135 research outputs found
The new very small angle neutron scattering spectrometer at Laboratoire Leon Brillouin
The design and characteristics of the new very small angle neutron scattering
spectrometer under construction at the Laboratoire Leon Brillouin is described.
Its goal is to extend the range of scattering vectors magnitudes towards
2x10{-4} /A. The unique feature of this new spectrometer is a high resolution
two dimensional image plate detector sensitive to neutrons. The wavelength
selection is achieved by a double reflection supermirror monochromator and the
collimator uses a novel multibeam design
The properties of the star-forming interstellar medium at z=0.84-2.23 from HiZELS: mapping the internal dynamics and metallicity gradients in high-redshift disc galaxies
We present adaptive optics assisted, spatially resolved spectroscopy of a sample of nine H�-selected galaxies at z =0.84–2.23 drawn from the HiZELS narrow-band survey.
These galaxies have star-formation rates of 1–27M⊙ yr−1 and are therefore representative of the typical high-redshift star-forming population. Our �kpc-scale resolution
observations show that approximately half of the sample have dynamics suggesting that the ionised gas is in large, rotating disks. We model their velocity fields to infer
the inclination-corrected, asymptotic rotational velocities.We use the absolute B-band magnitudes and stellar masses to investigate the evolution of the B-band and stellar mass Tully-Fisher relationships. By combining our sample with a number of similar measurements from the literature, we show that, at fixed circular velocity, the stellar mass of star-forming galaxies has increased by a factor 2.5 between z =2 and z =0, whilst the rest-frame B-band luminosity has decreased by a factor �6 over the same
period. Together, these demonstrate a change in mass-to-light ratio in the B-band of �(M/ LB) / (M/ LB)z=0 �3.5 between z =1.5 and z =0, with most of the evolution occuring below z =1. We also use the spatial variation of [Nii] /H� to show that the metallicity of the ionised gas in these galaxies declines monotonically with galactocentric radius, with an average �log(O/H) /�R=−0.027±0.005 dex kpc−1. This gradient is consistent with predictions for high-redshift disk galaxies from cosmologically based hydrodynamic simulations.
Key words: galaxies: evolution – galaxies: formation – galaxies: high-redshif
Extremely red radio galaxies
At least half the radio galaxies at z>1 in the 7C Redshift Survey have
extremely red colours (R-K>5), consistent with stellar populations which formed
at high redshift (z>5). We discuss the implications of this for the evolution
of massive galaxies in general and for the fraction of near-IR-selected EROs
which host AGN, a result which is now being tested by deep, hard X-ray surveys.
The conclusion is that many massive galaxies undergo at least two active
phases: one at z~5 when the black hole and stellar bulge formed and another at
z~1-2 when activity is triggered by an event such as an interaction or merger.Comment: 6 pages, 2 figures, to appear in the proceedings of the workshop on
"QSO hosts and their environments", IAA, Granada, 10-12 Jan 2001, Ed. I.
Marque
The inevitable youthfulness of known high-redshift radio galaxies
Radio galaxies can be seen out to very high redshifts, where in principle
they can serve as probes of the early evolution of the Universe. Here we show
that for any model of radio-galaxy evolution in which the luminosity decreases
with time after an initial rapid increase (that is, essentially all reasonable
models), all observable high-redshift radio-galaxies must be seen when the
lobes are less than 10^7 years old. This means that high-redshift radio
galaxies can be used as a high-time-resolution probe of evolution in the early
Universe. Moreover, this result helps to explain many observed trends of
radio-galaxy properties with redshift [(i) the `alignment effect' of optical
emission along radio-jet axes, (ii) the increased distortion in radio
structure, (iii) the decrease in physical sizes, (iv) the increase in radio
depolarisation, and (v) the increase in dust emission] without needing to
invoke explanations based on cosmology or strong evolution of the surrounding
intergalactic medium with cosmic time, thereby avoiding conflict with current
theories of structure formation.Comment: To appear in Nature. 4 pages, 2 colour figures available on request.
