arXiv subjects

Piero Madau

Publications and source records attributed to Piero Madau.

At least 181 records · Page 10Linked to original sources

arXiv2002

First Light and the Reionization of the Universe

The development of primordial inhomogeneities into the non-linear regime and the formation of the first bound objects mark the transition from a simple cooling universe -- described by just a few parameters -- to a very messy hot one -- the realm of radiative, hydrodynamic and star formation processes. In popular cold dark matter cosmological scenarios, stars and quasars may have first appeared in significant numbers around a redshift of 10 or so, as the gas within subgalactic halos cooled rapidly due to atomic processes and fragmented. This early generation of sources generated the ultraviolet radiation and kinetic energy that ended the cosmic ``dark ages'' and reheated and reionized most of the hydrogen in the cosmos by a redshift of 6. The detailed thermal history of the intergalactic medium -- the main repository of baryons at high redshift -- during and soon after these crucial formative stages depends on the power-spectrum of density fluctuations on small scales and on a complex network of poorly understood `feedback' mechanisms, and is one of the missing link in galaxy formation and evolution studies.

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arXiv2002

The Assembly and Merging History of Supermassive Black Holes in Hierarchical Models of Galaxy Formation

We assess models for the assembly of supermassive black holes (SMBHs) at the center of galaxies that trace their hierarchical build-up far up in the dark halo `merger tree'. We assume that the first `seed' black holes (BHs) formed in (mini)halos collapsing at z=20 from high-sigma density fluctuations. As these pregalactic holes become incorporated through a series of mergers into larger and larger halos, they sink to the center owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, form a binary system, and eventually coalesce. The merger history of dark matter halos and associated BHs is followed by cosmological Monte Carlo realizations of the merger hierarchy. A simple model, where quasar activity is driven by major mergers and SMBHs accrete at the Eddington rate a mass that scales with the fifth power of the velocity dispersion, is shown to reproduce the optical LF of quasars in the redshift range 1<z<4. Binary and triple BH interactions are followed in our merger tree. The assumptions underlying our scenario lead to the prediction of a population of massive BHs wandering in galaxy halos and the intergalactic medium at the present epoch, and contributing <10% to the total BH mass density. At all epochs the fraction of binary SMBHs in galaxy nuclei is of order 10%, while the fraction of binary quasars is less than 0.3%

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arXiv2002

Early Structure Formation and the Epoch of First Light

In popular cold dark matter cosmological scenarios, stars may have first appeared in significant numbers around a redshift of 10 or so, as the gas within protogalactic halos with virial temperatures in excess of 20,000 K (corresponding to masses comparable to those of present--day dwarf ellipticals) cooled rapidly due to atomic processes and fragmented. It is this `second generation' of subgalactic stellar systems, aided perhaps by an early population of accreting black holes in their nuclei, which may have generated the ultraviolet radiation and mechanical energy that ended the cosmic ``dark ages'' and reheated and reionized most of the hydrogen in the cosmos by a redshift of 6. The detailed history of the universe during and soon after these crucial formative stages depends on the power--spectrum of density fluctuations on small scales and on a complex network of poorly understood `feedback' mechanisms, and is one of the missing link in galaxy formation and evolution studies. The astrophysics of the epoch of first light is recorded in the thermal state, ionization degree, and chemical composition of the intergalactic medium, the main repository of baryons at high redshifts.

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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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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

Did Very Massive Stars Pre-enrich and Reionize the Universe?

Recent studies of heavy r-process elements in low [Fe/H] halo stars have suggested that an initial population of metal-free very massive stars (VMSs) may be required to provide early Fe enrichment without coproducing heavy r nuclei. We find similar abundance trends in alpha-elements (which should be copiously produced by VMSs), but not in other elements such as carbon (which should not), in agreement with this hypothesis. We then combine the corresponding level of prompt initial enrichment with models of VMS nucleosynthetic yields and spectra to estimate the corresponding ionizing fluxes. The result suggests that there may have been enough VMS activity to reionize the universe. The unusually hard spectrum of VMSs would imply a different reionization history from canonical models. HeII could have been reionized at high redshift, only to recombine as a subsequent generation of stars formed with a ``normal'' initial mass function.

