Near-infrared Background Research Articles

PROBING REIONIZATION WITH THE CROSS-POWER SPECTRUM OF 21 cm AND NEAR-INFRARED RADIATION BACKGROUNDS

The cross-correlation between the 21 cm emission from the high-redshift intergalactic medium and the near-infrared (NIR) background light from the high-redshift galaxies promises to be a powerful probe of cosmic reionization. In this paper, we investigate the cross power spectrum during the epoch of reionization. We employ an improved halo approach to derive the distribution of the density field and consider two stellar populations in the star formation model: metal-free stars and metal-poor stars. The reionization history is further generated to be consistent with the electron-scattering optical depth from cosmic microwave background measurements. Then the intensity of NIR background is estimated by collecting emission from stars in the first-light galaxies. On large scales, we find the 21 cm and NIR radiation backgrounds are positively correlated during the very early stages of reionization. However, these two radiation backgrounds quickly become anti-correlated as reionization proceeds. The maximum absolute value of the cross power spectrum is $|\Delta^2_{\rm{21,NIR}}|\sim10^{-4}$ mK nW $\rm{m}^{-2}$ $\rm{sr}^{-1}$ reached at $\ell\sim1000$, when the mean fraction of ionized hydrogen is $\bar{x}_{i}\sim0.9$. We find that SKA can measure the 21 cm-NIR cross power spectrum in conjunction with mild extensions to the existing CIBER survey, provided that the integration time independently adds up to 1000 and 1 hours for 21 cm and NIR observations, and that the sky coverage fraction of CIBER survey is extended from $4\times10^{-4}$ to 0.1. Measuring the cross-correlation signal as a function of redshift provides valuable information on reionization and helps confirm the origin of the "missing" NIR background.

THE CONTRIBUTION OFz≲ 6 SOURCES TO THE SPATIAL COHERENCE IN THE UNRESOLVED COSMIC NEAR-INFRARED AND X-RAY BACKGROUNDS

A spatial clustering signal has been established in Spitzer/IRAC measurements of the unresolved Cosmic near-Infrared Background (CIB) out to large angular scales, ~1 deg. This CIB signal, while significantly exceeding the contribution from the remaining known galaxies, was further found to be coherent at a highly statistically significant level with the unresolved soft Cosmic X-ray Background (CXB). This measurement probes the unresolved CXB to very faint source levels using deep near-IR source subtraction. We study contributions from extragalactic populations at low to intermediate redshifts to the measured positive cross-power signal of the CIB fluctuations with the CXB. We model the X-ray emission from AGN, normal galaxies and hot gas residing in virialized structures, calculating their CXB contribution including their spatial coherence with all infrared emitting counterparts. We use a halo model framework to calculate the auto and cross-power spectra of the unresolved fluctuations based on the latest constraints of the halo occupation distribution and the biasing of AGN, galaxies and diffuse emission. At small angular scales (<1'), the 4.5mic vs 0.5-2 keV coherence can be explained by shot noise from galaxies and AGN. However, at large angular scales (~10 arcmin) we find that the net contribution from the modeled populations is only able to account for ~3% of the measured CIBxCXB cross-power. The discrepancy suggests that the CIBxCXB signal originates from the same unknown source population producing the CIB clustering signal out to ~1 deg.

Stars and reionization: the cross-correlation of the 21 cm line and the near-infrared background

With improving telescopes, it may now be possible to observe the Epoch of Reionization in multiple ways. We examine two of these observables - the excess light in the near-infrared background that may be due to high redshift stars and ionized HII bubbles, and the 21 cm emission from neutral hydrogen. Because these two forms of emission should result from different, mutually exclusive regions, an anticorrelation should exist between them. We discuss the strengths of using cross-correlations between these observations to learn more about high redshift star formation and reionization history. In particular, we create simulated maps of emission from both the near-infrared background and 21 cm emission. We find that these observations are anticorrelated, with the strongest anticorrelation originating from times when the universe is half ionized. This result is robust and does not depend on the properties of the stars themselves. Rather, it depends on the ionization history. Cross-correlations can provide redshift information, which the near-infrared background cannot provide alone. In addition, cross-correlations can help separate foreground emission from the true high redshift component, making it possible to say with greater certainty that we are indeed witnessing the Epoch of Reionization.

