arXiv subjects

Abraham Loeb

Publications and source records attributed to Abraham Loeb.

At least 559 records · Page 31Linked to original sources

arXiv2010

Using Radio Halos and Minihalos to Measure the Distributions of Magnetic Fields and Cosmic-Rays in Galaxy Clusters

Some galaxy clusters show diffuse radio emission in the form of giant halos (GHs) on Mpc scales or minihalos (MHs) on smaller scales. Comparing VLA and XMM radial profiles of several such clusters, we find a universal linear correlation between radio and X-ray surface brightness, valid in both types of halos. It implies a halo central emissivity nu*j_nu=10^{-31.4+-0.2}*(n/10^-2cm^-3)^2*(T/T_0)^{0.2+-0.5} erg/s/cm^3, where T and T_0 are the local and central temperatures, and n is the electron number density. We argue that the tight correlation and the scaling of j_nu, combined with morphological and spectral evidence, indicate that both GHs and MHs arise from secondary electrons and positrons, produced in cosmic-ray ion (CRI) collisions with a strongly magnetized, B >3 muG intracluster gas. When the magnetic energy density drops below that of the microwave background, the radio emission weakens considerably, producing halos with a clumpy morphology (e.g. RXCJ2003.5-2323 and A2255) or a distinct radial break. We thus measure a magnetic field B=3 muG at a radius r~110 kpc in A2029 and r~50 kpc in Perseus. The spectrum of secondaries, produced from hadronic collisions of ~20 GeV CRIs, reflects the energy dependence of the collision cross section. We use the observed spectra of halos, in particular where they steepen with increasing radius or frequency, to (i) measure B~10(nu/700 MHz) muG, with nu the spectral break frequency; (ii) identify a correlation between the average spectrum and the central magnetic field; and (iii) infer a CRI spectral index s <-2.7 and energy fraction xi_p~10^{-3.6+-0.2} at particle energies above 10 GeV. Our results favor a model where CRIs diffuse away from their sources (which are probably supernovae, according to a preliminary correlation with star formation), whereas the magnetic fields are generated by mergers in GHs and by core sloshing in MHs.

astro-ph.CO↗

arXiv2010

Measuring the 3D Clustering of Undetected Galaxies Through Cross Correlation of their Cumulative Flux Fluctuations from Multiple Spectral Lines

We discuss a method for detecting the emission from high redshift galaxies by cross correlating flux fluctuations from multiple spectral lines. If one can fit and subtract away the continuum emission with a smooth function of frequency, the remaining signal contains fluctuations of flux with frequency and angle from line emitting galaxies. Over a particular small range of observed frequencies, these fluctuations will originate from sources corresponding to a series of different redshifts, one for each emission line. It is possible to statistically isolate the fluctuations at a particular redshift by cross correlating emission originating from the same redshift, but in different emission lines. This technique will allow detection of clustering fluctuations from the faintest galaxies which individually cannot be detected, but which contribute substantially to the total signal due to their large numbers. We describe these fluctuations quantitatively through the line cross power spectrum. As an example of a particular application of this technique, we calculate the signal-to-noise ratio for a measurement of the cross power spectrum of the OI(63 micron) and OIII(52 micron) fine structure lines with the proposed Space Infrared Telescope for Cosmology and Astrophysics. We find that the cross power spectrum can be measured beyond a redshift of z=8. Such observations could constrain the evolution of the metallicity, bias, and duty cycle of faint galaxies at high redshifts and may also be sensitive to the reionization history through its effect on the minimum mass of galaxies. As another example, we consider the cross power spectrum of CO line emission measured with a large ground based telescope like CCAT and 21-cm radiation originating from hydrogen in galaxies after reionization with an interferometer similar in scale to MWA, but optimized for post-reionization redshifts.

astro-ph.CO↗

arXiv2010

Constraining the Minimum Mass of High-Redshift Galaxies and Their Contribution to the Ionization State of the IGM

