arXiv subjects

Abraham Loeb

Publications and source records attributed to Abraham Loeb.

At least 523 records · Page 29Linked to original sources

arXiv2011

Constraints on the Redshift Evolution of the L_X-SFR Relation from the Cosmic X-Ray Backgrounds

Observations of local star forming galaxies have revealed a correlation between the rate at which galaxies form stars and their X-Ray luminosity. We combine this correlation with the most recent observational constraints on the integrated star formation rate density, and find that star forming galaxies account for 5-20% of the total soft and hard X-ray backgrounds, where the precise number depends on the energy band and the assumed average X-ray spectral energy distribution of the galaxies below ~20 keV. If we combine the L_X-SFR relation with recently derived star formation rate function, then we find that star forming galaxies whose X-ray flux falls well (more than a factor of 10) below the detection thresholds of the Chandra Deep Fields, can fully account for the unresolved soft X-ray background, which corresponds to ~6% of its total. Motivated by this result, we put limits on the allowed redshift evolution of the parameter c_X \equiv L_X/SFR, and/or its evolution towards lower and higher star formation rates. If we parametrize the redshift evolution of c_X ~ (1+z)^b, then we find that b \leq 1.3 (95% CL). On the other hand, the observed X-ray luminosity functions (XLFs) of star forming galaxies indicate that c_X may be increasing towards higher redshifts and/or higher star formation rates at levels that are consistent with the X-ray background, but possibly at odds with the locally observed L_X-SFR relation.

astro-ph.CO↗

arXiv2011

Photon Trapping Enables Super-Eddington Growth of Black-Hole Seeds in Galaxies at High Redshift

We identify a physical mechanism that would have resulted in rapid, obscured growth of seed super-massive black-holes in galaxies at z>6. Specifically, we find that the density at the centre of typical high redshift galaxies was at a level where the Bondi accretion rate implies a diffusion speed of photons that was slower than the gravitational infall velocity, resulting in photons being trapped within the accretion flow and advected into the black-hole. We show that there is a range of black-hole masses (M_bh ~ 10^3-10^5 solar masses) where the accretion flow traps radiation, corresponding to black-holes that were massive enough to generate a photon trapping accretion flow, but small enough that their Bondi radii did not exceed the isothermal scale height of self-gravitating gas. Under these conditions we find that the accretion reaches levels far in excess of the Eddington rate. A prediction of this scenario is that X-ray number counts of active galactic nuclei at z>6 would exhibit a cutoff at the low luminosities corresponding to black-hole masses below ~10^5 solar masses. At low redshifts we find photon trapping to be unimportant because it could only occur in rare low spin halos, and would require black-hole masses in excess of expectations from the observed black-hole - halo mass relation. The super-Eddington growth of ~10^5 solar mass seed black-holes at high redshift may have provided a natural acceleration towards the growth of super-massive black-holes at z~6-7, less than a billion years after the Big Bang.

astro-ph.CO↗

arXiv2011

Effects of supermassive binary black holes on gravitational lenses

Recent observations indicate that many if not all galaxies host massive central black holes (BHs). In this paper we explore the influence of supermassive binary black holes (SMBBHs) on their actions as gravitational lenses. When lenses are modelled as singular isothermal ellipsoids, binary black holes change the critical curves and caustics differently as a function of distance. Each black hole can in principle create at least one additional image, which, if observed, provides evidence of black holes. By studying how SMBBHs affect the cumulative distribution of magnification for images created by black holes, we find that the cross section for at least one such additional image to have a magnification larger than $10^{-5}$ is comparable to the cross section for producing multiple-images in singular isothermal lenses. Such additional images may be detectable with high-resolution and large dynamic range maps of multiply-imaged systems from future facilities, such as the SKA. The probability of detecting at least one image (two images) with magnification above $10^{-3}$ is $\sim 0.2 \fBH$ ($\sim 0.05 \fBH$) in a multiply-imaged lens system, where $\fBH$ is the fraction of galaxies housing binary black holes. We also study the effects of SMBBHs on the core images when galaxies have shallower central density profiles (modelled as non-singular isothermal ellipsoids). We find that the cross section of the usually faint core images is further suppressed by SMBBHs. Thus their presence should also be taken into account when one constrains the core radius from the lack of central images in gravitational lenses.

astro-ph.CO↗

arXiv2011

The imprint of the relative velocity between baryons and dark matter on the 21-cm signal from reionization