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arXiv2001

Winds from massive stars: implications for the afterglows of gamma-ray bursts

Recent observations suggest that long-duration gamma-ray bursts (GRBs) and their afterglows are produced by highly relativistic jets emitted in core-collapse explosions. The pre-explosive ambient medium provides a natural test for the most likely progenitors of GRBs. Those stars that shed their envelopes most readily have short jet crossing times and are more likely to produce a GRB. We construct a simple computational scheme to explore the expected contribution of the presupernova ejecta of single Wolf-Rayet (WR) stars to the circumstellar environment. Using detailed stellar tracks for the evolution of massive stars, we discuss the effects that the initial main sequence mass, metallicity, rotation and membership in a binary system have on the ambient medium. We extend the theory of GRB afterglows in winds to consider the effect of the relativistic fireball propagating through the WR ejecta. We make specific predictions for the interaction of the relativistic blast wave with the density bumps that arise when the progenitor star rapidly loses a large fraction of its initial mass or when the ejected wind interacts with the external medium and decelerates. A re-brightening of the afterglow with a spectrum redder than the typical synchrotron spectrum (as seen in GRB 970508, GRB 980326 and GRB 000911) is predicted. We also calculate the luminosity of the reflected echo that arises when circumstellar material Compton-scatters the prompt radiation and examine the spectral signatures expected from the interaction of the GRB afterglow with the ejected medium.

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arXiv2001

Compound gravitational lensing as a probe of dark matter substructure within galaxy halos

We show how observations of multiply-imaged quasars at high redshift can be used as a probe of dark matter clumps (subhalos with masses ~ 10^9 solar masses) within the virialized extent of more massive lensing halos. A large abundance of such satellites is predicted by numerical simulations of galaxy formation in cold dark matter (CDM) cosmogonies. Small-scale structure within galaxy halos affects the flux ratios of the images without appreciably changing their positions. We use numerical simulations to quantify the effect of dark matter substructure on the distribution of magnifications, and find that the magnification ratio of a typical image pair will deviate significantly from the value predicted by a smooth lensing potential if, near the Einstein radius, only a few percent of the lens surface density is contained in subhalos. The angular size of the continuum source dictates the range of subclump masses that can have a detectable effect: to avoid confusion with gravitational microlensing caused by stars in the lens galaxy, the background source must be larger than the optical continuum-emitting region of a QSO. We also find that substructure will cause distortions to images on milli-arcsecond scales and bias the distribution of QSO magnification ratios -- two other possible methods of detection.

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arXiv2001

Modelling the uv/x-ray cosmic background with CUBA

In this paper, I will describe the features of the numerical code CUBA, aimed at the solution of the radiative transfer equation in a cosmological context. CUBA will be soon available for public use at the URL http://pitto.mib.infn.it/~haardt/, allowing for several user-supplied input parameters, such as favourite cosmology, luminosity functions, Type II object evolution, stellar spectra, and many others. I will also present some new results of the UV/X-ray cosmic background as produced by the observed populations of QSOs and star forming galaxies, updating and extending our previous works. The background evolution is complemented with a number of derived quantities such as the ionization and thermal state of the IGM, the HeII opacity, the HI and HeII ionization rates, and the HI, HeII and Compton heating rates.

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arXiv2001

Massive Black Holes as Population III Remnants

Recent numerical simulations of the fragmentation of primordial molecular clouds in hierarchical cosmogonies have suggested that the very first stars (the so-called Population III) may have been rather massive. Here we point out that a numerous population of massive black holes (MBHs) -- with masses intermediate between those of stellar and supermassive holes -- may be the endproduct of such an episode of pregalactic star formation. If only one MBH with m > 150 msun formed in each of the `minihalos' collapsing at z=20 from 3-sigma fluctuations, then the mass density of Pop III MBHs would be comparable to that of the supermassive variety observed in the nuclei of galaxies. Since they form in high-sigma rare density peaks, relic MBHs are predicted to cluster in the bulges of present-day galaxies as they become incorporated through a series of mergers into larger and larger systems. Dynamical friction would cause more than 50 (m/150 msun)^{1/2} such objects to sink towards the center. The presence of a small cluster of MBHs in galaxy nuclei may have several interesting consequences associated with tidal captures of ordinary stars (likely followed by disruption), MBH capture by the central supermassive black hole, gravitational wave radiation from such coalescences. Accreting pregalactic MBHs may be detectable as ultra-luminous, off-nuclear X-ray sources.

