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Register a new userEvolution of globular-cluster systems of ultra-diffuse galaxies due to dynamical friction in MOND gravity
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(Abridged) Dynamical friction can be used to distinguish Newtonian gravity and modified Newtonian dynamics (MOND) because it works differently in these frameworks. This concept, however, has yet to be explored very much with MOND. Previous simulations showed weaker dynamical friction during major mergers for MOND than for Newtonian gravity with dark matter. Analytic arguments suggest the opposite for minor mergers. In this work, we verify the analytic predictions for MOND by high-resolution $N$-body simulations of globular clusters (GCs) moving in isolated ultra-diffuse galaxies (UDGs). We test the MOND analog of the Chandrasekhar formula for the dynamical friction proposed by Sanchez-Salcedo on a single GC. We also explore whether MOND allows GC systems of isolated UDGs to survive without sinking into nuclear star clusters. The simulations are run using the adaptive-mesh-refinement code Phantom of Ramses. The mass resolution is $20,M_odot$ and the spatial resolution $50,$pc. The GCs are modeled as point masses. Simulations including a single GC reveal that, as long as the apocenter of the GC is over about 0.5 effective radii, the Sanchez-Salcedo formula works excellently, with an effective Coulomb logarithm increasing with orbital circularity. Once the GC reaches the central kiloparsec, its sinking virtually stops, likely because of the core stalling mechanism. In simulations with multiple GCs, many of them sink toward the center, but the core stalling effect seems to prevent them from forming a nuclear star cluster. The GC system ends up with a lower velocity dispersion than the stars of the galaxy. By scaling the simulations, we extend these results to most UDG parameters, as long as these UDGs are not external-field dominated.
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Ultra-diffuse galaxies (UDGs) are unusual galaxies with low luminosities, similar to classical dwarf galaxies, but sizes up to $sim!5$ larger than expected for their mass. Some UDGs have large populations of globular clusters (GCs), something unexpected in galaxies with such low stellar density and mass. We have carried out a comprehensive study of GCs in both UDGs and classical dwarf galaxies at comparable stellar masses using HST observations of the Coma cluster. We present new imaging for 33 Dragonfly UDGs with the largest effective radii ($>2$ kpc), and additionally include 15 UDGs and 54 classical dwarf galaxies from the HST/ACS Coma Treasury Survey and the literature. Out of a total of 48 UDGs, 27 have statistically significant GC systems, and 11 have candidate nuclear star clusters. The GC specific frequency ($S_N$) varies dramatically, with the mean $S_N$ being higher for UDGs than for classical dwarfs. At constant stellar mass, galaxies with larger sizes (or lower surface brightnesses) have higher $S_N$, with the trend being stronger at higher stellar mass. At lower stellar masses, UDGs tend to have higher $S_N$ when closer to the center of the cluster, i.e., in denser environments. The fraction of UDGs with a nuclear star cluster also depends on environment, varying from $sim!40$% in the cluster core, where it is slightly lower than the nucleation fraction of classical dwarfs, to $lesssim20%$ in the outskirts. Collectively, we observe an unmistakable diversity in the abundance of GCs, and this may point to multiple formation routes.
We present Hubble Space Telescope imaging of two ultra diffuse galaxies (UDGs) with measured stellar velocity dispersions in the Coma cluster. The galaxies, Dragonfly 44 and DFX1, have effective radii of 4.7 kpc and 3.5 kpc and velocity dispersions of $47^{+8}_{-6}$ km/s and $30^{+7}_{-7}$ km/s, respectively. Both galaxies are associated with a striking number of compact objects, tentatively identified as globular clusters: $N_{rm gc}=74pm 18$ for Dragonfly 44 and $N_{rm gc}=62pm 17$ for DFX1. The number of globular clusters is far higher than expected from the luminosities of the galaxies but is consistent with expectations from the empirical relation between dynamical mass and globular cluster count defined by other galaxies. Combining our data for these two objects with previous HST observations of Coma UDGs we find that UDGs have a factor of $6.9^{+1.0}_{-2.4}$ more globular clusters than other galaxies of the same luminosity, in contrast to a recent study of a similar sample by Amorisco et al. (2017), but consistent with earlier results for individual galaxies. The Harris et al. (2017) relation between globular cluster count and dark matter halo mass implies a median halo mass of $M_{rm halo}sim 1.5times 10^{11},{rm M}_{odot}$ for the sixteen Coma UDGs that have been observed with HST so far, with the largest and brightest having $M_{rm halo}sim 5times 10^{11},{rm M}_{odot}$.
