arXiv subjects

Zoltan Haiman

Publications and source records attributed to Zoltan Haiman.

At least 109 records · Page 6Linked to original sources

arXiv2016

Intermediate-mass black holes from Population III remnants in the first galactic nuclei

We report the formation of intermediate-mass black holes (IMBHs) in suites of numerical $N$-body simulations of Population III remnant black holes (BHs) embedded in gas-rich protogalaxies at redshifts $z\gtrsim10$. We model the effects of gas drag on the BHs' orbits, and allow BHs to grow via gas accretion, including a mode of hyper-Eddington accretion in which photon trapping and rapid gas inflow suppress any negative radiative feedback. Most initial BH configurations lead to the formation of one (but never more than one) IMBH in the center of the protogalaxy, reaching a mass of $10^{3-5}\mathrm{M}_{\odot}$ through hyper-Eddington growth. Our results suggest a viable pathway to forming the earliest massive BHs in the centers of early galaxies. We also find that the nuclear IMBH typically captures a stellar-mass BH companion, making these systems observable in gravitational waves as extreme mass-ratio inspirals (EMRIs) with \textit{eLISA}.

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arXiv2016

A transition in circumbinary accretion discs at a binary mass ratio of 1:25

We study circumbinary accretion discs in the framework of the restricted three-body problem (R3Bp) and via numerically solving the height-integrated equations of viscous hydrodynamics. Varying the mass ratio of the binary, we find a pronounced change in the behaviour of the disc near mass ratio $q \equiv M_s/M_p \sim 0.04$. For mass ratios above $q=0.04$, solutions for the hydrodynamic flow transition from steady, to strongly-fluctuating; a narrow annular gap in the surface density around the secondary's orbit changes to a hollow central cavity; and a spatial symmetry is lost, resulting in a lopsided disc. This phase transition is coincident with the mass ratio above which stable orbits do not exist around the L4 and L5 equilibrium points of the R3B problem. Using the DISCO code, we find that for thin discs, for which a gap or cavity can remain open, the mass ratio of the transition is relatively insensitive to disc viscosity and pressure. The $q=0.04$ transition has relevance for the evolution of massive black hole binary+disc systems at the centers of galactic nuclei, as well as for young stellar binaries and possibly planets around brown dwarfs.

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arXiv2016

Formation of Massive Population III Galaxies through Photoionization Feedback: A Possible Explanation for CR7

We explore the formation of massive high-redshift Population III (Pop III) galaxies through photoionization feedback. We consider dark matter halos formed from progenitors that have undergone no star formation as a result of early reionization and photoevaporation caused by a nearby galaxy. Once such a halo reaches $\approx 10^9~M_\odot$, corresponding to the Jeans mass of the photoheated intergalactic medium (IGM) at $z\approx 7$, pristine gas is able to collapse into the halo, potentially producing a massive Pop III starburst. We suggest that this scenario may explain the recent observation of strong He II 1640~Å~line emission in CR7, which is consistent with $\sim 10^7~M_\odot$ of young Pop III stars. Such a large mass of Pop III stars is unlikely without the photoionization feedback scenario, because star formation is expected to inject metals into halos above the atomic cooling threshold ($\sim 10^8~M_\odot$ at $z \approx 7$). We use merger trees to analytically estimate the abundance of observable Pop III galaxies formed through this channel, and find a number density of $\approx 10^{-7}~{\rm Mpc^{-3}}$ at $z=6.6$ (the redshift of CR7). This is approximately a factor of ten lower than the density of Ly$α$ emitters as bright as CR7.

