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G Chabrier - One of the best experts on this subject based on the ideXlab platform.
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star formation statistical measure of the correlation between the prestellar core Mass Function and the stellar initial Mass Function
The Astrophysical Journal, 2010Co-Authors: G Chabrier, Patrick HennebelleAbstract:We present a simple statistical analysis of recent numerical simulations exploring the correlation between the core Mass Function (CMF) obtained from the fragmentation of a molecular cloud and the stellar Mass Function which forms from these collapsing cores. Our analysis shows that the distributions of bound cores and sink particles obtained in the simulations are consistent with the sinks being formed predominantly from their parent core Mass reservoir, with a statistical dispersion of the order of one-third of the core Mass. Such a characteristic dispersion suggests that the stellar initial Mass Function (IMF) is relatively tightly correlated to the parent CMF, leading to two similar distributions, as observed. This in turn argues in favor of the IMF being essentially determined at the early stages of core formation and being only weakly affected by the various environmental factors beyond the initial core Mass reservoir, at least in the Mass range explored in the present study. Accordingly, the final IMF of a star-forming region should be determined reasonably accurately, statistically speaking, from the initial CMF, provided some uniform efficiency factor. The calculations also show that these statistical fluctuations, due to, e.g., variations among the core properties, broaden the low-Mass tail of the IMF compared with the parent CMF, providing an explanation for the fact that the latter appears to underestimate the number of "pre brown dwarf" cores compared with the observationally derived brown dwarf IMF.
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galactic stellar and substellar initial Mass Function
Publications of the Astronomical Society of the Pacific, 2003Co-Authors: G ChabrierAbstract:We review recent determinations of the present-day Mass Function (PDMF) and initial Mass Function (IMF) in various components of the Galaxy—disk, spheroid, young, and globular clusters—and in conditions characteristic of early star formation. As a general feature, the IMF is found to depend weakly on the environment and to be well described by a power-law form forM , and a lognormal form below, except possibly for m!1 early star formation conditions. The disk IMF for single objects has a characteristic Mass around M , m!0.08 c and a variance in logarithmic Mass , whereas the IMF for multiple systems hasM , and . j!0.7 m!0.2 j!0.6 c The extension of the single MF into the brown dwarf regime is in good agreement with present estimates of L- and T-dwarf densities and yields a disk brown dwarf number density comparable to the stellar one, n!n! BD " pc !3 .T he IMF of young clusters is found to be consistent with the disk fi eld IMF, providing the same correction 0.1 for unresolved binaries, confirming the fact that young star clusters and disk field stars represent the same stellar population. Dynamical effects, yielding depletion of the lowest Mass objects, are found to become consequential for ages!130 Myr. The spheroid IMF relies on much less robust grounds. The large metallicity spread in the local subdwarf photometric sample, in particular, remains puzzling. Recent observations suggest that there is a continuous kinematic shear between the thick-disk population, present in local samples, and the genuine spheroid one. This enables us to derive only an upper limit for the spheroid Mass density and IMF. Within all the uncertainties, the latter is found to be similar to the one derived for globular clusters and is well represented also by a lognormal form with a characteristic Mass slightly larger than for the disk, M , ,e xcluding as ignif icant population of m!0.2-0.3 c brown dwarfs in globular clusters and in the spheroid. The IMF characteristic of early star formation at large redshift remains undetermined, but different observational constraints suggest that it does not extend below!1M , .T hese results suggest a characteristic Mass for star formation that decreases with time, from conditions prevailing at large redshift to conditions characteristic of the spheroid (or thick disk) to present-day conditions.Theseconclusions,however, remain speculative, given the large uncertainties in the spheroid and early star IMF determinations. These IMFs allow a reasonably robust determination of the Galactic present-day and initial stellar and brown dwarf contents. They also have important galactic implications beyond the Milky Way in yielding more accurate Mass-to-light ratio determinations. The Mass-to-light ratios obtained with the disk and the spheroid IMF yield values 1.8-1.4 times smaller than for a Salpeter IMF, respectively, in agreement with various recent dynamical determinations. This general IMF determination is examined in the context of star formation theory. None of the theories based on a Jeans-type mechanism, where fragmentation is due only to gravity, can fulfill all the observational constraints on star formation and predict a large number of substellar objects. On the other hand, recent numerical simulations of compressible turbulence, in particular in super-Alfvenic conditions, seem to reproduce both qualitatively and quantitatively the stellar and substellar IMF and thus provide an appealing theoretical foundation. In this picture, star formation is induced by the dissipation of large-scale turbulence to smaller scales through radiative MHD shocks, producing filamentary structures. These shocks produce local nonequilibrium structures with large density contrasts, which collapse eventually in gravitationally bound objects under the combined influence of turbulence and gravity. The concept of a single Jeans Mass is replaced by a distribution of local Jeans Masses, representative of the lognormal probability density Function of the turbulent gas. Objects below the mean thermal Jeans Mass still have a possibility to collapse, although with a decreasing probability.
