145,194 research outputs found
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
Labour Turnover and Firm Performance
We explore the impact of labour turnover on firm performance by analysing the predictions of an extension of the efficiency wage model of Salop (1979) developed by Garino and Martin (2007), which separates incumbent and newly hired workers in the production function. Within this theoretical framework, an exogenous increase in the turnover rate can increase profits if firms do not choose wages unilaterally. We test the theoretical predictions of the model using UK cross-section establishment-level data, the 2004 Workplace and Employee Relations Survey. In accordance with our theoretical priors, the empirical results support the standard inverse relationship between the quit rate and firm performance where firms unilaterally choose the wage and generally support a positive relationship between firm performance and the quit rate where trade unions influence wage setting
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
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
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
Probing gaseous halos of galaxies with radio jets
Reproduced with permission from Astronomy & Astrophysics. © 2019 ESOContext. Gaseous halos play a key role in understanding inflow, feedback, and the overall baryon budget in galaxies. Literature models predict transitions of the state of the gaseous halo between cold and hot accretion, winds, fountains, and hydrostatic halos at certain galaxy masses. Since luminosities of radio AGN are sensitive to halo densities, any significant transition would be expected to show up in the radio luminosities of large samples of galaxies. The LOw Frequency ARray (LOFAR) Two-Metre Sky Survey (LoTSS) has identified a galaxy stellar mass scale, 10 11 M ⊙, above which the radio luminosities increase disproportionately. Aims. We investigate if radio luminosities of galaxies, especially the marked rise at galaxy masses around 10 11 M ⊙, can be explained with standard assumptions regarding jet powers, scaling between black hole mass and galaxy mass, and gaseous halos. Methods. Based on observational data and theoretical constraints, we developed models for the radio luminosity of radio AGN in halos under infall, galactic wind, and hydrostatic conditions. We compared these models to LoTSS data for a large sample of galaxies in the mass range between 10 8.5 M ⊙ and 10 12 M ⊙. Results. Under the assumption that the same characteristic upper limit to jet powers known from high galaxy masses holds at all masses, we find the maximum radio luminosities for the hydrostatic gas halos to lie close to the upper envelope of the distribution of the LOFAR data. The marked rise in radio luminosity at 10 11 M ⊙ is matched in our model and is related to a significant change in halo gas density around this galaxy mass, which is a consequence of lower cooling rates at a higher virial temperature. Wind and infall models overpredict the radio luminosities for small galaxy masses and have no particular steepening of the run of the radio luminosities predicted at any galaxy mass. Conclusions. Radio AGN could have the same characteristic Eddington-scaled upper limit to jet powers in galaxies of all masses in the sample if the galaxies have hydrostatic gas halos in phases when radio AGN are active. We find no evidence of a change of the type of galaxy halo with the galaxy mass. Galactic winds and quasi-spherical cosmological inflow phases cannot frequently occur at the same time as powerful jet episodes unless the jet properties in these phases are significantly different from what we assumed in our model.Peer reviewedFinal Accepted Versio
Assessment and learning outcomes: the evaluation of deep learning in an on-line course
Using an online learning environment, students from European countries collaborated and communicated to carry out problem based learning in occupational therapy. The effectiveness of this approach was evaluated by means of the final assessments and published learning outcomes. In particular, transcripts from peer-to-peer sessions of synchronous communication were analysed. The SOLO taxonomy was used and the development of deep learning was studied week by week. This allowed the quality of the course to be appraised and showed, to a certain extent, the impact of this online international course on the learning strategies of the students. Results indicate that deep learning can be supported by synchronous communication and online meetings between course participants.</p
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