60 research outputs found
The First Galaxies: Assembly under Radiative Feedback from the First Stars
We investigate how radiative feedback from the first stars affects the
assembly of the first dwarf galaxies. We perform cosmological zoomed SPH
simulations of a dwarf galaxy assembling inside a halo of virial mass 10^9
solar at z = 10. The simulations follow the non-equilibrium chemistry/cooling
of primordial gas and the conversion of the gas into metal-free stars. To
quantify the radiative feedback, we compare a simulation in which stars emit
both molecular hydrogen dissociating and hydrogen/helium ionizing radiation
with a simulation in which stars emit only dissociating radiation, and with a
simulation in which stars remain dark. Photodissociation and -ionization exert
a strong negative feedback on the assembly of the simulated galaxy. Gas
condensation is strongly impeded, and star formation is strongly suppressed in
comparison with the simulation in which stars remain dark. The feedback on the
gas implies a suppression of the central dark matter densities in the minihalo
progenitor by factors of up to a few, which is a significant deviation from the
singular isothermal density profile characterizing the dark matter distribution
in the absence of radiative feedback. The evolution of gas densities, star
formation rates, and the distribution of dark matter becomes insensitive to the
inclusion of dissociating radiation in the late stages of the minihalo
assembly, and it becomes insensitive to the inclusion of ionizing radiation
once the minihalo turns into an atomically cooling galaxy. The formation of an
extended disk inside the dwarf galaxy is a robust outcome not affected by the
inclusion of radiation. We estimate that dwarf galaxies such as simulated here
will be among the faintest galaxies the upcoming James Webb Space Telescope
will detect. Our conclusions are subject to our neglect of feedback from
supernovae and chemical enrichment as well as to cosmic variance. [abridged]Comment: 25 pages (including 5 pages appendix), 13 figures. Accepted for
publication in Ap
Radiative Feedback from high mass X-ray binaries on the formation of the first galaxies and early reionization
Recent work suggests that the first generation of stars, the so-called
Population III (Pop III), could have formed primarily in binaries or as members
of small multiple systems. Here we investigate the impact of X-ray feedback
from High-Mass X-ray Binaries (HMXBs) left behind in stellar binary systems
after the primary forms a black hole (BH), accreting gas at a high rate from
the companion, a process that is thought to be favored at the low metallicities
characteristic of high-redshift gas. Thanks to their large mean free path,
X-rays are capable of preionizing and preheating the gas in the intergalactic
medium (IGM) and in haloes long before the reionization of the Universe is
complete, and thus could have strongly affected the formation of subsequent
generations of stars as well as reionization. We have carried out zoomed
hydrodynamical cosmological simulations of minihaloes, accounting for the
formation of Pop III stars and their collapse into BHs and HMXBs, and the
associated radiation-hydrodynamic feedback from UV and X-ray photons. We find
no strong net feedback from HMXBs on the simulated star formation history. On
the other hand, the preheating of the IGM by HMXBs leads to a strong
suppression of small-scale structures and significantly lowers the
recombination rate in the IGM, thus yielding a net positive feedback on
reionization. We further show that X-ray feedback from HMXBs can augment the
ionizing feedback from the Pop III progenitor stars to suppress gas accretion
onto the first BHs, limiting their growth into supermassive BHs. Finally, we
show that X-ray ionization by HMXBs leaves distinct signatures in the
properties of the high-redshift hydrogen that may be probed in upcoming
observations of the redshifted 21cm spin-flip line.Comment: 20 pages, 12 figures. Accepted for publication in MNRA
The Mutual Interaction Between Population III Stars and Self-Annihilating Dark Matter
We use cosmological simulations of high-redshift minihalos to investigate the
effect of dark matter annihilation (DMA) on the collapse of primordial gas. We
numerically investigate the evolution of the gas as it assembles in a
Population III stellar disk. We find that when DMA effects are neglected, the
disk undergoes multiple fragmentation events beginning at ~ 500 yr after the
appearance of the first protostar. On the other hand, DMA heating and
ionization of the gas speeds the initial collapse of gas to protostellar
densities and also affects the stability of the developing disk against
fragmentation, depending on the DM distribution. We compare the evolution when
we model the DM density with an analytical DM profile which remains centrally
peaked, and when we simulate the DM profile using N-body particles (the 'live'
DM halo). When utilizing the analytical DM profile, DMA suppresses disk
fragmentation for ~ 3500 yr after the first protostar forms, in agreement with
earlier work. However, when using a 'live' DM halo, the central DM density peak
is gradually flattened due to the mutual interaction between the DM and the
rotating gaseous disk, reducing the effects of DMA on the gas, and enabling
secondary protostars of mass ~ 1 M_sol to be formed within ~ 900 yr. These
