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Register a new userA distance of 13 Mpc resolves the claimed anomalies of the galaxy lacking dark matter
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The claimed detection of a diffuse galaxy lacking dark matter represents a possible challenge to our understanding of the properties of these galaxies and galaxy formation in general. The galaxy, already identified in photographic plates taken in the summer of 1976 at the UK 48-in Schmidt telescope, presents normal distance-independent properties (e.g. colour, velocity dispersion of its globular clusters). However, distance-dependent quantities are at odds with those of other similar galaxies, namely the luminosity function and sizes of its globular clusters, mass-to-light ratio and dark matter content. Here we carry out a careful analysis of all extant data and show that they consistently indicate a much shorter distance (13 Mpc) than previously indicated (20 Mpc). With this revised distance, the galaxy appears to be a rather ordinary low surface brightness galaxy (R_e=1.4+-0.1 kpc; M*=6.0+-3.6x10^7 Msun) with plenty of room for dark matter (the fraction of dark matter inside the half mass radius is >75% and M_halo/M*>20) corresponding to a minimum halo mass >10^9 Msun. At 13 Mpc, the luminosity and structural properties of the globular clusters around the object are the same as those found in other galaxies.
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We use hydrodynamical cosmological simulations to show that it is possible to create, via tidal interactions, galaxies lacking dark matter in a dark matter dominated universe. We select dwarf galaxies from the NIHAO project, obtained in the standard Cold Dark Matter model and use them as initial conditions for simulations of satellite-central interactions. After just one pericentric passage on an orbit with a strong radial component, NIHAO dwarf galaxies can lose up to 80 per~cent of their dark matter content, but, most interestingly, their central ($approx 8$~kpc) dark matter to stellar ratio changes from a value of ${sim}25$, as expected from numerical simulations and abundance matching techniques, to roughly unity as reported for NGC1052-DF2 and NGC1054-DF4. The stellar velocity dispersion drops from ${sim}30$ ${rm km,s^{-1}}$ before infall to values as low as $6pm 2$~ ${rm km,s^{-1}}$. These, and the half light radius around 3 kpc, are in good agreement with observations from van Dokkum and collaborators. Our study shows that it is possible to create a galaxy without dark matter starting from typical dwarf galaxies formed in a dark matter dominated universe, provided they live in a dense environment.
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.
(Abridged) Any viable cosmological model in which galaxies interact predicts the existence of primordial and tidal dwarf galaxies (TDGs). In particular, in the standard model of cosmology ($Lambda$CDM), according to the dual dwarf galaxy theorem, there must exist both primordial dark matter-dominated and dark matter-free TDGs with different radii. We study the frequency, evolution, and properties of TDGs in a $Lambda$CDM cosmology. We use the hydrodynamical cosmological Illustris-1 simulation to identify tidal dwarf galaxy candidates (TDGCs) and study their present-day physical properties. We also present movies on the formation of a few galaxies lacking dark matter, confirming their tidal dwarf nature. TDGCs can however also be formed via other mechanisms, such as from ram-pressure-stripped material or, speculatively, from cold-accreted gas. We find 97 TDGCs with $M_{stellar} >5 times 10^7 M_odot$ at redshift $z = 0$, corresponding to a co-moving number density of $2.3 times 10^{-4} h^3 cMpc^{-3}$. The most massive TDGC has $M_{total} = 3.1 times 10^9 M_odot$, comparable to that of the Large Magellanic Cloud. TDGCs are phase-space-correlated, reach high metallicities, and are typically younger than dark matter-rich dwarf galaxies. We report for the first time the verification of the dual dwarf theorem in a self-consistent $Lambda$CDM cosmological simulation. Simulated TDGCs and dark matter-dominated galaxies populate different regions in the radius-mass diagram in disagreement with observations of early-type galaxies. The dark matter-poor galaxies formed in Illustris-1 have comparable radii to observed dwarf galaxies and to TDGs formed in other galaxy-encounter simulations. In Illustris-1, only 0.17% of all selected galaxies with $M_{stellar} = 5 times 10^7-10^9 M_odot$ are TDGCs or dark matter-poor dwarf galaxies. The occurrence of NGC 1052-DF2-type objects is discussed.
Using ultra-deep imaging ($mu_g = 30.4$ mag/arcsec$^2$; 3$sigma$, 10x10), we probed the surroundings of the first galaxy lacking dark matter KKS2000[04] (NGC 1052-DF2). Signs of tidal stripping in this galaxy would explain its claimed low content of dark matter. However, we find no evidence of tidal tails. In fact, the galaxy remains undisturbed down to a radial distance of 80 arcsec. This radial distance triples previous spatial explorations of the stellar distribution of this galaxy. In addition, the distribution of its globular clusters (GCs) is not extended in relation to the bulk of the galaxy (the radius containing half of the GCs is 21 arcsec). We also found that the surface brightness radial profiles of this galaxy in the g and r bands decline exponentially from 35 to 80 arcsec. That, together with a constant ellipticity and position angle in the outer parts of the galaxy strongly suggests the presence of a low-inclination disk. This is consistent with the evidence of rotation found for this object. This finding implies that the dynamical mass of this galaxy is a factor of 2 higher than previously reported, bringing the dark matter content of this galaxy in line with galaxies of similar stellar mass.
The large and diffuse galaxies NGC1052-DF2 and NGC1052-DF4 have been found to have very low dark matter content and a population of luminous globular clusters. Accurate distance measurements are key to interpreting these observations. Recently, the distance to NGC1052-DF4 was found to be $20.0pm 1.6$ Mpc by identifying the tip of the red giant branch (TRGB) in 12 orbits of Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. Here we present 40 orbits of HST ACS data for NGC1052-DF2 and use these data to measure its TRGB. The TRGB is readily apparent in the color-magnitude diagram. Using a forward model that incorporates photometric uncertainties, we find a TRGB magnitude of $m_{rm F814W, TRGB} = 27.67 pm 0.10$ mag. The inferred distance is $D_{rm TRGB} = 22.1 pm 1.2$ Mpc, consistent with the previous surface brightness fluctuation distances to the bright elliptical galaxy NGC1052. The new HST distance rules out the idea that some of NGC1052-DF2s unusual properties can be explained if it were at $sim 13$ Mpc; instead, it implies that the galaxys globular clusters are even more luminous than had been derived using the previous distance of 20 Mpc. The distance from NGC1052-DF2 to NGC1052-DF4 is well-determined at $2.1pm 0.5$ Mpc, significantly larger than the virial diameter of NGC1052. We discuss the implications for formation scenarios of the galaxies and for the external field effect, which has been invoked to explain the intrinsic dynamics of these objects in the context of modified Newtonian dynamics.
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