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The motion of S2, one of the stars closest to the Galactic Centre, has been measured accurately and used to study the compact object at the centre of the Milky Way. It is commonly accepted that this object is a supermassive black hole, but the nature of its environment is open to discussion. Here, we investigate the possibility that dark matter in the form of an ultralight scalar field 'cloud' clusters around Sgr A*. We use the available data for S2 to perform a Markov Chain Monte Carlo analysis and find the best-fit estimates for a scalar cloud structure. Our results show no substantial evidence for such structures. When the cloud size is on the order of the size of the orbit of S2, we are able to constrain its mass to be smaller than 0.1 % of the central mass, setting a strong bound on the presence of new fields in the galactic centre. [ABSTRACT FROM AUTHOR]

Using general relativistic magnetohydrodynamic simulations of accreting black holes, we show that a suitable subtraction of the linear polarization per pixel from total intensity images can enhance the photon ring feature. We find that the photon ring is typically a factor of ≃2 less polarized than the rest of the image. This is due to a combination of plasma and general relativistic effects, as well as magnetic turbulence. When there are no other persistently depolarized image features, adding the subtracted residuals over time results in a sharp image of the photon ring. We show that the method works well for sample, viable GRMHD models of Sgr A* and M87*, where measurements of the photon ring properties would provide new measurements of black hole mass and spin, and potentially allow for tests of the 'no-hair' theorem of general relativity. [ABSTRACT FROM AUTHOR]

The Galactic centre black hole candidate Sgr A* is the best target for studies of low-luminosity accretion physics, including with near-infrared (NIR) and submillimetre wavelength long baseline interferometry experiments. Here, we compare images and spectra generated from a parameter survey of general relativistic MHD simulations to a set of radio to NIR observations of Sgr A*. Our models span the limits of weak and strong magnetization and use a range of sub-grid prescriptions for electron heating. We find two classes of scenarios can explain the broad shape of the submillimetre spectral peak and the highly variable NIR flaring emission. Weakly magnetized 'disc-jet' models where most of the emission is produced near the jet wall, consistent with past work, as well as strongly magnetized (magnetically arrested disc) models where hot electrons are present everywhere. Disc-jet models are strongly depolarized at submillimetre wavelengths as a result of strong Faraday rotation, inconsistent with observations of Sgr A*. We instead favour the strongly magnetized models, which provide a good description of the median and highly variable linear polarization signal. The same models can also explain the observed mean Faraday rotation measure and potentially the polarization signals seen recently in Sgr A* NIR flares. [ABSTRACT FROM AUTHOR]

The nature of the gaseous and dusty cloud G2 in the Galactic Centre is still under debate. We present three-dimensional hydrodynamical adaptive mesh refinement simulations of G2, modelled as an outflow from a ‘compact source’ moving on the observed orbit. The construction of mock position-velocity (PV) diagrams enables a direct comparison with observations and allows us to conclude that the observational properties of the gaseous component of G2 could be matched by a massive ($$\dot{M}_\mathrm{w}=5\times 10^{-7} \,\mathrm{ M}_{\odot }\, \mathrm{yr^{-1}}$$) and slow (50 km s−1) outflow, as observed for T Tauri stars. In order for this to be true, only the material at larger (>100 AU) distances from the source must be actually emitting; otherwise G2 would appear too compact compared to the observed PV diagrams. On the other hand, the presence of a central dusty source might be able to explain the compactness of G2’s dust component. In the present scenario, 5–10 years after pericentre the compact source should decouple from the previously ejected material, due to the hydrodynamic interaction of the latter with the surrounding hot and dense atmosphere. In this case, a new outflow should form, ahead of the previous one, which would be the smoking gun evidence for an outflow scenario. [ABSTRACT FROM AUTHOR]