The end of an era – the Population III to Population II transition and the near-infrared background

There are only a few ways to constrain the Era of Reionization and the properties of high redshift (z>6) stars through observations. Here, we discuss one of these observables - the spectrum of the Near Infrared Background - and how it is potentially affected by the transition from Population III to Population II stars. The stronger Lyman-alpha emission expected from massive Population III stars could result in a 'bump' in the spectrum of the Near Infrared Background (referred to in this work as the Lyman-alpha bump). The strength and shape of this bump can reveal properties of Population III stars. The Lyman-alpha bump is predicted to be higher if Population III stars are more massive and present at lower redshifts. The shape of the bump is governed by the star formation rate and the time it takes Population III stars to transition to Population II stars. If Population III stars are indeed massive, a bump is predicted as long as Population III stars exist at z < 15, even if their star formation rate is as low as 10^-7 M_sun yr^-1 Mpc^-3. This means that there may be some observational signature in the Near Infrared Background of small pockets of metal-free gas forming Population III stars at z ~ 6, even if they are quite rare.

Infrared background signatures of the first black holes

Angular fluctuations of the Near InfraRed Background (NIRB) intensity are observed up to scales $\simlt 1^{\ensuremath{^{\circ}}}$. Their interpretation is challenging as even after removing the contribution from detected sources, the residual signal is $>10$ times higher than expected from distant galaxies below the detection limit and first stars. We propose here a novel interpretation in which early, intermediate mass, accreting direct collapse black holes (DCBH), which are too faint to be detected individually in current surveys, could explain the observed fluctuations. We find that a population of highly obscured ($N_{\rm H}\simgt 10^{25} \rm cm^{-2}$) DCBHs formed in metal-free halos with virial temperature $10^4$ K at $z\simgt 12$, can explain the observed level $\approx 10^{-3}$ (nW m$^{-2}$ sr$^{-1})^2$ of the 3.6 and 4.5 $\mu$m fluctuations on scales $>100''$. The signal on smaller scales is instead produced by undetected galaxies at low and intermediate redshifts. Albeit Compton-thick, at scales $\theta> 100''$ DCBHs produce a CXB (0.5-2 keV)-NIRB ($4.5 \rm \mu m$) cross-correlation signal of $\simeq 10^{-11}$ erg s$^{-1}$ cm$^{-2}$ nW m$^{-2}$ sr$^{-1}$ slightly dependent on the specific value of the absorbing gas column ($N_{\rm H} \approx 10^{25} \rm cm^{-2}$) adopted and in agreement with the recent measurements by \cite{2012arXiv1210.5302C}. At smaller scales the cross-correlation is dominated by the emission of high-mass X-ray binaries (HMXB) hosted by the same low-$z$, undetected galaxies accounting for small scale NIRB fluctuations. These results outline the great potential of the NIRB as a tool to investigate the nature of the first galaxies and black holes.

The contribution of high-redshift galaxies to the near-infrared background

Several independent measurements have confirmed the existence of fluctuations ($\delta F_{\rm obs}\approx 0.1 \rm nW/m^{2}/sr$ at $3.6 \rm \mu m$) up to degree angular scales in the source-subtracted Near InfraRed Background (NIRB) whose origin is unknown. By combining high resolution cosmological N-body/hydrodynamical simulations with an analytical model, and by matching galaxy Luminosity Functions (LFs) and the constraints on reionization simultaneously, we predict the NIRB absolute flux and fluctuation amplitude produced by high-$z$ ($z > 5$) galaxies (some of which harboring Pop III stars, shown to provide a negligible contribution). This strategy also allows us to make an empirical determination of the evolution of ionizing photon escape fraction: we find $f_{\rm esc} = 1$ at $z \ge 11$, decreasing to $\approx 0.05$ at $z = 5$. In the wavelength range $1.0-4.5 \rm \mu m$, the predicted cumulative flux is $F =0.2-0.04 \rm nW/m^2/sr$. However, we find that the radiation from high-$z$ galaxies (including those undetected by current surveys) is insufficient to explain the amplitude of the observed fluctuations: at $l=2000$, the fluctuation level due to $z > 5$ galaxies is $\delta F = 0.01-0.002 \rm nW/m^2/sr$, with a relative wavelength-independent amplitude $\delta F/F = 4$%. The source of the missing power remains unknown. This might indicate that an unknown component/foreground, with a clustering signal very similar to that of high-$z$ galaxies, dominates the source-subtracted NIRB fluctuation signal.

