arXiv subjects

Andrea Ferrara

Publications and source records attributed to Andrea Ferrara.

At least 199 records · Page 11Linked to original sources

arXiv2011

First stars in Damped Lyman Alpha systems

In order to characterize Damped Lyman Alpha systems (DLAs) potentially hosting first stars, we present a novel approach to investigate DLAs in the context of Milky Way (MW) formation, along with their connection with the most metal-poor stars and local dwarf spheroidal (dSph) galaxies. The merger tree method previously developed is extended to include inhomogeneous reionization and metal mixing, and it is validated by matching both the Metallicity Distribution Function of Galactic halo stars and the Fe-Luminosity relation of dSph galaxies. The model explains the observed NHI-Fe relation of DLAs along with the chemical abundances of [Fe/H] < -2 systems. In this picture, the recently discovered z_abs ~ 2.34 C-enhanced DLA (Cooke et al. 2011a), pertains to a new class of absorbers hosting first stars along with second-generation long-living low-mass stars. These "PopIII DLAs" are the descendants of H2-cooling minihalos with Mh ~ 10^7 Msun, that virialize at z > 8 in neutral, primordial regions of the MW environment and passively evolve after a short initial period of star formation. The gas in these systems is warm Tg \sim (40-1000) K, and strongly C-enriched by long-living, extremely metal-poor stars of total mass M* \sim 10^{2-4} Msun.

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arXiv2011

Data-constrained reionization and its effects on cosmological parameters

We perform an analysis of the recent WMAP7 data considering physically motivated and viable reionization scenarios with the aim of assessing their effects on cosmological parameter determinations. The main novelties are: (i) the combination of CMB data with astrophysical results from quasar absorption line experiments; (ii) the joint variation of both the cosmological and astrophysical [governing the evolution of the free electron fraction x_e(z)] parameters. Including a realistic, data-constrained reionization history in the analysis induces appreciable changes in the cosmological parameter values deduced through a standard WMAP7 analysis. Particularly noteworthy are the variations in Omega_bh^2 = 0.02258^{+0.00057}_{-0.00056} (WMAP7) vs. Omega_bh^2$ = 0.02183\pm 0.00054 (WMAP7 + ASTRO), and the new constraints for the scalar spectral index, for which WMAP7 + ASTRO excludes the Harrison-Zel'dovich value n_s=1 at more than 3σ. Finally, the e.s. optical depth value is considerably decreased with respect to the standard WMAP7, i.e. τ_{e}=0.080\pm 0.012. We conclude that the inclusion of astrophysical datasets, allowing to robustly constrain the reionization history, in the extraction procedure of cosmological parameters leads to relatively important differences in the final determination of their values.

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arXiv2011

Joint QSO-CMB constraints on reionization history

Based on the work by Mitra, Choudhury & Ferrara (2010), we obtain model-independent constraints on reionization from cosmic microwave background (CMB) and QSO absorption line data by decomposing the function N_{ion}(z) (the number of photons entering the IGM per baryon in collapsed objects) into its principal components. The main addition in this work is that for the CMB data set, we explicitly include the angular power spectra C_l for TT, TE and EE modes in our analysis which seem to contain somewhat more information than taking the electron scattering optical depth tau_{el} as a single data point. Using Markov Chain Monte Carlo methods, we find that all the quantities related to reionization can be severely constrained at z < 6 whereas a broad range of reionization histories at z > 6 are still permitted by the current data sets. With currently available data from WMAP7, we constrain 0.080 < tau_{el} < 0.112 (95% CL) and also conclude that reionization is 50% complete between 9.0 < z(Q_{HII} = 0.5) < 11.8 (95% CL) and is 99% complete between 5.8 < z(Q_{HII} = 0.99) < 10.4 (95% CL). With the forthcoming PLANCK data on large-scale polarization (ignoring effect of foregrounds), the z > 6 constraints will be improved considerably, e.g., the 2-sigma error on tau_{el} will be reduced to 0.009 and the uncertainties on z(Q_{HII} = 0.5) and z(Q_{HII} = 0.99) would be \sim 1 and 3 (95% CL), respectively. For more stringent constraints on reionization at z > 6, one has to rely on data sets other than CMB. Our method will be useful in such case since it can be used for non-parametric reconstruction of reionization history with arbitrary data sets.

