arXiv subjects

P. Madau

Publications and source records attributed to P. Madau.

At least 37 records · Page 2Linked to original sources

arXiv2007

Interaction of massive black hole binaries with their stellar environment: II. Loss-cone depletion and binary orbital decay

We study the long-term evolution of massive black hole binaries (MBHBs) at the centers of galaxies using detailed scattering experiments to solve the full three-body problem. Ambient stars drawn from a isotropic Maxwellian distribution unbound to the binary are ejected by the gravitational slingshot. We construct a minimal, hybrid model for the depletion of the loss cone and the orbital decay of the binary, and show that secondary slingshots - stars returning on small impact parameter orbits to have a second super-elastic scattering with the MBHB - may considerably help the shrinking of the pair in the case of large binary mass ratios. In the absence of loss-cone refilling by two-body relaxation or other processes, the mass ejected before the stalling of a MBHB is half the binary reduced mass. About 50% of the ejected stars are expelled ejected in a "burst" lasting ~1E4 yrs M_6^1/4, where M_6 is the binary mass in units of 1E6 Msun. The loss cone is completely emptied in a few bulge crossing timescales, 1E7 yrs M_6^1/4. Even in the absence of two-body relaxation or gas dynamical processes, unequal mass and/or eccentric binaries with M_6 >0.1 can shrink to the gravitational wave emission regime in less than a Hubble time, and are therefore "safe" targets for the planned Laser Interferometer Space Antenna (LISA).

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arXiv2006

Interaction of massive black hole binaries with their stellar environment: I. Ejection of hypervelocity stars

We use full three-body scattering experiments to study the ejection of hypervelocity stars (HVSs) by massive black hole binaries (MBHBs) at the center of galaxies. Ambient stars drawn from a Maxwellian distribution unbound to the binary are expelled by the gravitational slingshot. Accurate measurements of thermally averaged hardening, mass ejection, and eccentricity growth rates (H, J, and K) for MBHBs in a fixed stellar background are obtained by numerical orbit integration from initial conditions determined by Monte Carlo techniques. Three-body interactions create a subpopulation of HVSs on nearly radial, corotating orbits, with a spatial distribution that is initially highly flattened in the inspiral plane of the MBHB, but becomes more isotropic with decreasing binary separation. The degree of anisotropy is smaller for unequal mass binaries and larger for stars with higher kick velocities. Eccentric MBHBs produce a more prominent tail of high-velocity stars and break planar symmetry, ejecting HVSs along a broad jet perpendicular to the semimajor axis. The jet two-sidedness decreases with increasing binary mass ratio, while the jet opening-angle increases with decreasing kick velocity and orbital separation. The detection of a numerous population of HVSs in the halo of the Milky Way by the next generation of large astrometric surveys like GAIA may provide a unique signature of the history, nature, and environment of the MBH at the Galactic center.

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arXiv2006

Hardening in a Time--Evolving Stellar Background: Hyper--Velocity Stars, Orbital Decay and Prediction for Lisa

We study the long-term evolution of massive black hole binaries (MBHBs) at the centers of galaxies using detailed full three-body scattering experiments. Stars, drawn from a distribution unbound to the binary, are ejected by the gravitational slingshot. We quantify the effect of secondary slingshots -- stars returning on small impact parameter orbits to have a second super-elastic scattering with the MBHB -- on binary separation. Even in the absence of two-body relaxation or gas dynamical processes, very unequal mass binaries of mass M=10^7 solar masses can shrink to the gravitational wave emission regime in less than a Hubble time, and are therefore a target for the planned Laser Interferometer Space Antenna (LISA). Three-body interactions create a subpopulation of hypervelocity stars on nearly radial, corotating orbits, with a spatial distribution that is initially highly flattened in the inspiral plane of the MBHB, but becomes more isotropic with decreasing binary separation. The mass ejected is ~0.7 times the binary reduced mass, and most of the stars are ejected in an initial burst lasting much less than a bulge crossing time.

