Ultra-diffuse Galaxy NGC 1052-DF2 Research Articles

DHOST gravity in ultra-diffuse galaxies – part I: the case of NGC1052-DF2

The Ultra-Diffuse galaxy NGC1052-DF2 has recently been revealed to be “extremely deficient” in dark matter, if not lacking it at all. This claim has raised many questions regarding the relationship between baryons and dark matter in Ultra-Diffuse galaxies. But there seems to be a quite unanimous belief that, if such very low dark matter content is confirmed and extended to other similar galactic objects, it might be a deathblow to theories which modify and extend General Relativity. Deficient dark matter galaxies thus represent a fertile ground to test both standard dark matter and modified gravity theories. In this work, we consider a specific Degenerate Higher-Order Scalar Tensor model to study the internal kinematics of NGC1052-DF2. Due to the partial breaking of the corresponding screening mechanism, this model can possibly have large cosmological scale effects influencing the dynamics of smaller structures like galaxies. We consider two scenarios: one in which the model only describes dark energy; and one in which it additionally entirely substitutes dark matter. We find that the best model to explain data is General Relativity with only stellar contribution. But while in the former scenario General Relativity is still statistically favoured, in the latter one the alternative model is as much successful and effective as General Relativity in matching observations. Thus, we can conclude that even objects like NGC1052-DF2 are not in contrast, and are not obstacles, to the study and the definition of a reliable alternative to General Relativity.

Globular clusters as tracers of the dark matter content of dwarfs in galaxy clusters

ABSTRACT Globular clusters (GCs) are often used to estimate the dark matter content of galaxies, especially dwarf galaxies, 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 catalogue of GCs tagged on to the Illustris simulation to study the accuracy of GC-based mass estimates. We focus on galaxies in the stellar mass range 108–1011.8 M⊙ identified in nine simulated Virgo-like clusters. Our results indicate that mass estimates are, on average, accurate in systems with GC numbers NGC ≥ 10 and where the uncertainty of individual GC line-of-sight velocities is smaller than the inferred velocity dispersion, σGC. In cases where NGC ≤ 10, however, biases may result, depending on how σGC 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 $M_{*} \sim 10^{8.5}\, \mathrm{M}_{\odot }$ – comparable with the ultra-diffuse galaxy NGC 1052-DF2 (DF2), notable for the low σGC of its 10 GCs – that have $\sigma _{\rm GC} \sim 7\!-\!15\, {\rm km \,s}^{-1}$. These DF2 analogues correspond to relatively massive systems at their infall time (M200 ∼ 1–3 × 1011 M⊙), which have retained only 3–17 GCs and have been stripped of more than 95 per cent of their dark matter. Our results suggest that extreme tidal mass loss in otherwise normal dwarf galaxies may be a possible formation channel for ultra-diffuse objects such as DF2.

On the Evolution of the Globular Cluster System in NGC 1052-DF2: Dynamical Friction, Globular–Globular Interactions, and Galactic Tides

Abstract The ultradiffuse 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 resimulating three of our earlier simulations of the GC system, where the GCs were modeled as single particles, with live GCs. Somewhat surprisingly, we infer a low merger rate of ∼0.03 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 ∼0.002 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.

Structural parameters for the globular-cluster-like objects in NGC 1052-DF2

ABSTRACT Recently, van Dokkum et al. have found an ultra-diffuse galaxy NGC 1052-DF2 with little or no dark matter, based on a spectroscopic study of its 11 constituent globular-cluster-like objects. In this paper, we analyse these 11 objects using Hubble Space Telescope imaging. We derive the structural parameters for each object by fitting the surface brightness profiles to three different models. Our results indicate that these objects are better fitted by the Wilson model than by the King and Sérsic models. Using the distance of D = 20 Mpc to NGC 1052-DF2 obtained by van Dokkum et al., these 11 objects have half-light radii in the range from ∼11 to ∼16 pc. These values are much larger than for normal globular clusters in the Milky Way, and are comparable to the sizes of an extended star cluster and of a typical ultra-compact dwarf. The half-light radii obtained here are larger than those obtained by van Dokkum et al. and Trujillo et al. The offset of the differences between the half-light radii of van Dokkum et al. and ours is 6.17 pc, and the offset of the differences between the half-light radii of Trujillo et al. and ours is 4.99 pc. Our half-light radii are, on average, 11.74 pc, which corresponds to 53 and 43 per cent larger than those obtained by van Dokkum et al. and Trujillo et al., respectively. The Rh versus MV diagram shows that these objects occupy the same areas of extended star clusters and ultra-compact dwarfs. Using the distance of D = 13 Mpc obtained by Trujillo et al., there are still five objects that do not lie in the area of normal globular clusters in the Milky Way. So, we suggest that these globular-cluster-like objects in NGC 1052-DF2 are not normal globular clusters like those in the Milky Way.

