92 research outputs found
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Observational aspects of galactic accretion at redshift 3.3
We investigate the origin of extragalactic continuum emission and its
relation to the stellar population of a recently discovered peculiar z=3.344
Lyman alpha emitter. Based on an analysis of the broad-band colors and
morphology we find further support for the idea that the underlying galaxy is
being fed by a large-scale (L > 35 kpc) accretion stream. Archival HST images
show small scale (~5 kpc) tentacular filaments converging near a hot-spot of
star-formation, possibly fueled by gas falling in along the filaments. The
spectral energy distribution of the tentacles is broadly compatible with either
(1) non-ionizing rest-frame far-UV continuum emission from stars formed in an
60 million-year-old starburst; (2) nebular 2-photon-continuum radiation,
arising from collisional excitation cooling, or (3) a recombination spectrum
emitted by hydrogen fluorescing in response to ionizing radiation escaping from
the galaxy. The latter possibility simultaneously accounts for the presence of
asymmetric Lyman alpha emission from the large-scale gaseous filament and the
nebular continuum in the smaller-scale tentacles as caused by the escape of
ionizing radiation from the galaxy. Possible astrophysical explanations for the
nature of the tentacles include: a galactic wind powered by the starburst;
in-falling gas during cold accretion, or tails of interstellar medium dragged
out of the galaxy by satellite halos that have plunged through the main halo.
The possibility of detecting extragalactic 2-photon continuum emission in
space-based, broad-band images suggests a tool for studying the gaseous
environment of high redshift galaxies at much greater spatial detail than
possible with Lyman alpha or other resonance line emission
Feedback from active galactic nuclei: Energy- versus momentum-driving
We employ hydrodynamical simulations using the moving-mesh code AREPO to
investigate the role of energy and momentum input from Active Galactic Nuclei
(AGN) in driving large-scale galactic outflows. We start by reproducing
analytic solutions for both energy- and momentum-driven outflowing shells in
simulations of a spherical isolated dark matter potential with gas in
hydrostatic equilibrium and with no radiative cooling. We confirm that for this
simplified setup, galactic outflows driven by a momentum input rate of order
L_Edd/c can establish an M_BH - sigma relation with slope and normalisation
similar to that observed. We show that momentum input at a rate of L_Edd/c is
however insufficient to drive efficient outflows once cooling and gas inflows
as predicted by cosmological simulations at resolved scales are taken into
account. We argue that observed large-scale AGN-driven outflows are instead
likely to be energy-driven and show that such outflows can reach momentum
fluxes exceeding 10 L_Edd/c within the innermost 10 kpc of the galaxy. The
outflows are highly anisotropic, with outflow rates and a velocity structure
found to be inadequately described by spherical outflow models. We verify that
the hot energy-driven outflowing gas is expected to be strongly affected by
metal-line cooling, leading to significant amounts (>10^9 M_sun) of entrained
cold gas
Probing delayed-end reionization histories with the 21-cm LAE cross-power spectrum
ABSTRACT
We model the 21-cm signal and Lyman-α emitter (LAE) population evolution during the epoch of reionization in order to predict the 21-cm LAE cross-power spectrum. We employ high-dynamic-range simulations of the intergalactic medium to create models that are consistent with constraints from the cosmic microwave background, Lyman-α forest, and LAE population statistics. Using these models we consider the evolution of the cross-power spectrum for a selection of realistic reionization histories and predict the sensitivity of current and upcoming surveys to measuring this signal. We find that the imprint of a delayed end to reionization can be observed by future surveys, and that strong constraints can be placed on the progression of reionization as late as z = 5.7 using a Subaru–SKA survey. We make predictions for the signal-to-noise ratios achievable by combinations of Subaru/PFS (Prime Focus Spectrograph) with the MWA, LOFAR, HERA, and SKA interferometers for an integration time of 1000 h. We find that a Subaru–SKA survey could measure the cross-power spectrum for a late reionization at z = 6.6 with a total signal-to-noise ratio greater than 5, making it possible to constrain both the timing and bubble size at the end of reionization. Furthermore, we find that expanding the current Subaru/PFS survey area and depth by a factor of three would double the total signal-to-noise ratio.</jats:p
Seeding high-redshift QSOs by collisional runaway in primordial star clusters
We study how runaway stellar collisions in high-redshift, metal-poor star
clusters form very massive stars (VMSs) that can directly collapse to
intermediate-mass black holes (IMBHs). We follow the evolution of a pair of