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arXiv2016

Hyper-Eddington accretion flows onto massive black holes

We study very-high rate spherically symmetric accretion flows onto a massive black hole (BH; 10^2 < M_BH < 10^6 Msun) embedded in a dense gas cloud with a low abundance of metals, performing one-dimensional hydrodynamical simulations which include multi-frequency radiation transfer and non-equilibrium primordial chemistry. We find that rapid gas supply from the Bondi radius at a hyper-Eddington rate can occur without being impeded by radiation feedback when (n/10^5 cm^-3) > (M_BH/10^4Msun)^{-1}(T/10^4 K)^{3/2}, where n and T are the density and temperature of ambient gas outside of the Bondi radius. The resulting accretion rate in this regime is steady, and larger than 3000 times the Eddington rate. At lower Bondi rates, the accretion is episodic due to radiative feedback and the average rate is limited below the Eddington rate. For the hyper-Eddington case, the steady solution consists of two parts: a radiation-dominated central core, where photon trapping due to electron scattering is important, and an accreting envelope which follows a Bondi profile with T~8000 K. When the emergent luminosity is limited below the Eddington luminosity because of photon trapping, radiation from the central region does not affect the gas dynamics at larger scales. We apply our result to the rapid formation of massive BHs in protogalaxies with a virial temperature of T_vir> 10^4 K. Once a seed BH forms at the center of the galaxy, it can grow up to a maximum ~10^5 (T_vir/10^4 K) Msun via gas accretion independent of the initial BH mass. Finally, we discuss possible observational signatures of rapidly accreting BHs with/without allowance for dust. We suggest that these systems could explain Lya emitters without X-rays and luminous infrared sources with hot dust emission, respectively.

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arXiv2015

A Statistical Method to Search for Recoiling Supermassive Black Holes in Active Galactic Nuclei

We propose an observational test for gravitationally recoiling supermassive black holes (BHs) in active galactic nuclei, based on a correlation between the velocities of BHs relative to their host galaxies, |Δv|, and their obscuring dust column densities, Σ_{dust} (both measured along the line of sight). We use toy models for the distribution of recoil velocities, BH trajectories, and the geometry of obscuring dust tori in galactic centres, to simulate 2.5x10^5 random observations of recoiling quasars. BHs with recoil velocities comparable to the escape velocity from the galactic centre remain bound to the nucleus, and do not fully settle back to the centre of the torus due to dynamical friction in a typical quasar lifetime. We find that |Δv| and Σ_ {dust} for these BHs are positively correlated. For obscured (Σ_{dust}>0) and for partially obscured (0<Σ_{dust}<~2.3 g/m^2) quasars with |Δv|>=45 km/s, the sample correlation coefficient between log10(|Δv|) and Σ_{dust} is r_{45} = 0.28+/-0.02 and r_{45} = 0.13+/-0.02, respectively. Allowing for random +/-100 km/s errors in |Δv| unrelated to the recoil dilutes the correlation for the partially obscured quasars to r_{45} = 0.026+/-0.004 measured between |Δv| and Σ_{dust}. A random sample of >~3,500 obscured quasars with |Δv|>=45 km/s would allow rejection of the no-correlation hypothesis with 3 sigma significance 95% of the time. Finally, we find that the fraction of obscured quasars, F_{obs}(|Δv|), decreases with |Δv| from F_{obs}(<10 km/s)>~0.8 to F_{obs}(>10^3 km/s)<~0.4. This predicted trend can be compared to the observed fraction of type II quasars, and can further test combinations of recoil, trajectory, and dust torus models.

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arXiv2015

Cosmology Constraints from the Weak Lensing Peak Counts and the Power Spectrum in CFHTLenS

