328 research outputs found
Cosmological constraints from the cluster contribution to the power spectrum of the soft X-ray background. New evidence for a low sigma_8
We use the X-ray power spectrum of the ROSAT all-sky survey in the R6 band
(approximately 0.9-1.3 keV) to set an upper limit on the galaxy cluster power
spectrum. The cluster power spectrum is modelled with a minimum number of
robust assumptions regarding the structure of the clusters. The power spectrum
of ROSAT sets an upper limit on the Omega_m-sigma_8 plane which excludes all
the models with sigma_8 above sigma_8 = 0.5/(Omega_m^0.38) in a flat LCDM
universe. We discuss the possible sources of systematic errors in our
conclusions, mainly dominated by the assumed L_x-T relation. Alternatively,
this relation could be constrained by using the X-ray power spectrum, if the
cosmological model is known. Our conclusions suggest that only models with a
low value of sigma_8 (sigma_8 < 0.8 for Omega_m = 0.3) may be compatible with
our upper limit. We also find that models predicting lower luminosities in
galaxy clusters are favoured. Reconciling our cosmological constraints with
these arising by other methods might require either a high entropy floor or
wide-spread presence of cooling flows in the low-redshift clusters.Comment: 14 pages, 19 plots (2 as gif files). MNRAS submitte
Constraining the Lifetime of Quasars from their Spatial Clustering
The lifetime t_Q of the luminous phase of quasars is constrained by current
observations to be between 10^6 and 10^8 years, but is otherwise unkown. We
model the quasar luminosity function in detail in the optical and X-ray bands
using the Press-Schechter formalism, and show that the expected clustering of
quasars depends strongly on their assumed lifetime. We quantify this
dependence, and find that existing measurements of the correlation length of
quasars are consistent with the range 10^6 < t_Q < 10^8 years. We then show
that future measurements of the power spectrum of quasars out to z=3, from the
2dF or Sloan Digital Sky Survey, can significantly improve this constraint, and
in principle allow a precise determination of t_Q. We estimate the systematic
errors introduced by uncertainties in the modeling of the quasar-halo
relationship, as well as by the possible existence of obscured quasars.Comment: ApJ, in press (emulateapj
Constraints on the distribution of absorption in the X-ray selected AGN population found in the 13H XMM-Newton/Chandra deep field
We present an analysis of the X-ray properties of sources detected in the 13H
XMM-Newton deep (200ks) field. In order to constrain the absorbed AGN
population, we use extensive Monte Carlo simulations to directly compare the
X-ray colours of observed sources with those predicted by several model
distributions. We have tested the simplest form of the AGN unified scheme,
whereby the intrinsic XLF of absorbed AGN is set to be the same as that of
their unabsorbed brethren, coupled with various model distributions of
absorption. The best fitting of these models sets the fraction of AGN with
absorbing column NH, proportional to (logNH)^8. We have also tested two
extensions to the unified scheme: an evolving absorption scenario, and a
luminosity dependent model distribution. Both of these provide poorer matches
to the observed X-ray colour distributions than the best fitting simple unified
model. We find that a luminosity dependent density evolution XLF reproduces
poorly the 0.5-2 keV source counts seen in the 13H field. Field to field
variations could be the cause of this disparity. Computing the simulated X-ray
colours with a simple absorbed power-law + reflection spectral model is found
to over-predict, by a factor of two, the fraction of hard sources that are
completely absorbed below 0.5 keV, implying that an additional source of
soft-band flux must be present for a number of the absorbed sources. Finally,
we show that around 40% of the 13H sample are expected to be AGN with NH>10^22
cm^-2.Comment: 13 pages, 9 figures, Accepted for publication in MNRA
The late stages of evolution of helium star-neutron star binaries and the formation of double neutron star systems
With a view to understanding the formation of double neutron-stars (DNS), we
investigate the late stages of evolution of helium stars with masses of 2.8 -
6.4 Msun in binary systems with a 1.4 Msun neutron-star companion. We found
that mass transfer from 2.8 - 3.3 Msun helium stars and from 3.3 - 3.8 Msun in
very close orbits (P_orb > 0.25d) will end up in a common-envelope (CE) and
spiral-in phase due to the development of a convective helium envelope. If the
neutron star has sufficient time to complete the spiraling-in process before
the core collapses, the system will produce very tight DNSs (P_orb ~ 0.01d)
with a merger timescale of the order of 1 Myr or less. These systems would have
important consequences for the detection rate of GWR and for the understanding
of GRB progenitors. On the other hand, if the time left until the explosion is
shorter than the orbital-decay timescale, the system will undergo a SN
explosion during the CE phase. Helium stars with masses 3.3 - 3.8 Msun in wider
orbits (P_orb > 0.25d) and those more massive than 3.8 Msun do not go through
CE evolution. The remnants of these massive helium stars are DNSs with periods
in the range of 0.1 - 1 d. This suggests that this range of mass includes the
progenitors of the galactic DNSs with close orbits (B1913+16 and B1534+12). A
minimum kick velocity of 70 km/s and 0 km/s (for B1913+16 and B1534+12,
respectively) must have been imparted at the birth of the pulsar's companion.
