arXiv subjects

Abraham Loeb

Publications and source records attributed to Abraham Loeb.

At least 631 records · Page 35Linked to original sources

arXiv2007

Detecting Reionization in the Star Formation Histories of High-Redshift Galaxies

The reionization of cosmic hydrogen, left over from the big bang, increased its temperature to >~ 1.e4 K. This photo-heating resulted in an increase of the minimum mass of galaxies and hence a suppression of the cosmic star formation rate. The affected population of dwarf galaxies included the progenitors of massive galaxies that formed later. We show that a massive galaxy at a redshift z >~ 6 should show a double-peaked star formation history marked by a clear break. This break reflects the suppression signature from reionization of the region in which the galaxy was assembled. Since massive galaxies originate in overdense regions where cosmic evolution is accelerated, their environment reionizes earlier than the rest of the universe. For a galaxy of ~ 1.e12 M_solar in stars at a redshift of z ~ 6.5, the star formation rate should typically be suppressed at a redshift z >~ 10 since the rest of the universe is known to have reionized by z >~ 6.5. Indeed, this is inferred to be the case for HUDF-JD2, a massive galaxy which is potentially at z ~ 6.5 but is inferred to have formed the bulk of its 3.e11 M_solar in stars at z >~ 9.

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arXiv2007

Light-cone anisotropy in 21cm fluctuations during the epoch of reionization

The delay in light travel time along the line of sight generates an anisotropy in the power spectrum of 21cm brightness fluctuations from the epoch of reionization. We show that when the fluctuations in the neutral hydrogen fraction become non-linear at the later stages of reionization, the light-cone anisotropy becomes of order unity on scales >50 comoving Mpc. During this period the density fluctuations and the associated anisotropy generated by peculiar velocities are negligible in comparison.

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arXiv2007

Dynamics of triple black hole systems in hierarchically merging massive galaxies

Galaxies with stellar bulges are generically observed to host supermassive black holes (SMBHs). The hierarchical merging of galaxies should therefore lead to the formation of SMBH binaries. Merging of old massive galaxies with little gas promotes the formation of low-density nuclei where SMBH binaries are expected to survive over long times. If the binary lifetime exceeds the typical time between mergers, then triple black hole (BH) systems may form. We study the statistics of close triple-SMBH encounters in galactic nuclei by computing a series of 3-body orbits with physically-motivated initial conditions appropriate for giant elliptical galaxies. Our simulations include a smooth background potential consisting of a stellar bulge plus a dark matter halo, drag forces due to gravitational radiation and dynamical friction on the stars and dark matter, and a simple model of the time evolution of the inner density profile under heating and mass ejection by the SMBHs. We find that the binary pair coalesces as a result of repeated close encounters in ~85% of our runs, and in ~15% of cases a new eccentric binary forms from the third SMBH and binary remnant and coalesces during the run time. In about 40% of the runs the lightest BH is left wandering through the galactic halo or escapes the galaxy altogether, but escape of all three SMBHs is exceedingly rare. The triple systems typically scour out cores with mass deficits ~1-2 times their total mass, which can help to account for the large cores observed in some massive elliptical galaxies, such as M87. The high coalescence rate, prevalence of very high-eccentricity orbits, and gravitational radiation ``spikes'' during close encounters in our runs, may provide interesting signals for the future Laser Interferometer Space Antenna (LISA).

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arXiv2007

Properties of the Radio-Emitting Gas Around SgrA*

We show that the radial profiles of the temperature and density of the electrons as well as the magnetic field strength around the massive black hole at the Galactic center, SgrA*, may be constrained directly from existing radio data without any need to make prior assumptions about the dynamics of the emitting gas. The observed spectrum and wavelength-dependent angular size of SgrA* indicate that the synchrotron emission originates from an optically-thick plasma of quasi-thermal electrons. We find that the electron temperature rises above the virial temperature within tens of Schwarzschild radii from the black hole, suggesting that the emitting plasma may be outflowing. Constraints on the electron density profile are derived from polarization measurements. Our best-fit results differ from expectations based on existing theoretical models. However, these models cannot be ruled out as of yet due to uncertainties in the source size measurements. Our constraints could tighten considerably with future improvements in the size determination and simultaneous polarization measurements at multiple wavelengths.

