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Andrea Ferrara

Publications and source records attributed to Andrea Ferrara.

At least 181 records · Page 10Linked to original sources
arXiv2013

Infrared background signatures of the first black holes

Angular fluctuations of the Near InfraRed Background (NIRB) intensity are observed up to scales $\simlt 1^{\ensuremath{^{\circ}}}$. Their interpretation is challenging as even after removing the contribution from detected sources, the residual signal is $>10$ times higher than expected from distant galaxies below the detection limit and first stars. We propose here a novel interpretation in which early, intermediate mass, accreting direct collapse black holes (DCBH), which are too faint to be detected individually in current surveys, could explain the observed fluctuations. We find that a population of highly obscured ($N_{\rm H}\simgt 10^{25} \rm cm^{-2}$) DCBHs formed in metal-free halos with virial temperature $10^4$ K at $z\simgt 12$, can explain the observed level $\approx 10^{-3}$ (nW m$^{-2}$ sr$^{-1})^2$ of the 3.6 and 4.5 $μ$m fluctuations on scales $>100''$. The signal on smaller scales is instead produced by undetected galaxies at low and intermediate redshifts. Albeit Compton-thick, at scales $θ> 100''$ DCBHs produce a CXB (0.5-2 keV)-NIRB ($4.5 \rm μm$) cross-correlation signal of $\simeq 10^{-11}$ erg s$^{-1}$ cm$^{-2}$ nW m$^{-2}$ sr$^{-1}$ slightly dependent on the specific value of the absorbing gas column ($N_{\rm H} \approx 10^{25} \rm cm^{-2}$) adopted and in agreement with the recent measurements by \cite{2012arXiv1210.5302C}. At smaller scales the cross-correlation is dominated by the emission of high-mass X-ray binaries (HMXB) hosted by the same low-$z$, undetected galaxies accounting for small scale NIRB fluctuations. These results outline the great potential of the NIRB as a tool to investigate the nature of the first galaxies and black holes.

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arXiv2013

FIR line emission from high redshift galaxies

By combining high resolution, radiative transfer cosmological simulations of z~6 galaxies with a sub-grid multi-phase model of their interstellar medium we derive the expected intensity of several far infrared (FIR) emission lines ([C II] 158 micron, [O I] 63 micron, and [N II] 122 micron) for different values of the gas metallicity, Z. For Z = Z_sun the [C II] spectrum is very complex due to the presence of several emitting clumps of individual size < 3 kpc; the peak is displaced from the galaxy center by ~100 km/s. While the [O I] spectrum is also similarly displaced, the [N II] line comes predominantly from the central ionized regions of the galaxy. When integrated over ~500 km/s, the [C II] line flux is 185 mJy km/s; 95% of such flux originates from the cold (T~250 K) H I phase, and only 5% from the warm (T~5000 K) neutral medium. The [O I] and [N II] fluxes are ~6 and ~90 times lower than the [C II] one, respectively. By comparing our results with observations of Himiko, the most extended and luminous Lyman Alpha Emitter (LAE) at z = 6.6, we find that the gas metallicity in this source must be sub-solar. We conclude that the [C II] line from z~6 galaxies is detectable by the ALMA full array in 1.9 < t_ON < 7.7 hr observing time, depending on Z.

