arXiv subjects

Andrea Ferrara

Publications and source records attributed to Andrea Ferrara.

At least 235 records · Page 13Linked to original sources

arXiv2003

The Detectability of the First Stars and Their Cluster Enrichment Signatures

We conduct a comprehensive investigation of the detectability of the first stars and their enrichment signatures in galaxy clusters. We show that the mean metallicity of outflows from objects containing these Population III (PopIII) stars is well above the critical transition metallicity (Z_cr \sim 10^-4) that marks the formation of normal stars. Thus the fraction of PopIII objects formed as a function of redshift is heavily dependent on the distribution of metals and fairly independent of the precise value of Z_cr. Using an analytical model of inhomogenous structure formation, we study the evolution of PopIII objects as a function of the star formation efficiency, IMF, and efficiency of outflow generation. For all models, PopIII objects tend to be in the 10^6.5-10^7.0 solar mass range, just large enough to cool within a Hubble time, but small enough that they are not clustered near areas of previous star formation. Although the mean metallicity exceeds Z_cr at a redshift of 15 in all models, the peak of PopIII star formation occurs at z \sim 10, and such stars continue to form well into the observable range. We discuss the observational properties of these objects, some of which may have already been detected in ongoing surveys of high-redshift Lyman-alpha emitters. Finally, we combine our PopIII distributions with the yield models of Heger and Woosley (2002) to study their impact on the intracluster medium (ICM) in galaxy clusters. We find that PopIII stars can contribute no more than 20% of the iron observed in the ICM, although their peculiar elemental yields help to reconcile theoretical models with the observed Fe and Si/Fe abundances. However, these stars tend to overproduce S/Fe and their associated SN heating falls far short of the observed level of 1 keV per ICM gas particle.

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arXiv2002

On the Direct Detectability of the Cosmic Dark Ages: 21-cm Emission from Minihalos

In the standard Cold Dark Matter (CDM) theory of structure formation, virialized minihalos (with T_vir}<10^4 K) form in abundance at high redshift (z>6), during the cosmic ``dark ages.'' The hydrogen in these minihalos, the first nonlinear baryonic structures to form in the universe, is mostly neutral and sufficiently hot and dense to emit strongly at the 21-cm line. We calculate the emission from individual minihalos and the radiation background contributed by their combined effect. Minihalos create a ``21-cm forest'' of emission lines. We predict that the angular fluctuations in this 21-cm background should be detectable with the planned LOFAR and SKA radio arrays, thus providing a direct probe of structure formation during the ``dark ages.'' Such a detection will serve to confirm the basic CDM paradigm while constraining the shape of the power-spectrum of primordial density fluctuations down to much smaller scales than have previously been constrained, the onset and duration of the reionization epoch, and the conditions which led to the first stars and quasars. We present results here for the currently-favored, flat $Λ$CDM model, for different tilts of the primordial power spectrum.

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arXiv2002

What regulates the velocity distribution of interstellar clouds?

Kinetic energy stored in ISM bulk/turbulent motions is a crucial ingredient to properly describe most properties of observed galaxies. By using Monte Carlo simulations, we investigate how this energy is injected by supernovae and dissipated via cloud collisions and derive the corresponding ISM velocity probability distribution function (PDF). The functional form of the PDF for the modulus of the velocity dispersion is $$p(v) \propto v^2 \exp[-(v/σ)^β].$$ The power-law index of the PDF depends only on the value of the average cloud collision elasticity < ε> as β= 2\exp(<ε> -1). If βand the gas velocity dispersion σare known, the specific kinetic energy dissipated by collisions is found to be \propto σ^2 \ln (2 / β)/(β-0.947); in steady state, this is equal to the energy input from SNe. We predict that in a multiphase, low metallicity (Z \approx 5 \times 10^{-3} Z_\odot) ISM the PDF should be close to a Maxwellian (β= 2) with velocity dispersion σ\simgt 11$ km s\m; in more metal rich systems (Z \simgt 5 \times 10^{-2} Z_\odot), instead, we expect to observe almost exponential PDFs. This is in good agreement with a number of observations that we review and might explain the different star formation modes seen in dwarfs and spiral galaxies.

