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Register a new userUltra deep AKARI observations of Abell 2218: resolving the 15 um extragalactic background light
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We present extragalactic number counts and a lower limit estimate for the cosmic infrared background at 15 um from AKARI ultra deep mapping of the gravitational lensing cluster Abell 2218. This data is the deepest taken by any facility at this wavelength, and uniquely samples the normal galaxy population. We have de-blended our sources, to resolve photometric confusion, and de-lensed our photometry to probe beyond AKARIs blank-field sensitivity. We estimate a de-blended 5 sigma sensitivity of 28.7 uJy. The resulting 15 um galaxy number counts are a factor of three fainter than previous results, extending to a depth of ~ 0.01 mJy and providing a stronger lower limit constraint on the cosmic infrared background at 15 um of 1.9 +/- 0.5 nW m^-2 sr^-1.
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The Extragalactic Background Light (EBL) as an integrated light from outside of our Galaxy includes information of the early universe and the Dark Ages. We analyzed the spectral data of the astrophysical diffuse emission obtained with the low-resolution spectroscopy mode on the AKARI Infra-Red Camera (IRC) in 1.8-5.3 um wavelength region. Although the previous EBL observation in this wavelength region is restricted to the observations by DIRBE and IRTS, this study adds a new independent result with negligible contamination of Galactic stars owing to higher sensitivity for point sources. Other two major foreground components, the zodiacal light (ZL) and the diffuse Galactic light (DGL), were subtracted by taking correlations with ZL brightness estimated by the DIRBE ZL model and with the 100 um dust thermal emission, respectively. The isotropic emission was obtained as EBL, which shows significant excess over integrated light of galaxies at <4 um. The obtained EBL is consistent with the previous measurements by IRTS and DIRBE.
Gravitational lensing by massive galaxy clusters allows study of the population of intrinsically faint infrared galaxies that lie below the sensitivity and confusion limits of current infrared and submillimeter telescopes. We present ultra-deep PACS 100 and 160 microns observations toward the cluster lens Abell 2218, to penetrate the Herschel confusion limit. We derive source counts down to a flux density of 1 mJy at 100 microns and 2 mJy at 160 microns, aided by strong gravitational lensing. At these levels, source densities are 20 and 10 beams/source in the two bands, approaching source density confusion at 160 microns. The slope of the counts below the turnover of the Euclidean-normalized differential curve is constrained in both bands and is consistent with most of the recent backwards evolutionary models. By integrating number counts over the flux range accessed by Abell 2218 lensing (0.94-35 mJy at 100 microns and 1.47-35 mJy at 160 microns, we retrieve a cosmic infrared background (CIB) surface brightness of ~8.0 and ~9.9 nW m^-2 sr^-1, in the respective bands. These values correspond to 55% (+/- 24%) and 77% (+/- 31%) of DIRBE direct measurements. Combining Abell 2218 results with wider/shallower fields, these figures increase to 62% (+/- 25%) and 88% (+/- 32%) CIB total fractions, resolved at 100 and 160 microns, disregarding the high uncertainties of DIRBE absolute values.
We present results from two observations (combined exposure of ~17 ks) of galaxy cluster A2218 using the Advanced CCD Imaging Spectrometer on board the Chandra X-ray Observatory that were taken on October 19, 1999. Using a Raymond-Smith single temperature plasma model corrected for galactic absorption we find a mean cluster temperature of kT = 6.9+/-0.5 keV, metallicity of 0.20+/-0.13 (errors are 90 % CL) and rest-frame luminosity in the 2-10 keV energy band of 6.2x10^{44} erg/s in a LambdaCDM cosmology with H_0=65 km/s/Mpc. The brightness distribution within 4.2 of the cluster center is well fit by a simple spherical beta model with core radius 66.4 and beta = 0.705 . High resolution Chandra data of the inner 2 of the cluster show the x-ray brightness centroid displaced ~22 from the dominant cD galaxy and the presence of azimuthally asymmetric temperature variations along the direction of the cluster mass elongation. X-ray and weak lensing mass estimates are in good agreement for the outer parts (r > 200h^{-1}) of the cluster; however, in the core the observed temperature distribution cannot reconcile the x-ray and strong lensing mass estimates in any model in which the intracluster gas is in thermal hydrostatic equilibrium. Our x-ray data are consistent with a scenario in which recent merger activity in A2218 has produced both significant non-thermal pressure in the core and substructure along the line of sight; each of these phenomena probably contributes to the difference between lensing and x-ray core mass estimates.
This review covers the measurements related to the extragalactic background light (EBL) intensity from gamma-rays to radio in the electromagnetic spectrum over 20 decades in the 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), centered at 1 microns, 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 Gamma-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 microns established an energetically important background with an intensity comparable to the optical background. This discovery paved the path for large aperture far-infrared and sub-millimeter 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 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 to 5 microns 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.
Data from (non-) attenuation of gamma rays from active galactic nuclei (AGN) and gamma ray bursts (GRBs) give upper limits on the extragalactic background light (EBL) from the UV to the mid-IR that are only a little above the lower limits from observed galaxies. These upper limits now rule out some EBL models and purported observations, with improved data likely to provide even stronger constraints. We present EBL calculations both based on multiwavelength observations of thousands of galaxies and also based on semi-analytic models, and show that they are consistent with these lower limits from observed galaxies and with the gamma-ray upper limit constraints. Such comparisons close the loop on cosmological galaxy formation models, since they account for all the light, including that from galaxies too faint to see. We compare our results with those of other recent works, and discuss the implications of these new EBL calculations for gamma ray attenuation. Catching a few GRBs with groundbased atmospheric Cherenkov Telescope (ACT) arrays or water Cherenkov detectors could provide important new constraints on the high-redshift star formation history of the universe.
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