arXiv subjects

Massimo Ricotti

Publications and source records attributed to Massimo Ricotti.

At least 109 records · Page 6Linked to original sources

arXiv2007

Is the Concentration of Dark Matter Halos at Virialization Universal ?

Several recent studies suggest a correlation between dark matter halo mass and the shape of the density profile. We re-analyze simulations from Ricotti (2003) in which such a correlation was proposed. We use a standard analysis of the halo density profiles and compare the old simulations to new ones performed with Gadget2, including higher resolution runs. We confirm Ricotti's result that, at virialization, the central log slopes alpha, at 5%-10% of the virial radius are correlated with the halo mass and that the halo concentration is a universal constant. Our results do not contradict the majority of published papers: when using a split power law to fit the density profiles, due to the alpha-concentration degeneracy, the fits are consistent with halos having a universal shape with alpha=1 or 1.5 and concentrations that depend on the mass, in agreement with results published elsewhere. Recently, several groups have found no evidence for convergence of the inner halo profile to a constant power law. The choice of a split power law parameterization used in this letter is motivated by the need to compare our results to previous ones and is formally valid because we are not able to resolve regions where the slope of the fitting function reaches its asymptotic constant value. Using a non-parameterized technique, we also show that the density profiles of dwarf galaxies at z ~ 10 have a log slope shallower than 0.5 within 5% of the virial radius.

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arXiv2006

Growth of structure seeded by primordial black holes

We discuss the possibilities for primordial black holes (PBHs) to grow via the accretion of dark matter. In agreement with previous works, we find that accretion during the radiation-dominated era does not lead to a significant mass increase. However, during matter-domination, PBHs may grow by up to two orders of magnitude in mass through the acquisition of large dark matter halos. We discuss the possibility of PBHs being an important component in dark matter halos of galaxies as well as their potential to explain the ultra-luminous x-ray sources (ULXs) observed in nearby galactic disks. We point out that although PBHs are ruled out as the dominant component of dark matter, there is still a great deal of parameter space open to them playing a role in the modern-day universe. For example, a primordial halo population of PBHs each at $10^{2.5} M_\odot$ making up 0.1% of the dark matter grow to $10^{4.5} M_\odot$ via the accumulation of dark matter halos to account for $\sim 10%$ of the dark matter mass by a redshift of $z \approx 30$. These intermediate mass black holes may then ``light up'' when passing through molecular clouds, becoming visible as ULXs at the present day, or they may form the seeds for supermassive black holes at the centers of galaxies.

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arXiv2005

Formation Histories of Dwarf Galaxies in the Local Group

We compare the properties of dwarf galaxies in the Local Group with the simulated galaxies formed before reionization in a cosmological simulation of unprecedented spatial and mass resolution,including radiative feedback effects. We find that a subset of the Local Group dwarfs are remarkably similar to the simulated dwarf galaxies in all their properties already before reionization. Based on this similarity, we propose the hypothesis that Local Group dwarfs form in a variety of ways: some of them are ``true fossils'' of the pre-reionization era, some of them form most of their stars later, after reionization (we call them ``survivors'' of the reionization era), and the rest of them form an intermediate group of ``polluted fossils''. We also identify a simple observational test that is able to falsify our hypothesis.

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arXiv2004

X-ray Preionisation Powered by Accretion on the First Black Holes. II: Cosmological Simulations and Observational Signatures

