Extragalactic Background Light: new constraints from the study of the photon-photon absorption on blazar spectra

The study of the Extragalactic Background Light (EBL) is crucial to understand many astrophysical problems (as the formation of first stars, the evolution of galaxies and the role of dust emission). At present, one of the most powerful ways to put constraints on EBL is represented by the study of the photon-photon absorption on gamma-ray spectra of TeV blazars. Adopting this method, we found that, if the only contribution to the optical and Near Infrared (NIR) background is given by galaxies, the spectrum of the blazar H1426+428 cannot be fitted. To reproduce the observational data of H1426+428 a Near Infrared excess with respect to galaxy counts is required, with amplitude consistent with both the Matsumoto et al. (2000) data with Kelsall's model of zodiacal light (ZL) subtraction and the DIRBE data with Wright's model of ZL subtraction. The derived constraints on the optical EBL are weaker, because the experimental errors on blazar data are still bigger than the differences among various optical EBL models. In the mid-infrared the SPITZER measurement at 24 micron provides the best fit of the blazar spectrum.Comment: 8 pages, 5 figures, to appear in Proceedings of "Baryons in Dark Matter Halos", 5-9 October 2004, Novigrad, Croati

First Stars Contribution to the Near Infrared Background Fluctuations

We show that the emission from the first, metal-free stars inside Population III objects (PopIIIs) are needed to explain the level of fluctuations in the Near Infrared Background (NIRB) recently discovered by Kashlinsky et al. (2002), at least at the shortest wavelengths. Clustering of (unresolved) Pop IIIs can in fact account for the entire signal at almost all the ~1-30 arcsec scales probed by observations in the J band. Their contribution fades away at shorter frequencies and becomes negligible in the K band. ``Normal'', highly clustered, ~3 galaxies undergoing intense star-formation such as those found in the Hubble Deep Fields can 'fill in' this gap and provide for the missing signal. It is in fact found that their contribution to the intensity fluctuations is the dominant one at lambda=2.17 mum, while it gradually looses importance in the H and J bands. The joint contribution from these two populations of cosmic objects is able, within the errors, to reproduce the observed power spectrum in the whole Near Infrared range on small angular scales (theta < 200" for Pop III protogalaxies). Signals on larger scales detected by other experiments instead require the presence of more local sources.Comment: 6 pages, 4 figures, submitted to MNRA

Missing cosmic metals revealed by X-ray absorption towards distant sources

The census of heavy elements (metals) produced by all stars through cosmic times up to present-day is limited to ~50%; of these only half are still found within their parent galaxy. The majority of metals is expelled from galaxies into the circumgalactic (or even more distant, intergalactic) space by powerful galactic winds, leaving unpleasant uncertainty on the amount, thermal properties and distribution of these key chemical species. These dispersed metals unavoidably absorb soft X-ray photons from distant sources. We show that their integrated contribution can be detected in the form of increasing X-ray absorption with distance, for all kinds of high-energy cosmic sources. Based on extensive cosmological simulations, we assess that $\sim$ 10\% of all cosmic metals reside in the intergalactic medium. Most of the X-ray absorption arises instead from a few discrete structures along the line of sight. These extended structures, possibly pin-pointing galaxy groups, contain million degree, metal-enriched gas, 100-1,000 times denser than the cosmic mean. An additional ~10% of cosmic metals could reside in this phase.Comment: Accepted for publication in Astronomy & Astrophysics. 9 pages, 4 figures, 1 tabl

XMM-Newton Slew Survey observations of the gravitational wave event GW150914

The detection of the first gravitational wave (GW) transient GW150914 prompted an extensive campaign of follow-up observations at all wavelengths. Although no dedicated XMM-Newton observations have been performed, the satellite passed through the GW150914 error box during normal operations. Here we report the analysis of the data taken during these satellite slews performed two hours and two weeks after the GW event. Our data cover 1.1 square degrees and 4.8 square degrees of the final GW localization region. No credible X-ray counterpart to GW150914 is found down to a sensitivity of 6E-13 erg/cm2/s in the 0.2-2 keV band. Nevertheless, these observations show the great potential of XMM-Newton slew observations for the search of the electromagnetic counterparts of GW events. A series of adjacent slews performed in response to a GW trigger would take <1.5 days to cover most of the typical GW credible region. We discuss this scenario and its prospects for detecting the X-ray counterpart of future GW detections.Comment: 6 pages, 3 figures, 2 tables. Accepted for publication in ApJ Letter

