arXiv subjects

A. Ferrara

Publications and source records attributed to A. Ferrara.

At least 199 records · Page 11Linked to original sources

arXiv2005

A very extended reionization epoch ?

The recent observations of cross temperature-polarization power spectra of the Cosmic Microwave Background (CMB) made by the WMAP satellite are in better agreement with a high value of the Thomson scattering optical depth τ\approx 0.17. This value is close to τ= 0.3, which is taken as the upper limit in the parameter extraction analysis made by the WMAP team. However, models with τ\sim 0.3 provide a good fit to current CMB data and are not significantly excluded when combined with Large Scale Structure data. By making use of a self-consistent reionization model, we verify the astrophysical feasibility of models with τ\sim 0.3. It turns out that current data on various observations related to the thermal and ionization history of the intergalactic medium are not able to rule out τ\sim 0.3. The possibility of a very extended reionization epoch can significantly undermine the WMAP constraints on crucial cosmological parameters such as the Hubble constant, the spectral index of primordial fluctuations and the amplitude of dark matter clustering.

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arXiv2005

Background radiation from sterile neutrino decay and reionization

Sterile neutrinos are one of the most promising Warm Dark Matter candidates. By considering their radiative- and pion-decay channels, we derive the allowed contribution of sterile neutrinos to the X-ray, optical and near-infrared cosmic backgrounds. The X-ray background puts a strong constraint on the mass of radiatively decaying neutrinos (m <= 14 keV), whereas the allowed mass range for pion-decay neutrinos (for a particle lifetime > 4 X 10^17 s) is 150 <= m/MeV <= 500. Taking into account these constraints, we find that sterile neutrinos do not significantly contribute to the optical and near-infrared background. We further consider the impact of sterile neutrinos on reionization. We find that the Thomson optical depth due to sterile neutrinos is tau_e = (0.4-3) X 10^-2 in the case of radiative decays, and it is ~10^-3 for the pion-decay channel. We conclude that these particles must have played only a minor role in cosmic reionization history.

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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

Experimental Constraints on Self-consistent Reionization Models

A self-consistent formalism to jointly study cosmic reionization and thermal history of the IGM is presented. The model implements most of the relevant physics governing these processes, such as the inhomogeneous IGM density distribution, three different sources of ionizing photons (PopIII stars, PopII stars and QSOs), and radiative feedback. By constraining the free parameters with available data on redshift evolution of Lyman-limit systems, Gunn-Peterson and electron scattering optical depths, Near InfraRed Background (NIRB), and cosmic star formation history, we select a fiducial model, whose main predictions are: (i) H was completely reionized at z \approx 15, while HeII must have been reionized by z \approx 12. At z \approx 7, HeIII suffered an almost complete recombination as a result of the extinction of PopIII stars, as required by the interpretation of the NIRB. (ii) A QSO-induced complete HeII reionization occurs at z=3.5; a similar double H reionization does not take place due to the large number of photons above 1 Ryd from PopII stars and QSOs, even after PopIII stars have disappeared. (iii) Following reionization, the temperature of the IGM corresponding to the mean gas density is boosted to 15000 K. Observations of T_0 are consistent with the fact that He is singly ionized at z > 3.5, while they are consistent with He being doubly ionized at z < 3.5. This might be interpreted as a signature of (second) HeII reionization. (iv) Only 0.3% of the stars produced by z=2 need to be PopIII stars in order to achieve the hydrogen reionization. Such model not only relieves the tension between the Gunn-Peterson optical depth and WMAP observations, but also accounts self-consistently for all known observational constraints (abridged).

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arXiv2005

Thermal and Fragmentation Properties of Star-forming Clouds in Low-metallicity Environments

The thermal and chemical evolution of star-forming clouds is studied for different gas metallicities, Z, using the model of Omukai (2000), updated to include deuterium chemistry and the effects of cosmic microwave background (CMB) radiation. HD-line cooling dominates the thermal balance of clouds when Z \~ 10^{-5}-10^{-3} Z_sun and density ~10^{5} cm^{-3}. Early on, CMB radiation prevents the gas temperature to fall below T_CMB, although this hardly alters the cloud thermal evolution in low-metallicity gas. From the derived temperature evolution, we assess cloud/core fragmentation as a function of metallicity from linear perturbation theory, which requires that the core elongation E := (b-a)/a > E_NL ~ 1, where a (b) is the short (long) core axis length. The fragment mass is given by the thermal Jeans mass at E = E_NL. Given these assumptions and the initial (gaussian) distribution of E we compute the fragment mass distribution as a function of metallicity. We find that: (i) For Z=0, all fragments are very massive, > 10^{3}M_sun, consistently with previous studies; (ii) for Z>10^{-6} Z_sun a few clumps go through an additional high density (> 10^{10} cm^{-3}) fragmentation phase driven by dust-cooling, leading to low-mass fragments; (iii) The mass fraction in low-mass fragments is initially very small, but at Z ~ 10^{-5}Z_sun it becomes dominant and continues to grow as Z is increased; (iv) as a result of the two fragmentation modes, a bimodal mass distribution emerges in 0.01 < Z/Z_sun < 0.1. (v) For > 0.1Z_sun, the two peaks merge into a singly-peaked mass function which might be regarded as the precursor of the ordinary Salpeter-like IMF.