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arXiv2007

An Empirically-Calibrated Model For Interpreting the Evolution of Galaxies During the Reionization Era

[Abridged] We develop a simple star formation model whose goal is to interpret the emerging body of observational data on star-forming galaxies at z>~6. The efficiency and duty cycle of the star formation activity within dark matter halos are determined by fitting the luminosity functions of Lya emitter and Lyman-break galaxies at redshifts z~5-6. Using our model parameters we predict the likely abundance of star forming galaxies at earlier epochs and compare these to the emerging data in the redshift interval 7<z<10. We find that the abundance of luminous Lyman-break galaxies in the 500 Myr between z~6 and 10 can be naturally explained by the hierarchical assembly of dark matter haloes; there is only marginal evidence for strong physical evolution. In contrast, the first estimates of the abundance of less luminous star forming galaxies at z=9-10 are higher than predicted and, if verified by further data, may suggest a top-heavy stellar mass function at these early epochs. Although these abundances remain uncertain because of the difficulty of spectroscopic confirmation and cosmic variance, even a modest improvement in survey capability with present or upcoming facilities should yield great progress. In this context, we use our model to consider those observational techniques that hold the most promise and make predictions for specific surveys that are, or will soon be, underway. We conclude that narrowband Lya emitter surveys should be efficient on searches at z~7-8; however, such conventional surveys are unlikely to detect sufficient galaxies at z~10 to provide useful constraints. For this reason, gravitational lensing offers the best prospect for probing the z~10 universe prior to JWST.

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arXiv2007

Missing Pages in Our Photo Album of the Infant Universe

Existing data sets include an image of the Universe when it was 0.4 million years old (in the form of the cosmic microwave background), as well as images of individual galaxies when the Universe was older than a billion years. But there is a serious challenge: in between these two epochs was a period when the Universe was dark, stars had not yet formed, and the cosmic microwave background no longer traced the distribution of matter. And this is precisely the most interesting period, when the primordial soup evolved into the rich zoo of objects we now see. In this popular-level overview, I describe how astronomers plan to observe this nearly-invisible yet crucial period.

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arXiv2007

Hypervelocity Collisions of Binary Stars at the Galactic Centre

Recent surveys have identified seven hypervelocity stars (HVSs) in the halo of the Milky Way. Most of these stars may have originated from the breakup of binary star systems by the nuclear black hole SgrA*. In some instances, the breakup of the binary may lead to a collision between its member stars. We examine the dynamical properties of these collisions by simulating thousands of different binary orbits around SgrA* with a direct N-body integration code. For some orbital parameters, the two stars collide with an impact velocity lower than their escape velocity and may therefore coalesce. It is possible for a coalescing binary to have sufficient velocity to escape the galaxy. Furthermore, some of the massive S-stars near Sgr A* might be the merger remnants of binary systems, however this production method can not account for most of the S-stars.

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arXiv2006

Eavesdropping on Radio Broadcasts from Galactic Civilizations with Upcoming Observatories for Redshifted 21cm Radiation

The question of whether intelligent life exists elsewhere is one of the fundamental unknowns about our Universe. Over the past decade more than 200 extra-solar planets have been discovered, providing new urgency for addressing this question in these or other planetary systems. Independently of this perspective, new radio observatories for cosmology are currently being constructed with the goal of detecting 21cm emission from cosmic hydrogen in the redshift range 6<z<15. For example, the Low-Frequency Demonstrator (LFD) of the Mileura Wide-Field Array (MWA), will cover in 8 kHz bins the entire frequency range of 80-300 MHz, which is perfectly matched to the band over which our civilization emits most of its radio power. We show that this and other low-frequency observatories (culminating with the Square Kilometer Array [SKA]) will be able to detect radio broadcast leakage from an Earth-like civilization out to a distance of ~10^{1-2.7}pc, within a spherical volume containing 10^{3-8}x(Omega_b/4*pi)^alpha stars, where alpha=1 (or 1.5) for a radar beam of solid angle Omega_b that remains steady (or sweeps) across the sky. Such a radio signal will show-up as a series of narrow spectral lines that do not coincide with known atomic or molecular lines. The high spectral resolution attainable with the upcoming observatories will allow to monitor the periodic Doppler shift of the broadcasted lines over the planet's orbital period around the parent star. Determination of the parent star mass through observations of its spectrum could then be used to infer the inclination, semi-major axis and eccentricity of the planet's orbit. This, in turn, will allow to estimate the temperature on the planet's surface and to assess whether it can support liquid water or life as we know it.

