arXiv subjects

Eli Visbal

Publications and source records attributed to Eli Visbal.

65 records · Page 4Linked to original sources

arXiv2014

A No-Go Theorem for Direct Collapse Black Holes Without a Strong Ultraviolet Background

Explaining the existence of supermassive black holes (SMBHs) larger than $\sim 10^9 M_\odot$ at redshifts $z >\sim 6$ remains an open theoretical question. One possibility is that gas collapsing rapidly in pristine atomic cooling halos ($T_{\rm vir} >\sim 10^4 \rm{K}$) produces $10^4-10^6 M_\odot$ black holes. Previous studies have shown that the formation of such a black hole requires a strong UV background to prevent molecular hydrogen cooling and gas fragmentation. Recently it has been proposed that a high UV background may not be required for halos that accrete material extremely rapidly or for halos where gas cooling is delayed due to a high baryon-dark matter streaming velocity. In this work, we point out that building up a halo with $T_{\rm vir} >\sim 10^4 \rm{K}$ before molecular cooling becomes efficient is not sufficient for forming a direct collapse black hole (DCBH). Though molecular hydrogen formation may be delayed, it will eventually form at high densities leading to efficient cooling and fragmentation. The only obvious way that molecular cooling could be avoided in the absence of strong UV radiation, is for gas to reach high enough density to cause collisional dissociation of molecular hydrogen ($\sim 10^4 ~ {\rm cm}^{-3}$) before cooling occurs. However, we argue that the minimum core entropy, set by the entropy of the intergalactic medium (IGM) when it decouples from the CMB, prevents this from occurring for realistic halo masses. This is confirmed by hydrodynamical cosmological simulations without radiative cooling. We explain the maximum density versus halo mass in these simulations with simple entropy arguments. The low densities found suggest that DCBH formation indeed requires a strong UV background.

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arXiv2014

The observable signature of late heating of the Universe during cosmic reionization

Models and simulations of the epoch of reionization predict that spectra of the 21-cm transition of atomic hydrogen will show a clear fluctuation peak, at a redshift and scale, respectively, that mark the central stage of reionization and the characteristic size of ionized bubbles. This is based on the assumption that the cosmic gas was heated by stellar remnants - particularly X-ray binaries - to temperatures well above the cosmic microwave background at that time (~ 30 K). Here we show instead that the hard spectra (that is, spectra with more high-energy photons than low-energy photons) of X-ray binaries make such heating ineffective, resulting in a delayed and spatially uniform heating that modifies the 21-cm signature of reionization. Rather than looking for a simple rise and fall of the large-scale fluctuations (peaking at several millikelvin), we must expect a more complex signal also featuring a distinct minimum (at less than a millikelvin) that marks the rise of the cosmic mean gas temperature above the microwave background. Observing this signal, possibly with radio telescopes in operation today, will demonstrate the presence of a cosmic background of hard X-rays at that early time.

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arXiv2013

Complete history of the observable 21-cm signal from the first stars during the pre-reionization era

We present the first complete calculation of the history of the inhomogeneous 21-cm signal from neutral hydrogen during the era of the first stars. We use hybrid computational methods to capture the large-scale distribution of the first stars, whose radiation couples to the neutral hydrogen emission, and to evaluate the 21-cm signal from z ~ 15-35. In our realistic picture large-scale fluctuations in the 21-cm signal are sourced by the inhomogeneous density field and by the Ly-alpha and X-ray radiative backgrounds. The star formation is suppressed by two spatially varying effects: negative feedback provided by the Lyman-Werner radiative background, and supersonic relative velocities between the gas and dark matter. Our conclusions are quite promising: we find that the fluctuations imprinted by the inhomogeneous Ly-alpha background in the 21-cm signal at z ~ 25 should be detectable with the Square Kilometer Array.

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arXiv2012

The 21-cm signature of the first stars during the Lyman-Werner feedback era

The formation of the first stars is an exciting frontier area in astronomy. Early redshifts z ~ 20 have become observationally promising as a result of a recently recognized effect of a supersonic relative velocity between the dark matter and gas. This effect produces prominent structure on 100 comoving Mpc scales, which makes it much more feasible to detect 21-cm fluctuations from the epoch of first heating. We use semi-numerical hybrid methods to follow for the first time the joint evolution of the X-ray and Lyman-Werner radiative backgrounds, including the effect of the supersonic streaming velocity on the cosmic distribution of stars. We incorporate self-consistently the negative feedback on star formation induced by the Lyman-Werner radiation, which dissociates molecular hydrogen and thus suppresses gas cooling. We find that the feedback delays the X-ray heating transition by a Delta z ~ 2, but leaves a promisingly large fluctuation signal over a broad redshift range. The large-scale power spectrum is predicted to reach a maximal signal-to-noise ratio of S/N ~ 3-4 at z ~ 18 (for a projected first-generation instrument), with S/N > 1 out to z ~ 22-23. We hope to stimulate additional numerical simulations as well as observational efforts focused on the epoch prior to cosmic reionization.