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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

Lensing Constraints on the Cores of Massive Dark Matter Halos

The statistics of wide-separation (6'' < theta < 15'') gravitational lenses constrain the amount of mass in the cores of dark matter halos on group and cluster mass scales. For a family of halo models with a central cusp $ρ\propto r^{-alpha}$ (1.0 <= alpha <= 1.9), the lack of wide-separation lenses in the large Cosmic Lens All-Sky Survey yields an upper limit on the fraction of the halo mass that is contained within ~4% of the virial radius, f_{core} < 0.023 (95% confidence level, LCDM). This limit offers an important test of the cold dark matter paradigm. While the halo profiles derived from numerical simulations appear to be consistent with this upper limit, larger surveys currently underway such as the 2dF and SDSS should detect wide-separation lenses and thus provide a measurement of the core mass fraction in massive dark matter halos.

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arXiv2000

Extragalactic Background Light, MACHOs, and the Cosmic Stellar Baryon Budget

The optical/far-IR extragalactic background light (EBL) from both resolved and unresolved extragalactic sources is an indicator of the total luminosity of cosmic structures, as the cumulative emission from young and evolved galactic systems, as well as from active galactic nuclei (AGNs), is recorded in this radiation. This is a brief review of some of the implications of the observed brightness of the night sky for the stellar mass density and average metallicity of the universe today, and of the possible contribution of MACHO progenitors and QSOs to the EBL. Assuming a Salpeter initial mass function with a cutoff below 0.6 M_sun, a lower limit of Omega_{g+s}h^2>0.0015 I_{60} can be derived to the visible (recycled gas + stars) mass density required to generate an EBL at a level of I_{EBL}=60 I_{60} nW/m^2/sr: our latest, best-guess estimate is Omega_{g+s}h^2 ~ 0.0023 I_{60}, which implies a mean metallicity at the present-epoch of y_Z Omega_{g+s}/Omega_b ~ 0.2 Z_sun. If massive dark halos around spiral galaxies are partially composed of faint, old white dwarfs, i.e. if a non-negligible fraction (a few percent) of the nucleosynthetic baryons is locked in the remnants (MACHOs) of intermediate-mass stars forming at very high redshifts, then the bright early phases of such halos should contribute significantly to the observed EBL. Assuming a standard black hole accretion model for quasar activity and using recent observations of the quasar population and new synthesis models for the cosmic X-ray background, we estimate a present mass density of QSO remnants of rho_{BH} ~ 3e5 M_sun/Mpc^3 for a 10% efficiency of accreted mass-to-radiation conversion. The quasar contribution to the brightness of the night sky is I_QSO ~ 2 nW/m^2/sr.

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arXiv2000

Photon Consumption in Minihalos during Cosmological Reionization

At the earliest epochs of structure formation in cold dark matter (CDM) cosmologies, the smallest nonlinear objects are the numerous small halos that condense with virial temperatures below 10,000 K. Such ``minihalos'' are not yet resolved in large-scale three-dimensional cosmological simulations. Here we employ a semi-analytic method, combined with three-dimensional simulations of individual minihalos, to examine their importance during cosmological reionization. We show that, depending on when reionization takes place, they potentially play an important role as sinks of ionizing radiation. If reionization occurs at sufficiently high redshifts (z_r > 20), the intergalactic medium is heated to 10,000 K and most minihalos never form. On the other hand, if z_r < 20, a significant fraction (>10 percent) of all baryons have already collapsed into minihalos, and are subsequently removed from the halos by photoevaporation as the ionizing background flux builds up. We show that this process can require a significant budget of ionizing photons; exceeding the production by a straightforward extrapolation back in time of known quasar and galaxy populations by a factor of up to 10 and 3, respectively.

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arXiv2000

The Optical Extragalactic Background Light from Resolved Galaxies

We discuss the ultraviolet to near-IR galaxy counts from the deepest imaging surveys, including the northern and southern Hubble Deep Fields. The logarithmic slope of the galaxy number-magnitude relation is flatter than 0.4 in all seven UBVIJHK optical bandpasses at faint magnitudes, i.e. the light from resolved galaxies has converged from the UV to the near-IR. Most of the galaxy contribution to the extragalactic background light (EBL) comes from relatively bright, low-redshift objects (50% at V_AB<21 and 90% at V_AB<25.5). We find a lower limit to the surface brightness of the optical EBL of about 15 nW/m^2/sterad, comparable to the intensity of the far-IR background from COBE data. Diffuse light, lost because of surface brightness selection effects, may add substantially to the EBL.