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arXiv2013

Signatures of X-rays in the early Universe

[abridged] With their long mean free paths and efficient heating of the intergalactic medium (IGM), X-rays could have a dramatic impact on the thermal and ionization history of the Universe. We explore this in various signals: (i) Reionization history: including X-rays results in an earlier, more extended reionization. Efficient thermal feedback from X-ray heating could yield an extended, ~10% ionized epoch. (ii) Reionization morphology: a sizable (~10%) contribution of X-rays to reionization results in a more uniform morphology, though the impact is modest when compared at the same global neutral fraction, xH. However, changes in morphology cannot be countered by increasing the bias of the ionizing sources, making them a robust signature. (iii) The kinetic Sunyaev-Zel'dovich (kSZ) effect: at a fixed reionization history, X-rays decrease the kSZ power at l=3000 by ~0.5 microK^2. Our extreme model in which X-rays dominate reionization is the only one that is marginally consistent with upper limits from the South Pole Telescope, assuming no thermal Sunyaev-Zel'dovich (tSZ) - dusty galaxy correlation. Since this extreme model is unlikely, we conclude that there should be a sizable tSZ-dusty galaxy signal. (iv) The cosmic 21cm signal: the impact of X-rays on the 21cm power spectrum during the advanced stages of reionization (xH<0.7) is modest, except in extreme, X-ray dominated models. The largest impact of X-rays is to govern IGM heating. In fact, unless thermal feedback is efficient, the epoch of X-ray heating likely overlaps with the beginning of reionization (xH>0.9). This results in a 21cm power spectrum which is ~ 10-100 times higher than obtained from naive estimates ignoring this overlap. However, if thermal feedback is efficient, the resulting extended epoch between X-ray heating and reionization could provide a clean probe of the matter power spectrum in emission.

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arXiv2013

Constraints on Warm Dark Matter models from high-redshift long gamma-ray bursts

Structures in Warm Dark Matter (WDM) models are exponentially suppressed below a certain scale, characterized by the dark matter particle mass, $m_{\rm x}$. Since structures form hierarchically, the presence of collapsed objects at high-redshifts can set strong lower limits on $m_{\rm x}$. We place robust constraints on $m_{\rm x}$ using recent results from the {\it Swift} database of high-redshift gamma-ray bursts (GRBs). We parameterize the redshift evolution of the ratio between the cosmic GRB rate and star formation rate (SFR) as $\propto (1+z)^α$, thereby allowing astrophysical uncertainties to partially mimic the cosmological suppression of structures in WDM models. Using a maximum likelihood estimator on two different $z>4$ GRB subsamples (including two bursts at $z>8$), we constrain $m_{\rm x} \gtrsim 1.6$-1.8 keV at 95% C.L., when marginalized over a flat prior in $α$. We further estimate that 5 years of a SVOM-like mission would tighten these constraints to $m_{\rm x} \gtrsim 2.3 $ keV. Our results show that GRBs are a powerful probe of high-redshift structures, providing robust and competitive constraints on $m_{\rm x}$.

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arXiv2013

Reionization and the Cosmic Dawn with the Square Kilometre Array

The Square Kilometre Array (SKA) will have a low frequency component (SKA-low) which has as one of its main science goals the study of the redshifted 21cm line from the earliest phases of star and galaxy formation in the Universe. This 21cm signal provides a new and unique window on both the formation of the first stars and accreting black holes and the later period of substantial ionization of the intergalactic medium. The signal will teach us fundamental new things about the earliest phases of structure formation, cosmology and even has the potential to lead to the discovery of new physical phenomena. Here we present a white paper with an overview of the science questions that SKA-low can address, how we plan to tackle these questions and what this implies for the basic design of the telescope.

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arXiv2013

Identifying Lyman Alpha Emitters powered by AGNs

Lyman Alpha Emitters (LAEs) are usually thought to be powered by star formation. It has been recently reported that a fraction of LAEs at redshift z~3-4 hosts an Active Galactic Nuclei (AGN). If an AGN is present it could be obscured and undetectable in X-rays, but yet dominate the Lya luminosity. We examine the properties of these AGN-powered LAEs at high redshift (z>=6) using radiative transfer cosmological simulations and obtain a reliable criterion to identify them from their observed \Lya line and surface brightness. We find that these sources should have: (a) negative line weighted skewness, S_w <0, and (b) surface brightness profiles FWHM >= 1.5". This parameter space cannot be populated by starburst LAEs. Thus, LAEs satisfying this criterion would be strong candidates for the presence of a hidden AGN powering their luminosity.