We study the evolution of star clusters located in the outer regions of a galaxy undergoing a sudden mass loss through gas expulsion in the framework of Milgromian dynamics (MOND) by means of N-body simulations. We find that, to leave a bound star cluster, the star formation efficiency (SFE) of an embedded cluster dominated by deep MOND gravity can be reduced down to $2.5%$. For a given SFE, the star clusters that survive in MOND can bind a larger fraction of mass compared to the Newtonian dynamics. Moreover, the more diffuse the embedded cluster is, the less substantial the size expansion of the final star cluster is. The density profiles of a surviving star cluster are more cuspy in the centre for more massive embedded clusters, and the central density profiles are flatter for less massive embedded clusters or for lower SFE. This work may help to understand the low concentration and extension of the distant low-density globular clusters (GCs) and ultra-faint and diffuse satellite galaxies around the Milky Way.
We present a study of ultra-diffuse galaxies (UDGs) in the Virgo Cluster based on deep imaging from the Next Generation Virgo Cluster Survey (NGVS). Applying a new definition for the UDG class based on galaxy scaling relations, we define samples of 44 and 26 UDGs using expansive and restrictive selection criteria, respectively. Our UDG sample includes objects that are significantly fainter than previously known UDGs: i.e., more than half are fainter than $langlemurangle_e sim27.5$ mag arcsec$^{-2}$. The UDGs in Virgos core region show some evidence for being structurally distinct from normal dwarf galaxies, but this separation disappears when considering the full sample of galaxies throughout the cluster. UDGs are more centrally concentrated in their spatial distribution than other Virgo galaxies of similar luminosity, while their morphologies demonstrate that at least some UDGs owe their diffuse nature to physical processes---such as tidal interactions or low-mass mergers---that are at play within the cluster environment. The globular cluster (GC) systems of Virgo UDGs have a wide range in specific frequency ($S_N$), with a higher mean $S_N$ than normal Virgo dwarfs, but a lower mean $S_N$ than Coma UDGs at fixed luminosity. Their GCs are predominantly blue, with a small contribution from red clusters in the more massive UDGs. The combined GC luminosity function is consistent with those observed in dwarf galaxies, showing no evidence of being anomalously luminous. The diversity in their morphologies and their GC properties suggests no single process has given rise to all objects within the UDG class. Based on the available evidence, we conclude that UDGs are simply those systems that occupy the extended tails of the galaxy size and surface brightness distributions.
The ultra-diffuse galaxy NGC 1052-DF2 has an overabundance of luminous globular clusters (GCs), and its kinematics is consistent with the presence of little to no dark matter. As the velocity dispersion among the GCs is comparable to the expected internal dispersions of the individual GCs, the galaxy might be highly conducive to GC-GC merging. If true, this could explain the puzzling luminosity function of its GCs. Here, we examine this possibility by re-simulating three of our earlier simulations of the GC system (Dutta Chowdhury et al. 2019), where the GCs were modeled as single particles, with live GCs. Somewhat surprisingly, we infer a low merger rate of $sim 0.03 rm Gyr^{-1}$. The main reason is that the GCs are too dense for tidal shock capture, caused by impulsive encounters among them, to operate efficiently (we infer a tidal capture rate of only $sim 0.002 rm Gyr^{-1}$). Therefore, whatever mergers occur are driven by other mechanisms, which we find to be captures induced by dynamical friction and compressive tides from other GCs. The low merger rate inferred here makes it unlikely that the unusually large luminosities of the GCs can be explained as a result of past GC-GC mergers. Our simulations also indicate that, if NGC 1052-DF2 is indeed largely devoid of dark matter, its tidal field is too weak to induce any significant mass loss from the GCs. Therefore, in such a scenario, we predict that it is improbable for the GCs to reveal tidal features, something that can be tested with future deep observations.
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