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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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arXiv2005

Constraining the thermal history of the Warm-Hot Intergalactic Medium

We have identified a large-scale structure traced by galaxies at z=0.8, within the Lockman Hole, by means of multi-object spectroscopic observations. By using deep XMM images we have investigated the soft X-ray emission from the Warm-Hot Intergalactic Medium (WHIM) expected to be associated with this large-scale structure and we set a tight upper limit to its flux in the very soft 0.2-0.4 keV band. The non-detection requires the WHIM at these redshifts to be cooler than 0.1 keV. Combined with the WHIM emission detections at lower redshift, our result indicates that the WHIM temperature is rapidly decreasing with redshift, as expected in popular cosmological models.

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arXiv2004

Molecular hydrogen in damped Ly-alpha systems: clues to interstellar physics at high-redshift

In order to interpret H2 (molecular hydrogen) quasar absorption line observations of damped Ly-alpha systems (DLAs) and sub-DLAs, we model their H2 abundance as a function of dust-to-gas ratio, including H2 self-shielding and dust extinction against dissociating photons. Then, we constrain the physical state of gas by using H2 data. Using H2 excitation data for DLA with H2 detections, we derive a gas density 1.5 < log n [cm^-3] < 2.5, temperature 1.5 < log T [K] < 3, and internal UV radiation field (in units of the Galactic value) 0.5 < log χ< 1.5. We then find that the observed relation between molecular fraction and dust-to-gas ratio of the sample is naturally explained by the above conditions. However, it is still possible that H2 deficient DLAs and sub-DLAs with H2 fractions less than ~ 10^-6 are in a more diffuse and warmer state. The efficient photodissociation by the internal UV radiation field explains the extremely small H2 fraction (< 10^-6) observed for κ< 1/30 (κis the dust-to-gas ratio in units of the Galactic value); H2 self-shielding causes a rapid increase and the large variations of H2 abundance for κ> 1/30. We finally propose an independent method to estimate the star formation rates of DLAs from H2 abundances; such rates are then critically compared with those derived from other proposed methods. The implications for the contribution of DLAs to the cosmic star formation history are briefly discussed.

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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

A supernova origin for dust in a high-redshift quasar

Interstellar dust plays a crucial role in the evolution of the Universe by assisting the formation of molecules, by triggering the formation of the first low-mass stars, and by absorbing stellar ultraviolet-optical light and subsequently re-emitting it at infrared/millimetre wavelengths. Dust is thought to be produced predominantly in the envelopes of evolved (age >1 Gyr), low-mass stars. This picture has, however, recently been brought into question by the discovery of large masses of dust in the host galaxies of quasars at redshift z>6, when the age of the Universe was less than 1 Gyr. Theoretical studies, corroborated by observations of nearby supernova remnants, have suggested that supernovae provide a fast and efficient dust formation environment in the early Universe. Here we report infrared observations of a quasar at redshift 6.2, which are used to obtain directly its dust extinction curve. We then show that such a curve is in excellent agreement with supernova dust models. This result demonstrates a supernova origin for dust in this high-redshift quasar, from which we infer that most of the dust at high redshifts has probably the same origin.

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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

A Lyman alpha emitter at z = 6.5 found with slitless spectroscopy

We report the discovery of a Lyman alpha emitting galaxy at z = 6.518. The single line was found in the 43 square arcmin VLT/FORS field by slitless spectroscopy limited to the atmospheric window at 9100 A (sampling Lya at 6.4 < z < 6.6). Its counterpart is undetected in a deep I band image and the line has an asymmetric appearance in a deeper follow-up spectrum. There are no plausible line identifications except for Lya with a flux of 1.9x10^-17 erg/s/cm2 and rest frame equivalent width > 80 A. The lower limit to the star formation rate density at z = 6.5 derived from our complete sample is 5x10^-4 M_sol/year/Mpc^3, consistent with measurements in the Subaru Deep Field and Hubble Ultra Deep Field but approximately ten times higher than in the Large Area Lyman Alpha survey. This Lya emitter is among the very small sample of highest redshift galaxies known.

