88 research outputs found
High redshift AGNs and HI reionisation: limits from the unresolved X-ray background
The rapidly declining population of bright quasars at z~3 appears to make an
increasingly small contribution to the ionising background at the HI Lyman
limit. It is then generally though that massive stars in (pre-)galactic systems
may provide the additional ionising flux needed to complete HI reionisation by
z>6. A galaxy dominated background, however, may require that the escape
fraction of Lyman continuum radiation from high redshift galaxies is as high as
10%, a value somewhat at odds with (admittedly scarce) observational
constraints. High escape fractions from dwarf galaxies have been advocated, or,
alternatively, a so-far undetected (or barely detected) population of
unobscured, high-redshift faint AGNs. Here we question the latter hypothesis,
and show that such sources, to be consistent with the measured level of the
unresolved X-ray background at z=0, can provide a fraction of the HII filling
factor not larger than 13% by z=6. The fraction rises to <27% in the somewhat
extreme case of a constant comoving redshift evolution of the AGN emissivity.
This still calls for a mean escape fraction of ionising photons from high-z
galaxies >10%.Comment: A&A Letter, accepted (4 pages, 2 figures
Constraints on the Accretion History of Massive Black Holes from Faint X-ray Counts
We investigate how hierarchical models for the co-evolution of the massive
black hole (MBH) and AGN population can reproduce the observed faint X-ray
counts. We find that the main variable influencing the theoretical predictions
is the Eddington ratio of accreting sources. We compare three different models
proposed for the evolution of AGN Eddington ratio, f_Edd: constant f_Edd=1,
f_Edd decreasing with redshift, and f_Edd depending on the AGN luminosity, as
suggested by simulations of galactic mergers including BHs and AGN feedback. We
follow the full assembly of MBHs and host halos from early times to the present
in a LambdaCDM cosmology. AGN activity is triggered by halo major mergers and
MBHs accrete mass until they satisfy the observed correlation with velocity
dispersion. We find that all three models can reproduce fairly well the total
faint X-ray counts. The redshift distribution is however poorly matched in the
first two models. The Eddington ratios suggested by merger simulations predicts
no turn-off of the faint end of the AGN optical luminosity function at
redshifts z>=1 down to very low luminosity.Comment: MNRAS in press, 7 pages, 8 figure
Ultra-faint high-redshift galaxies in the Frontier Fields
By combining cosmological simulations with Frontier Fields project lens
models we find that, in the most optimistic case, galaxies as faint as (AB magnitude at ) can be detected in the
Frontier Fields. Such faint galaxies are hosted by dark matter halos of mass
and dominate the ionizing photon budget over currently
observed bright galaxies, thus allowing for the first time the investigation of
the dominant reionization sources. In addition, the observed number of these
galaxies can be used to constrain the role of feedback in suppressing star
formation in small halos: for example, if galaxy formation is suppressed in
halos with circular velocity km s, galaxies fainter than
should not be detected in the FFs.Comment: 5 pages, 7 figures, accepted for publication in MNRAS Letter
Counts of high-redshift GRBs as probe of primordial non-Gaussianities
We propose to use high-redshift long -ray bursts (GRBs) as
cosmological tools to constrain the amount of primordial non-Gaussianity in the
density field. By using numerical, N-body, hydrodynamic, chemistry simulations
of different cosmological volumes with various Gaussian and non-Gaussian
models, we self-consistently relate the cosmic star formation rate density to
the corresponding GRB rate. Assuming that GRBs are fair tracers of cosmic star
formation, we find that positive local non-Gaussianities, described in terms of
the non-linear parameter, \fnl, might boost significantly the GRB rate at high
redshift, . Deviations with respect to the Gaussian case account for a
few orders of magnitude if \fnl, one order of magnitude for
\fnl, and a factor of for \fnl. These differences
are found only at large redshift, while at later times the rates tend to
converge. Furthermore, a comparison between our predictions and the observed
GRB data at allows to exclude large negative \fnl, consistently with
previous works. Future detections of any long GRB at extremely high redshift
() could favor non-Gaussian scenarios with positive \fnl. More
stringent constraints require much larger high- GRB complete samples,
currently not available in literature. By distinguishing the contributions to
the GRB rate from the metal-poor population III regime, and the metal-enriched
population II-I regime, we conclude that the latter is a more solid tracer of
the underlying matter distribution, while the former is strongly dominated by
feedback mechanisms from the first, massive, short-lived stars, rather than by
possible non-Gaussian fluctuations. This holds quite independently of the
assumed population III initial mass function.Comment: 12 pages; MNRAS in press. Chi ha paura muore tutti i giorn
The brief era of direct collapse black hole formation
It has been proposed that the first, intermediate-mass () black holes might form through direct collapse of unpolluted
gas in atomic-cooling halos exposed to a strong Lyman-Werner (LW) or
near-infrared (NIR) radiation. As these systems are expected to be
Compton-thick, photons above 13.6 eV are largely absorbed and re-processed into
lower energy bands. It follows that direct collapse black holes (DCBHs) are
very bright in the LW/NIR bands, typically outshining small high-redshift
galaxies by more than 10 times. Once the first DCBHs form, they then trigger a
runaway process of further DCBH formation, producing a sudden rise in their
cosmic mass density. The universe enters the "DCBH era" at when
a large fraction of atomic-cooling halos are experiencing DCBH formation. By
combining the clustering properties of the radiation sources with Monte Carlo
simulations we show that in this scenario the DCBH mass density rises from
~ Mpc at to the peak value Mpc at in our fiducial model. However, the
abundance of \textit{active} (accreting) DCBHs drops after , as gas
in the potential formation sites (unpolluted halos with virial temperature
slightly above ~K) is photoevaporated. This effect almost completely
suppresses DCBH formation after . The DCBH formation era lasts only
Myr, but it might crucially provide the seeds of the supermassive
black holes (SMBHs) powering quasars.Comment: 26 pages, 4 figures, 2 tables. Accepted for publication in MNRA
Infrared background signatures of the first black holes
Angular fluctuations of the Near InfraRed Background (NIRB) intensity are
observed up to scales \simlt 1^{\ensuremath{^{\circ}}}. Their interpretation
is challenging as even after removing the contribution from detected sources,
the residual signal is times higher than expected from distant galaxies
below the detection limit and first stars. We propose here a novel
interpretation in which early, intermediate mass, accreting direct collapse
black holes (DCBH), which are too faint to be detected individually in current
surveys, could explain the observed fluctuations. We find that a population of
highly obscured (N_{\rm H}\simgt 10^{25} \rm cm^{-2}) DCBHs formed in
metal-free halos with virial temperature K at z\simgt 12, can explain
the observed level (nW m sr of the 3.6 and
4.5 m fluctuations on scales . The signal on smaller scales is
instead produced by undetected galaxies at low and intermediate redshifts.
