51 research outputs found
Simulating the Impact of X-ray Heating during the Cosmic Dawn
Upcoming observations of the 21-cm signal from the Epoch of Reionization will
soon provide the first direct detection of this era. This signal is influenced
by many astrophysical effects, including long range X-ray heating of the
intergalactic gas. During the preceding Cosmic Dawn era the impact of this
heating on the 21-cm signal is particularly prominent, especially before spin
temperature saturation. We present the largest-volume (349\,Mpc
comoving=244~Mpc) full numerical radiative transfer simulations to date
of this epoch that include the effects of helium and multi-frequency heating,
both with and without X-ray sources. We show that X-ray sources contribute
significantly to early heating of the neutral intergalactic medium and, hence,
to the corresponding 21-cm signal. The inclusion of hard, energetic radiation
yields an earlier, extended transition from absorption to emission compared to
the stellar-only case. The presence of X-ray sources decreases the absolute
value of the mean 21-cm differential brightness temperature. These hard sources
also significantly increase the 21-cm fluctuations compared the common
assumption of temperature saturation. The 21-cm differential brightness
temperature power spectrum is initially boosted on large scales, before
decreasing on all scales. Compared to the case of the cold, unheated
intergalactic medium, the signal has lower rms fluctuations and increased
non-Gaussianity, as measured by the skewness and kurtosis of the 21-cm
probability distribution functions. Images of the 21-cm signal with resolution
around 11~arcmin still show fluctuations well above the expected noise for deep
integrations with the SKA1-Low, indicating that direct imaging of the X-ray
heating epoch could be feasible.Comment: 13 pages, 8 figure
Clues to the nature of dark matter from first galaxies
We use thirty-eight high-resolution simulations of galaxy formation between
redshift 10 and 5 to study the impact of a 3 keV warm dark matter (WDM)
candidate on the high-redshift Universe. We focus our attention on the stellar
mass function and the global star formation rate and consider the consequences
for reionization, namely the neutral hydrogen fraction evolution and the
electron scattering optical depth. We find that three different effects
contribute to differentiate warm and cold dark matter (CDM) predictions: WDM
suppresses the number of haloes with mass less than few M; at
a fixed halo mass, WDM produces fewer stars than CDM; and finally at halo
masses below M, WDM has a larger fraction of dark haloes than
CDM post-reionization. These three effects combine to produce a lower stellar
mass function in WDM for galaxies with stellar masses at and below
M. For , the global star formation density is lower by a
factor of two in the WDM scenario, and for a fixed escape fraction, the
fraction of neutral hydrogen is higher by 0.3 at . This latter
quantity can be partially reconciled with CDM and observations only by
increasing the escape fraction from 23 per cent to 34 per cent. Overall, our
study shows that galaxy formation simulations at high redshift are a key tool
to differentiate between dark matter candidates given a model for baryonic
physics.Comment: 11 pages, 8 figures, submitted to MNRA
NIHAO XX: The impact of the star formation threshold on the cusp-core transformation of cold dark matter haloes
We use cosmological hydrodynamical galaxy formation simulations from the
NIHAO project to investigate the impact of the threshold for star formation on
the response of the dark matter (DM) halo to baryonic processes. The fiducial
NIHAO threshold, , results in strong expansion of the DM
halo in galaxies with stellar masses in the range . We find that lower thresholds such as (as employed
by the EAGLE/APOSTLE and Illustris/AURIGA projects) do not result in
significant halo expansion at any mass scale. Halo expansion driven by
supernova feedback requires significant fluctuations in the local gas fraction
on sub-dynamical times (i.e., < 50 Myr at galaxy half-light radii), which are
themselves caused by variability in the star formation rate. At one per cent of
the virial radius, simulations with have gas fractions of
and variations of , while simulations have order of
magnitude lower gas fractions and hence do not expand the halo. The observed DM
circular velocities of nearby dwarf galaxies are inconsistent with CDM
simulations with and , but in reasonable agreement with .