Infrared light from wandering stars

An explanation has been proposed for the observed excess of cosmic light at infrared wavelengths. It invokes stars that are cast into the dark-matter haloes of their parent galaxies during powerful galaxy collisions. See Letter p.514 During the past two decades, space telescopes have detected variations in the near-infrared background radiation that cannot be explained by emissions from known galaxies. Suggested sources for this excess radiation have included contributions from the earliest galaxies during the epoch of reionization, and faint, dwarf galaxies at intermediate redshifts. These sources would leave characteristic imprints on the spatial variation of the near-infrared background, but previous measurements have not sampled at sufficiently large spatial scales to distinguish these signatures. Asantha Cooray et al. now report measurements from the Spitzer telescope that sample at angular scales of as much as one degree, and find that the previously suggested sources do not explain the data. Rather, they suggest that the fluctuations arise from intrahalo stars that were stripped from their host galaxies during galactic collisions and have migrated to distant orbits in the galaxies' dark-matter haloes.

Near-infrared background anisotropies from diffuse intrahalo light of galaxies

Unresolved anisotropies of the cosmic near-infrared background radiation are expected to have contributions from the earliest galaxies during the epoch of reionization and from faint, dwarf galaxies at intermediate redshifts. Previous measurements were unable to pinpoint conclusively the dominant origin because they did not sample spatial scales that were sufficiently large to distinguish between these two possibilities. Here we report a measurement of the anisotropy power spectrum from subarcminute to one-degree angular scales, and find the clustering amplitude to be larger than predicted by the models based on the two existing explanations. As the shot-noise level of the power spectrum is consistent with that expected from faint galaxies, a new source population on the sky is not necessary to explain the observations. However, a physical mechanism that increases the clustering amplitude is needed. Motivated by recent results related to the extended stellar light profile in dark-matter haloes, we consider the possibility that the fluctuations originate from intrahalo stars of all galaxies. We find that the measured power spectrum can be explained by an intrahalo light fraction of 0.07 to 0.2 per cent relative to the total luminosity in dark-matter haloes of 10(9) to 10(12) solar masses at redshifts of about 1 to 4.

THE NEAR-INFRARED BACKGROUND INTENSITY AND ANISOTROPIES DURING THE EPOCH OF REIONIZATION

A fraction of the extragalactic near-infrared (near-IR) background light involves redshifted photons from the ultraviolet (UV) emission from galaxies present during reionization at redshifts above 6. The absolute intensity and the anisotropies of the near-IR background provide an observational probe of the first-light galaxies and their spatial distribution. We estimate the extragalactic background light intensity during reionization by accounting for the stellar and nebular emission from first-light galaxies. We require the UV photon density from these galaxies to generate a reionization history that is consistent with the optical depth to electron scattering from cosmic microwave background measurements. We also require the bright-end luminosity function of galaxies in our models to reproduce the measured Lyman drop-out luminosity functions at redshifts of 6 to 8. The absolute intensity is about 0.1 to 0.3 nW m$^{-2}$ sr$^{-1}$ at the peak of its spectrum at $\sim$ 1.1 $\mu$m. We also discuss the anisotropy power spectrum of the near-IR background using a halo model to describe the galaxy distribution. We compare our predictions for the anisotropy power spectrum to existing measurements from deep near-IR imaging data from {\it Spitzer}/IRAC, {\it Hubble}/NICMOS, and {\it AKARI}. The predicted rms fluctuations at tens of arcminute angular scales are roughly an order of magnitude smaller than the existing measurements. While strong arguments have been made that the measured fluctuations do not have an origin involving faint low-redshift galaxies, we find that the existing measurements are also incompatible with an origin during the era of reionization. The measured near-IR background anisotropies remain unexplained and could be associated with an unidentified non-astrophysical origin.