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arXiv2005

The Rest-Frame FUV Morphologies of Star-Forming Galaxies at z ~ 1.5 and z ~ 4

We apply a new approach to quantifying galaxy morphology and identifying galaxy mergers to the rest-frame far-ultraviolet images of 82 z ~ 4 Lyman break galaxies (LBGs) and 55 1.2 < z < 1.8 emission-line galaxies in the GOODS and Ultra Deep Fields. We compare the distributions of the Gini coefficient (G), second-order moment of the brightest 20% of galaxy light (M20), and concentration (C) for high-redshift and low-redshift galaxies with average signal to noise per pixel > 2.5 and Petrosian radii > 0.3 arcsec. Ten of the 82 LBGs have M20 >= -1.1 and possess bright double or multiple nuclei, implying a major-merger fraction of star-forming galaxies ~ 10-25% at M_{FUV} < -20, depending on our incompleteness corrections. Galaxies with bulge-like morphologies (G >= 0.55, M20 < -1.6) make up ~ 30% of the z ~ 4 LBG sample, while the remaining ~ 50% have G and M20 values higher than expected for smooth bulges and disks and may be star-forming disks, minor-mergers or post-mergers. The star-forming z ~ 1.5 galaxy sample has a morphological distribution which is similar to the UDF z ~ 4 LBGs, with an identical fraction of major-merger candidates but fewer spheroids. The observed morphological distributions are roughly consistent with current hierarchical model predictions for the major-merger rates and minor-merger induced starbursts at z ~ 1.5 and ~4. We also examine the rest-frame FUV-NUV and FUV-B colors as a function of morphology and find no strong correlations at either epoch.

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arXiv2005

The First Miniquasar

We investigate the environmental impact of the first active galactic nuclei that may have formed ~150 Myr after the big bang in low-mass ~10^6 Msun minihaloes. Using Enzo, an adaptive-mesh refinement cosmological hydrodynamics code, we carry out three-dimensional simulations of the radiative feedback from `miniquasars' powered by intermediate-mass black holes. We follow the non-equilibrium multispecies chemistry of primordial gas in the presence of a point source of X-ray radiation, which starts shining in a rare high-sigma peak at z=21 and emits a power-law spectrum in the 0.2-10 keV range. We find that, after one Salpeter time-scale, the miniquasar has heated up the simulation box to a volume-averaged temperature of 2800 K. The mean electron and H2 fractions are now 0.03 and 4e-5: the latter is 20 times larger than the primordial value, and will delay the buildup of a uniform UV photodissociating background. The net effect of the X-rays is to reduce gas clumping in the IGM by as much as a factor of 3. While the suppression of baryonic infall lowers the gas mass fraction at overdensities delta in the range 20-2000, enhanced molecular cooling increases the amount of dense material at delta>2000. In many haloes within the proximity of our miniquasar the H2-boosting effect of X-rays is too weak to overcome heating, and the cold and dense gas mass actually decreases. We find little evidence for an entropy floor in gas at intermediate densities preventing gas contraction and H2 formation. Overall, the radiative feedback from X-rays enhances gas cooling in lower-sigma peaks that are far away from the initial site of star formation, thus decreasing the clustering bias of the early pregalactic population, but does not appear to dramatically reverse or promote the collapse of pregalactic clouds as a whole. (abridged)

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arXiv2005

ELT Observations of Supernovae at the Edge of the Universe

We discuss the possibility of using Supernovae as tracers of the star formation history of the Universe for the range of stellar masses $\sim 3-30$ M$_\odot$ and possibly beyond. We simulate the observations of 350 SNe, up to $z\sim 15$, made with OWL (100m) telescope.

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arXiv2005

Low-Frequency Gravitational Radiation from Coalescing Massive Black Holes

We compute the expected low-frequency gravitational wave signal from coalescing massive black hole (MBH) binaries at the center of galaxies. We follow the merging history of halos and associated holes via cosmological Monte Carlo realizations of the merger hierarchy from early times to the present in a LCDM cosmology. MBHs get incorporated through a series of mergers into larger and larger halos, sink to the centre owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become more massive, and form a binary system. Stellar dynamical processes dominates the orbital evolution of the binary at large separations, while gravitational wave emission takes over at small radii, causing the final coalescence of the system. We discuss the observability of inspiraling MBH binaries by a low-frequency gravitational wave experiment such as the planned Laser Interferometer Space Antenna (LISA), discriminating between resolvable sources and unresolved confusion noise. Over a 3-year observing period LISA should resolve this GWB into discrete sources, detecting ~90 individual events above a S/N=5 confidence level, while expected confusion noise is well below planned LISA capabilities.

