A co-rotating gas and satellite structure around the interacting galaxy pair NGC 4490/85

ABSTRACT

The interacting binary system NGC 4490/85 = Arp 269 is intermediate in mass between the Milky Way/Large Magellanic Cloud and the Large/Small Magellanic Cloud binary systems. It is a system of 14 known galaxies. We estimate the total Newtonian gravitating mass of the NGC 4490/85 group to be |$M_\mathrm{ T} = (1.37\pm 0.43) \times 10^{12} \, \mathrm{M}_{\odot }$| using radial velocities and projected separations of its 13 candidate members. The system of dwarf satellites in the group demonstrates signs of coherent rotation in the same direction as that of the extended H i-shell surrounding the central interacting galaxy pair. The origin of this phase-space correlated population of star-forming late-type satellite galaxies raises questions in view of the planes-of-satellites observed around more massive galaxy pairs that are, however, made up of old early-type dwarf galaxies. We also report the detection of a candidate stellar Plume near the binary. This elongated structure of low surface brightness is a likely optical counterpart to the H i-tail north of NGC 4490/85, recently discovered by the Five-Hundred-meter Aperture Spherical radio telescope.

1 INTRODUCTION

The discovery of planes of satellites around nearby luminous galaxies has become one of the most important developments in 100 kpc-scale cosmology. A significant portion of Milky Way’s (MW’s) dwarf satellites are concentrated in a thin disc of satellites (DoS), the members of which show signs of co-rotation (Lynden-Bell 1976; Kroupa, Theis & Boily 2005; Metz, Kroupa & Libeskind 2008; Pawlowski & Kroupa 2020). A similar planes-of-satellites structure has been found around the nearest spiral galaxy M31 (Koch & Grebel 2006; Metz, Kroupa & Jerjen 2007; Ibata et al. 2013). The rotating planar satellite structures of the MW and of M31 appear to be correlated with each other (Pawlowski, Kroupa & Jerjen 2013). This has been interpreted to be evidence for an encounter between the two galaxies about 8–10 Gyr ago (Bílek et al. 2018; Banik et al. 2022). Moreover, the evidence for planar structures composed of dwarf satellites has been noted to also exist around neighbouring galaxies: Centaurus-A (Tully et al. 2015; Müller et al. 2021), M81 (Chiboucas et al. 2013), and NGC 253 (Martínez-Delgado et al. 2021). All these 5 galaxies have a high K-band luminosity in the range of (0.5–1.0) × 10¹¹ L_K⊙ and are located in a sphere with a radius of 4 Mpc around us. In fact, every single galaxy brighter than L_K = 5 × 10¹⁰ L_K⊙ in the nearest volume has been found to have a flattened satellite–galaxy system. The MW, M31, and NGC 253 systems have a ratio of thickness to diameter of about 0.1, being seen nearly edge-on, while the Cen A and M81 systems are observed at an inclination. In addition to the satellite-systems within a few Mpc distance of us, a survey of regions spanning 150 kpc around galaxies within a redshift z < 0.05 shows host galaxies to prefer, with 99.994 per cent confidence, to have phase-space correlated satellite systems (Ibata et al. 2014, 2015). Additional flattened and disk-like satellite systems have been reported (Heesters et al. 2021; Crosby et al. 2023).

Here we present evidence for the presence of co-moving satellites around the spiral galaxy NGC 4490 with a K-band luminosity of 1.9 × 10¹⁰ L_K⊙.

The paper is organized as follows: Section 2 presents the basic properties of the 14 dwarf galaxies forming a group together with NGC 4490. Section 3 presents some peculiar features of the group. Section 4 reports the discovery of a new candidate stellar structure in the vicinity of the NGC 4490/85, which we call the ‘Plume’. Section 5 contains a discussion and the conclusions.

2 PROPERTIES OF THE NGC 4490/85 GALAXY GROUP

The spiral galaxy NGC 4490 together with its neighbour, NGC 4485, form the known pair Holm 414 = Arp 269 = VV 30 = KPG 341. The galaxies NGC 4490 and NGC 4485 have, respectively, radial velocities of 623 km s⁻¹ and 517 km s⁻¹ in the Local Group rest frame. According to the Extragalactic Distance Data base (=EDD; Anand et al. 2021), the pair is located at a distance of 8.91 Mpc, which we use in our calculations. Thus |$1^{\prime }=2.59\,$| kpc at that distance. NGC 4490 and its in-contact-neighbour, NGC 4485, have K-band luminosities of, respectively, log₁₀(L_K/L_K⊙) = 10.28 and 8.99 (Table 1 below). For comparison, the corresponding log-luminosities of M31, the MW, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are, respectively, 10.73, 10.78, 9.42, and 8.85 (Karachentsev, Makarov & Kaisina 2013a).¹ Thus, given their luminosities, the NGC 4490/85 pair is intermediate between the MW/LMC and the LMC/SMC interacting binaries. The projected on-sky distance between NGC 4490 and NGC 4485 is 8.8 kpc.