astro-ph.CO↗

arXiv2012

The signature of the first stars in atomic hydrogen at redshift 20

Dark and baryonic matter moved at different velocities in the early Universe, which strongly suppressed star formation in some regions. This was estimated to imprint a large-scale fluctuation signal of about 2 mK in the 21-cm spectral line of atomic hydrogen associated with stars at a redshift of 20, although this estimate ignored the critical contribution of gas heating due to X-rays and major enhancements of the suppression. A large velocity difference reduces the abundance of halos and requires the first stars to form in halos of about a million solar masses, substantially greater than previously expected. Here we report a simulation of the distribution of the first stars at z=20 (cosmic age of ~180 Myr), incorporating all these ingredients within a 400 Mpc box. We find that the 21-cm signature of these stars is an enhanced (10 mK) fluctuation signal on the 100-Mpc scale, characterized by a flat power spectrum with prominent baryon acoustic oscillations. The required sensitivity to see this signal is achievable with an integration time of a thousand hours with an instrument like the Murchison Wide-field Array or the Low Frequency Array but designed to operate in the range of 50-100 MHz.

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arXiv2012

A Simple Model for the Density Profiles of Isolated Dark Matter Halos

We explore the possibility that the density profiles of elliptical galaxies and cold dark matter (CDM) halos found in cosmological simulations can be understood in terms of the same physical process, collisionless gravitational collapse. To investigate this, we study a simplified model, the collapse of a perfectly cold Plummer sphere. First, we examine an N-body simulation of this model with particles constrained to purely radial orbits. This results in a final state characterized by a profile slightly steeper than ρ\propto r^{-2} at small radii and behaving as ρ\propto r^{-4} at large radii, which can be understood in terms of simple analytic arguments. Next, we repeat our simulation without the restriction of radial orbits. This results in a shallower inner density profile, like those found in elliptical galaxies and CDM halos. We attribute this change to the radial orbit instability (ROI) and propose a form of the distribution function (DF) motivated by a physical picture of collapse. As evidence of the link between our model and CDM halos, we find that our collapse simulation has a final state with pseudo-phase-space density which scales roughly as ρ/σ^3 \propto r^{-1.875}, like that observed in CDM halos from cosmological simulations (Navarro et al. 2010). The velocity anisotropy profile is also qualitatively similar to that found near the centers of these halos. We argue that the discrepancy at large radii (where CDM halos scale as ρ\propto r^{-3}) is due to the presence of the cosmological background or continued infall. This leads us to predict that the outer CDM halo density profile is not "universal," but instead depends on cosmological environment (be it an underdense void or overdense region).

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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.

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arXiv2011

Demonstrating the Feasibility of Line Intensity Mapping Using Mock Data of Galaxy Clustering from Simulations

Visbal & Loeb (2010) have shown that it is possible to measure the clustering of galaxies by cross correlating the cumulative emission from two different spectral lines which originate at the same redshift. Through this cross correlation, one can study galaxies which are too faint to be individually resolved. This technique, known as intensity mapping, is a promising probe of the global properties of high redshift galaxies. Here, we test the feasibility of such measurements with synthetic data generated from cosmological dark matter simulations. We use a simple prescription for associating galaxies with dark matter halos and create a realization of emitted radiation as a function of angular position and wavelength over a patch of the sky. This is then used to create synthetic data for two different hypothetical instruments, one aboard the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) and another consisting of a pair of ground based radio telescopes designed to measure the CO(1-0) and CO(2-1) emission lines. We find that the line cross power spectrum can be measured accurately from the synthetic data with errors consistent with the analytical prediction of Visbal & Loeb (2010). Removal of astronomical backgrounds and masking bright line emission from foreground contaminating galaxies do not prevent accurate cross power spectrum measurements.

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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↗

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.

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arXiv2007

On the Search for Quasar Light Echoes

The UV radiation from a quasar leaves a characteristic pattern in the distribution of ionized hydrogen throughout the surrounding space. This pattern or light echo propagates through the intergalactic medium at the speed of light, and can be observed by its imprint on the Ly-alpha forest spectra of background sources. As the echo persists after the quasar has switched off, it offers the possibility of searching for dead quasars, and constraining their luminosities and lifetimes. We outline a technique to search for and characterize these light echoes. To test the method, we create artificial Ly-alpha forest spectra from cosmological simulations at z=3, apply light echoes and search for them. We show how the simulations can also be used to quantify the significance level of any detection. We find that light echoes from the brightest quasars could be found in observational data. With absorption line spectra of 100 redshift z~3-3.5 quasars or galaxies in a 1 square degree area, we expect that ~10 echoes from quasars with B band luminosities L_B=3x10^45 ergs/s exist that could be found at 95% confidence, assuming a quasar lifetime of ~10^7 yr. Even a null result from such a search would have interesting implications for our understanding of quasar luminosities and lifetimes.

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