| 1. |
- Ball, William T., et al.
(författare)
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Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery
- 2018
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 18:2, s. 1379-1394
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Tidskriftsartikel (refereegranskat)abstract
- Ozone forms in the Earth's atmosphere from the photodissociation of molecular oxygen, primarily in the tropical stratosphere. It is then transported to the extratropics by the Brewer-Dobson circulation (BDC), forming a protective ozone layer around the globe. Human emissions of halogen-containing ozone-depleting substances (hODSs) led to a decline in stratospheric ozone until they were banned by the Montreal Protocol, and since 1998 ozone in the upper stratosphere is rising again, likely the recovery from halogen-induced losses. Total column measurements of ozone between the Earth's surface and the top of the atmosphere indicate that the ozone layer has stopped declining across the globe, but no clear increase has been observed at latitudes between 60 degrees S and 60 degrees N outside the polar regions (60-90 degrees). Here we report evidence from multiple satellite measurements that ozone in the lower stratosphere between 60 degrees S and 60 degrees N has indeed continued to decline since 1998. We find that, even though upper stratospheric ozone is recovering, the continuing downward trend in the lower stratosphere prevails, resulting in a downward trend in stratospheric column ozone between 60 degrees S and 60 degrees N. We find that total column ozone between 60 degrees S and 60 degrees N appears not to have decreased only because of increases in tropospheric column ozone that compensate for the stratospheric decreases. The reasons for the continued reduction of lower stratospheric ozone are not clear; models do not reproduce these trends, and thus the causes now urgently need to be established.
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| 2. |
- Barnett, R., et al.
(författare)
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Euclid preparation V. Predicted yield of redshift 7 < z < 9 quasars from the wide survey
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 631
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Tidskriftsartikel (refereegranskat)abstract
- We provide predictions of the yield of 7 < z < 9 quasars from the Euclid wide survey, updating the calculation presented in the Euclid Red Book in several ways. We account for revisions to the Euclid near-infrared filter wavelengths; we adopt steeper rates of decline of the quasar luminosity function (QLF; Phi) with redshift, Phi proportional to 10(k(z-6)), k = 0:72, and a further steeper rate of decline, k = 0:92; we use better models of the contaminating populations (MLT dwarfs and compact early-type galaxies); and we make use of an improved Bayesian selection method, compared to the colour cuts used for the Red Book calculation, allowing the identification of fainter quasars, down to J(AB) similar to 23. Quasars at z > 8 may be selected from Euclid OYJH photometry alone, but selection over the redshift interval 7 < z < 8 is greatly improved by the addition of z-band data from, e.g., Pan-STARRS and LSST. We calculate predicted quasar yields for the assumed values of the rate of decline of the QLF beyond z = 6. If the decline of the QLF accelerates beyond z = 6, with k = 0.92, Euclid should nevertheless find over 100 quasars with 7.0 < z < 7.5, and similar to 25 quasars beyond the current record of z = 7.5, including similar to 8 beyond z = 8.0. The first Euclid quasars at z > 7.5 should be found in the DR1 data release, expected in 2024. It will be possible to determine the bright-end slope of the QLF, 7 < z < 8, M-1450 < 25, using 8m class telescopes to confirm candidates, but follow-up with JWST or E-ELT will be required to measure the faint-end slope. Contamination of the candidate lists is predicted to be modest even at J(AB) similar to 23. The precision with which k can be determined over 7 < z < 8 depends on the value of k, but assuming k = 0.72 it can be measured to a 1 sigma uncertainty of 0.07.
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| 3. |
- Barnett, R., et al.
(författare)
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Observations of the Lyman series forest towards the redshift 7.1 quasar ULAS J1120+0641
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 601
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Tidskriftsartikel (refereegranskat)abstract
- We present a 30 h integration Very Large Telescope X-shooter spectrum of the Lyman series forest towards the z = 7.084 quasar ULAS J1120+0641. The only detected transmission at S/N > 5 is confined to seven narrow spikes in the Ly alpha forest, over the redshift range 5.858 < z < 6.122, just longward of the wavelength of the onset of the Ly beta forest. There is also a possible detection of one further unresolved spike in the Ly beta forest at z = 6.854, with S/N = 4.5. We also present revised Hubble Space Telescope F814W photometry of the source. The summed flux from the transmission spikes is in agreement with the F814W photometry, so all the transmission in the Lyman series forest may have been detected. There is a Gunn-Peterson (GP) trough in the Ly alpha forest from z = 6.122 all the way to the quasar near zone at z = 7.04. The trough, of comoving length 240 h(-1) Mpc, is over twice as long as the next longest known GP trough. We combine the spectroscopic and photometric results to constrain the evolution of the Ly alpha effective optical depth (tau(eff)(GP)) with redshift, extending a similar analysis by Simpson et al. We find tau(eff)(GP) alpha (1 + z)(xi) where xi = 11.2(-0.6)(+0.4) for z > 5.5. The data nevertheless provide only a weak limit on the volume-weighted intergalactic medium (IGM) hydrogen neutral fraction at z similar to 6.5, x(HI) > 10(-4), similar to limits at redshift z similar to 6 from less distant quasars. The new observations cannot extend measurements of the neutral fraction of the IGM to higher values because absorption in the Ly alpha forest is already saturated near z similar to 6. For higher neutral fractions, other methods such as measuring the red damping wing of the IGM will be required.
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| 4. |
- O'Riordan, C. M., et al.
(författare)
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Galaxy mass profiles from strong lensing I : the circular power-law model
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 487:4, s. 5143-5154
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Tidskriftsartikel (refereegranskat)abstract
- In this series of papers, we develop a formalism for constraining mass profiles in strong gravitational lenses with extended images, using fluxes in addition to positional information. We start in this paper with a circular power-law profile and show that the slope gamma is uniquely determined by only two observables: the flux ratio f(1)/f(2) and the image position ratio theta(1)/theta(2) of the two images. We derive an analytic expression relating these two observables to the slope, a result that does not depend on the Einstein angle or the structure or brightness of the source. We then find an expression for the uncertainty on the slope sigma(gamma) that depends only on the position ratio theta(1)/theta(2) and the total signal-to-noise ratio (S/N) in the images. For example, in a system with position ratio theta(1)/theta(2)= 0.5, S/N=100, and gamma= 2 we find that gamma is constrained to a precision of +/- 0.03. We then test these results against a series of mock observations. We invert the images and fit an 11-parameter model, including ellipticity and position angle for both lens and source and measure the uncertainty on gamma. We find agreement with the theoretical estimate for all mock observations. In future papers, we will examine the radial range of the galaxy over which the constraint on the slope applies, and extend the analysis to elliptical lenses.
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