Testing Reionization with Gamma Ray Burst Absorption Spectra

We propose to study cosmic reionization using absorption line spectra of high-redshift Gamma Ray Burst (GRB) afterglows. We show that the statistics of the dark portions (gaps) in GRB absorption spectra represent exquisite tools to discriminate among different reionization models. We then compute the probability to find the largest gap in a given width range [Wmax, Wmax + dW] at a flux threshold Fth for burst afterglows at redshifts 6.3 < z < 6.7. We show that different reionization scenarios populate the (Wmax, Fth) plane in a very different way, allowing to distinguish among different reionization histories. We provide here useful plots that allow a very simple and direct comparison between observations and model results. Finally, we apply our methods to GRB 050904 detected at z = 6.29. We show that the observation of this burst strongly favors reionization models which predict a highly ionized intergalactic medium at z~6, with an estimated mean neutral hydrogen fraction xHI = 6.4 \pm 0.3 \times 10^-5 along the line of sight towards GRB 050904.Comment: 5 pages, 3 figures, revised to match the accepted version; major change: gap statistics is now studied in terms of the flux threshold Fth, instead of the observed J-band flux FJ; MNRAS in pres

Lyman Alpha Emitter Evolution in the Reionization Epoch

Combining cosmological SPH simulations with a previously developed Lyman Alpha production/transmission model and the Early Reionization Model (ERM, reionization ends at redshift z~7), we obtain Lyman Alpha and UV Luminosity Functions (LFs) for Lyman Alpha Emitters (LAEs) for redshifts between 5.7 and 7.6. Matching model results to observations at z~5.7 requires escape fractions of Lyman Alpha, f_alpha=0.3, and UV (non-ionizing) continuum photons, f_c=0.22, corresponding to a color excess, E(B-V)=0.15. We find that (i) f_c increases towards higher redshifts, due the decreasing mean dust content of galaxies, (ii) the evolution of f_alpha/f_c hints at the dust content of the ISM becoming progressively inhomogeneous/clumped with decreasing redshift. The clustering photoionization boost is important during the initial reionization phases but has little effect on the Lyman Alpha LF for a highly ionized IGM. Halo (stellar) masses are in the range 10.0 < \log M_h < 11.8 (8.1 < \log M_* < 10.4) with M_h \propto M_*^{0.64}. The star formation rates are between 3-120 solar masses per year, mass-weighted mean ages are greater than 20 Myr at all redshifts, while the mean stellar metallicity increases from Z=0.12 to 0.22 solar metallicity from z~7.6 to z~5.7; both age and metallicity positively correlate with stellar mass. The brightest LAEs are all characterized by large star formation rates and intermediate ages (~200 Myr), while objects in the faint end of the Lyman Alpha LF show large age and star formation rate spreads. With no more free parameters, the Spectral Energy Distributions of three LAE at z~5.7 observed by Lai et al. (2007) are well reproduced by an intermediate age (182-220 Myr) stellar population and the above E(B-V) value.Comment: 13 pages, 9 figures, accepted to MNRA

Where are the sources of the Near Infrared Background?

The observed near infrared background excess over light from known galaxies is commonly ascribed to redshifted radiation from early, very massive, PopIII stars. We show here that this interpretation must be discarded as it largely overpredicts the number of J-dropouts and Ly\alpha emitters in ultra deep field searches. Independently of the detailed physics of Ly\alpha line emission, J-dropouts limit the background excess fraction due to PopIII sources to be (at best) < 1/24. As alternative explanations can either be rejected (e.g. miniquasars, decaying neutrinos) or appear unlikely (zodiacal light), whereas 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.Comment: 5 pages, 3 figures; MNRAS in pres

Is Primordial He Truly from Big Bang ?

Population III stars are now believed to contribute to the observed Near Infrared Background (NIRB) and heavy element pollution of the intergalactic medium. Here we show that a Pop III contribution to the primordial He abundance consistent with NIRB constraints, might mask a lower value for the Big Bang Nucleosynthesis (BBN) He abundance.Comment: 5 pages, 2 figures. Accepted for publication on MNRA

The Imprint of the Cosmic Dark Ages on the Near Infrared Background

The redshifted light of the first (Pop III) stars might substantially contribute to the near infrared background (NIRB). By fitting recent data with models including up-to-date Pop III stellar spectra, we find that such stars can indeed account for the whole NIRB residual (i.e. after `normal' galaxy contribution subtraction) if the high redshift star formation efficiency is f = 10%-50%, depending on the IMF (the top-heaviest requiring lowest efficiency) and on the unknown galaxy contribution in the L band (our models, however, suggest it to be negligible). Such epoch of Pop III star formation ends in all models by z_end ~ 8.8, with a hard limit z_end < 9 set by J band observations. To prevent an associated IGM over-enrichment with heavy elements compared to observed levels in the IGM, pair-instability supernovae must be the dominant heavy element sources. Alternative explanations must break the light-metal production link by advocating very massive stars (M > 260 M_sun) locking their nucleosynthetic products in the compact remnant or by postulating an extremely inhomogeneous metal enrichment of the Ly alpha forest. We discuss these possibilities in detail along with the uncertainties related to the adopted zodiacal light model.Comment: 11 pages, 7 figures, submitted to MNRA