61 research outputs found
Radiation Backgrounds at Cosmic Dawn: X-Rays from Compact Binaries
We compute the expected X-ray diffuse background and radiative feedback on
the intergalactic medium (IGM) from X-ray binaries prior and during the epoch
of reionization. The cosmic evolution of compact binaries is followed using a
population synthesis technique that treats separately neutron stars and black
hole binaries in different spectral states and is calibrated to reproduce the
observed X-ray properties of galaxies at z<4. Together with an updated
empirical determination of the cosmic history of star formation, recent
modeling of the stellar mass-metallicity relation, and a scheme for absorption
by the IGM that accounts for the presence of ionized HII bubbles during the
epoch of reionization, our detailed calculations provide refined predictions of
the X-ray volume emissivity and filtered radiation background from "normal"
galaxies at z>6. Radiative transfer effects modulate the background spectrum,
which shows a characteristic peak between 1 and 2 keV. While the filtering of
X-ray radiation through the IGM slightly increases the mean excess energy per
photoionization, it also weakens the radiation intensity below 1 keV, lowering
the mean photoionization and heating rates. Numerical integration of the rate
and energy equations shows that the contribution of X-ray binaries to the
ionization of the bulk IGM is negligible, with the electron fraction never
exceeding 1%. Direct HeI photoionizations are the main source of IGM heating,
and the temperature of the largely neutral medium in between HII cavities
increases above the temperature of the cosmic microwave background (CMB) only
at z0.1.
Therefore, in this scenario, it is only at relatively late epochs that the bulk
of neutral intergalactic hydrogen may be observable in 21-cm emission against
the CMB.Comment: 14 pages, 8 figures, accepted for publication in The Astrophysical
Journa
Energy Feedback from X-ray Binaries in the Early Universe
X-ray photons, because of their long mean-free paths, can easily escape the
galactic environments where they are produced, and interact at long distances
with the inter-galactic medium, potentially having a significant contribution
to the heating and reionization of the early Universe. The two most important
sources of X-ray photons in the Universe are active galactic nuclei (AGN) and
X-ray binaries (XRBs). In this Letter we use results from detailed, large scale
population synthesis simulations to study the energy feedback of XRBs, from the
first galaxies (z~ 20) until today. We estimate that X-ray emission from XRBs
dominates over AGN at z>6-8. The shape of the spectral energy distribution of
the emission from XRBs shows little change with redshift, in contrast to its
normalization which evolves by ~4 orders of magnitude, primarily due to the
evolution of the cosmic star-formation rate. However, the metallicity and the
mean stellar age of a given XRB population affect significantly its X-ray
output. Specifically, the X-ray luminosity from high-mass XRBs per unit of
star-formation rate varies an order of magnitude going from solar metallicity
to less than 10% solar, and the X-ray luminosity from low-mass XRBs per unit of
stellar mass peaks at an age of ~300 Myr and then decreases gradually at later
times, showing little variation for mean stellar ages > 3 Gyr. Finally, we
provide analytical and tabulated prescriptions for the energy output of XRBs,
that can be directly incorporated in cosmological simulations.Comment: Accepted for publication to ApJ Letters, 6 pages, 2 figures, 2 table.
Significant changes to figure 2
r-process enrichment of ultra-faint dwarf galaxies by fast merging double neutron stars
The recent aLIGO/aVirgo discovery of gravitational waves from the neutron
star merger (NSM) GW170817 and the follow up kilonova observations have shown
that NSMs produce copious amount of r-process material. However, it is
difficult to reconcile the large natal kicks and long average merging times of
Double Neutron Stars (DNSs), with the levels of r-process enrichment seen in
ultra faint dwarf (UFD) galaxies such as Reticulum II and Tucana III. Assuming
that such dwarf systems have lost a significant fraction of their stellar mass
through tidal stripping, we conclude that contrary to most current models, it
is the DNSs with rather large natal kicks but very short merging timescales
that can enrich UFD-type galaxies. These binaries are either on highly
eccentric orbits, or form with very short separations due to an additional
mass-transfer between the first-born neutron star and a naked helium star,
progenitor of the second-born neutron star. These DNSs are born with a
frequency that agrees with the statistics of the r-process UFDs, and merge well
within the virial radius of their host halos, therefore contributing
significantly to their r-process enrichment.Comment: Accepted for publication in Ap
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