The group of galaxies around NGC 4490 has in total currently-known 15 proposed members as listed in Table 1. The columns of Table 1 contain: (1) – the name of the galaxy; (2) – equatorial coordinates in degrees; (3) – super-galactic coordinates; (4) – morphological type in de Vaucouleurs digital scale; (5) – distance to the galaxy; (6) – the method by which the distance is estimated: via the tip of the red giant branch (trgb), by the Tully–Fisher relation (TF), by radial velocity of the galaxy in the Numerical Action Method (NAM; Shaya et al. 2017), by probable membership in the group (mem); (7) – radial velocity of the galaxy relative to the Local Group centroid; (8) – luminosity of the galaxy in the K-band; (9) – projected separation of the galaxy from NGC 4490; (10) – radial velocity of the galaxy relative to NGC 4490; (11) – orbital estimate of the total Newtonian gravitating mass of the group via each satellite,

where G is the gravitational constant. The estimate of M_T assumes an isotropic distribution of orbits of satellites with an average eccentricity 〈e²〉 = 0.5 (Karachentsev & Kashibadze 2021). The values given in Table 1 are taken from the latest version of the Updated Nearby Galaxy Catalogue (=UNGC; Karachentsev, Makarov & Kaisina 2013a) available at http://www.sao.ru/lv/lvgdb. Links to individual distance estimates are provided in the last column of the table and notes to it.

It should be noted here that for the dominant member of the group, NGC 4490, the EDD data base (Anand et al. 2021) gives a trgb-distance of 7.07 Mpc (rather than 8.91 Mpc which is the distance of NGC 4485 adopted here). In this case, the components of the interacting pair NGC 4490/85 would be separated by a 3D distance of 1.84 Mpc, which would question them being a physical pair and would disfavour a relatively recent intense interaction of the galaxies. Analysis of the colour–magnitude diagram presented in EDD shows that the position of the trgb on it is determined unreliably. Therefore, we adopted a single value of 8.91 Mpc for both components of the pair.

Relative to NGC 4490, the 12 companions (Table 1) with measured radial velocities have an average velocity of 〈ΔV〉 = −27 ± 26 km s⁻¹. The average distance of 5 companions with accurately measured trgb-distances is 8.80 ± 0.27 Mpc, in agreement with the adopted distance of 8.91 Mpc for the central galaxy pair. This can be regarded as evidence that the galaxies in Table 1 (except Dw 1224+39) are real members of a single group.

Elmegreen et al. (1998) took deep images of the NGC4490/85 pair with the 0.6-m Burrell telescope and noted several tidal features around the object and star-forming regions. Huchtmeier, Seiradakis & Materne (1980) and Clemens, Alexander & Green (1998) investigated this interacting pair in the H i 21-cm line using the 100-m Effelsberg radio telescope and the Very Large Array (VLA). They found that the pair is immersed in a common prolate H i-shell, the size of which reaches 60 kpc. The last authors suggested that the distribution of neutral hydrogen can best be explained by a galactic-scale bipolar outflow of H i driven by supernovae in NGC 4490. The recent deeper H i observations with the Five-Hundred-meter Aperture Spherical radio Telescope (FAST) demonstrate that the H i-envelope has an extension of more than 100 kpc (Liu et al. 2023). At the northern end of the H i-shell, the authors found an H i-tail that stretches in the direction of the nearby dwarf galaxy KK 149.

The distribution of the 15 candidate members of the NGC 4490/85 group is presented in Fig. 1 with circles, the sizes of which correspond to the luminosity of the galaxy with colour indicating the galaxy radial velocity according to the scale in the left upper corner. The numbers near the galaxies correspond to radial velocity differences in km s⁻¹ relative to the central galaxy NGC 4490. The new dwarf system, Plume (see Section 4), is shown by the asterisk. Two dwarf galaxies without measured velocities are marked by grey circles. The small ellipse in the centre of Fig. 1 reproduces the size and orientation of the H i-shell around the interacting pair NGC 4490/85 according to the data by Liu et al. (2023). The large ellipse encloses the main body of the group.

