1,721,210 research outputs found
The inprint of cosmic dark ages on the near-infrared background
No full textThe redshifted light of the first (Population III) stars might contribute substantially to the near-infrared background (NIRB). By fitting recent data with models including up-to-date Population 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 per cent, depending on the initial mass function (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 an epoch of Population III star formation ends in all models by zend≈ 8.8, with a hard limit zend < 9 set by J-band observations. To prevent an associated intergalactic medium (IGM) overenrichment with heavy elements compared with observed levels in the IGM, pair-instability supernovae must be the dominant heavy element source. Alternative explanations must break the light—metal production link by advocating very massive stars M > 260 M⊙, locking their nucleosynthetic products in the compact remnant or by postulating an extremely inhomogeneous metal enrichment of the Lyα forest. We discuss these possibilities in detail along with the uncertainties related to the adopted zodiacal light model
Where Are the Sources of the Near Infrared Background ?
No full textThe 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
Induced formation of primordial low-mass stars
No full textWe show that the explosion of the first supernovae can trigger low-mass star formation via gravitational fragmentation of the supernova-driven gas shell. If the shell mass does not exceed the host galaxy gas mass, all explosions with energies ESN>=1051 erg can lead to shell fragmentation. However, the minimum ambient density required to induce such fragmentation is much larger, n0>300 cm-3, for Type II supernovae than for pair-instability ones, which can induce star formation even at lower ambient densities. The typical mass of the unstable fragments is ~104-7Msolar their density is in the range 110-6×107 cm-3. Fragments have a metallicity strictly lower than 10-2.6Zsolar and large values of the gravitational-to-pressure force ratio f~=8. Based on these findings, we conclude that the second generation of stars produced by such self-propagating star formation is predominantly constituted by low-mass, long-living, extremely metal-poor (or even metal-free, if mixing is suppressed) stars. We discuss the implications of such results for Pop III star formation scenarios and for the most iron-poor halo star HE0107-5240
Counts of high-redshift GRBs as probes of primordial non-Gaussianities
No full textWe propose to use high-redshift long γ-ray bursts (GRBs) as cosmological tools to constrain the amount of primordial non-Gaussianity in the density field. By using numerical, N-body, hydrodynamic, chemistry simulations of different cosmological volumes with various Gaussian and non-Gaussian models, we self-consistently relate the cosmic star formation rate density to the corresponding GRB rate. Assuming that GRBs are fair tracers of cosmic star formation, we find that positive local non-Gaussianities, described in terms of the non-linear parameter, fNL, might boost significantly the GRB rate at high redshift, z ≫ 6. Deviations with respect to the Gaussian case account for a few orders of magnitude if fNL ̃ 1000, one order of magnitude for fNL ̃ 100 and a factor of ̃2 for fNL ̃ 50. These differences are found only at large redshift, while at later times the rates tend to converge. Furthermore, a comparison between our predictions and the observed GRB data at z > 6 allows us to exclude large negative fNL, consistently with previous works. Future detections of any long GRB at extremely high redshift (z ̃ 15-20) could favour non-Gaussian scenarios with positive fNL. More stringent constraints require much larger high-z GRB complete samples, currently not available in the literature. By distinguishing the contributions to the GRB rate from the metal-poor Population III regime, and the metal-enriched Population II-I regime, we conclude that the latter is a more solid tracer of the underlying matter distribution, while the former is strongly dominated by feedback mechanisms from the first, massive, short-lived stars, rather than by possible non-Gaussian fluctuations. This holds quite independently of the assumed Population III initial mass function
Evidence for Luminosity Evolution of Long Gamma-ray Bursts in Swift Data
No full textWe compute the luminosity function (LF) and the formation rate of long gamma ray bursts (GRBs) by fitting the observed differential peak flux distribution obtained by the BATSE experiment in two different scenarios: i) the GRB luminosity evolves with redshift and ii) GRBs form preferentially in low-metallicity environments. In both cases, model predictions are consistent with the Swift number counts and with the number of detections at z>2.5 and z>3.5. To discriminate between the two evolutionary scenarios, we compare the model results with the number of luminous bursts (i.e. with isotropic peak luminosity in excess of 10^53 erg s^-1) detected by Swift in its first three years of mission. Our sample conservatively contains only bursts with good redshift determination and measured peak energy. We find that pure luminosity evolution models can account for the number of sure identifications. In the case of a pure density evolution scenario, models with Z_th>0.3 Zsun are ruled out with high confidence. For lower metallicity thresholds, the model results are still statistically consistent with available lower limits. However, many factors can increase the discrepancy between model results and data, indicating that some luminosity evolution in the GRB LF may be needed also for such low values of Z_th. Finally, using these new constraints, we derive robust upper limits on the bright-end of the GRB LF, showing that this cannot be steeper than ~2.6
Gamma-ray constraints on the infrared background excess
No full textMotivated 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 νIν = 2.7 nW m-2 sr-1 at 24 μm gives a good fit for all the considered blazars
On the search of electromagnetic cosmological counterparts to coalescences of massive black hole binaries
No full textReionization history from coupled cosmic microwave background/21-cm line data
No full textWe study cosmic microwave background (CMB) secondary anisotropies produced by inhomogeneous reionization by means of cosmological simulations coupled with the radiative transfer code crash. The reionization history is consistent with the Wilkinson Microwave Anisotropy Probe Thomson optical depth determination. We find that the signal arising from this process dominates over the primary CMB component for l≳ 4000 and reaches a maximum amplitude of l(l+ 1)Cl/2π≃ 1.6 × 10−13 on arcmin scales (i.e. l as large as several thousands). We then cross-correlate secondary CMB anisotropy maps with neutral hydrogen 21-cm line emission fluctuations obtained from the same simulations. The two signals are highly anticorrelated on angular scales corresponding to the typical size of H ii regions (including overlapping) at the 21-cm map redshift. We show how the CMB/21-cm cross-correlation can be used: (i) to study the nature of the reionization sources; (ii) to reconstruct the cosmic reionization history; (iii) to infer the mean cosmic ionization level at any redshift. We discuss the feasibility of the proposed experiment with forthcoming facilities
Dust formation in very massive primordial supernovae
No full textAt redshift z>~ 5, Type II supernovae (SNeII) are the only known dust sources with evolutionary time-scales shorter than the Hubble time. We extend the model of dust formation in the ejecta of SNeII by Todini & Ferrara to investigate the same process in pair-instability supernovae (PISNe), which are though to arise from the explosion of the first, metal-free, very massive (140-260 Msolar) cosmic stars.
We find that 15-30 per cent of the PISN progenitor mass is converted into dust, a value >10 times higher than for SNeII; PISN dust depletion factors (the fraction of produced metals locked into dust grains) range between 0.3 and 0.7. These conclusions depend very weakly on the mass of the PISN stellar progenitor, which in contrast affects considerably the composition and size distribution. For the assumed temperature evolution, grain condensation starts 150-200 d after the explosion; the dominant compounds for all progenitor masses are SiO2 and Mg2SiO4 while the contribution of amorphous carbon and magnetite grains grows with progenitor mass; typical grain sizes range between 10-3 and a few times 0.1 μm and are always smaller than 1 μm. We give a brief discussion of the implications of dust formation for the initial mass function evolution of the first stars, cosmic reionization and the intergalactic medium
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