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arXiv2011

Probing cosmic star formation up to z = 9.4 with GRBs

We propose a novel approach, based on Principal Components Analysis, to the use of Gamma-Ray Bursts (GRBs) as probes of cosmic star formation history (SFH) up to very high redshifts. The main advantage of such approach is to avoid the necessity of assuming an \textit{ad hoc} parameterization of the SFH. We first validate the method by reconstructing a known SFH from Monte Carlo-generated mock data. We then apply the method to the most recent \textit{Swift} data of GRBs with known redshift and compare it against the SFH obtained by independent methods. The main conclusion is that the level of star formation activity at $z \approx 9.4$ could have been already as high as the present-day one ($\approx 0.01 M_\odot$ yr$^{-1}$ Mpc$^{-3}$). This is a factor 3-5 times higher than deduced from high-$z$ galaxy searches through drop-out techniques. If true, this might alleviate the long-standing problem of a photon-starving reionization; it might also indicate that galaxies accounting for most of the star formation activity at high redshift go undetected by even the most deep searches.

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arXiv2011

Ancient giants: on the farthest galaxy at z=8.6

The observational frontiers for the detection of high-redshift galaxies have recently been pushed to unimaginable distances with the record-holding Lyman Alpha Emitter (LAE) UDFy-38135539 discovered at redshift z=8.6. However, the physical nature and the implications of this discovery have yet to be assessed. By selecting galaxies with observed luminosities similar to UDFy-38135539 in state-of-the-art cosmological simulations tuned to reproduce the large scale properties of LAEs, we bracket the physical nature of UDFy-38135539: it has a star formation rate ~ 2.7-3.7 solar masses/yr, it contains ~ 10^{8.3-8.7} solar mass of stars 50-80 Myr old, with stellar metallicity ~ 0.03-0.12 of the solar value. For any of the simulated galaxies to be visible as a LAE in the observed range, the intergalactic neutral hydrogen fraction at z=8.6 must be <= 0.2 and extra ionizing radiation from sources clustered around UDFy-38135539 is necessary. Finally, we predict that there is a 70% (15%) probability of detecting at least 1 such source from JWST (HST/WFC3) observations in a physical radius ~ 0.4 Mpc around UDFy-38135539.

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arXiv2011

Cosmic density field reconstruction from Ly-alpha forest data

We present a novel, fast method to recover the density field through the statistics of the transmitted flux in high redshift quasar absorption spectra. The proposed technique requires the computation of the probability distribution function of the transmitted flux (P_F) in the Ly-alpha forest region and, as a sole assumption, the knowledge of the probability distribution function of the matter density field (P_Delta). We show that the probability density conservation of the flux and matter density unveils a flux-density (F-Delta) relation which can be used to invert the Ly-alpha forest without any assumption on the physical properties of the intergalactic medium. We test our inversion method at z=3 through the following steps: [i] simulation of a sample of synthetic spectra for which P_Delta is known; [ii] computation of P_F; [iii] inversion of the Ly-alpha forest through the F-Delta relation. Our technique, when applied to only 10 observed spectra characterized by a signal-to noise ratio S/N >= 100 provides an exquisite (relative error epsilon_Delta <~ 12 % in >~ 50 % of the pixels) reconstruction of the density field in >~ 90 % of the line of sight. We finally discuss strengths and limitations of the method.

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arXiv2011

Enabling parallel computing in CRASH

We present the new parallel version (pCRASH2) of the cosmological radiative transfer code CRASH2 for distributed memory supercomputing facilities. The code is based on a static domain decomposition strategy inspired by geometric dilution of photons in the optical thin case that ensures a favourable performance speed-up with increasing number of computational cores. Linear speed-up is ensured as long as the number of radiation sources is equal to the number of computational cores or larger. The propagation of rays is segmented and rays are only propagated through one sub-domain per time step to guarantee an optimal balance between communication and computation. We have extensively checked pCRASH2 with a standardised set of test cases to validate the parallelisation scheme. The parallel version of CRASH2 can easily handle the propagation of radiation from a large number of sources and is ready for the extension of the ionisation network to species other than hydrogen and helium.

