scholarly journals What is the Evidence for Dark Matter?

2004 ◽  
Vol 220 ◽  
pp. 27-37 ◽  
Author(s):  
J. A. Sellwood
Keyword(s):  
Dark Matter ◽  
Galaxy Formation ◽  
Alternative Hypothesis ◽  
Fine Tuning ◽  
Current Evidence ◽  
Newtonian Mechanics ◽  
Dark Matter Halos ◽  
Gravitational Fields ◽  
Decisive Advantage ◽  
Satisfactory Theory

Newtonian mechanics indicates that galaxies and galaxy clusters are much more massive than we would have guessed from their luminosities, with the discrepancy being generally attributed to dark matter halos. An alternative hypothesis is that accelerations in very weak gravitational fields are larger than predicted by Newton's laws, and there is no need for dark matter. Even though we do not currently have a satisfactory theory associated with this rival hypothesis, we can ask whether any observational tests could rule it out or prefer it over the dark matter hypothesis. Current evidence suggests that neither hypothesis enjoys a decisive advantage over the other. If dark matter turns out to be the correct interpretation however, then theories of galaxy formation face some quite severe fine-tuning problems.

scholarly journals Rotation of Galaxy Dark Matter Halos

2006 ◽  
Vol 2 (S235) ◽  
pp. 104-104
Author(s):  
Stéphane Herbert-Fort ◽  
Dennis Zaritsky ◽  
Yeun Jin Kim ◽  
Jeremy Bailin ◽  
James E. Taylor
Keyword(s):  
Dark Matter ◽  
Angular Momentum ◽  
Numerical Simulations ◽  
Galaxy Formation ◽  
Spiral Galaxy ◽  
Observational Studies ◽  
Spiral Galaxies ◽  
Satellite System ◽  
Dark Matter Halos ◽  
The Mean

AbstractThe degree to which outer dark matter halos of spiral galaxies rotate with the disk is sensitive to their accretion history and may be probed with associated satellite galaxies. We use the Steward Observatory Bok telescope to measure the sense of rotation of nearby isolated spirals and combine these data with those of their associated satellites (drawn from SDSS) to directly test predictions from numerical simulations. We aim to constrain models of galaxy formation by measuring the projected component of the halo angular momentum that is aligned with that of spiral galaxy disks, Jz. We find the mean bulk rotation of the ensemble satellite system to be co-rotating with the disk with a velocity of 22 ± 13 km/s, in general agreement with previous observational studies and suggesting that galaxy disks could be formed by halo baryons collapsing by a factor of ≈10. We also find a prograde satellite fraction of 51% and Jz, of the satellite system to be positively correlated with the disk, albeit at low significance (2655 ± 2232 kpc km/s).

scholarly journals Observing merger trees in a new light

2018 ◽  
Vol 35 ◽  
Author(s):  
Rhys J. J. Poulton ◽  
Aaron S. G. Robotham ◽  
Chris Power ◽  
Pascal J. Elahi
Keyword(s):  
Dark Matter ◽  
Galaxy Formation ◽  
Diagnostic Tool ◽  
Cosmic Time ◽  
Analytical Models ◽  
Dark Matter Halos ◽  
Assembly Simulation ◽  
Novel Method ◽  
Tree Building

AbstractMerger trees harvested from cosmologicalN-body simulations encode the assembly histories of dark matter halos over cosmic time and are a fundamental component of semi-analytical models of galaxy formation. The ability to compare the tools used to construct merger trees, namely halo finders and tree building algorithms, in an unbiased and systematic manner is critical to assess the quality of merger trees. In this paper, we present the dendrogram, a novel method to visualise merger trees, which provides a comprehensive characterisation of a halo’s assembly history—tracking subhalo orbits, halo merger events, and the general evolution of halo properties. We show the usefulness of thedendrogramas a diagnostic tool of merger trees by comparing halo assembly simulation analysed with three different halo finders—VELOCIraptor, AHF, and Rockstar—and their associated tree builders. Based on our analysis of the resulting dendrograms, we highlight how they have been used to motivate improvements to VELOCIraptor. Thedendrogramsoftware is publicly available online, at:https://github.com/rhyspoulton/MergerTree-Dendrograms.

scholarly journals Mass-Discrepancy Acceleration Relation: A Natural Outcome of Galaxy Formation in Cold Dark Matter Halos

