arXiv subjects

Massimo Ricotti

Publications and source records attributed to Massimo Ricotti.

At least 91 records · Page 5Linked to original sources

arXiv2013

Rayleigh-Taylor Instability at Ionization Fronts: Perturbation Analysis

The linear growth rate of the Rayleigh-Taylor instability (RTI) at ionization fronts is investigated via perturbation analysis in the limit of incompressible fluids. In agreement with previous numerical studies is found that absorption of ionizing radiation inside the HII region due to hydrogen recombinations suppresses the growth of instabilities. In the limit of a large density contrast at the ionization front the RTI growth rate has the simple analytical solution n=-nur+(nur^2+gk)^(1/2), where nur is the hydrogen recombination rate inside the HII region, k is the perturbation's wavenumber and g is the effective acceleration in the frame of reference of the front. Therefore, the growth of surface perturbations with wavelengths lambda >> lambda_{cr} = 2πg/nur^2 is suppressed by a factor (lambda_{cr}/4lambda)^(1/2) with respect to the non-radiative incompressible RTI. Implications on stellar and black hole feedback are briefly discussed.

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arXiv2013

Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback. III. Enhanced Luminosity of Intermediate Mass Black Holes Moving at Supersonic Speeds

In this third paper of a series, we study the growth and luminosity of black holes (BHs) in motion with respect to their surrounding medium. We run a large set of two-dimensional (2D) axis-symmetric simulations to explore a large parameter space of initial conditions and formulate an analytical model for the accretion. Contrary to the case without radiation feedback, the accretion rate increases with increasing BH velocity $\vbh$ reaching a maximum value at $\vbh=2c_{\rm s,in}\sim 50$ km/s, where $c_{\rm s,in}$ is the sound speed inside the "cometary-shaped" \hii region around the BH, before decreasing as $\vbh^{-3}$ when the I-front becomes {\it R}-type (rarefied) and the accretion rate approaches the classical Bondi-Hoyle-Lyttleton solution. The increase of the accretion rate with $\vbh$ is produced by the formation of a {\it D}-type (dense) ionization front (I-front) preceded by a standing bow-shock that reduces the downstream gas velocity to transonic values. There is a range of densities and velocities where the dense shell is unstable producing periodic accretion rate peaks which can significantly increase the detectability of intermediate mass BHs. We find that the mean accretion rate for a moving BH is larger than that of a stationary BH of the same mass if the medium temperature is $T_\infty<10^4$ K. This result could be important for the growth of seed BHs in the multi-phase medium of the first galaxies and for building and early X-ray background that may affect the formation of the first galaxies and the reionization process.

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arXiv2012

Two Epochs of Globular Cluster Formation from Deep Fields Luminosity Functions: Implications for Reionization and the Milky Way Satellites

The ages of globular clusters in our own Milky Way are known with precision of about \pm 1 Gyr, hence their formation at redshifts z>~3 and their role in hierarchical cosmology and the reionization of the intergalactic medium remain relatively undetermined. Here we analyze the effect of globular cluster formation on the observed rest-frame UV luminosity functions (LFs) and UV continuum slopes of high redshift galaxies in the Hubble Ultra Deep Fields. We find that the majority of present day globular clusters have formed during two distinct epochs: at redshifts z ~ 2-3 and at redshifts z>~6. The birth of proto-GC systems produce the steep, faint-end slopes of the galaxy LFs, and because the brightness of proto-GCs fades 5 Myrs after their formation, their blue colors are in excellent agreement with observations. Our results suggest that: i) the bulk of the old globular cluster population with estimated ages >~12 Gyr (about 50% of the total population), formed in the relatively massive dwarf galaxies at redshifts z>~6; ii) proto-GC formation was an important mode of star formation in those dwarf galaxies, and likely dominated the reionization process. Another consequence of this scenario is that some of the most massive Milky Way satellites may be faint and yet undiscovered because tidal stripping of a dominant GC population precedes significant stripping of the dark matter halos of these satellites. This scenario may alleviate some remaining tensions between CDM simulations and observations.