We model the latest HST WFPC3/IR observations of > 100 galaxies at redshifts z=7-8 in terms of a hierarchical galaxy formation model with starburst activity. Our model provides a distribution of UV luminosities per dark matter halo of a given mass and a natural explanation for the fraction of halos hosting galaxies. The observed luminosity function is best fit with a minimum halo mass per galaxy of 10^{9.4+0.3-0.9} Msun, corresponding to a virial temperature of 10^{4.9+0.2-0.7} K. Extrapolating to faint, undetected galaxies, the total production rate of ionizing radiation depends critically on this minimum mass. Future measurements with JWST should determine whether the entire galaxy population can comfortably account for the UV background required to keep the intergalactic medium ionized.

astro-ph.CO↗

arXiv2010

Scale-Dependent Bias of Galaxies from Baryonic Acoustic Oscillations

Baryonic acoustic oscillations (BAOs) modulate the density ratio of baryons to dark matter across large regions of the Universe. We show that the associated variation in the mass-to-light ratio of galaxies should generate an oscillatory, scale-dependent bias of galaxies relative to the underlying distribution of dark matter. A measurement of this effect would calibrate the dependence of the characteristic mass-to-light ratio of galaxies on the baryon mass fraction in their large scale environment. This bias, though, is unlikely to significantly affect measurements of BAO peak positions.

astro-ph.CO↗

arXiv2010

Prompt Tidal Disruption of Stars as an Electromagnetic Signature of Supermassive Black Hole Coalescence

A precise electromagnetic measurement of the sky coordinates and redshift of a coalescing black hole binary holds the key for using its gravitational wave (GW) signal to constrain cosmological parameters and to test general relativity. Here we show that the merger of ~10^{6-7}M_sun black holes is generically followed over a period of years by multiple electromagnetic flares from tidally disrupted stars. The sudden recoil imparted to the merged black hole by GW emission promptly fills its loss cone and results in a tidal disruption rate of stars as high as ~0.1 per year. The prompt disruption of a star within a single galaxy over a short period provides a unique electromagnetic flag of a recent black hole coalescence event, and sequential disruptions could be used on their own to calibrate the expected rate of GW sources for pulsar timing arrays or the proposed Laser Interferometer Space Antenna (LISA).

astro-ph.CO↗

arXiv2010

Taking "The Road Not Taken'': On the Benefits of Diversifying Your Academic Portfolio

It is common practice among young astrophysicists these days to invest research time conservatively in mainstream ideas that have already been explored extensively in the literature. This tendency is driven by peer pressure and job market prospects, and is occasionally encouraged by senior researchers. Although the same phenomenon existed in past decades, it is alarmingly more prevalent today because a growing fraction of observational and theoretical projects are pursued in large groups with rigid research agendas. In addition, the emergence of a ``standard model'' in cosmology (albeit with unknown dark components) offers secure ``bonds'' for a safe investment of research time. In this short essay, which summarizes a banquet lecture at a recent conference, I give examples for both safe and risky topics in astrophysics (which I split into categories of ``bonds,'' ``stocks,'' and ``venture capital''), and argue that young researchers should always allocate a small fraction of their academic portfolio to innovative projects with risky but potentially highly profitable returns. In parallel, selection and promotion committees must find new strategies for rewarding candidates with creative thinking.

astro-ph.IM↗

arXiv2010

Dynamics and Magnetization in Galaxy Cluster Cores Traced by X-ray Cold Fronts

Cold fronts (CFs) - density and temperature plasma discontinuities - are ubiquitous in cool cores of galaxy clusters, where they appear as X-ray brightness edges in the intracluster medium, nearly concentric with the cluster center. We analyze the thermodynamic profiles deprojected across core CFs found in the literature. While the pressure appears continuous across these CFs, we find that all of them require significant centripetal acceleration beneath the front. This is naturally explained by a tangential, nearly sonic bulk flow just below the CF, and a tangential shear flow involving a fair fraction of the plasma beneath the front. Such shear should generate near-equipartition magnetic fields on scales ~<50 pc from the front, and could magnetize the entire core. Such fields would explain the apparent stability of cool-core CFs and the recently reported CF-radio minihalo association.

astro-ph.CO↗

arXiv2010

Constraining the unexplored period between reionization and the dark ages with observations of the global 21 cm signal