2017 ◽  
Vol 118 (16) ◽  
Author(s):  
Aaron D. Ludlow ◽  
Alejandro Benítez-Llambay ◽  
Matthieu Schaller ◽  
Tom Theuns ◽  
Carlos S. Frenk ◽  
...  
Keyword(s):  
Dark Matter ◽  
Galaxy Formation ◽  
Cold Dark Matter ◽  
Dark Matter Halos ◽  
Natural Outcome

THE LINK BETWEEN GALAXIES AND DARK MATTER

2011 ◽  
Vol 20 (10) ◽  
pp. 1771-1777
Author(s):  
HOUJUN MO
Keyword(s):  
Dark Matter ◽  
Galaxy Formation ◽  
Luminosity Function ◽  
The Other ◽  
Dark Matter Halos ◽  
Galaxy Formation And Evolution ◽  
The Galaxy ◽  
Galaxy Halo ◽  
Formation And Evolution ◽  
The Universe

Given that dark matter is gravitationally dominant in the universe, and that galaxy formation is closely related to dark matter halos, a key first step in understanding galaxy formation and evolution in the CDM paradigm is to quantify the galaxy-halo connection for galaxies of different properties. Here I will present results about the halo/galaxy connection obtained from two different methods. One is based on the conditional luminosity function, which describes the occupation of galaxies in halos of different masses, and the other is based on galaxy systems properly selected to represent dark halos.

scholarly journals Dwarf Galaxies as Cosmological Probes

2018 ◽  
Vol 14 (S344) ◽  
pp. 455-463
Author(s):  
Julio F. Navarro
Keyword(s):  
Dark Matter ◽  
Galaxy Formation ◽  
Cold Dark Matter ◽  
Dwarf Galaxies ◽  
Dark Matter Halos ◽  
Too Big To Fail ◽  
Sharp Minimum ◽  
Luminous Galaxies ◽  
Small Scales ◽  
Low Mass

AbstractThe Lambda Cold Dark Matter (LCDM) paradigm makes specific predictions for the abundance, structure, substructure and clustering of dark matter halos, the sites of galaxy formation. These predictions can be directly tested, in the low-mass halo regime, by dark matter-dominated dwarf galaxies. A number of potential challenges to LCDM have been identified when confronting the expected properties of dwarfs with observation. I review our understanding of a few of these issues, including the “missing satellites” and the “too-big-to-fail” problems, and argue that neither poses an insurmountable challenge to LCDM. Solving these problems requires that most dwarf galaxies inhabit halos of similar mass, and that there is a relatively sharp minimum halo mass threshold to form luminous galaxies. These predictions are eminently falsifiable. In particular, LCDM predicts a large number of “dark” low-mass halos, some of which should have retained enough primordial gas to be detectable in deep 21 cm or Hα surveys. Detecting this predicted population of “mini-halos” would be a major discovery and a resounding success for LCDM on small scales.

scholarly journals Hi and Galaxy Formation in Loose Groups

2004 ◽  
Vol 21 (4) ◽  
pp. 390-392
Author(s):  
Daniel J. Pisano
Keyword(s):  
Dark Matter ◽  
Galaxy Formation ◽  
Local Group ◽  
Dark Matter Halos ◽  
Early Results ◽  
Large Numbers ◽  
The Galaxy ◽  
Order Of Magnitude ◽  
Low Mass ◽  
High Velocity Clouds

AbstractModels of hierarchical galaxy formation predict that large numbers of low-mass, dark matter halos remain around galaxies today. These models predict an order of magnitude more halos than observed stellar satellites in the Local Group. One possible solution to this discrepancy is that the high-velocity clouds (HVCs) around the Milky Way may be associated with the excess dark matter halos and be the gaseous remnants of the galaxy formation process. If this is the case, then analogues to the HVCs should be visible in other groups. In this paper, I review the observations of Hi clouds lacking stars around other galaxies and in groups, present early results from our Hi survey of loose groups analogous to the Local Group, and discuss implications for the nature of HVCs and galaxy formation.

scholarly journals Galaxy formation from dry and hydro simulations

2009 ◽  
Vol 5 (H15) ◽  
pp. 85-85
Author(s):  
Luca Ciotti
Keyword(s):  
Dark Matter ◽  
Galaxy Formation ◽  
Scaling Laws ◽  
High Redshift ◽  
Elliptical Galaxies ◽  
Dark Matter Halos ◽  
Galaxy Mergers ◽  
Lower Redshift

AbstractThe effects of dry and wet merging on the Scaling Laws (SLs) of elliptical galaxies (Es) are discussed. It is found that the SLs, possibly established at high redshift by the fast collapse of gas-rich and clumpy stellar distributions in preexisting dark matter halos following the cosmological SLs, are compatible with a (small) number of galaxy mergers at lower redshift.

