arXiv subjects

R. Salvaterra

Publications and source records attributed to R. Salvaterra.

At least 181 records · Page 10Linked to original sources

arXiv2007

CMB polarization constraints on radiative feedback

We compute the imprints left on the CMB by two cosmic reionization models consistent with current observations but characterized by alternative radiative feedback prescriptions (suppression and filtering) resulting in a different suppression of star formation in low-mass halos. The models imply different ionization and thermal histories and 21 cm background signals. The derived Comptonization, u, and free-free distortion, y_B, parameters are below current observational limits for both models. However, the value of u = 1.69 * 10^-7 (9.65 * 10^-8) for the suppression (filtering) model is in the detectability range of the next generation of CMB spectrum experiments. Through the dedicated Boltzmann code CMBFAST, modified to include the above ionization histories, we compute the CMB angular power spectrum (APS) of the TT, TE, and EE modes. For the EE mode the differences between these models are significantly larger than the cosmic and sampling variance over the multipole range l = 5-15, leaving a good chance of discriminating between these feedback mechanisms with forthcoming/future CMB polarization experiments. The main limitations come from foreground contamination: it should be subtracted at per cent level in terms of APS, a result potentially achievable by novel component separation techniques and mapping of Galactic foreground.

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arXiv2007

Detectable Signatures of Cosmic Radiative Feedback

We use a semi-analytical model to study the impact of reionization, and the associated radiative feedback, on galaxy formation. Two feedback models have been considered: (i) a standard prescription, according to which star formation is totally suppressed in galaxies with circular velocity below a critical threshold (model CF06) and (ii) a characterization based on the filtering scale (model G00), allowing for a gradual reduction of the gas available for star formation in low-mass galaxies. In model CF06 reionization starts at z ~ 15-20, is 85% complete by z ~ 10; at the same z, the ionized fraction is 16% in model G00. The models match SDSS constraints on the evolution of the neutral hydrogen fraction at z < 7, but predict different Thomson optical depths, tau_e = 0.1017 (CF06), and 0.0631 (G00); such values are within 1 sigma of the WMAP 3-yr determination. Both models are in remarkable good agreement with additional existing data (evolution of Lyman-limit systems, cosmic star formation history, high-z galaxy counts, IGM thermal history), which therefore cannot be used to discriminate among different feedback models. Deviations among radiative feedback prescriptions emerge when considering the expected HI 21 cm background signal, where a ~ 15 mK absorption feature in the range 75-100 MHz is present in model G00 and a global shift of the emission feature preceding reionization towards larger frequencies occurs in the same model. Single dish observations with existing or forthcoming low-frequency radio telescopes can achieve mK sensitivity, allowing the identification of these features provided that foregrounds can be accurately subtracted.

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arXiv2007

Gamma Ray Bursts from the early Universe: predictions for present-day and future instruments

Long Gamma Ray Bursts (GRBs) constitute an important tool to study the Universe near and beyond the epoch of reionization. We delineate here the characteristics of an 'ideal' instrument for the search of GRBs at z>6-10. We find that the detection of these objects requires soft band detectors with a high sensitivity and moderately large FOV. In the light of these results, we compare available and planned GRB missions, deriving conservative predictions on the number of high-z GRBs detectable by these instruments along with the maximum accessible redshift. We show that the Swift satellite will be able to detect various GRBs at z>6, and likely at z>10 if the trigger threshold is decreased by a factor of ~2. Furthermore, we find that INTEGRAL and GLAST are not the best tool to detect bursts at z>6: the former being limited by the small FOV, and the latter by its hard energy band and relatively low sensitivity. Finally, future missions (SVOM, EDGE, but in particular EXIST) will provide a good sample of GRBs at z>6 in a few years of operation.

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arXiv2007

On the detection of very high redshift Gamma Ray Bursts with Swift

We compute the probability to detect long Gamma Ray Bursts (GRBs) at z>5 with Swift, assuming that GRBs form preferentially in low-metallicity environments. The model fits well both the observed BATSE and Swift GRB differential peak flux distribution and is consistent with the number of z>2.5 detections in the 2-year Swift data. We find that the probability to observe a burst at z>5 becomes larger than 10% for photon fluxes P<1 ph s^{-1} cm^{-2}, consistent with the number of confirmed detections. The corresponding fraction of z>5 bursts in the Swift catalog is ~10%-30% depending on the adopted metallicity threshold for GRB formation. We propose to use the computed probability as a tool to identify high redshift GRBs. By jointly considering promptly-available information provided by Swift and model results, we can select reliable z>5 candidates in a few hours from the BAT detection. We test the procedure against last year Swift data: only three bursts match all our requirements, two being confirmed at z>5. Other three possible candidates are picked up by slightly relaxing the adopted criteria. No low-z interloper is found among the six candidates.