The group of galaxies around NGC 4490 has the following parameters: an average projected separation of satellites 〈R_p〉 = 182 kpc, a mean-square radial velocity of satellites σ_v = 85 km s⁻¹, the average estimate of the total Newtonian gravitating mass 〈M_T〉 = (1.37 ± 0.43) × 10¹² M_⊙. According to Tully (2015), the total (virial) Newtonian mass of a group is related to the virial radius as

This gives for the NGC 4490 group the value R_v = 239 kpc, marked with a green circle in Fig. 1. With a total luminosity of the group members |$\Sigma L_\mathrm{ K} = 2.20 \times 10^{10}\, L_{{\rm K}\odot }$|⁠, the total Newtonian mass-to-luminosity ratio of the group is |$\langle M_\mathrm{ T}\rangle /\Sigma L_\mathrm{ K} = (62\pm 20) \, \mathrm{M}_{\odot }/L_{{\rm K}\odot }$|⁠. This value is about half the global mass-to-luminosity ratio: With the mean cosmic density of matter in the standard Λcold dark matter (ΛCDM) cosmological model being |$4.38 \times 10^{10} \, \mathrm{M}_{\odot }/$|Mpc³ with Ω_m = 0.3, and the observed mean density of K-band luminosity being (4.3 ± 0.2) × 10⁸ L_K⊙/Mpc³ (table 4 in Driver et al. 2012), the global ratio is equal to |$(102\pm 5)\, \mathrm{M}_{\odot }/L_{{\rm K}\odot }$|⁠. Note that with |$M_{\rm T} \approx 1.4 \times 10^{12}\,M_\odot$| within |$R \approx 240\,{\rm kpc}$|⁠, the dynamical acceleration is |$g \approx 0.03\,a_0$|⁠, where |$a_0 \approx 3.9\,{\rm pc/Myr}^2$| is Milgrom's critical acceleration. The Milgromian gravitating mass thus comes out to be |$M_{\rm M} = (0.03\,R)^2 \, a_0 / G \approx 4 \times 10^{10}\,M_\odot$|⁠, i.e., |$M_{\rm M}=(g/a_0)\,M_{\rm T} \approx 0.03\,M_{\rm T}$|⁠, such that the K-band mass to light ratio of the system is about |$2\,M_\odot/L_{K\odot}$| in Milgromian dynamics (Milgrom 1983).

Away from the main body of the NGC 4490 group there is a dwarf galaxy Dw 1224+39. It is located 15′ North of the spiral galaxy NGC 4369 = Mrk 439 that has a radial velocity of V_LG = 1038 km s⁻¹ and a TF-distance of 29.7 Mpc, and is possibly a satellite of NGC 4369.

The group in question is located near the Local Supercluster equator, where the number density of galaxies at the line of sight is rather high. Nevertheless, the NGC 4490 group appears to be quite isolated. There are no other galaxies with radial velocities in the range (450–750) km s⁻¹ in the indicated area of size, 7° × 4°. According to Karachentsev, Nasonova & Courtois (2013b), the spatially closest neighbour to the NGC 4490 group is the group of 18 galaxies around NGC 4258 (12^h19^m + 47°18′) at a trgb-distance of 7.66 Mpc from the observer. The spatial separation between the group centres is 1.56 Mpc.

3 THE SYSTEM OF SATELLITES AROUND NGC 4490

The group of galaxies around the interacting pair NGC 4490/85 is characterized by the following features.

• The group can be classified as a ‘fossil’ group because the luminosity of each satellite does not exceed 1/20th of the luminosity of the principal galaxy. This difference in luminosities will intensify if the components of the central pair were to merge. This is expected to occur on a time scale of ≈5 × 10⁸ yr, provided both are immersed in dark matter haloes (but see Kroupa 2015; Roshan et al. 2021; Haslbauer et al. 2022; Kroupa et al. 2023).

• Almost the entire population of the group consists of late-type galaxies with ongoing star formation. Only one dwarf galaxy, Dw 1229+41, near the interacting pair, is classified by us as a quiescent dSph object. Within the Fig. 1 area, we did not find other dSph galaxies with a luminosity brighter than |$L_\mathrm{ K} = 3\times 10^6\, L_{{\rm K}\odot }$|⁠.