scholarly journals Magnetic Field Generation in Protogalactic Halos

1990 ◽  
Vol 140 ◽  
pp. 519-526 ◽  
Author(s):  
Ralph E. Pudritz
Keyword(s):  
Dark Matter ◽  
Magnetic Fields ◽  
Galaxy Formation ◽  
Galactic Halos ◽  
Dark Matter Halos ◽  
Magnetic Field Generation ◽  
Dynamo Action ◽  
Potential Wells ◽  
Fundamental Point ◽  
Amplification Process

It has become quite clear that dynamically significant magnetic fields of order 10−6 Gauss are present in galactic halos (Perry, these Proceedings) at redshifts up to z ≥ 2. Comparable fields have been detected on cluster and even on supercluster scales (Giovannini, these Proceedings). The existence of such high fields is surprising because it is highly unlikely that primeval magnetic fields could have been much stronger than 10–18 Gauss. These observations require that there be an epoch of very vigorous dynamo action that can amplify this seed primordial field up to the observed values. Since strong fields are already in place by redshifts of z ≃ 2, this amplification process must operate very quickly. This argues against the idea that such fields could be generated on huge intercluster scales since the dynamical times are far too long. A much more promising site for the dynamo process is in the dark matter halos of young galaxies during the epoch of galaxy formation. The fundamental point is that vigorous dynamo action can take place in shocks that are expected to occur throughout the dark matter potential wells of protogalaxies (Pudritz and Silk 1989 (≡ PS), Pudritz 1989).

scholarly journals Extracting Galaxy Merger Timescales II: A new fitting formula

2020 ◽  
Author(s):  
R J J Poulton ◽  
C Power ◽  
A S G Robotham ◽  
P J Elahi ◽  
C D P Lagos
Keyword(s):  
Dark Matter ◽  
Angular Momentum ◽  
Structure Formation ◽  
Structural Properties ◽  
Galaxy Formation ◽  
Fundamental Problem ◽  
Dark Matter Halos ◽  
New Model ◽  
Orbital Parameters ◽  
Improved Model

Abstract Predicting the merger timescale (τmerge) of merging dark matter halos, based on their orbital parameters and the structural properties of their hosts, is a fundamental problem in gravitational dynamics that has important consequences for our understanding of cosmological structure formation and galaxy formation. Previous models predicting τmerge have shown varying degrees of success when compared to the results of cosmological N-body simulations. We build on this previous work and propose a new model for τmerge that draws on insights derived from these simulations. We find that published predictions can provide reasonable estimates for τmerge based on orbital properties at infall, but tend to underpredict τmerge inside the host virial radius (R200) because tidal stripping is neglected, and overpredict it outside R200 because the host mass is underestimated. Furthermore, we find that models that account for orbital angular momentum via the circular radius Rcirc underpredict (overpredict) τmerge for bound (unbound) systems. By fitting for the dependence of τmerge on various orbital and host halo properties, we derive an improved model for τmerge that can be applied to a merging halo at any point in its orbit. Finally, we discuss briefly the implications of our new model for τmerge for semi-analytical galaxy formation modelling.

Neutrinos and the dark matter problem

1994 ◽  
Vol 346 (1678) ◽  
pp. 121-135 ◽  
Keyword(s):  
Dark Matter ◽  
Galaxy Formation ◽  
Cold Dark Matter ◽  
Mass Density ◽  
Rest Mass ◽  
Density Fluctuations ◽  
Fine Tuning ◽  
Cosmological Argument ◽  
The Mean ◽  
The Universe

Theoretical and experimental arguments suggest that the mean mass density of our universe is close to the closure value and that most of the mass in the universe consists of weakly interacting non-baryonic particles. Among the plethora of candidates that have been proposed as the dark matter, the neutrino remains the only particle known to exist, even though the issue of a neutrino mass remains unresolved. It was shown several years ago that neutrinos alone cannot provide the dark matter because physical processes in the early universe would have wiped out primordial density fluctuations on the scale of galaxies and below. The idea that cosmic strings or textures may seed galaxy formation in a neutrino-dominated universe has not yet been demonstrated to be viable. On the other hand, a model in which the bulk of the dark matter is cold and neutrinos with a mass of ca . 10 eV provide a ca . 30% contribution can, in principle, overcome many of the objections against the standard cold dark matter cosmogony. Although subject to the usual ‘fine-tuning’ criticism, these mixed dark matter models represent the best cosmological argument in favour of a non-zero rest mass for the neutrino.

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