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, distancedependent 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±3.6×10 7 M ) 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 M . At 13 Mpc, the luminosity and structural properties of the globular clusters around the object are the same as those found in other galaxies.

We present the first application of a spherical collapse model to a supercluster of galaxies. Positions and redshifts of ∼ 3000 galaxies in the Shapley Supercluster (SSC) are used to define velocity caustics that limit the gravitationally collapsing structure in its central part. This is found to extend at least to 8h −1 Mpc of the central cluster, A 3558, enclosing 11 ACO clusters. Infall velocities reach ∼ 2000 km s −1 . Dynamical models of the collapsing region are used to estimate its mass profile. An upper bound on the mass, based on a pure spherical infall model, gives M (< 8h −1 Mpc) < ∼ 1.3 × 10 16 h −1 M ⊙ for an Einstein-de Sitter (critical) Universe and M (< 8h −1 Mpc) < ∼ 8.5 × 10 15 h −1 M ⊙ for an empty Universe. The model of Diaferio & Geller (1997), based on estimating the escape velocity, gives a significantly lower value, M (< 8h −1 Mpc) ≈ 2.1 × 10 15 h −1 M ⊙ , very similar to the mass found around the Coma cluster by the same method (Geller et al. 1999), and comparable to or slightly lower than the dynamical mass in the virialized regions of clusters enclosed in the same region of the SSC. In both models, the overdensity in this region is substantial, but far from the value required to account for the peculiar motion of the Local Group with respect to the cosmic microwave background.

We present results of our wide-field redshift survey of galaxies in a 285 square degree region of the Shapley Supercluster (SSC), based on a set of 10 529 velocity measurements (including 1201 new ones) on 8632 galaxies obtained from various telescopes and from the literature. Our data reveal that the main plane of the SSC (v ≈ 14 500 km s −1 ) extends further than previous estimates, filling the whole extent of our survey region of 12 degrees by 30 degrees on the sky (30 × 75 h −1 Mpc). There is also a connecting structure associated with the slightly nearer Abell 3571 cluster complex (v ≈ 12 000 km s −1 ). These galaxies seem to link two previously identified sheets of galaxies and establish a connection with a third one at v = 15 000 km s −1 near RA = 13 h . They also tend to fill the gap of galaxies between the foreground Hydra-Centaurus region and the more distant SSC. In the velocity range of the Shapley Supercluster (9000 km s −1 < cz < 18 000 km s −1 ), we found redshift-space overdensities with b j < 17.5 of 5.4 over the 225 square degree central region and 3.8 in a 192 square degree region excluding rich clusters. Over the large region of our survey, we find that the intercluster galaxies make up 48 per cent of the observed galaxies in the SSC region and, accounting for the different completeness, may contribute nearly twice as much mass as the cluster galaxies. In this paper, we discuss the completeness of the velocity catalogue, the morphology of the supercluster, the global overdensity, and some properties of the individual galaxy clusters in the Supercluster.

A unique signature for the presence of massive black holes in very dense stellar regions is occasional giant-amplitude outbursts of multiwavelength radiation from tidal disruption and subsequent accretion of stars that make a close approach to the black holes 1 . Previous strong tidal disruption event (TDE) candidates were all associated with the centers of largely isolated galaxies 2-6 . Here we report the discovery of a luminous X-ray outburst from a massive star cluster at a projected distance of 12.5 kpc from the center of a large lenticular galaxy. The luminosity peaked at ∼10 43 erg s −1 and decayed systematically over 10 years, approximately following a trend that supports the identification of the event as a TDE. The X-ray spectra were all very soft, with emission confined to be 3.0 keV, and could be described with a standard thermal disk. The disk cooled significantly as the luminosity decreased, a key thermal-state signature often observed in accreting stellar-mass black holes. This thermal-state signature, coupled with very high luminosities, ultrasoft X-ray spectra and the characteristic powerlaw evolution of the light curve, provides strong evidence that the source contains an intermediate-mass black hole (IMBH) with a mass of a few ten thousand solar mass. This event demonstrates that one of the most effective means to detect IMBHs is through X-ray flares from TDEs in star clusters.We discovered the X-ray source 3XMM J215022.4−055108 (referred to as J2150−0551 hereafter) in our project of searching for TDEs from the XMM-Newton X-ray source catalog. The source was serendipitously detected in 2006-2009 in two XMM-Newton observa-tions and one Chandra observation of a field in the second Canadian Network for Observational Cosmology Field Galaxy Redshift Survey 7 . It was still detected, but with much lower X-ray fluxes, in our followup Swift observation in 2014 and Chandra observation in 2016. Figure 1 shows the Hubble Space Telescope (HST) ACS F775W image around the field of J2150−0551 taken in 2003. The source lies at an angular offset of 11.6 arcsec from the center of the barred lenticular galaxy 6dFGS gJ215022.2-055059 (referred to as Gal1 hereafter) and is spatially coincident with a faint optical object. Gal1 is at a redshift of z = 0.055 or a luminosity distance of DL = 247 Mpc (for H0 = 70 km s −1 Mpc −1 , ΩM = 0.3, ΩΛ = 0.7). The chance probability for J2150−0551 to be within 11.6 arcsec from the center of a bright galaxy like Gal1 is very small (0.01%, see SI), strongly supporting the association of J2150−0551 with Gal1.The fits to the X-ray spectra with a standard thermal thin disk are shown in the lower panels in Figure 2 (see also Table 1), and the inferred long-term evolution of the bolometric disk luminosity is plotted in the upper panel. The dependence of the bolometric disk luminosity on the apparent maximum disk temperature is plotted in Figure 3. The most striking feature is that the disk luminosity approximately scales with the temperature as L ∝ T 4 (i.e., a constant inner disk radi...

We report 2868 new multi-object spectroscopic measurements of galaxy redshifts in an area roughly 12 • × 6 • (right ascension × declination) centered on the Shapley Supercluster (SSC). These correspond to 2627 different galaxies. Including other measurements reported in the literature, the total number of galaxies with measured redshifts in a 19 • × 16 • area centered on the supercluster now reaches 5090. Of these, 2949 lie in the redshift range 9,000 -18,000 km s −1 , which we tentatively identify with the SSC. This unprecedented sample allows a quite detailed qualitative morphological study of the SSC. Based on the threedimensional distribution of galaxies and clusters of galaxies, we identify several sub-condensations of the supercluster, as well as walls and filaments connecting them. We also find another supercluster in the background, at redshift ∼ 23,000 km s −1 .