The source G2 has already completed its pericentre passage around Sgr A*, the supermassive black hole in the centre of our Galaxy. Although it has been monitored for 15 yr, its astrophysical nature and origin still remain unknown. In this work, we aim to test the hypothesis of G2 being the result of a stellar wind collision. To do so, we study the motion and final fate of gas clumps formed as a result of collisions of stellar winds in massive binaries. Our approach is based on a test-particle model in order to describe the trajectories of such clumps. The model takes into account the gravitational field of Sgr A*, the interaction of the clumps with the interstellar medium as well as their finite lifetimes. Our analysis allows us to reject the hypothesis based on four arguments: (i) if G2 has followed a purely Keplerian orbit since its formation, it cannot have been produced in any of the known massive binaries since their motions are not consistent, (ii) in general, gas clumps are evaporated through thermal conduction on very short timescale ($$\lt100\,\rm yr$$) before getting close enough to Sgr A*, (iii) IRS 16SW, the best candidate for the origin of G2, cannot generate clumps as massive as G2, and (iv) clumps ejected from IRS 16SW describe trajectories significantly different to the observed motion of G2. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence and precise measurement of the astrometric baseline vector between any two telescopes are two major challenges in implementing phase-referenced interferometric astrometry and imaging. They limit the performance of a fibre-fed interferometer by degrading the instrument sensitivity and the precision of astrometric measurements and by introducing image reconstruction errors due to inaccurate phases. A multiple-beam acquisition and guiding camera was built to meet these challenges for a recently commissioned four-beam combiner instrument, GRAVITY, at the European Southern Observatory Very Large Telescope Interferometer. For each telescope beam, it measures (a) field tip-tilts by imaging stars in the sky, (b) telescope pupil shifts by imaging pupil reference laser beacons installed on each telescope using a 2 × 2 lenslet and (c) higher-order aberrations using a 9 × 9 Shack. Hartmann. The telescope pupils are imaged to provide visual monitoring while observing. These measurements enable active field and pupil guiding by actuating a train of tip-tilt mirrors placed in the pupil and field planes, respectively. The Shack. Hartmann measured quasi-static aberrations are used to focus the auxiliary telescopes and allow the possibility of correcting the non-common path errors between the adaptive optics systems of the unit telescopes and GRAVITY. The guiding stabilizes the light injection into single-mode fibres, increasing sensitivity and reducing the astrometric and image reconstruction errors. The beam guiding enables us to achieve an astrometric error of less than 50 ƒÊas. Here, we report on the data reduction methods and laboratory tests of the multiple-beam acquisition and guiding camera and its performance on-sky. [ABSTRACT FROM AUTHOR]

We present the first fully simultaneous fits to the near-infrared (NIR) and X-ray spectral slope (and its evolution) during a very bright flare from Sgr A*, the supermassive black hole at the Milky Way's centre. Our study arises from ambitious multiwavelength monitoring campaigns with XMM--Newton, NuSTAR and SINFONI. The average multiwavelength spectrum is well reproduced by a broken power law with ΓNIR = 1.7 ± 0.1 and ΓX = 2.27 ± 0.12. The difference in spectral slopes (ΔΓ = 0.57 ± 0.09) strongly supports synchrotron emission with a cooling break. The flare starts first in the NIR with a flat and bright NIR spectrum, while X-ray radiation is detected only after about 103 s, when a very steep X-ray spectrum (ΔΓ = 1.8 ± 0.4) is observed. These measurements are consistent with synchrotron emission with a cooling break and they suggest that the high-energy cut-off in the electron distribution (γmax) induces an initial cut-off in the optical--UV band that evolves slowly into the X-ray band. The temporal and spectral evolution observed in all bright X-ray flares are also in line with a slow evolution of γmax. We also observe hints for a variation of the cooling break that might be induced by an evolution of the magnetic field (from B ~ 30 ± 8 G to B ~ 4.8 ± 1.7 G at the X-ray peak). Such drop of the magnetic field at the flare peak would be expected if the acceleration mechanism is tapping energy from the magnetic field, such as in magnetic reconnection. We conclude that synchrotron emission with a cooling break is a viable process for Sgr A*'s flaring emission. [ABSTRACT FROM AUTHOR]

Near-infrared observations of stellar orbits at the Galactic Center provide conclusive evidence for a massive black hole associated with the compact radio source Sgr A*. The astrometric reference frame for these observations is tied to a set of red giant stars, which are also detectable at radio wavelengths through SiO maser emission in their envelopes. We have improved the precision and long-term stability of this reference frame, in which Sgr A* is localized to within a factor 5 better than previously: ~0.17 mas in position (in 2009) and ~0.07 mas yr-1 in velocity. This improvement is the result of modelling and correcting optical distortion in the VLT/NACO imager to a submas level and including new infrared and radio measurements, which now both span more than a decade in time. A further improvement will follow future observations and facilitate the detection of relativistic orbital effects. [ABSTRACT FROM AUTHOR]

The mean absolute extinction towards the central parsec of the Milky Way is AK ≃ 3 mag, including both foreground and Galactic Centre dust. Here we present a measurement of dust extinction within the Galactic old nuclear star cluster (NSC), based on combining differential extinctions of NSC stars with their νl proper motions along Galactic longitude. Extinction within the NSC preferentially affects stars at its far side, and because the NSC rotates, this causes higher extinctions for NSC stars with negative νl, as well as an asymmetry in the νl-histograms. We model these effects using an axisymmetric dynamical model of the NSC in combination with simple models for the dust distribution. Comparing the predicted asymmetry to data for ~7100 stars in several NSC fields, we find that dust associated with the Galactic Centre mini-spiral with extinction AK ≃ 0.15-0.8 mag explains most of the data. The largest extinction AK ≃ 0.8 mag is found in the region of the Western arm of the mini-spiral. Comparing with total AK determined from stellar colours, we determine the extinction in front of the NSC. Finally, we estimate that for a typical extinction ofAK ≃ 0.4 the statistical parallax of the NSC changes by ~0.4per cent. [ABSTRACT FROM AUTHOR]