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arXiv2006

Thermal Evaporation of Gas from X-ray Clusters

A fraction of the thermal protons in the outer envelope of an X-ray cluster have velocities that exceed the local escape speed from the cluster gravitational potential. The Coulomb mean-free-path of these protons is larger than the virial radius of the cluster at temperatures >2.5 keV. The resulting leakage of suprathermal particles generates a collisionless shock in neighboring voids and fills them with heat and magnetic fields. The momentum flux of suprathermal particles cannot be confined by magnetic tension at the typical field strength in the periphery of cluster halos (<<1 micro-gauss). Over a Hubble time, thermal evaporation could drain up to a tenth of the cluster gas at its virial temperature. The evaporated fraction could increase dramatically if additional heat is deposited into the gas by cluster mergers, active galactic nuclei or supernovae. Thermal evaporation is not included in existing cosmological simulations since they are based on the fluid approximation. Measurements of the baryon mass fraction in the outer envelopes of hot clusters (through their Sunyaev-Zel'dovich effect or X-ray emission) can be used to empirically constrain their evaporation rate.

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arXiv2006

CMB Anisotropies from Outflows in Lyman Break Galaxies

Thomson scattering of the Cosmic Microwave Background (CMB) on moving electrons in the outflows of Lyman Break Galaxies (LBGs) at redshifts 2-8 contributes to the small-scale CMB anisotropies. The net effect produced by each outflow depends on its level of deviation from spherical symmetry, caused either by an anisotropic energy injection from the nuclear starburst or quasar activity, or by an inhomogeneous intergalactic environment. We find that for plausible outflow parameters consistent with spectroscopic observations of LBGs, the induced CMB anisotropies on arcminute scales reach up to $\sim 1 μ$K, comparable to the level produced during the epoch of reionization.

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arXiv2006

Assessing The Starburst Contribution to the Gamma-Ray and Neutrino Backgrounds

If cosmic ray protons interact with gas at roughly the mean density of the interstellar medium in starburst galaxies, then pion decay in starbursts is likely to contribute significantly to the diffuse extra-galactic background in both gamma-rays and high energy neutrinos. We describe the assumptions that lead to this conclusion and clarify the difference between our estimates and those of Stecker (2006). Detection of a single starburst by GLAST would confirm the significant contribution of starburst galaxies to the extra-galactic neutrino and gamma-ray backgrounds.

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arXiv2006

Testing General Relativity with High-Resolution Imaging of Sgr A*

Submilliarcsecond astrometry and imaging of the black hole Sgr A* at the Galactic Center may become possible in the near future at infrared and submillimetre wavelengths. This resolution is sufficient to observe the silhouette the supermassive black hole in the Galactic center casts upon background emission. However, more exciting is the prospect of observing ``hot spots'' in the accretion flow. Here we discuss how such measurements may be used to test not only the consistency of General Relativity, but also the validity of the Kerr metric in particular.

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arXiv2006

Smooth Boundaries to Cosmological HII Regions from Galaxy Clustering

The HII regions around quasars and galaxies at redshifts beyond the epoch of reionisation will provide prime targets for upcoming 21cm campaigns using a new generation of low-frequency radio observatories. Here we show that the boundaries of these HII regions will not be sharp. Rather, the clustering of sources near massive galaxies results in a neutral fraction that rises gradually towards large radii from an interior value near zero. A neutral fraction corresponding to the global background value is typically reached at a distance of 2-5 times the radius of the HII region around the central massive galaxy.

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arXiv2006

An Observational Test for the Anthropic Origin of the Cosmological Constant

The existence of multiple regions of space beyond the observable Universe (within the so-called "multiverse") where the vacuum energy density takes different values, has been postulated as an explanation for the low non-zero value observed for it in our Universe. It is often argued that our existence pre-selects regions where the cosmological constant is sufficiently small to allow galaxies like the Milky Way to form and intelligent life to emerge. Here we propose a simple empirical test for this anthropic argument within the boundaries of the observable Universe. We make use of the fact that dwarf galaxies formed in our Universe at redshifts as high as z~10 when the mean matter density was larger by a factor of ~10^3 than today. Existing technology enables to check whether planets form in nearby dwarf galaxies and globular clusters by searching for microlensing or transit events of background stars. The oldest of these nearby systems may have formed at z~10. If planets are as common per stellar mass in these descendents as they are in the Milky Way galaxy, then the anthropic argument would be weakened considerably since planets could have formed in our Universe even if the cosmological constant was three orders of magnitude larger than observed. For a flat probability distribution, this would imply that the probability for us to reside in a region where the cosmological constant obtains its observed value is lower than \~10^{-3}. A precise version of the anthropic argument could then be ruled-out at a confidence level of ~99.9%, which constitutes a satisfactory measure of a good experimental test.