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the galactic disk Mass budget i stellar Mass Function and density
The Astrophysical Journal, 2001Co-Authors: G ChabrierAbstract:In this paper, we use the general theory worked out within the past few years for the structure and the evolution of low-Mass stars to derive the stellar Mass Function in the Galactic disk down to the vicinity of the hydrogen-burning limit from the observed nearby luminosity Functions. The accuracy of the Mass-magnitude relationships derived from the aforementioned theory is examined by comparison with recent, accurate observational relationships in the M-dwarf domain. The Mass Function is shown to flatten out below ~1 M☉ but to keep rising down to the bottom of the main sequence. Combining the present determination below 1 M☉ and J. M. Scalo's Mass Function for larger Masses, we show that the Mass Function is well described over the entire stellar Mass range, from ~100 M☉ to ~0.1 M☉, by three Functional forms, namely, a two-segment power law, a lognormal form, or an exponential form, all normalized to the Hipparcos sample at 0.8 M☉. Integration of this Mass Function yields a reasonably accurate census of the entire stellar population in the Galactic disk and its volume and surface Mass density.
Xi Kang - One of the best experts on this subject based on the ideXlab platform.
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the effect of warm dark matter on galaxy properties constraints from the stellar Mass Function and the tully fisher relation
The Astrophysical Journal, 2013Co-Authors: Xi Kang, Andrea V Maccio, Aaron A DuttonAbstract:In this paper, we combine high-resolution N-body simulations with a semi-analytical model of galaxy formation to study the effects of a possible warm dark matter (WDM) component on the observable properties of galaxies. We compare three WDM models with a dark matter (DM) Mass of 0.5, 0.75, and 2.0 keV with the standard cold dark matter case. For a fixed set of parameters describing the baryonic physics, the WDM models predict fewer galaxies at low (stellar) Masses, as expected due to the suppression of power on small scales, while no substantial difference is found at the high-Mass end. However, these differences in the stellar Mass Function vanish when a different set of parameters is used to describe the (largely unknown) galaxy formation processes. We show that it is possible to break this degeneracy between DM properties and the parameterization of baryonic physics by combining observations on the stellar Mass Function with the Tully-Fisher relation (the relation between stellar Mass and the rotation velocity at large galactic radii as probed by resolved H I rotation curves). WDM models with a too warm candidate (m(nu) < 0.75 keV) cannot simultaneously reproduce the stellar Mass Function and the Tully-Fisher relation. We conclude that accurate measurements of the galaxy stellar Mass Function and the link between galaxies and DM halos down to the very low Mass end can give very tight constraints on the nature of DM candidates.
