Measurements Of Extragalactic Background Light Research Articles

Prospects of additional contribution at optical-NIR band of EBL in the light of VHE spectra

ABSTRACT The extragalactic background light (EBL) that spans the ultraviolet-infrared (UV-IR) band originates from direct and dust-reprocessed starlight integrated over the history of the Universe. EBL measurements are very challenging due to foreground emission like the zodiacal light and interplanetary dust emission. Indeed, some optical/NIR (near infrared) direct measurements overpredict EBL models based on galaxy counts. On the other hand, there is some debate on possible additional components of the optical-NIR photon density, e.g. population-III stars, axion-photon decay, direct collapse of black holes, intrahalo light, etc. Owing to the absorption of very high energy (VHE) gamma-rays by interaction with EBL photons, we study the prospects of accommodating an additional population of EBL sources in the optical-NIR band on top of the standard galaxy-count–based component. To this aim we use 105 VHE spectra of 37 blazars with known redshifts, 0.03 < z < 0.94. We correct the observed spectra for absorption by our model EBL. By requiring the intrinsic spectra to be non-concave and with a VHE spectral index >1.5, we estimate, at different wavelengths, upper limits to the additional low-energy photon fields that would contribute to the absorption of gamma-rays. Considering these limits, we suggest that there is room for photons from Pop III stars and axion-like particle annihilation. However, these additional hypothetical photon fields are bound to fall significantly below direct published EBL measurements by several instruments, and therefore, our limits are either in tension or even inconsistent with such measurements.

Looking for the signals of the missing baryons in the extragalactic background light

The missing baryons in the universe are assumed to be hidden in the whole space as a warm–hot intergalactic medium (WHIM). Finding them is one of the important subjects in modern cosmology. In this paper, we point out that the very high-energy electron–positron rays may light up the WHIM due to the anomalous bremsstrahlung according to an improved Bethe–Heitler formula. The resulting excess of the extragalactic background light (EBL) can be observed by the direct measurement method. A possible explanation on the difference between the direct and indirect measurements of EBL is also proposed. Thus, we find a new window to probe the WHIM properties via the EBL.

Extragalactic background light: inventory of light throughout the cosmic history

ABSTRACTThe Extragalactic Background Light (EBL) stands for the mean surface brightness of the sky as we would see it from a representative vantage point in the intergalactic space outside of our Milky Way Galaxy. Averaged over the whole 4π solid angle it represents the collective light from all luminous matter radiated throughout the cosmic history. Part of the EBL is resolved into galaxies that, with the increasing detecting power of giant telescopes and sensitive detectors, are seen to deeper and deeper limiting magnitudes. This resolved part is now known to contribute a substantial or even the major part of the EBL. There still remains, however, the challenge of finding out to what extent galaxies too faint or too diffuse to be discerned individually, individual stars or emission by gas outside the galaxies, or – more speculatively – some hitherto unknown light sources such as decaying elementary particles are accounting for the remaining EBL. We review the recent progress that has been made in the measurement of EBL. The current photometric results suggest that there is, beyond the resolved galaxies, an EBL component that cannot be explained by diffuse galaxy halos or intergalactic stars.

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.

Spectrophotometric measurement of the Extragalacic Background Light

AbstractThe Extragalactic Background Light (EBL) at UV, optical and NIR wavelengths consists of the integrated light of all unresolved galaxies along the line of sight plus any contributions by intergalactic matter including hypothetical decaying relic particles. The measurement of the EBL has turned out to be a tedious problem. This is because of the foreground components of the night sky brightness, much larger than the EBL itself: the Zodiacal Light (ZL), Integrated Starlight (ISL), Diffuse Galactic Light (DGL) and, for ground-based observations, the Airglow (AGL) and the tropospheric scattered light. We have been developing a method for the EBL measurement which utilises the screening effect of a dark nebula on the EBL. A differential measurement in the direction of a high-latitude dark nebula and its surrounding area provides a signal that is due to two components only, i.e. the EBL and the diffusely scattered ISL from the cloud. We present a progress report of this method where we are now utilising intermediate resolution spectroscopy with ESO's VLT telescope. We detect and remove the scattered ISL component by using its characteristic Fraunhofer line spectral signature. In contrast to the ISL, in the EBL spectrum all spectral lines are washed out. We present a high quality spectrum representing the difference between an opaque position within our target cloud and several clear OFF positions around the cloud. We derive a preliminary EBL value at 400 nm and an upper limit to the EBL at 520 nm. These values are in the same range as the EBL lower limits derived from galaxy counts.Unit: We will use in this paper the abbreviation 1 cgs = 10−9erg s−1cm−2sr−1Å−1