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arXiv2012

Can the Near-IR Fluctuations Arise from Known Galaxy Populations?

Spatial Fluctuations in the Cosmic Infrared Background have now been measured out to sub-degree scales showing a strong clustering signal from unresolved sources. We attempt to explain these measurement by considering faint galaxy populations at z<6 as the underlying sources for this signal using 233 measured UV, optical and NIR luminosity functions (LF) from a variety of surveys covering a wide range of redshifts. We populate the lightcone and calculate the total emission redshifted into the near-IR bands in the observer frame and recover the observed optical and near-IR galaxy counts to a good accuracy. Using a halo model for the clustering of galaxies with an underlying LCDM density field, we find that fluctuations from known galaxy populations are unable to account for the large scale CIB clustering signal seen by HST/NICMOS, Spitzer/IRAC and AKARI/IRC and continue to diverge out to larger angular scales. Our purely empirical reconstruction shows that known galaxy populations are not responsible for the bulk of the fluctuation signal seen in the measurements and suggests an unknown population of very faint and highly clustered sources dominating the signal.

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arXiv2012

Reconstructing the Near-IR Background Fluctuations from known Galaxy Populations using Multiband Measurements of Luminosity Functions

We model fluctuations in the Cosmic Infrared Background (CIB) arising from known galaxy populations using 233 measured UV, optical and NIR luminosity functions (LF) from a variety of surveys spanning a wide range of redshifts. We compare best-fit Schechter parameters across the literature and find clear indication of evolution with redshift. Providing fitting formulae for the multi-band evolution of the LFs out to z~5, we calculate the total emission redshifted into the near-IR bands in the observer frame and recover the observed optical and near-IR galaxy counts to a good accuracy. Our empirical approach, in conjunction with a halo model describing the clustering of galaxies, allows us to compute the fluctuations of the unresolved CIB and compare the models to current measurements. We find that fluctuations from known galaxy populations are unable to account for the large scale CIB clustering signal seen by Spitzer/IRAC and AKARI/IRC and continue to diverge out to larger angular scales. This holds true even if the LFs are extrapolated out to faint magnitudes with a steep faint-end slope all the way to z=8. We also show that removing resolved sources to progressively fainter magnitude limits, isolates CIB fluctuations to increasingly higher redshifts. Our empirical approach suggests that known galaxy populations are not responsible for the bulk of the fluctuation signal seen in the measurements and favors a very faint population of highly clustered sources.

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arXiv2012

Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback. II. Growth Rate and Duty Cycle

This paper, the second of a series on radiation-regulated accretion onto black holes (BHs) from galactic scales, focuses on the effects of radiation pressure and angular momentum of the accreting gas. We simulate accretion onto intermediate-mass black holes, but we derive general scaling relationships that are solutions of the Bondi problem with radiation feedback valid for any mass of the BH $M_{\rm bh}$. Thermal pressure of the ionized sphere around the BH regulates the accretion rate producing periodic and short-lived luminosity bursts. We find that for ambient gas densities exceeding $n^{\rm cr}_{\rm H,\infty} \propto M_{\rm bh}^{-1}$, the period of the oscillations decreases rapidly and the duty cycle increases from 6%, in agreement with observations of the fraction of active galactic nuclei at $z\sim 3$, to 50%. The mean accretion rate becomes Eddington limited for $n_{\rm H,\infty}>n_{\rm H,\infty}^{\rm Edd} \simeq n_{\rm H,\infty}^{\rm cr} T_{\infty,4}^{-1}$ where $T_{\infty,4}$ is the gas temperature in units of $10^4$ K. In the sub-Eddington regime, the mean accretion rate onto BHs is about $1% T_{\infty,4}^{2.5}$ of the Bondi rate, thus is proportional to the thermal pressure of the ambient medium. The period of the oscillations coincides with depletion timescale of the gas inside the ionized bubble surrounding the BH. Gas depletion is dominated by a pressure gradient pushing the gas outward if $n_{\rm H,\infty}<n^{\rm cr}_{\rm H,\infty}$ and by accretion onto the BH otherwise. Generally, for $n_{\rm H,\infty}<n^{\rm cr}_{\rm H,\infty}$ angular momentum does not affect significantly the accretion rate and period of the oscillations.

