161 research outputs found
Interpreting radiative efficiency in radio-loud AGNs
Author submitted version of unrefereed Nature Astronomy comment. Version in journal format available at https://rdcu.be/KH6WRadiative efficiency in radio-loud active galactic nuclei is governed by the accretion rate onto the central black hole rather than directly by the type of accreted matter; while it correlates with real differences in host galaxies and environments, it does not provide unambiguous information about particular objects.Non peer reviewedFinal Accepted Versio
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 Hïżœ-selected galaxies at z =0.84â2.23 drawn from the HiZELS narrow-band survey.
These galaxies have star-formation rates of 1â27Mâ yrâ1 and are therefore representative of the typical high-redshift star-forming population. Our ïżœkpc-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 ïżœ6 over the same
period. Together, these demonstrate a change in mass-to-light ratio in the B-band of ïżœ(M/ LB) / (M/ LB)z=0 ïżœ3.5 between z =1.5 and z =0, with most of the evolution occuring below z =1. We also use the spatial variation of [Nii] /Hïżœ to show that the metallicity of the ionised gas in these galaxies declines monotonically with galactocentric radius, with an average ïżœlog(O/H) /ïżœR=â0.027±0.005 dex kpcâ1. This gradient is consistent with predictions for high-redshift disk galaxies from cosmologically based hydrodynamic simulations.
Key words: galaxies: evolution â galaxies: formation â 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 âŒ80 ”Jy beamâ1
(at 144 MHz) over an area of ⌠5720 deg2
. We find
that 41 sources (36%) are detected in LoTSS-DR2 at > 2Ï 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 â0.29+0.10
â0.09 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
Evolution of star formation in the UKIDSS Ultra Deep Survey Field - I. Luminosity functions and cosmic star formation rate out to z = 1.6
We present new results on the cosmic star formation history in the Subaru/XMMâNewton Deep Survey (SXDS)âUltra Deep Survey (UDS) field out to z = 1.6. We compile narrowband
data from the Subaru Telescope and the Visible and Infrared Survey Telescope for Astronomy (VISTA) in conjunction with broad-band data from the SXDS and UDS, to make a selection of 5725 emission-line galaxies in 12 redshift slices, spanning 10 Gyr of cosmic time. We determine photometric redshifts for the sample using 11-band photometry, and use
a spectroscopically confirmed subset to fine tune the resultant redshift distribution. We use the maximum-likelihood technique to determine luminosity functions in each redshift slice and model the selection effects inherent in any narrow-band selection statistically, to obviate the retrospective corrections ordinarily required. The deep narrow-band data are sensitive to very low star formation rates (SFRs), and allow an accurate evaluation of the faint end slope of the Schechter function, α. We find that α is particularly sensitive to the assumed faintest
broad-band magnitude of a galaxy capable of hosting an emission line, and propose that this limit should be empirically motivated. For this analysis, we base our threshold on the limiting observed equivalent widths of emission lines in the local Universe. We compute the
characteristic SFR of galaxies in each redshift slice, and the integrated SFR density, ÏSFR. We find our results to be in good agreement with the literature and parametrize the evolution of the SFR density as ÏSFR â (1 + z)4.58 confirming a steep decline in star formation activity since z ⌠1.6.
Keywords: surveys â galaxies: evolution â galaxies: formation â galaxies: high-redshift â
galaxies: star formation â cosmology: observations
Cosmology from LOFAR Two-metre Sky Survey data release 2: angular clustering of radio sources
Covering ⌠5600 deg2 to rms sensitivities of âŒ70â100 ÎŒJy beamâ1, the LOFAR Two-metre Sky Survey Data Release 2 (LoTSS-DR2) provides the largest low-frequency (âŒ150 MHz) radio catalogue to date, making it an excellent tool for large-area radio cosmology studies. In this work, we use LoTSS-DR2 sources to investigate the angular two-point correlation function of galaxies within the survey. We discuss systematics in the data and an improved methodology for generating random catalogues, compared to that used for LoTSS-DR1, before presenting the angular clustering for âŒ900 000 sources â„1.5 mJy and a peak signal-to-noise â„ 7.5 across âŒ80 per cent of the observed area. Using the clustering, we infer the bias assuming two evolutionary models. When fitting angular scales of 0.5 †Ξ < 5âŠ, using a linear bias model, we find LoTSS-DR2 sources are biased tracers of the underlying matter, with a bias of bC = 2.14+0.22 â0.20 (assuming constant bias) and bE(z = 0) = 1.79+0.15 â0.14 (for an evolving model, inversely proportional to the growth factor), corresponding to bE = 2.81+0.24 â0.22 at the median redshift of our sample, assuming the LoTSS Deep Fields redshift distribution is representative of our data. This reduces to bC = 2.02+0.17 â0.16 and bE(z = 0) = 1.67+0.12 â0.12 when allowing preferential redshift distributions from the Deep Fields to model our data. Whilst the clustering amplitude is slightly lower than LoTSS-DR1 (â„2 mJy), our study benefits from larger samples and improved redshift estimates
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 âŒ1800 QSOs at 0.2 z 1000 km sâ1) 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
The LOFAR Two-metre Sky Survey: Deep Fields data release 1. V. Survey description, source classifications, and host galaxy properties
Source classifications, stellar masses, and star-formation rates are presented for â80â000 radio sources from the first data release of the Low Frequency Array Two-metre Sky Survey (LoTSS) Deep Fields, which represents the widest deep radio survey ever undertaken. Using deep multi-wavelength data spanning from the ultraviolet to the far-infrared, spectral energy distribution (SED) fitting is carried out for all of the LoTSS Deep host galaxies using four different SED codes, two of which include modelling of the contributions from an active galactic nucleus (AGN). Comparing the results of the four codes, galaxies that host a radiative AGN are identified, and an optimized consensus estimate of the stellar mass and star-formation rate for each galaxy is derived. Those galaxies with an excess of radio emission over that expected from star formation are then identified, and the LoTSS Deep sources are divided into four classes: star-forming galaxies, radio-quiet AGN, and radio-loud high-excitation and low-excitation AGN. Ninety-five perâcent of the sources can be reliably classified, of which more than two-thirds are star-forming galaxies, ranging from normal galaxies in the nearby Universe to highly-starbursting systems at z > 4. Star-forming galaxies become the dominant population below 150-MHz flux densities of â1âmJy, accounting for 90 perâcent of sources at S150MHz ⌠100 ÎŒJy. Radio-quiet AGN comprise â10 perâcent of the overall population. Results are compared against the predictions of the SKADS and T-RECS radio sky simulations, and improvements to the simulations are suggested
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