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arXiv2011

Turbulence in the Intergalactic Medium

We study supernova-driven galactic outflows as a mechanism for injecting turbulence in the intergalactic medium (IGM) far from galaxies. To this aim we follow the evolution of a 10^13 Msun galaxy along its merger tree, with carefully calibrated prescriptions for star formation and wind efficiencies. At z~3 the majority of the bubbles around galaxies are old (ages >1Gyr), i.e. they contain metals expelled by their progenitors at earlier times; their filling factor increases with time reaching about 10% at z<2. The energy deposited by these expanding shocks in the IGM is predominantly in kinetic form (mean energy density of 1 μeV cm^-3, about 2-3 x the thermal one), which is rapidly converted in disordered motions by instabilities, finally resulting in a fully developed turbulent spectrum whose evolution is followed through a spectral transfer function approach. The derived mean IGM turbulent Doppler parameter, b_t, peaks at z~1 at about 1.5 km/s with maximum b_t = 25 km/s. The shape of the b_t distribution does not significantly evolve with redshift but undergoes a continuous shift towards lower b_t values with time, as a result of bubble aging. We find also a clear trend of decreasing b_t with N_HI and a more complex dependence on R_s resulting from the age spread of the bubbles. We have attempted a preliminary comparison with the data, hampered by the scarcity of the latter and by the challenge provided by the subtraction of peculiar and thermal motions. Finally we comment on the implications of turbulence for various cosmological studies.

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arXiv2010

Reionization constraints using Principal Component Analysis

Using a semi-analytical model developed by Choudhury & Ferrara (2005) we study the observational constraints on reionization via a principal component analysis (PCA). Assuming that reionization at z>6 is primarily driven by stellar sources, we decompose the unknown function N_{ion}(z), representing the number of photons in the IGM per baryon in collapsed objects, into its principal components and constrain the latter using the photoionization rate obtained from Ly-alpha forest Gunn-Peterson optical depth, the WMAP7 electron scattering optical depth and the redshift distribution of Lyman-limit systems at z \sim 3.5. The main findings of our analysis are: (i) It is sufficient to model N_{ion}(z) over the redshift range 2 6 are relatively loose. (iii) The weak constraints on N_{ion}(z) at z>6 do not allow to disentangle different feedback models with present data. There is a clear indication that N_{ion}(z) must increase at z>6, thus ruling out reionization by a single stellar population with non-evolving IMF, and/or star-forming efficiency, and/or photon escape fraction. The data allows for non-monotonic N_{ion}(z) which may contain sharp features around z \sim 7. (iv) The PCA implies that reionization must be 99% completed between 5.8 6 constraints will be significantly improved.

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arXiv2010

The earliest galaxies seen in 21 cm line absorption

We investigate the 21 cm absorption lines produced by non-linear structures during the early stage of reionization, i.e. the starless minihalos and the dwarf galaxies. After a detailed modelling of their properties, with particular attention to the coupling physics, we determine their 21 cm absorption line profiles. The infalling gas velocity around minihalos/dwarf galaxies strongly affects the line shape, and with the low spin temperatures outside the virial radii of the systems, gives rise to horn-like line profiles. The optical depth of a dwarf galaxy is reduced for lines of sight penetrating through its HII region, and especially, a large HII region created by a dwarf galaxy with higher stellar mass and/or a top-heavy initial mass function results in an optical depth trough rather than an absorption line. We compute synthetic spectra of 21 cm forest for both high redshift quasars and radio afterglows of gamma ray bursts (GRBs). Even with the planned SKA, radio afterglows of most if not all GRBs would still be too dim to be the background sources for high resolution (1 kHz) observations, but absorption lines can be easily detected towards a high-z quasar. Broadband observation against GRB afterglows can also be used to reveal the evolving 21 cm signal from both minihalos and dwarf galaxies if there was no X-ray background or it was extremely weak, but it becomes difficult if an early X-ray background existed. Hence the 21 cm absorption could be a powerful probe of the presence/intensity of the X-ray background and the thermal history of the early universe.

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arXiv2010

The faintest galaxies

We investigate the nature of Ultra Faint dwarf spheroidal galaxies (UF dSphs) in a general cosmological context, simultaneously accounting for various "classical" dSphs and Milky Way (MW) properties, including their Metallicity Distribution Function (MDF). The model successfully reproduces both the observed [Fe/H]-Luminosity relation and the mean MDF of UFs. According to our results UFs are the living fossils of H2-cooling minihaloes formed at z>8.5, i.e. before the end of reionization. They are the oldest and the most dark matter-dominated (M/L > 100) dSphs in the MW system, with a total mass of M = 10^(7-8) Msun. The model allows to interpret the different shape of UFs and classical dSphs MDF, along with the frequency of extremely metal-poor stars in these objects. We discuss the "missing satellites problem" by comparing the UF star formation efficiencies with those derived for minihaloes in the Via Lactea simulation.

