242 research outputs found
Supermassive black holes as sources for LISA
Some issues relevant for the formation of supermassive black holes are
discused and estimates of the event rates for the emission of gravitational
waves by coalescing supermassive black hole binaries are given. The models take
into account recent improvements in our knowledge of galaxy and star formation
in the high-redshift universe. Estimated event rates range from a few to a
hundred per year. Typical events will occur at redshift three or larger in
galaxies lying at the (very) faint end of the luminosity function at these
redshifts.Comment: 5 pages, LaTeX, 2 postscript figures included; invited talk at the
Second International LISA Symposium, Pasadena, July 1998 (ed. W. Folkner,
American Institute of Physics
Using the kinematic Sunyaev-Zeldovich effect to determine the peculiar velocities of clusters of galaxies
We have investigated the possibility of inferring peculiar velocities for
clusters of galaxies from the Doppler shift of scattered cosmic microwave
background (CMB) photons. We find that if the core radius of the gas
distribution or the beam size of the instrument is larger than 3-7 arcminutes,
then the maximum attainable signal-to-noise ratio is determined by confusion
with primary fluctuations. For smaller angular scales, ``cosmic confusion'' is
less important and instrumental noise and/or foreground emission will be the
limiting factor. For a cluster with the optical depth of the Coma cluster and
for an optimal filtering technique, typical one-sigma errors span the wide
range from 400 to 1600 km/s, depending on the cosmological model, the
resolution of the instrument and the core radius of the cluster. The results
have important implications for the design of future high-resolution surveys of
the CMB. Individual peculiar velocities will be measurable only for a few fast
moving clusters at intermediate redshift unless cosmic fluctuations are smaller
than most standard cosmological scenarios predict. However, a reliable
measurement of bulk velocities of ensembles of X-ray bright clusters will be
possible on very large scales (100-500 Mpc/h).Comment: 34 pages, with 11 figures included. Postscript. Submitted to MNRAS.
Latest version (recommended) at http://www.mpa-garching.mpg.de/~max/sz.html
or from [email protected]
The nature and evolution of the highly ionized near-zones in the absorption spectra of z~6 quasars
We use state-of-the-art hydrodynamical simulations combined with a 1D
radiative transfer code to assess the extent to which the highly ionized
regions observed close to z~6 quasars, which we refer to as near-zones, can
constrain the ionization state of the surrounding IGM. We find the appearance
in Lya absorption of a quasar HII ionization front expanding into a neutral IGM
can be very similar to a classical proximity zone, produced by the enhancement
in ionizing flux close to a quasar embedded in a highly ionized IGM. The
observed sizes of these highly ionized near-zones and their redshift evolution
can be reproduced for a wide range of IGM neutral hydrogen fractions for
plausible values of the luminosity and lifetime of the quasars. The observed
near-zone sizes at the highest observed redshifts are equally consistent with a
significantly neutral and a highly ionized surrounding IGM. Stronger
constraints on the IGM neutral hydrogen fraction can be obtained by considering
the relative size of the near-zones in the Lya and Lyb regions of a quasar
spectrum. A large sample of high quality quasar absorption spectra with
accurate determinations of near-zone sizes and their redshift evolution in both
the Lya and Lyb regions should confirm or exclude the possibility that the
Universe is predominantly neutral at the highest observed redshifts. The width
of the discrete absorption features in these near-zones will contain important
additional information on the ionization state and the previous thermal history
of the IGM at these redshifts.Comment: 25 pages, 11 figures, accepted for publication in MNRA
The observed ionization rate of the intergalactic medium and the ionizing emissivity at z >5: Evidence for a photon starved and extended epoch of reionization
We use a large set of hydrodynamical simulations, combined with measurements
of the Lyman alpha opacity of the IGM taken from the literature, to obtain
robust estimates for the photoionization rate per hydrogen atom at z=5 and 6.
We find the photoionization rate drops by a factor of two and four,
respectively, compared to our recent measurements at z = 2 - 4. The number of
ionizing photons emitted by known sources at z=5 and 6, based on an
extrapolation of source numbers below the detection limit and standard
assumptions for the relationship between the ionizing emissivity and observed
luminosity density at 1500 Angstroms, are in reasonable agreement with the
photoionization rates inferred from the Lyman alpha forest if the escape
fraction of ionizing photons from galaxies is large (>= 20 per cent). Claims to
the contrary may be attributed to the adoption of an unduly high value for the
clumping factor of ionized hydrogen. Using physically motivated assumptions for
the mean free path of ionizing photons our measurements of the photoionization
rate can be turned into an estimate of the ionizing emissivity. In comoving
units the inferred ionizing emissivity is nearly constant over the redshift
range 2-6 and corresponds to 1.5-3 photons emitted per hydrogen atom over a
time interval corresponding to the age of the Universe at z=6. This strongly
suggests that the epoch of reionization was photon-starved and extended.