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arXiv2011

Where are the Fossils of the First Galaxies? I. Local Volume Maps and Properties of the Undetected Dwarfs

We present a new method for generating initial conditions for LCDM N-body simulations which provides the dynamical range necessary to follow the evolution and distribution of the fossils of the first galaxies on Local Volume, 5-10 Mpc, scales. The initial distribution of particles represents the position, velocity and mass distribution of the dark and luminous halos extracted from pre-reionization simulations. We confirm previous results that ultra-faint dwarfs have properties compatible with being well preserved fossils of the first galaxies. However, because the brightest pre-reionization dwarfs form preferentially in biased regions, they most likely merge into non-fossil halos with circular velocities >20-30 km/s. Hence, we find that the maximum luminosity of true-fossils in the Milky Way is L_V<10^5 L_solar, casting doubts on the interpretation that some classical dSphs are true-fossils. In addition, we argue that most ultra-faints at small galactocentric distance, R<50 kpc, had their stellar properties modified by tides, while a large population of fossils is still undetected due to their extremely low surface brightness log(Sigma_V) < -1.4. We estimate that the region outside R_50 (~ 400 kpc) up to 1 Mpc from the Milky Way contains about a hundred true fossils of the first galaxies with V-band luminosities 10^3 - 10^5 L_solar and half-light radii, r_hl ~ 100-1000 pc.

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arXiv2011

Where are the Fossils of the First Galaxies? II. True Fossils, Ghost Halos, and the Missing Bright Satellites

We use a new set of cold dark matter simulations of the local universe to investigate the distribution of fossils of primordial dwarf galaxies within, and around the Milky Way. Throughout, we build upon previous results showing agreement between the observed stellar properties of a subset of the ultra-faint dwarfs and our simulated fossils. Here, we show that fossils of the first galaxies have galactocentric distributions and cumulative luminosity functions consistent with observations. In our model there are ~ 300 luminous satellites orbiting the Milky Way, ~50-70% of which are well preserved fossils, with this fraction decreasing with galactocentric distance. Within the Milky Way virial radius, the majority of these fossils have luminosities L_V<10^5 L_solar. This work produces an overabundance of bright dwarf satellites (L_V > 10^4 L_solar) with respect to observations where observations are nearly complete. The "bright satellite problem" is most evident in the outer parts of the Milky Way. We estimate that, although relatively bright, the primordial stellar populations are very diffuse, producing a population with surface brightnesses below surveys` detection limits and are easily stripped by tidal forces. Although we cannot yet present unmistakable evidence for the existence of the fossils of first galaxies in the Local Group, the results of our studies suggest observational strategies that may demonstrate their existence. Primarily, the detection of "ghost halos" of primordial stars around isolated dwarfs would prove that stars formed in minihalos (M<10^8 M_solar) before reionization, and strongly suggest that at least a fraction of the ultra-faint dwarfs are fossils of the first galaxies.

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arXiv2011

The Halo Model of Large Scale Structure for Warm Dark Matter

We present a comprehensive analysis of the halo model of cosmological large to small-scale structure statistics in the case of warm dark matter (WDM) structure formation scenarios. We include the effects of WDM on the linear matter power spectrum, halo density profile, halo concentration relation, halo mass function, subhalo density profile, subhalo mass function and biasing of the smooth dark matter component. As expected, we find large differences at the smallest physical scales in the nonlinear matter power spectrum predicted in the halo model between WDM and cold dark matter even for reasonably high-scale WDM particle masses. We find that significant effects are contributed from the alteration of the halo density profile and concentration, as well as the halo mass function. We further find that the effects of WDM on the subhalo population are important but sub-dominant. Clustering effects of the biasing of the smooth component in WDM is not largely significant.

