1,228 research outputs found
Hints against the cold and collisionless nature of dark matter from the galaxy velocity function
The observed number of dwarf galaxies as a function of rotation velocity is
significantly smaller than predicted by the standard model of cosmology. This
discrepancy cannot be simply solved by assuming strong baryonic feedback
processes, since they would violate the observed relation between maximum
circular velocity () and baryon mass of galaxies. A speculative
but tantalising possibility is that the mismatch between observation and theory
points towards the existence of non-cold or non-collisionless dark matter (DM).
In this paper, we investigate the effects of warm, mixed (i.e warm plus cold),
and self-interacting DM scenarios on the abundance of dwarf galaxies and the
relation between observed HI line-width and maximum circular velocity. Both
effects have the potential to alleviate the apparent mismatch between the
observed and theoretical abundance of galaxies as a function of .
For the case of warm and mixed DM, we show that the discrepancy disappears,
even for luke-warm models that evade stringent bounds from the Lyman-
forest. Self-interacting DM scenarios can also provide a solution as long as
they lead to extended ( kpc) dark matter cores in the density
profiles of dwarf galaxies. Only models with velocity-dependent cross sections
can yield such cores without violating other observational constraints at
larger scales.Comment: Matches published versio
Another baryon miracle? Testing solutions to the 'missing dwarfs' problem
The dearth of dwarf galaxies in the local universe is hard to reconcile with
the large number of low mass haloes expected within the concordance
CDM paradigm. In this paper we perform a systematic evaluation of the
uncertainties affecting the measurement of DM halo abundance using galaxy
kinematics. Using a large sample of dwarf galaxies with spatially resolved
kinematic data we derive a correction to obtain the observed abundance of
galaxies as a function of their halo maximum circular velocity from the
line-of-sight velocity function in the Local Volume. This estimate provides a
direct means of comparing the predictions of theoretical models and simulations
(including nonstandard cosmologies and novel galaxy formation physics) to the
observational constraints. The new "galactic " function is steeper
than the line-of-sight velocity function but still shallower than the
theoretical CDM expectation, showing that some unaccounted physical process is
necessary to reduce the abundance of galaxies and/or drastically modify their
density profiles compared to CDM haloes. Using this new galactic
function, we investigate the viability of baryonic solutions such as
feedback-powered outflows and photoevaporation of gas from an ionising
radiation background. At the 3- confidence level neither energetic
feedback nor photoevaporation are effective enough to reconcile the
disagreement. In the case of maximum baryonic effects, the theoretical estimate
still deviates significantly from the observations for km/s. CDM
predicts at least 1.8 times more galaxies with km/s and 2.5
times more than observed at km/s. Recent hydrodynamic simulations seem to
resolve the discrepancy but disagree with the properties of observed galaxies
with resolved kinematics. (abridged)Comment: 17 pages, 22 figures; major revisions include clarification of the
method, expanded comparison with simulations with a new figure, analysis of
uncertainties in model as well as pressure support corrections, and a new
table with nomenclatur
Where did the globular clusters of the Milky Way form? Insights from the E-MOSAICS simulations
Globular clusters (GCs) are typically old, with most having formed at z >~ 2. This makes understanding their birth environments difficult, as they are typically too distant to observe with sufficient angular resolution to resolve GC birth sites. Using 25 cosmological zoom-in simulations of Milky Way-like galaxies from the E-MOSAICS project, with physically-motivated models for star formation, feedback, and the formation, evolution, and disruption of GCs, we identify the birth environments of present-day GCs. We find roughly half of GCs in these galaxies formed in-situ (52.0 +/- 1.0 per cent) between z ~ 2 - 4, in turbulent, high-pressure discs fed by gas that was accreted without ever being strongly heated through a virial shock or feedback. A minority of GCs form during mergers (12.6 +/- 0.6 per cent in major mergers, and 7.2 +/- 0.5 per cent in minor mergers), but we find that mergers are important for preserving the GCs seen today by ejecting them from their natal, high density interstellar medium (ISM), where proto-GCs are rapidly destroyed due to tidal shocks from ISM substructure. This chaotic history of hierarchical galaxy assembly acts to mix the spatial and kinematic distribution of GCs formed through different channels, making it difficult to use observable GC properties to distinguish GCs formed in mergers from ones formed by smooth accretion, and similarly GCs formed in-situ from those formed ex-situ. These results suggest a simple picture of GC formation, in which GCs are a natural outcome of normal star formation in the typical, gas-rich galaxies that are the progenitors of present-day galaxies
A comparison of cut points for measuring risk factors for adolescent substance use and antisocial behaviors in the U.S. and Colombia
As the identification and targeting of salient risk factors for adolescent substance use become more widely used globally, an essential question arises as to whether U.S.-based cut points in the distributions of these risk factors that identify high risk can be used validly in other countries as well. This study examined proportions of youth at high risk using different empirically derived cut points in the distributions of 18 measured risk factors. Data were obtained from large-scale samples of adolescents in Colombia and the United States. Results indicated that significant (p \u3c 0.05) differences in the proportions of high risk youth were found in 38.9% of risk factors for 6th graders, 61.1% for 8th graders, and 66.6% for 10th graders. Colombian-based cut points for determining the proportion of Colombian youth at high risk were preferable to U.S.-based cut points in almost all comparisons that exhibited a significant difference. Our findings suggest that observed differences were related to the type of risk factor (e.g., drug specific vs. non-drug specific). Findings from this study demonstrate the need for collecting large-scale national data on risk factors for adolescent substance use and developing country-specific cut points based on the distributions of these measures to avoid misidentification of youth at high risk