neighbouring high-redshift mini-haloes with high-resolution, cosmological
hydrodynamical zoom-in simulations using the adaptive mesh refinement code
RAMSES combined with the non-equilibrium chemistry package KROME. The first
collapsing mini-halo is assumed to enrich the central nuclear star cluster
(NSC) of the other to a critical metallicity, sufficient for Population II
(Pop. II) star formation at redshift . Using the spatial
configuration of the flattened, asymmetrical gas cloud forming in the core of
the metal enriched halo, we set the initial conditions for simulations of an
initially non-spherical star cluster with the direct summation code NBODY6
which are compared to about 2000 NBODY6 simulations of spherical star clusters
for a wide range of star cluster parameters. The final mass of the VMS that
forms depends strongly on the initial mass and initial central density of the
NSC. For the initial central densities suggested by our RAMSES simulations,
VMSs with mass > 400 M can form in clusters with stellar masses of
M, and this can increase to well over 1000 M
for more massive and denser clusters. The high probability we find for forming
a VMS in these mini-haloes at such an early cosmic time makes collisional
runaway of Pop. II star clusters a promising channel for producing large
numbers of high-redshift IMBHs that may act as the seeds of supermassive black
holes
Fast cold gas in hot AGN outflows
Observations of the emission from spatially extended cold gas around bright
high-redshift QSOs reveal surprisingly large velocity widths exceeding 2000 km
s^(-1), out to projected distances as large as 30 kpc. The high velocity widths
have been interpreted as the signature of powerful AGN-driven outflows.
Naively, these findings appear in tension with hydrodynamic models in which
AGN-driven outflows are energy-driven and thus very hot with typical
temperatures T = 10^6-7 K. Using the moving-mesh code Arepo, we perform
'zoom-in' cosmological simulations of a z = 6 QSO and its environment,
following black hole growth and feedback via energy-driven outflows. In the
simulations, the QSO host galaxy is surrounded by a clumpy circum-galactic
medium pre-enriched with metals due to supernovae-driven galactic outflows. As
a result, part of the AGN-driven hot outflowing gas can cool radiatively,
leading to large amounts (> 10^9 M_sun) of cold gas comoving with the hot
bipolar outflow. This results in velocity widths of spatially extended cold gas
similar to those observed. We caution, however, that gas inflows, random
motions in the deep potential well of the QSO host galaxy and cooling of
supernovae-driven winds contribute significantly to the large velocity width of
the cold gas in the simulations, complicating the interpretation of
observational data
Numerical resolution effects on simulations of massive black hole seeds
We have performed high-resolution numerical simulations with the
hydrodynamical AMR code Enzo to investigate the formation of massive seed black
holes in a sample of six dark matter haloes above the atomic cooling threshold.
The aim of this study is to illustrate the effects of varying the maximum
refinement level on the final object formed. The virial temperatures of the
simulated haloes range from and
they have virial masses in the range
to at . The outcome of
our six fiducial simulations is both generic and robust. A rotationally
supported, marginally gravitationally stable, disk forms with an exponential
profile. The mass and scale length of this disk depends strongly on the maximum
refinement level used. Varying the maximum refinement level by factors between
1 / 64 to 256 times the fiducial level illustrates the care that must be taken
in interpreting the results. The lower resolution simulations show tentative
evidence that the gas may become rotationally supported out to 20 pc while the
highest resolution simulations show only weak evidence of rotational support
due to the shorter dynamical times for which the simulation runs. The higher
resolution simulations do, however, point to fragmentation at small scales of
the order of AU. In the highest resolution simulations a central
object of a few times forms with multiple strongly
bound, Jeans unstable, clumps of and radii of 10 - 20
AU suggesting the formation of dense star clusters in these haloes
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Modelling the observed luminosity function and clustering evolution of Ly α emitters: Growing evidence for late reionization
We model the high redshift (z > 5) Lyman- emitting (LAE) galaxy
population using the empirical rest-frame equivalent width distribution. We
calibrate to the observed luminosity function and angular correlation function
at z = 5.7 as measured by the SILVERRUSH survey. This allows us to populate the
high-dynamic-range Sherwood simulation suite with LAEs, and to calculate the
transmission of their Ly emission through the inter-galactic medium
(IGM). We use this simulated population to explore the effect of the IGM on
high-redshift observations of LAEs, and make predictions for the narrowband