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arXiv2002

Early Enrichment of the Intergalactic Medium and its Feedback on Galaxy Formation

Supernova-driven outflows from early galaxies may have had a large impact on the kinetic and chemical structure of the intergalactic medium (IGM). We use three-dimensional Monte Carlo cosmological realizations of a simple linear peaks model to track the time evolution of such metal-enriched outflows and their feedback on galaxy formation. We find that at most 30% of the IGM by volume is enriched to values above 10^-3 solar in models that only include objects that cool by atomic transitions. The majority of enrichment occurs relatively early (5 < z < 12) and resulting in a mass-averaged cosmological metallicity between 10^-3 and 10^-1.5 solar. The inclusion of Population III objects that cool through H2 line emission has only a minor impact on these results: increasing the mean metallicity and filling factor by at most a factor of 1.4, and moving the dawn of the enrichment epoch to a redshift of approximately 14 at the earliest. Thus enrichment by outflowing galaxies is likely to have been incomplete and inhomogeneous, biased to the areas near the starbursting galaxies themselves. Models with a 10% star formation efficiency can satisfactorily reproduce the nearly constant (2 < z < 5, Z approximately 3.5 x 10^-4 solar) metallicity of the low column density Ly-alpha forest derived by Songaila (2001), an effect of the decreasing efficiency of metal loss from larger galaxies. Finally, we show that IGM enrichment is intimately tied to the ram-pressure stripping of baryons from neighboring perturbations. This results in the suppression of at least 20% of the dwarf galaxies in the mass range 10^8.5 to 10^9.5 solar, in all models with filling factors greater than 2%, and an overall suppression of approximately 50% of dwarf galaxies in the most observationally-favored model.

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arXiv2002

Radiation from the first forming stars

The evolution of radiation emitted during the dynamical collapse of metal-free protostellar clouds is investigated within a spherically symmetric hydrodynamical scheme that includes the transfer of radiation and the chemistry of the primordial gas. The cloud centre collapses on a time scale of about 10^5-10^6 years, thanks to line cooling from molecular hydrogen (H2). For most of the collapse time, when the evolution proceeds self-similarly, the luminosity slowly rises up to about 10^36 erg/s and is essentially due to H2 IR line emission. Later, continuum IR radiation provides an additional contribution, which is mostly due to the accretion of an infalling envelope upon a small hydrostatic protostellar core which develops in the centre. We follow the beginning of the accretion phase, when the enormous accretion rate (~ 0.1 Msun/yr) produces a very high continuum luminosity of about 10^36 erg/s. Despite the high luminosities, the radiation field is unable to affect the gas dynamics during the collapse and the first phases of accretion, because the opacity of the infalling gas is too small; this is quite different from present-day star formation. We also find that the protostellar evolution is similar among clouds with different initial configurations, including those resulting from 3D cosmological simulations of primordial objects; in particular, the shape of the molecular spectra is quite universal. Finally, we briefly discuss the detectability of this pristine cosmic star formation activity.

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arXiv2002

Gamma Ray Bursts from the First Stars: Neutrino Signals

If the first (PopIII) stars were very massive, their final fate is to collapse into very massive black holes. Once a proto-black hole has formed into the stellar core, accretion continues through a disk. It is widely accepted, although not confirmed, that magnetic fields drive an energetic jet which produces a burst of TeV neutrinos by photon-meson interaction, and eventually breaks out of the stellar envelope appearing as a Gamma Ray Burst (GRB). Based on recent numerical simulations and neutrino emission models, we predict the expected neutrino diffuse flux from these PopIII GRBs and compare it with the capabilities of present and planned detectors as AMANDA and IceCube. If beamed into 1% of the sky, we find that the rate of PopIII GRBs is $\le 4 \times 10^6$ yr$^{-1}$. High energy neutrinos from PopIII GRBs could dominate the overall flux in two energy bands [$10^4 - 10^5$] GeV and [$10^5 - 10^6$] GeV of neutrino telescopes. The enhanced sensitivities of forthcoming detectors in the high-energy band (AMANDA-II, IceCube) will provide a fundamental insight on the characteristic explosion energies of PopIII GRBs and will constitute a unique probe of the the Initial Mass Function (IMF) of the first stars and of the redshift $z_f$ marking the metallicity-driven transition from a top-heavy to a normal IMF. The current upper limit set by AMANDA-B10 implies that such transition must have occurred not later than $z_f =9.2$ for the most plausible jet energies. Based on such results, we speculate that PopIII GRBs, if not chocked, could be associated with a new class of events detected by BeppoSax, the Fast X-ray Transient (FXTs), which are bright X-ray sources, with peak energies in the 2-10 keV band and durations between 10-200 s.

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arXiv2002

How is the Reionization Epoch Defined?

