each wavelength, with the faintest limits in the DIRBE spectral range being at 3.5 μm (νIν < 64 nW m-2 sr-1, 95% confidence level) and at 240 μm (νIν < 28 nW m-2 sr-1, 95% confidence level). The bright foregrounds from interplanetary dust scattering and emission, stars, and interstellar dust emission are the principal impediments to the DIRBE measurements of the CIB. These foregrounds have been modeled and removed from the sky maps. Assessment of the random and systematic uncertainties in the residuals and tests for isotropy show that only the 140 and 240 μm data provide candidate detections of the CIB. The residuals and their uncertainties provide CIB upper limits more restrictive than the dark sky limits at wavelengths from 1.25 to 100 μm. No plausible solar system or Galactic source of the observed 140 and 240 μm residuals can be identified, leading to the conclusion that the CIB has been detected at levels of νIν = 25 ± 7 and 14 ± 3 nW m-2 sr-1 at 140 and 240 μm, respectively. The integrated energy from 140 to 240 μm, 10.3 nW m-2 sr-1, is about twice the integrated optical light from the galaxies in the Hubble Deep Field, suggesting that star formation might have been heavily enshrouded by dust at high redshift. The detections and upper limits reported here provide new constraints on models of the history of energy-releasing processes and dust production since the decoupling of the cosmic microwave background from matter.

The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background. II. Model of the Interplanetary Dust Cloud

The COBE Diffuse Infrared Background Experiment (DIRBE) was designed to search for the cosmic infrared background (CIB) radiation. For an observer confined to the inner solar system, scattered light and thermal emission from the interplanetary dust (IPD) are major contributors to the diffuse sky brightness at most infrared wavelengths. Accurate removal of this zodiacal light foreground is a necessary step toward a direct measurement of the CIB. The zodiacal light foreground contribution in each of the 10 DIRBE wavelength bands ranging from 1.25 to 240 μm is distinguished by its apparent seasonal variation over the whole sky. This contribution has been extracted by fitting the brightness calculated from a parameterized physical model to the time variation of the all-sky DIRBE measurements over 10 months of liquid He cooled observations. The model brightness is evaluated as the integral along the line of sight of the product of a source function and a three-dimensional dust density distribution function. The dust density distribution is composed of multiple components: a smooth cloud, three asteroidal dust bands, and a circumsolar ring near 1 AU. By using a directly measurable quantity that relates only to the IPD cloud, we exclude other contributors to the sky brightness from the IPD model. High-quality maps of the infrared sky with the zodiacal foreground removed have been generated using the IPD model described here. Imperfections in the model reveal themselves as low-level systematic artifacts in the residual maps that correlate with components of the IPD. The most evident of these artifacts are located near the ecliptic plane in the mid-IR and are less than 2% of the zodiacal foreground brightness. Uncertainties associated with the model are discussed, including implications for the CIB search.

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).

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. 31 refs.

Preliminary spectral observations of the Galaxy with a 7 deg beam by the Cosmic Background Explorer (COBE)

The FIR absolute spectrophotometer (FIRAS) on the Cosmic Background Explorer (COBE) has carried out the first all-sky spectral line survey in the FIR region, as well as mapping spectra of the Galactic dust distribution at below 100 microns. Lines of forbidden C I, C II, and N II, as well as of CO are all clearly detected. The mean line intensities are interpreted in terms of the heating and cooling of the multiple phases of the interstellar gas. In addition, an average spectrum of the galaxy is constructed and searched for weak lines. The spectrum of the galaxy observed by FIRAS has two major components: a continuous spectrum due to interstellar dust heated by starlight, and a line spectrum dominated by the strong 158-micron line from singly ionized carbon, with a spatial distribution similar to the dust distribution, and a luminosity of 0.3 percent of the dust luminosity. There are in addition moderately strong 122- and 205.3-micron lines, identified as coming from singly-ionized nitrogen. Maps of the emission by dust and forbidden C II and N II are presented.

Detection and Characterization of Cold Interstellar Dust and Polycyclic Aromatic Hydrocarbon Emission, from COBE Observations

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.

The COBE mission - its design and performance two years after launch

COBE, NASAs first space mission devoted primarily to cosmology, carries three scientific instruments to make precise measurements of the spectrum and anisotropy of the cosmic microwave background radiation on angular scales greater than 7° and to conduct a search for a diffuse cosmic infrared background radiation with 0°.7 angular resolution. The mission goal is to make these measurements to the limit imposed by the local astrophysical foregrounds. The COBE instruments cover the wavelength range from 1.2 μm to 1 cm. The instruments are calibrated periodically in orbit using internal calibrators and celestial standards

The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background. IV. Cosmological Implications