We use cosmological simulations to study the X-ray ionisation and heating of the intergalactic medium by an early population of accreting black holes. By considering observational constraints from the X-ray background, we find an upper limit for the optical depth to Thompson scattering tau_e~0.17. The redshifted soft X-ray background from these early sources produces:(i) fully ionised atomic hydrogen in the low density intergalactic medium before redshift z~7 (consequently stellar reionisation is characterised by an instantaneous overlap phase of HII regions),(ii) a second HeII reionisation at z~3 and (iii) heats the intergalactic medium to near 10000 K at low redshifts. The typical luminosity in the soft X-ray band of the galaxies hosting the black holes is about one order of magnitude below the sensitivity limit of the Chandra deep field. About a thousand of these sources may be present per square arcmin of the sky, producing detectable fluctuations. Few rarer objects could be luminous enough to be visible in the Chandra deep field. XEUS and Con-X satellites will be able to detect more of these sources that, if radio loud, could be used to study the 21cm forest in absorption.A signature of an early X-ray preionisation is the production of secondary CMB anisotropies on small angular scales. We find that in these models the power spectrum of temperature fluctuations increases with decreasing angular scale (dT~16 muK at 1arcsec scales), while for stellar reionisation scenarios the power decreases on smaller scales. We also show that the redshifted 21 cm radiation from neutral hydrogen can be marginally detected in emission at redshifts 7<z<12. At a redshift of about z~30 a stronger and narrower (in redshift space) signal in absorption against the CMB, that is peculiar to these models,could be detectable.[abridged]

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arXiv2004

X-ray Preionisation Powered by Accretion on the First Black Holes. I: a Model for the WMAP Polarisation Measurement

We investigate the possibility that there is a first phase of partial ionisation due to X-rays produced by black hole accretion in small-mass galaxies at redshifts 7<z<20. This is followed by complete reionisation by stellar sources at z~7. This scenario is motivated by the large optical depth to Thompson scattering, tau_e=0.17, measured by WMAP. But it is also consistent with the observed Gunn-Peterson trough in the spectra of quasars at z~5-6. We use a semianalytic code to explore models with different black hole accretion histories and cosmological parameters.We find that ``preionisation'' by X-rays can increase the intergalactic medium (IGM) optical depth from tau_e~0.06 given by stellar sources only, to 0.1<tau_e<0.2, if a fraction of baryons 5x10^-5 is accreted onto seed black holes produced in the collapse of low metallicity, high mass stars before z~15. To be effective, preionisation requires a non-negligible star formation in the first small-mass galaxies in hich seed black holes are formed.The model predicts that dwarf spheroidal galaxies may host a mass in black holes that is 5-40% of their stellar mass. The redshifted X-ray background produced by this early epoch of black hole accretion constitutes about 5-10% of the X-ray background in the 2-50 keV bands and roughtly half of the currently estimated black hole mass density was formed at early times.Moreover, in most models, the photons from the redshifted background are sufficient to fully reionise HeII at redshift z~3 without any additional contribution from quasars at lower redshifts and the temperature of the mean density intergalactic medium remains close to 10^4 K down to redshift z~1.[abridged]

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arXiv2004

Have we detected one of the sources responsible for an early reionisation of the Universe?

In a recent paper Pell'o et al. have reported a candidate z=10 galaxy, A1835#1916, which was found in a near-infrared survey of the central regions of the gravitational lensing cluster A1835. If this detection is confirmed and the detection rate turns out to be typical, then the volume averaged ultraviolet emissivity must be rising rapidly with increasing redshift. For a magnification due to gravitational lensing by a factor M>25 estimated by Pell'o et al., the inferred star formation rate density at z=10 would be about one order of magnitude higher than estimates of the star formation rate density at z=6. Objects at z=10 would contribute substantially to the total source counts at 1.6 micronm and the estimated space density of sources may exceed the space density of dark matter haloes in a LCDM model. We therefore argue that if A1835#1916 is indeed at z=10 then either the magnification factor may have been overestimated or the galaxy has a top-heavy initial mass function. Sources with the ultraviolet flux and space density of A1835#1916 may be sufficient to reionise most of the diffuse hydrogen in the Universe at z=10. We further use a correlation between the equivalent width and the redshift of the Ly-alpha emission line with respect to the systemic redshift observed in Lyman break galaxies to obtain constraints on the ionisation state of the surrounding intergalactic medium (IGM) from the Gunn-Peterson absorption. These constraints also argue in favour of the surrounding IGM being partially or fully ionised.