We study the effect of a prolonged epoch of reionization on the angular power spectrum of the Cosmic Microwave Background. Typically reionization studies assume a sudden phase transition, with the intergalactic gas moving from a fully neutral to a fully ionized state at a fixed redshift. Such models are at odds, however, with detailed investigations of reionization, which favor a more extended transition. We have modified the code CMBFAST to allow the treatment of more realistic reionization histories and applied it to data obtained from numerical simulations of reionization. We show that the prompt reionization assumed by CMBFAST in its original form heavily contaminates any constraint derived on the reionization redshift. We find, however, that prompt reionization models give a reasonable estimate of the epoch at which the mean cosmic ionization fraction was ~50%, and provide a very good measure of the overall Thomson optical depth. The overall differences in the temperature (polarization) angular power spectra between prompt and extended models with equal optical depths are less than 1% (10%).

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arXiv2001

Early Metal Enrichment by Pregalactic Outflows: II. Simulations of Blow-away

We present results from three-dimensional numerical simulations of the dynamics of SN-driven bubbles as they propagate through and escape the grasp of subgalactic halos with masses M ~ 10^8 Msun at redshift z=9. The hydrodynamic simulations use a nested grid method to follow the evolution of explosive multi-SN events operating on the characteristic timescale of a few x 10^7 yr, the lifetime of massive stars. The results confirm that, if the star formation efficiency of subgalactic halos is close to 10%, a significant fraction of the halo gas will be lifted out of the potential well (`blow-away'), shock the intergalactic medium, and pollute it with metal-enriched material, a scenario recently advocated by Madau, Ferrara & Rees (2001). The volume filling factor of the ejecta is close to unity. Depending on the stellar distribution, we find that less than 30% of the available SN energy is converted into kinetic energy of the blown away material, the remainder being radiated away. It appears that mechanical feedback is less efficient than expected from simple energetic arguments, as off-nuclear SN explosions drive inward-propagating shocks that tend to collect and pile up cold gas in the central regions of the host halo. Low-mass galaxies at early epochs may survive multiple SN events and continue forming stars.

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arXiv2001

Scintillation as a Probe of the Intergalactic Medium

Most of the baryons in the low-redshift universe reside in a warm/hot component which is difficult to detect with standard absorption/emission line techniques. We propose to use quasar refractive scintillation as an useful, complementary probe of such ionized, intergalactic gas. In particular, an application to the case of the intracluster medium is presented. We show that clusters located at z approx 0.02 should produce a source rms intensity fluctuation at 50-100 GHz of several tens of percent and on time scales ranging from days to months, depending on the projected location of the source on the foreground cluster. However, in order to produce such a signal, the source needs to be very compact. This effect, if observed, can be used as an independent test of the baryonic mass fraction in clusters.

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arXiv2001

Early Metal Enrichment of the Intergalactic Medium by Pregalactic Outflows

We assess supernova (SN)-driven pregalactic outflows as a mechanism for distributing the product of stellar nucleosynthesis over large cosmological volumes prior to the reionization epoch. SN ejecta will escape the grasp of halos with virial temperatures T_vir>10^{4.3} K (corresponding to masses M>10^8 h^{-1} M_sun at redshift z=9 when they collapse from 2-sigma fluctuations) if rapid cooling can take place, and a significant fraction of their baryonic mass is converted into stars over a dynamical timescale. We study the evolution of SN-driven bubbles as they blow out from subgalactic halos and propagate into the intergalactic medium (IGM), and show that to lift the halo gas out of the potential well the energy injection must continue at least until blow-away occurs. The collective explosive output of about ten thousands SNe per halo at these early epochs could pollute vast regions of intergalactic space to a mean metallicity =Omega_Z/Omega_b>0.003 (comparable to the levels observed in the Lyman-alpha forest at z=3) without much perturbing the IGM hydrodynamically, i.e. producing large variations of the baryons relative to the dark matter. The volume filling factor of the ejecta is higher than 20% if the star formation efficiency is of order 10%. When the filling factor of the ejecta becomes significant, enriched material will typically be at a higher adiabat than expected from photoionization.