Also available at http://www-astro.physics.ox.ac.uk/~km
Low frequency radio properties of the z>5 quasar population
Optically luminous quasars at z > 5 are important probes of super-massive black hole (SMBH) formation. With new and future
radio facilities, the discovery of the brightest low-frequency radio sources in this epoch would be an important new probe of cosmic
reionization through 21-cm absorption experiments. In this work, we systematically study the low-frequency radio properties of a
sample of 115 known spectroscopically confirmed z > 5 quasars using the second data release of the Low Frequency Array (LOFAR)
Two Metre Sky survey (LoTSS-DR2), reaching noise levels of ∼80 µJy beam−1
(at 144 MHz) over an area of ∼ 5720 deg2
. We find
that 41 sources (36%) are detected in LoTSS-DR2 at > 2σ significance and we explore the evolution of their radio properties (power,
spectral index, and radio loudness) as a function of redshift and rest-frame ultra-violet properties. We obtain a median spectral index
of −0.29+0.10
−0.09 by stacking 93 quasars using LoTSS-DR2 and Faint Images of the Radio Sky at Twenty Centimetres (FIRST) data at
1.4 GHz, in line with observations of quasars at z < 3. We compare the radio loudness of the high-z quasar sample to a lower-z quasar
sample at z ∼ 2 and find that the two radio loudness distributions are consistent with no evolution, although the low number of high-z
quasars means that we cannot rule out weak evolution. Furthermore, we make a first order empirical estimate of the z = 6 quasar radio
luminosity function, which is used to derive the expected number of high-z sources that will be detected in the completed LoTSS
survey. This work highlights the fact that new deep radio observations can be a valuable tool in selecting high-z quasar candidates for
follow-up spectroscopic observations by decreasing contamination of stellar dwarfs and reducing possible selection biases introduced
by strict colour cuts
The multiwavelength properties of red QSOs: Evidence for dusty winds as the origin of QSO reddening
Fundamental differences in the radio properties of red quasars (QSOs), as compared to blue QSOs, have been recently discovered, positioning them as a potential key population in the evolution of galaxies and black holes across cosmic time. To elucidate the nature of these objects, we exploited a rich compilation of broad-band photometry and spectroscopic data to model their spectral energy distributions (SEDs) from the ultraviolet to the far-infrared and characterise their emission-line properties. Following a systematic comparison approach, we characterise the properties of the QSO accretion, obscuration, and host galaxies in a sample of ∼1800 QSOs at 0.2 z 1000 km s−1) in red QSOs as compared to the control sample. We find that red QSOs that exhibit evidence for high-velocity wind components present a stronger signature of the infrared excess, suggesting a causal connection between QSO reddening and the presence of hot dust distributions in QSO winds. We propose that dusty winds at nuclear scales are potentially the physical ingredient responsible for the optical colours in red QSOs, as well as a key parameter for the regulation of accretion material in the nucleus.</p
Extragalactic Radio Continuum Surveys and the Transformation of Radio Astronomy
Next-generation radio surveys are about to transform radio astronomy by
discovering and studying tens of millions of previously unknown radio sources.
These surveys will provide new insights to understand the evolution of
galaxies, measuring the evolution of the cosmic star formation rate, and
rivalling traditional techniques in the measurement of fundamental cosmological
parameters. By observing a new volume of observational parameter space, they
are also likely to discover unexpected new phenomena. This review traces the
evolution of extragalactic radio continuum surveys from the earliest days of
radio astronomy to the present, and identifies the challenges that must be
overcome to achieve this transformational change.Comment: To be published in Nature Astronomy 18 Sept 201
Stability or democracy: on the role of monitors, media and miracles
This book brings together studies on the broad theme of elections and democratization in Africa since roughly 1989. It is based on a seminar held in The Netherlands in February 1997, and includes chapters on both electoral processes, especially the role of foreign observers therein, and the historical and sociocultural backgrounds or contexts of democratization, elections and political legitimacy. Part 1 deals with elections and election observation in Africa in general (contributions by O. van Cranenburgh, S. Ellis, I. van Kessel, B. de Gaay Fortman). Part 2 consists of country studies (M. Doornbos on Uganda, D. Foeken en T. Dietz on Kenya, J. Abbink on Ethiopia, R. van Dijk on Malawi, R. Buijtenhuijs on Chad, and M.-F. Lange on Mali). Part 3 includes a chapter that reflects the discussions held at the seminar between observers, academics and policymakers in the Netherlands Ministry of Foreign Affairs (W. van Binsbergen en J. Abbink); a review of Dutch policies on election observation in Africa during the period 1992-1997 (O. van Cranenburgh); and a discussion of the 1997 general elections in Kenya, where a new approach of election observation was introduced (M. Rutten).ASC – Publicaties niet-programma gebonde
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