Refracting into ultra-diffuse galaxy NGC 1052-DF2 by passing near the centre of NGC 1052

ABSTRACT The recent observation of the ultra-diffuse galaxy NGC 1052-DF2 shows a galaxy may lack dark matter, which becomes a challenge to the standard structure formation theory. Here, we show that such phenomena can be a natural consequence if the NGC 1052-DF2 had experienced a single passage within a few kpc to the centre of the galaxy NGC 1052. The tidal effect of NGC 1052 in the encounter will heat the NGC 1052-DF2, and stretch the previous dwarf galaxy significantly into its current size. The relative lack of dark matter in the observed region is a natural consequence of the dark matter limited total amount in the corresponding small central region before the encounter, together with a systematic underestimation of the trace mass estimator method during revirialization after the encounter. The observed flat distribution of the ultra-diffuse galaxy can be reproduced with a cored initial star profile, which is a major improvement compared with the previous work. Our results show no need for introducing any new physical mechanism, as well as an alternative origin of an ultra-diffuse galaxy without repeated pericentre passage.

On the Orbital Decay of Globular Clusters in NGC 1052-DF2: Testing a Baryon-only Mass Model

Abstract The dark matter content of the ultra-diffuse galaxy NGC 1052-DF2, as inferred from globular cluster (GC) and stellar kinematics, carries a considerable amount of uncertainty, with current constraints also allowing for the complete absence of dark matter. We test the viability of such a scenario by examining whether in a “baryon-only” mass model the observed GC population experiences rapid orbital decay due to dynamical friction. Using a suite of 50 multi-GC N-body simulations that match observational constraints on both the stellar component of NGC 1052-DF2 and its GC population but differ in the initial line-of-sight positions and the tangential velocities of the GCs, we show that there is a substantial amount of realization-to-realization variance in the evolution of the GCs. Nevertheless, over ∼10 Gyr, some of the GCs experience significant orbital evolution. Others evolve less. A combination of reduced dynamical friction in the galaxy core and GC–GC scattering keeps the GCs afloat, preventing them from sinking all the way to the galaxy center. While the current phase-space coordinates of the GCs are not unlikely for a baryon-only mass model, the GC system does evolve over time. Therefore, if NGC 1052-DF2 has no dark matter, some of its GCs must have formed farther out, and the GC system must have been somewhat more extended in the past. The presence of a low-mass cuspy halo, while allowed by the kinematics, seems improbable, as significantly shorter inspiral timescales in the central region would quickly lead to the formation of a nuclear star cluster.

The ultra-diffuse galaxy NGC 1052-DF2 with MUSE

The so-called ultra-diffuse galaxy NGC 1052-DF2 was announced to be a galaxy lacking dark matter based on a spectroscopic study of its constituent globular clusters. Here we present the first spectroscopic analysis of the stellar body of this galaxy using the MUSE integral-field spectrograph at the (ESO) Very Large Telescope. The MUSE datacube simultaneously provides DF2’s stellar velocity field and systemic velocities for seven globular clusters (GCs). We further discovered three planetary nebulae (PNe) that are likely part of this galaxy. While five of the clusters had velocities measured in the literature, we were able to confirm the membership of two more candidates through precise radial velocity measurements, which increases the measured specific frequency of GCs in DF2. The mean velocity of the diffuse stellar body, 1792.9+1.4−1.8 km s−1, is consistent with the mean globular cluster velocity. We detect a weak but significant velocity gradient within the stellar body, with a kinematic axis close to the photometric major axis, making it a prolate-like rotator. We estimate a velocity dispersion from the clusters and PNe of σint = 10.6−2.3+3.9 km s−1. The velocity dispersion σDF2⋆(Re) for the stellar body within one effective radius is 10.8+3.2−4.0 km s−1. Considering various sources of systemic uncertainties, this central value varies between 5 and 13 km s−1, and we conservatively report a 95% confidence upper limit to the dispersion within one Re of 21 km s−1. We provide updated mass estimates based on these dispersions corresponding to the different distances to NGC 1052-DF2 that have been reported in the recent literature.

Still Missing Dark Matter: KCWI High-resolution Stellar Kinematics of NGC1052-DF2

Abstract The velocity dispersion of the ultra diffuse galaxy NGC1052-DF2 was found to be km s−1, much lower than expected from the stellar mass–halo mass relation and nearly identical to the expected value from the stellar mass alone. This result was based on the radial velocities of 10 luminous globular clusters that were assumed to be associated with the galaxy. A more precise measurement is possible from high-resolution spectroscopy of the diffuse stellar light. Here we present an integrated spectrum of the diffuse light of NGC1052-DF2 obtained with the Keck Cosmic Web Imager (KCWI), with an instrumental resolution of σ instr ≈ 12 km s−1. The systemic velocity of the galaxy is v sys = 1805 ± 1.1 km s−1, in very good agreement with the average velocity of the globular clusters ( km s−1). There is no evidence for rotation within the KCWI field of view. We find a stellar velocity dispersion of km s−1, consistent with the dispersion that was derived from the globular clusters. The implied dynamical mass within the half-light radius r 1/2 = 2.7 kpc is M dyn = (1.3 ± 0.8) × 108 M ⊙, similar to the stellar mass within that radius (M stars = (1.0 ± 0.2) × 108 M ⊙). With this confirmation of the low velocity dispersion of NGC1052-DF2, the most urgent question is whether this “missing dark matter problem” is unique to this galaxy or applies more widely.