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arXiv2001

Cosmological Origin of the Lowest Metallicity Halo Stars

We explore the predictions of the standard hierarchical clustering scenario of galaxy formation, regarding the numbers and metallicities of PopIII stars likely to be found within our Galaxy today. By PopIII we shall be referring to stars formed at large redshift ($z>4$), with low metallicities ($[Z/Z_{\odot}]<-2.5$) and in small systems (total mass $\simlt$ $2\times 10^{8} M_{\odot}$) that are extremely sensitive to stellar feedback, and which through a prescribed merging history (Lacey & Cole 1993) end up becoming part of the Milky Way today. An analytic, extended Press-Schechter formalism is used to get the mass functions of halos which will host PopIII stars at a given redshift, and which will end up in Milky Way sized systems today. Each of these is modeled as a mini galaxy, with a detailed treatment of the dark halo structure, angular momentum distribution, final gas temperature and disk instabilities, all of which determine the fraction of the baryons which are subject to star formation. Use of new primordial metallicity stellar evolutionary models allows us to trace the history of the stars formed, give accurate estimates of their expected numbers today, and their location in $L/L_{\odot}$ vs. $T/K$ HR diagrams. A first comparison with observational data suggests that the IMF of the first stars was increasingly high mass weighted towards high redshifts, levelling off at $z\simgt 9$ at a characteristic stellar mass scale $m_s=10-15 M_\odot$.

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arXiv2000

Detecting the First Objects in the Mid-IR with NGST

We calculate the expected Mid-IR molecular hydrogen line emission from the first objects in the universe. Due to their low masses, the stellar feedback from massive stars is able to blowaway their gas content and collect it into a cooling shell where H_2 rapidly forms and IR roto-vibrational (as for example the restframe 2.12 micron) lines carry away a large fraction (up to 10%) of the explosion energy. The fluxes from these sources are in the range 10^{-21}-10^{-17} erg/s/cm^2. The highest number counts are expected in the 20 micron band, where about 10^5 sources/deg^2 are predicted at the limiting flux of 3 \times 10^{-18} erg/s/cm^2. Among the planned observational facilities, we find that the best detection perspectives are offered by NGST, which should be able to reveal about 200 first objects in one hour observation time at its limiting flux in the above band. Therefore, Mid-IR instruments appear to represent perfect tools to trace star formation and stellar feedback in the high (z>5) redshift universe.

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arXiv2000

Dust Formation in Primordial Type II Supernovae

We have investigated the formation of dust in the ejecta of Type II supernovae (SNe), mostly of primordial composition, to answer the question of where are the first solid particles formed in the universe. However, we have also considered non-zero progenitor's metallicity values up to Z=Zsun. The calculations are based on standard nucleation theory and the scheme has been first tested on the well studied case of SN1987A, yielding results that are in agreement with the available data. We find that: i) the first dust grains are predominantly made of silicates, amorphous carbon (AC), magnetite, and corundum; ii) the largest grains are the AC ones, with sizes around 300 A, whereas other grain types have smaller radii, around 10-20 A. The grain size distribution depends somewhat on the thermodynamics of the ejecta expansion and variations in the results by a factor ~ 2 might occur within reasonable estimates of the relevant parameters. Also, and for the same reason, the grain size distribution, is essentially unaffected by metallicity changes. The predictions on the amount of dust formed are very robust: for Z=0, we find that SNe with masses in the range (12-35) Msun produce about 0.08 Msun < M_dust < 0.3 Msun of dust/SN. The above range increases by roughly 3 times as the metallicity is increased to solar values. We discuss the implications and the cosmological consequences of the results.

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arXiv2000

Physical Processes at Cosmic Dawn

In this summary paper I will focus on the four main topics discussed at this Conference: (i) First luminous sources in the universe, (ii) Reionization, (iii) Intergalactic medium and (iv) Galaxy formation. In addition to provide a necessarily sketchy summary of the various talks on these subjects, I will also outline some of potentially important issues that still need to be fully addressed.

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arXiv2000

Mixing Metals in the Early Universe

We investigate the evolution of the metallicity of the intergalactic medium (IGM) with particular emphasis on its spatial distribution. We propose that metal enrichment occurs as a two step process. First, supernova (SN) explosions eject metals into relatively small regions confined to the surroundings of star-forming galaxies. From a comprehensive treatment of blowout we show that SNae by themselves fail by more than one order of magnitude to distribute the products of stellar nucleosynthesis over volumes large enough to pollute the whole IGM to the metallicity levels observed. Thus, a additional (but as yet unknown) physical mechanism must be invoked to mix the metals on scales comparable to the mean distance between the galaxies which are most efficient pollutants. From this simple hypothesis we derive a number of testable predictions for the evolution of the IGM metallicity. Specifically, we find that: (i) the fraction of metals ejected over the star formation history of the universe is about 50% at z=0; that is, approximately half of the metals today are found in the IGM; (ii) if the ejected metals were homogeneously mixed with the baryons in the universe, the average IGM metallicity would be ~ 1/25 Z(solar) at z=3. However, due to spatial inhomogeneities, the mean of the distribution of metallicities in the diffusive zones has a wide (more than 2 orders of magnitude) spread around this value; (iii) if metals become more uniformly distributed at z < 1, as assumed, at z = 0 the metallicity of the IGM is narrowly confined within the range Z ~ 0.1 +/- 0.03 Z(solar). Finally, we point out that our results can account for the observed metal content of the intracluster medium.

