arXiv subjects

Abraham Loeb

Publications and source records attributed to Abraham Loeb.

At least 577 records · Page 32Linked to original sources

arXiv2009

Signatures of Relativistic Helical Motion in the Rotation Measures of AGN Jets

Polarization has proved an invaluable tool for probing magnetic fields in relativistic jets. Maps of the intrinsic polarization vectors have provided the best evidence to date for uniform, toroidally dominated magnetic fields within jets. More recently, maps of the rotation measure (RM) in jets have for the first time probed the field geometry of the cool, moderately relativistic surrounding material. In most cases, clear signatures of toroidal magnetic field are detected, corresponding to gradients in RM profiles transverse to the jet. However, in many objects these profiles also display marked asymmetries which are difficult to explain in simple helical jet models. Furthermore, in some cases the RM profiles are strongly frequency and/or time dependent. Here we show that these features may be naturally accounted for by including relativistic helical motion in the jet model. In particular, we are able to reproduce bent RM profiles observed in a variety of jets, frequency dependent RM profile morphologies and even the time dependence of the RM profiles of knots in 3C 273. Finally, we predict that some sources may show reversals in their RM profiles at sufficiently high frequencies, depending upon the the ratio of the components of jet sheath velocity transverse and parallel to the jet. Thus, multi-frequency RM maps promise a novel way in which to probe the velocity structure of relativistic outflows.

astro-ph.HE↗

arXiv2009

The 21cm Power Spectrum After Reionization

We discuss the 21cm power spectrum (PS) following the completion of reionization. In contrast to the reionization era, this PS is proportional to the PS of mass density fluctuations, with only a small modulation due to fluctuations in the ionization field on scales larger than the mean-free-path of ionizing photons. We derive the form of this modulation, and demonstrate that its effect on the 21cm PS will be smaller than 1% for physically plausible models of damped Ly-alpha systems. In contrast to the 21cm PS observed prior to reionization, in which HII regions dominate the ionization structure, the simplicity of the 21cm PS after reionization will enhance its utility as a cosmological probe by removing the need to separate the PS into physical and astrophysical components. As a demonstration, we consider the Alcock-Paczynski test and show that the next generation of low-frequency arrays could measure the angular distortion of the PS at the percent level for z~3-5.

astro-ph↗

arXiv2009

Probing the Epoch of Reionization with the Lyman Alpha Forest at z~4-5

The inhomogeneous cosmological reionization process leaves tangible imprints in the intergalactic medium down to z=4-5. The Lyman-alpha forest flux power spectrum provides a potentially powerful probe of the epoch of reionization. With the existing SDSS I/II quasar sample we show that two cosmological reionization scenarios, one completing reionization at z=6 and the other at z=9, can be distinguished at ~7 sigma level by utilizing Lyman alpha forest absorption spectra at z=4.5+-0.5, in the absence of other physical processes that may also affect the Lyman alpha flux power spectrum. The redshift range z=4-5 may provide the best window, because there is still enough transmitted flux and quasars to measure precise statistics of the flux fluctuations, and the IGM still retains a significant amount of memory of reionization.

astro-ph.CO↗

arXiv2009

Concentrating the Dark Matter in Galaxy Clusters through Tidal Stripping of Baryonically-Compressed Galactic Halos

Gravitational lensing observations of massive X-ray clusters imply a steep characteristic density profile marked by a central concentration of dark matter. The observed mass fraction within a projected radius of 150 kpc is twice that found in state-of-the-art dark matter simulations of the standard Lambda-CDM cosmology. A central baryon enhancement that could explain this discrepancy is not observed, leaving a major puzzle. We propose a solution based on the merger histories of clusters. A significant fraction of the final dark matter content of a cluster halo originates within galaxy-sized halos, in which gas can cool and compress the dark matter core to high densities. The subsequent tidal stripping of this compressed dark matter occurs in denser regions that are closer to the center of the cluster halo. Eventually, the originally cooled gas must be dispersed into the intracluster medium through feedback, for consistency with observations that do not find central baryon enhancements in clusters. Still, the early adiabatic compression of the galactic dark matter leaves a net effect on the cluster. Using a simple model for this process, we show that the central cluster profile is substantially modified, potentially explaining the observed discrepancy.

