During the last interglacial period, ~125,000 years ago, sea level was at least several meters higher than at present, with substantial variability observed for peak sea level at geographically ...diverse sites. Speculation that the West Antarctic ice sheet collapsed during the last interglacial period has drawn particular interest to understanding climate and ice-sheet dynamics during this time interval. We provide an internally consistent database of coral U-Th ages to assess last interglacial sea-level observations in the context of isostatic modeling and stratigraphie evidence. These data indicate that global (eustatic) sea level peaked 5.5 to 9 meters above present sea level, requiring smaller ice sheets in both Greenland and Antarctica relative to today and indicating strong sea-level sensitivity to small changes in radiative forcing.
We present the evolution of the structure of relaxed cold dark matter (CDM) haloes in the cosmology from the Planck satellite. Our simulations cover five decades in halo mass, from dwarf galaxies to ...galaxy clusters. Because of the increased matter density and power spectrum normalization the concentration–mass relation in the Planck
cosmology has a ∼20 per cent higher normalization at redshift z = 0 compared to Wilkinson Microwave Anisotropy Probe cosmology. We confirm that CDM haloes are better described by the Einasto profile; for example, at scales near galaxy half-light radii CDM haloes have significantly steeper density profiles than implied by Navarro–Frenk–White (NFW) fits. There is a scatter of ∼0.2 dex in the Einasto shape parameter at fixed halo mass, adding further to the diversity of CDM halo profiles. The evolution of the concentration–mass relation in our simulations is not reproduced by any of the analytic models in the literature. We thus provide a simple fitting formula that accurately describes the evolution between redshifts z = 5 and 0 for both NFW and Einasto fits. Finally, the observed concentrations and halo masses of spiral galaxies, groups and clusters of galaxies at low redshifts are in good agreement with our simulations, suggesting only mild halo response to galaxy formation on these scales.
Abstract
Simulating thin and extended galactic disks has long been a challenge in computational astrophysics. We introduce the NIHAO-UHD suite of cosmological hydrodynamical simulations of Milky Way ...mass galaxies and study stellar disk properties such as stellar mass, size and rotation velocity which agree well with observations of the Milky Way and local galaxies. In particular, the simulations reproduce the age-velocity dispersion relation and a multi-component stellar disk as observed for the Milky Way. Half of our galaxies show a double exponential vertical profile, while the others are well described by a single exponential model which we link to the disk merger history. In all cases, mono-age populations follow a single exponential whose scale height varies monotonically with stellar age and radius. The scale length decreases with stellar age while the scale height increases. The general structure of the stellar disks is already set at time of birth as a result of the inside-out and upside-down formation. Subsequent evolution modifies this structure by increasing both the scale length and height of all mono-age populations. Thus, our results put tight constraints on how much dynamical memory stellar disks can retain over cosmological timescales. Our simulations demonstrate that it is possible to form thin galactic disks in cosmological simulations provided there are no significant stellar mergers at low redshifts. Most of the stellar mass is formed in-situ with only a few percent ($\lesssim 5\%$) brought in by merging satellites at early times. Redshift zero snapshots and halo catalogues are publicly available.
Recent studies have shown that massive elliptical galaxies have total mass density profiles within an effective radius that can be approximated as
, with mean slope 〈γ′〉 = 2.08 ± 0.03 and scatter
. ...The small scatter of the slope (known as the bulge-halo conspiracy) is not generic in Λ cold dark matter (ΛCDM) based models and therefore contains information about the galaxy formation process. We compute the distribution of γ′ for ΛCDM-based models that reproduce the observed correlations between stellar mass, velocity dispersion, and effective radius of early-type galaxies in the Sloan Digital Sky Survey. The models have a range of stellar initial mass functions (IMFs) and dark halo responses to galaxy formation. The observed distribution of γ′ is well reproduced by a model with cosmologically motivated but uncontracted dark matter haloes, and a Salpeter-type IMF. Other models are on average ruled out by the data, even though they may happen in individual cases. Models with adiabatic halo contraction (and lighter IMFs) predict too small values of γ′. Models with halo expansion, or mass-follows-light predict too high values of γ′. Our study shows that the non-homologous structure of massive early-type galaxies can be precisely reproduced by ΛCDM models if the IMF is not universal and if mechanisms, such as feedback from active galactic nuclei, or dynamical friction, effectively on average counterbalance the contraction of the halo expected as a result of baryonic cooling.
Warming climate, melting ice, rising seas
We know that the sea level will rise as climate warms. Nevertheless, accurate projections of how much sea-level rise will occur are difficult to make based ...solely on modern observations. Determining how ice sheets and sea level have varied in past warm periods can help us better understand how sensitive ice sheets are to higher temperatures. Dutton
et al.
review recent interdisciplinary progress in understanding this issue, based on data from four different warm intervals over the past 3 million years. Their synthesis provides a clear picture of the progress we have made and the hurdles that still exist.
Science
, this issue
10.1126/science.aaa4019
Reconstructing past magnitudes, rates, and sources of sea-level rise can help project what our warmer future may hold.