Observations of the frequency dependence of the global brightness temperature of the redshifted 21 cm line of neutral hydrogen may be possible with single dipole experiments. In this paper, we develop a Fisher matrix formalism for calculating the sensitivity of such instruments to the 21 cm signal from reionization and the dark ages. We show that rapid reionization histories with duration delta z< 2 can be constrained, provided that local foregrounds can be well modelled by low order polynomials. It is then shown that observations in the range nu = 50 - 100 MHz can feasibly constrain the Lyman alpha and X-ray emissivity of the first stars forming at z = 15 - 25, provided that systematic temperature residuals can be controlled to less than 1 mK. Finally, we demonstrate the difficulty of detecting the 21 cm signal from the dark ages before star formation.

astro-ph.CO↗

arXiv2010

Constraining Parity Violation in Gravity with Measurements of Neutron-Star Moments of Inertia

Neutron stars are sensitive laboratories for testing general relativity, especially when considering deviations where velocities are relativistic and gravitational fields are strong. One such deviation is described by dynamical, Chern-Simons modified gravity, where the Einstein-Hilbert action is modified through the addition of the gravitational parity-violating Pontryagin density coupled to a field. This four-dimensional effective theory arises naturally both in perturbative and non-perturbative string theory, loop quantum gravity, and generic effective field theory expansions. We calculate here Chern-Simons modifications to the properties and gravitational fields of slowly spinning neutron stars. We find that the Chern-Simons correction affects only the gravitomagnetic sector of the metric to leading order, thus introducing modifications to the moment of inertia but not to the mass-radius relation. We show that an observational determination of the moment of inertia to an accuracy of 10%, as is expected from near-future observations of the double pulsar, will place a constraint on the Chern-Simons coupling constant of ξ^{1/4} < 5 km, which is at least three-orders of magnitude stronger than the previous strongest bound.

gr-qc↗

arXiv2010

Galaxy Statistics in Pencil-beam Surveys at High Redshifts

Surveys of faint galaxies at high redshifts often result in a "pencil-beam" geometry that is much longer along the line-of-sight than across the sky. We explore the effects of this geometry on the abundance and clustering of Lyman-break galaxies (LBGs) and Lyman-alpha emitters (LAEs) in current and future surveys based on cosmological N-body simulations which adequately describe the nonlinear growth of structure on small scales and compare to linear theory. We find that the probability distribution of the LBG abundance is skewed toward low values since the narrow transverse dimension of the survey is more likely to probe underdense regions. Over a range that spans 1--2 orders of magnitude in galaxy luminosities, the variance in the number of objects differs from the commonly used analytic prediction and is not dominated by Poisson noise. Additionally, nonlinear bias on small scales results in a one-dimensional power spectrum of LAEs using a James Webb Space Telescope field-of-view that is relatively flat, markedly different from the expectation of linear perturbation theory. We discuss how these results may affect attempts to measure the UV background at high redshifts, estimate the relationship between halo mass and galaxy luminosity, and probe reionization by measuring the power-modulating effect of ionized regions.

astro-ph.CO↗

arXiv2009

The Pulsar Contribution to the Gamma-Ray Background

We estimate the contribution of Galactic pulsars, both ordinary and millisecond pulsars (MSPs), to the high-energy (>100 MeV) gamma-ray background. We pay particular attention to the high-latitude part of the background that could be confused with an extragalactic component in existing analyses that subtract a Galactic cosmic-ray model. Our pulsar population models are calibrated to the results of large-scale radio surveys and we employ a simple empirical gamma-ray luminosity calibration to the spin-down rate that provides a good fit to existing data. We find that while ordinary pulsars are expected to contribute only a fraction ~10^-3 of the high-latitude gamma-ray intensity (I_X~1x10^-5 ph s^-1 cm^-2 sr^-1), MSPs could provide a much larger contribution and even potentially overproduce it, depending on the model parameters. We explore these dependences using a range of MSP models as a guide to how gamma-ray measurements can usefully constrain the MSP population. Existing gamma-ray background measurements and source counts already rule out several models. Finally, we show how fluctuations in the gamma-ray sky can be used to distinguish between different sources of the background.