Albeit Compton-thick, at scales DCBHs produce a CXB (0.5-2
keV)-NIRB () cross-correlation signal of erg
s cm nW m sr slightly dependent on the specific
value of the absorbing gas column ()
adopted and in agreement with the recent measurements by
\cite{2012arXiv1210.5302C}. At smaller scales the cross-correlation is
dominated by the emission of high-mass X-ray binaries (HMXB) hosted by the same
low-, undetected galaxies accounting for small scale NIRB fluctuations.
These results outline the great potential of the NIRB as a tool to investigate
the nature of the first galaxies and black holes.Comment: 27 pages, 8 figures, accepted for publication in MNRA
High redshift Gamma-Ray Bursts
Ten years of operations of the Swift satellite have allowed us to collect a small sample of long Gamma-Ray Bursts (GRBs) at redshift larger than 6. I will review here the present status of this research field and discuss the possible use of GRBs as a fundamental new tool to explore the early Universe, complementary to quasar and galaxy surveys. <P /
XMM-Newton and INTEGRAL observations of the bright GRB 230307A : vanishing of the local absorption and limits on the dust in the Magellanic Bridge
230307A is the second brightest gamma ray burst detected in more than 50
years of observations and is located in the direction of the Magellanic Bridge.
Despite its long duration, it is most likely the result of the compact merger
of a binary ejected from a galaxy in the local universe (redshift z=0.065). Our
XMM-Newton observation of its afterglow at 4.5 days shows a power-law spectrum
with photon index , unabsorbed flux erg cm s and no absorption in
excess of that produced in our Galaxy and in the Magellanic Bridge. We derive a
limit of cm on the absorption
at the GRB redshift, which is a factor 5 below the value measured
during the prompt phase. We searched for the presence of dust scattering rings
with negative results and set an upper limit of the order of on the
absorption from dust in the Magellanic Bridge.Comment: Version accepted for publication on The Astrophysical Journal (a few
changes and more figures
The first orbital period of a very bright and fast Nova in M31: M31N 2013-01b
We present the first X-ray and UV/optical observations of a very bright and
fast nova in the disc of M31, M31N 2013-01b. The nova reached a peak magnitude
15 mag and decayed by 2 magnitudes in only 3 days, making it one of the
brightest and fastest novae ever detected in Andromeda. From archival
multi-band data we have been able to trace its fast evolution down to
mag in less than two weeks and to uncover for the first time the Super-Soft
X-ray phase, whose onset occurred 10-30 days from the optical maximum. The
X-ray spectrum is consistent with a blackbody with a temperature of 50 eV
and emitting radius of 4 cm, larger than a white dwarf
radius, indicating an expanded region. Its peak X-ray luminosity, 3.5 erg s, locates M31N 2013-01b among the most luminous novae in
M31. We also unambiguously detect a short 1.280.02 h X-ray periodicity
that we ascribe to the binary orbital period, possibly due to partial eclipses.
This makes M31N 2013-01b the first nova in M31 with an orbital period
determined. The short period also makes this nova one of the few known below
the 2-3 h orbital period gap. All the observed characteristics strongly
indicate that M31N 2013-01b harbours a massive white dwarf and a very low-mass
companion, consistent with being a nova belonging to the disc population of the
Andromeda Galaxy.Comment: 9 pages, 3 figures, 2 tables; accepted by the Astrophysical Journa
Probing intergalactic radiation fields during cosmic reionization through gamma-ray absorption
We discuss expectations for the absorption of high-energy gamma-rays by
gamma-gamma pair production with intergalactic radiation fields (IRFs) at very
high redshifts (z~5-20), and the prospects thereof for probing the cosmic
reionization era. For the evolving IRF, a semi-analytical model incorporating
both Population II and Population III stars is employed, which is consistent
with a wide variety of existing high-z observations including QSO spectral
measurements, WMAP Thomson depth constraints, near-IR source count limits, etc.
We find that the UV IRF below the Lyman edge energy with intensities in the
range of a few times 10^{-19} erg cm^{-2} s^{-1} Hz^{-1} sr^{-1} can cause
appreciable attenuation above ~12 GeV at z~5, down to ~6-8 GeV at z>~8-10. This
may be observable in the spectra of blazars or gamma-ray bursts by the Fermi
Gamma-ray Space Telescope or next generation facilities such as the Cherenkov
Telescope Array, Advanced Gamma-ray Imaging System or 5@5, providing invaluable
insight into early star formation and cosmic reionization.Comment: MNRAS in press with minor revisions, 5 pages, 5 figures. Numerical
data of the model results will be available at
http://www-tap.scphys.kyoto-u.ac.jp/~inoue/hizabs
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