The post-recombination streaming of baryons through dark matter keeps baryons out of low mass (<10^6 solar masses) halos coherently on scales of a few comoving Mpc. It has been argued that this will have a large impact on the 21-cm signal before and after reionization, as it raises the minimum mass required to form ionizing sources. Using a semi-numerical code, we show that the impact of the baryon streaming effect on the 21-cm signal during reionization (redshifts z approximately 7-20) depends strongly on the cooling scenario assumed for star formation, and the corresponding virial temperature or mass at which stars form. For the canonical case of atomic hydrogen cooling at 10^4 Kelvin, the minimum mass for star formation is well above the mass of halos that are affected by the baryon streaming and there are no major changes to existing predictions. For the case of molecular hydrogen cooling, we find that reionization is delayed by a change in redshift of approximately 2 and that more relative power is found in large modes at a given ionization fraction. However, the delay in reionization is degenerate with astrophysical assumptions, such as the production rate of UV photons by stars.

astro-ph.CO↗

arXiv2011

Recoiled star clusters in the Milky Way halo: N-body simulations and a candidate search through SDSS

During the formation of the Milky Way, > 100 central black holes (BHs) may have been ejected from their small host galaxies as a result of asymmetric gravitational wave emission. We previously showed that many of these BHs are surrounded by a compact cluster of stars that remained bound to the BH during the ejection process. In this paper, we perform long term N-body simulations of these star clusters to determine the distribution of stars in these clusters today. These numerical simulations, reconciled with our Fokker-Planck simulations, show that stellar density profile follows a power-law with slope ~ -2.15, and show that large angle scattering and tidal disruptions remove 20 - 90% of the stars by ~10^10 yr. We then analyze the photometric and spectroscopic properties of recoiled clusters accounting for the small number of stars in the clusters. We use our results to perform a systematic search for candidates in the Sloan Digital Sky Survey. We find no spectroscopic candidates, in agreement with our expectations from the completeness of the survey. Using generic photometric models of present day clusters we identify ~100 recoiling cluster candidates. Follow-up spectroscopy would be able to determine the nature of these candidates.

astro-ph.CO↗

arXiv2011

Gauging the Contribution of X-ray Sources to Reionization Through the Kinetic Sunyaev-Zel'dovich Effect

Measurements of the kinetic Sunyaev-Zel'dovich (kSZ) effect from instruments such as the South Pole Telescope (SPT) and the Atacama Cosmology Telescope (ACT) will soon put improved constraints on reionization. Popular models assume that UV photons alone are responsible for reionization of the intergalactic medium. We explore the effects of a significant contribution of X-rays to reionization on the kSZ signal. Because X-rays have a large mean free path through the neutral intergalactic medium, they introduce partial ionization in between the sharp-edged bubbles created by UV photons. This smooth ionization component changes the power spectrum of the cosmic microwave background (CMB) temperature anisotropies. We quantify this effect by running semi-numerical simulations of reionization. We test a number of different models of reionization without X-rays that have varying physical parameters, but which are constrained to have similar total optical depths to electron scattering. These are then compared to models with varying levels of contribution to reionization from X-rays. We find that models with more than a 10% contribution from X-rays produce a significantly lower power spectrum of temperature anisotropies than all the UV-only models tested. The expected sensitivity of SPT and ACT may be insufficient to distinguish between our models, however, a non-detection of the kSZ signal from the epoch of reionization could result from the contribution of X-rays. It will be important for future missions with improved sensitivity to consider the impact of X-ray sources on reionization.

astro-ph.CO↗

arXiv2011

Empirical Constraints on the Coevolution of Supermassive Black Holes and their Host Spheroids

We investigate the evolution of the MBH-σ relation by examining the relationship between the intrinsic scatter in the MBH-σ relation and galaxy bolometric nuclear luminosity, the latter being a probe of the accretion rate of the black hole (BH). Our sample is composed of galaxies with classical bulges when possible, of which 38 have dynamically measured BHs masses, and 17 have BHs masses measured by reverberation mapping. In order to obtain the bolometric nuclear luminosity for galaxies with low nuclear luminosity, we convert the X-ray nuclear luminosity measured by Chandra to bolometric luminosity. We find that the scatter in the MBH-σ relation is uncorrelated with nuclear luminosity over seven orders of magnitude in luminosity, with the high luminosity end approaching the Eddington luminosity. This suggests that at the present epoch galaxies evolve along the MBH-σ relation. This conclusion is consistent with the standard paradigm that BHs grow contemporaneously with their host stellar spheroids.

astro-ph.CO↗

arXiv2011

Recoiling Black Holes in Merging Galaxies: Relationship to AGN Lifetimes and Merger Remnant Properties