astro-ph.HE↗

arXiv2009

An Electromagnetic Signature of Galactic Black Hole Binaries That Enter Their Gravitational-Wave Induced Inspiral

Mergers of gas-rich galaxies lead to black hole binaries that coalesce as a result of dynamical friction on the ambient gas. Once the binary tightens to <10^3 Schwarzschild radii, its merger is driven by the emission of gravitational waves (GWs). We show that this transition occurs generically at orbital periods of ~1-10 years and an orbital velocity V of a few thousand km/s, with a very weak dependence on the supply rate of gas (V proportional to Mdot^{1/8}). Therefore, as binaries enter their GW-dominated inspiral, they inevitably induce large periodic shifts in the broad emission lines of any associated quasar(s). The probability of finding a binary in tighter configurations scales as V^{-8} owing to their much shorter lifetimes. Systematic monitoring of the broad emission lines of quasars on timescales of months to decades can set a lower limit on the expected rate of GW sources for LISA.

astro-ph.CO↗

arXiv2009

Cosmological constraints from 21cm surveys after reionization

21cm emission from residual neutral hydrogen after the epoch of reionization can be used to trace the cosmological power spectrum of density fluctuations. Using a Fisher matrix formulation, we provide a detailed forecast of the constraints on cosmological parameters that are achievable with this probe. We consider two designs: a scaled-up version of the MWA observatory as well as a Fast Fourier Transform Telescope. We find that 21cm observations dedicated to post-reionization redshifts may yield significantly better constraints than next generation Cosmic Microwave Background (CMB) experiments. We find the constraints on $Ω_Λ$, $Ω_{\rm m}h^2$, and $Ω_νh^2$ to be the strongest, each improved by at least an order of magnitude over the Planck CMB satellite alone for both designs. Our results do not depend as strongly on uncertainties in the astrophysics associated with the ionization of hydrogen as similar 21cm surveys during the epoch of reionization. However, we find that modulation of the 21cm power spectrum from the ionizing background could potentially degrade constraints on the spectral index of the primordial power spectrum and its running by more than an order of magnitude. Our results also depend strongly on the maximum wavenumber of the power spectrum which can be used due to non-linearities.

astro-ph↗

arXiv2009

Detecting Changing Polarization Structures in Sagittarius A* with High Frequency VLBI

Sagittarius A* is the source of near infrared, X-ray, radio, and (sub)millimeter emission associated with the supermassive black hole at the Galactic Center. In the submillimeter regime, Sgr A* exhibits time-variable linear polarization on timescales corresponding to <10 Schwarzschild radii of the presumed 4 million solar mass black hole. In previous work, we demonstrated the potential for total-intensity (sub)millimeter-wavelength VLBI to detect time-variable -- and periodic -- source structure changes in the Sgr A* black hole system using nonimaging analyses. Here we extend this work to include full polarimetric VLBI observations. We simulate full-polarization (sub)millimeter VLBI data of Sgr A* using a hot-spot model that is embedded within an accretion disk, with emphasis on nonimaging polarimetric data products that are robust against calibration errors. Although the source-integrated linear polarization fraction in the models is typically only a few percent, the linear polarization fraction on small angular scales can be much higher, enabling the detection of changes in the polarimetric structure of Sgr A* on a wide variety of baselines. The shortest baselines track the source-integrated linear polarization fraction, while longer baselines are sensitive to polarization substructures that are beam-diluted by connected-element interferometry. The detection of periodic variability in source polarization should not be significantly affected even if instrumental polarization terms cannot be calibrated out. As more antennas are included in the (sub)mm-VLBI array, observations with full polarization will provide important new diagnostics to help disentangle intrinsic source polarization from Faraday rotation effects in the accretion and outflow region close to the black hole event horizon.

astro-ph.GA↗

arXiv2009

Lyman Alpha Blobs as an Observational Signature of Cold Accretion Streams into Galaxies