Star formation rates are more variable for higher , lower galaxy masses, and
when star formation is measured on shorter time scales. For example,
simulations with have up to 0.4 dex higher scatter in specific star
formation rates than simulations with . Thus observationally
constraining the sub-grid model for star formation, and hence the nature of DM,
should be possible in the near future.Comment: 18 pages, 13 figures, accepted to MNRA
The edge of galaxy formation III: The effects of warm dark matter on Milky Way satellites and field dwarfs
In this third paper of the series, we investigate the effects of warm dark
matter with a particle mass of on the smallest
galaxies in our Universe. We present a sample of 21 hydrodynamical cosmological
simulations of dwarf galaxies and 20 simulations of satellite-host galaxy
interaction that we performed both in a Cold Dark Matter (CDM) and Warm Dark
Matter (WDM) scenario. In the WDM simulations, we observe a higher critical
mass for the onset of star formation. Structure growth is delayed in WDM, as a
result WDM haloes have a stellar population on average two Gyrs younger than
their CDM counterparts. Nevertheless, despite this delayed star formation, CDM
and WDM galaxies are both able to reproduce the observed scaling relations for
velocity dispersion, stellar mass, size, and metallicity at . WDM
satellite haloes in a Milky Way mass host are more susceptible to tidal
stripping due to their lower concentrations, but their galaxies can even
survive longer than the CDM counterparts if they live in a dark matter halo
with a steeper central slope. In agreement with our previous CDM satellite
study we observe a steepening of the WDM satellites' central dark matter
density slope due to stripping. The difference in the average stellar age for
satellite galaxies, between CDM and WDM, could be used in the future for
disentangling these two models.Comment: 10 pages, 11 figures, accepted for publication on MNRA
Relative baryon-dark matter velocities in cosmological zoom simulations
Supersonic relative motion between baryons and dark matter due to the
decoupling of baryons from the primordial plasma after recombination affects
the growth of the first small-scale structures. Large box sizes (greater than a
few hundred Mpc) are required to sample the full range of scales pertinent to
the relative velocity, while the effect of the relative velocity is strongest
on small scales (less than a few hundred kpc). This separation of scales
naturally lends itself to the use of `zoom' simulations, and here we present
our methodology to self-consistently incorporate the relative velocity in zoom
simulations, including its cumulative effect from recombination through to the
start time of the simulation. We apply our methodology to a large-scale
cosmological zoom simulation, finding that the inclusion of relative velocities
suppresses the halo baryon fraction by -- per cent between and
, in qualitative agreement with previous works. In addition, we find that
including the relative velocity delays the formation of star particles by Myr on average (of the order of the lifetime of a Population III star) and suppresses the final stellar mass by as much
as per cent at .Comment: 14 pages, 12 figures. Accepted for publication in MNRA
The large-scale observational signatures of low-mass galaxies during reionization
Observations of the epoch of reionization give us clues about the nature and
evolution of the sources of ionizing photons, or early stars and galaxies. We
present a new suite of structure formation and radiative transfer simulations
from the PRACE4LOFAR project designed to investigate whether the mechanism of
radiative feedback, or the suppression of star formation in ionized regions
from UV radiation, can be inferred from these observations. Our source halo
mass extends down to , with sources in the mass range to
expected to be particularly susceptible to feedback from
ionizing radiation, and we vary the aggressiveness and nature of this
suppression. Not only do we have four distinct source models, we also include
two box sizes (67 Mpc and 349 Mpc), each with two grid resolutions. This suite
of simulations allows us to investigate the robustness of our results. All of
our simulations are broadly consistent with the observed electron-scattering
optical depth of the cosmic microwave background and the neutral fraction and
photoionization rate of hydrogen at . In particular, we investigate the
redshifted 21-cm emission in anticipation of upcoming radio interferometer
observations. We find that the overall shape of the 21-cm signal and various
statistics are robust to the exact nature of source suppression, the box size,
and the resolution. There are some promising model discriminators in the
non-Gaussianity and small-scale power spectrum of the 21-cm signal.Comment: 21 pages, 15 figures, MNRAS accepte
Evaluating the QSO contribution to the 21-cm signal from the Cosmic Dawn
The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the redshifted 21-cm signal from the neutral hydrogen present during the Cosmic Dawn. Temperature fluctuations from X-ray heating of the neutral intergalactic medium can dominate the fluctuations in the 21-cm signal from this time. This heating depends on the abundance, clustering, and properties of the X-ray sources present, which remain highly uncertain. We present a suite of three new large-volume, 349 Mpc a side, fully numerical radiative transfer simulations including QSO-like sources, extending the work previously presented in Ross et al. (2017). The results show that our QSOs have a modest contribution to the heating budget, yet significantly impact the 21-cm signal. Initially, the power spectrum is boosted on large scales by heating from the biased QSO-like sources, before decreasing on all scales. Fluctuations from images of the 21-cm signal with resolutions corresponding to SKA1-Low at the appropriate redshifts are well above the expected noise for deep integrations, indicating that imaging could be feasible for all the X-ray source models considered. The most notable contribution of the QSOs is a dramatic increase in non-Gaussianity of the signal, as measured by the skewness and kurtosis of the 21-cm probability distribution functions. However, in the case of late Lyman-α saturation, this non-Gaussianity could be dramatically decreased particularly when heating occurs earlier. We conclude that increased non-Gaussianity is a promising signature of rare X-ray sources at this time, provided that Lyman-α saturation occurs before heating dominates the 21-cm signal
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