[Abridged]Comment: 20 pages, 9 figures, accepted for publication in MNRA
On the rapid demise of Lyman-alpha emitters at z>7 due to the increasing incidence of optically thick absorption systems
A variety of independent observational studies have now reported a
significant decline in the fraction of Lyman-break galaxies which exhibit Ly-a
emission over the redshift interval z=6-7. In combination with the strong
damping wing extending redward of Ly-a in the spectrum of the bright z=7.085
quasar ULAS 1120+0641, this has strengthened suggestions that the hydrogen in
the intergalactic medium (IGM) is still substantially neutral at z~7. Current
theoretical models imply HI fractions as large as 40-90 per cent may be
required to explain these data assuming there is no intrinsic evolution in the
Ly-a emitter population. We propose that such large neutral fractions are not
necessary. Based on a hydrodynamical simulation which reproduces the absorption
spectra of high-redshift (z~6-7) quasars, we demonstrate that the opacity of
the intervening IGM redward of rest-frame Ly-a can rise rapidly in average
regions of the Universe simply because of the increasing incidence of
absorption systems which are optically thick to Lyman continuum photons as the
tail-end of reionisation is approached. Our simulations suggest these data do
not require a large change in the IGM neutral fraction by several tens of per
cent from z=6-7, but may instead be indicative of the rapid decrease in the
typical mean free path for ionising photons expected during the final stages of
reionisation.Comment: 11 pages, 6 figures, accepted to MNRA
Probing the growth of supermassive black holes at z>6 with LOFAR
HII regions surrounding supermassive black holes (BHs) in an otherwise still
neutral intergalactic medium (IGM) are likely to be the most easily detectable
sources by future 21cm experiments like LOFAR. We have made predictions for the
size distribution of such HII regions for several physically motivated models
for BH growth at high redshift and compared this to the expected LOFAR
sensitivity to these sources. The number of potentially detectable HII regions
does not only depend on the ionisation state of the intergalactic medium and
the decoupling of the spin temperature of the neutral hydrogen from the cosmic
microwave background (CMB) temperature, but is also strongly sensitive to the
rate of growth of BHs at high redshift. If the supermassive BHs at redshift 6
were built up via continuous Eddington-limited accretion from low mass seed BHs
at high redshift, then LOFAR is not expected to detect isolated QSO HII regions
at redshifts much larger than 6, and only if the IGM is still significantly
neutral. If the high-redshift growth of BHs starts with massive seed BHs and is
driven by short-lived accretion events following the merging of BH hosting
galaxies then the detection of HII regions surrounding supermassive BHs may
extend to redshifts as large as 8-9 but is still very sensitive to the redshift
to which the IGM remains significantly neutral. The most optimistic predictions
are for a model where the supermassive BHs at z>6 have grown slowly. HII
regions around supermassive BHs may then be detected to significantly larger
redshifts.Comment: 11 pages, 6 figures, accepted for publication in MNRA
A closer look at using quasar near-zones as a probe of neutral hydrogen in the intergalactic medium
We examine a large set of synthetic quasar spectra to realistically assess
the potential of using the relative sizes of highly ionized near-zones in the
Lya and Lyb forest as a probe of the neutral hydrogen content of the
intergalactic medium (IGM) at z>6. The scatter in the relative near-zone size
distribution, induced by underlying fluctuations in the baryonic density field
and the filtering of ionizing radiation, is considerable even for fixed
assumptions about the IGM neutral fraction. As a consequence, the current
observational data cannot distinguish between an IGM which is significantly
neutral or highly ionized just above z=6. Under standard assumptions for quasar
ages and ionizing luminosities, a future sample of several tens of high
resolution Lya and Lyb near-zone spectra should be capable of distinguishing
between a volume weighted neutral hydrogen fraction in the IGM which is greater
or less than 10 per cent.Comment: 6 pages, 3 figures, accepted for publication in MNRAS letter
The bias of DLAs at z ~ 2.3: contraining stellar feedback in shallow potential wells
We discuss the recent Baryon Oscillation Spectroscopic Survey measurement of
a rather high bias factor for the host galaxies/haloes of Damped Lyman-alpha
Absorbers (DLAs), in the context of our previous modelling of the physical
properties of DLAs within the cold dark matter paradigm. Joint
modelling of the column density distribution, the velocity width distribution
of associated low ionization metal absorption, and the bias parameter suggests
that DLAs are hosted by galaxies with dark matter halo masses in the range , with a rather sharp cutoff at the lower mass end,
corresponding to virial velocities of 35 km/sec. The observed properties of
DLAs appear to suggest efficient (stellar) feedback in haloes with
masses/virial velocities below the cutoff and a large retained baryon fraction
(> 35 %) in haloes above the cutoff.Comment: 10 pages, 9 figures. Published in MNRAS, May 21, 2014. 440 (3):
2313-2321. v3: Corrections in light of errata: MNRAS, 454(1), p. 218. Note,
in particular, the changes to Figure 5 and the virial velocity cut-of
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