The black hole at the Galactic Centre exhibits regularly flares of radiation, the origin of which is still not understood. In this paper, we study the ability of the near-future GRAVITY infrared instrument to constrain the nature of these events. We develop realistic simulations of GRAVITY astrometric data sets for various flare models. We show that the instrument will be able to distinguish an ejected blob from alternative flare models, provided the blob inclination is ≳45°, the flare brightest magnitude is 14 ≲ mK ≲ 15 and the flare duration is ≳1 h 30 min. [ABSTRACT FROM AUTHOR]

ABSTRACT In order to understand which process (e.g. galactic winds, cold accretion) is responsible for the cool ( T ∼ 104 K) halo gas around galaxies, we embarked on a programme to study the star formation properties of galaxies selected by their Mg ii absorption signature in quasar spectra. Specifically, we searched for the Hα line emission from galaxies near very strong z ≃ 2 Mg ii absorbers (with rest-frame equivalent width Å) because these could be the signposts of outflows or inflows. Surprisingly, we detect Hα from only four hosts out of 20 sightlines (and two out of the 19 H i-selected sightlines), despite reaching a star formation rate (SFR) sensitivity limit of 2.9 M⊙ yr−1 (5σ) for a Chabrier initial mass function. This low success rate (4/20) is in contrast with our z ≃ 1 survey where we detected 66 per cent (14/21) of the Mg ii hosts (down to 0.6 M⊙ yr−1; 5σ). Taking into account the difference in sensitivity between the two surveys, we should have been able to detect ≥11.4 (≥7.6) of the 20 z ≃ 2 hosts - assuming that SFR evolves as ∝(1 + z)γ with γ= 2.5 (or γ= 0) respectively - whereas we found only four galaxies. Interestingly, all the z = 2 detected hosts have observed SFRs ≳ 9 M⊙ yr−1, well above our sensitivity limit, while at z = 1 they all have SFR < 9 M⊙ yr−1, an evolution that is in good agreement with the evolution of the SFR main sequence, i.e. with γ= 2.5. Moreover, we show that the z = 2 undetected hosts are not hidden under the quasar continuum after stacking our data. They also cannot be outside our surveyed area as this latter option runs against our sample selection criteria ( Å) and the known Wλ2796r-impact parameter relation for low-ionization ions. Hence, strong Mg ii absorbers could trace star-formation-driven winds in low-mass haloes ( Mh≤ 1010.6 M⊙), provided that the winds do not extend beyond 20 kpc in order not to violate the evolution of the absorber number density d N/d z (Mg ii). Alternatively, our results imply that z = 2 galaxies traced by strong Mg ii absorbers do not form stars at a rate expected (3-10 M⊙ yr−1) for their (halo or stellar) masses, supporting the existence of a transition in accretion efficiency at Mh≃ 1011 M⊙. This scenario can explain both the detections and the non-detections. [ABSTRACT FROM AUTHOR]

The Galactic Central black hole Sgr A* exhibits outbursts of radiation in the near-infrared (so-called IR flares). One model of these events consists in a hotspot orbiting on the innermost stable circular orbit (ISCO) of the hole. These outbursts can be used as a probe of the central gravitational potential. One main scientific goal of the second-generation VLTI instrument GRAVITY is to observe these flares astrometrically. Here, the astrometric precision of GRAVITY is investigated in the imaging mode, which consists in analysing the image computed from the interferometric data. The capability of the instrument to put in light the motion of a hotspot orbiting on the ISCO of our central black hole is then discussed. We find that GRAVITY's astrometric precision for a single star in the imaging mode is smaller than the Schwarzschild radius of Sgr A*. The instrument can also demonstrate that a body orbiting on the last stable orbit of the black hole is indeed moving. It yields a typical size of the orbit, if the source is as bright as . These results show that GRAVITY allows one to study the close environment of Sgr A*. Having access to the ISCO of the central massive black hole probably allows constraining general relativity in its strong regime. Moreover, if the hotspot model is appropriate, the black hole spin can be constrained. [ABSTRACT FROM AUTHOR]

In this article, we identify and discuss various statistical and systematic effects influencing the astrometric accuracy achievable with Multi-adaptive optics Imaging CAmera for Deep Observations, the near-infrared (NIR) imaging camera proposed for the 42-m European Extremely Large Telescope. These effects are instrumental (e.g. geometric distortion), atmospheric (e.g. chromatic differential refraction) and astronomical (reference source selection). We find that there are several phenomena having impact on scales, meaning they can be substantially larger than the theoretical statistical astrometric accuracy of an optical/NIR 42-m telescope. Depending on type, these effects need to be controlled via dedicated instrumental design properties or via dedicated calibration procedures. We conclude that if this is done properly, astrometric accuracies of 40 μas or better – with 40 μas yr−1 in proper motions corresponding to ≈20 km s−1 at 100 kpc distance – can be achieved in one epoch of actual observations. [ABSTRACT FROM AUTHOR]