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arXiv2007

The Gamma Ray Burst Luminosity Function in the Light of the Swift 2-year Data

We 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 satellite in three different scenarios: i) GRBs follow the cosmic star formation and their LF is constant in time; ii) GRBs follow the cosmic star formation but the LF varies with redshift; iii) GRBs form preferentially in low-metallicity environments. We find that the differential peak flux number counts obtained by BATSE and by Swift can be reproduced using the same LF and GRB formation rate, indicating that the two satellites are observing the same GRB population. We then check the resulting redshift distributions in the light of Swift 2-year data, focusing in particular on the relatively large sample of GRBs detected at z>2.5. We show that models in which GRBs trace the cosmic star formation and are described by a constant LF are ruled out by the number of high-z Swift detections. This conclusion does not depend on the redshift distribution of bursts that lack of optical identification, nor on the existence of a decline in star formation rate at z>2, nor on the adopted faint-end of the GRB LF. Swift observations can be explained by assuming that the LF varies with redshift and/or that GRB formation is limited to low-metallicity environments.

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arXiv2006

Unresolved X-ray background: clues on galactic nuclear activity at z>6

We study, by means of dedicated simulations of massive black hole build-up, the possibility to constraint the existence and nature of the AGN population at z>6 with available and planned X-ray and near infrared space telescopes. We find that X-ray deep-field observations can set important constraints to the faint-end of the AGN luminosity function at very high redshift. Planned X-ray telescopes should be able to detect AGN hosting black holes with masses down to >10^5 Msun (i.e., X-ray luminosities in excess of 10^42 erg s^-1), and can constrain the evolution of the population of massive black hole at early times (6 6 and we discuss the possible role of AGN in the reionization of the Universe at these early epochs, showing that AGN alone can provide enough ionizing photons only in the (improbable) case of an almost completely homogeneous inter-galactic medium. Finally, we show that super-Eddington accretion, suggested by the observed QSOs at z>6, must be a very rare event, confined to black holes living in the highest density peaks.

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arXiv2006

On the search of electromagnetic cosmological counterparts to coalescences of massive black hole binaries

We explore the nature of possible electromagnetic counterparts of coalescences of massive black hole binaries at cosmological distances detectable by the Laser Interferometer Space Antenna (LISA). An electromagnetic precursor, during the last year of gravitational wave (GW)-driven inspiral, or an afterglow within few years after coalescence, may highlight the position in the sky of galaxies hosting LISA sources. We show that observations of precursors and afterglows are mutually exclusive, depending on the mass of the primary black hole. Precursors are expected to occur in binaries where the primary (more massive) black hole is heavier than ~10^7 \Msun. They may correspond to on-off states of accretion, i.e., to a bright X-ray source decaying into quiescence before black hole coalescence, and are likely associated to disturbed galaxies showing signs of ongoing starbursts. Coalescences of lighter binaries, with masses <5x10^6 \Msun, lack of any precursor, as gas is expected to be consumed long before the GW-driven orbital decay. Such events would not be hosted by (massive) galaxies with an associated starburst, given the slow binary inspiral time compared to the typical time scale of starbursts. By contrast, coalescence, for such light binaries, is followed by an electromagnetic afterglow, i.e., an off-on accretion state rising in <20 yrs. Using a cosmological merger tree algorithm, we show that future X-ray missions such as XEUS will be able to identify, in 20 yrs operation, almost all the massive BH binary detectable by LISA, and, in only 5 yrs, all the LISA sources at z>6.

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arXiv2006

The Infrared Glow of First Stars

Kashlinsky et al. (2005) find a significant cosmic infrared background fluctuation excess on angular scales >50 arcsec that cannot be explained by instrumental noise or local foregrounds. The excess has been tentatively attributed to emission from primordial very massive (PopIII) stars formed <200 Myr after the Big Bang. Using an evolutionary model motivated by independent observations and including various feedback processes, we find that PopIII stars can contribute <40% of the total background intensity (νJ_ν~ 1-2 nW m^-2 sr^-1 in the 0.8-8 \mum range) produced by all galaxies (hosting both PopIII and PopII stars) at z>5. The infrared fluctuation excess is instead very precisely accounted by the clustering signal of galaxies at z>5, predominantly hosting PopII stars with masses and properties similar to the present ones.

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arXiv2005

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 Lyαemitters in ultra deep field searches. Independently of the detailed physics of Lyαline 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.

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arXiv2005

Gamma-ray constraints on the infrared background excess

Motivated by the idea that the recently detected near-infrared (1.2-4 micron) excess over the contribution of known galaxies is due to redshifted light from the first cosmic stars (Salvaterra & Ferrara 2003), 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 at 24 micron gives a good fit for all the considered blazars.