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arXiv2011

Accretion onto Intermediate Mass Black Holes Regulated by Radiative Feedback I. Parametric Study for Spherically Symmetric Accretion

We study the effect of radiative feedback on accretion onto intermediate mass black holes (IMBHs) using the hydrodynamical code ZEUS-MP with a radiative transfer algorithm. In this paper, the first of a series, we assume accretion from a uniformly dense gas with zero angular momentum and extremely low metallicity. Our 1D and 2D simulations explore how X-ray and UV radiation emitted near the black hole regulates the gas supply from large scales. Both 1D and 2D simulations show similar accretion rate and period between peaks in accretion, meaning that the hydro-instabilities that develop in 2D simulations do not affect the mean flow properties. We present a suite of simulations exploring accretion across a large parameter space, including different radiative efficiencies and radiation spectra, black hole masses, density and temperature, $T_\infty$, of the neighboring gas. In agreement with previous studies we find regular oscillatory behavior of the accretion rate, with duty cycle $\sim 6%$, mean accretion rate 3% $(T_{\infty}/10^4 {\rm K})^{2.5}$ of the Bondi rate and peak accretion $\sim 10$ times the mean for $T_{\infty}$ ranging between 3000K and 15000K. We derive parametric formulas for the period between bursts, the mean accretion rate and the peak luminosity of the bursts and thus provide a formulation of how feedback regulated accretion operates. The temperature profile of the hot ionized gas is crucial in determining the accretion rate, while the period of the bursts is proportional to the mean size of the Strömgren sphere and we find qualitatively different modes of accretion in the high vs. low density regimes. We also find that softer spectrum of radiation produces higher mean accretion rate.

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arXiv2011

Constraints on the Dark Matter Particle Mass from the Number of Milky Way Satellites

We have conducted N-body simulations of the growth of Milky Way-sized halos in cold and warm dark matter cosmologies. The number of dark matter satellites in our simulated Milky Ways decreases with decreasing mass of the dark matter particle. Assuming that the number of dark matter satellites exceeds or equals the number of observed satellites of the Milky Way we derive lower limits on the dark matter particle mass. We find with 95% confidence m_s > 13.3 keV for a sterile neutrino produced by the Dodelson and Widrow mechanism, m_s > 8.9 keV for the Shi and Fuller mechanism, m_s > 3.0 keV for the Higgs decay mechanism, and m_{WDM} > 2.3 keV for a thermal dark matter particle. The recent discovery of many new dark matter dominated satellites of the Milky Way in the Sloan Digital Sky Survey allows us to set lower limits comparable to constraints from the complementary methods of Lyman-alpha forest modeling and X-ray observations of the unresolved cosmic X-ray background and of dark matter halos from dwarf galaxy to cluster scales. Future surveys like LSST, DES, PanSTARRS, and SkyMapper have the potential to discover many more satellites and further improve constraints on the dark matter particle mass.

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arXiv2009

The First Galaxies and the Likely Discovery of their Fossils in the Local Group

In cold dark matter cosmologies, small mass halos outnumber larger mass halos at any redshift. However, the lower bound for the mass of a galaxy is unknown, as are the typical luminosity of the smallest galaxies and their numbers in the universe. The answers depend on the extent to which star formation in the first population of small mass halos may be suppressed by radiative feedback loops operating over cosmological distance scales. If early populations of dwarf galaxies did form in significant number, their relics should be found today in the Local Group. These relics have been termed "fossils of the first galaxies". This paper is a review that summarizes our ongoing efforts to simulate and identify these fossils around the Milky Way and Andromeda. It is widely believed that reionization of the intergalactic medium would have stopped star formation in the fossils of the first galaxies. Thus, they should be among the oldest objects in the Universe. However, here we dispute this idea and discuss a physical mechanism whereby relatively recent episodes of gas accretion and star formation would be produced in some fossils of the first galaxies. We argue that fossils may be characterized either by a single old population of stars or by a bimodal star formation history. We also propose that the same mechanism could turn small mass dark halos formed before reionization into gas-rich but starless "dark galaxies". We believe that current observational data support the thesis that a fraction of the new ultra-faint dwarfs recently discovered in the Local Group are in fact fossils of the first galaxies.