Central supermassive black holes (SMBHs) are a ubiquitous feature of locally-observed galaxies, and ample evidence suggests that the growth of SMBHs and their host galaxies is closely linked. However, in the event of a merger, gravitational-wave (GW) recoil may displace a SMBH from its galactic center, or eject it entirely. To explore the consequences of this phenomenon, we use hydrodynamic simulations of gaseous galaxy mergers that include a range of BH recoil velocities. We have generated a suite of over 200 simulations with more than 60 merger models, enabling us to identify systematic trends in the behavior of recoiling BHs -- specifically (i) their dynamics, (ii) their observable signatures, and (iii) their effects on BH/galaxy co-evolution. (i) Recoiling BH trajectories depend heavily on the gas content of the host galaxy; maximal BH displacements from the center may vary by up to an order of magnitude between gas-rich and gas-poor mergers. In some cases, recoil trajectories also depend on the timing of the BH merger relative to the formation of the galaxy merger remnant. (ii) Recoiling BHs may be observable as offset active galactic nuclei (AGN) via either kinematic offsets (v > 800 km s^-1) or spatial offsets (R > 1 kpc) for lifetimes of about 1 - 100 Myr. In addition, recoil events affect the total AGN lifetime. GW recoil generally reduces the lifetimes of bright AGN, but may extend lower-luminosity AGN lifetimes. (iii) Rapidly-recoiling BHs may be up to about 5 times less massive than their stationary counterparts. These mass deficits lower the normalization of the M - sigma relation and contribute to both intrinsic and overall scatter. Furthermore, recoil events displace AGN feedback from the galactic center, which enhances central star formation rates. This results in longer starburst phases and higher central stellar densities in merger remnants.

astro-ph.CO↗

arXiv2011

The Shocking Truth: The small contribution to hydrogen reionization from gravitational infall

It is commonly thought that stars are responsible for reionizing the Universe. However, deep searches for star-forming galaxies during the epoch of reionization have not yet found sufficient galaxies to provide the necessary ionising flux. Fast accretion shocks associated with gravitational infall of baryons during the formation of galaxies have recently been proposed as an alternative method of generating the required ionising photons. In this Letter we assess the contribution to hydrogen reionization from shocked gas associated with gravitational infall. We find that shocks can ionize no more than a few percent of the cosmic hydrogen by z~6. However, the small fraction of ionizing radiation produced by fast accretion shocks would be significantly more biased than that associated with stars, leading to a modification of the luminosity weighted source clustering by ~10%. This modification of the bias may be measurable with future precision experiments utilising the redshifted 21cm line to study the distribution of hydrogen during the reionization era.

astro-ph.CO↗

arXiv2011

Rating Growth of Scientific Knowledge and Risk from Theory Bubbles

In physics the value of a theory is measured by its agreement with experimental data. But how should the physics community gauge the value of an emerging theory that has not been tested experimentally as of yet? With no reality check, a hypothesis like string theory may linger for a while before physicists will know its actual value in describing nature. In this short article, I advocate the need for a website operated by graduate students that will use various measures of publicly available data (such as the growth rate of newly funded experiments, research grants, publications, and faculty jobs) to gauge the future dividends of various research frontiers. The analysis can benefit from past experience (e.g. in research areas that suffered from limited experimental data over long periods of time) and aim to alert the community of the risk from future theory bubbles.

astro-ph.IM↗

arXiv2011

Evidence Against Dark Matter Halos Surrounding the Globular Clusters MGC1 and NGC 2419

The conjecture that the ancient globular clusters (GCs) formed at the center of their own dark matter halos was first proposed by Peebles (1984), and has recently been revived to explain the puzzling abundance patterns observed within many GCs. In this paper we demonstrate that the outer stellar density profile of isolated GCs is very sensitive to the presence of an extended dark halo. The GCs NGC 2419, located at 90 kpc from the center of our Galaxy, and MGC1, located at ~200 kpc from the center of M31, are ideal laboratories for testing the scenario that GCs formed at the centers of massive dark halos. Comparing analytic models to observations of these GCs, we conclude that these GCs cannot be embedded within dark halos with a virial mass greater than 10^6 Msun, or, equivalently, the dark matter halo mass-to-stellar mass ratio must be Mdm/M_*<1. If these GCs have indeed orbited within weak tidal fields throughout their lifetimes, then these limits imply that these GCs did not form within their own dark halos. Recent observations of an extended stellar halo in the GC NGC 1851 are also interpreted in the context of our analytic models. Implications of these results for the formation of GCs are briefly discussed.

astro-ph.GA↗

arXiv2011

Formation of galactic nuclei with multiple supermassive black holes at high redshifts

