64 research outputs found
Swirling around filaments: are large-scale structure vortices spinning up dark halos?
The kinematic analysis of dark matter and hydrodynamical simulations suggests
that the vorticity in large-scale structure is mostly confined to, and
predominantly aligned with their filaments, with an excess of probability of 20
per cent to have the angle between vorticity and filaments direction lower than
60 degrees relative to random orientations. The cross sections of these
filaments are typically partitioned into four quadrants with opposite vorticity
sign, arising from multiple flows, originating from neighbouring walls. The
spins of halos embedded within these filaments are consistently aligned with
this vorticity for any halo mass, with a stronger alignment for the most
massive structures up to an excess of probability of 165 per cent. On large
scales, adiabatic/cooling hydrodynamical simulations display the same vorticity
in the gas as in the dark matter. The global geometry of the flow within the
cosmic web is therefore qualitatively consistent with a spin acquisition for
smaller halos induced by this large-scale coherence, as argued in Codis et al.
(2012). In effect, secondary anisotropic infall (originating from the
vortex-rich filament within which these lower-mass halos form) dominates the
angular momentum budget of these halos. The transition mass from alignment to
orthogonality is related to the size of a given multi-flow region with a given
polarity. This transition may be reconciled with the standard tidal torque
theory if the latter is augmented so as to account for the larger scale
anisotropic environment of walls and filaments.Comment: 17 pages, 19 figures, 3 tables. accepted for publication in MNRA
Why do extremely massive disc galaxies exist today?
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 4, June 2020, Pages 5568â5575, https://doi.org/10.1093/mnras/staa970. ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.Galaxy merger histories correlate strongly with stellar mass, largely regardless of morphology. Thus, at fixed stellar mass, spheroids and discs share similar assembly histories, both in terms of the frequency of mergers and the distribution of their mass ratios. Since mergers are the principal drivers of disc-to-spheroid morphological transformation, and the most massive galaxies typically have the richest merger histories, it is surprising that discs exist at all at the highest stellar masses (e.g. beyond the knee of the mass function). Using Horizon-AGN, a cosmological hydro-dynamical simulation, we show that extremely massive (M*> 10^11.4 MSun) discs are created via two channels. In the primary channel (accounting for ~70% of these systems and ~8% of massive galaxies) the most recent, significant merger (stellar mass ratio > 1:10) between a massive spheroid and a gas-rich satellite `spins up' the spheroid by creating a new rotational stellar component, leaving a massive disc as the remnant. In the secondary channel (accounting for ~30% of these systems and ~3% of massive galaxies), a system maintains a disc throughout its lifetime, due to an anomalously quiet merger history. Not unexpectedly, the fraction of massive discs is larger at higher redshift, due to the Universe being more gas-rich. The morphological mix of galaxies at the highest stellar masses is, therefore, a strong function of the gas fraction of the Universe. Finally, these massive discs have similar black-hole masses and accretion rates to massive spheroids, providing a natural explanation for why a minority of powerful AGN are surprisingly found in disc galaxies.Peer reviewedFinal Published versio
Intrinsic alignments of galaxies in the Horizon-AGN cosmological hydrodynamical simulation
The intrinsic alignments of galaxies are recognised as a contaminant to weak
gravitational lensing measurements. In this work, we study the alignment of
galaxy shapes and spins at low redshift () in Horizon-AGN, an
adaptive-mesh-refinement hydrodynamical cosmological simulation box of 100
Mpc/h a side with AGN feedback implementation. We find that spheroidal galaxies
in the simulation show a tendency to be aligned radially towards over-densities
in the dark matter density field and other spheroidals. This trend is in
agreement with observations, but the amplitude of the signal depends strongly
on how shapes are measured and how galaxies are selected in the simulation.