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arXiv2009

Late Gas accretion onto Primordial Minihalos: a Model for Leo T, Dark Galaxies and Extragalactic High-Velocity Clouds

In this letter we revisit the idea of reionization feedback on dwarf galaxy formation. We show that primordial minihalos with v_cir<20 km/s stop accreting gas after reionization, as it is usually assumed, but in virtue of their increasing concentration and the decreasing temperature of the intergalactic medium as redshift decreases below z=3, they have a late phase of gas accretion and possibly star formation. We expect that pre-reionization fossils that evolved on the outskirts of the Milky Way or in isolation show a bimodal star formation history with 12 Gyr old and <10 Gyr old population of stars. Leo T fits with this scenario. Another prediction of the model is the possible existence of a population of gas rich minihalos that never formed stars. More work is needed to understand whether a subset of compact high-velocity clouds can be identified as such objects or whether an undiscovered population exists in the voids between galaxies.

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arXiv2009

Pre-reionization Fossils, Ultra-faint Dwarfs and the Missing Galactic Satellite Problem

We argue that, at least a fraction of the newly discovered population of ultra-faint dwarf spheroidal galaxies in the Local Group constitute the fossil relic of a once ubiquitous population of dwarf galaxies formed before reionization with circular velocities smaller than $v_{c}^{cr} \sim 20$ km/s. We present several arguments in support of this model. The number of luminous Milky Way satellites inferred from observations is larger than the estimated number of dark halos in the Galaxy that have, or had in the past, circular velocity $>v_{c}^{cr}$, as predicted by the "Via Lactea" simulation. This implies that some ultra-faint dwarfs are fossils. However, this argument is weakened by recent results from the "Aquarius" simulations showing that the number of Galactic dark matter satellites is 2.5 larger than previously believed. Secondly, the existence of a population of ultra-faint dwarfs was predicted by cosmological simulations in which star formation in the first minihalos is reduced -- but not suppressed -- by radiative feedback. Here, we show the statistical properties of the fossil galaxies in those simulations are consistent with observations of the new dwarf population and with the number and radial distribution of Milky Way satellites as a function of their luminosity. Finally, the observed Galactocentric distribution of dwarfs is consistent with a fraction of dSphs being fossils. To make our case more compelling, future work should determine whether stellar chemical abundances of simulated "fossils" can reproduce observations and whether the tidal scenarios for the formation of Local Group dwarf spheroidals are equally consistent with all available observations.

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arXiv2009

Properties of Dark Matter Revealed by Astrometric Measurements of the Milky Way and Local Galaxies

The fact that dark matter (DM), thus far, has revealed itself only on scales of galaxies and larger, again thrusts onto astrophysics the opportunity and the responsibility to confront the age old mystery "What is the nature of matter?" By deriving basic data on the nature of DM - e.g., mass of its particle(s), present mean temperature, distribution in galaxies and other structures in the universe, and capacity for dissipational collapse - we will be uncovering the properties of the dominant species of matter in the universe and significantly extending the standard models of particle physics. Determining the mass of the DM particle to an order of magnitude would help to sort out the particle family to which it (or they) belongs. Beyond mass, there are issues of stability. The DM particle may be unstable with a measurable half-life, or it may become unstable after absorbing a certain amount of energy from collisions. In both cases it would contribute to the present hot dark matter component. Some key parameters of DM can most accurately be measured in the very nearby universe because DM dominates the mass in the outer Milky Way (MW), in other galaxies in the Local Group, and in the Local Group in its entirety. The presence and amount of DM can be quantified by study of dynamical processes observable in fine detail within these entities. Precise measurements of 3-D velocities for stars, coherent star streams, and stars in satellite stellar systems out to the edge of the Galaxy can reveal "what is the shape, orientation, density law, and lumpiness of the dark matter halo" as well as "what is the total mass of the Galaxy?"