We examine the formation of groups of multiple supermassive black holes (SMBHs) in gas-poor galactic nuclei due to the high merger rate of galaxies at high redshifts. We calculate the relative likelihood of binary, triple, and quadruple SMBH systems, by considering the timescales for relevant processes and combining merger trees with N-body simulations for the dynamics of stars and SMBHs in galactic nuclei. Typical haloes today with mass $M_0\approx 10^{14}$ M$_\odot$ have an average mass $M_{z=6}=5\times 10^{11}$ M$_\odot$ at $z\sim 6$, while rare haloes with current mass $M_0\gtrsim 10^{15}$ M$_\odot$ have an average mass $M_{z=6}=5\times 10^{12}$ M$_\odot$ at that redshift. These cluster-size haloes are expected to host single galaxies at $z\sim 6$. We expect about 30% galaxies within haloes with present-day mass $M_0\approx 10^{14}$ M$_\odot$ to contain more than two SMBHs at redshifts $2\lesssim z\lesssim 6$. For larger present-day haloes, with $M_0\gtrsim 10^{15}$ M$_\odot$, this fraction is almost 60%. The existence of multiple SMBHs at high redshifts can potentially explain the mass deficiencies observed in the cores of massive elliptical galaxies, which are up to 5 times the mass of their central BHs. Multiple SMBHs would also lead to an enhanced rate of tidal disruption of stars, modified gravitational wave signals compared to isolated BH binaries, and slingshot ejection of SMBHs from galaxies at high speeds in excess of 2000 km s$^{-1}$.

astro-ph.CO↗

arXiv2011

Constraining the Stellar Mass Function in the Galactic Center via Mass Loss from Stellar Collisions

The dense concentration of stars and high velocity dispersions in the Galactic centre imply that stellar collisions frequently occur. Stellar collisions could therefore result in significant mass loss rates. We calculate the amount of stellar mass lost due to indirect and direct stellar collisions and find its dependence on the present-day mass function of stars. We find that the total mass loss rate in the Galactic centre due to stellar collisions is sensitive to the present-day mass function adopted. We use the observed x-ray luminosity in the Galactic centre to preclude any present-day mass functions that result in mass loss rates $> 10^{-5} \mathrm{M_{\odot} yr^{-1}}$ in the vicinity of $\sim 1"$. For present-day mass functions of the form, $dN/dM \propto M^{-α}$, we constrain the present-day mass function to have a minimum stellar mass $\lesssim 7\mathrm{M_{\odot}}$ and a power law slope $\gtrsim 1.25$. We also use this result to constrain the initial mass function in the Galactic centre by considering different star formation scenarios.

astro-ph.GA↗

arXiv2011

Constraining the Structure of Sagittarius A*'s Accretion Flow with Millimeter-VLBI Closure Phases

Millimeter wave Very Long Baseline Interferometry (mm-VLBI) provides access to the emission region surrounding Sagittarius A*, the supermassive black hole at the center of the Milky Way, on sub-horizon scales. Recently, a closure phase of 0+-40 degrees was reported on a triangle of Earth-sized baselines (SMT-CARMA-JCMT) representing a new constraint upon the structure and orientation of the emission region, independent from those provided by the previously measured 1.3mm-VLBI visibility amplitudes alone. Here, we compare this to the closure phases associated with a class of physically motivated, radiatively inefficient accretion flow models, and present predictions for future mm-VLBI experiments with the developing Event Horizon Telescope (EHT). We find that the accretion flow models are capable of producing a wide variety of closure phases on the SMT-CARMA-JCMT triangle, and thus not all models are consistent with the recent observations. However, those models that reproduce the 1.3mm-VLBI visibility amplitudes overwhelmingly have SMT-CARMA-JCMT closure phases between +-30 degrees, and are therefore broadly consistent with all current mm-VLBI observations. Improving station sensitivity by factors of a few, achievable by increases in bandwidth and phasing together multiple antennas at individual sites, should result in physically relevant additional constraints upon the model parameters and eliminate the current 180 degree ambiguity on the source orientation. When additional stations are included, closure phases of order 45--90 degrees are typical. In all cases the EHT will be able to measure these with sufficient precision to produce dramatic improvements in the constraints upon the spin of Sgr A*.

