194 research outputs found
Reionization and Cosmic Dawn: theory and simulations
We highlight recent progress in the sophistication and diversification of
cosmic dawn and reionization simulations. The application of these modeling
tools to current observations has allowed us narrow down the timing of
reionization, which we now know to within dz ~ 1 for the bulk of reionization.
The strongest constraints come from the optical depth to the CMB measured with
the {\it Planck} satellite and the first detection of ongoing reionization from
the spectra of the z=7.1 QSOs ULASJ1120+0641. However, we still know virtually
nothing about the astrophysical sources during the first billion years. The
revolution in our understanding will be led by upcoming interferometric
observations of the cosmic 21-cm signal. The properties of the sources and
sinks of UV and X-ray photons are encoded in the 3D patterns of the signal. The
development of Bayesian parameter recovery techniques, which tap into the
wealth of the 21-cm signal, will soon usher in an era of precision
astrophysical cosmology.Comment: Invited review for the IAU Symposium 333 "Peering towards Cosmic
Dawn", Dubrovnik, October 2-6, 2017; to appear in the proceedings, eds. Vibor
Jelic and Thijs van der Hulst [8 pages, 3 figures
How does radiative feedback from a UV background impact reionization?
An ionizing UV background (UVB) inhibits gas accretion and photo-evaporates
gas from the shallow potential wells of small, dwarf galaxies. During
cosmological reionization, this effect can result in negative feedback:
suppressing star-formation inside HII regions, thus impeding their continued
growth. It is difficult to model this process, given the enormous range of
scales involved. We tackle this problem using a tiered approach: combining
parameterized results from single-halo collapse simulations with large-scale
models of reionization. In the resulting reionization models, the ionizing
emissivity of galaxies depends on the local values of the reionization redshift
and the UVB intensity. We present a physically-motivated analytic expression
for the average minimum mass of star-forming galaxies, which can be readily
used in modeling galaxy formation. We find that UVB feedback: (i) delays the
end stages of reionization by less than 0.5 in redshift; (ii) results in a more
uniform distribution of HII regions, peaked on smaller-scales (with large-scale
ionization power suppressed by tens of percent); and (iii) suppresses the
global photoionization rate per baryon by a factor of < 2 towards the end of
reionization. However, the impact is modest, since the hydrodynamic response of
the gas to the UVB occurs on a time-scale comparable to reionization. In
particular, the popular approach of modeling UVB feedback with an instantaneous
transition in the minimum mass of star-forming galaxies, dramatically
overestimates its importance. UVB feedback does not significantly affect
reionization unless: (i) molecularly-cooled galaxies contribute significantly
to reionization; or (ii) internal feedback processes strongly couple with UVB
feedback in the early Universe. Since both are considered unlikely, we conclude
that there is no significant self-regulation of reionization by UVB feedback.Comment: 9 pages, 9 figure
Simultaneously constraining the astrophysics of reionisation and the epoch of heating with 21CMMC
The cosmic 21 cm signal is set to revolutionise our understanding of the
early Universe, allowing us to probe the 3D temperature and ionisation
structure of the intergalactic medium (IGM). It will open a window onto the
unseen first galaxies, showing us how their UV and X-ray photons drove the
cosmic milestones of the epoch of reionisation (EoR) and epoch of heating
(EoH). To facilitate parameter inference from the 21 cm signal, we previously
developed 21CMMC: a Monte Carlo Markov Chain sampler of 3D EoR simulations.
Here we extend 21CMMC to include simultaneous modelling of the EoH, resulting
in a complete Bayesian inference framework for the astrophysics dominating the
observable epochs of the cosmic 21 cm signal. We demonstrate that second
generation interferometers, the Hydrogen Epoch of Reionisation Array (HERA) and
Square Kilometre Array (SKA) will be able to constrain ionising and X-ray
source properties of the first galaxies with a fractional precision of order
-10 per cent (1). The ionisation history of the Universe can be
constrained to within a few percent. Using our extended framework, we quantify
the bias in EoR parameter recovery incurred by the common simplification of a
saturated spin temperature in the IGM. Depending on the extent of overlap
between the EoR and EoH, the recovered astrophysical parameters can be biased
by .Comment: 20 pages, 10 figures, 4 tables. Accepted to MNRAS (matches online
version). Movies showing the imprint of the astrophysical parameters on the
21cm signal can be found at http://homepage.sns.it/mesinger/21CMMC.htm
The depletion of gas in high-redshift dwarf galaxies from an inhomogeneous reionization
The reionization of the intergalactic medium (IGM) was likely inhomogeneous
and extended. By heating the IGM and photo-evaporating gas from the outskirts
of galaxies, this process can have a dramatic impact on the growth of
structures. Using a suite of spherically-symmetric collapse simulations
spanning a large parameter space, we study the impact of an ionizing
ultraviolet background (UVB) on the condensation of baryons onto dark matter
halos. We present an expression for the halo baryon fraction, which is an
explicit function of: (i) halo mass; (ii) UVB intensity; (iii) redshift; (iv)
redshift at which the halo was exposed to a UVB. We also present a
corresponding expression for the characteristic or critical mass, defined as
the halo mass which retains half of its baryons compared to the global value.