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arXiv2006

The Cumulative Bakground of High-Energy Neutrinos from Starburst Galaxies

We show that starburst galaxies convert efficiently cosmic-rays into pions, which in turn decay into high-energy neutrinos and photons. The cumulative background of GeV neutrinos is 10^{-7}GeV/cm^2/s/sr. Its extrapolation to higher neutrino energies depends on the energy spectrum of the injected cosmic-rays and is proportional to E^{-0.15+-0.1} up to E~0.3PeV and possibly higher neutrino energies. This flux, which constitutes a lower limit to the high energy extra-Galactic neutrino flux, is likely to be detectable by forthcoming km-scale neutrino telescopes.

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arXiv2006

Dwarf Galaxy Formation Was Suppressed By Cosmic Reionization

A large number of faint galaxies, born less than a billion years after the big bang, have recently been discovered. The fluctuations in the distribution of these galaxies contributed to a scatter in the ionization fraction of cosmic hydrogen on scales of tens of Mpc, as observed along the lines of sight to the earliest known quasars. Theoretical simulations predict that the formation of dwarf galaxies should have been suppressed after cosmic hydrogen was reionized, leading to a drop in the cosmic star formation rate. Here we present evidence for this suppression. We show that the post-reionization galaxies which produced most of the ionizing radiation at a redshift z~5.5, must have had a mass in excess of ~10^{10.6+/-0.4} solar masses or else the aforementioned scatter would have been smaller than observed. This limiting mass is two orders of magnitude larger than the galaxy mass that is thought to have dominated the reionization of cosmic hydrogen (~10^8 solar masses). We predict that future surveys with space-based infrared telescopes will detect a population of smaller galaxies that reionized the Universe at an earlier time, prior to the epoch of dwarf galaxy suppression.

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arXiv2006

Imprint of Inhomogeneous Reionization on the Power Spectrum of Galaxy Surveys at High Redshifts

We consider the effects of inhomogeneous reionization on the distribution of galaxies at high redshifts. Modulation of the formation process of the ionizing sources by large scale density modes makes reionization inhomogeneous and introduces a spread to the reionization times of different regions with the same size. After sources photo-ionize and heat these regions to a temperature $\ga 10^4$K at different times, their temperatures evolve as the ionized intergalactic medium (IGM) expands. The varying IGM temperature makes the minimum mass of galaxies spatially non-uniform with a fluctuation amplitude that increases towards small scales. These scale-dependent fluctuations modify the shape of the power spectrum of low-mass galaxies at high redshifts in a way that depends on the history of reionization. The resulting distortion of the primordial power spectrum is significantly larger than changes associated with uncertainties in the inflationary parameters, such as the spectral index of the scalar power spectrum or the running of the spectral index. Future surveys of high-redshift galaxies will offer a new probe of the thermal history of the IGM but might have a more limited scope in constraining inflation.

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arXiv2006

First Light

The first dwarf galaxies, which constitute the building blocks of the collapsed objects we find today in the Universe, had formed hundreds of millions of years after the big bang. This pedagogical review describes the early growth of their small-amplitude seed fluctuations from the epoch of inflation through dark matter decoupling and matter-radiation equality, to the final collapse and fragmentation of the dark matter on all mass scales above \~10^{-4} solar masses. The condensation of baryons into halos in the mass range of ~10^5-10^{10} solar masses led to the formation of the first stars and the re-ionization of the cold hydrogen gas, left over from the big bang. The production of heavy elements by the first stars started the metal enrichment process that eventually led to the formation of rocky planets and life. A wide variety of instruments currently under design [including large-aperture infrared telescopes on the ground or in space (JWST), and low-frequency arrays for the detection of redshifted 21cm radiation], will establish better understanding of the first sources of light during an epoch in cosmic history that was largely unexplored so far. Numerical simulations of reionization are computationally challenging, as they require radiative transfer across large cosmological volumes as well as sufficently high resolution to identify the sources of the ionizing radiation. The technological challenges for observations and the computational challenges for numerical simulations, will motivate intense work in this field over the coming decade.

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