The alignment of SDSS satellites with the VPOS: effects of the survey footprint shape

It is sometimes argued that the uneven sky coverage of the Sloan Digital Sky Survey (SDSS) biases the distribution of satellite galaxies discovered by it to align with the polar plane defined by the 11 brighter, classical Milky Way (MW) satellites. This might prevent the SDSS satellites from adding significance to the MW's Vast Polar Structure (VPOS). We investigate whether this argument is valid by comparing the observed situation with model satellite distributions confined to the exact SDSS footprint area. We find that the SDSS satellites indeed add to the significance of the VPOS and that the survey footprint rather biases away from a close alignment between the plane fitted to the SDSS satellites and the plane fitted to the 11 classical satellites. Finding the observed satellite phase-space alignments of both the classical and SDSS satellites is a ~5{\sigma} event with respect to an isotropic distribution. This constitutes a robust discovery of the VPOS and makes it more significant than the Great Plane of Andromeda (GPoA). Motivated by the GPoA, which consists of only about half of M31's satellites, we also estimate which fraction of the MW satellites is consistent with being part of an isotropic distribution. Depending on the underlying satellite plane width, only 2 to 6 out of the 27 considered MW satellites are expected to be drawn from isotropy, and an isotropic component of >50% of the MW satellite population is excluded at 95% confidence.Comment: 12 pages, 6 figures, 1 table. Accepted for publication in MNRA

The Planes of Satellite Galaxies Problem, Suggested Solutions, and Open Questions

Satellite galaxies of the Milky Way and of the Andromeda galaxy have been found to preferentially align in significantly flattened planes of satellite galaxies, and available velocity measurements are indicative of a preference of satellites in those structures to co-orbit. There is increasing evidence that such kinematically correlated satellite planes are also present around more distant hosts. Detailed comparisons show that similarly anisotropic phase-space distributions of sub-halos are exceedingly rare in cosmological simulations based on the $\Lambda$CDM paradigm. Analogs to the observed systems have frequencies of $\leq 0.5$ per cent in such simulations. In contrast to other small-scale problems, the satellite planes issue is not strongly affected by baryonic processes because the distribution of sub-halos on scales of hundreds of kpc is dominated by gravitational effects. This makes the satellite planes one of the most serious small-scale problem for $\Lambda$CDM. This review summarizes the observational evidence for planes of satellite galaxies in the Local Group and beyond, and provides an overview of how they compare to cosmological simulations. It also discusses scenarios which aim at explaining the coherence of satellite positions and orbits, and why they all are currently unable to satisfactorily resolve the issue.Comment: Invited review for MPLA, accepted for publication. 26 pages, 3 figure

The rotationally stabilized VPOS and predicted proper motions of the Milky Way satellite galaxies

The satellite galaxies of the Milky Way (MW) define a vast polar structure (VPOS), a thin plane perpendicular to the MW disc. Proper motion (PM) measurements are now available for all of the 11 brightest, `classical' satellites and allow an updated analysis of the alignment of their orbital poles with this spatial structure. The coherent orbital alignment of 7 to 9 out of 11 satellites demonstrates that the VPOS is a rotationally stabilized structure and not only a pressure-supported, flattened ellipsoid. This allows us to empirically and model independently predict the PMs of almost all satellite galaxies by assuming that the MW satellite galaxies orbit within the VPOS. As a test of our method, the predictions are best met by satellites whose PMs are already well constrained, as expected because more uncertain measurements tend to deviate more from the true values. Improved and new PM measurements will further test these predictions. A strong alignment of the satellite galaxy orbital poles is not expected in dark matter based simulations of galaxy formation. Coherent orbital directions of satellite galaxies are, however, a natural consequence of tidal dwarf galaxies formed together in the debris of a galaxy collision. The orbital poles of the MW satellite galaxies therefore lend further support to tidal scenarios for the origin of the VPOS and are a very significant challenge for the standard LCDM model of cosmology. We also note that the dependence of the MW satellite speeds on Galactocentric distance appear to map an effective potential with a constant velocity of approximately 240 km/s to about 250 kpc. The individual satellite velocities are only mildly radial.Comment: 17 pages, 8 figures, 4 tables, accepted for publication in MNRA

Future measurements of the Lense-Thirring effect in the Double Pulsar

The Double Pulsar system PSR J0737-3039A/B has proven to be an excellent laboratory for high precision tests of general relativity. With additional years of timing measurements and new telescopes like the Square Kilometre Array (SKA), the precision of these tests will increase and new effects like the Lense-Thirring precession of the orbit will become measurable. Here, we discuss the prospects of measuring the Lense-Thirring effect and thereby constraining the equations of state at supra-nuclear densities in neutron stars using the Double Pulsar.Comment: 6 pages, 3 figures; Contribution to the proceedings of "The Fourteenth Marcel Grossmann Meeting", University of Rome "La Sapienza", Rome, July 12-18, 201