Disc galaxies show a tendency to be oriented tangentially around spheroidals in
three-dimensions. While this signal seems suppressed in projection, this does
not guarantee that disc alignments can be safely ignored in future weak lensing
surveys. The shape alignments of luminous galaxies in Horizon-AGN are in
agreement with observations and other simulation works, but we find less
alignment for lower luminosity populations. We also characterize the
systematics of galaxy shapes in the simulation and show that they can be safely
neglected when measuring the correlation of the density field and galaxy
ellipticities.Comment: 20 pages, 23 figure
Low-Surface-Brightness Galaxies are missing in the observed Stellar Mass Function
We investigate the impact of the surface brightness (SB) limit on the galaxy
stellar mass functions (GSMFs) using mock surveys generated from the Horizon
Run 5 (HR5) simulation. We compare the stellar-to-halo-mass relation, GSMF, and
size-stellar mass relation of the HR5 galaxies with empirical data and other
cosmological simulations. The mean SB of simulated galaxies are computed using
their effective radii, luminosities, and colors. To examine the cosmic SB
dimming effect, we compute -corrections from the spectral energy
distributions of individual simulated galaxy at each redshift, apply the
-corrections to the galaxies, and conduct mock surveys based on the various
SB limits. We find that the GSMFs are significantly affected by the SB limits
at a low-mass end. This approach can ease the discrepancy between the GSMFs
obtained from simulations and observations at . We also find
that a redshift survey with a SB selection limit of \left^e = 28
mag arcsec will miss 20% of galaxies with at . The missing fraction of low-surface-brightness galaxies
increases to 50%, 70%, and 98% at , 1.1, and 1.9, respectively, at the
SB limit.Comment: 27 pages, 30 figures, accepted for publication in Ap
The mean state and variability of the North Atlantic circulation: a perspective from ocean reanalyses
The observational network around the North Atlantic has improved significantly over the last few decades with subsurface profiling floats and satellite observations, and the recent efforts to monitor the Atlantic Meridional Overturning Circulation (AMOC). These have shown decadal timescale changes across the North Atlantic including in heat content, heat transport and the circulation. However there are still significant gaps in the observational coverage. Ocean reanalyses integrate the observations with a dynamically consistent ocean model and can be used to understand the observed changes. However the ability of the reanalyses to represent the dynamics must also be assessed.
We use an ensemble of global ocean reanalyses to examine the time mean state and interannualâdecadal variability of the North Atlantic ocean since 1993. We assess how well the reanalyses are able to capture processes and whether any understanding can be gained. In particular we examine aspects of the circulation including convection, AMOC and gyre strengths, and transports. We find that reanalyses show some consistency, in particular showing a weakening of the subpolar gyre and AMOC at 50oN from the midâ90s until at least 2009 (related to decadal variability in previous studies), a strengthening and then weakening of the AMOC at 26.5oN since 2000, and impacts of circulation changes on transports. These results agree with model studies and the AMOC observations at 26.5oN since 2005. We also see less spread across the ensemble in AMOC strength and mixed layer depth, suggesting improvements as the observational coverage has improved
Mediterranean Sea response to climate change in an ensemble of twenty first century scenarios
The Mediterranean climate is expected to become warmer and drier during the twenty-first century. Mediterranean Sea response to climate change could be modulated by the choice of the socio-economic scenario as well as the choice of the boundary conditions mainly the Atlantic hydrography, the river runoff and the atmospheric fluxes. To assess and quantify the sensitivity of the Mediterranean Sea to the twenty-first century climate change, a set of numerical experiments was carried out with the regional ocean model NEMOMED8 set up for the Mediterranean Sea. The model is forced by airâsea fluxes derived from the regional climate model ARPEGE-Climate at a 50-km horizontal resolution. Historical simulations representing the climate of the period 1961â2000 were run to obtain a reference state. From this baseline, various sensitivity experiments were performed for the period 2001â2099, following different socio-economic scenarios based on the Special Report on Emissions Scenarios. For the A2 scenario, the main three boundary forcings (river runoff, near-Atlantic water hydrography and airâsea fluxes) were changed one by one to better identify the role of each forcing in the way the ocean responds to climate change. In two additional simulations (A1B, B1), the scenario is changed, allowing to quantify the socio-economic uncertainty. Our 6-member scenario simulations display a warming and saltening of the Mediterranean. For the 2070â2099 period compared to 1961â1990, the sea surface temperature anomalies range from +1.73 to +2.97 °C and the SSS anomalies spread from +0.48 to +0.89. In most of the cases, we found that the future Mediterranean thermohaline circulation (MTHC) tends to reach a situation similar to the eastern Mediterranean