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Register a new userThe Post-Periapse Evolution of Galactic Center Source G1: The second case of a resolved tidal interaction with a supermassive black hole
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We present new Adaptive Optics (AO) imaging and spectroscopic measurements of Galactic Center source G1 from W. M. Keck Observatory. Our goal is to understand its nature and relationship to G2, which is the first example of a spatially-resolved object interacting with the supermassive black hole (SMBH). Both objects have been monitored with AO for the past decade (2003 - 2014) and are comparatively close to the black hole ($a_{rm{min}} sim$200-300 AU) on very eccentric orbits ($e_{rm{G1}}sim$0.99; $e_{rm{G2}}sim$0.96). While G2 has been tracked before and during periapse passage ($T_{0} sim$ 2014.2), G1 has been followed since soon after emerging from periapse ($T_{0} sim$ 2001.3). Our observations of G1 double the previously reported observational time baseline, which improves its orbital parameter determinations. G1s orbital trajectory appears to be in the same plane as that of G2, but with a significantly different argument of periapse ($Deltaomega$ = 21$pm$4 degrees). This suggests that G1 is an independent object and not part of a gas stream containing G2 as has been proposed. Furthermore, we show for the first time that: (1) G1 is extended in the epochs closest to periapse along the direction of orbital motion and (2) G1 becomes significantly smaller over time, (450 AU in 2004 to less than 170 AU in 2009). Based on these observations, G1 appears to be the second example of an object tidally interacting with a SMBH. G1s existence 14 years after periapse, along with its compactness in epochs further from the time of periapse, suggest that this source is stellar in nature.
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Pulsars, if existing and detectable in the immediate vicinity of the massive black hole (MBH) in the Galactic center (GC), may be used as a superb tool to probe both the environment and the metric of the central MBH. The recent discovery of a magnetized pulsar in the GC suggests that many more pulsars should exist near the MBH. In this paper, we estimate the number and the orbital distribution of pulsars in the vicinity of the MBH in the GC by assuming that the pulsar progenitors, similar to the GC S-stars, were captured to orbits tightly bound to the MBH through the tidal breakup of stellar binaries. We use the current observations on both the GC S-stars and the hypervelocity stars to calibrate the injection rate(s) of and the dynamical model(s) for the stellar binaries. By including the relaxation processes, supernova kicks, and gravitational wave radiation in our simulations, we estimate that ~97-190 (9-14) pulsars may presently orbit the central MBH with semimajor axes <=4000AU (<=1000AU), which is compatible with the current observational constraints on the number of the GC pulsars. The semimajor axis and the pericenter distance of the pulsar closest to the central MBH are probably in the range of ~120-460AU and ~2-230AU, respectively. Future telescopes, such as the SKA, may be able to detect a significant number of pulsars with semimajor axis smaller than a few thousand AU in the GC. Long-term monitoring of these pulsars would be helpful in constraining both the environment and the metric of the central MBH. Our preferred model also results in about ten hyperfast pulsars with velocity >~1500km/s moving away from the Milky Way.
In early 2014 the fast-moving near-infrared source G2 reached its closest approach to the supermassive black hole Sgr A* in the Galactic Center. We report on the evolution of the ionized gaseous component and the dusty component of G2 immediately after this event, revealed by new observations obtained in 2015 and 2016 with the SINFONI integral field spectrograph and the NACO imager at the ESO VLT. The spatially resolved dynamics of the Br$gamma$ line emission can be accounted for by the ballistic motion and tidal shearing of a test-particle cloud that has followed a highly eccentric Keplerian orbit around the black hole for the last 12 years. The non-detection of a drag force or any strong hydrodynamic interaction with the hot gas in the inner accretion zone limits the ambient density to less than a few 10$^3$ cm$^{-3}$ at the distance of closest approach (1500 $R_s$), assuming G2 is a spherical cloud moving through a stationary and homogeneous atmosphere. The dust continuum emission is unresolved in L-band, but stays consistent with the location of the Br$gamma$ emission. The total luminosity of the Br$gamma$ and L emission has remained constant to within the measurement uncertainty. The nature and origin of G2 are likely related to that of the precursor source G1, since their orbital evolution is similar, though not identical. Both object are also likely related to a trailing tail structure, which is continuously connected to G2 over a large range in position and radial velocity.
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.
As it was pointed out recently in Hees et al. (2017), observations of stars near the Galactic Center with current and future facilities provide an unique tool to test general relativity (GR) and alternative theories of gravity in a strong gravitational field regime. In particular, the authors showed that the Yukawa gravity could be constrained with Keck and TMT observations. Some time ago, Dadhich et al. (2001) showed that the Reissner -- Nordstrom metric with a tidal charge is naturally appeared in the framework of Randall -- Sundrum model with an extra dimension ($Q^2$ is called tidal charge and it could be negative in such an approach). Astrophysical consequences of of presence of black holes with a tidal charge are considerered, in particular, geodesics and shadows in Kerr -- Newman braneworld metric are analyzed in (Schee and Stuchlik, 2009a), while profiles of emission lines generated by rings orbiting braneworld Kerr black hole are considered in (Schee and Stuchlik, 2009b). Possible observational signatures of gravitational lensing in a presence of the Reissner -- Nordstrom black hole with a tidal charge at the Galactic Center are discussed in papers by Bin-Nun (2010a, 2010b, 2011). Here we are following such an approach and we obtain analytical expressions for orbital precession for Reissner -- Nordstrom -- de-Sitter solution in post-Newtonian approximation and discuss opportunities to constrain parameters of the metric from observations of bright stars with current and future astrometric observational facilities such as VLT, Keck, GRAVITY, E-ELT and TMT.
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.
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