A&A 627, L1 (2019) Astronomy https://doi.org/10.1051/0004-6361/201935840 & c M. Bílek et al. 2019 Astrophysics LETTER TO THE EDITOR Discussing the first velocity dispersion profile of an ultra-diffuse galaxy in MOND Michal Bílek, Oliver Müller, and Benoit Famaey Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg (ObAS), UMR 7550, 67000 Strasbourg, France e-mail: [email protected] Received 4 May 2019 / Accepted 3 June 2019 ABSTRACT Using Jeans modeling, we calculated the velocity dispersion profile of the ultra-diffuse galaxy (UDG) Dragonfly 44 in MOND. For the nominal mass-to-light ratio from the literature and an isotropic profile, the agreement with the data is excellent near the center of the galaxy. However, in modified gravity, close to the cluster core, the gravitational environment should bring the galaxy back toward Newtonian behavior. The success of the isolated MOND prediction for the central velocity dispersion could then mean that the galaxy is at a great distance (5 Mpc) from the cluster core, as hinted by the fact that nearby UDGs share similar velocities with a dispersion well below that of the cluster itself. There is, however, a 2σ tension in the outer part of the UDG due to an increase in the observed dispersion profile with respect to the flat MOND prediction. This deviation could simply be a measurement error. Other possibilities could be, for a UDG far from the cluster, a higher-than-nominal baryonic mass with a tangentially anisotropic dispersion profile or it could even be a dark baryonic halo. If the UDG is closer to the cluster core, the deviation could be a sign that it is in the process of disruption. Key words. galaxies: individual: Dragonfly 44 – galaxies: clusters: individual: Coma cluster – galaxies: kinematics and dynamics 1. Introduction van Dokkum et al. 2018b; Trujillo et al. 2019; Martin et al. 2018; Kroupa et al. 2018; Famaey et al. 2018; Emsellem et al. 2019; Ultra-diffuse galaxies (UDGs) are very extended (reff > 1:5 kpc) Fensch et al. 2019; Müller et al. 2019a,b; Nusser 2019; Laporte −2 low surface brightness (µV >25 mag arcsec ) objects. They have et al. 2019). been identified in different galactic environments for decades As low surface brightness objects, UDGs are expected to (e.g.,Sandage&Binggeli1984;Impeyetal.1988).Thesegalaxies show complex dynamical behavior in the context of MOdified have recently experienced a revival (Crnojevic´ et al. 2014; van Newtonian Dynamics (MOND, Milgrom 1983; here we only Dokkum et al. 2015a; Koda et al. 2015; van der Burg et al. consider MOND modified gravity theories and not modified 2016; Martínez-Delgado et al. 2016; Merritt et al. 2016; Román inertia theories). As a historical note, MOND was proposed to & Trujillo 2017a; Venhola et al. 2017; Wittmann et al. 2017; solve the missing mass problem in high surface brightness spi- Müller et al. 2018; Mancera Piña et al. 2019). In galaxy clusters, ral galaxies (e.g., Rubin et al. 1978; Bosma 1981; Rubin et al. UDGs do not contain gas, while in sparser environments they 1982) and has since excelled in reproducing the dynamics of can be gas dominated (van Dokkum et al. 2015a; Shi et al. 2017; a much wider range of galaxies (e.g., McGaugh & de Blok Leisman et al. 2017; Papastergis et al. 2017; Sardone et al. 2019), 1998; McGaugh & Milgrom 2013; Lelli et al. 2017, see also the following the well-known density–morphology relation (Dressler extensive of review in Famaey & McGaugh 2012). At the time 1980). Several formation scenarios have been proposed in the MOND was proposed, the existence of UDGs was unknown. ΛCDM context; they might be tidal dwarf galaxies, galaxies In MOND, a dynamical system appears Newtonian when the formed by collapse of gas in galaxy outflows, dwarf galaxies that gravitational acceleration is greater than about the threshold of −13 −2 experienced strong tidal stripping or repeated episodes of intense a0 = 1:2 × 10 km s (Milgrom 1983; Begeman et al. 1991). star formation, among others (e.g. van Dokkum et al. 2015a; Yozin Below this threshold, a departure from Newtonian dynamics &Bekki2015;Amorisco&Loeb2016;DiCintioetal.2017;Chan occurs such that the system appears to host dark matter. The et al. 2018; Toloba et al. 2018). Observations suggest that at least dark matter behavior is at its full strength only if the galaxy is some UDGs form in the field and are then accreted to galaxy clus- isolated. In the case when the object resides in a strong gravita- ters where they experience environmental quenching (Román & tional field of the environment, the deviations from Newtonian Trujillo 2017b; Ferré-Mateu et al. 2018; Alabi et al. 2018). dynamics can effectively be suppressed (e.g., Milgrom 1983; The dynamics of these systems provides exciting insights Bekenstein & Milgrom 1984), which is due to the non-linearity to the discussion. For instance, van Dokkum et al.