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arXiv2010

Searching for the earliest galaxies in the 21 cm forest

We use a model developed by Xu et al. (2010) to compute the 21 cm line absorption signatures imprinted by star-forming dwarf galaxies (DGs) and starless minihalos (MHs). The method, based on a statistical comparison of the equivalent width (W_ν) distribution and flux correlation function, allows us to derive a simple selection criteria for candidate DGs at very high (z >= 8) redshift. We find that ~ 18% of the total number of DGs along a line of sight to a target radio source (GRB or quasar) can be identified by the condition W_ν< 0; these objects correspond to the high-mass tail of the DG distribution at high redshift, and are embedded in large HII regions. The criterion W_ν> 0.37 kHz instead selects ~ 11% of MHs. Selected candidate DGs could later be re-observed in the near-IR by the JWST with high efficiency, thus providing a direct probe of the most likely reionization sources.

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arXiv2010

The visibility of Lyman Alpha Emitters during reionization

We present the first Lyman Alpha Emitter (LAE) study that combines: (i) cosmological SPH simulations run using GADGET-2, (ii) radiative transfer simulations (CRASH), and (iii) a previously developed LAE model. This complete LAE model accounts for the intrinsic LAE Lyman Alpha/continuum luminosity, dust enrichment and Lyman Alpha transmission through the intergalactic medium (IGM), to quantify the effects of reionization, dust and velocity fields on the Lyman Alpha and UV Luminosity Functions (LF). We find that a model neglecting dust sorely fails to reproduce either the slope or the magnitude of the observed Lyman Alpha and UV LFs. Clumped dust is required to simultaneously fit the observed UV and Lyman Alpha LFs, such that the intrinsic Lyman Alpha-to-continuum luminosity is enhanced by a factor f_alpha/f_c ~ 1.5 (3.7) excluding (including) peculiar velocities. The higher value including velocity fields arises since LAEs reside in large potential wells and inflows decrease their Lyman Alpha transmission. For the first time, a degeneracy is found between the the ionization state of the IGM and the clumping of dust inside high-redshift galaxies. The Lyman Alpha LF at z ~ 5.7 can be well reproduced (to within a 5 sigma error) by a wide range of IGM average neutral hydrogen fraction, 3.4/10^3 to 0.16, provided that the increase in the Lyman Alpha transmission through a more ionized IGM is compensated by a decrease in the Lyman Alpha escape fraction from the galaxy due to dust absorption. The physical properties of LAEs are presented, along with a discussion of the assumptions adopted.

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arXiv2010

Probing intergalactic radiation fields during cosmic reionization through gamma-ray absorption

We discuss expectations for the absorption of high-energy gamma-rays by gamma-gamma pair production with intergalactic radiation fields (IRFs) at very high redshifts (z~5-20), and the prospects thereof for probing the cosmic reionization era. For the evolving IRF, a semi-analytical model incorporating both Population II and Population III stars is employed, which is consistent with a wide variety of existing high-z observations including QSO spectral measurements, WMAP Thomson depth constraints, near-IR source count limits, etc. We find that the UV IRF below the Lyman edge energy with intensities in the range of a few times 10^{-19} erg cm^{-2} s^{-1} Hz^{-1} sr^{-1} can cause appreciable attenuation above ~12 GeV at z~5, down to ~6-8 GeV at z>~8-10. This may be observable in the spectra of blazars or gamma-ray bursts by the Fermi Gamma-ray Space Telescope or next generation facilities such as the Cherenkov Telescope Array, Advanced Gamma-ray Imaging System or 5@5, providing invaluable insight into early star formation and cosmic reionization.