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arXiv2008

The Fate of the First Galaxies. III. Properties of Primordial Dwarf Galaxies and their Impact on the Intergalactic Medium

In two previous papers, we presented simulations of the first galaxies in a representative volume of the Universe. The simulations are unique because we model feedback-regulated galaxy formation, using time-dependent, spatially-inhomogeneous radiative transfer coupled to hydrodynamics. Here, we study the properties of simulated primordial dwarf galaxies with masses <2x10^8 Msolar and investigate their impact on the intergalactic medium. While many primordial galaxies are dark, about 100--500 per comoving Mpc^3 are luminous but relatively faint. They form preferentially in chain structures, and have low surface brightness stellar spheroids extending to 20% of the virial radius. Their interstellar medium has mean density n_H~10--100 cm^-3, metallicity Z~ 0.01--0.1 Zsolar and can sustain a multi-phase structure. With large scatter, the mean efficiency of star formation scales with halo mass, \propto M_dm^2, independent of redshift. Because of feedback, halos smaller than a critical mass, M_crit(z), are devoid of most of their baryons. More interestingly, we find that dark halos have always a smaller M_crit(z) than luminous ones. Metal enrichment of the intergalactic medium is inhomogeneous, with only a 1%--10% volume filling factor of enriched gas with [Z/H]>-3.0 and 10%--50% with [Z/H]>-5.0. At z=10, the fraction of stars with metallicity Z<10^-3 Zsolar is 10^-6 of the total stellar mass. Although detections of high-redshift dwarf galaxies with the James Webb Space Telescope will be a challenge, studies of their fossil records in the local Universe are promising because of their large spatial density.

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arXiv2007

Effect of Primordial Black Holes on the Cosmic Microwave Background and Cosmological Parameter Estimates

We investigate the effect of non-evaporating primordial black holes (PBHs) on the ionization and thermal history of the universe. X-rays emitted by gas accretion onto PBHs modify the cosmic recombination history, producing measurable effects on the spectrum and anisotropies of the Cosmic Microwave Background (CMB). Using the third-year WMAP data and FIRAS data we improve existing upper limits on the abundance of PBHs with masses >0.1 Msun by several orders of magnitude. Fitting WMAP3 data with cosmological models that do not allow for non-standard recombination histories, as produced by PBHs or other early energy sources, may lead to an underestimate of the best-fit values of the amplitude of linear density fluctuations (sigma_8) and the scalar spectral index (n_s). Cosmological parameter estimates are affected because models with PBHs allow for larger values of the Thomson scattering optical depth, whose correlation with other parameters may not be correctly taken into account when PBHs are ignored. Values of tau_e=0.2, n_s=1 and sigma_8=0.9 are allowed at 95% CF. This result that may relieve recent tension between WMAP3 data and clusters data on the value of sigma_8. PBHs may increase the primordial molecular hydrogen abundance by up to two orders of magnitude, this promoting cooling and star formation. The suppression of galaxy formation due to X-ray heating is negligible for models consistent with the CMB data. Thus, the formation rate of the first galaxies and stars would be enhanced by a population of PBHs.

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arXiv2007

Bondi accretion in the early universe

This paper presents a study of quasi-steady spherical accretion in the early Universe, before the formation of the first stars and galaxies. The main motivation is to derive the basic formulas that will be used in a companion paper to calculate the accretion luminosity of primordial black holes and their effect on the cosmic ionization history. The following cosmological effects are investigated: the coupling of the gas to the CMB photon fluid (i.e., Compton drag), Hubble expansion, and the growth of the dark matter halo seeded by the gravitational potential of the central point mass. The gas equations of motion are solved assuming either a polytropic or an isothermal equation of state. We consider the cases in which the accreting object is a point mass or a spherical dark matter halo with power-law density profile, as predicted by the theory of "secondary infall''. Analytical solutions for the sonic radius and fitting formulas for the accretion rate are provided. Different accretion regimes exist depending on the mass of the accreting object. If the black hole mass is smaller than 50-100 Msun, gas accretion is unaffected by Compton drag. A point mass and an extended dark halo of equal mass accrete at the same rate if M>5000 Msun, while smaller mass dark halos accrete less efficiently than the equivalent point mass. For masses M>3 x 10^4 Msun, the viscous term due to the Hubble expansion becomes important and the assumption of quasi-steady flow fails. Hence, the steady Bondi solutions transition to the time-dependent self-similar solutions for "cold cosmological infall".

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