Lensing peaks have been proposed as a useful statistic, containing cosmological information from non-Gaussianities that is inaccessible from traditional two-point statistics such as the power spectrum or two-point correlation functions. Here we examine constraints on cosmological parameters from weak lensing peak counts, using the publicly available data from the 154 deg$^2$ CFHTLenS survey. We utilize a new suite of ray-tracing N-body simulations on a grid of 91 cosmological models, covering broad ranges of the three parameters $Ω_m$, $σ_8$, and $w$, and replicating the Galaxy sky positions, redshifts, and shape noise in the CFHTLenS observations. We then build an emulator that interpolates the power spectrum and the peak counts to an accuracy of $\leq 5\%$, and compute the likelihood in the three-dimensional parameter space ($Ω_m$, $σ_8$, $w$) from both observables. We find that constraints from peak counts are comparable to those from the power spectrum, and somewhat tighter when different smoothing scales are combined. Neither observable can constrain $w$ without external data. When the power spectrum and peak counts are combined, the area of the error "banana'' in the ($Ω_m$, $σ_8$) plane reduces by a factor of $\approx2$, compared to using the power spectrum alone. For a flat $Λ$ cold dark matter model, combining both statistics, we obtain the constraint $σ_8(Ω_m/0.27)^{0.63}=0.85\substack{+0.03 \\ -0.03}$.

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arXiv2015

Limits on Population III star formation in minihaloes implied by Planck

Recently, Planck measured a value of the cosmic microwave background (CMB) optical depth due to electron scattering of $τ=0.066 \pm 0.016$. Here we show that this low value leaves essentially no room for an early partial reionisation of the intergalactic medium (IGM) by high-redshift Population III (Pop III) stars, expected to have formed in low-mass minihaloes. We perform semi-analytic calculations of reionisation which include the contribution from Pop II stars in atomic cooling haloes, calibrated with high-redshift galaxy observations, and Pop III stars in minihaloes with feedback due to Lyman-Werner (LW) radiation and metal enrichment. We find that without LW feedback or prompt metal enrichment (and assuming a minihalo escape fraction of 0.5) the Pop III star formation efficiency cannot exceed $\sim{\rm a~few}\times 10^{-4}$, without violating the constraints set by Planck data. This excludes massive Pop III star formation in typical $10^6 M_\odot$ minihaloes. Including LW feedback and metal enrichment alleviates this tension, allowing large Pop III stars to form early on before they are quenched by feedback. We find that the total density of Pop III stars formed across cosmic time is $\lesssim 10^{4-5}~M_\odot~{\rm Mpc^{-3}}$ and does not depend strongly on the feedback prescription adopted. Additionally, we perform a simple estimate of the possible impact on reionisation of X-rays produced by accretion onto black hole remnants of Pop III stars. We find that unless the accretion duty cycle is very low ($\lesssim 0.01$), this could lead to an optical depth inconsistent with Planck.

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arXiv2015

Looking for Population III stars with He II line intensity mapping

Constraining the properties of Population III (Pop III) stars will be very challenging because they reside in small galaxies at high redshift which will be difficult to directly detect. In this paper, we suggest that intensity mapping may be a promising method to study Pop III stars. Intensity mapping is a technique proposed to measure large-scale fluctuations of galaxy line emission in three dimensions without resolving individual sources. This technique is well suited for observing many faint galaxies because it can measure their cumulative emission even if they cannot be directly detected. We focus on intensity mapping of He II recombination lines. These lines are much stronger in Pop III stars than Pop II stars because the harder spectra of Pop III stars are expected to produce many He II ionizing photons. Measuring the He II 1640 Å intensity mapping signal, along with the signals from other lines such as Ly$α$, H$α$, and metal lines, could give constraints on the initial mass function (IMF) and star formation rate density (SFRD) of Pop III stars as a function of redshift. To demonstrate the feasibility of these observations, we estimate the strength of the Pop III He II 1640 Å intensity mapping signal from $z=10-20$. We show that at $z\approx10$, the signal could be measured accurately by two different hypothetical future instruments, one which cross-correlates He II 1640 Å with CO(1-0) line emission from galaxies and the other with 21 cm emission from the intergalactic medium (IGM).

