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Register a new userOrbital decay of Globular Clusters in the galaxy with little dark matter
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Authors
Adi Nusser
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Recently, cite{vanDokkum2018} have presented an important discovery of an ultra diffuse galaxy, NGC1052-DF2, with a dark matter content significantly less than predicted from its stellar mass alone. The analysis relies on measured radial velocities of 10 Globular Clusters (GCs), of estimated individual masses of a few $ times 10^6 M_odot$. This is about $1%$ of the inferred mass of NGC1052-DF2 of $2times 10^8 M_odot$ within a half-light radius, $R_mathrm{e}=2.2, mathrm{kpc}$. The large relative mass and the old age of these objects imply that they might be susceptible to orbital decay by dynamical friction. Using analytic estimates and N-body simulations of an isolated system matching the inferred mass profile of NGC1052-DF2, we show that orbits of the most massive GCs should already have decayed on a time scale of a few Gyrs. These findings should help in constraining mass profile and formation scenarios of NGC1052-DF2.
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Globular clusters (GCs) are often used to estimate the dark matter content of galaxies, especially dwarfs, where other kinematic tracers are lacking. These estimates typically assume spherical symmetry and dynamical equilibrium, assumptions that may not hold for the sparse GC population of dwarfs in galaxy clusters. We use a catalog of GCs tagged onto the Illustris simulation to study the accuracy of GC-based mass estimates. We focus on galaxies in the stellar mass range 10$^{8} - 10^{11.8}$ M$_{odot}$ identified in $9$ simulated Virgo-like clusters. Our results indicate that mass estimates are, on average, quite accurate in systems with GC numbers $N_{rm GC} geq 10$ and where the uncertainty of individual GC line-of-sight velocities is smaller than the inferred velocity dispersion, $sigma_{rm GC}$. In cases where $N_{rm GC} leq 10$, however, biases may result depending on how $sigma_{rm GC}$ is computed. We provide calibrations that may help alleviate these biases in methods widely used in the literature. As an application, we find a number of dwarfs with $M_{*} sim 10^{8.5}, M_{odot}$ (comparable to the ultradiffuse galaxy DF2, notable for the low $sigma_{GC}$ of its $10$ GCs) with $sigma_{rm GC} sim 7$ - $15; rm km rm s^{-1}$. These DF2 analogs correspond to relatively massive systems at their infall time ($M_{200} sim 1$ - $3 times 10^{11}$ $M_{odot}$) which have retained only $3$-$17$ GCs and have been stripped of more than 95$%$ of their dark matter. Our results suggest that extreme tidal mass loss in otherwise normal dwarf galaxies may be a possible formation channel for ultradiffuse objects like DF2.
We recently found an ultra diffuse galaxy (UDG) with a half-light radius of R_e = 2.2 kpc and little or no dark matter. The total mass of NGC1052-DF2 was measured from the radial velocities of bright compact objects that are associated with the galaxy. Here we analyze these objects using a combination of HST imaging and Keck spectroscopy. Their average size is <r_h> = 6.2+-0.5 pc and their average ellipticity is <{epsilon}> = 0.18+-0.02. From a stacked Keck spectrum we derive an age >9 Gyr and a metallicity of [Fe/H] = -1.35+-0.12. Their properties are similar to {omega} Centauri, the brightest and largest globular cluster in the Milky Way, and our results demonstrate that the luminosity function of metal-poor globular clusters is not universal. The fraction of the total stellar mass that is in the globular cluster system is similar to that in other UDGs, and consistent with failed galaxy scenarios where star formation terminated shortly after the clusters were formed. However, the galaxy is a factor of ~1000 removed from the relation between globular cluster mass and total galaxy mass that has been found for other galaxies, including other UDGs. We infer that a dark matter halo is not a prerequisite for the formation of metal-poor globular cluster-like objects in high redshift galaxies.
The proper motions of stars in the outskirts of globular clusters are used to estimate cluster velocity dispersion profiles as far as possible within their tidal radii. We use individual color-magnitude diagrams to select high probability cluster stars for 25 metal-poor globular clusters within 20 kpc of the sun, 19 of which have substantial numbers of stars at large radii. Of the 19, 11 clusters have a falling velocity dispersion in the 3-6 half mass radii range, 6 are flat, and 2 plausibly have a rising velocity dispersion. The profiles are all in the range expected from simulated clusters started at high redshift in a zoom-in cosmological simulation. The 11 clusters with falling velocity dispersion profiles are consistent with no dark matter above the Galactic background. The 6 clusters with approximately flat velocity dispersion profiles could have local dark matter, but are ambiguous. The 2 clusters with rising velocity dispersion profiles are consistent with a remnant local dark matter halo, but need membership confirmation and detailed orbital modeling to further test these preliminary results.
A cosmological zoom-in simulation which develops into a Milky Way-like halo is started at redshift 7. The initial dark matter distribution is seeded with dense star clusters, median mass $5times 10^5 M_sun$, placed in the largest sub-halos present, which have a median peak circular velocity of 25 kms. Three simulations are initialized using the same dark matter distribution, with the star clusters started on approximately circular orbits having initial median radii 6.8 kpc, 0.14 kpc, and, at the exact center of the sub-halos. The simulations are evolved to the current epoch at which time the median galactic orbital radii of the three sets of clusters are 30, 5 and 16 kpc, with the clusters losing about 2, 50 and 15% of their mass, respectively. Clusters started at small orbital radii have so much tidal forcing that they are often not in equilibrium. Clusters started at larger sub-halo radii have a velocity dispersion that declines smoothly to $simeq$20% of the central value at $simeq$20 half mass radii. The clusters started at the sub-halo centers can show a rise in velocity dispersion beyond 3-5 half mass radii. That is, the clusters formed without local dark matter always have stellar mass dominated kinematics at all radii, whereas about 25% of the clusters started at sub-halo centers have remnant local dark matter.
Small distortions in the images of Einstein rings or giant arcs offer the exciting prospect of detecting dark matter haloes or subhaloes of mass below $10^9$M$_{odot}$, most of which are too small to have made a visible galaxy. A very large number of such haloes are predicted to exist in the cold dark matter model of cosmogony; in contrast other models, such as warm dark matter, predict no haloes below a mass of this order which depends on the properties of the warm dark matter particle. Attempting to detect these small perturbers could therefore discriminate between different kinds of dark matter particles, and even rule out the cold dark matter model altogether. Globular clusters in the lens galaxy also induce distortions in the image which could, in principle, contaminate the test. Here, we investigate the population of globular clusters in six early type galaxies in the Virgo cluster. We find that the number density of globular clusters of mass $sim10^6$M$_{odot}$ is comparable to that of the dark matter perturbers (including subhaloes in the lens and haloes along the line-of-sight). We show that the very different degrees of mass concentration in globular clusters and dark matter haloes result in different lensing distortions. These are detectable with milli-arcsecond resolution imaging which can distinguish between globular cluster and dark matter halo signals.
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