astro-ph.CO↗

arXiv2009

The Kinetic Sunyaev-Zel'dovich effect of the Milky Way Halo

We calculate the expected imprint of the ionized gas in the Milky-Way halo on the Cosmic Microwave Background (CMB) through the kinetic Sunyaev-Zel'dovich (kSZ) effect. Unlike other Galactic foregrounds, the halo kSZ signature covers the full sky, generates anisotropies on large angular scales, is not accompanied by spectral distortions, and could therefore be confused with primordial CMB anisotropies. We construct theoretical models for various halo components, including smooth diffuse gas, filaments of cold inflowing gas and high velocity clouds. We find that the kSZ effect for all components is above the sensitivity of the Planck satellite, over a range of angular scales. However, the typical halo contribution is well below the cosmic variance noise in the primordial CMB power spectrum. High velocity clouds could dominate the halo contribution and better observational data is required to mask them out. We derive expected kSZ maps based on existing data from tracers of the halo gas distribution, such as 21cm maps of neutral hydrogen and H_alpha maps of recombining gas. The cross-correlation of these maps with the WMAP5 data does not yield any statistically significant signal.

astro-ph.CO↗

arXiv2009

The hyperfine transition of 3He+ as a probe of the intergalactic medium

We explore the prospects of using the hyperfine transition of 3He+ as a probe of the intergalactic medium. The emission signal from ionized regions during reionization is expected to be anti-correlated with 21cm maps. The predicted emission signal from Lyman-alpha blobs at lower redshifts is detectable with future radio observatories.

astro-ph.CO↗

arXiv2009

Star Clusters Around Recoiled Black Holes in the Milky Way Halo

Gravitational wave emission by coalescing black holes (BHs) kicks the remnant BH with a typical velocity of hundreds of km/s. This velocity is sufficiently large to remove the remnant BH from a low-mass galaxy but is below the escape velocity from the Milky Way (MW) galaxy. If central BHs were common in the galactic building blocks that merged to make the MW, then numerous BHs that were kicked out of low-mass galaxies should be freely floating in the MW halo today. We use a large statistical sample of possible merger tree histories for the MW to estimate the expected number of recoiled BH remnants present in the MW halo today. We find that hundreds of BHs should remain bound to the MW halo after leaving their parent low-mass galaxies. Each BH carries a compact cluster of old stars that populated the core of its original host galaxy. Using the time-dependent Fokker-Planck equation, we find that present-day clusters are ~< 1 pc in size, and their central bright regions should be unresolved in most existing sky surveys. These compact systems are distinguishable from globular clusters by their internal (Keplerian) velocity dispersion greater than one hundred km/s and their high mass-to-light ratio owing to the central BH. An observational discovery of this relic population of star clusters in the MW halo, would constrain the formation history of the MW and the dynamics of BH mergers in the early Universe. A similar population should exist around other galaxies, and may potentially be detectable in M31 and M33.

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arXiv2009

Gravitational waves from scattering of stellar-mass black holes in galactic nuclei

Stellar mass black holes (BHs) are expected to segregate and form a steep density cusp around supermassive black holes (SMBHs) in galactic nuclei. We follow the evolution of a multi-mass system of BHs and stars by numerically integrating the Fokker-Planck energy diffusion equations for a variety of BH mass distributions. We find that the BHs "self-segregate'', and that the rarest, most massive BHs dominate the scattering rate closest to the SMBH (< 0.1 pc). BH--BH binaries form out of gravitational wave emission during BH encounters. We find that the expected rate of BH coalescence events detectable by Advanced LIGO is ~1 - 100/yr, depending on the initial mass function of stars in galactic nuclei and the mass of the most massive BHs. We find that the actual merger rate is likely ~10 times larger than this due to the intrinsic scatter of stellar densities in many different galaxies. The BH binaries that form this way in galactic nuclei have significant eccentricities as they enter the LIGO band (90% with e > 0.9), and are therefore distinguishable from other binaries, which circularise before becoming detectable. We also show that eccentric mergers can be detected to larger distances and greater BH masses than circular mergers, up to ~ 700 solar masses. Future ground-based gravitational wave observatories will be able to constrain both the mass function of BHs and stars in galactic nuclei.

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arXiv2009

The Event Horizon of Sagittarius A*

Black hole event horizons, causally separating the external universe from compact regions of spacetime, are one of the most exotic predictions of General Relativity (GR). Until recently, their compact size has prevented efforts to study them directly. Here we show that recent millimeter and infrared observations of Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, all but requires the existence of a horizon. Specifically, we show that these observations limit the luminosity of any putative visible compact emitting region to below 0.4% of Sgr A*'s accretion luminosity. Equivalently, this requires the efficiency of converting the gravitational binding energy liberated during accretion into radiation and kinetic outflows to be greater than 99.6%, considerably larger than those implicated in Sgr A*, and therefore inconsistent with the existence of such a visible region. Finally, since we are able to frame this argument entirely in terms of observable quantities, our results apply to all geometric theories of gravity that admit stationary solutions, including the commonly discussed f(R) class of theories.