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arXiv2000

An ionizing UV background dominated by massive stars

We discuss implications of a stellar-dominated UV background at high redshifts. The composite spectrum of 29 LBGs evaluated by Steidel etal (2000) at =3.4 can be well fit by a stellar population with ongoing star formation, a Salpeter initial mass function, modest or negligible dust reddening, and no intrinsic HI photoelectric absorption. Fading starbursts in which star formation has ceased for 10^7 yr or more cannot reproduce the observed flux shortward of 1 Ryd.The escape fraction of H-ionizing photons must be close to 100 percent for the observed sample of LBGs. The spectrum of ionizing photons produced by a stellar population with ongoing star formation is similar to that of QSOs between 1 and 3 Ryd, becomes softer between 3 and 4 Ryd and drops sharply shortward of 4 Ryd. A galaxy-dominated UV background appears inconsistent with the observed HeII/HI opacity ratio at z=2.4, but might be able to explain the SiIV/CIV abundances measured at z>3 in absorption spectra. A scenario may be emerging where star-forming galaxies reionized intergalactic hydrogen at $z>6$ and dominate the 1 Ryd metagalactic flux at z>3, with quasi-stellar sources taking over at low redshifts. If the large amplitude of the H-ionizing flux estimated by Steidel etal is correct, hydrodynamical simulations of structure formation in the IGM require a baryon density (to explain the observed Lyalpha opacity in absorption spectra) which is similar to or larger than that favoured by recent CMB experiments, and is inconsistent with standard nucleosynthesis values.

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arXiv2000

On the Association of Gamma-ray Bursts with Massive Stars: Implications for Number Counts and Lensing Statistics

Recent evidence appears to link gamma-ray bursts (GRBs) to star-forming regions in galaxies at cosmological distances. If short-lived massive stars are the progenitors of GRBs, the rate of events per unit cosmological volume should be an unbiased tracer (i.e. unaffected by dust obscuration and surface brightness limits) of the cosmic history of star formation. Here we use realistic estimates for the evolution of the stellar birthrate in galaxies to model the number counts, redshift distribution, and time-delay factors of GRBs. We present luminosity function fits to the BATSE log N-log P relation for different redshift distributions of the bursts. Our results imply about 1-2 GRBs every one million Type II supernovae, and a characteristic `isotropic-equivalent' burst luminosity in the range 3-20 x 1e51 ergs/s (for H_0=65 km/s/Mpc). We compute the rate of multiple imaging of background GRBs due to foreground mass condensations in a Lambda-dominated cold dark matter cosmology, assuming that dark halos approximate singular isothermal spheres on galaxy scales and Navarro-Frenk-White profiles on group/cluster scales, and are distributed in mass according to the Press-Schechter model. We show that the expected sensivity increase of Swift relative to BATSE could result in a few strongly lensed individual bursts detected down to a photon flux of 0.1 phot/s/cm^2 in a 3-year survey. Because of the partial sky coverage, however, it is unlikely that the Swift satellite will observe recurrent events (lensed pairs).

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arXiv2000

Probing the End of the Dark Ages

In currently popular cosmological scenarios -- all variants of the cold dark matter (CDM) cosmogony -- some time beyond a redshift of 15, stars within the numerous small halos that condense with virial temperatures ~1e4 K created the first heavy elements; these protogalactic systems, together perhaps with an early population of mini-quasars, generated the ultraviolet radiation and mechanical energy that reheated and reionized the cosmos. The history of the Universe during and soon after these crucial formative stages is recorded in the thermal, ionization, and chemical state of the all-pervading intergalactic medium (IGM), which contains most of the ordinary baryonic material left over from the big bang. Throughout the epoch of structure formation, the IGM becomes clumpy under the influence of gravity, and acts as a source for the gas that gets accreted, cools, and forms stars within subgalactic fragments, and as a sink for the metal enriched material, energy, and radiation which they eject.

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