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arXiv2004

The gravitational wave signal from massive black hole binaries and its contribution to the LISA data stream

Massive black hole binaries, with masses in the range 1E3-1E8 Msun, are expected to be the most powerful sources of gravitational radiation at mHz frequencies, and hence are among the primary targets for the planned Laser Interferometer Space Antenna (LISA). We extend and refine our previous analysis (Sesana et al. 2004), detailing the gravitational wave signal expected from a cosmological population of massive black hole binaries. As done in our previous paper, we follow the merger history of dark matter halos, the dynamics of the massive black holes they host, and their growth via gas accretion and binary coalescences in a LCDM cosmology. Stellar dynamical processes dominates the orbital evolution of black hole binaries at large separations, while gravitational wave emission takes over at small radii, causing the final coalescence of the pairs. We show that the GW signal from this population, in a 3 year LISA observation, will be resolved into approx 90 discrete events with S/N>5, among which approx. 35 will be observed above threshold until coalescence. These "merging events" involve relatively massive binaries, M=10E5 Msun, in the redshift range 2 6) and, although their S/N integrated over the duration of the observation can be substantial, the final coalescence phase is at too high frequency to be directly observable by space-based interferometers such as LISA. LISA will be able to detect a fraction approx. 90% of all the coalescences of massive black hole binaries occurring at z=5. The residual confusion noise from unresolved massive black hole binaries is expected to be at least an order of magnitude below the estimated stochastic noise.

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arXiv2003

The Merging History of Massive Black Holes

We investigate a hierarchical structure formation scenario describing the evolution of a Super Massive Black Holes (SMBHs) population. The seeds of the local SMBHs are assumed to be 'pregalactic' black holes, remnants of the first POPIII stars. 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. A simple model in which the damage done to a stellar cusps by decaying BH pairs is cumulative is able to reproduce the observed scaling relation between galaxy luminosity and core size. An accretion model connecting quasar activity with major mergers and the observed BH mass-velocity dispersion correlation reproduces remarkably well the observed luminosity function of optically-selected quasars in the redshift range 1<z<5. We finally asses the potential observability of the gravitational wave background generated by the cosmic evolution of SMBH binaries by the planned space-born interferometer LISA.

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arXiv2003

Gravitational Lensing Statistics in Universes Dominated by Dark Energy

We study lens statistics in flat, low-density universes with different equations of state $w=p_Q/ρ_Q$ for the dark energy component. Dark energy modifies the distance-redshift relation and the mass function of dark matter halos leading to changes in the lensing optical depth as a function of image separation. Those effects must, however, be distinguished from effects associated with the structure of dark matter halos. Baryonic cooling causes galaxy-mass halos to have different central density profiles than group- and cluster-mass halos, which causes the distribution of normal arcsecond-scale lenses to differ from the distribution of ``wide-separation'' ($Δθ\gtrsim 4\arcsec$) lenses. Fortunately, the various parameters related to cosmology and halo structure have very different effects on the overall image separation distribution: (1) the abundance of wide-separation lenses is exremely sensitive (by orders of magnitude) to the distribution of ``concentration'' parameters for massive halos modeled with the Navarro-Frenk-White profile; (2) the transition between normal and wide-separation lenses depends mainly on the mass scale where baryonic cooling ceases to be efficient; and (3) dark energy has effects at all image separation scales. While current lens samples cannot usefully constrain all of the parameters, ongoing and future imaging surveys should discover hundreds or thousands of lenses and make it possible to disentangle the various effects and constrain all of the parameters simultaneously. (abridged)

