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Register a new userA geometric distance measurement to the Galactic Center black hole with 0.3% uncertainty
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We present a 0.16% precise and 0.27% accurate determination of R0, the distance to the Galactic Center. Our measurement uses the star S2 on its 16-year orbit around the massive black hole Sgr A* that we followed astrometrically and spectroscopically for 27 years. Since 2017, we added near-infrared interferometry with the VLTI beam combiner GRAVITY, yielding a direct measurement of the separation vector between S2 and Sgr A* with an accuracy as good as 20 micro-arcsec in the best cases. S2 passed the pericenter of its highly eccentric orbit in May 2018, and we followed the passage with dense sampling throughout the year. Together with our spectroscopy, in the best cases with an error of 7 km/s, this yields a geometric distance estimate: R0 = 8178 +- 13(stat.) +- 22(sys.) pc. This work updates our previous publication in which we reported the first detection of the gravitational redshift in the S2 data. The redshift term is now detected with a significance level of 20 sigma with f_redshift = 1.04 +- 0.05.
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The angular size of the broad line region (BLR) of the nearby active galactic nucleus (AGN) NGC 3783 has been spatially resolved by recent observations with VLTI/GRAVITY. A reverberation mapping (RM) campaign has also recently obtained high quality light curves and measured the linear size of the BLR in a way that is complementary to the GRAVITY measurement. The size and kinematics of the BLR can be better constrained by a joint analysis that combines both GRAVITY and RM data. This, in turn, allows us to obtain the mass of the supermassive black hole in NGC3783 with an accuracy that is about a factor of two better than that inferred from GRAVITY data alone. We derive $M_mathrm{BH}=2.54_{-0.72}^{+0.90}times 10^7,M_odot$. Finally, and perhaps most notably, we are able to measure a geometric distance to NGC 3783 of $39.9^{+14.5}_{-11.9}$ Mpc. We are able to test the robustness of the BLR-based geometric distance with measurements based on the Tully-Fisher relation and other indirect methods. We find the geometric distance is consistent with other methods within their scatter. We explore the potential of BLR-based geometric distances to directly constrain the Hubble constant, $H_0$, and identify differential phase uncertainties as the current dominant limitation to the $H_0$ measurement precision for individual sources.
The hierarchical nature of galaxy formation suggests that a supermassive black hole binary could exist in our galactic center. We propose a new approach to constraining the possible orbital configuration of such a binary companion to the galactic center black hole Sgr A* through the measurement of stellar orbits. Focusing on the star S0-2, we show that requiring its orbital stability in the presence of a companion to Sgr A* yields stringent constraints on the possible configurations of such a companion. Furthermore, we show that precise measurements of {it time variations} in the orbital parameters of S0-2 could yield stronger constraints. Using existing data on S0-2 we derive upper limits on the binary black hole separation as a function of the companion mass. For the case of a circular orbit, we can rule out a 10^5 M_sun companion with a semimajor axis greater than 170 astronomical units or 0.8 mpc. This is already more stringent than bounds obtained from studies of the proper motion of Sgr A*. Including other stars orbiting the galactic center should yield stronger constraints that could help uncover the presence of a companion to Sgr A*. We show that a companion can also affect the accretion process, resulting in a variability which may be consistent with the measured infrared flaring timescales and amplitudes. Finally, if such a companion exists, it will emit gravitational wave radiation, potentially detectable with LISA.