• The satellite system is stretched along the super-galactic plane. In the equatorial coordinates, the positional angle of its major axis (the large ellipse) is PA = 158° ± 3°. The positional angle of the H i-shell around the central interacting pair (small ellipse in Fig. 1) is practically oriented in the same direction, PA = 159° ± 2°. It is noteworthy that the major axis of the Plume (Section 4) has also a similar direction, PA = 155° ± 5°.

• The satellite system reveals a distinct asymmetry in the distribution of radial velocities relative to the main galaxy NGC 4490. All the objects on the northern side of the group (bluish circles on the right side of Fig. 1) have negative relative velocities, whereas all the objects on the opposite side (reddish circles) have positive ones. This suggests the system of satellites to be arranged in an inclined similar rotationally supported DoS structure as observed around the MW, M31, and Cen A.

• Moreover, as is seen on VLA- and FAST-maps, the H i-shell around NGC 4490/85 is also characterized by an increase in radial velocity from the northern side to the South, conforming with the satellite system.

The listed geometric and kinematic coincidences do not look accidental. The observational data show the gas component of the central part of the group and the system of satellites as a whole to have mutual coherent motions.

4 THE PLUME: THE DISCOVERY OF A NEW STELLAR STRUCTURE NEAR NGC 4490

About a dozen dwarf galaxies with radial velocities close to the velocity of the centroid of the NGC 4490/85 pair were known in its vicinity. Based on data from a recent Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys of the sky (Dey et al. 2019), Karachentsev & Kaisina (2022) performed a search for new dwarf satellites of NGC 4490/85 and found three objects of low surface brightness that could be companions of the pair. The recent publication (Liu et al. 2023) gave us a reason to repeat the search for faint dwarfs around NGC 4490 using the DESI Legacy Imaging Survey data. As a result, we discovered an object of extremely low surface brightness, which is located in the region of the northern H i-tail indicated by Liu et al. (2023) on their figures. The object has dimensions of ≈4′ × 1′ and is elongated in the direction of NGC 4485 and KK 149. It will be important to quantify this object using other deep imaging surveys (such as the Canada-France Imagine Survey in the r-band, CFIS-r, at the Canada-France-Hawaii Telescope, CFHT; Ibata et al. 2017). It is not an instrumental artefact, but could in principle be MW cirrus. However, its location in the direction of a H i-tidal tail makes it a possible candidate new stellar structure, a stellar plume (hereinafter referred to as the Plume), associated with the NGC 4490/85 system. Judging by its structure, the new object may have been a past dwarf galaxy, destroyed in the pair’s gravitational field and surrounded by tidal material, being similar to the time-dependent models of such systems (Kroupa 1997; Casas et al. 2012).

The DESI Legacy Imaging Survey image of the object Plume can be found in Fig. 2. The figure shows the interacting binary galaxies NGC 4490 and NGC 4485 together with their dwarf companions KK 149 and MAPS1231+42 taken from the DESI Legacy Imaging Survey (Dey et al. 2019). The contour of the diffuse tail of H i recently mapped by the FAST radio telescope (Liu et al. 2023) is indicated. This apparently tidal system is barely seen in the Sloan Digital Sky Survey and very barely visible in the UV-survey GALEX. It is extremely difficult to determine the integral apparent magnitude of the Plume. We estimated it by the average brightness contrast of the Plume above the night sky background: With a contrast of about 1 per cent and a night sky brightness of 22.5 mag/sq.arcsec, and with the dimensions of 4.2′ × 1.2′, the integral magnitude of the Plume is |$B \approx 16{_{.}^{\rm m}}9$|⁠, the Plume's central surface brightness in the B-band being |${\approx} 27{_{.}^{\rm m}}5\, /$|space″. The basic parameters of the Plume are presented in Table 2. The magnitude of the H i-flux and the radial velocity of the Plume are estimated by us using the VLA- and FAST-maps presented by Clemens, Alexander & Green (1998) and Liu et al. (2023). The H i-flux is very approximate due to unknown projection factors. Assuming a distance to the Plume of 8.91 Mpc, we roughly estimate its hydrogen and stellar masses as 2.0 × 10⁷ M_⊙ and 8.0 × 10⁷ M_⊙, respectively. The latter can be larger by an order of magnitude for a young system if the galaxy-wide IMF varies systematically as indicated by other observations (Jerabkova et al. 2018).