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arXiv2004

On the Origin of Dark Matter Cores in Dwarf Galaxies

In this paper, we study the dynamical stability and time evolution of the central dark matter cores of low-mass (about 10^8-10^9 M_solar) galactic haloes found in recent cold dark matter simulations at high redshift. From those simulations we extract three haloes, assembled by hierarchical merging, that at redshift z>10 display a core and we evolve them without further merging to low redshift using direct N-body integration. The central core in the dark matter profile is found to be dynamically stable: it survives for many crossing times without evolution into a cusp. This result supports the claim that the mass dependence of the central dark matter profiles of simulated haloes is a direct consequence of the power spectrum of primordial density fluctuations. In addition, we show that the simulated dark matter profiles, if they evolved in isolation, are consistent with the observed velocity dispersion profile of stars in the inner parts of the Draco dwarf spheroidal galaxy. Simple scaling arguments are reviewed which explain the evolution of the concentration parameter with redshift. We also review some arguments used to derive the logarithmic slope of the inner and outer density profile.

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arXiv2004

Reionisation: the role of Globular Clusters

In this talk I discuss the role of proto-globular clusters as the dominant sources of radiation that reionised hydrogen in the intergalactic medium (IGM) at redshift z~6. Observations at lower redshifts indicate that only a small fraction, f_esc, of hydrogen ionising radiation emitted from massive stars can escape unabsorbed by the galaxy into the IGM. High redshift galaxies are expected to be more compact and gas rich than present day galaxies, consequently f_esc from their disks or spheroids might have been very small. But if the sites of star formation in the galaxies are off-centre and if the star formation efficiency of the proto-clusters is high, then the mean f_esc calculated for these objects only, is expected to be close to unity. Here I argue that this mode of star formation is consistent with several models for globular clusters formation. Using simple arguments based on the observed number of globular cluster systems in the local universe and assuming that the oldest globular clusters formed before reionisation and had f_esc~1, I show that they produced enough ionising photons to reionise the IGM at z~6. I also enphasize that globular cluster formation might have been the dominant mode of star formation at redshifts from 6 to 12.

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arXiv2004

Reionisation, chemical enrichment and seed black holes from the first stars: is Population III important?

We investigate the effects of a top-heavy stellar initial mass function on the reionisation history of the intergalactic medium (IGM). We use cosmological simulations that include self-consistently the feedback from ionising radiation, H_2 dissociating radiation and supernova (SN) explosions. We find that it is difficult to reionise the IGM at z_rei>10 with stellar sources even after making extreme assumptions. If star formation in 10^9 M_\odot galaxies is not suppressed by SN explosions, the optical depth to Thomson scattering is tau_e< 0.13. If we allow for the normal energy input from SNe or if pair-instability SNe are dominant, we find taue<0.09. Assuming normal yields for the first stars (popIII), the mean metallicity of the IGM is already Z/Z_\odot=2x10^-3 (10^-3 260 M_\odot, they collapse directly onto black holes without exploding as SNe.If metal-poor stars are initially important and collapse to black holes is the typical outcome, then the secondary emission of ionising radiation from accretion on SN induced seed black holes, might be more important than the primary emission. Surprisingly, including feedback effects, we estimate that a warm dark matter scenario (with particle mass of 1.25 keV) reduces tau_e by only approximately 10%.[abridged]