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arXiv2013

The contribution of high redshift galaxies to the Near-Infrared Background

Several independent measurements have confirmed the existence of fluctuations ($δF_{\rm obs}\approx 0.1 \rm nW/m^{2}/sr$ at $3.6 \rm μm$) up to degree angular scales in the source-subtracted Near InfraRed Background (NIRB) whose origin is unknown. By combining high resolution cosmological N-body/hydrodynamical simulations with an analytical model, and by matching galaxy Luminosity Functions (LFs) and the constraints on reionization simultaneously, we predict the NIRB absolute flux and fluctuation amplitude produced by high-$z$ ($z > 5$) galaxies (some of which harboring Pop III stars, shown to provide a negligible contribution). This strategy also allows us to make an empirical determination of the evolution of ionizing photon escape fraction: we find $f_{\rm esc} = 1$ at $z \ge 11$, decreasing to $\approx 0.05$ at $z = 5$. In the wavelength range $1.0-4.5 \rm μm$, the predicted cumulative flux is $F =0.2-0.04 \rm nW/m^2/sr$. However, we find that the radiation from high-$z$ galaxies (including those undetected by current surveys) is insufficient to explain the amplitude of the observed fluctuations: at $l=2000$, the fluctuation level due to $z > 5$ galaxies is $δF = 0.01-0.002 \rm nW/m^2/sr$, with a relative wavelength-independent amplitude $δF/F = 4$%. The source of the missing power remains unknown. This might indicate that an unknown component/foreground, with a clustering signal very similar to that of high-$z$ galaxies, dominates the source-subtracted NIRB fluctuation signal.

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arXiv2013

The physics of the fundamental metallicity relation

We present a simple, redshift-independent analytic model that explains the local Fundamental Metallicity Relation (FMR), taking into account the physical processes of star formation, inflow of metal-poor intergalactic medium (IGM) gas, and the outflow of metal rich interstellar medium (ISM) gas. We show that the physics of the FMR can be summarised as follows: for massive galaxies with stellar mass M_* >= 10^11 solar masses, ISM metal enrichment due to star formation is compensated by inflow of metal poor IGM gas, leading to a constant value of the gas metallicity with star formation rate (SFR); outflows are rendered negligible as a result of the large potential wells of these galaxies. On the other hand, as a result of their smaller SFR, less massive galaxies produce less heavy elements that are also more efficiently ejected due to their shallow potential wells; as a result, for a given M_*, the gas metallicity decreases with SFR. For such galaxies, the outflow efficiency determines both the slope, and the knee of the metallicity-SFR relation. Without changing any parameters, this simple model is also successfully matched to the gas fraction - gas metallicity relation observed for a sample of about 260 nearby galaxies.

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arXiv2012

The nature of dark matter from the global high redshift HI 21 cm signal

We study the imprint of dark matter (DM) annihilation on the global 21 cm signal from the Dark Ages to Cosmic Reionization. Motivated by recent observations, we focus on three DM candidates: (i) a 10 GeV Bino-like neutralino (ii) a 200 GeV Wino and (iii) a 1 TeV heavier particle annihilating into leptons. For each DM candidate we assume two values for the thermally averaged annihilation cross section σv, the standard thermal value σv_th = 3 x 10^-26 cm^3 s^-1 and the maximum value allowed by WMAP7 data, σv_max. We include the enhancement of DM annihilations due to collapsed structures, detailed estimates of energy deposition into the intergalactic medium (IGM), as well realistic prescriptions for astrophysical sources of UV and X-ray radiation. In these models, the additional heat input from DM annihilation suppresses the mean 21cm brightness temperature offset by δTb ~ few-100 mK. In particular, the very deep δTb ~ -150 mK absorption feature at ~20 < z < 25 predicted by popular models of the first galaxies is considerably reduced or totally erased by some of the considered DM candidates. Such an enhancement in IGM heating could come from either DM annihilations or a stronger-than-expected astrophysical component (i.e. abundant early X-ray sources). However, we find that the two signatures are not degenerate, since the DM heating is dominated by halos several orders of magnitude smaller than those hosting galaxies, whose fractional abundance evolves more slowly resulting in a smaller gradient: d δTb/d ν< 4 mK/MHz in the range ν~60-80 MHz. The detection of such signals by future radio telescopes would be clear evidence of DM energy injection at high-redshifts.