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arXiv2015

Emulating the CFHTLenS Weak Lensing data: Cosmological Constraints from moments and Minkowski functionals

Weak gravitational lensing is a powerful cosmological probe, with non--Gaussian features potentially containing the majority of the information. We examine constraints on the parameter triplet $(Ω_m,w,σ_8)$ from non-Gaussian features of the weak lensing convergence field, including a set of moments (up to $4^{\rm th}$ order) and Minkowski functionals, using publicly available data from the 154deg$^2$ CFHTLenS survey. We utilize a suite of ray--tracing N-body simulations spanning 91 points in $(Ω_m,w,σ_8)$ parameter space, replicating the galaxy sky positions, redshifts and shape noise in the CFHTLenS catalogs. We then build an emulator that interpolates the simulated descriptors as a function of $(Ω_m,w,σ_8)$, and use it to compute the likelihood function and parameter constraints. We employ a principal component analysis to reduce dimensionality and to help stabilize the constraints with respect to the number of bins used to construct each statistic. Using the full set of statistics, we find $Σ_8\equivσ_8(Ω_m/0.27)^{0.55}=0.75\pm0.04$ (68% C.L.), in agreement with previous values. We find that constraints on the $(Ω_m,σ_8)$ doublet from the Minkowski functionals suffer a strong bias. However, high-order moments break the $(Ω_m,σ_8)$ degeneracy and provide a tight constraint on these parameters with no apparent bias. The main contribution comes from quartic moments of derivatives.

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arXiv2015

High-Redshift Star Formation in a Time-Dependent Lyman-Werner Background

The first generation of stars produces a background of Lyman-Werner (LW) radiation which can photo-dissociate molecular hydrogen, increasing the mass of dark matter halos required to host star formation. Previous studies have determined the critical mass required for efficient molecular cooling with a constant LW background. However, the true background is expected to increase rapidly at early times. Neglecting this evolution could underestimate star formation in small halos that may have started to cool in the past when the LW intensity was much lower. Background evolution is a large source of uncertainty in pre-reionization predictions of the cosmological 21cm signal, which can be observed with future radio telescopes. To address this, we perform zero-dimentional one-zone calculations that follow the density, chemical abundances, and temperature of gas in the central regions of dark matter halos, including hierarchical growth and an evolving LW background. We begin by studying the physics of halos subjected to a background that increases exponentially with redshift. We find that when the intensity increases more slowly than $J_{\rm LW}(z) \propto 10^{-z/5}$, cooling in the past is a relatively small effect. We then self-consistently compute the cosmological LW background over $z=15-50$ and find that cooling in the past due to an evolving background has a modest impact. Finally, we compare these results to three-dimensional hydrodynamical cosmological simulations with varying LW histories. While only a small number of halos were simulated, the results are consistent with our one-zone calculations.

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arXiv2015

Direct collapse black hole formation from synchronized pairs of atomic cooling halos

High-redshift quasar observations imply that supermassive black holes (SMBHs) larger than $\sim 10^9 ~ M_\odot$ formed before $z=6$. That such large SMBHs formed so early in the Universe remains an open theoretical problem. One possibility is that gas in atomic cooling halos exposed to strong Lyman-Werner (LW) radiation forms $10^4-10^6 ~ M_\odot$ supermassive stars which quickly collapse into black holes. We propose a scenario for direct collapse black hole (DCBH) formation based on synchronized pairs of pristine atomic cooling halos. We consider halos at very small separation with one halo being a subhalo of the other. The first halo to surpass the atomic cooling threshold forms stars. Soon after these stars are formed, the other halo reaches the cooling threshold and due to its small distance from the newly formed galaxy, is exposed to the critical LW intensity required to form a DCBH. The main advantage of this scenario is that synchronization can potentially prevent photoevaporation and metal pollution in DCBH-forming halos. Since the halos reach the atomic cooling threshold at nearly the same time, the DCBH-forming halo is only exposed to ionizing radiation for a brief period. Tight synchronization could allow the DCBH to form before stars in the nearby galaxy reach the end of their lives and generate supernovae winds. We use N-body simulations to estimate the abundance of DCBHs formed in this way. The largest source of uncertainty in our estimate is the initial mass function (IMF) of metal free stars formed in atomic cooling halos. We find that even for tight synchronization, the density of DCBHs formed in this scenario could explain the SMBHs implied by $z=6$ quasar observations. Metal pollution and photoevaporation could potentially reduce the abundance of DCBHs below that required to explain the observations in other models that rely on a high LW flux.