BACKGROUND
Although thermal expansion of seawater and melting of mountain glaciers have dominated global mean sea level (GMSL) rise over the last century, mass loss from the Greenland and Antarctic ice sheets is expected to exceed other contributions to GMSL rise under future warming. To better constrain polar ice-sheet response to warmer temperatures, we draw on evidence from interglacial periods in the geologic record that experienced warmer polar temperatures and higher GMSLs than present. Coastal records of sea level from these previous warm periods demonstrate geographic variability because of the influence of several geophysical processes that operate across a range of magnitudes and time scales. Inferring GMSL and ice-volume changes from these reconstructions is nontrivial and generally requires the use of geophysical models.
ADVANCES
Interdisciplinary studies of geologic archives have ushered in a new era of deciphering magnitudes, rates, and sources of sea-level rise. Advances in our understanding of polar ice-sheet response to warmer climates have been made through an increase in the number and geographic distribution of sea-level reconstructions, better ice-sheet constraints, and the recognition that several geophysical processes cause spatially complex patterns in sea level. In particular, accounting for glacial isostatic processes helps to decipher spatial variability in coastal sea-level records and has reconciled a number of site-specific sea-level reconstructions for warm periods that have occurred within the past several hundred thousand years. This enables us to infer that during recent interglacial periods, small increases in global mean temperature and just a few degrees of polar warming relative to the preindustrial period resulted in ≥6 m of GMSL rise. Mantle-driven dynamic topography introduces large uncertainties on longer time scales, affecting reconstructions for time periods such as the Pliocene (~3 million years ago), when atmospheric CO
2
was ~400 parts per million (ppm), similar to that of the present. Both modeling and field evidence suggest that polar ice sheets were smaller during this time period, but because dynamic topography can cause tens of meters of vertical displacement at Earth’s surface on million-year time scales and uncertainty in model predictions of this signal are large, it is currently not possible to make a precise estimate of peak GMSL during the Pliocene.
OUTLOOK
Our present climate is warming to a level associated with significant polar ice-sheet loss in the past, but a number of challenges remain to further constrain ice-sheet sensitivity to climate change using paleo–sea level records. Improving our understanding of rates of GMSL rise due to polar ice-mass loss is perhaps the most societally relevant information the paleorecord can provide, yet robust estimates of rates of GMSL rise associated with polar ice-sheet retreat and/or collapse remain a weakness in existing sea-level reconstructions. Improving existing magnitudes, rates, and sources of GMSL rise will require a better (global) distribution of sea-level reconstructions with high temporal resolution and precise elevations and should include sites close to present and former ice sheets. Translating such sea-level data into a robust GMSL signal demands integration with geophysical models, which in turn can be tested through improved spatial and temporal sampling of coastal records.
Further development is needed to refine estimates of past sea level from geochemical proxies. In particular, paired oxygen isotope and Mg/Ca data are currently unable to provide confident, quantitative estimates of peak sea level during these past warm periods. In some GMSL reconstructions, polar ice-sheet retreat is inferred from the total GMSL budget, but identifying the specific ice-sheet sources is currently hindered by limited field evidence at high latitudes. Given the paucity of such data, emerging geochemical and geophysical techniques show promise for identifying the sectors of the ice sheets that were most vulnerable to collapse in the past and perhaps will be again in the future.
Peak global mean temperature, atmospheric CO
2
, maximum global mean sea level (GMSL), and source(s) of meltwater.
Light blue shading indicates uncertainty of GMSL maximum. Red pie charts over Greenland and Antarctica denote fraction (not location) of ice retreat.
Interdisciplinary studies of geologic archives have ushered in a new era of deciphering magnitudes, rates, and sources of sea-level rise from polar ice-sheet loss during past warm periods. Accounting for glacial isostatic processes helps to reconcile spatial variability in peak sea level during marine isotope stages 5e and 11, when the global mean reached 6 to 9 meters and 6 to 13 meters higher than present, respectively. Dynamic topography introduces large uncertainties on longer time scales, precluding robust sea-level estimates for intervals such as the Pliocene. Present climate is warming to a level associated with significant polar ice-sheet loss in the past. Here, we outline advances and challenges involved in constraining ice-sheet sensitivity to climate change with use of paleo–sea level records.
Abstract
We address the origin of ultra-diffuse galaxies (UDGs), which have stellar masses typical of dwarf galaxies but effective radii of Milky Way-sized objects. Their formation mechanism, and ...whether they are failed L
⋆ galaxies or diffuse dwarfs, are challenging issues. Using zoom-in cosmological simulations from the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project, we show that UDG analogues form naturally in dwarf-sized haloes due to episodes of gas outflows associated with star formation. The simulated UDGs live in isolated haloes of masses 1010–11 M⊙, have stellar masses of 107–8.5 M⊙, effective radii larger than 1 kpc and dark matter cores. They show a broad range of colours, an average Sérsic index of 0.83, a typical distribution of halo spin and concentration, and a non-negligible H i gas mass of 107 − 9 M⊙, which correlates with the extent of the galaxy. Gas availability is crucial to the internal processes which form UDGs: feedback-driven gas outflows, and subsequent dark matter and stellar expansion, are the key to reproduce faint, yet unusually extended, galaxies. This scenario implies that UDGs represent a dwarf population of low surface brightness galaxies and should exist in the field. The largest isolated UDGs should contain more H i gas than less extended dwarfs of similar M
⋆.