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arXiv2012

Dark Matter Halo Environment for Primordial Star Formation

We study the statistical properties (such as shape and spin) of high-z halos likely hosting the first (PopIII) stars with cosmological simulations including detailed gas physics. In the redshift range considered ($11 < z < 16$) the average sphericity is $ = 0.3 \pm 0.1$, and for more than 90% of halos the triaxiality parameter is $T \lesssim 0.4$, showing a clear preference for oblateness over prolateness. Larger halos in the simulation tend to be both more spherical and prolate: we find $s \propto M_h^{α_s}$ and $T \propto M_h^{α_T}$, with $α_s \approx 0.128$ and $α_T= 0.276$ at z = 11. The spin distributions of dark matter and gas are considerably different at $z=16$, with the baryons rotating slower than the dark matter. At lower redshift, instead, the spin distributions of dark matter and gas track each other almost perfectly, as a consequence of a longer time interval available for momentum redistribution between the two components. The spin of both the gas and dark matter follows a lognormal distribution, with a mean value at z=16 of $<λ> =0.0184$, virtually independent of halo mass. This is in good agreement with previous studies. Using the results of two feedback models (MT1 and MT2) by McKee & Tan (2008) and mapping our halo spin distribution into a PopIII IMF, we find that at high-$z$ the IMF closely tracks the spin lognormal distribution. Depending on the feedback model, though, the distribution can be centered at $\approx 65 M_\odot$ (MT1) or $\approx 140 M_\odot$ (MT2). At later times, model MT1 evolves into a bimodal distribution with a second prominent peak located at $35-40 M_\odot$ as a result of the non-linear relation between rotation and halo mass. We conclude that the dark matter halo properties might be a key factor shaping the IMF of the first stars.

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arXiv2012

The escape fraction of ionizing photons from high redshift galaxies from data-constrained reionization models

The escape fraction, f_{esc}, of ionizing photons from high-redshift galaxies is a key parameter to understand cosmic reionization and star formation history. Yet, in spite of many efforts, it remains largely uncertain. We propose a novel, semi-empirical approach based on a simultaneous match of the most recently determined Luminosity Functions (LF) of galaxies in the redshift range 6 \leq z \leq 10 with reionization models constrained by a large variety of experimental data. From this procedure we obtain the evolution of the best-fit values of f_{esc} along with their 2-sigma limits. We find that, averaged over the galaxy population, (i) the escape fraction increases from f_{esc} = 0.068_{-0.047}^{+0.054} at z=6 to f_{esc} = 0.179_{-0.132}^{+0.331} at z=8; (ii) at z=10 we can only put a lower limit of f_{esc} > 0.146. Thus, although errors are large, there is an indication of a 2.6 times increase of the average escape fraction from z=6 to z=8 which might partially release the "starving reionization" problem.

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arXiv2012

Multiscale Inference of Matter Fields and Baryon Acoustic Oscillations from the Ly-alpha Forest

We present a novel Bayesian method for the joint reconstruction of cosmological matter density fields, peculiar velocities and power-spectra in the quasi-nonlinear regime. We study its applicability to the Ly-alpha forest based on multiple quasar absorption spectra. Our approach to this problem includes a multiscale, nonlinear, two-step scheme since the statistics describing the matter distribution depends on scale, being strongly non-Gaussian on small scales (< 0.1 h^{-1} Mpc) and closely lognormal on scales >~10 h^{-1} Mpc. The first step consists on performing 1D highly resolved matter density reconstructions along the line-of-sight towards z~2-3 quasars based on an arbitrary non-Gaussian univariate model for matter statistics. The second step consists on Gibbs-sampling based on conditional PDFs. The matter density field is sampled in real space with Hamiltonian-sampling using the Poisson/Gamma-lognormal model, while redshift distortions are corrected with linear Lagrangian perturbation theory. The power-spectrum of the lognormal transformed variable which is Gaussian distributed (and thus close to the linear regime) can consistently be sampled with the inverse Gamma distribution function. We test our method through numerical N-body simulations with a computational volume large enough (> 1 h^{-3} Gpc^3) to show that the linear power-spectra are nicely recovered over scales larger than >~20 h^{-1} Mpc, i.e. the relevant range where features imprinted by the baryon-acoustics oscillations (BAOs) appear.