The DNSs with wider orbits (J1518+4904 and probably J1811-1736) are produced
from helium star-neutron star binaries which avoid RLOF, with the helium star
more massive than 2.5 Msun. For these systems the minimum kick velocities are
50 km/s and 10 km/s (for J1518+4904 and J1811-1736, respectively).Comment: 16 pages, latex, 12 figures, accepted for publication in MNRA
The FMOS-COSMOS survey of star-forming galaxies at z~1.6. IV: Excitation state and chemical enrichment of the interstellar medium
We investigate the physical conditions of ionized gas in high-z star-forming
galaxies using diagnostic diagrams based on the rest-frame optical emission
lines. The sample consists of 701 galaxies with an Ha detection at , from the FMOS-COSMOS survey, that represent the normal
star-forming population over the stellar mass range with those at being
well sampled. We confirm an offset of the average location of star-forming
galaxies in the BPT diagram ([OIII]/Hb vs. [NII]/Ha), primarily towards higher
[OIII]/Hb, compared with local galaxies. Based on the [SII] ratio, we measure
an electron density (), that is higher
than that of local galaxies. Based on comparisons to theoretical models, we
argue that changes in emission-line ratios, including the offset in the BPT
diagram, are caused by a higher ionization parameter both at fixed stellar mass
and at fixed metallicity with additional contributions from a higher gas
density and possibly a hardening of the ionizing radiation field. Ionization
due to AGNs is ruled out as assessed with Chandra. As a consequence, we revisit
the mass-metallicity relation using [NII]/Ha and a new calibration including
[NII]/[SII] as recently introduced by Dopita et al. Consistent with our
previous results, the most massive galaxies ()
are fully enriched, while those at lower masses have metallicities lower than
local galaxies. Finally, we demonstrate that the stellar masses, metallicities
and star formation rates of the FMOS sample are well fit with a
physically-motivated model for the chemical evolution of star-forming galaxies.Comment: 38 pages; Accepted for publication in Ap
Solving discrete logarithms on a 170-bit MNT curve by pairing reduction
Pairing based cryptography is in a dangerous position following the
breakthroughs on discrete logarithms computations in finite fields of small
characteristic. Remaining instances are built over finite fields of large
characteristic and their security relies on the fact that the embedding field
of the underlying curve is relatively large. How large is debatable. The aim of
our work is to sustain the claim that the combination of degree 3 embedding and
too small finite fields obviously does not provide enough security. As a
computational example, we solve the DLP on a 170-bit MNT curve, by exploiting
the pairing embedding to a 508-bit, degree-3 extension of the base field.Comment: to appear in the Lecture Notes in Computer Science (LNCS
A Classic Type 2 QSO
In the Chandra Deep Field South 1Msec exposure we have found, at redshift
3.700 +- 0.005, the most distant Type 2 AGN ever detected. It is the source
with the hardest X-ray spectrum with redshift z>3. The optical spectrum has no
detected continuum emission to a 3sigma detection limit of ~3 10^{-19}
ergs/s/cm^2/AA and shows narrow lines of Ly_alpha, CIV, NV, HeII, OVI, [OIII],
and CIII]. Their FWHM line widths have a range of ~700-2300 km/s with an
average of approximately ~1500 km/s. The emitting gas is metal rich (Z ~2.5-3
Z_solar). In the X-ray spectrum of 130 counts in the 0.5-7 keV band there is
evidence for intrinsic absorption with N_H > 10^{24} cm^{-2}. An iron K_alpha
line with rest frame energy and equivalent width of ~6.4 keV and ~1 keV,
respectively, in agreement with the obscuration scenario, is detected at a
2sigma level. If confirmed by our forthcoming XMM observations this would be
the highest redshift detection of FeK_alpha. Depending on the assumed cosmology
and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for
absorption is ~10^{45 +- 0.5} ergs/s, which makes our source a classic example
of the long sought Type 2 QSOs. From standard population synthesis models,
these sources are expected to account for a relevant fraction of the
black-hole-powered QSO distribution at high redshift.Comment: 24 LaTeX pages including 6 postscript figures. Revised version,
accepted by Ap
Ultr-Luminous Infrared Galaxies: QSOs in Formation?