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arXiv2003

The Rest-Frame UV Luminosity Density of Star-Forming Galaxies at Redshifts z>3.5

We have measured the rest--frame lambda~1500 Ang comoving specific luminosity density of star--forming galaxies at redshift 3.5<z<6.5 from deep images taken with the Hubble Space Telescope (HST)and the Advanced Camera for Surveys (ACS), obtained as part of the Great Observatories Origins Deep Survey (GOODS). We used color selection criteria to construct samples of star--forming galaxies at redshifts z~4, 5 and 6, identified by the signature of the 912 Ang Lyman continuum discontinuity and Lyman-alpha forest blanketing in their rest--frame UV colors (Lyman--break galaxies). The ACS samples cover ~0.09 square degree, and are also relatively deep, reaching between 0.2 and 0.5 L_3^*, depending on the redshift, where $L_3^*$ is the characteristic UV luminosity of Lyman--break galaxies at z~3. The specific luminosity density of Lyman--break galaxies appears to be nearly constant with redshift from z~3 to z~6, although the measure at z~6 remains relatively uncertain, because it depends on the accurate estimate of the faint counts of the z~6 sample. If Lyman--break galaxies are fair tracers of the cosmic star formation activity, our results suggest that at z~6 the universe was already producing stars as vigorously as it did near its maximum several Gyr later, at 1<~z<~3. Thus, the onset of large--scale star formation in the universe is to be sought at around z~6 or higher, namely at less than ~7% of the current cosmic age.

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arXiv2003

Probing Beyond the Epoch of Hydrogen Reionization with 21 Centimeter Radiation

We use numerical simulations of hydrogen reionization by stellar sources in the context of LCDM cosmogonies to investigate the 21 (1+z) cm radio signal expected from the diffuse intergalactic medium (IGM) prior to the epoch of reionization breakthrough at redshift z_ion. Two reionization scenarios are analyzed in details: an `early reionization' case with z_ion~13, consistent with the recent discovery by the WMAP satellite of a large optical depth to Thomson scattering, and a `late reionization' case with z_ion~8. It is a generic prediction of these models that the background of Lyalpha photons produced by the early generation of stars which ultimately ionize the universe will be sufficiently intense to make intergalactic neutral hydrogen visible against the cosmic microwave background during the `grey age', i.e. z_ion<z<20. Depending on the redshift of reionization breakthrough, broad-beam observations at frequencies <150 MHz (below 100 MHz for the `early reionization' scenario) with the next generation of radio telescopes should reveal angular fluctuations in the sky brightness temperature in the range 5-20 mK (1 sigma) on scales below 5 arcmin.

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arXiv2003

Early Preheating and Galaxy Formation

Winds from pregalactic starbursts and 'miniquasars' may pollute the IGM with metals and raise its temperature to a high adiabat, and so inhibit the formation of early galaxies. We compute the thermal history of the IGM when it experiences a period of rapid, homogeneous "preheating" at high redshifts. Measurements of the temperature of the Lyamn-alpha forest at z~3 constrain the redshift and energy of preheating, and rule out models that preheat too late or to too high a temperature. We predict galaxy luminosity functions in preheated universes. The results depend crucially on whether the baryonic smoothing scale in the IGM is computed globally, or in a local, density-dependent fashion. Using a globally averaged smoothing scale, we find that models with excessive preheating produce too few L_* and fainter galaxies, and are therefore inconsistent with observational data. A density-dependent smoothing scale requires more energetic preheating to achieve the same degree of suppression in the faint-end slope. All models, however, appear unable to explain the sharp cut-off in the luminosity function at bright magnitudes. Supernova-driven preheating scenarios tend to raise the mean metallicity of the universe well above the minimum levels observed in the Lyman-alpha clouds. We find that ionizing photon escape fractions must be significantly higher than 10% in order to explain the low inferred HI fraction at z~6. While early preheating causes strong suppression of dwarf galaxy formation we show that it is not able to reproduce the observed abundance of satellite galaxies in the Local Group in detail.

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arXiv1999

Can the reionization epoch be detected as a global signature in the cosmic background?

The reionization of the Universe is expected to leave a signal in the form of a sharp step in the spectrum of the sky. If reionization occurs at 5 < z < 20, a feature should appear in the radio sky at 70 - 240 MHz due to redshifted HI 21-cm line emission, accompanied by another feature in the optical/near-IR at 0.7 - 2.6 micron due to hydrogen recombination radiation. The expected amplitude is well above fundamental detection limits, and the sharpness of the feature may make it distinguishable from variations due to terrestrial, galactic and extragalactic foregrounds. Because this is essentially a continuum measurement of a signal which occurs over the whole sky, relatively small telescopes may suffice for detection in the radio. In the optical/near-IR, a space telescope is needed with the lowest possible background conditions, since the experiment will be severely background-limited.