A direct measurement of the extragalactic background light (EBL) can provide important constraints on the integrated cosmological history of star formation, metal and dust production, and the conversion of starlight into infrared emission by dust. In this paper we examine the cosmological implications of the recent detection of the EBL in the 125 to 5000 ?m wavelength region by the Diffuse Infrared Background Experiment (DIRBE) and Far Infrared Absolute Spectrophotometer (FIRAS) on board the Cosmic Background Explorer (COBE). We first show that the 140 and 240 ?m isotropic residual emission found in the DIRBE data cannot be produced by foreground emission sources in the solar system or the Galaxy. The DIRBE 140 and 240 ?m isotropic residuals, and by inference the FIRAS residuals as well, are therefore extragalactic. Assuming that most of the 140 and 240 ?m emission is from dust yields a 2 ? lower limit of ?I(?) ? 5 nW m-2 sr-1 for the EBL at 100 ?m. The integrated EBL detected by the COBE between 140 and 5000 ?m is ~16 nW m-2 sr-1, roughly 20%-50% of the integrated EBL intensity expected from energy release by nucleosynthesis throughout cosmic history. This also implies that at least ~5%-15% of the baryonic mass density implied by big bang nucleosynthesis has been processed through stars. The COBE observations provide important constraints on the cosmic star formation rate, and we calculate the EBL spectrum for various star formation histories. The results show that the UV and optically determined cosmic star formation rates fall short in producing the observed 140 to 5000 ?m background. The COBE observations require the star formation rate at redshifts of z ? 1.5 to be larger than that inferred from UV-optical observations by at least a factor of 2. This excess stellar energy must be mainly generated by massive stars, since it otherwise would result in a local K-band luminosity density that is larger than observed. The energy sources could either be yet undetected dust-enshrouded galaxies, or extremely dusty star-forming regions in observed galaxies, and they may be responsible for the observed iron enrichment in the intracluster medium. The exact star formation history or scenarios required to produce the EBL at far-IR wavelengths cannot be unambiguously resolved by the COBE observations and must await future observations.

A Three-dimensional Decomposition of the Infrared Emission from Dust in the Milky Way

We have constructed a three-dimensional model of the Galactic large-scale infrared emission from dust associated with the molecular neutral atomic (H I), and extended low-density cm~3) (H 2 ), (n e D 1E100 ionized (H II) gas phases of the interstellar medium. The model incorporates a three-dimensional map of the molecular and neutral atomic hydrogen gas distributions, derived from available 12CO and H I surveys by using the radial velocity information in the spectral lines as a distance indicator, and available 5 and 19 GHz radio continuum surveys to trace the column density of ionized gas. We use the model to decompose the Di†use Infrared Background Experiment (DIRBE) 12E240 km obserCOBE5 vations of the Galactic plane region ( o b o „ 5i), from which the zodiacal light and stellar emission have been subtracted, into distinct emission components associated with each gas phase within selected ranges of Galactocentric distance. An interstellar dust model is ‐tted to the resulting infrared spectra to derive the following quantities within each Galactocentric distance interval: (1) the abundance and equilibrium temperature of the large dust grain component within each gas phase; (2) estimates of the abundance of very small (\200 transiently heated dust grains and polycyclic aromatic hydrocarbon (PAH) mol”) ecules; and (3) constraints on various model parameters, such as the energy density of the ambient interstellar radiation ‐eld, which heats the dust within the H I gas phase. Our results show steep negative Galactocentric gradients in the equilibrium temperature of the large dust grain component within the H I, and H II gas phases, the GalaxyIs ambient interstellar radiH 2 , ation ‐eld, and the dust-to-gas mass ratio for each gas phase. The intensity of the ambient interstellar radiation ‐eld increases by a factor of D3 between the solar circle (8.5 kpc) and the molecular ring at a Galactocentric distance of D5 kpc. The dust abundance gradient of ([0.05 ^ 0.03) dex kpc~1 is equivalent, within the uncertainties, to the metallicity gradient in the Galactic disk. The derived emission spectra are consistent with a model in which very small transiently heated dust grains and PAHs are abundant and the dominant contributors to the mid-infrared (5 km \j\ 40 km) luminosity from a Galactocentric distance of 2 kpc out to a Galactocentric distance of at least 12 kpc, and indicate that the relative abundance of the PAHs is signi‐cantly higher in the outer region of the Galactic disk than inside the solar circle. We combine the results of our decomposition algorithm with the results of a study of optical extinction at high Galactic latitude to derive the radial distribution of optical opacity in the Galactic disk and ‐nd that our Galaxy would be e†ectively transparent Galaxy) \ 0.2 mag] to an external obser[A B (total ver viewing it at a low inclination (i \ 30i). All of the Galactic infrared emission observed by the DIRBE can be accounted for by dust associated with gas that is detected by current radio surveys, refuting the recent suggestion that a large fraction of the dynamically inferred hidden mass in spiral galaxies may be due to unseen gas and stars in the disk of the galaxies. Subject headings: di†use radiation E dust, extinction E Galaxy: structure E infrared: ISM: continuum E ISM: abundances E ISM: molecules

COBE diffuse infrared background experiment observations of the galactic bulge

Low angular resolution maps of the Galactic bulge at 1.25, 2.2, 3.5, and 4.9 micrometers obtained by the Diffuse Infrared Background Experiment (DIRBE) onboard NASAs Cosmic Background Explorer (COBE) are presented. After correction for extinction and subtraction of an empirical model for the Galactic disk, the surface brightness distribution of the bulge resembles a flattened ellipse with a minor-to-major axis ratio of approximately 0.6. The bulge minor axis scale height is found to be 2.1 deg +/- 0.2 deg for all four near-infrared wavelengths. Asymmetries in the longitudinal distribution of bulge brightness contours are qualitatively consistent with those expected for a triaxial bar with its near end in the first Galactic quadrant (0 deg less than l less than 90 deg). There is no evidence for an out-of-plane tilt of such a bar.