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arXiv2000

The Influence of Galactic Outflows on the Formation of Nearby Dwarf Galaxies

We show that the gas in growing density perturbations is vulnerable to the influence of winds outflowing from nearby collapsed galaxies that have already formed stars. This suggests that the formation of nearby galaxies with masses less than 10^9 solar masses is likely to be suppressed, irrespective of the details of galaxy formation. An impinging wind may shock heat the gas of a nearby perturbation to above the virial temperature, thereby mechanically evaporating the gas, or the baryons may be stripped from the perturbation entirely if they are accelerated to above the escape velocity. We show that baryonic stripping is the most effective of these two processes, because shock-heated clouds that are too large to be stripped are able to radiatively cool within a sound-crossing time, limiting evaporation. The intergalactic medium temperatures and star-formation rates required for outflows to have a significant influence on the formation of low-mass galaxies are consistent with current observations, but may soon be examined directly via associated distortions in the cosmic microwave background, and with near-infrared observations from the Next Generation Space Telescope, which may detect the supernovae from early-forming stars.

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arXiv2000

Generation of the Primordial Magnetic Fields during Cosmological Reionization

We investigate the generation of magnetic field by the Biermann battery in cosmological ionization fronts, using new simulations of the reionization of the universe by stars in protogalaxies. Two mechanisms are primarily responsible for magnetogenesis: i) the breakout of I-fronts from protogalaxies, and ii) the propagation of I-fronts through the high density neutral filaments which are part of the cosmic web. The first mechanism is dominant prior to overlapping of ionized regions (z ~ 7), whereas the second continues to operate even after that epoch. However, after overlap the field strength increase is largely due to the gas compression occurring as cosmic structures form. As a consequence, the magnetic field at z ~ 5 closely traces the gas density, and it is highly ordered on megaparsec scales. The mean mass-weighted field strength is B_0 ~ 10^{-19} G in the simulation box. There is a relatively well-defined, nearly linear correlation between B_0 and the baryonic mass of virialized objects, with B_0 ~ 10^{-18} G in the most massive objects (M ~ 10^9 M_sun) in our simulations. This is a lower limit, as lack of numerical resolution prevents us from following small scale dynamical processes which could amplify the field in protogalaxies. Although the field strengths we compute are probably adequate as seed fields for a galactic dynamo, the field is too small to have had significant effects on galaxy formation, on thermal conduction, or on cosmic ray transport in the intergalactic medium. It could, however, be observed in the intergalactic medium through innovative methods based on analysis of gamma-ray burst photon arrival times.

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arXiv2000

Gravitational Lensing Effects on High Redshift Type II Supernova Studies with NGST

We derive the expected Type II SN differential number counts, N(m), and Hubble diagram for SCDM and LCDM cosmological models, taking into account the effects of gravitational lensing (GL) produced by the intervening cosmological mass. The mass distribution of dark matter halos (ie the lenses) is obtained by means of a Monte Carlo method applied to the Press-Schechter mass function. The halos are assumed to have a NFW density profile, in agreement with recent simulations of hierarchical cosmological models. Up to z=15, the (SCDM, LCDM) models predict a total number of (857, 3656) SNII/yr in 100 surveyed 4' times 4' fields of the Next Generation Space Telescope. NGST will be able to reach the peak of the N(m) curve, located at AB approx 30(31) for SCDM(LCDM) in J and K wavelength bands and detect (75%, 51%) of the above SN events. This will allow a detailed study of the early cosmic star formation history, as traced by SNIIe. N(m) is only very mildly affected by the inclusion of lensing effects. In addition, GL introduces a moderate uncertainty in the determination of cosmological parameters from Hubble diagrams, when these are pushed to higher $z$. For example, for a ``true'' LCDM with (Omega_M= 0.4, Omega_Lambda=0.6), without proper account of GL, one would instead derive (Omega_{M}=0.36^{+0.15}_{-0.12}, Omega_{Lambda}=0.60^{+0.12}_{-0.24}). We briefly compare our results with previous similar work and discuss the limitations of the model.

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