(2018a) of any MOND theory (Milgrom 2014)1. In other words, the announced the discovery of a UDG free of dark matter in the object will appear dark matter-free, even though its internal field of NGC 1052. Soon, in the same field around NGC 1052 a acceleration is below a0, thanks to the so-called external field second galaxy lacking dark matter was found (van Dokkum et al. 2019a). These two galaxies, NGC 1052-DF2 and NGC 1052- 1 However, the external field effect can be weak, or even almost negli- DF4, have sparked a vast discussion in the literature (e.g., gible, in some modified inertia versions of MOND (Milgrom 2011). L1, page 1 of5 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A&A 627, L1 (2019) effect (EFE). To explain the dynamics of two UDGs of the NGC 1052 group in MOND, NGC 1052-DF2 and NGC 1052- 50 MOND isotropic DF4 (if they are not in the foreground of their putative host Newtonian isotropic group, see Trujillo et al. 2019), it is crucial to take the EFE into account, mostly removing the tension for NGC 1052-DF2 40 (Kroupa et al. 2018; Famaey et al. 2018), and lessening it for ] NGC 1052-DF4 (Müller et al. 2019a). Interestingly, the UDG s / 30 Dragonfly 44 in the Coma cluster behaves very differently from m k [ the UDGs of the NGC 1052 group. It exhibits a pronounced deviation from Newtonian dynamics. Early measurements of 20 its stellar velocity dispersion yielded ∼47 km s−1 (van Dokkum et al. 2016), well above the nominal isolated MOND prediction. However, all these previous studies relied on the global velocity 10 dispersion estimated at the half-light radius of the galaxy, but did not take into account the overall shape and profile of the velocity 0 dispersion, due to the difficulty in measuring it. 0 1 2 3 4 5 The first velocity dispersion profile of a UDG, namely Dragonfly 44, was recently presented by van Dokkum et al. R [kpc] (2019b). The data was taken with the Keck Cosmic Web Imager Fig. 1. Comparison of the measured velocity dispersion profile of the (KCWI) on Mauna Kea during 25.3 h of observations. The veloc- ultra-diffuse galaxy Dragonfly 44 to the MOND no-fitting model and ity dispersion profile was studied in the context of Newtonian the Newtonian model. A fixed mass-to-light ratio of 1.3 and the best 8 gravity with several types of dark matter profiles in van Dokkum estimate of the galaxy luminosity as 3:0×10 L were assumed, together et al.(2019b). Here we use these data to discuss Dragonfly 44 in with isotropy. We illustrate the effect of relaxing some of these assump- MOND. tions in Fig. 2. 2. Velocity dispersion profile expected in MOND in In the dark matter framework, it is important to realize that this central velocity dispersion could have been very different isolation from the MOND prediction, and there would actually be no obvi- We first study the velocity dispersion profile of Dragonfly 44 as ous reason for this agreement to be so good given the unclear expected by MOND, assuming that the galaxy is far away from origin of UDGs. Indeed, for comparison, the dotted line is an the Coma cluster center and any other galaxies, so that the EFE isotropic model employing Newtonian gravity without dark mat- is negligible. We calculated it using the spherically symmetric ter, which clearly underestimates the velocity dispersion by more Jeans equation (e.g., Binney & Tremaine 2008) than 3σ. On the other hand, while the deviation of the MOND mod- 1 d(ρσ2) β(r) els from observation increases with radius, the data points are r + 2 σ2 = a(r); (1) ρ dr r r still within the 2σ uncertainty limit so that the no-fitting model is consistent with the data. We discuss the implications of our where ρ is the density of tracers, σr the radial velocity disper- results and possible reasons (beyond the possible measurement sion, β the anisotropy parameter, and a the radial gravitational error) for the deviation in the outer part of the profile in the fol- acceleration. In our case, ρ is the star density of the galaxy lowing section. that we approximated by a Sérsic sphere whose parameters were obtained by fitting the photometric profile (van Dokkum et al. 2015a, 2016, 2019b), i.e., the I-band luminosity of LI = 3. Discussion ± × 8 3:0 0:6 10 L , the effective radius of Re = 4:7 kpc, and the 3.1.