ABSTRACT We present a summary of recent assessments of the mass distribution in disk galaxies. In order to characterize galaxy formation models the relative fraction of baryons and dark matter at all ...radii in galaxies must be determined. For disk galaxies, various measurements of the mass distribution in galaxies based on vertical kinematics, strong lensing, residuals of scaling relations, fluid dynamical modeling, bar strength and pattern speed, warps, and others have called for either a maximal or sub-maximal contribution of the baryons in the inner parts of the disk. We propose a global picture whereby all galaxies are typically baryon-dominated (maximal) at the center and dark-matter dominated (sub-maximal) in their outskirts. Using as a fiducial radius the peak of the rotation curve of a pure baryonic exponential disk at , where is the disk scale length, the transition from maximal to sub-maximal baryons occurs within or near for low-mass disk galaxies (with 200 ) and beyond for more massive systems. The mass fraction's inverse dependence on circular velocity in disk galaxies is largely insensitive to the presence of a bar at . The mean mass fractions of late- and early-type galaxies are shown to be different at the same fiducial radius and circular velocity, pointing to different galaxy formation mechanisms. Feedback, dynamical friction, size, and IMF variations are key ingredients for understanding these differences.
ABSTRACT
We study ultra-diffuse galaxies (UDGs) in zoom in cosmological simulations, seeking the origin of UDGs in the field versus galaxy groups. We find that while field UDGs arise from dwarfs in a ...characteristic mass range by multiple episodes of supernova feedback (Di Cintio et al.), group UDGs may also form by tidal puffing up and they become quiescent by ram-pressure stripping. The field and group UDGs share similar properties, independent of distance from the group centre. Their dark-matter haloes have ordinary spin parameters and centrally dominant dark-matter cores. Their stellar components tend to have a prolate shape with a Sérsic index n ∼ 1 but no significant rotation. Ram pressure removes the gas from the group UDGs when they are at pericentre, quenching star formation in them and making them redder. This generates a colour/star-formation-rate gradient with distance from the centre of the dense environment, as observed in clusters. We find that ∼20 per cent of the field UDGs that fall into a massive halo survive as satellite UDGs. In addition, normal field dwarfs on highly eccentric orbits can become UDGs near pericentre due to tidal puffing up, contributing about half of the group-UDG population. We interpret our findings using simple toy models, showing that gas stripping is mostly due to ram pressure rather than tides. We estimate that the energy deposited by tides in the bound component of a satellite over one orbit can cause significant puffing up provided that the orbit is sufficiently eccentric. We caution that while the simulations produce UDGs that match the observations, they under-produce the more compact dwarfs in the same mass range, possibly because of the high threshold for star formation or the strong feedback.
We use a series of cosmological N-body simulations for a flat Λ cold dark matter (ΛCDM) cosmology to investigate the structural properties of dark matter haloes, at redshift zero, in the mass range 3 ...× 109 h−1 ≲Mvir ≲ 3 × 1013 h−1 M⊙. These properties include the concentration parameter, c, the spin parameter, λ, and the mean axis ratio, . For the concentration–mass relation we find c∝ in agreement with the model proposed by Bullock et al., but inconsistent with the alternative model of Eke et al. The normalization of the concentration–mass relation, however, is 15 per cent lower than suggested by Bullock et al. The results for λ and are in good agreement with previous studies, when extrapolated to the lower halo masses probed here, while c and λ are anticorrelated, in that high-spin haloes have, on average, lower concentrations. In an attempt to remove unrelaxed haloes from the sample, we compute for each halo the offset parameter, xoff, defined as the distance between the most bound particle and the centre of mass, in units of the virial radius. Removing haloes with large xoff increases the mean concentration by ∼10 per cent, lowers the mean spin parameter by ∼15 per cent, and removes the most prolate haloes. In addition, it largely removes the anticorrelation between c and λ, though not entirely. We also investigate the relation between halo properties and their large-scale environment density. For low-mass haloes we find that more concentrated haloes live in denser environments than their less concentrated counterparts of the same mass, consistent with recent correlation function analyses. Note, however, that the trend is weak compared to the scatter. For the halo spin parameters we find no environment dependence, while there is a weak indication that the most spherical haloes reside in slightly denser environments. Finally, using a simple model for disc galaxy formation we show that haloes that host low surface brightness galaxies are expected to be hosted by a biased subset of haloes. Not only do these haloes have spin parameters that are larger than average, they also have concentration parameters that are ∼15 per cent lower than the average at a given halo mass. We discuss the implications of all these findings for the claimed disagreement between halo concentrations inferred from low surface brightness rotation curves, and those expected for a ΛCDM cosmology.
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