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arXiv2015

The impact of spurious shear on cosmological parameter estimates from weak lensing observables

Residual errors in shear measurements, after corrections for instrument systematics and atmospheric effects, can impact cosmological parameters derived from weak lensing observations. Here we combine convergence maps from our suite of ray-tracing simulations with random realizations of spurious shear. This allows us to quantify the errors and biases of the triplet $(Ω_m,w,σ_8)$ derived from the power spectrum (PS), as well as from three different sets of non-Gaussian statistics of the lensing convergence field: Minkowski functionals (MF), low--order moments (LM), and peak counts (PK). Our main results are: (i) We find an order of magnitude smaller biases from the PS than in previous work. (ii) The PS and LM yield biases much smaller than the morphological statistics (MF, PK). (iii) For strictly Gaussian spurious shear with integrated amplitude as low as its current estimate of $σ^2_{sys}\approx 10^{-7}$, biases from the PS and LM would be unimportant even for a survey with the statistical power of LSST. However, we find that for surveys larger than $\approx 100$ deg$^2$, non-Gaussianity in the noise (not included in our analysis) will likely be important and must be quantified to assess the biases. (iv) The morphological statistics (MF,PK) introduce important biases even for Gaussian noise, which must be corrected in large surveys. The biases are in different directions in $(Ω_m,w,σ_8)$ parameter space, allowing self-calibration by combining multiple statistics. Our results warrant follow-up studies with more extensive lensing simulations and more accurate spurious shear estimates.

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arXiv2014

Lyman edges in supermassive black hole binaries

We propose a new spectral signature for supermassive black hole binaries (SMBHBs) with circumbinary gas disks: a sharp drop in flux blueward of the Lyman limit. A prominent edge is produced if the gas dominating the emission in the Lyman continuum region of the spectrum is sufficiently cold (T < 20,000 K) to contain significant neutral hydrogen. Circumbinary disks may be in this regime if the binary torques open a central cavity in the disk and clear most of the hot gas from the inner region, and if any residual UV emission from the individual BHs is either dim or intermittent. We model the vertical structure and spectra of circumbinary disks using the radiative transfer code TLUSTY, and identify the range of BH masses and binary separations producing a Lyman edge. We find that compact supermassive binaries with orbital periods of ~0.1 - 10 yr, whose gravitational waves (GWs) are expected to be detectable by pulsar timing arrays (PTAs), could have prominent Lyman edges. Such strong spectral edge features are not typically present in AGN spectra and could serve as corroborating evidence for the presence of a SMBHB.

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arXiv2014

Does disk fragmentation prevent the formation of supermassive stars in protogalaxies?

Supermassive stars (SMSs; >10^5 Msun) formed in the first protogalaxies with virial temperature T_vir>10^4 K are expected to collapse into seeds of supermassive black hole (SMBHs) in the high-redshift universe (z>7). Fragmentation of the primordial gas is, however, a possible obstacle to SMS formation. We discuss the expected properties of a compact, metal-free, marginally unstable nuclear protogalactic disk, and the fate of the clumps formed in the disk by gravitational instability. Interior to a characteristic radius R_f=few*10^{-2} pc, the disk fragments into massive clumps with M_c~30 Msun. The clumps grow via accretion and migrate inward rapidly on a timescale of ~10^4 yr, which is comparable or shorter than the Kelvin-Helmholz time >10^4 yr. Some clumps may evolve to zero-age main sequence stars and halt gas accretion by radiative feedback, but most of the clumps can migrate inward and merge with the central protostar before forming massive stars. Moreover, we found that dust-induced-fragmentation in metal-enriched gas does not modify these conclusions unless Z> 3*10^{-4} Zsun, because clump migration below this metallicity remains as rapid as in the primordial case. Our results suggest that fragmentation of a compact, metal--poor disk can not prevent the formation of a SMS.