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arXiv2003

Dependence of the Inner DM Profile on the Halo Mass

I compare the density profile of dark matter (DM) halos in cold dark matter (CDM) N-body simulations with 1 Mpc, 32 Mpc, 256 Mpc and 1024 Mpc box sizes. In dimensionless units the simulations differ only for the initial power spectrum of density perturbations. I compare the profiles when the most massive halos are composed of about 10^5 DM particles. The DM density profiles of the halos in the 1 Mpc box show systematically shallower cores with respect to the corresponding halos in the 32 Mpc simulation that have masses, M_{dm}, typical of the Milky Way and are fitted by a NFW profile. The DM density profiles of the halos in the 256 Mpc box are consistent with having steeper cores than the corresponding halos in the 32 Mpc simulation, but higher mass resolution simulations are needed to strengthen this result. Combined, these results indicate that the density profile of DM halos is not universal, presenting shallower cores in dwarf galaxies and steeper cores in clusters. Physically the result sustains the hypothesis that the mass function of the accreting satellites determines the inner slope of the DM profile. In comoving coordinates, r, the profile ρ_{dm} \propto 1/(X^α(1+X)^{3-α}), with X=c_Δr/r_Δ, r_Δis the virial radius and α=α(M_{dm}), provides a good fit to all the DM halos from dwarf galaxies to clusters at any redshift with the same concentration parameter c_Δ~ 7. The slope, γ, of the outer parts of the halo appears to depend on the acceleration of the universe: when the scale parameter is a=(1+z)^{-1} < 1, the slope is γ~ 3 as in the NFW profile, but γ~ 4 at a > 1 when Ω_Λ~ 1 and the universe is inflating.[abridged]

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arXiv2002

Dwarf Sphs/First-galaxies connection

I analyze the properties of the first galaxies in cosmological simulations with radiative feedback. Preliminary results indicate similarities with the observed properties of the bulk of dwarf spheroidal galaxies (dSphs) in the Local Group and Andromeda. I briefly discuss observational tests that could help in understanding the impact of a population of small primordial objects on the cosmic evolution.

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arXiv2002

Did Globular Clusters Reionize the Universe?

A problem still unsolved in cosmology is the identification of the sources of radiation able to reionize HI in the intergalactic medium (IGM) by z~6. Theoretical works and observations seem to indicate that the fraction, f_esc, of HI ionizing radiation emitted from galaxies that escapes into the IGM is small in the local universe (f_esc < 10%). At high redshift galaxies are more compact and probably gas rich implying smaller values of f_esc from their disks or spheroids. But if the sites of star formation are displaced from the disk or spheroid and the star formation efficiency of the proto-clusters is high, then f_esc should be about one. This star formation scenario is consistent with several models for globular clusters formation. Using simple arguments based on the observed number of globular cluster systems in the local universe and assuming that the oldest globular clusters formed before reionization and had f_esc~1, I show that they produced enough ionizing photons to reionize the IGM at z=6.

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arXiv2002

The Fate of the First Galaxies. I. Self-Consistent Cosmological Simulations with Radiative Transfer

In cold dark matter (CDM) cosmogonies, low-mass objects play an important role in the evolution of the universe. Not only are they the first luminous objects to shed light in a previously dark universe, but, if their formation is not inhibited by their own feedback, they dominate the galaxy mass function until redshift z \sim 5. In this paper we present and discuss the implementation of a 3D cosmological code that includes most of the needed physics to simulate the formation and evolution of the first galaxies with a self-consistent treatment of radiative feedback. The simulation includes continuum radiative transfer using the ``Optically Thin Variable Eddington Tensor'' (OTVET) approximation and line-radiative transfer in the H_2 Lyman-Werner bands of the background radiation. We include detailed chemistry for H_2 formation/destruction, molecular and atomic cooling/heating processes, ionization by secondary electrons, and heating by Lyαresonant scattering. We find that the first galaxies ("small-halos") are characterized by a bursting star formation, self-regulated by a feedback process that acts on cosmological scales. Their formation is not suppressed by feedback processes; therefore, their impact on cosmic evolution cannot be neglected. The main focus of this paper is on the methodology of the simulations, and we only briefly introduce some of the results. An extensive discussion of the results and the nature of the feedback mechanism are the focus of a companion paper.