filter redshifts at z = 6.6, 7.0 and 7.3. Comparing our model with existing
observations, we find a late reionization is suggested, consistent with the
recent low optical depth derived from the cosmic microwave background (CMB) by
the Planck collaboration and the opacity fluctuations in the Ly
forest. We also explore the role of the circum-galactic medium (CGM) and the
larger volume of gas which is infalling onto the host halo versus the IGM in
attenuating the Ly signal, finding that a significant fraction of the
attenuation is due to the CGM and infalling gas, which increases towards the
end of reionization, albeit with a large scatter across the mock LAE
population
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Warm dark matter as a solution to the small scale crisis: New constraints from high redshift Lyman-α forest data
We present updated constraints on the free-streaming of warm dark matter
(WDM) particles derived from an analysis of the Lya flux power spectrum
measured from high-resolution spectra of 25 z > 4 quasars obtained with the
Keck High Resolution Echelle Spectrometer (HIRES) and the Magellan Inamori
Kyocera Echelle (MIKE) spectrograph. We utilize a new suite of high-resolution
hydrodynamical simulations that explore WDM masses of 1, 2 and 4 keV (assuming
the WDM consists of thermal relics), along with different physically motivated
thermal histories. We carefully address different sources of systematic error
that may affect our final results and perform an analysis of the Lya flux power
with conservative error estimates. By using a method that samples the
multi-dimensional astrophysical and cosmological parameter space, we obtain a
lower limit mwdm > 3.3 keV (2sigma) for warm dark matter particles in the form
of early decoupled thermal relics. Adding the Sloan Digital Sky Survey (SDSS)
Lya flux power spectrum does not improve this limit. Thermal relics of masses 1
keV, 2 keV and 2.5 keV are disfavoured by the data at about the 9sigma, 4sigma
and 3sigma C.L., respectively. Our analysis disfavours WDM models where there
is a suppression in the linear matter power spectrum at (non-linear) scales
corresponding to k=10h/Mpc which deviates more than 10% from a LCDM model.
Given this limit, the corresponding "free-streaming mass" below which the mass
function may be suppressed is 2x10^8 Msun/h. There is thus very little room for
a contribution of the free-streaming of WDM to the solution of what has been
termed the small scale crisis of cold dark matter
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Evolution of the Quasar Luminosity Function: Implications for EoR-21cm
AbstractWe present predictions for the spatial distribution of 21 cm brightness temperature fluctuations from high-dynamic-range simulations for AGN-dominated reionization histories that have been tested against available Lyα and CMB data. We model AGN by extrapolating the observed Mbh–σ relation to high redshifts and assign them ionizing emissivities consistent with recent UV luminosity function measurements. AGN-dominated reionization histories increase the variance of the 21 cm emission by a factor of up to ten compared to similar reionization histories dominated by faint galaxies, to values close to 100 mK2 at scales accessible to experiments (k ≲ 1 cMpc−1h). This is lower than the sensitivity reached by ongoing experiments by only a factor of about two or less. AGN dominated reionization should be easily detectable by LOFAR (and later HERA and SKA1) at their design sensitivity.</jats:p
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Probing the metallicity and ionization state of the circumgalactic medium at z ~ 6 and beyond with OI absorption
Low ionization metal absorption due to OI has been identified as an important
probe of the physical state of the inter-/circumgalactic medium at the tail-end
of reionization. We use here high-resolution hydrodynamic simulations to
interpret the incidence rate of OI absorbers at z~6 as observed by Becker et
al. (2011). We infer weak OI absorbers (EW > 0.1 A) to have typical HI column
densities in the range of sub-DLAs, densities of 80 times the mean baryonic
density and metallicities of about 1/500 th solar. This is similar to the
metallicity inferred at similar overdensities at z~3, suggesting that the metal
enrichment of the circumgalactic medium around low-mass galaxies has already
progressed considerably by z~6. The apparently rapid evolution of the incidence
rates for OI absorption over the redshift range 5<z<6 mirrors that of
self-shielded Lyman-Limit systems at lower redshift and is mainly due to the
rapid decrease of the meta-galactic photo-ionization rate at z>5. We predict
the incidence rate of OI absorbers to continue to rise rapidly with increasing
redshift as the IGM becomes more neutral. If the distribution of metals extends
to lower density regions, OI absorbers will allow the metal enrichment of the
increasingly neutral filamentary structures of the cosmic web to be probed
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