Radiosurgery in the treatment of recurrent malignant gliomas: Experience at the Carlos Haya Hospital
The globular cluster system mass-halo mass relation in the E-MOSAICS simulations
Linking globular clusters (GCs) to the assembly of their host galaxies is an overarching goal in GC studies. The inference of tight scaling relations between GC system properties and the mass of both the stellar and dark halo components of their host galaxies are indicative of an intimate physical connection, yet have also raised fundamental questions about how and when GCs form. Specifically, the inferred correlation between the mass of a GC system (Mgc) and the dark matter halo mass (Mhalo) of a galaxy has been posited as a consequence of a causal relation between the formation of dark matter mini-haloes and GC formation during the early epochs of galaxy assembly. We present the first results from a new simulation of a cosmological volume (~cMpc on a side) from the E-MOSAICS suite, which includes treatments of the formation and evolution of GCs within the framework of a detailed galaxy formation model. The simulated Mgc-Mhalo relation is linear for halo masses , and is driven by the hierarchical assembly of galaxies. Below this halo mass, the simulated relation features a downturn, which we show is consistent with observations, and is driven by the underlying stellar mass-halo mass relation of galaxies. Our fiducial model reproduces the observed Mgc-Mstar relation across the full mass range, which we argue is more physically relevant than the Mgc-Mhalo relation. We also explore the physical processes driving the observed constant value of and find that it is the result of a combination of cluster formation physics and cluster disruption
Predicting accreted satellite galaxy masses and accretion redshifts based on globular cluster orbits in the E-MOSAICS simulations
The ages and metallicities of globular clusters (GCs) are known to be powerful tracers of the properties of their progenitor galaxies, enabling their use in determining the merger histories of galaxies. However, while useful in separating GCs into individual accretion events, the orbits of GC groups themselves have received less attention as probes of their progenitor galaxy properties. In this work, we use simulations of galaxies and their GC systems from the E-MOSAICS project to explore how the present-day orbital properties of GCs are related to the properties of their progenitor galaxies. We find that the orbits of GCs deposited by accretion events are sensitive to the mass and merger redshift of the satellite galaxy. Earlier mergers and larger galaxy masses deposit GCs at smaller median apocentres and lower total orbital energy. The orbital properties of accreted groups of GCs can therefore be used to infer the properties of their progenitor galaxy, though there exists a degeneracy between galaxy mass and accretion time. Combining GC orbits with other tracers (GC ages, metallicities) will help to break the galaxy mass/accretion time degeneracy, enabling stronger constraints on the properties of their progenitor galaxy. In situ GCs generally orbit at lower energies (small apocentres) than accreted GCs, however they exhibit a large tail to high energies and even retrograde orbits (relative to the present-day disc), showing significant overlap with accreted GCs. Applying the results to Milky Way GCs groups suggests a merger redshift for the Gaia Sausage/Enceladus and for the `low-energy'/Kraken group, adding further evidence that the Milky Way had two significant mergers in its past
NGC 5846-UDG1: A Galaxy Formed Mostly by Star Formation in Massive, Extremely Dense Clumps of Gas
It has been shown that ultra-diffuse galaxies (UDGs) have higher specific frequencies of globular clusters, on average, than other dwarf galaxies with similar luminosities. The UDG NGC 5846-UDG1 is among the most extreme examples of globular cluster-rich galaxies found so far. Here we present new Hubble Space Telescope observations and analysis of this galaxy and its globular cluster system. We find that NGC 5846-UDG1 hosts 54 ± 9 globular clusters, three to four times more than any previously known galaxy with a similar luminosity and higher than reported in previous studies. With a galaxy luminosity of L V,gal ≈ 6 × 107 L ⊙ (M ⋆ ≈ 1.2 × 108 M ⊙) and a total globular cluster luminosity of L V,GCs ≈ 7.6 × 106 L ⊙, we find that the clusters currently comprise ∼13% of the total light. Taking into account the effects of mass loss from clusters during their formation and throughout their lifetime, we infer that most of the stars in the galaxy likely formed in globular clusters, and very little to no normal low-density star formation occurred. This result implies that the most extreme conditions during early galaxy formation promoted star formation in massive and dense clumps, in contrast to the dispersed star formation observed in galaxies today
The kinematics of globular cluster populations in the E-MOSAICS simulations and their implications for the assembly history of the Milky Way
We present a detailed comparison of the Milky Way (MW) globular cluster (GC) kinematics with the 25 Milky Way-mass cosmological simulations from the E-MOSAICS project. While the MW falls within the kinematic distribution of GCs spanned by the simulations, the relative kinematics of its metal-rich () versus metal-poor () populations are atypical for its mass. To understand the origins of these features, we perform a comprehensive statistical analysis of the simulations, and find 18 correlations describing the assembly of galaxies and their dark matter haloes based on their GC population kinematics. The correlations arise because the orbital distributions of accreted and in-situ GCs depend on the masses and accretion redshifts of accreted satellites, driven by the combined effects of dynamical fraction, tidal stripping, and dynamical heating. Because the kinematics of in-situ/accreted GCs are broadly traced by the metal-rich/metal-poor and inner/outer populations, the observed GC kinematics are a sensitive probe of galaxy assembly. We predict that relative to the population of galaxies, the MW assembled its dark matter and stellar mass rapidly through a combination of in-situ star formation, more than a dozen low-mass mergers, and early () major merger. The rapid assembly period ended early, limiting the fraction of accreted stars. We conclude by providing detailed quantitative predictions for the assembly history of the MW
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