astro-ph.HE↗

arXiv2009

Estimating the Parameters of Sgr A*'s Accretion Flow Via Millimeter VLBI

Recent millimeter-VLBI observations of Sagittarius A* (Sgr A*) have, for the first time, directly probed distances comparable to the horizon scale of a black hole. This provides unprecedented access to the environment immediately around the horizon of an accreting black hole. We leverage both existing spectral and polarization measurements and our present understanding of accretion theory to produce a suite of generic radiatively inefficient accretion flow (RIAF) models of Sgr A*, which we then fit to these recent millimeter-VLBI observations. We find that if the accretion flow onto Sgr A* is well described by a RIAF model, the orientation and magnitude of the black hole's spin is constrained to a two-dimensional surface in the spin, inclination, position angle parameter space. For each of these we find the likeliest values and their 1-sigma & 2-sigma errors to be a=0(+0.4+0.7), inclination=50(+10+30)(-10-10) degrees, and position angle=-20(+31+107)(-16-29) degrees, when the resulting probability distribution is marginalized over the others. The most probable combination is a=0(+0.2+0.4), inclination=90(-40-50) degrees and position angle=-14(+7+11)(-7-11) degrees, though the uncertainties on these are very strongly correlated, and high probability configurations exist for a variety of inclination angles above 30 degrees and spins below 0.99. Nevertheless, this demonstrates the ability millimeter-VLBI observations, even with only a few stations, to significantly constrain the properties of Sgr A*.

astro-ph↗

arXiv2009

Imaging the Black Hole Silhouette of M87: Implications for Jet Formation and Black Hole Spin

The silhouette cast by the horizon of the supermassive black hole in M87 can now be resolved with the emerging millimeter very-long baseline interferometry (VLBI) capability. Despite being ~2000 times farther away than SgrA* (the supermassive black hole at the center of the Milky-Way and the primary target for horizon-scale imaging), M87's much larger black hole mass results in a horizon angular scale roughly half that of SgrA*'s, providing another practical target for direct imaging. However, unlike SgrA*, M87 exhibits a powerful radio jet, providing an opportunity to study jet formation physics on horizon scales. We employ a simple, qualitatively correct force-free jet model to explore the expected high-resolution images of M87 at wavelengths of 1.3mm and 0.87mm (230GHz and 345GHz), for a variety of jet parameters. We show that future VLBI data will be able to constrain the size of the jet footprint, the jet collimation rate, and the black hole spin. Polarization will further probe the structure of the jet's magnetic field and its effect on the emitting gas. Horizon-scale imaging of M87 and SgrA* will enable for the first time the empirical exploration of the relationship between the mass and spin of a black hole and the characteristics of the gas inflow/outflow around it.

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arXiv2009

The Race Between Stars and Quasars in Reionizing Cosmic Hydrogen

The cosmological background of ionizing radiation has been dominated by quasars once the Universe aged by ~2 billion years. At earlier times (redshifts z>3), the observed abundance of bright quasars declined sharply, implying that cosmic hydrogen was reionized by stars instead. Here, we explain the physical origin of the transition between the dominance of stars and quasars as a generic feature of structure formation in the concordance LCDM cosmology. At early times, the fraction of baryons in galaxies grows faster than the maximum (Eddington-limited) growth rate possible for quasars. As a result, quasars were not able to catch up with the rapid early growth of stellar mass in their host galaxies.

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arXiv2009

Upper Limit on Dimming of Cosmological Sources by Intergalactic Grey Dust from the Soft X-ray Background

Active Galactic Nuclei (AGN) produce a dominant fraction (~80%) of the Soft X-ray background (SXB) at photon energies 0.5<E<2 keV. If dust pervaded throughout the intergalactic medium, its scattering opacity would have produced diffuse X-ray halos around AGN. Taking account of known galaxies and galaxy clusters, only a fraction F_halo <10% of the SXB can be in the form of diffuse X-ray halos around AGN. We therefore limit the intergalactic opacity to optical/infrared photons from large dust grains (with radii in the range a=0.2-2.0 mum) to a level tau_GD<0.15(F_halo/10%) to a redshift z~1. Our results are only weakly dependent on the grain size distribution or the redshift evolution of the intergalactic dust. Stacking X-ray images of AGN can be used to improve our constraints and diminish the importance of dust as a source of systematic uncertainty for future supernova surveys which aim to improve the precision on measuring the redshift evolution of the dark energy equation-of-state.