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the effect of warm dark matter on galaxy properties constraints from the stellar Mass Function and the tully fisher relation
arXiv: Cosmology and Nongalactic Astrophysics, 2012Co-Authors: Xi Kang, Andrea V Maccio, Aaron A DuttonAbstract:In this paper we combine high resolution N-body simulations with a semi analytical model of galaxy formation to study the effects of a possible Warm Dark Matter (WDM) component on the observable properties of galaxies. We compare three WDM models with a dark matter Mass of 0.5, 0.75 and 2.0 keV, with the standard Cold Dark Matter case. For a fixed set of parameters describing the baryonic physics the WDM models predict less galaxies at low (stellar) Masses, as expected due to the suppression of power on small scales, while no substantial difference is found at the high Mass end. However these differences in the stellar Mass Function, vanish when different set of parameters are used to describe the (largely unknown) galaxy formation processes. We show that is possible to break this degeneracy between DM properties and the parameterization of baryonic physics by combining observations on the stellar Mass Function with the Tully-Fisher relation (the relation between stellar Mass and the rotation velocity at large galactic radii as probed by resolved HI rotation curves). WDM models with a too warm candidate (m<0.75 keV) cannot simultaneously reproduce the stellar Mass Function and the Tully-Fisher relation. We conclude that accurate measurements of the galaxy stellar Mass Function and the link between galaxies and dark matter haloes down to the very low-Mass end can give very tight constraints on the nature of DM candidates.
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galaxy formation with cold gas accretion and evolving stellar initial Mass Function
The Astrophysical Journal, 2010Co-Authors: Xi Kang, Weipeng Lin, Ramin A Skibba, Dongni ChenAbstract:The evolution of the galaxy stellar Mass Function is especially useful to test the current model of galaxy formation. Observational data have revealed a few inconsistencies with predictions from the.CDM model. For example, most Massive galaxies have already been observed at very high redshifts, and they have experienced only mild evolution since then. In conflict with this, semi-analytical models (SAMs) of galaxy formation predict an insufficient number of Massive galaxies at high redshift and a rapid evolution between redshift 1 and 0. In addition, there is a strong correlation between star formation rate (SFR) and stellar Mass for star-forming galaxies, which can be roughly reproduced with the model, but with a normalization that is too low at high redshift. Furthermore, the stellar Mass density obtained from the integral of the cosmic star formation history is higher than the measured one by a factor of 2. In this paper, we study these issues using an SAM that includes (1) cold gas accretion in Massive halos at high redshift; (2) tidal stripping of stellar Mass from satellite galaxies; and (3) an evolving stellar initial Mass Function (IMF; bottom-light) with a higher gas recycle fraction. Our results show that the combined effects from (1) and (2) can predict sufficiently Massive galaxies at high redshifts and reproduce their mild evolution at low redshift, while the combined effects of (1) and (3) can reproduce the correlation between SFR and stellar Mass for star-forming galaxies across a wide range of redshifts. A bottom-light/top-heavy stellar IMF could partly resolve the conflict between the stellar Mass density and cosmic star formation history.
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galaxy formation with cold gas accretion and evolving stellar initial Mass Function
arXiv: Cosmology and Nongalactic Astrophysics, 2010Co-Authors: Xi Kang, Weipeng Lin, Ramin A Skibba, Dongni ChenAbstract:The evolution of the galaxy stellar Mass Function is especially useful to test the current model of galaxy formation. Observational data have revealed a few inconsistencies with predictions from the $\Lambda {\rm CDM}$ model. For example, most Massive galaxies have already been observed at very high redshifts, and they have experienced only mild evolution since then. In conflict with this, semi-analytical models of galaxy formation predict an insufficient number of Massive galaxies at high redshift and a rapid evolution between redshift 1 and 0 . In addition, there is a strong correlation between star formation rate and stellar Mass for star-forming galaxies, which can be roughly reproduced with the model, but with a normalization that is too low at high redshift. Furthermore, the stellar Mass density obtained from the integral of the cosmic star formation history is higher than the measured one by a factor of 2. In this paper, we study these issues using a semi-analytical model that includes: 1) cold gas accretion in Massive halos at high redshift; 2) tidal stripping of stellar Mass from satellite galaxies; and 3) an evolving stellar initial Mass Function (bottom-light) with a higher gas recycle fraction. Our results show that the combined effects from 1) and 2) can predict sufficiently Massive galaxies at high redshifts and reproduce their mild evolution at low redshift, While the combined effects of 1) and 3) can reproduce the correlation between star formation rate and stellar Mass for star-forming galaxies across wide range of redshifts. A bottom-light/top-heavy stellar IMF could partly resolve the conflict between the stellar Mass density and cosmic star formation history.