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arXiv1998

VLT and HST observations of a candidate high redshift elliptical galaxy in the Hubble Deep Field South

The combined use of the ESO Very Large Telescope (VLT) UT1 Science Verification (SV) images and of the Hubble Space Telescope (HST) Hubble Deep Field South observations allows us to strengthen the identification as a candidate elliptical galaxy of the Extremely Red Object HDFS 223251-603910 previously identified by us on the basis of NICMOS and Cerro Tololo Interamerican Observatory imaging. The photometry presented here includes VLT data in U, B, V, R, I, a STIS unfiltered image, NICMOS J, H, and K band data, thus combining the 16.5 hours of VLT SV exposures with 101 hours of HST observing. The object is detected in all images except the VLT U band and is one of the reddest known with B-K=9.7+-0.5. We consider a wide range of models with different ages, metallicities, star formation histories and dust content, and conclude that the observed spectral energy distribution agrees best with that of an old elliptical galaxy at redshift just below 2. Alternative possibilities are discussed in light of their likelihood and of the perspective of spectroscopic confirmation.

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arXiv1998

An Extremely Red r^{1/4} Galaxy in the Test Image of the Hubble Deep Field South

We report the serendipitous discovery of an extremely red object in the Hubble Deep Field South (HDFS) Test NICMOS (Near Infrared Camera and Multi Object Spectrograph) field of view. The object is resolved in the NICMOS image and has a light profile very well described by an r^{1/4} law with effective radius r_e=0."20+-0."05 and H_{AB}=21.7+-0.1 magnitudes. In contrast, the galaxy is undetected in the R and I band ground based images taken at the CTIO 4 m Blanco Telescope, giving a lower limit to the color of (R-H)_{AB}>3.9 and (I-H)_{AB}>3.5 at the 95% confidence level. The colors of a range of synthetic galactic spectra are computed, showing that the object is likely to be an ``old'' elliptical galaxy at redshift z>~1.7. Alternatively the colors can be reproduced by an ``old'' elliptical galaxy at somewhat lower redshift (z>~1) with significant amount of dust, or by a younger galaxy at higher redshift. This object represents a very interesting target for future VLT observations.

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arXiv1998

On the Evolution of the Cosmic Supernova Rates

Ongoing searches for supernovae (SNe) at cosmological distances have recently started to provide a link between SN Ia statistics and galaxy evolution. We use recent estimates of the global history of star formation to compute the theoretical Type Ia and Type II SN rates as a function of cosmic time from the present epoch to high redshifts. We show that accurate measurements of the frequency of SN events in the range 0<z<1 will be valuable probes of the nature of Type Ia progenitors and the evolution of the stellar birthrate in the universe. The Next Generation Space Telescope should detect of order 20 Type II SNe per 4'x 4' field per year in the interval 1<z<4.

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arXiv1998

High Redshift Galaxies in the Hubble Deep Field: II. Colours and Number Counts

We discuss the deep galaxy counts from the Hubble Deep Field (HDF) imaging survey. At faint magnitudes, the slope of the differential number-magnitude relation is flatter than 0.2 in all four HDF bandpasses. In the ultraviolet, a fluctuation analysis shows that the flattening observed below U=26 mag is not due to incompleteness and is more pronounced than in the other bands, consistent with the idea that a redshift limit has been reached in the galaxy distribution. A reddening trend of about 0.5 magnitude is observed at faint fluxes in the colour-magnitude diagram, (U-V)_{eff} vs. V. We interpret these results as the effect of intergalactic attenuation on distant galaxies. At flux levels of AB=27 mag and in agreement with the fluctuation analysis and the colour-magnitude relation, about 7% of the sources in U, 30% in B and 35% in V are Lyman-break ``dropouts'', i.e. candidate star-forming galaxies at z>2. By integrating the number counts to the limits of the HDF survey we find that the mean surface brightness of the extragalactic sky is dominated by galaxies that are relatively bright and are known to have =0.6. To AB=29 mag, the integrated light from resolved galaxies in the I-band is 2.1\pm 0.4 \times 10^{-20} erg/cm^2/s/Hz/sr, and its spectrum is well described by a broken power-law. We discuss the predictions for the counts, colours, and luminosity densities from standard low-q_0 pure-luminosity-evolution models without dust obscuration, and find that they are unable to reproduce all the observed properties of faint field galaxies.

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