RESOLVED MAGNETIC FIELD MAPPING OF A MOLECULAR CLOUD USING GPIPS

We present the first resolved map of plane-of-sky magnetic field strength for a quiescent molecular cloud. GRSMC 45.60+0.30 subtends 40 x 10 pc at a distance of 1.88 kpc, masses 16,000 M_sun, and exhibits no star formation. Near-infrared background starlight polarizations were obtained for the Galactic Plane Infrared Polarization Survey using the 1.8 m Perkins telescope and the Mimir instrument. The cloud area of 0.78 deg2 contains 2684 significant starlight polarizations for Two Micron All Sky Survey matched stars brighter than 12.5 mag in the H band. Polarizations are generally aligned with the cloud's major axis, showing an average position angle dispersion of 15 \pm 2{\deg} and polarization of 1.8 \pm 0.6%. The polarizations were combined with Galactic Ring Survey 13CO spectroscopy and the Chandrasekhar-Fermi method to estimate plane-of-sky magnetic field strengths, with an angular resolution of 100 arcsec. The average plane-of-sky magnetic field strength across the cloud is 5.40 \pm 0.04 {\mu}G. The magnetic field strength map exhibits seven enhancements or "magnetic cores." These cores show an average magnetic field strength of 8.3 \pm 0.9 {\mu}G, radius of 1.2 \pm 0.2 pc, intercore spacing of 5.7 \pm 0.9 pc, and exclusively subcritical mass-to-flux ratios, implying their magnetic fields continue to suppress star formation. The magnetic field strength shows a power-law dependence on gas volume density, with slope 0.75 \pm 0.02 for n_{H_2} >=10 cm-3. This power-law index is identical to those in studies at higher densities, but disagrees with predictions for the densities probed here.

THE COSMIC NEAR INFRARED BACKGROUND. III. FLUCTUATIONS, REIONIZATION, AND THE EFFECTS OF MINIMUM MASS AND SELF-REGULATION

Current observations suggest that the universe was reionized sometime before z~6. One way to observe this epoch of the universe is through the Near Infrared Background (NIRB), which contains information about galaxies which may be too faint to be observed individually. We calculate the angular power spectrum (C_l) of the NIRB fluctuations caused by the distribution of these galaxies. Assuming a complete subtraction of any post-reionization component, C_l will be dominated by galaxies responsible for completing reionization (e.g., z~6). The shape of C_l at high l is sensitive to the amount of non-linear bias of dark matter halos hosting galaxies. As the non-linear bias depends on the mass of these halos, we can use the shape of C_l to infer typical masses of dark matter halos responsible for completing reionization. We extend our previous study by using a higher-resolution N-body simulation, which can resolve halos down to 10^8 M_sun. We also include improved radiative transfer, which allows for the suppression of star formation in small-mass halos due to photo-ionization heating. As the non-linear bias enhances the dark-matter-halo power spectrum on small scales, we find that C_l is steeper for the case with a complete suppression of small sources or partial suppression of star formation in small halos (the minimum galaxy mass is M_min=10^9 M_sun in ionized regions and M_min=10^8 M_sun in neutral regions) than the case in which these small halos were unsuppressed. In all cases, we do not see a turn-over toward high l in the shape of l^2 C_l.

Constraining the near-infrared background light from Population III stars using high-redshift gamma-ray sources

The Fermi satellite has detected GeV emission from a number of gamma-ray bursts and active galactic nuclei at high redshift, z > 1.5. We examine the constraints that the detections of gamma rays from several of these sources place on the contribution of population-III stars to the extragalactic background light. Emission from these primordial stars, particularly redshifted Lyman-alpha emission, can interact with gamma rays to produce electron-positron pairs and create an optical depth to the propagation of gamma-ray emission, and the detection of emission above 10 GeV can therefore constrain the production of this background. We consider two initial mass functions for the early stars, and use derived SEDs for each to put upper limits on the star-formation rate density of massive early stars from redshifts 6 to 10. Our limits are complementary to those set on a high near-IR background flux by ground-based TeV-scale observations, and show that current data can limit star-formation in the late stages of reionization to less than 0.5 M_solar yr^-1 Mpc^-3. Our results also show that the total background flux from population-III stars must be considerably less than that from resolved galaxies at wavelengths below 1.5 microns.