Antonio Riotto - One of the best experts on this subject based on the ideXlab platform.
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the halo Mass Function from excursion set theory iii non gaussian fluctuations
The Astrophysical Journal, 2010Co-Authors: Michele Maggiore, Antonio RiottoAbstract:We compute the effect of primordial non-Gaussianity on the halo Mass Function, using excursion set theory. In the presence of non-Gaussianity, the stochastic evolution of the smoothed density field, as a Function of the smoothing scale, is non-Markovian and beside "local" terms that generalize Press-Schechter (PS) theory, there are also "memory" terms, whose effect on the Mass Function can be computed using the formalism developed in the first paper of this series. We find that, when computing the effect of the three-point correlator on the Mass Function, a PS-like approach which consists in neglecting the cloud-in-cloud problem and in multiplying the final result by a fudge factor sime2, is in principle not justified. When computed correctly in the framework of excursion set theory, in fact, the "local" contribution vanishes (for all odd-point correlators the contribution of the image Gaussian cancels the PS contribution rather than adding up), and the result comes entirely from non-trivial memory terms which are absent in PS theory. However it turns out that, in the limit of large halo Masses, where the effect of non-Gaussianity is more relevant, these memory terms give a contribution which is the same as that computed naively with PS theory, plus subleading terms depending on derivatives of the three-point correlator. We finally combine these results with the diffusive barrier model developed in the second paper of this series, and we find that the resulting Mass Function reproduces recent N-body simulations with non-Gaussian initial conditions, without the introduction of any ad hoc parameter.
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the halo Mass Function from excursion set theory ii the diffusing barrier
The Astrophysical Journal, 2010Co-Authors: Michele Maggiore, Antonio RiottoAbstract:In excursion set theory, the computation of the halo Mass Function is mapped into a first-passage time process in the presence of a barrier, which in the spherical collapse model is a constant and in the ellipsoidal collapse model is a fixed Function of the variance of the smoothed density field. However, N-body simulations show that dark matter halos grow through a mixture of smooth accretion, violent encounters, and fragmentations, and modeling halo collapse as spherical, or even as ellipsoidal, is a significant oversimplification. In addition, the very definition of what is a dark matter halo, both in N-body simulations and observationally, is a difficult problem. We propose that some of the physical complications inherent to a realistic description of halo formation can be included in the excursion set theory framework, at least at an effective level, by taking into account that the critical value for collapse is not a fixed constant δ c , as in the spherical collapse model, nor a fixed Function of the variance σ of the smoothed density field, as in the ellipsoidal collapse model, but rather is itself a stochastic variable, whose scatter reflects a number of complicated aspects of the underlying dynamics. Solving the first-passage time problem in the presence of a diffusing barrier we find that the exponential factor in the Press-Schechter Mass Function changes from exp{–δ2 c /2σ2} to exp{–aδ2 c /2σ2}, where a = 1/(1 + DB ) and DB is the diffusion coefficient of the barrier. The numerical value of DB , and therefore the corresponding value of a, depends among other things on the algorithm used for identifying halos. We discuss the physical origin of the stochasticity of the barrier and, from recent N-body simulations that studied the properties of the collapse barrier, we deduce a value DB 0.25. Our model then predicts a 0.80, in excellent agreement with the exponential fall off of the Mass Function found in N-body simulations, for the same halo definition. Combining this result with the non-Markovian corrections computed in Paper I of this series, we derive an analytic expression for the halo Mass Function for Gaussian fluctuations and we compare it with N-body simulations.
Holger Baumgardt - One of the best experts on this subject based on the ideXlab platform.