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arXiv2012

The infant Milky Way

We investigate the physical properties of the progenitors of today living Milky Way-like galaxies that are visible as Damped Lya Absorption systems and Lya Emitters at higher redshifts (z ~ 2.3,5.7). To this aim we use a statistical merger-tree approach that follows the formation of the Galaxy and its dwarf satellites in a cosmological context, tracing the chemical evolution and stellar population history of the progenitor halos. The model accounts for the properties of the most metal-poor stars and local dwarf galaxies, providing insights on the early cosmic star-formation. Fruitful links between Galactic Archaeology and more distant galaxies are presented.

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arXiv2012

The formation of the extremely primitive star SDSS J102915+172927 relies on dust

The relative importance of metals and dust grains in the formation of the first low-mass stars has been a subject of debate. The recently discovered Galactic halo star SDSS J102915+172927 (Caffau et al. 2011) has a mass less than 0.8 Msun and a metallicity of Z = 4.5 10^{-5} Zsun. We investigate the origin and properties of this star by reconstructing the physical conditions in its birth cloud. We show that the observed elemental abundance trend of SDSS J102915+172927 can be well fitted by the yields of core-collapse supernovae with metal-free progenitors of 20 Msun and 35 Msun. Using these selected supernova explosion models, we compute the corresponding dust yields and the resulting dust depletion factor taking into account the partial destruction by the supernova reverse shock. We then follow the collapse and fragmentation of a star forming cloud enriched by the products of these SN explosions at the observed metallicity of SDSS J102915+172927. We find that [0.05 - 0.1] Msun mass fragments, which then lead to the formation of low-mass stars, can occur provided that the mass fraction of dust grains in the birth cloud exceeds 0.01 of the total mass of metals and dust. This, in turn, requires that at least 0.4 Msun of dust condense in the first supernovae, allowing for moderate destruction by the reverse shock. If dust formation in the first supernovae is less efficient or strong dust destruction does occur, the thermal evolution of the SDSS J102915+172927 birth cloud is dominated by molecular cooling, and only > 8 Msun fragments can form. We conclude that the observed properties of SDSS J102915+172927 support the suggestion that dust must have condensed in the ejecta of the first supernovae and played a fundamental role in the formation of the first low-mass stars.

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arXiv2012

Supermassive black hole ancestors

We study a model in which supermassive black holes (SMBHs) can grow by the combined action of gas accretion on heavy seeds and mergers of both heavy (m_s^h=10^5 Msol) and light (m_s^l = 10^2 Msol) seeds. The former result from the direct collapse of gas in T_s^h >1.5x10^4K, H_2-free halos; the latter are the endproduct of a standard H_2-based star formation process. The H_2-free condition is attained by exposing halos to a strong (J_21 > 10^3) Lyman-Werner UV background produced by both accreting BHs and stars, thus establishing a self-regulated growth regime. We find that this condition is met already at z close to 18 in the highly biased regions in which quasars are born. The key parameter allowing the formation of SMBHs by z=6-7 is the fraction of halos that can form heavy seeds: the minimum requirement is that f_heavy>0.001; SMBH as large as 2x10^10 Msol can be obtained when f_heavy approaches unity. Independently of f_heavy, the model produces a high-z stellar bulge-black hole mass relation which is steeper than the local one, implying that SMBHs formed before their bulge was in place. The formation of heavy seeds, allowed by the Lyman-Werner radiative feedback in the quasar-forming environment, is crucial to achieve a fast growth of the SMBH by merger events in the early phases of its evolution, i.e. z>7. The UV photon production is largely dominated by stars in galaxies, i.e. black hole accretion radiation is sub-dominant. Interestingly, we find that the final mass of light BHs and of the SMBH in the quasar is roughly equal by z=6; by the same time only 19% of the initial baryon content has been converted into stars. The SMBH growth is dominated at all epochs z > 7.2 by mergers (exceeding accretion by a factor 2-50); at later times accretion becomes by far the most important growth channel. We finally discuss possible shortcomings of the model.