Recent hydrodynamic simulations of galaxy formation reveal streams of cold (T ~ 1e4 K) gas flowing into the centers of dark matter halos as massive as 1e12-1e13.5 M_sun at redshifts z~1-3. In this paper we show that if > 20% of the gravitational binding energy of the gas is radiated away, then the simulated cold flows are spatially extended Lyman Alpha (Lya) sources with luminosities, Lya line widths, and number densities that are comparable to those of observed Lya blobs. Furthermore, the filamentary structure of the cold flows can explain the wide range of observed Lya blob morphologies. Since the most massive halos form in dense environments, the association of Lya blobs with overdense regions arise naturally. We argue that Lya blobs - even those which are clearly associated with starburst galaxies or quasars - provide direct observational support for the cold accretion mode of galaxies. We discuss various testable predictions of this association.

astro-ph.CO↗

arXiv2009

Will the Large Synoptic Survey Telescope detect extra-solar planetesimals entering the solar system?

Planetesimal formation is a common by-product of the star formation process. Taking the dynamical history of the Solar system as a guideline -- in which the planetesimal belts were heavily depleted due to graviational perburbation with the giant planets -- and assuming similar processes have take place in other planetary systems, one would expect the interestellar space to be filled with extra-solar planetesimals. However, not a single one of these objects has been detected so far entering the Solar system, even though it would clearly be distinguishable from a Solar system comet due to its highly hyperbolic orbit. The Large Synoptic Survey Telescope (LSST) will provide wide coverage maps of the sky to a very high sensitivity, ideal to detect moving objects like comets, both active and inactive. In anticipation of these observations, we estimate how many inactive "interstellar comets" might be detected during the duration of the survey. The calculation takes into account estimates (from observations and models) of the number density of stars, the amount of solids available to form planetesimals, the frequency of planet and planetesimal formation, the efficiency of planetesimal ejection, and the possible size distribution of these small bodies.

astro-ph.EP↗

arXiv2009

Probing the Epoch of Reionization with Milky Way Satellites

While the connection between high-redshift star formation and the local universe has recently been used to understand the observed population of faint dwarf galaxies in the Milky Way (MW) halo, we explore how well these nearby objects can probe the epoch of first light. We construct a detailed, physically motivated model for the MW satellites based on the state-of-the-art Via Lactea II dark-matter simulations. Our model incorporates molecular hydrogen (H_2) cooling in low-mass systems and inhomogeneous photo-heating feedback during the internal reionization of our own galaxy. We find that the existence of MW satellites fainter than M_V ~ -5 is strong evidence for H_2 cooling in low-mass halos, while satellites with -5 > M_V > -9 were affected by hydrogen cooling and photoheating feedback. The age of stars in very low-luminosity systems and the minimum luminosity of these satellites are key predictions of our model. Most of the stars populating the brightest MW satellites could have formed after the epoch of reionization. Our models also predict a significantly larger dispersion in M_300 values than observed and a number of luminous satellites with M_300 as low as 10^6 solar masses.

astro-ph.CO↗

arXiv2009

Constraining reionization using 21 cm observations in combination with CMB and Lyman-alpha forest data

In this paper, we explore the constraints on the reionization history that are provided by current observations of the Lyman-alpha forest and the CMB. Rather than using a particular semi-analytic model, we take the novel approach of parametrizing the ionizing sources with arbitrary functions, and perform likelihood analyses to constrain possible reionization histories. We find model independent conclusions that reionization is likely to be mostly complete by z=8 and that the IGM was 50% ionized at z=9-10. Upcoming low-frequency observations of the redshifted 21 cm line of neutral hydrogen are expected to place significantly better constraints on the hydrogen neutral fraction at 6<z<12. We use our constraints on the reionization history to predict the likely amplitude of the 21 cm power spectrum and show that observations with the highest signal-to-noise ratio will most likely be made at frequencies corresponding to z=9-10. This result provides an important guide to the upcoming 21 cm observations. Finally, we assess the impact that measurement of the neutral fraction will have on our knowledge of reionization and the early source population. Our results show that a single measurement of the neutral fraction mid-way through the reionization era will significantly enhance our knowledge of the entire reionization history.

astro-ph.CO↗