We systematically investigate the error sources for high-precision astrometry from adaptive optics (AO) based near-infrared imaging data. We focus on the application in the crowded stellar field in the Galactic Centre. We show that at the level of as a number of effects are limiting the accuracy. Most important are the imperfectly subtracted seeing haloes of neighbouring stars, residual image distortions and unrecognized confusion of the target source with fainter sources in the background. Further contributors to the error budget are the uncertainty in estimating the point-spread function, the signal-to-noise ratio induced statistical uncertainty, coordinate transformation errors, the chromaticity of refraction in Earth's atmosphere, the post-AO differential tilt jitter and anisoplanatism. For stars as bright as , residual image distortions limit the astrometry, for fainter stars the limitation is set by the seeing haloes of the surrounding stars. In order to improve the astrometry substantially at the current generation of telescopes, an AO system with high performance and weak seeing haloes over a relatively small field is suited best. Furthermore, techniques to estimate or reconstruct the seeing halo could be promising. [ABSTRACT FROM AUTHOR]

In this article we summarize and discuss the infrared, radio and X-ray emission from the supermassive black hole in the Galactic Centre, SgrA*. We include new results from near-infrared polarimetric imaging observations obtained on 2006 May 31. In that night, a strong flare in Ks band (2.08 μm) reaching top fluxes of ∼16 mJy could be observed. This flare was highly polarized (up to ∼40 per cent) and showed clear substructure on a time-scale of 15 min, including a swing in the polarization angle of about 70°. For the first time we were able to observe both polarized flux and short-time variability, with high significance in the same flare event. This result adds decisive information to the puzzle of the SgrA* activity. The observed polarization angle during the flare peak is the same as observed in two events in 2004 and 2005. Our observations strongly support the dynamical emission model of a decaying plasma hotspot orbiting SgrA* on a relativistic orbit. The observed polarization parameters and their variability with time might allow to constrain the orientation of accretion disc and spin axis with respect to the Galaxy. [ABSTRACT FROM AUTHOR]

ABSTRACT In order to understand which process (e.g. galactic winds, cold accretion) is responsible for the cool ( T ∼ 104 K) halo gas around galaxies, we embarked on a programme to study the star formation properties of galaxies selected by their Mg ii absorption signature in quasar spectra. Specifically, we searched for the Hα line emission from galaxies near very strong z ≃ 2 Mg ii absorbers (with rest-frame equivalent width Å) because these could be the signposts of outflows or inflows. Surprisingly, we detect Hα from only four hosts out of 20 sightlines (and two out of the 19 H i-selected sightlines), despite reaching a star formation rate (SFR) sensitivity limit of 2.9 M⊙ yr−1 (5σ) for a Chabrier initial mass function. This low success rate (4/20) is in contrast with our z ≃ 1 survey where we detected 66 per cent (14/21) of the Mg ii hosts (down to 0.6 M⊙ yr−1; 5σ). Taking into account the difference in sensitivity between the two surveys, we should have been able to detect ≥11.4 (≥7.6) of the 20 z ≃ 2 hosts - assuming that SFR evolves as ∝(1 + z)γ with γ= 2.5 (or γ= 0) respectively - whereas we found only four galaxies. Interestingly, all the z = 2 detected hosts have observed SFRs ≳ 9 M⊙ yr−1, well above our sensitivity limit, while at z = 1 they all have SFR < 9 M⊙ yr−1, an evolution that is in good agreement with the evolution of the SFR main sequence, i.e. with γ= 2.5. Moreover, we show that the z = 2 undetected hosts are not hidden under the quasar continuum after stacking our data. They also cannot be outside our surveyed area as this latter option runs against our sample selection criteria ( Å) and the known Wλ2796r-impact parameter relation for low-ionization ions. Hence, strong Mg ii absorbers could trace star-formation-driven winds in low-mass haloes ( Mh≤ 1010.6 M⊙), provided that the winds do not extend beyond 20 kpc in order not to violate the evolution of the absorber number density d N/d z (Mg ii). Alternatively, our results imply that z = 2 galaxies traced by strong Mg ii absorbers do not form stars at a rate expected (3-10 M⊙ yr−1) for their (halo or stellar) masses, supporting the existence of a transition in accretion efficiency at Mh≃ 1011 M⊙. This scenario can explain both the detections and the non-detections. [ABSTRACT FROM AUTHOR]