astro-ph.CO↗

arXiv2009

Acceleration of Galactic Supershells by Lyman Alpha Radiation

Scattering of Lyman Alpha (hereafter Lya) photons by neutral hydrogen gas in a single outflowing 'supershell' around star forming regions often explains the shape and offset of the observed Lya emission line from galaxies. We compute the radiation pressure that is exerted by this scattered Lya radiation on the outflowing material. We show that for reasonable physical parameters, Lya radiation pressure alone can accelerate supershells to velocities in the range v_sh=200-400 km/s. These supershells possibly escape from the gravitational potential well of their host galaxies and contribute to the enrichment of the intergalactic medium. We compute the physical properties of expanding supershells that are likely to be present in a sample of known high-redshift (z=2.7-5.0) galaxies, under the assumption that they are driven predominantly by Lya radiation pressure. We predict ranges of radii r_sh=0.1-10 kpc, ages t_sh=1-100 Myr, and energies E_sh=1e53-1e55 ergs, which are in reasonable agreement with the properties of local galactic supershells. Furthermore, we find that the radius, r_sh, of a Lya-driven supershell of constant mass depends uniquely on the intrinsic Lya luminosity of the galaxy, L_alpha, the HI column density of the supershell, N_HI, and the shell speed, v_sh, through the scaling relation r_sh ~ L_alpha/(N_HI v_sh^2). We derive mass outflow rates in supershells that reach ~10-100% of the star formation rates of their host galaxies.

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arXiv2009

In Situ Probes of the First Galaxies and Reionization: Gamma-ray Bursts

The first structures in the Universe formed at z>7, at higher redshift than all currently known galaxies. Since GRBs are brighter than other cosmological sources at high redshift and exhibit simple power-law afterglow spectra that is ideal for absorption studies, they serve as powerful tools for studying the early universe. New facilities planned for the coming decade will be able to obtain a large sample of high-redshift GRBs. Such a sample would constrain the nature of the first stars, galaxies, and the reionization history of the Universe.

astro-ph.CO↗

arXiv2009

Strong gravitational lensing probes of the particle nature of dark matter

There is a vast menagerie of plausible candidates for the constituents of dark matter, both within and beyond extensions of the Standard Model of particle physics. Each of these candidates may have scattering (and other) cross section properties that are consistent with the dark matter abundance, BBN, and the most scales in the matter power spectrum; but which may have vastly different behavior at sub-galactic "cutoff" scales, below which dark matter density fluctuations are smoothed out. The only way to quantitatively measure the power spectrum behavior at sub-galactic scales at distances beyond the local universe, and indeed over cosmic time, is through probes available in multiply imaged strong gravitational lenses. Gravitational potential perturbations by dark matter substructure encode information in the observed relative magnifications, positions, and time delays in a strong lens. Each of these is sensitive to a different moment of the substructure mass function and to different effective mass ranges of the substructure. The time delay perturbations, in particular, are proving to be largely immune to the degeneracies and systematic uncertainties that have impacted exploitation of strong lenses for such studies. There is great potential for a coordinated theoretical and observational effort to enable a sophisticated exploitation of strong gravitational lenses as direct probes of dark matter properties. This opportunity motivates this white paper, and drives the need for: a) strong support of the theoretical work necessary to understand all astrophysical consequences for different dark matter candidates; and b) tailored observational campaigns, and even a fully dedicated mission, to obtain the requisite data.

astro-ph.CO↗

arXiv2009

Cosmology from the Highly-Redshifted 21 cm Line

One of the next decade's most exciting prospects is to explore the cosmic "dark ages," during which the first stars in the Universe formed, with the 21 cm line of neutral hydrogen. At z>6, this light redshifts into the low-frequency radio regime and can be observed with new generations of wide-field arrays. These experiments have the potential to observe enormous volumes and to improve both cosmological and astrophysical constraints dramatically. In this white paper, we describe how the next decade will see the first steps toward unlocking this potential, allowing us to answer two key questions about the fundamental physics of our Universe and the intergalactic medium: Does the standard cosmological model describe the Universe during the "dark ages?" How did the intergalactic medium evolve during this important time, ending with the reionization of hydrogen?

astro-ph.CO↗

arXiv2009

Astrophysics from the Highly-Redshifted 21 cm Line

The cosmic dark ages and the epoch of reionization, during which the first generations of stars and galaxies formed, are among the most compelling frontiers of extragalactic astrophysics and cosmology. Here we describe an exciting new probe of these eras: the 21 cm line of neutral hydrogen, which will allow us to map the neutral intergalactic medium throughout the era of first galaxy formation. In this white paper, we describe how these studies can answer two key questions about galaxy formation: What were the properties of high-redshift galaxies? How did they affect the Universe around them?

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