Tal Alexander

Monitoring stellar orbits around the Massive Black Hole in the Galactic Center

We present the results of 16 years of monitoring stellar orbits around the massive black hole in the center of the Milky Way, using high-resolution near-infrared techniques. This work refines our previous analysis mainly by greatly improving the definition of the coordinate system, which reaches a long-term astrometric accuracy of 300 μas, and by investigating in detail the individual systematic error contributions. The combination of a long-time baseline and the excellent astrometric accuracy of adaptive optics data allows us to determine orbits of 28 stars, including the star S2, which has completed a full revolution since our monitoring began. Our main results are: all stellar orbits are fit extremely well by a single-point-mass potential to within the astrometric uncertainties, which are now 6× better than in previous studies. The central object mass is , where the fractional statistical error of 1.5% is nearly independent from R 0, and the main uncertainty is due to the uncertainty in R 0. Our current best estimate for the distance to the Galactic center is R 0 = 8.33 ± 0.35 kpc. The dominant errors in this value are systematic. The mass scales with distance as (3.95 ± 0.06) × 106(R 0/8 kpc)2.19 M ☉. The orientations of orbital angular momenta for stars in the central arcsecond are random. We identify six of the stars with orbital solutions as late-type stars, and six early-type stars as members of the clockwise-rotating disk system, as was previously proposed. We constrain the extended dark mass enclosed between the pericenter and apocenter of S2 at less than 0.066, at the 99% confidence level, of the mass of Sgr A*. This is two orders of magnitudes larger than what one would expect from other theoretical and observational estimates.

SINFONI in the galactic center: young stars and infrared flares in the central light-month

We report 75 milli-arcsec resolution, near-IR imaging spectroscopy within the central 30 light days of the Galactic Center [...]. To a limiting magnitude of K~16, 9 of 10 stars in the central 0.4 arcsec, and 13 of 17 stars out to 0.7 arcsec from the central black hole have spectral properties of B0-B9, main sequence stars. [...] all brighter early type stars have normal rotation velocities, similar to solar neighborhood stars. We [...] derive improved 3d stellar orbits for six of these S-stars in the central 0.5 arcsec. Their orientations in space appear random. Their orbital planes are not co-aligned with those of the two disks of massive young stars 1-10 arcsec from SgrA*. We can thus exclude [...] that the S-stars as a group inhabit the inner regions of these disks. They also cannot have been located/formed in these disks [...]. [...] we conclude that the S-stars were most likely brought into the central light month by strong individual scattering events. The updated estimate of distance to the Galactic center from the S2 orbit fit is Ro = 7.62 +/- 0.32 kpc, resulting in a central mass value of 3.61 +/- 0.32 x 10^6 Msun. We happened to catch two smaller flaring events from SgrA* [...]. The 1.7-2.45 mum spectral energy distributions of these flares are fit by a featureless, red power law [...]. The observed spectral slope is in good agreement with synchrotron models in which the infrared emission comes from [...] radiative inefficient accretion flow in the central R~10 Rs region.

A geometric determination of the distance to the Galactic center

We report new astrometric and spectroscopic observations of the star S2 orbiting the massive black hole in the Galactic center that were taken at the ESO VLT with the adaptive optics-assisted, near-IR camera NAOS/CONICA and the near-IR integral field spectrometer SPIFFI. We use these data to determine all the orbital parameters of the star with high precision, including the Sun-Galactic center distance, which is a key parameter for calibrating stellar standard candles and an important rung in the extragalactic distance ladder. Our deduced value of R0 = 7.94 ± 0.42 kpc is the most accurate primary distance measurement to the center of the Milky Way and has minimal systematic uncertainties of astrophysical origin. It is in excellent agreement with other recent determinations of R0.