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arXiv2009

Detecting Lyman Alpha Emitters in the Sub-millimeter

Using the results from a previously developed Lyman Alpha/continuum production/transmission and dust enrichment model for Lyman Alpha Emitters (LAEs), based on cosmological SPH simulations, we assess the detectability of their dust-reprocessed sub-millimeter (submm) radiation. As supernovae (rather than evolved stars) control dust formation and destruction processes, LAEs are relatively dust-poor with respect to local galaxies: they have low dust-to-gas ratios (0.05 times the dust-to-gas ratio of the Milky Way) in spite of their relatively high metallicity, which is approximately 10-50% of the solar value. Using the derived escape fraction of ultraviolet (UV) continuum photons we compute the UV luminosity absorbed by dust and re-emitted in the far infrared. The LAE submm fluxes correlate with their Lyman Alpha luminosity: about (3%, 1%) at z=(5.7, 6.6) of the LAEs in our simulated sample (those with with Log L_α> 43.1) would have fluxes at 850 micrometer (the optimal band for detection) in excess of 0.12 mJy and will be therefore detectable at 5 sigma with ALMA with an integration time of only 1 hour. Such detections would open a new window on the physical conditions prevailing in these most distant galaxies.

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arXiv2009

Particle energy cascade in the Intergalactic Medium

We study the development of high energy (E_in < 1TeV) cascades produced by a primary electron of energy E_in injected into the intergalactic medium (IGM). To this aim we have developed the new code MEDEA (Monte Carlo Energy DEposition Analysis) which includes Bremsstrahlung and Inverse Compton (IC) processes, along with H/He collisional ionizations and excitations, and electron-electron collisions. The cascade energy partition into heating, excitations and ionizations depends primarily on the IGM ionized fraction, x_e, but also on redshift, z, due to IC on CMB photons. While Bremsstrahlung is unimportant under most conditions, IC becomes largely dominant at energies E_in > 1MeV. The main effect of IC at injection energies E_in < 100MeV is a significant boost of the fraction of energy converted into low energy photons (hν< 10.2eV) which do not further interact with the IGM. For energies E_in > 1GeV CMB photons are preferentially upscattered within the X-ray spectrum ($hν> 10^4eV) and can free stream to the observer. Complete tables of the fractional energy depositions as a function of redshift, E_in and ionized fraction are given. Our results can be used in many astrophysical contexts, with an obvious application related to the study of decaying/annihilating Dark Matter (DM) candidates in the high-z Universe.

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arXiv2009

The cool side of Lyman Alpha Emitters

We extend a previous study of Lyman Alpha Emitters (LAEs) based on hydrodynamical cosmological simulations, by including two physical processes important for LAEs: (a) Lyman Alpha and continuum luminosities produced by cooling of collisionally excited HI in the galaxy, (b) dust formation and evolution; we follow these processes on a galaxy-by-galaxy basis. HI cooling on average contributes 16-18% of the Lyman Alpha radiation produced by stars, but this value can be much higher in low mass LAEs and further increased if the HI is clumpy. The continuum luminosity is instead almost completely dominated by stellar sources. The dust content of galaxies scales with their stellar mass, M_{dust} is proportional to M_*^0.7 and stellar metallicity, Z_*, such that M_{dust} is proportional to Z_*^1.7. As a result, the massive galaxies have Lyman Alpha escape fraction as low as f_alpha=0.1, with a LAE-averaged value decreasing with redshift such that it is (0.33,0.23) at z =(5.7,6.6). The UV continuum escape fraction shows the opposite trend with redshift, possibly resulting from clumpiness evolution. The model successfully reproduces the observed Lyman Alpha and UV luminosity functions at different redshifts and the Lyman Alpha equivalent width scatter to a large degree, although the observed distribution appears to be more more extended than the predicted one. We discuss possible reasons for such tension.

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arXiv2009

Ultra faint dwarfs: probing early cosmic star formation

We investigate the nature of the newly discovered Ultra Faint dwarf spheroidal galaxies (UF dSphs) in a general cosmological context simultaneously accounting for various ``classical`` dSphs and Milky Way properties including their Metallicity Distribution Function (MDF). To this aim we extend the merger tree approach previously developed to include the presence of star-forming minihaloes, and an heuristic prescription for radiative feedback. The model successfully reproduces both the observed [Fe/H]-Luminosity relation and the mean MDF of UFs. In this picture UFs are the oldest, most dark matter-dominated (M/L > 100) dSphs with a total mass M= 10^{7-8}Msun; they are leftovers of H_2-cooling minihaloes formed at z > 8.5, i.e. before reionization. Their MDF is broader (because of a more prolonged SF) and shifted towards lower [Fe/H] (as a result of a lower gas metallicity at the time of formation) than that of classical dSphs. These systems are very ineffectively star-forming, turning into stars by z=0 only <3% of the potentially available baryons. We provide a useful fit for the star formation efficiency of dSphs.

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