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Register a new userThe geodesic motion of S2 and G2 as a test of the fermionic dark matter nature of our galactic core
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[Abridged] The S-stars motion around the Galactic center (Sgr A*) implies the existence of a compact source with a mass of about $4times 10^6 M_odot$, traditionally assumed to be a massive black hole (BH). Important for any model is the explanation of the multiyear, accurate astrometric data of S2 and the challenging G2: its post-pericenter velocity decelerates faster than expected from a Keplerian orbit around the putative BH. This has been reconciled in the literature by acting on G2 a drag force by an accretion flow. Alternatively, we show that the S2 and G2 motion is explained by the core-halo fermionic dark matter (DM) profile of the fully-relativistic Ruffini-Arguelles-Rueda (RAR) model. It has been already shown that for 48-345 keV fermions, it accurately fits the rotation curves of the Milky-Way halo. We here show that, for a fermion mass of 56 keV, it explains the time-dependent data of the position (orbit) and light-of-sight radial velocity (redshift function $z$) of S2 and G2, the latter without a drag force. We find the RAR model fits better the data: the mean of reduced chi-squares of the orbit and $z$ data are, for S2, $langlebar{chi}^2rangle_{rm S2, RAR}approx 3.1$ and $langlebar{chi}^2rangle_{rm S2, BH}approx 3.3$ while, for G2, $langlebar{chi}^2rangle_{rm G2, RAR}approx 20$ and $langlebar{chi}^2rangle_{rm G2, BH}approx 41$. For S2 the fits of the $z$ data are comparable, $bar{chi}^2_{z,rm RAR}approx 1.28$ and $bar{chi}^2_{z,rm BH}approx 1.04$, for G2 only the RAR model fits, $bar{chi}^2_{z,rm RAR}approx 1.0$ and $bar{chi}^2_{z,rm BH}approx 26$. In addition, the critical mass for the gravitational collapse of a degenerate 56 keV-fermion DM core into a BH is $sim 10^8 M_odot$, which may be the initial seed for the formation of the observed central supermassive BH in active galaxies, such as M87.
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Within the dark matter paradigm, explaining observed orbital dynamics at galactic level through the inclusion of a dominant dark halo, implies also the necessary appearance of dynamical friction effects. Satellite galaxies, globular clusters and even stars orbiting within these galactic halos, will perturb the equilibrium orbits of dark matter particles encountered, to produce a resulting trailing wake of slightly enhanced dark matter density associated with any perturber in the halo. The principal effect of this gravitational interaction between an orbiting body and the dark matter particles composing it, is the appearance of a frictional drag force slowly removing energy and angular momentum from the perturber. Whilst this effect might be relevant to help bring about the actual merger of the components of interacting forming galaxies, at smaller stellar scales, it becomes negligible. However, the trailing wake will still be present. In this letter I show that the corresponding dark matter wake associated to the Sun, will constitute a small but resonant perturbation on solar system dynamics which can be ruled out, as current laser and radio ranging measurements are now over an order of magnitude more precise than the amplitude of the orbital perturbations which said wake implies. The absence of any such detection implies the nonexistence of the dynamical friction trailing wake on the sun, which in turn strongly disfavours dark matter as an explanation for the observed gravitational anomalies at galactic scales.
The highly elliptical, 16-year-period orbit of the star S2 around the massive black hole candidate Sgr A* is a sensitive probe of the gravitational field in the Galactic centre. Near pericentre at 120 AU, ~1400 Schwarzschild radii, the star has an orbital speed of ~7650 km/s, such that the first-order effects of Special and General Relativity have now become detectable with current capabilities. Over the past 26 years, we have monitored the radial velocity and motion on the sky of S2, mainly with the SINFONI and NACO adaptive optics instruments on the ESO Very Large Telescope, and since 2016 and leading up to the pericentre approach in May 2018, with the four-telescope interferometric beam-combiner instrument GRAVITY. From data up to and including pericentre, we robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for S2 of z ~ 200 km/s / c with different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factor f, with f = 0 and f = 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemes f = 0.90 +/- 0.09 (stat) +- 0.15 (sys). The S2 data are inconsistent with pure Newtonian dynamics.
Searching for space-time variations of the constants of Nature is a promising way to search for new physics beyond General Relativity and the standard model motivated by unification theories and models of dark matter and dark energy. We propose a new way to search for a variation of the fine-structure constant using measurements of late-type evolved giant stars from the S-star cluster orbiting the supermassive black hole in our Galactic Center. A measurement of the difference between distinct absorption lines (with different sensitivity to the fine structure constant) from a star leads to a direct estimate of a variation of the fine structure constant between the stars location and Earth. Using spectroscopic measurements of 5 stars, we obtain a constraint on the relative variation of the fine structure constant below $10^{-5}$. This is the first time a varying constant of Nature is searched for around a black hole and in a high gravitational potential. This analysis shows new ways the monitoring of stars in the Galactic Center can be used to probe fundamental physics.
Precise measurements of the S-stars orbiting SgrA* have set strong constraints on the nature of the compact object at the centre of the Milky Way. The presence of a black hole in that region is well established, but its neighboring environment is still an open debate. In that respect, the existence of dark matter in that central region may be detectable due to its strong signatures on the orbits of stars: the main effect is a Newtonian precession which will affect the overall pericentre shift of S2, the latter being a target measurement of the GRAVITY instrument. The exact nature of this dark matter (e.g., stellar dark remnants or diffuse dark matter) is unknown. This article assumes it to be an scalar field of toroidal distribution, associated with ultra-light dark matter particles, surrounding the Kerr black hole. Such a field is a form of hair expected in the context of superradiance, a mechanism that extracts rotational energy from the black hole. Orbital signatures for the S2 star are computed and shown to be detectable by GRAVITY. The scalar field can be constrained because the variation of orbital elements depends both on the relative mass of the scalar field to the black hole and on the field mass coupling parameter.
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