The observational parameters of the Plume, given in Table 2, are poorly determined due to its very low surface brightness. Dedicated deep follow-up observations in different (u, g, r, i) filters will be needed to establish Plume’s spectral energy distribution and to clarify whether it is young, as expected for a stellar population recently born from gaseous debris. The physical dimension of the Plume is |${\approx} 10\, {\rm kpc} \times 2.5\,$| kpc if it is at the distance of the NGC 4490/85 binary (Table 2). If this were to be the physical scale of this object, then it would be larger by a factor of ten than known tidal-dwarf galaxies of similar mass (c.f. fig. 2 in Dabringhausen & Kroupa 2013; Haslbauer et al. ). It is thus likely unbound, consisting of an evolved version of the type of star-forming regions and stellar populations observed to be in tidal debris extended over dozens of kpc in interacting galaxies. Examples of such tidal structures, plumes, and streams are the putative tidal dwarf KK 208 (PC166170) near NGC 5236 with a Holmberg diameter of 5.9 arcmin (8.8 kpc) (Karachentsev, Makarov & Kaisina 2013a), the eastern tidal structure of NGC 5907 with a diameter of 12 arcmin (Lanzetta et al. 2023) corresponding to a linear dimension of 53 kpc at a distance of 14.3 Mpc, the western stream near NGC 5907 being fainter and about twice as extended. Other examples of such structures are the dog-leg tidal stream of NGC 1097 (Galianni et al. 2010), tadpole galaxies (Elmegreen et al. 2012), the Leo Triplet (Wu et al. 2022), and other interacting galaxies (Rodruck et al. 2023) and the umbrella-like stellar stream of NGC 922 (Martínez-Delgado et al. 2023a). Such tidally produced structures of low-surface brightness constrain the galaxy encounters that produced them which test cosmological structure formation theory (e.g. Kroupa 2015) and systematic surveys for them such as through the Stellar Stream Legacy Survey are underway (Martínez-Delgado et al. 2023b). On the other hand, with a central surface brightness, |${\approx} 27.5\,$| mag/square arcsec and an effective linear diameter of |${\approx} 10\,$| kpc, the Plume might also classify as being an ultra-diffuse galaxy (UDG; van Dokkum et al. 2015), probably having been assembled during a past encounter between NGC 4490 and NGC 4485. We emphasize that the formation of UDGs remains not understood.

The H i peaks at an RA of about 12:30:00 (Liu et al. 2023), while the optical object is at 12:30:13, which is about 3′ away. In most interacting systems able to produce tidal dwarf galaxies (TDGs), the very young TDG coincides with the peak of the H i. More generally, with rare exceptions, the optical and H i tidal features overlap in space in such freshly formed dwarf satellite galaxies. This is clearly not the case here, further pointing to the Plume being a stellar stream. In this interpretation, the question arises why the southern H i tail (to the lower left but of Fig. 2) which seems to have a similar column density as the northern one does not have an optical counterpart.

5 DISCUSSION AND CONCLUSION

The discovery of a hitherto not know stellar structure, the Plume, in the NGC 4490/85 group of galaxies is reported (Section 4). The Plume might be part of a stellar stream or a UDG possibly formed during a past encounter between NGC4490 and NGC 4485 (Section 4). The Plume needs dedicated follow-up observations to improve understanding of its nature.

This work also reports the discovery that the NGC 4490/85 group has a rotating and thus highly phase-space correlated population of satellite galaxies. The kinematic pattern observed in the NGC 4490 group indicates a joint co-rotation of the system of satellites and the gas envelope surrounding the central interacting pair NGC 4490/85. Assuming regular rotation, the satellite system will have a significant angular momentum J_sat = Σ M_*R_p|ΔV|. We estimated J_sat, comparing it with the angular rotational momentum of the principal galaxy, J_N4490 = M_*V_mR₂₆. Here, the amplitude of the galaxy rotation, V_m = 86 km s⁻¹, and the galaxy optical radius, R₂₆ = 8.1 kpc, are taken from UNGC (Karachentsev, Makarov & Kaisina 2013a). Assuming the same stellar mass-to-luminosity ratio for the central galaxy and for its satellites, we obtained the value J_sat/J_N4490 ≈ 2, which is approximate due to uncertain projection factors in the motion of the satellites. Thus, the total angular momentum of the satellites turns out to be comparable with the momentum of rotation of the dominant galaxy in the group.

The satellite galaxies not only appear to lie in a rotating plane, but also seem to be concentrated in a bar-like configuration, which is slightly offset from the line b_SG = 6^o.0 (see Fig. 1). Moreover, from the data in Table 1 the mass of the satellite system may be concentrated on the east (left) side of this configuration. If this is true, then it seems likely that the satellite system would exert a tidal (and m = 1) force on the outer H i distribution (which also seems aligned along about the same line). Thus, another source of tidal H i distortion could be the outer satellite system (see Criswell & Struck 2019 for a discussion of such effects).