Transient. However, this response is varying depending on the chosen boundary conditions and socio-economic scenarios. Our numerical experiments suggest that the choice of the near-Atlantic surface water evolution, which is very uncertain in General Circulation Models, has the largest impact on the evolution of the Mediterranean water masses, followed by the choice of the socio-economic scenario. The choice of river runoff and atmospheric forcing both have a smaller impact. The state of the MTHC during the historical period is found to have a large influence on the transfer of surface anomalies toward depth. Besides, subsurface currents are substantially modified in the Ionian Sea and the Balearic region. Finally, the response of thermosteric sea level ranges from +34 to +49 cm (2070â2099 vs. 1961â1990), mainly depending on the Atlantic forcing
The Copernicus Marine Environment Monitoring Service Ocean State Report
The Copernicus Marine Environment Monitoring Service (CMEMS) Ocean State Report (OSR) provides an annual report of the state of the global ocean and European regional seas for policy and decision-makers with the additional aim of increasing general public awareness about the status of, and changes in, the marine environment. The CMEMS OSR draws on expert analysis and provides a 3-D view (through reanalysis systems), a view from above (through remote-sensing data) and a direct view of the interior (through in situ measurements) of the global ocean and the European regional seas. The report is based on the unique CMEMS monitoring capabilities of the blue (hydrography, currents), white (sea ice) and green (e.g. Chlorophyll) marine environment. This first issue of the CMEMS OSR provides guidance on Essential Variables, large-scale changes and specific events related to the physical ocean state over the period 1993â2015. Principal findings of this first CMEMS OSR show a significant increase in global and regional sea levels, thermosteric expansion, ocean heat content, sea surface temperature and Antarctic sea ice extent and conversely a decrease in Arctic sea ice extent during the 1993â2015 period. During the year 2015 exceptionally strong large-scale changes were monitored such as, for example, a strong El Niño Southern Oscillation, a high frequency of extreme storms and sea level events in specific regions in addition to areas of high sea level and harmful algae blooms. At the same time, some areas in the Arctic Ocean experienced exceptionally low sea ice extent and temperatures below average were observed in the North Atlantic Ocean
The Role of diapycnal mixing in coupled atmosphere-ocean general circulation models
The value of ocean diapycnal diffusivity (v) sets the rate at which dense bottom water can be mixedup through the stratified water column and thus plays an important role in the meridional overturningcirculation (MOC). Previous idealised experiments and simplified theory suggest that the strength ofthe MOC and the ocean heat transport scale with the v. This study investigates the dependence ofthe MOC and other parameters on v using atmosphere-ocean general circulation models (AOGCM).Firstly, the dependence of the MOC strength on v is studied using a low resolution AOGCM withrealistic geometry, FORTE, with spatially constant v values ranging from 0.1 cm2/s to an unrealistichigh value of 5 cm2/s. At the cyclostationary state, global MOC strength is found to scale with v(in agreement with previous studies) according to a power law of 0.5. No power law is found for theMOC in the individual basins. The increase in MOC strength in the Atlantic and Pacific Oceans isassociated with an increase in the ocean heat transport. The atmosphere responds to the change inthe ocean state by a decrease of its energy transport and surface winds. Only a partial compensationis found between the ocean and atmosphere energy transport. The strength of v is found to have astrong impact on coupled phenomena, such as a cessation of El Niño at high v.Secondly, similar experiments are conducted with a state-of-the-art AOGCM, ECHAM5/ MPIOM.In this model, v is derived from a constant background diapycnal diffusion (b), wind inducedmixing, the Richardson number and the convective adjustment. A set of 3 coupled experiments isconducted, with b = 0.1, 0.25 and 1 cm2/s. The scaling law from simple theory and the previousexperiments with FORTE is not observed with this coupled model. At the cyclostationary state, theMOC strength weakens by 16% as b increases from 0.1 to 1 cm2/s. This behavior is not foundwhen the experiments are repeated with an ocean-only model. The reduction in MOC in the coupledmodel is linked to a strong reduction in the convective mixing at high latitudes. The convectivemixing is reduced by a continuous strong freshening in the Arctic region due to an increase in surfaceair temperature and melting of the sea-ice in the coupled experiments, which is not observed in theocean-only experiments.The responses of the two coupled models show many similarities as b increases. Both modelsshow convection in the Pacific for high values of b. The main difference is the response of the MOCin the Atlantic is linked to the different locations of the deep convection and their relative changes inthe models.I conclude that the diapycnal mixing and the ocean-atmosphere interactions both control the strengthof the MOC, and their influences cannot be considered separately
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