Since our results are general and physically-motivated, they can be broadly
applied to inhomogeneous reionization models.Comment: 5 pages, 3 figure
Early galaxy formation in warm dark matter cosmologies
We present a framework for high-redshift () galaxy formation that
traces their dark matter (DM) and baryonic assembly in four cosmologies: Cold
Dark Matter (CDM) and Warm Dark Matter (WDM) with particle masses of
1.5, 3 and 5 . We use the same astrophysical parameters regulating
star formation and feedback, chosen to match current observations of the
evolving ultra violet luminosity function (UV LF). We find that the assembly of
observable (with current and upcoming instruments) galaxies in CDM and WDM results in similar halo mass to light ratios (M/L),
stellar mass densities (SMDs) and UV LFs. However the suppression of
small-scale structure leads to a notably delayed and subsequently more rapid
stellar assembly in the WDM model. Thus galaxy assembly in WDM cosmologies is characterized by: (i) a dearth of
small-mass halos hosting faint galaxies; and (ii) a younger, more UV bright
stellar population, for a given stellar mass. The higher M/L ratio (effect ii)
partially compensates for the dearth of small-mass halos (effect i), making the
resulting UV LFs closer to CDM than expected from simple estimates of halo
abundances. We find that the redshift evolution of the SMD is a powerful probe
of the nature of DM. Integrating down to a limit of for the
James Webb Space Telescope (JWST), the SMD evolves as (SMD) in WDM, as compared to (SMD) in CDM. Thus high-redshift stellar assembly provides a powerful testbed
for WDM models, accessible with the upcoming JWST.Comment: Accepted for publication in Ap
The Cosmic 21-cm Revolution Charting the first billion years of our universe
The redshifted 21-cm signal is set to transform astrophysical cosmology, bringing a historically data-starved field into the era of Big Data. Corresponding to the spin-flip transition of neutral hydrogen, the 21-cm line is sensitive to the temperature and ionization state of the cosmic gas, as well as to cosmological parameters. Crucially, with the development of new interferometers it will allow us to map out the first billion years of our universe, enabling us to learn about the properties of the unseen first generations of galaxies. Rapid progress is being made on both the observational and theoretical fronts, and important decisions on techniques and future direction are being made. The Cosmic 21-cm Revolution gathers contributions from current leaders in this fast-moving field, providing both an overview for graduate students and a reference point for current researchers
Feedback-regulated Super Massive Black Hole Seed Formation
The nature of the seeds of high-redshift supermassive black holes (SMBHs) is
a key question in cosmology. Direct collapse black holes (DCBH) that form in
pristine, atomic-line cooling halos, illuminated by a Lyman-Werner (LW) UV flux
exceeding a critical threshold J_crit, represent an attractive possibility. We
investigate when and where these conditions are met during cosmic evolution.
For the LW intensity, J_LW, we account for departures from the background value
in close proximity to star forming galaxies. For the pristine halo fraction, we
account for both (i) supernova driven outflows, and (ii) the inherent pollution
from progenitor halos. We estimate the abundance of DCBH formation sites,
n_DCBH(z), and find that it increases with cosmic time from n_DCBH(z=20) ~
1e-12 -1e-7 cMpc^-3 to n_DCBH(z=10) ~ 1e-10 - 1e-5 cMpc^-3. Our analysis shows
the possible importance of galactic winds, which can suppress the predicted
n_DCBH by several orders of magnitude, and cause DCBH formation to
preferentially occur around the UV-brightest (M_UV ~ -22 to -20) star forming
galaxies. Our analysis further highlights the dependence of these predictions
on (i) the escape fraction of LW photons, (ii) J_crit, and (iii) the galactic
outflow prescription.Comment: 13 pages, 9 figures, accepted to MNRA
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