The Galactic Center is an excellent laboratory for studying phenomena and physical processes that may be occurring in many other galactic nuclei. The Center of our Milky Way is by far the closest galactic nucleus, and observations with exquisite resolution and sensitivity cover 18 orders of magnitude in energy of electromagnetic radiation. Theoretical simulations have become increasingly more powerful in explaining these measurements. This review summarizes the recent progress in observational and theoretical work on the central parsec, with a strong emphasis on the current empirical evidence for a central massive black hole and on the processes in the surrounding dense nuclear star cluster. We present the current evidence, from the analysis of the orbits of more than two dozen stars and from the measurements of the size and motion of the central compact radio source, Sgr A*, that this radio source must be a massive black hole of about 4.4 times 1e6 Msun, beyond any reasonable doubt. We report what is known about the structure and evolution of the dense nuclear star cluster surrounding this black hole, including the astounding fact that stars have been forming in the vicinity of Sgr A* recently, apparently with a top-heavy stellar mass function. We discuss a dense concentration of fainter stars centered in the immediate vicinity of the massive black hole, three of which have orbital peri-bothroi of less than one light day. This S-star cluster appears to consist mainly of young early-type stars, in contrast to the predicted properties of an equilibrium stellar cusp around a black hole. This constitutes a remarkable and presently not fully understood paradox of youth. We also summarize what is known about the emission properties of the accreting gas onto Sgr A* and how this emission is beginning to delineate the physical properties in the hot accretion zone around the event horizon.
We present 1-resolution ALMA observations of the circumnuclear disk (CND) and the environment around SgrA*. The images unveil the presence of small spatial scale CO (J=3-2) molecular cloudlets within the central pc of the Milky Way, moving at high speeds, up to 300 km/s along the line-of-sight. The CO-emitting structures show intricate morphologies: extended and filamentary at high negative-velocities (v_LSR < -150 km/s), more localized and clumpy at extreme positive-velocities (v_LSR > +200 km/s). Based on the pencil-beam CO absorption spectrum toward SgrA* synchrotron emission, we also present evidence for a diffuse gas component producing absorption features at more extreme negative-velocities (v_LSR < -200 km/s). The CND shows a clumpy spatial distribution. Its motion requires a bundle of non-uniformly rotating streams of slightly different inclinations. The inferred gas density peaks are lower than the local Roche limit. This supports that CND molecular cores are transient. We apply the two standard orbit models, spirals vs. ellipses, invoked to explain the kinematics of the ionized gas streamers around SgrA*. The location and velocities of the CO cloudlets are inconsistent with the spiral model, and only two of them are consistent with the Keplerian ellipse model. Most cloudlets, however, show similar velocities that are incompatible with the motions of the ionized streamers or with gas bounded to the central gravity. We speculate that they are leftovers of more massive, tidally disrupted, clouds that fall into the cavity, or that they originate from instabilities in the inner rim of the CND and infall from there. Molecular cloudlets, all together with a mass of several 10 M_Sun, exist around SgrA*. Most of them must be short-lived: photoevaporated by the intense stellar radiation field, blown away by winds from massive stars, or disrupted by strong gravitational shears.
We present 3D kinematic observations of stars within the central 0.5 pc of the Milky Way nuclear star cluster using adaptive optics imaging and spectroscopy from the Keck telescopes. Recent observations have shown that the cluster has a shallower surface density profile than expected for a dynamically relaxed cusp, leading to important implications for its formation and evolution. However, the true three dimensional profile of the cluster is unknown due to the difficulty in de-projecting the stellar number counts. Here, we use spherical Jeans modeling of individual proper motions and radial velocities to constrain for the first time, the de-projected spatial density profile, cluster velocity anisotropy, black hole mass ($M_mathrm{BH}$), and distance to the Galactic center ($R_0$) simultaneously. We find that the inner stellar density profile of the late-type stars, $rho(r)propto r^{-gamma}$ to have a power law slope $gamma=0.05_{-0.60}^{+0.29}$, much more shallow than the frequently assumed Bahcall $&$ Wolf slope of $gamma=7/4$. The measured slope will significantly affect dynamical predictions involving the cluster, such as the dynamical friction time scale. The cluster core must be larger than 0.5 pc, which disfavors some scenarios for its origin. Our measurement of $M_mathrm{BH}=5.76_{-1.26}^{+1.76}times10^6$ $M_odot$ and $R_0=8.92_{-0.55}^{+0.58}$ kpc is consistent with that derived from stellar orbits within 1$^{primeprime}$ of Sgr A*. When combined with the orbit of S0-2, the uncertainty on $R_0$ is reduced by 30% ($8.46_{-0.38}^{+0.42}$ kpc). We suggest that the MW NSC can be used in the future in combination with stellar orbits to significantly improve constraints on $R_0$.
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