92 research outputs found
Decoding the X-ray Properties of Pre-Reionization Era Sources
Evolution in the X-ray luminosity -- star formation rate (-SFR) relation
could provide the first evidence of a top-heavy stellar initial mass function
in the early universe, as the abundance of high-mass stars and binary systems
are both expected to increase with decreasing metallicity. The sky-averaged
(global) 21-cm signal has the potential to test this prediction via constraints
on the thermal history of the intergalactic medium, since X-rays can most
easily escape galaxies and heat gas on large scales. A significant complication
in the interpretation of upcoming 21-cm measurements is the unknown spectrum of
accreting black holes at high-, which depends on the mass of accreting
objects and poorly constrained processes such as how accretion disk photons are
processed by the disk atmosphere and host galaxy interstellar medium. Using a
novel approach to solving the cosmological radiative transfer equation (RTE),
we show that reasonable changes in the characteristic BH mass affects the
amplitude of the 21-cm signal's minimum at the mK level ---
comparable to errors induced by commonly used approximations to the RTE ---
while modifications to the intrinsic disk spectrum due to Compton scattering
(bound-free absorption) can shift the position of the minimum of the global
signal by (), and modify its
amplitude by up to mK ( mK) for a given accretion
history. Such deviations are larger than the uncertainties expected of current
global 21-cm signal extraction algorithms, and could easily be confused with
evolution in the -SFR relation.Comment: 14 pages, 7 figures, accepted for publication in MNRA
The Global 21-cm Signal in the Context of the High-z Galaxy Luminosity Function
Motivated by recent progress in studies of the high- Universe, we build a
new model for the global 21-cm signal that is explicitly calibrated to
measurements of the galaxy luminosity function (LF) and further tuned to match
the Thomson scattering optical depth of the cosmic microwave background,
. Assuming that the galaxy population can be smoothly
extrapolated to higher redshifts, the recent decline in best-fit values of
and the inefficient heating induced by X-ray binaries (HMXBs; the
presumptive sources of the X-ray background at high-) imply that the
entirety of cosmic reionization and reheating occurs at redshifts . In contrast to past global 21-cm models, whose (
MHz) absorption features and strong mK emission features were driven
largely by the assumption of efficient early star-formation and X-ray heating,
our new fiducial model peaks in absorption at MHz at a depth of
mK and has a negligible emission component. As a result, a strong
emission signal would provide convincing evidence that HMXBs are not the only
drivers of cosmic reheating. Shallow absorption troughs should accompany strong
heating scenarios, but could also be caused by a low escape fraction of
Lyman-Werner photons. Generating signals with troughs at MHz
requires a floor in the star-formation efficiency in halos below , which is equivalent to steepening the faint-end of the galaxy LF.
These findings demonstrate that the global 21-cm signal is a powerful
complement to current and future galaxy surveys and efforts to better
understand the interstellar medium in high- galaxies.Comment: 17 pages, 9 figures, in pres
Effects of self-consistent rest-ultraviolet colours in semi-empirical galaxy formation models
Connecting the observed rest-ultraviolet (UV) luminosities of high-
galaxies to their intrinsic luminosities (and thus star formation rates)
requires correcting for the presence of dust. We bypass a common
dust-correction approach that uses empirical relationships between infrared
(IR) emission and UV colours, and instead augment a semi-empirical model for
galaxy formation with a simple -- but self-consistent -- dust model and use it
to jointly fit high- rest-UV luminosity functions (LFs) and colour-magnitude
relations (-). In doing so, we find that UV colours
evolve with redshift (at fixed UV magnitude), as suggested by observations,
even in cases without underlying evolution in dust production, destruction,
absorption, or geometry. The observed evolution in our model arises due to the
reduction in the mean stellar age and rise in specific star formation rates
with increasing . The UV extinction, , evolves similarly
with redshift, though we find a systematically shallower relation between
and than that predicted by IRX-
relationships derived from galaxy samples. Finally, assuming that
high transmission () is a reliable LAE indicator,
modest scatter in the effective dust surface density of galaxies can explain
the evolution both in - and LAE fractions. These
predictions are readily testable by deep surveys with the James Webb Space
Telescope.Comment: 14+4 pages, 11+5 figures, accepted for publication in MNRA
The Persistence of Population III Star Formation
We present a semi-analytic model of star formation in the early universe,
beginning with the first metal-free stars. By employing a completely
feedback-limited star formation prescription, stars form at maximum efficiency
until the self-consistently calculated feedback processes halt formation. We
account for a number of feedback processes including a meta-galactic
Lyman-Werner background, supernovae, photoionization, and chemical feedback.
Halos are evolved combining mass accretion rates found through abundance
matching with our feedback-limited star formation prescription, allowing for a
variety of Population III (Pop III) initial mass functions (IMFs). We find
that, for a number of models, massive Pop III star formation can continue on
until at least and potentially past at rates of around
to M yr Mpc, assuming these stars
form in isolation. At this point Lyman-Werner feedback pushes the minimum halo
mass for star formation above the atomic cooling threshold, cutting off the
formation of massive Pop III stars. We find that, in most models, Pop II and
Pop III star formation co-exist over cosmological time-scales, with the total
star formation rate density and resulting radiation background strongly
dominated by the former before Pop III star formation finally ends. These halos
form at most M of massive Pop III stars during this phase
and typically have absolute magnitudes in the range of to . We also briefly discuss how future observations from telescopes such as
JWST or WFIRST and 21-cm experiments may be able to constrain unknown
parameters in our model such as the IMF, star formation prescription, or the
physics of massive Pop III stars.Comment: 16 pages, 13 figures, submitted to MNRA
On the expected purity of photometric galaxy surveys targeting the Cosmic Dawn
Over the last three decades, photometric galaxy selection using the
Lyman-break technique has transformed our understanding of the high-z Universe,
providing large samples of galaxies at 3 < z < 8 with relatively small
contamination. With the advent of the James Webb Space Telescope, the
Lyman-break technique has now been extended to z ~ 17. However, the purity of
the resulting samples has not been tested. Here we use a simple model, built on
the robust foundation of the dark matter halo mass function, to show that the
expected level of contamination rises dramatically at z > 10, especially for
luminous galaxies, placing stringent requirements on the selection process. The
most luminous sources at z > 12 are likely at least ten thousand times rarer
than potential contaminants, so extensive spectroscopic followup campaigns may
be required to identify a small number of target sources.Comment: 6 pages, 2 figures, submitted to MNRA
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