astro-ph.HE↗

arXiv2011

Observational selection effects and the M-sigma relation

We examine the possibility that the observed relation between black-hole mass and host-galaxy stellar velocity dispersion (the M-sigma relation) is biased by an observational selection effect, the difficulty of detecting a black hole whose sphere of influence is smaller than the telescope resolution. In particular, we critically investigate recent claims that the M-sigma relation only represents the upper limit to a broad distribution of black-hole masses in galaxies of a given velocity dispersion. We find that this hypothesis can be rejected at a high confidence level, at least for the early-type galaxies with relatively high velocity dispersions (median 268 km/s) that comprise most of our sample. We also describe a general procedure for incorporating observational selection effects in estimates of the properties of the M-sigma relation. Applying this procedure we find results that are consistent with earlier estimates that did not account for selection effects, although with larger error bars. In particular, (i) the width of the M-sigma relation is not significantly increased; (ii) the slope and normalization of the M-sigma relation are not significantly changed; (iii) most or all luminous early-type galaxies contain central black holes at zero redshift. Our results may not apply to late-type or small galaxies, which are not well-represented in our sample.

astro-ph.CO↗

arXiv2011

The cosmic X-ray and gamma-ray background from dark matter annihilation

(Abridged) The extragalactic background light (EBL) observed at multiple wavelengths is a promising tool to probe the nature of dark matter since it might contain a significant contribution from gamma-rays produced promptly by dark matter annihilation. Additionally, the electrons and positrons produced in the annihilation give energy to the CMB photons to populate the EBL with X-rays and gamma-rays. We here create full-sky maps of the radiation from both of these contributions using the high-resolution Millennium-II simulation. We use upper limits on the contributions of unknown sources to the EBL to constrain the intrinsic properties of dark matter using a model-independent approach that can be employed as a template to test different particle physics models (including those with a Sommerfeld enhancement). These upper limits are based on observations spanning eight orders of magnitude in energy (from soft X-rays measured by CHANDRA to gamma-rays measured by Fermi), and on expectations for the contributions from blazars and star forming galaxies. To exemplify this approach, we analyze a set of benchmark Sommerfeld-enhanced models that give the correct dark matter abundance, satisfy CMB constraints, and fit the cosmic ray spectra measured by PAMELA and Fermi without any contribution from local subhalos. We find that these models are in conflict with the EBL constraints unless the contribution of unresolved subhalos is small and the annihilation signal dominates the EBL. We conclude that provided the collisionless cold dark matter paradigm is accurate, even for conservative estimates of the contribution from unresolved subhalos and astrophysical backgrounds, the EBL is at least as sensitive a probe of these types of scenarios as the CMB. Our results disfavor an explanation of the positron excess measured by PAMELA based only on dark matter annihilation in the smooth Galactic halo.

astro-ph.CO↗

arXiv2011

Tidal Disruption Flares of Stars From Moderately Recoiled Black Holes

We analyze stellar tidal disruption events as a possible observational signature of gravitational wave induced recoil of supermassive black holes. As a black hole wanders through its galaxy, it will tidally disrupt bound and unbound stars at rates potentially observable by upcoming optical transient surveys. To quantify these rates, we explore a broad range of host galaxy and black hole kick parameters. We find that emission from a transient accretion disk can produce ~1 event per year which LSST would identify as spatially offset, while super-Eddington tidal flares, if they exist, are likely to produce ~10 spatially offset events per year. A majority of tidal disruption flares, and a large majority of flares with an observable spatial offset, are due to bound rather than unbound stars. The total number of disruption events due to recoiled black holes could be almost 1% of the total stellar tidal disruption rate.

astro-ph.CO↗

arXiv2011

Measuring the Redshift of Reionization with a Modest Array of Low-Frequency Dipoles

The designs of the first generation of cosmological 21-cm observatories are split between single dipole experiments which integrate over a large patch of sky in order to find the global (spectral) signature of reionization, and interferometers with arcminute-scale angular resolution whose goal is to measure the 3D power spectrum of ionized regions during reionization. We examine whether intermediate scale instruments with complete Fourier (uv) coverage are capable of placing new constraints on reionization. We find that even without using a full power spectrum analysis, the global redshift of reionization, z_reion, can in principle be measured from the variance in the 21-cm signal among multiple beams as a function of frequency at a roughly 1 degree angular scale. At this scale, the beam-to-beam variance in the differential brightness temperature peaks when the average neutral fraction was around 50%, providing a convenient flag of z_reion. We choose a low angular resolution of order 1 degree to exploit the physical size of the ionized regions and maximize the signal-to-noise ratio. Thermal noise, foregrounds, and instrumental effects should also be manageable at this angular scale, as long as the uv coverage is complete within the compact core required for low-resolution imaging. For example, we find that z_reion can potentially be detected to within a redshift uncertainty of less than around 1 in around 500 hours of integration on the existing MWA prototype (with only 32x16 dipoles), operating at an angular resolution of around 1 degree and a spectral resolution of 2.4 MHz.

astro-ph.CO↗