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arXiv2005

Constraints on the IMF of the first stars

Motivated by theoretical predictions that first stars were predominantly very massive, we investigate the physics of the transition from an early epoch dominated by massive Pop III stars to a later epoch dominated by familiar low-mass Pop II/I stars by means of a numerically-generated catalogue of dark matter halos coupled with a self-consistent treatment of chemical and radiative feedback. Depending on the strength of the chemical feedback, Pop III stars can contribute a substantial fraction (several percent) of the cosmic star formation activity even at moderate redshifts, z = 5. We find that the three z = 10 sources tentatively detected in NICMOS UDFs should be powered by Pop III stars, if these are massive; however, this scenario fails to reproduce the derived WMAP electron scattering optical depth. Instead, both the UDFs and WMAP constraints can be fulfilled if stars at any time form with a more standard, slightly top-heavy, Larson IMF in the range 1 Msun < M < 100 Msun.

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arXiv2005

Cosmic Backgrounds from Miniquasars

A large population of Intermediate Mass Black Holes (IMBHs) might be produced at early cosmic times as a left over of the evolution of the very massive first stars. Accretion onto IMBHs provides a source of (re)ionizing radiation. We show that the baryon mass fraction locked into IMBHs and their growth is strongly constrained by the observed residual Soft X-ray Background (SXRB) intensity. Thus, unless they are extremely X-ray quiet, miniquasars must be quite rare and/or have a short shining phase. As a byproduct, we find that miniquasars can not be the only source of reionization and that their alleged contribution to the near infrared bands is completely negligible.

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arXiv2005

Reionization History from Coupled CMB/21cm Line Data

We study 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 WMAP 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)C_l/2π~ 1.6 x 10^{-13} on arcmin scale, i.e. l as large as several thousands. We then cross-correlate secondary CMB anisotropy maps with neutral hydrogen 21cm line emission fluctuations obtained from the same simulations. The two signals are highly anti-correlated on angular scales corresponding to the typical size of HII regions (including overlapping) at the 21cm map redshift. We show how the CMB/21cm cross-correlation can be used to: (a) study the nature of the reionization sources, (b) reconstruct the cosmic reionization history, (c) infer the mean cosmic ionization level at any redshift. We discuss the feasibility of the proposed experiment with forthcoming facilities.

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arXiv2004

Cosmological reionization after WMAP: perspectives from PLANCK and future CMB missions

The WMAP first year detection of a high redshift reionization through its imprints on CMB anisotropy T and TE mode angular power spectra calls for a better comprehension of the universe ionization and thermal history after the standard recombination. Different reionization mechanisms predict different signatures in the CMB, both in temperature and polarization anisotropies and in spectral distortions. The Planck capability to distinguish among different scenarios through its sensitivity to T, TE, and E mode angular power spectra is discussed. Perspectives open by future high sensitivity experiments on the CMB polarization anisotropy and spectrum are also presented.

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arXiv2004

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.

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arXiv2004

Feedback Processes at Cosmic Dawn

The consequences of energy injection associated with stellar activity in galaxies have been often referred to as ``feedback effects''. Assessing the detailed physics and the role of such effects are central issues in modern theories of galaxy formation and evolution. In these Lectures we present a brief review of various aspects related to feedback with particular emphasis to those occurring when the universe was only 100 Myr old, an epoch now commonly known as the the Cosmic Dawn, when the first galaxies and the stars within them brought the universe back into the visible ages.

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arXiv2004

Induced Formation of Primordial Low-Mass Stars

We 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 E_SN > 10^{51} erg can lead to shell fragmentation. However, the minimum ambient density required to induce such fragmentation is much larger, n_0 > 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 10^{4-7} Msun; their density is in the range 110-6x10^7 cm{-3}. Fragments have a metallicity strictly lower than 10^{-2.6} Zsun 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.

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arXiv2004

Dust Formation in Very Massive Primordial Supernovae

At redshift z>5 Type II supernovae (SNII) are the only known dust sources with evolutionary timescales shorter than the Hubble time. We extend the model of dust formation in the ejecta of SNII by Todini & Ferrara (2001) to investigate the same process in pair-instability supernovae (PISN), which are though to arise from the explosion of the first, metal free, very massive (140-260 Msun) cosmic stars. We find that 15%-30% of the PISN progenitor mass is converted into dust, a value >10 times higher than for SNII; PISN dust depletion factors (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 instead affects considerably the composition and size distribution. For the assumed temperature evolution, grain condensation starts 150-200 days 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 0.001 and a few 0.1 micron and are always smaller than 1 micron. We give a brief discussion of the implications of dust formation for the IMF evolution of the first stars, cosmic reionization and the intergalactic medium.

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