73 research outputs found
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 , , and 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 , corresponding to the Jeans mass of the
photoheated intergalactic medium (IGM) at , 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 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 ( at ). We use merger trees to analytically estimate the abundance of
observable Pop III galaxies formed through this channel, and find a number
density of at (the redshift of CR7).
This is approximately a factor of ten lower than the density of Ly
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 formed before . 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 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 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, H, 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 . We show that at , 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
at redshifts remains an open theoretical
question. One possibility is that gas collapsing rapidly in pristine atomic
cooling halos () produces
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 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 () 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 . 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 ,
without violating the constraints set by Planck data. This excludes massive Pop
III star formation in typical 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 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 (), 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 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 , 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 (). We also show that when halos (the potential sites of DCBH formation) merge with much
larger nearby halos (), 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 .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 . 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 Hz band, and also of
ionizing radiation in the early Universe. We quantify the relation between the
amplitude of the GWB () and the electron scattering optical
depth (), produced by PopIII stars, assuming that 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 , consistent with the recent
Planck measurement of . Moreover, the spectral index of
the background from PopIII BBHs becomes as small as at Hz, which is significantly
flatter than the value 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 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
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