simulations demonstrate that DMA is ineffective in suppressing gas collapse and
subsequent fragmentation, rendering the formation of long-lived dark stars
unlikely. However, DMA effects may still be significant in the early collapse
and disk formation phase of primordial gas evolution.Comment: 17 pages, 11 figures, to appear in MNRA
The first galaxies: simulating their feedback-regulated assembly
We investigate the formation of a galaxy reaching a virial mass of
solar mass at by carrying out a zoomed radiation-hydrodynamical
cosmological simulation. This simulation traces Population~III (Pop~III) star
formation, characterized by a modestly top-heavy initial mass function (IMF),
and considers stellar feedback such as photoionization heating from Pop III and
Population~II (Pop~II) stars, mechanical and chemical feedback from supernovae
(SNe), and X-ray feedback from accreting black holes (BHs) and high-mass X-ray
binaries (HMXBs). We self-consistently impose a transition in star formation
mode from top-heavy Pop III to low-mass Pop~II, and find that the star
formation rate in the computational box is dominated by Pop~III until ,
and by Pop~II thereafter. The simulated galaxy experiences bursty star
formation, with a substantially reduced gas content due to photoionization
heating from Pop~III and Pop~II stars, together with SN feedback. All the gas
within the simulated galaxy is metal-enriched above solar, such that
there are no remaining pockets of primordial gas. The simulated galaxy has an
estimated observed flux of , which is too low to be detected by
the James Webb Space Telescope (JWST) without strong lensing amplification. We
also show that our simulated galaxy is similar in terms of stellar mass to
Segue 2, the least luminous dwarf known in the Local Group.Comment: 18 pages 14 figures, Accepted for publication in MNRA
The impact of local stellar radiation on the HI column density distribution
It is often assumed that local sources of ionizing radiation have little
impact on the distribution of HI in the post-reionization Universe. While this
is a good assumption for the IGM, analytic arguments suggest that local sources
may typically be more important than the background radiation for high column
density absorbers (N_HI > 10^17 /cm^2). We post-process cosmological
simulations with accurate radiation transport to investigate the impact of
local stellar sources on the HI distribution. We demonstrate that the limited
numerical resolution and the simplified treatment of the ISM that are typical
of current cosmological simulations provide significant challenges, but that
many of the problems can be overcome by taking two steps. First, using ISM
particles rather than stellar particles as sources results in a much better
sampling of the source distribution. Second, by rescaling the source
luminosities so that the amount of radiation escaping into the IGM agrees with
that required to produce the observed background, many of the results become
insensitive to errors in the predicted fraction of the radiation that escapes
the immediate vicinity of the sources. By adopting this strategy and by varying
the assumptions about the structure of the unresolved ISM, we conclude that we
can robustly estimate the effect of local sources for column densities N_HI <<
10^21 /cm^2. However, neither the escape fraction of ionizing radiation nor the
effect of local sources on the abundance of N_HI >~ 10^21 systems can be
predicted with confidence. We find that local stellar radiation is unimportant
for N_HI << 10^17, but that it can affect Lyman Limit and DLA systems. For
10^18 < N_HI < 10^21 the impact of local sources increases with redshift. At z
= 5 the abundance of absorbers with N_HI >> 10^17 is substantially reduced, but
at z = 0 the reduction only becomes significant for N_HI >~ 10^21 /cm^2.Comment: 19 pages, 12 figures; Accepted for publication in MNRA
The Source Density And Observability Of Pair-Instability Supernovae From The First Stars
Theoretical models predict that some of the first stars ended their lives as extremely energetic pair-instability supernovae (PISNe). With energies approaching 10(53) erg, these supernovae are expected to be within the detection limits of the upcoming James Webb Space Telescope (JWST), allowing observational constraints to be placed on the properties of the first stars. We estimate the source density of PISNe using a semi-analytic halo mass function based approach, accounting for the effects of feedback from star formation on the PISN rate using cosmological simulations. We estimate an upper limit of similar to 0.2 PISNe per JWST field of view at any given time. Feedback can reduce this rate significantly, e. g., lowering it to as little as one PISN per 4000 JWST fields of view for the most pessimistic explosion models. We also find that the main obstacle to observing PISNe from the first stars is their scarcity, not their faintness; exposures longer than a few times 10(4) s will do little to increase the number of PISNe found. Given this, we suggest a mosaic style search strategy for detecting PISNe from the first stars. Even rather high-redshift PISNe are unlikely to be missed by moderate exposures, and a large number of pointings will be required to ensure a detection.NSF AST-0708795, AST-1009928NASA ATFP NNX09AJ33GAstronom
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