CONSTRAINTS ON THE EXTRAGALACTIC BACKGROUND LIGHT FROM VERY HIGH ENERGY GAMMA-RAY OBSERVATIONS OF BLAZARS

The extragalactic background light (EBL) from the infrared to the ultraviolet is difficult to measure directly, but can be constrained with a variety of methods. EBL photons absorb gamma-rays from distant blazars, allowing one to use blazar spectra from atmospheric Cherenkov telescopes (ACTs) to put upper limits on the EBL by assuming a blazar source spectrum. Here we apply a simple technique, similar to the one developed by Schroedter (2005), to the most recent very-high energy (VHE) gamma-ray observations of blazars to put upper limits on the EBL energy density. This technique is independent of the EBL model and has well-defined errors on the constraints. Our results are consistent with EBL measurements and constraints but marginally inconsistent with several EBL models.

The Optical Extragalactic Background Light: Revisions and Further Comments

We revise the measurements in our previous work of foreground zodiacal light (ZL) and diffuse Galactic light (DGL) that were used to measure the extragalactic background light (EBL). These changes result in a decrease of 8 and an increase of 0.3 in units of 10-9 ergs s-1 cm-2 sr-1 A-1 (cgs units) in the ZL and DGL flux, respectively. We therefore obtain revised values for the EBL of 6 ± 4, 10 ± 5, and 7 ± 4 cgs in the HST WFPC2 U (F300W), V (F555W), and I (F814W) bands, respectively, from sources fainter than mV ~ 23 AB mag. The revisions are dominated by the details of the tropospheric scattering models used to measure the ZL. We discuss these results in the context of faint number counts and diffuse EBL measurements at other wavelengths. In particular, we note that unless the slope of the galaxy counts increases beyond mV ~ 30 AB mag, unresolved sources will contribute <0.2 cgs, which is far below the uncertainties achievable for any diffuse EBL measurement in the foreseeable future. Therefore, the best constraints on faint sources come from the resolved sources themselves. As in our earlier work, models are still required to derive the bolometric EBL (0.1-1000 μm) due to uncertainties in the mid-infrared; consequently, our previous discussions of the bolometric EBL are not affected by the revisions presented here. Finally, we discuss the nature of the extended point-spread function (PSF) of ground-based telescopes and its impact on surface brightness measurements. In particular, we show that the slope and amplitude of extended PSFs vary considerably between telescopes and with time. We find no conclusive, single cause of extended PSFs, although atmospheric scattering is ruled out.

Subaru Super Deep Field with Adaptive Optics. I. Observations and First Implications

We present a deep K'-band (2.12 μm) imaging of the 1' × 1' Subaru Super Deep Field (SSDF) taken with the Subaru adaptive optics (AO) system. Total integration time of 26.8 hr results in the limiting magnitude of K' ~ 24.7 (5 σ, 02 aperture) for point sources and K' ~ 23.5 (5 σ, 06 aperture) for galaxies, which is the deepest limit ever achieved in the K' band. The average stellar full width at half-maximum (FWHM) of the co-added image is 018. Based on the photometric measurements of detected galaxies, we obtained the differential galaxy number counts, for the first time, down to K' ~ 25, which is more than 0.5 mag deeper than the previous data. We found that the number count slope d log N/dm is about 0.15 at 22 < K' < 25, which is flatter than the previous data. Therefore, detected galaxies in the SSDF have only negligible contribution to the near-infrared extragalactic background light (EBL), and the discrepancy claimed so far between the diffuse EBL measurements and the estimated EBL from galaxy count integration has become more serious. The size distribution of detected galaxies was obtained down to the area size of less than 0.1 arcsec2, which is less than half of that of the previous data in the K' band. We compared the observed size-magnitude relation with a simple pure luminosity evolution model allowing for intrinsic size evolution and found that a model with no size evolution gives the best fit to the data. It implies that the surface brightness of galaxies at high redshift is not much different from that expected from the size-luminosity relation of present-day galaxies.