THE COSMIC NEAR-INFRARED BACKGROUND. II. FLUCTUATIONS

The near-infrared background (NIRB) is one of a few methods that can be used to observe the redshifted light from early stars at a redshift of 6 and above, and thus it is imperative to understand the significance of any detection or nondetection of the NIRB. Fluctuations of the NIRB can provide information on the first structures, such as halos and their surrounding ionized regions in the intergalactic medium (IGM). We combine, for the first time, N-body simulations, radiative transfer code, and analytic calculations of luminosity of early structures to predict the angular power spectrum (Cl) of fluctuations in the NIRB. We study in detail the effects of various assumptions about the stellar mass, the initial mass spectrum of stars, the metallicity, the star formation efficiency (f*), the escape fraction of ionizing photons (fesc), and the star formation timescale (tSF), on the amplitude as well as the shape of Cl. The power spectrum of NIRB fluctuations is maximized when f* is the largest (as Cl ∝ f2*) and fesc is the smallest (as more nebular emission is produced within halos). A significant uncertainty in the predicted amplitude of Cl exists due to our lack of knowledge of tSF of these early populations of galaxies, which is equivalent to our lack of knowledge of the mass-to-light ratio of these sources. We do not see a turnover in the NIRB angular power spectrum of the halo contribution, which was claimed to exist in the literature, and explain this as the effect of high levels of nonlinear bias that was ignored in the previous calculations. This is partly due to our choice of the minimum mass of halos contributing to NIRB (∼2 × 109 M☉), and a smaller minimum mass, which has a smaller nonlinear bias, may still exhibit a turnover. Therefore, our results suggest that both the amplitude and shape of the NIRB power spectrum provide important information regarding the nature of sources contributing to the cosmic reionization. The angular power spectrum of the IGM, in most cases, is much smaller than the halo angular power spectrum, except when fesc is close to unity, tSF is longer, or the minimum redshift at which the star formation is occurring is high. In addition, low levels of the observed mean background intensity tend to rule out high values of f* ≳ 0.2.

The Effect of AGN and SNe Feedback on Star Formation, Reionization and the Near Infrared Background

Feedback from supernovae (SNe) and from active galactic nuclei (AGN) accompanies the history of star formation and galaxy evolution. We present an analytic model to explain how and when the SNe and AGN exert their feedback effects on the star formation and galaxy evolution processes. By using SNe and AGN kinetic feedback mechanisms based on the Lambda Cold Dark Matter (LCDM) model, we explore how these feedback mechanisms affect the star formation history (SFH), the Near-Infrared Background (NIRB) flux and the cosmological reionization. We find the values of the feedback strengths, AGN = 1.0+0.5−0.3 and SN = 0.04+0.02−0.02, can provide a reasonable explanation of most of the observational results, and that the AGN feedback effect on star formation history is quite different from the SNe feedback at high redshifts. Our conclusions manifest quantitatively that these feedback effects decrease star formation rate density (SFRD) and the NIRB flux (in 1.4 - 4.0 μm), and postpone the time of completion of the cosmological reionization.

적외선 우주배경복사 관측 실험 검교정

The international cooperation project CIBER (Cosmic Infrared Background ExpeRiment) is a rocket-borne instrument, of which the scientific goal is to measure the cosmic near-infrared extra-galactic background to search for signatures of primordial galaxy formation. CIBER consists of a wide-field two-color camera, a low-resolution absolute spectrometer, and a high-resolution narrow-band imaging spectrometer. Currently, all the subsystems have been built, and the integration, testing, and calibration of the CIBER system are on process for the scheduled launch in June 2008.

Constraints on the Cosmic Near‐Infrared Background Excess from NICMOS Deep Field Observations

NICMOS observations of the resolved object fluxes in the Hubble Deep Field-North and the Hubble Ultra Deep Field are significantly below the fluxes attributed to a 1.4-1.8 μm near-infrared background excess (NIRBE) from previous low spatial resolution NIRS measurements. Tests placing sources in the NICMOS image with fluxes sufficient to account for the NIRBE indicate that the NIRBE flux must be either flat on scales greater than 100'' or clumped on scales of several arcminutes to avoid detection in the NICMOS image. A fluctuation analysis of the new NICMOS data shows a fluctuation spectrum consistent with that found at the same wavelength in deep 2MASS calibration images. The fluctuation analysis shows that the majority of the fluctuation power comes from resolved galaxies at redshifts of 1.5 and less and that the fluctuations observed in the earlier deep 2MASS observations can be completely accounted for with normal low-redshift galaxies. Neither the NICMOS direct flux measurements nor the fluctuation analysis require an additional component of near-infrared flux other than the flux from normal resolved galaxies in the redshift range between 0 and 7. The residual fluctuations in the angular range between 1'' and 10'' is 1-2 nW m-2 sr-1, which is at or above several predictions of fluctuations from high redshift Population III objects, but inconsistent with attributing the entire NIRBE to high-redshift galaxies.

