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 $\Omega_\Lambda$, $\Omega_{\rm m}h^2$, and $\Omega_\nu h^2$ 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.Comment: 12 pages, 4 figures; Replaced with version accepted by JCAP; Significant changes regarding treatment of scale dependent bias due to UV backgroun

Direct collapse black hole formation via high-velocity collisions of protogalaxies

We propose high-velocity collisions of protogalaxies as a new pathway to form supermassive stars (SMSs) with masses of ~ 10^5 Msun at high redshift (z > 10). When protogalaxies hosted by dark matter halos with a virial temperature of ~ 10^4 K collide with a relative velocity > 200 km/s, the gas is shock-heated to ~ 10^6 K and subsequently cools isobarically via free-free emission and He^+, He, and H line emission. Since the gas density (> 10^4 cm^{-3}) is high enough to destroy H_2 molecules by collisional dissociation, the shocked gas never cools below ~ 10^4 K. Once a gas cloud of ~ 10^5 Msun reaches this temperature, it becomes gravitationally unstable and forms a SMS which will rapidly collapse into a super massive black hole (SMBH) via general relativistic instability. We perform a simple analytic estimate of the number density of direct-collapse black holes (DCBHs) formed through this scenario (calibrated with cosmological N-body simulations) and find n_{DCBH} ~ 10^{-9} Mpc^{-3} (comoving) by z = 10. This could potentially explain the abundance of bright high-z quasars.Comment: 9 pages, 5 figures, accepted for publication in MNRA

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 \rho \propto r^{-2} at small radii and behaving as \rho \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 \rho/\sigma^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 \rho \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).Comment: 9 pages, 5 figures, submitted to Ap

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~\AA~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$\alpha$ emitters as bright as CR7.Comment: 5 pages, 4 figures, replaced with version accepted by MNRAS Letter

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.Comment: 8 pages, 4 figures, replaced with version accepted by MNRA

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 \AA{} intensity mapping signal, along with the signals from other lines such as Ly$\alpha$, H$\alpha$, 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 \AA{} 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 \AA{} with CO(1-0) line emission from galaxies and the other with 21 cm emission from the intergalactic medium (IGM).Comment: 8 pages, 5 figures, replaced with version accepted by MNRA

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.Comment: 5 pages, 5 figures, replaced with version accepted by MNRA

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 $\tau=0.066 \pm 0.016$. 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.Comment: 12 pages, 10 figures, replaced with version accepted by MNRAS. Added simple treatment of metal enrichmen

Identifying Direct Collapse Black Hole Seeds through their Small Host Galaxies

Observations of high-redshift quasars indicate that super massive black holes (SMBHs) with masses greater than ${\sim}10^9~M_\odot$ were assembled within the first billion years after the Big Bang. It is unclear how such massive black holes formed so early. One possible explanation is that these SMBHs were seeded by `heavy' direct collapse black holes (DCBHs) with masses of $M_{\rm BH}\approx10^5~M_\odot$, but observations have not yet confirmed or refuted this scenario. In this Letter, we utilize a cosmological N-body simulation to demonstrate that before they grow roughly an order of magnitude in mass, DCBHs will have black hole mass to halo mass ratios much higher than expected for black hole remnants of Population III (Pop III) stars which have grown to the same mass (${\sim}10^6~M_\odot$). We also show that when $T_{\rm vir}\approx 10^4~{\rm K}$ halos (the potential sites of DCBH formation) merge with much larger nearby halos ($M_{\rm h} > 10^{10}~M_\odot$), they almost always orbit their larger host halos with a separation of a few kpc, which is sufficient to be spatially resolved with future X-ray and infrared telescopes. Thus, we propose that a future X-ray mission such as Lynx combined with infrared observations will be able to distinguish high-redshift DCBHs from smaller black hole seeds due to the unusually high black hole mass to stellar mass ratios of the faintest observed quasars, with inferred BH masses below ${\sim} 10^6 M_\odot$.Comment: 7 pages, 3 figures, accepted by ApJ

Gravitational wave background from Population III binary black holes consistent with cosmic reionization

The recent discovery of the gravitational wave source GW150914 has revealed a coalescing binary black hole (BBH) with masses of $\sim 30~M_\odot$. Previous proposals for the origin of such a massive binary include Population III (PopIII) stars. PopIII stars are efficient producers of BBHs and of a gravitational wave background (GWB) in the $10-100$ Hz band, and also of ionizing radiation in the early Universe. We quantify the relation between the amplitude of the GWB ($\Omega_{\rm gw}$) and the electron scattering optical depth ($\tau_{\rm e}$), produced by PopIII stars, assuming that $f_{\rm esc}\approx 10\%$ of their ionizing radiation escapes into the intergalactic medium. We find that PopIII stars would produce a GWB that is detectable by the future O5 LIGO/Virgo if $\tau_{\rm e} \gtrsim 0.07$, consistent with the recent Planck measurement of $\tau_e=0.055 \pm 0.09$. Moreover, the spectral index of the background from PopIII BBHs becomes as small as ${\rm d}\ln \Omega_{\rm gw}/{\rm d}\ln f\lesssim 0.3$ at $f \gtrsim 30$ Hz, which is significantly flatter than the value $\sim 2/3$ generically produced by lower-redshift and less-massive BBHs. A detection of the unique flattening at such low frequencies by the O5 LIGO/Virgo will indicate the existence of a high-chirp mass, high-redshift BBH population, which is consistent with the PopIII origin. A precise characterization of the spectral shape near $30-50$ Hz by the Einstein Telescope could also constrain the PopIII initial mass function and star formation rate.Comment: 6 pages, 5 figures, accepted for publication in MNRAS, discussions and a figure adde