The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000 Å. II. Measurement of Foreground Zodiacal Light

We present a measurement of the absolute surface brightness of the zodiacal light (3900-5100 A) toward a fixed extragalactic target at high ecliptic latitude based on moderate-resolution (~1.3 A pixel-1) spectrophotometry obtained with the du Pont 2.5 m Telescope at the Las Campanas Observatory in Chile. This measurement and contemporaneous Hubble Space Telescope data from the Wide Field Planetary Camera 2 and Faint Object Spectrograph comprise a coordinated program to measure the mean flux of the diffuse extragalactic background light (EBL). The zodiacal light at optical wavelengths results from scattering by interplanetary dust so that the zodiacal light flux toward any extragalactic target varies seasonally with the position of the Earth. This measurement of zodiacal light is therefore relevant to the specific observations (date and target field) under discussion. To obtain this result, we have developed a technique that uses the strength of the zodiacal Fraunhofer lines to identify the absolute flux of the zodiacal light in the multiple-component night-sky spectrum. Statistical uncertainties in the result are 0.6% (1 σ). However, the dominant source of uncertainty is systematic errors, which we estimate to be 1.1% (1 σ). We discuss the contributions included in this estimate explicitly. The systematic errors in this result contribute 25% in quadrature to the final error in our coordinated EBL measurement, which is presented in the first paper of this series.

The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000 Å. III. Cosmological Implications

We have used the Hubble Space Telescope Wide Field and Planetary Camera 2 in combination with ground-based spectroscopy to measure the integrated flux of galaxies at optical wavelengths—the extragalactic background light (EBL). We have also computed the integrated light from individual galaxy counts in the images used to measure the EBL and in the Hubble Deep Field. We find that flux in galaxies as measured by standard galaxy photometry methods has generally been underestimated by about 50%, resulting from missed flux in the outer, lower surface brightness parts of galaxies and from associated errors in the estimated sky level. Comparing the corrected, integrated flux from individual galaxies with our total EBL measurement, we find that there is yet further light that contributes to the background that is not represented by galaxy counts and that the total flux in individually detected sources is a factor of 2-3 less than the EBL from 8000 to 3000 A. We show that a significant fraction of the EBL may come from normal galaxies at z 1; while significant flux comes from galaxies beyond the current detection limits, this evolution cannot be tightly constrained by our data. Based on our measurements of the optical EBL, combined with previously published measurements of the UV and IR EBL, we estimate that the total EBL from 0.1 to 1000 μm is 100 ± 20 nW m-2 sr-1. If the total EBL were produced entirely by stellar nucleosynthesis, then we estimate that the total baryonic mass processed through stars is Ω* = 0.0062(±0.0022) h-2 in units of the critical density. For currently favored values of the baryon density ΩB this corresponds to 0.33 ± 0.12ΩB. This estimate is smaller by roughly 7% if we allow for a contribution of 7 h0.7 nW m-2 sr-1 to the total EBL from accretion onto central black holes. This estimate of Ω* suggests that the universe has been enriched to a total metal mass of 0.21 (±0.13) Z☉ ΩB, which is consistent with other observational estimates of the cumulative metal mass fraction of stars, stellar remnants, and the intracluster medium of galaxy clusters in the local universe.

Faint Submillimeter Counts from Deep 850 Micron Observations of the Lensing Clusters A370, A851, and A2390

We present deep 850 micron maps of three massive lensing clusters, A370, A851, and A2390, with well-constrained mass models. Our cluster exposure times are more than 2 to 5 times longer than any other published cluster field observations. We catalog the sources and determine the submillimeter number counts. The counts are best determined in the 0.3 to 2 mJy range where the areas are large enough to provide a significant sample. At 0.3 mJy the cumulative counts are 3.3 (1.3,6.3) 10^4 per square degree, where the upper and lower bounds in the brackets are the 90% confidence range. The surface density at these faint count limits enters the realm of significant overlap with other galaxy populations.The corresponding percentage of the extragalactic background light (EBL) residing in this flux range is about 45-65%, depending on the EBL measurement used. Given that 20-30% of the EBL is resolved at flux densities between 2 and 10 mJy, most of the submillimeter EBL is arising in sources above 0.3 mJy. We also performed a noise analysis to obtain an independent estimate of the counts. The upper bounds on the counts determined from the noise analysis closely match the upper limits obtained from the direct counts. The differential counts from this and other surveys can reasonably be described by the parameterization n(S)=3 10^4/(0.7 + S^3) per square degree per mJy with S in mJy, which also integrates to match the EBL.

Detecting the Low Surface Brightness Universe: the Extragalactic Background Light and LSB Galaxies

AbstractIndividual sources are detected ideally to a confusion limit at a given wavelength, but there is still much information beyond that. Absolute background brightness measurements provide a crucial constraint to models describing the undetected population of sources in the distant – and/or low surface brightness – universe. We calculate how much low surface brightness galaxies (LSB) would contribute to the the overall extragalactic background light (EBL) and review the status of EBL measurements in the optical and infrared wavelengths. To be able to push deeper the detection limits on very faint sources a fluctuation analysis method is introduced. The use of these different approaches together is essential when studying the very faint and low surface brightness universe.