We present new near-infrared Keck and VLT spectroscopic data on the stellar
dynamics in late stage, ultra-luminous infrared galaxy (ULIRG) mergers . We now
have information on the structural and kinematic properties of 18 ULIRGs, 8 of
which contain QSO-like active galactic nuclei. The host properties (velocity
dispersion, effective radius, effective surface brightness, M_K) of
AGN-dominated and star formation dominated ULIRGs are similar. ULIRGs fall
remarkably close to the fundamental plane of early type galaxies. They populate
a wide range of the plane, are on average similar to L*-rotating ellipticals,
but are well offset from giant ellipticals and optically/UV bright, low-z
QSOs/radio galaxies. ULIRGs and local QSOs/radio galaxies are very similar in
their distributions of bolometric and extinction corrected near-IR
luminosities, but ULIRGs have smaller effective radii and velocity dispersions
than the local QSO/radio galaxy population. Hence, their host masses and
inferred black hole masses are correspondingly smaller. The latter are more
akin to those of local Seyfert galaxies. ULIRGs thus resemble local QSOs in
their near-IR and bolometric luminosities because they are (much more)
efficiently forming stars and/or feeding their black holes, and not because
they have QSO-like, very massive black holes. We conclude that ULIRGs as a
class cannot evolve into optically bright QSOs. They will more likely become
quiescent, moderate mass field ellipticals or, when active, might resemble the
X-ray bright, early type galaxies that have recently been found by the Chandra
Observatory.Comment: accepted to be published in ApJ, 7 figure
Chandra Detection of a TypeII Quasar at z=3.288
We report on observations of a TypeII quasar at redshift z=3.288, identified
as a hard X-ray source in a 185 ks observation with the Chandra X-ray
Observatory and as a high-redshift photometric candidate from deep, multiband
optical imaging. CXOJ084837.9+445352 (hereinafter CXO52) shows an unusually
hard X-ray spectrum from which we infer an absorbing column density N(H) =
(4.8+/-2.1)e23 / cm2 (90% confidence) and an implied unabsorbed 2-10 keV
rest-frame luminosity of L(2-10) = 3.3e44 ergs/s, well within the quasar
regime. Hubble Space Telescope imaging shows CXO52 to be elongated with slight
morphological differences between the WFPC2 F814W and NICMOS F160W bands.
Optical and near-infrared spectroscopy of CXO52 show high-ionization emission
lines with velocity widths ~1000 km/s and flux ratios similar to a Seyfert2
galaxy or radio galaxy. The latter are the only class of high-redshift TypeII
luminous AGN which have been extensively studied to date. Unlike radio
galaxies, however, CXO52 is radio quiet, remaining undetected at radio
wavelengths to fairly deep limits, f(4.8GHz) < 40 microJy. High-redshift TypeII
quasars, expected from unification models of active galaxies and long-thought
necessary to explain the X-ray background, are poorly constrained
observationally with few such systems known. We discuss recent observations of
similar TypeII quasars and detail search techniques for such systems: namely
(1) X-ray selection, (2) radio selection, (3) multi-color imaging selection,
and (4) narrow-band imaging selection. Such studies are likely to begin
identifying luminous, high-redshift TypeII systems in large numbers. We discuss
the prospects for these studies and their implications to our understanding of
the X-ray background.Comment: 28 pages, 5 figures; to appear in The Astrophysical Journa
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