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arXiv2004

Cosmological Feedback from High-Redshift Dwarf Galaxies

We model how repeated supernova explosions in high-redshift dwarf starburst galaxies drive superbubbles and winds out of the galaxies. We compute the efficiencies of metal and mass ejection and energy transport from the galactic potentials, including the effect of cosmological infall of external gas. The starburst bubbles quickly blow out of small, high-redshift, galactic disks, but must compete with the ram pressure of the infalling gas to escape into intergalactic space. We show that the assumed efficiency of the star formation rate dominates the bubble evolution and the metal, mass, and energy feedback efficiencies. With star formation efficiency f*=0.01, the ram pressure of infall can confine the bubbles around high-redshift dwarf galaxies with circular velocities v_c>52 km/s. We can expect high metal and mass ejection efficiencies, and moderate energy transport efficiencies in halos with v_c~30-50 km/s and f*~0.01 as well as in halos with v_c~100 km/s and f*>>0.01. Such haloes collapse successively from 1-2 sigma peaks in LambdaCDM Gaussian density perturbations as time progresses. These dwarf galaxies can probably enrich low and high-density regions of intergalactic space with metals to 10^-3-10^-2 Zsun as they collapse at z~8 and z<5 respectively. They also may be able to provide adequate turbulent energy to prevent the collapse of other nearby halos, as well as to significantly broaden Lyman-alpha absorption lines to v_rms~20-40 km/s. We compute the timescales for the next starbursts if gas freely falls back after a starburst, and find that, for star formation efficiencies as low as f*<0.01, the next starburst should occur in less than half the Hubble time at the collapse redshift. This suggests that episodic star formation may be ubiquitous in dwarf galaxies.

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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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arXiv2004

Detections of Warm-Hot Intergalactic Medium

Several popular cosmological models predict that most of the baryonic mass in the local universe is located in filamentary and sheet-like structures associated with groups and clusters of galaxies. This gas is expected to be gravitationally heated to ~10^6 K and therefore emitting in the soft X-rays. We have investigated three fields with large scale structures of galaxies at redshifts 0.1, 0.45, 0.79 and found signatures of warm-hot thermal emission (kT< 1 keV) correlated with the distribution of galaxies for the first two. The correlation and the properties of both X-ray and galaxy distribution strongly suggest that the diffuse X-ray flux is due to extragalactic emission by the Warm-Hot Intergalactic Medium (WHIM) predicted by cosmological models.

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arXiv2003

The Proximity Effect Around High Redshift Galaxies

Recent observations have shown that the intergalactic medium (IGM) is more transparent to Lyalpha photons close to Lyman Break Galaxies (LBGs) than at large distance from them, ie a proximity effect. Cosmological simulations including winds from LBGs have been so far unable to explain this trend. By coupling such simulations with the radiative transfer code CRASH, we investigate whether the addition of the ionizing radiation emitted by LBGs can increase the transmissivity by decreasing the neutral hydrogen fraction in the inner Mpc of the galaxy halo. The transmissivity as a function of distance is roughly reproduced only if LBGs are identified with dwarf galaxies (with masses < 10^9 solar masses), which are undergoing a vigorous (50 solar masses/yr) burst of star formation. Similar star formation rates in larger galaxies are not sufficient to overwhelm the large recombination rates associated with their denser environment. If so, photoionization partly reconciles theory with observations, although we discuss a number of uncertainties affecting both approaches.

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arXiv2003

CRASH: a Radiative Transfer Scheme

We present a largely improved version of CRASH, a 3-D radiative transfer code that treats the effects of ionizing radiation propagating through a given inhomogeneous H/He cosmological density field, on the physical conditions of the gas. The code, based on a Monte Carlo technique, self-consistently calculates the time evolution of gas temperature and ionization fractions due to an arbitrary number of point/extended sources and/or diffuse background radiation with given spectra. In addition, the effects of diffuse ionizing radiation following recombinations of ionized atoms have been included. After a complete description of the numerical scheme, to demonstrate the performances, accuracy, convergency and robustness of the code we present four different test cases designed to investigate specific aspects of radiative transfer: (i) pure hydrogen isothermal Stromgren sphere; (ii) realistic Stromgren spheres; (iii) multiple overlapping point sources, and (iv) shadowing of background radiation by an intervening optically thick layer. When possible, detailed quantitative comparison of the results against either analytical solutions or 1-D standard photoionization codes has been made showing a good level of agreement. For more complicated tests the code yields physically plausible results, which could be eventually checked only by comparison with other similar codes. Finally, we briefly discuss future possible developments and cosmological applications of the code.

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