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arXiv2014

Binary Black Hole Accretion During Inspiral and Merger

We present the results of 2D, moving mesh, viscous hydrodynamical simulations of accretion onto merging supermassive black hole (SMBH) binaries. We include viscous heating, shock heating, and radiative cooling, and simulate the transition from the "pre-decoupling" epoch, where the inspiral timescale is longer than the viscous timescale, to the "post-decoupling" epoch, where the inspiral timescale is shorter than the viscous timescale. We find that there is no abrupt halt to the accretion at decoupling, but rather the accretion shows a slow decay, with significant accretion well after the expected decoupling. Moreover, we find that the luminosity in X-rays is significantly higher prior to the merger, as orbital energy from the SMBH binary is converted to heat via strong shocks inside the cavity, and radiated away. Following the merger, the cavity refills viscously and the accretion rate relaxes to the Shakura-Sunyaev value, while the X-ray luminosity drops as the shocks quickly dissipate.

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arXiv2014

Close companions to two high-redshift quasars

We report the serendipitous discoveries of companion galaxies to two high-redshift quasars. SDSS J025617.7+001904 is a z=4.79 quasar included in our recent survey of faint quasars in the SDSS Stripe 82 region. The initial MMT slit spectroscopy shows excess Lyman alpha emission extending well beyond the quasar's light profile. Further imaging and spectroscopy with LBT/MODS1 confirms the presence of a bright galaxy (i_AB = 23.6) located 2arcsec (12 kpc projected) from the quasar with strong Lyman alpha emission (EW_0 ~ 100Ang) at the redshift of the quasar, as well as faint continuum. The second quasar, CFHQS J005006.6+344522 (z=6.25), is included in our recent HST SNAP survey of z~6 quasars searching for evidence of gravitational lensing. Deep imaging with ACS and WFC3 confirms an optical dropout ~4.5 mag fainter than the quasar (Y_AB=25) at a separation of 0.9 arcsec. The red i_775-Y_105 color of the galaxy and its proximity to the quasar (5 kpc projected if at the quasar redshift) strongly favor an association with the quasar. Although it is much fainter than the quasar it is remarkably bright when compared to field galaxies at this redshift, while showing no evidence for lensing. Both systems may represent late-stage mergers of two massive galaxies, with the observed light for one dominated by powerful ongoing star formation and for the other by rapid black hole growth. Observations of close companions are rare; if major mergers are primarily responsible for high-redshift quasar fueling then the phase when progenitor galaxies can be observed as bright companions is relatively short.

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arXiv2014

The Migration of Gap-Opening Planets is not Locked to Viscous Disk Evolution

Most standard descriptions of Type II migration state that massive, gap-opening planets must migrate at the viscous drift rate. This is based on the idea that the disk is separated into an inner and outer region and gas is considered unable to cross the gap. In fact, gas easily crosses the gap on horseshoe orbits, nullifying this necessary premise which would set the migration rate. In this work, it is demonstrated using highly accurate numerical calculations that the actual migration rate is dependent on disk and planet parameters, and can be significantly larger or smaller than the viscous drift rate. In the limiting case of a disk much more massive than the secondary, the migration rate saturates to a constant which is sensitive to disk parameters and is not necessarily of order viscous rate. In the opposite limit of a low-mass disk, the migration rate decreases linearly with disk mass. Steady-state solutions in the low disk mass limit show no pile-up outside the secondary's orbit, and no corresponding drainage of the inner disk.

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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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