A Second Galaxy Missing Dark Matter in the NGC 1052 Group

Abstract The ultra-diffuse galaxy NGC1052-DF2 has a very low velocity dispersion, indicating that it has little or no dark matter. Here we report the discovery of a second galaxy in this class, residing in the same group. NGC1052-DF4 closely resembles NGC1052-DF2 in terms of its size, surface brightness, and morphology; has a similar distance of ; and also has a population of luminous globular clusters extending out to from the center of the galaxy. Accurate radial velocities of the diffuse galaxy light and seven of the globular clusters were obtained with the Low Resolution Imaging Spectrograph on the Keck I telescope. The velocity of the diffuse light is identical to the median velocity of the clusters, , and close to the central velocity of the NGC 1052 group. The rms spread of the globular cluster velocities is very small at . Taking observational uncertainties into account we determine an intrinsic velocity dispersion of , consistent with the expected value from the stars alone ( ) and lower than expected from a standard NFW halo ( ). We conclude that NGC1052-DF2 is not an isolated case but that a class of such objects exists. The origin of these large, faint galaxies with an excess of luminous globular clusters and an apparent lack of dark matter is, at present, not understood.

NGC 1052-DF2 and modified gravity (MOG) without dark matter

ABSTRACT We model the velocity dispersion of the ultradiffuse galaxy NGC 1052-DF2 using Newtonian gravity and modified gravity (MOG). The velocity dispersion predicted by MOG is higher than the Newtonian gravity prediction, but it is fully consistent with the observed velocity dispersion that is obtained from the motion of 10 globular clusters (GCs).

Does the galaxy NGC1052-DF2 falsify Milgromian dynamics?

Galaxies normally have far more dark matter than normal matter, but the dynamics of objects within the ultra-diffuse galaxy NGC1052–DF2 suggest that it has a very little dark matter component. Most galaxies studied need a component of dark matter associated with them in order to explain their observed properties. Normally there is far more dark matter than normal matter, with an average of 30 times more for galaxies about the mass of the Milky Way. Dwarf galaxies are thought to have even higher ratios of dark to normal matter (more than 400 times as much). Pieter van Dokkum and colleagues report that the ultra-diffuse galaxy NGC1052–DF2 has a dynamical mass, determined by the motions of globular-cluster-like objects, that is essentially the same as the mass in stars, meaning that it does not have a dark matter component. Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio Mhalo/Mstars has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 1010 solar masses) and increases both towards lower masses and towards higher masses1,2. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies3,4. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy5 NGC1052–DF2, which has a stellar mass of approximately 2 × 108 solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4 × 108 solar masses. This implies that the ratio Mhalo/Mstars is of order unity (and consistent with zero), a factor of at least 400 lower than expected2. NGC1052–DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.

A Deficit of Dark Matter from Jeans Modeling of the Ultra-diffuse Galaxy NGC 1052-DF2

Abstract The discovery of the ultra-diffuse galaxy NGC 1052-DF2 and its peculiar population of star clusters has raised new questions about the connections between galaxies and dark matter (DM) halos at the extremes of galaxy formation. In light of debates over the measured velocity dispersion of its star clusters and the associated mass estimate, we constrain mass models of DF2 using its observed kinematics with a range of priors on the halo mass. Models in which the galaxy obeys a standard stellar-halo mass relation are in tension with the data and also require a large central density core. Better fits are obtained when the halo mass is left free, even after accounting for increased model complexity. The dynamical mass-to-light ratio for our model with a weak prior on the halo mass is , consistent with the stellar population estimate for DF2. We use tidal analysis to find that the low-mass models are consistent with the undisturbed isophotes of DF2. Finally, we compare with Local Group dwarf galaxies and demonstrate that DF2 is an outlier in both its spatial extent and its relative DM deficit.

The Maybe Stream: A Possible Cold Stellar Stream in the Ultra-diffuse Galaxy NGC1052-DF2

We report tentative evidence for a cold stellar stream in the ultra-diffuse galaxy NGC1052-DF2. If confirmed, this stream (which we refer to as Maybe Stream) would be the first cold stellar stream detected outside of the Local Group. The candidate stream is very narrow and has an unusual and highly curved shape.

A galaxy lacking dark matter.

Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio Mhalo/Mstars has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 1010 solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2 × 108 solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4 × 108 solar masses. This implies that the ratio Mhalo/Mstars is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.