The dwarf satellites may be in primary infall towards the massive central galaxy from both its sides along a putative diffuse cosmic filament. Such an assumption implies the possible presence of more distant companions at distances ≈(2–4)R_v, which, in principle, is accessible to observational verification. This explanation however raises the question where all the primordial dwarf satellite galaxies are that ought to be evident in the dark matter haloes of the NGC 4490/85 galaxy pair according to the ΛCDM model.

On the other hand, the system of phase-space correlated satellite galaxies around the NGC 4490/85 pair plus the co-rotating gas envelope is reminiscent of the similar arrangement of MW satellite galaxies and young halo globular star clusters in a vast polar structure that co-aligns and co-orbits with the gaseous Magellanic Stream (Pawlowski, Kroupa & Jerjen 2013; Pawlowski & Kroupa 2020). The similarity of the angular momenta of the NGC 4490/85 satellites and the dominant galaxy pair may be due to the former forming as a population of tidal dwarf galaxies from the latter. Such a process of the formation of new dwarf galaxies in gaseous tidal tails produced by an encounter of two late-type galaxies has been documented through simulations (Wetzstein, Naab & Burkert 2007). It has also been shown that the satellite distributions around the MW and of M31, on being correlated (Pawlowski, Kroupa & Jerjen 2013), suggest their formation through a past encounter between the MW and M31 as shown by simulations (Bílek et al. 2018; Banik et al. 2022). The NGC 4490/85 satellite system differs from the other known phase-space-correlated satellite systems that are made up of old early-type dwarf-spheroidal galaxies, by being composed of late-type star-forming dwarfs. The satellites of NGC 4490/85 would therefore be a freshly formed system of tidal dwarf galaxies. Theoretical work via computer simulations is needed to study if the orbit of NGC4485 around NGC4490 can have expelled tidal tails that are sufficiently massive to form the observed population of satellite galaxies sufficiently recently to allow them to be still star-forming. The observational finding that even the relatively low-mass interacting binary system NGC 4490/85 can have an associated phase-space correlated satellite systems composed of star-forming dwarf galaxies thus poses an interesting problem to understand theoretically. It might be related to the distribution of (the non-satellite) star-forming dwarf galaxies in a highly symmetrical structure around the MW/M31 pair in the Local Group (Pawlowski, Kroupa & Jerjen 2013), a structure that has not yet found a theoretical understanding.

While the individual plane-of-satellite system or DoS around the MW is being understood to not be a challenge by it being ‘common (at the 1/231 level or at the 1 per 50 Mpc cubed level)’ (Xu, Kang & Libeskind 2023, see also Sawala et al. 2023), according to Asencio et al. (2022) the MW, M31, and Cen A phase-space correlated satellite systems are, combined, in |$5.27\, \sigma$| tension with the ΛCDM model. This is confirmed by Kanehisa, Pawlowski & Mueller (2023) who show mergers cannot produce correlated systems of pre-merger satellite galaxies. Phase-space-correlated satellite systems arise from tidal-dwarf galaxies that form in galaxy-galaxy encounters though, with Haslbauer et al. (2019) therewith confirming the dual dwarf galaxy theorem for the ΛCDM structure formation model. The real tension would be higher if the mutual correlation of the MW and M31 systems were taken into account. The additional two systems (M81, NGC 253) together with the present discovery of another highly phase-space correlated satellite plus circum-galactic gas system (NGC 4490/85), imply the planes of satellite problem to be an until now not solved failure of the current cosmological structure formation theory (Kroupa, Theis & Boily 2005; Pawlowski 2018, 2021a, b; Kroupa et al. 2023).

ACKNOWLEDGEMENTS

We thank the referee and Curtis Struck, Mordehai Milgrom, Marcel Pawlowski and Oliver Mueller for constructive and helpful comments. Igor D. Karachentsev is grateful to Ming Zhu, Serafim Kaisin, and Dmitry Makarov for useful discussions. This work has made use of Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Survey data, and the revised version of the Local Volume galaxy data base. The Local Volume galaxies data base has been updated within the framework of grant 075–15–2022–262 (13.MNPMU.21.0003) of the Ministry of Science and Higher Education of the Russian Federation.

DATA AVAILABILITY

The data on which this work is based is publicly available and is detailed in the corresponding section of this manuscript.

Footnotes

References