적외선 우주배경복사 관측 실험 국제 공동 연구

A Korean team (Korea Astronomy and Space Science Institute, Korea Basic Science Institute, and Kyung Hee University) takes part in an international cooperation project called CIBER (Cosmic Infrared Background ExpeRiment), which has begun with Jet Propulsion Laboratory (JPL) in USA and Institute of Space and Astronautical Science (ISAS) in Japan. CIBER is a rocket-borne instrument, of which the scientific goal is to measure the cosmic near-infrared extra-galactic background to search for signatures of primordial galaxy formation. CIBER consists of a wide-field two-color camera, a low-resolution absolute spectrometer, and a high-resolution narrow-band imaging spectrometer. The Korean team is in charge of the ground support electronics and manufacturing of optical parts of the narrow-band spectrometer, which will provide excellent opportunities for science and technology to Korean infrared groups.

Where are the sources of the near-infrared background?

Abstract The observed near-infrared background excess over light from known galaxies is commonly ascribed to redshifted radiation from early, very massive, Population III (Pop III) stars. We show here that this interpretation must be discarded as it largely overpredicts the number of J-dropouts and Lyα emitters in ultradeep field searches. Independently of the detailed physics of Lyα line emission, J-dropouts limit the background excess fraction due to Pop III sources to be (at best) ⩽1/24. As alternative explanations can either be rejected (e.g. miniquasars, decaying neutrinos) or appear unlikely (zodiacal light), but the reality of the excess is supported by the interpretation of the angular fluctuations, the origin of this component remains very puzzling. We briefly discuss possible hints to solve the problem.

The Near‐Infrared Background: Interplanetary Dust or Primordial Stars?

The intensity of the diffuse ~ 1 - 4 micron sky emission from which solar system and Galactic foregrounds have been subtracted is in excess of that expected from energy released by galaxies and stars that formed during the z < 5 redshift interval (Arendt & Dwek 2003, Matsumoto et al. 2005). The spectral signature of this excess near-infrared background light (NIRBL) component is almost identical to that of reflected sunlight from the interplanetary dust cloud, and could therefore be the result of the incomplete subtraction of this foreground emission component from the diffuse sky maps. Alternatively, this emission component could be extragalactic. Its spectral signature is consistent with that of redshifted continuum and recombination line emission from HII regions formed by the first generation of very massive stars. In this paper we analyze the implications of this spectral component for the formation rate of these Population III stars, the redshift interval during which they formed, the reionization of the universe and evolution of collapsed halo masses. We find that to reproduce the intensity and spectral shape of the NIRBL requires a peak star formation rate that is higher by about a factor of 4 to 10 compared to those derived from hierarchical models. Furthermore, an extragalactic origin for the NIRBL leads to physically unrealistic absorption-corrected spectra of distant TeV blazars. All these results suggest that Pop III stars contribute only a fraction of the NIRBL intensity with zodiacal light, star forming galaxies, and/or non-nuclear sources giving rise to the remaining fraction.

Gamma-ray constraints on the infrared background excess

Motivated by the idea that the recently detected near-infrared (1.2–4 μm) excess over the contribution of known galaxies is due to redshifted light from the first cosmic stars [MNRAS 339 (2003) 973], we have used the effect caused by photon–photon absorption on gamma-ray spectra of blazars to put constraints on extragalactic background light (EBL) from the optical to the far-IR bands. Our analysis is mainly based on the blazar H 1426+428, for which we assume a power-law unabsorbed spectrum. We find that an EBL model with no excess over known galaxies in the near-infrared background (NIRB) is in agreement with all the considered blazars; however, it implies a very peculiar intrinsic spectrum for H 1426+428. Additional data on the blazars 1ES1101-232, H 2356-309 and PKS 2155-304 exclude the existence of a strong NIRB excess consistent with Kelsall’s model of zodiacal light subtraction (ZL); the COBE/DIRBE measurements, after Wright’s model ZL subtraction, represent a firm NIRB upper limit. The constraints on the optical EBL are weaker, due to the fact that predictions from different optical EBL models are often comparable to the experimental errors. In the mid-infrared the SPITZER measurement of νI ν = 2.7 nW m −2 sr −1 at 24 μm gives a good fit for all the considered blazars.