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the velocity dispersion and Mass Function of the outer halo globular cluster palomar 4
Monthly Notices of the Royal Astronomical Society, 2012Co-Authors: Matthias J Frank, M Hilker, Holger Baumgardt, Patrick Cote, E K Grebel, Hosein Haghi, Andreas H W Kupper, S G DjorgovskiAbstract:We obtained precise line-of-sight radial velocities of 23 member stars of the remote halo globular cluster Palomar 4 (Pal 4) using the High Resolution Echelle Spectrograph (HIRES) at the Keck I telescope. We also measured the Mass Function of the cluster down to a limiting magnitude of V 28 mag using archival HST /WFPC2 imaging. We derived the cluster’s surface brightness prole based on the WFPC2 data and on broad-band imaging with the Low-Resolution Imaging Spectrometer (LRIS) at the Keck II telescope. We nd a mean cluster velocity of 72 :55 0:22 km s 1 and a velocity dispersion of 0:87 0:18 km s 1 . The global Mass Function of the cluster, in the Mass range 0:55 6 M 6 0:85 M , is shallower than a Kroupa Mass Function and the cluster is signicantly depleted in low-Mass stars in its center compared to its outskirts. Since the relaxation time of Pal 4 is of the order of a Hubble time, this points to primordial Mass segregation in this cluster. Extrapolating the measured Mass Function towards lower-Mass stars and including the contribution of compact remnants, we derive a total cluster Mass of 29,800 M . For this Mass, the measured velocity dispersion is consistent with the expectations of Newtonian dynamics and below the prediction of MOND. Pal 4 adds to the growing body of evidence that the dynamics of star clusters in the outer Galactic halo can hardly be explained by MOND.
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the velocity dispersion and Mass Function of the outer halo globular cluster palomar 4
arXiv: Astrophysics of Galaxies, 2012Co-Authors: Matthias J Frank, M Hilker, Holger Baumgardt, Patrick Cote, E K Grebel, Hosein Haghi, Andreas H W Kupper, S G DjorgovskiAbstract:We obtained precise line-of-sight radial velocities of 23 member stars of the remote halo globular cluster Palomar 4 (Pal 4) using the High Resolution Echelle Spectrograph (HIRES) at the Keck I telescope. We also measured the Mass Function of the cluster down to a limiting magnitude of V~28 mag using archival HST/WFPC2 imaging. We derived the cluster's surface brightness profile based on the WFPC2 data and on broad-band imaging with the Low-Resolution Imaging Spectrometer (LRIS) at the Keck II telescope. We find a mean cluster velocity of 72.55+/-0.22 km/s and a velocity dispersion of 0.87+/-0.18 km/s. The global Mass Function of the cluster, in the Mass range 0.55<=M<=0.85 M_solar, is shallower than a Kroupa Mass Function and the cluster is significantly depleted in low-Mass stars in its center compared to its outskirts. Since the relaxation time of Pal 4 is of the order of a Hubble time, this points to primordial Mass segregation in this cluster. Extrapolating the measured Mass Function towards lower-Mass stars and including the contribution of compact remnants, we derive a total cluster Mass of 29800 M_solar. For this Mass, the measured velocity dispersion is consistent with the expectations of Newtonian dynamics and below the prediction of Modified Newtonian Dynamics (MOND). Pal 4 adds to the growing body of evidence that the dynamics of star clusters in the outer Galactic halo can hardly be explained by MOND.
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the influence of gas expulsion and initial Mass segregation on the stellar Mass Function of globular star clusters
Monthly Notices of the Royal Astronomical Society, 2008Co-Authors: Michael Marks, Pavel Kroupa, Holger BaumgardtAbstract:Recently, De Marchi, Paresce & Pulone studied a sample of 20 globular clusters and found that all clusters with high concentrations have steep stellar Mass Functions while clusters with low concentration have comparatively shallow Mass Functions. No globular clusters were found with a flat Mass Function and high concentration. This seems curious since more concentrated star clusters are believed to be dynamically more evolved and should have lost more low-Mass stars via evaporation, which would result in a shallower Mass Function in the low-Mass part.
