| 1. |
- Snodgrass, C., et al.
(författare)
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The 67P/Churyumov-Gerasimenko observation campaign in support of the Rosetta mission
- 2017
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Ingår i: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962. ; 375:2097
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Tidskriftsartikel (refereegranskat)abstract
- We present a summary of the campaign of remote observations that supported the European Space Agency's Rosetta mission. Telescopes across the globe (and in space) followed comet 67P/ Churyumov-Gerasimenko from before Rosetta's arrival until nearly the end of the mission in September 2016. These provided essential data for mission planning, large-scale context information for the coma and tails beyond the spacecraft and a way to directly compare 67P with other comets. The observations revealed 67P to be a relatively 'well-behaved' comet, typical of Jupiter family comets and with activity patterns that repeat from orbit to orbit. Comparison between this large collection of telescopic observations and the in situ results from Rosetta will allow us to better understand comet coma chemistry and structure. This work is just beginning as the mission ends-in this paper, we present a summary of the ground-based observations and early results, and point to many questions that will be addressed in future studies. This article is part of the themed issue 'Cometary science after Rosetta'.
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| 2. |
- Kehoe, Laura, et al.
(författare)
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Make EU trade with Brazil sustainable
- 2019
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 364:6438, s. 341-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 3. |
- Schober, Jennifer, et al.
(författare)
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GALACTIC SYNCHROTRON EMISSION AND THE FAR-INFRARED-RADIO CORRELATION AT HIGH REDSHIFT
- 2016
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 827:2
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Tidskriftsartikel (refereegranskat)abstract
- Theoretical scenarios, including the turbulent small-scale dynamo, predict that strong magnetic fields already exist in young galaxies. Based on the assumption of energy equipartition between magnetic fields and turbulence, we determine the galactic synchrotron flux as a function of redshift z. Galaxies in the early universe are different from local galaxies, in particular, the former have more intense star formation. To cover a large range of conditions, we consider two different systems: one model galaxy comparable to the Milky Way and one typical high-z starburst galaxy. We include a model of the steady-state cosmic ray spectrum and find that synchrotron emission can be detected up to cosmological redshifts with current and future radio telescopes. The turbulent dynamo theory is in agreement with the origin of the observed correlation between the far-infrared (FIR) luminosity L-FIR and the radio luminosity L-radio. Our model reproduces this correlation well at z = 0. We extrapolate the FIR-radio correlation to higher redshifts and predict a time evolution with a significant deviation from its present-day appearance already at z approximate to 2 for a gas density that increases strongly with z. In particular, we predict a decrease of the radio luminosity with redshift which is caused by the increase of cosmic ray energy losses at high z. The result is an increase of the ratio between L-FIR and L-radio. Simultaneously, we predict that the slope of the FIR-radio correlation becomes shallower with redshift. This behavior of the correlation could be observed in the near future with ultra-deep radio surveys.
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| 4. |
- Schober, Jennifer, et al.
(författare)
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Saturation of the turbulent dynamo
- 2015
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 92:2
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Tidskriftsartikel (refereegranskat)abstract
- The origin of strong magnetic fields in the Universe can be explained by amplifying weak seed fields via turbulent motions on small spatial scales and subsequently transporting the magnetic energy to larger scales. This process is known as the turbulent dynamo and depends on the properties of turbulence, i.e., on the hydrodynamical Reynolds number and the compressibility of the gas, and on the magnetic diffusivity. While we know the growth rate of the magnetic energy in the linear regime, the saturation level, i.e., the ratio of magnetic energy to turbulent kinetic energy that can be reached, is not known from analytical calculations. In this paper we present a scale-dependent saturation model based on an effective turbulent resistivity which is determined by the turnover time scale of turbulent eddies and the magnetic energy density. The magnetic resistivity increases compared to the Spitzer value and the effective scale on which the magnetic energy spectrum is at its maximum moves to larger spatial scales. This process ends when the peak reaches a characteristic wave number k(star) which is determined by the critical magnetic Reynolds number. The saturation level of the dynamo also depends on the type of turbulence and differs for the limits of large and small magnetic Prandtl numbers Pm. With our model we find saturation levels between 43.8% and 1.3% for Pm >> 1 and between 2.43% and 0.135% for Pm << 1, where the higher values refer to incompressible turbulence and the lower ones to highly compressible turbulence.
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| 5. |
- Schober, Jennifer, et al.
(författare)
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Tracing star formation with non-thermal radio emission
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 468:1, s. 946-958
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Tidskriftsartikel (refereegranskat)abstract
- A key for understanding the evolution of galaxies and in particular their star formation history will be future ultradeep radio surveys. While star formation rates (SFRs) are regularly estimated with phenomenological formulas based on the local FIR-radio correlation, we present here a physically motivated model to relate star formation with radio fluxes. Such a relation holds only in frequency ranges where the flux is dominated by synchrotron emission, as this radiation originates from cosmic rays produced in supernova remnants, therefore reflecting recent star formation. At low frequencies, synchrotron emission can be absorbed by the free-free mechanism. This suppression becomes stronger with increasing number density of the gas, more precisely of the free electrons. We estimate the critical observing frequency below which radio emission is not tracing the SFR, and use the three well-studied local galaxies M51, M82, and Arp 220 as test cases for our model. If the observed galaxy is at high redshift, this critical frequency moves along with other spectral features to lower values in the observing frame. In the absence of systematic evolutionary effects, one would therefore expect that the method can be applied at lower observing frequencies for high-redshift observations. However, in case of a strong increase of the typical gas column densities towards high redshift, the increasing free-free absorption may erase the star formation signatures at low frequencies. At high radio frequencies both, free-free emission and the thermal bump, can dominate the spectrum, also limiting the applicability of this method.
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| 6. |
- Schober, Jennifer, et al.
(författare)
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Tracing star formation with radio emission
- 2017
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Ingår i: Memorie della Societa Astronomica Italiana - Journal of the Italian Astronomical Society. - : Societa Astronomica Italiana. ; , s. 749-750
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Konferensbidrag (refereegranskat)abstract
- Synchrotron emission from cosmic rays, produced in supernova remnants, contains information about a galaxy's star formation rate (SFR). For a quantitative estimate, we construct a model of non-thermal galactic radio flux, including the effect of free-free absorption. The latter can lead to a breakdown of the relation between the SFR and the radio flux at low frequencies and high gas densities. We employ our model to local disk and starburst galaxies and discuss the evolution of SFR-radio relations with redshift.
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