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arXiv2002

The Fate of the First Galaxies. II. Effects of Radiative Feedback

[abridged]We use 3D cosmological simulations with radiative transfer to study the formation and evolution of the first galaxies in a LCDM cosmology. We find that the first luminous objects ("small-halos") are characterized by a bursting star formation (SF) that is self-regulated by a feedback process acting on cosmological instead of galactic scales. The global star formation history is regulated by the mean number of ionizing photons that escape from each source, ε_{UV}\fesc. It is almost independent of the assumed star formation efficiency parameter, ε_*, and the intensity of the dissociating background. The main feedback process that regulates the SF is the re-formation of H_2 in front of HII regions and inside relic HII regions. The HII regions remain confined inside filaments, maximizing the production of H_2 in overdense regions through cyclic destruction/reformation of H_2. If ε_{UV}\fesc > 10^{-8}/ε_* the SF is self-regulated, photo-evaporation of "small-halo" objects dominate the metal pollution of the low density IGM, and the mass of produced metals depends only on \fesc. If ε_{UV}\fesc \simlt 10^{-8}/ε_*, positive feedback dominates, and "small-halo" objects constitute the bulk of the mass in stars and metals at least until redshift z \sim 10. "Small-halo" objects cannot reionize the universe because the feedback mechanism confines the HII regions inside the large scale structure filaments. In contrast to massive objects, which can reionize voids, "small-halo" objects partially ionize only the dense filaments while leaving the voids mostly neutral.

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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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arXiv2001

Feedback from Galaxy Formation: Production and Photodissociation of Primordial Molecular Hydrogen

We use one-dimensional radiative transfer simulations to study the evolution of H_2 gas-phase (H^- catalyzed) formation and photo-dissociation regions in the primordial universe. We find a new positive feedback mechanism capable of producing shells of H_2 in the intergalactic medium, which are optically thick in some Lyman-Werner bands. While these shells exist, this feedback effect is important in reducing the H_2 dissociating background flux and the size of photo-dissociation spheres around each luminous object. The maximum background opacity of the IGM in the H_2 Lyman-Werner bands is τ_{H_2} ~ 1-2 for a relic molecular fraction x_{H_2}=2 x 10^{-6}, about 6 times greater than found by Haiman, Abel & Rees. Therefore, the relic molecular hydrogen can decrease the photo-dissociation rate by about an order of magnitude. The problem is relevant to the formation of small primordial galaxies with masses M_{DM} < 10^8 M_\odot, that rely on molecular hydrogen cooling to collapse. Alternatively, the universe may have remained dark for several hundred million years after the birth of the first stars, until galaxies with virial temperature T_{vir} > 10^4 K formed.

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arXiv2000

Feedback from Galaxy Formation: Escaping Ionizing Radiation from Galaxies at High Redshift

Several observational and theoretical arguments suggest that starburst galaxies may rival quasars as sources of metagalactic ionizing radiation at redshifts z > 3. Reionization of the intergalactic medium (IGM) at z > 5 may arise, in part, from the first luminous massive stars. To be an important source of radiative feedback from star formation, a substantial fraction (\sim 1-10%) of the ionizing photons must escape the gas layers of the galaxies. Using models of smoothly distributed gas confined by dark-matter (DM) potentials, we estimate the fraction, f_esc, of Lyc flux that escapes the halos of spherical galaxies as a function of their mass and virialization redshift. The gas density profile is found by solving the equation of hydrostatic equilibrium for the baryonic matter in the potential well of a DM halo with the density profile found by Navarro et. al. (1996). We then perform a parametric study to understand the dependence of fesc on redshift, mass, star-formation efficiency (SFE), stellar density distribution, and OB association luminosity function. We give useful analytical formulas for f_esc(z,M_{DM}). Using the Press-Schechter formalism, we find that stellar reionization at z \sim 7 is probably dominated by small galactic sub-units, with M_{DM} \simlt 10^7 M_\odot and SFE \sim 10 times that in the Milky Way. This may affect the distribution of heavy elements throughout the intergalactic medium.

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arXiv1999

The Evolution of the Effective Equation of State of the IGM

We develop a method to extract the "effective equation of state" of the intergalactic medium from the doppler b parameter distribution of the low-density Lyman-alpha forest. We test the method on numerical simulations and then apply it to published observations of the Lyman-alpha forest at redshifts z from 0 to 4. We find that the effective equation of state is close to isothermal at redshift z=3, indicating that a second reheating of the IGM took place at z=3. This reheating can plausibly be identified with the reionization of HeII observed to occur at z about 3.

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