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the influence of gas expulsion and initial Mass segregation on the stellar Mass Function of globular star clusters
arXiv: Astrophysics, 2008Co-Authors: Michael Marks, Pavel Kroupa, Holger BaumgardtAbstract:Recently de Marchi, Paresce & Pulone (2007) studied a sample of twenty globular clusters and found that all clusters with high concentrations have steep stellar Mass-Functions while clusters with low concentration have comparatively shallow Mass-Functions. No globular clusters were found with a flat Mass-Function and high concentration. This seems curious since more concentrated star clusters are believed to be dynamically more evolved and should have lost more low-Mass stars via evaporation, which would result in a shallower Mass-Function in the low-Mass part. We show that this effect can be explained by residual-gas expulsion from initially Mass-segregated star clusters, and is enhanced further through unresolved binaries. If gas expulsion is the correct mechanism to produce the observed trend, then observation of these parameters would allow to constrain cluster starting conditions such as star formation efficiency and the time-scale of gas expulsion.
S G Djorgovski - One of the best experts on this subject based on the ideXlab platform.
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the velocity dispersion and Mass Function of the outer halo globular cluster palomar 4
Monthly Notices of the Royal Astronomical Society, 2012Co-Authors: Matthias J Frank, M Hilker, Holger Baumgardt, Patrick Cote, E K Grebel, Hosein Haghi, Andreas H W Kupper, S G DjorgovskiAbstract:We obtained precise line-of-sight radial velocities of 23 member stars of the remote halo globular cluster Palomar 4 (Pal 4) using the High Resolution Echelle Spectrograph (HIRES) at the Keck I telescope. We also measured the Mass Function of the cluster down to a limiting magnitude of V 28 mag using archival HST /WFPC2 imaging. We derived the cluster’s surface brightness prole based on the WFPC2 data and on broad-band imaging with the Low-Resolution Imaging Spectrometer (LRIS) at the Keck II telescope. We nd a mean cluster velocity of 72 :55 0:22 km s 1 and a velocity dispersion of 0:87 0:18 km s 1 . The global Mass Function of the cluster, in the Mass range 0:55 6 M 6 0:85 M , is shallower than a Kroupa Mass Function and the cluster is signicantly depleted in low-Mass stars in its center compared to its outskirts. Since the relaxation time of Pal 4 is of the order of a Hubble time, this points to primordial Mass segregation in this cluster. Extrapolating the measured Mass Function towards lower-Mass stars and including the contribution of compact remnants, we derive a total cluster Mass of 29,800 M . For this Mass, the measured velocity dispersion is consistent with the expectations of Newtonian dynamics and below the prediction of MOND. Pal 4 adds to the growing body of evidence that the dynamics of star clusters in the outer Galactic halo can hardly be explained by MOND.
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the velocity dispersion and Mass Function of the outer halo globular cluster palomar 4
arXiv: Astrophysics of Galaxies, 2012Co-Authors: Matthias J Frank, M Hilker, Holger Baumgardt, Patrick Cote, E K Grebel, Hosein Haghi, Andreas H W Kupper, S G DjorgovskiAbstract:We obtained precise line-of-sight radial velocities of 23 member stars of the remote halo globular cluster Palomar 4 (Pal 4) using the High Resolution Echelle Spectrograph (HIRES) at the Keck I telescope. We also measured the Mass Function of the cluster down to a limiting magnitude of V~28 mag using archival HST/WFPC2 imaging. We derived the cluster's surface brightness profile based on the WFPC2 data and on broad-band imaging with the Low-Resolution Imaging Spectrometer (LRIS) at the Keck II telescope. We find a mean cluster velocity of 72.55+/-0.22 km/s and a velocity dispersion of 0.87+/-0.18 km/s. The global Mass Function of the cluster, in the Mass range 0.55<=M<=0.85 M_solar, is shallower than a Kroupa Mass Function and the cluster is significantly depleted in low-Mass stars in its center compared to its outskirts. Since the relaxation time of Pal 4 is of the order of a Hubble time, this points to primordial Mass segregation in this cluster. Extrapolating the measured Mass Function towards lower-Mass stars and including the contribution of compact remnants, we derive a total cluster Mass of 29800 M_solar. For this Mass, the measured velocity dispersion is consistent with the expectations of Newtonian dynamics and below the prediction of Modified Newtonian Dynamics (MOND). Pal 4 adds to the growing body of evidence that the dynamics of star clusters in the outer Galactic halo can hardly be explained by MOND.