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arXiv2012

Molecular hydrogen in Lyman Alpha Emitters

We present a physically motivated model to estimate the molecular hydrogen (H2) content of high-redshift (z~5.7,6.6) Lyman Alpha Emitters (LAEs) extracted from a suite of cosmological simulations. We find that the H2 mass fraction, (f_H2), depends on three main LAE physical properties: (a) star formation rate, (b) dust mass, and (c) cold neutral gas mass. At z~5.7, the value of f_H2 peaks and ranges between 0.5-0.9 for intermediate mass LAEs with stellar mass M_* ~ 10^{9-10} solar mass, decreasing for both smaller and larger galaxies. However, the largest value of the H2 mass is found in the most luminous LAEs. These trends also hold at z\sim6.6, although, due to a lower dust content, f_H2(z=6.6)\sim0.5 f_H2(z=5.7) when averaged over all LAEs; they arise due to the interplay between the H2 formation/shielding controlled by dust and the intensity of the ultraviolet (UV) Lyman-Werner photo-dissociating radiation produced by stars. We then predict the carbon monoxide (CO) luminosities for such LAEs and check that they are consistent with the upper limits found by Wagg et al. (2009) for two z>6 LAEs. At z\sim(5.7, 6.6), the lowest CO rotational transition observable for both samples with the actual capabilities of Atacama Large Millimeter Array (ALMA) is the CO(6-5). We find that at z\sim5.7, about 1-2% of LAEs, i.e., those with an observed Lyman Alpha luminosity larger than 10^{43.2} erg/s would be detectable with an integration time of 5-10 hours (S/N=5); at z\sim6.6 none of the LAEs would be detectable in CO, even with an ALMA integration time of 10 hours. We also build the CO `flux function', i.e., the number density of LAEs as a function of the line-integrated CO flux, S_CO, and show that it peaks at S_CO = 0.1 mJy at z\sim5.7, progressively shifting to lower values at higher redshifts. We end by discussing the model uncertainties.

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arXiv2012

Lyman Alpha Emitters and Lyman Break Galaxies: dichotomous twins?

(Abridged) We extend our previous studies aimed at modeling Lyman Alpha Emitters (LAEs) to the second population of high redshift sources, Lyman Break Galaxies (LBGs), with the final goal of investigating the physical relationship between them at 6 < z <8. We use a set of large (~10^6 Mpc^3) cosmological SPH simulations that include a detailed treatment of star formation, feedback, metal enrichment and supernova dust production; these same simulations have already been shown to successfully reproduce a large number of observed properties of LAEs (Dayal et al. 2010). We identify LBGs as galaxies with an absolute ultraviolet (UV) magnitude M_{UV} <= -17, consistent with current observational criteria. With no further parameter tuning, the model reproduces the redshift evolution of the LBG UV luminosity function, stellar mass function, and star formation rate (SFR) density extremely well, compared to the data at z~6-8. It predicts a z-independent average beta value of about -2.2, consistent with the most recent observational results. The mean LBG specific SFR increases from 6.7 Gyr^-1 at z ~ 6 to 13.9 Gyr^-1 at z ~ 8, and is largest for the smallest LBGs. From a comparison of the simulated LAE and LBG populations, we find no appreciable differences between them in terms of the stellar masses, ages, SFR, and dust content; only the faintest LBGs with M_{UV} >= -18 (-19) at z ~ 6 (8) do not show an observable Lyman Alpha line. LAEs hence represent a luminous LBG subset, whose relative extent depends only on the adopted equivalent width (EW) selection criteria. Further, in line with current observations, we find that the fraction of LBGs showing a Lyman Alpha line decreases with increasing redshift due to the combined effects of dust and reionization. We conclude with a brief critical model discussion, which emphasizes the uncertainties inherent to theoretical EW determinations.

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