Massive Perturber-driven Interactions between Stars and a Massive Black Hole

We study the role of massive perturbers (MPs) in deflecting stars and binaries to almost radial (loss cone) orbits, where they pass near the central massive black hole (MBH), interact with it at periapse, and are ultimately destroyed. MPs dominate dynamical relaxation when the ratio of the second moments of the MP and star mass distributions, ?2 ? NpM/NM, satisfies ?2 1. We compile the MP mass function from published observations and show that MPs in the nucleus of the Galaxy (mainly giant molecular clouds), and plausibly in late-type galaxies generally, have 102 ?2 108. MPs thus shorten the relaxation timescale by 101-107 relative to two-body relaxation by stars alone. We show that this increases by 101-103 the rate of large-periapse interactions with the MBH, where loss cone refilling by stellar two-body relaxation is inefficient. We extend the Fokker-Planck loss cone formalism to approximately account for relaxation by rare encounters with MPs. We show that binary star-MBH exchanges driven by MPs can explain the origin of the young main-sequence B stars that are observed very near the Galactic MBH and can increase by orders of magnitude the ejection rate of hypervelocity stars. In contrast, the rate of small-periapse interactions of single stars with the MBH, such as tidal disruption, is only increased by a factor of a few. We suggest that MP-driven relaxation plays an important role in the three-body exchange capture of stars on very tight orbits around the MBH. These captured stars may later be disrupted by the MBH via tidal orbital decay or direct scattering into the loss cone; captured compact objects may inspiral into the MBH by the emission of gravitational waves from zero-eccentricity orbits.

Stellar processes near the massive black hole in the Galactic Center

A massive black hole resides in the center of most, perhaps all galaxies. The one in the center of our home galaxy, the Milky Way, provides a uniquely accessible laboratory for studying in detail the connections and interactions between a massive black hole and the stellar system in which it grows; for investigating the effects of extreme density, velocity and tidal fields on stars; and for using stars to probe the central dark mass and probe post-Newtonian gravity in the weak- and strong-field limits. Recent results, open questions and future prospects are reviewed in the wider context of the theoretical framework and physical processes that underlie them. Contents: [1] Introduction (1.1) Astrophysical context (1.2) Science questions (1.3) Scope and connections to related topics [2] Observational overview: Stars in the Galactic center (2.1) The central 100 parsecs (2.2) The central parsec [3] Stellar dynamics at extreme densities (3.1) Physical processes and scales (3.2) The stellar cusp in the Galactic center (3.3) Mass segregation (3.4) Stellar Collisions [4] Probing the dark mass with stellar dynamics (4.1) Weighing and pinpointing the dark mass (4.2) Constraints on non-BH dark mass alternatives (4.3) Limits on MBH binarity (4.4) High-velocity runaway stars [5] Probing post-Newtonian gravity near the MBH (5.1) Relativistic orbital effects (5.2) Gravitational lensing [6] Strong star-MBH interactions (6.1) Tidal disruption (6.2) Dissipative interactions with the MBH [7] The riddle of the young stars (7.1) The difficulties of forming or importing stars near a MBH (7.2) Proposed solutions (7.3) Feeding the MBH with stellar winds [8] Outlook (8.1) Progress report (8.2) Future directions

Resonant relaxation near a massive black hole: the stellar distribution and gravitational wave sources

Resonant relaxation (RR) of orbital angular momenta occurs near massive black holes (MBHs) where the potential is spherical and stellar orbits are nearly Keplerian and so do not precess significantly. The resulting coherent torques efficiently change the magnitude of the angular momenta and rotate the orbital inclination in all directions. As a result, many of the tightly bound stars very near the MBH are rapidly destroyed by falling into the MBH on low angular momentum orbits, while the orbits of the remaining stars are efficiently randomized. We solve numerically the Fokker-Planck equation in energy for the steady state distribution of a single-mass population with an RR sink term. We find that the steady state current of stars, which sustains the accelerated drainage close to the MBH, can be 10 larger than that due to noncoherent two-body relaxation alone. RR mostly affects tightly bound stars, and so it increases only moderately the total tidal disruption rate, which is dominated by stars originating from less bound orbits farther away. We show that the event rate of gravitational wave (GW) emission from inspiraling stars, originating much closer to the MBH, is dominated by RR dynamics. The GW event rate depends on the uncertain efficiency of RR. The efficiency indicated by the few available simulations implies rates 10 times higher than those predicted by two-body relaxation, which would improve the prospects of detecting such events by future GW detectors, such as LISA. However, a higher, but still plausible, RR efficiency can lead to the drainage of all tightly bound stars and strong suppression of GW events from inspiraling stars. We apply our results to the Galactic MBH and show that the observed dynamical properties of stars there are consistent with RR.

Evidence for a long‐standing top‐heavy initial mass function in the central parsec of the galaxy

We classify 329 late-type giants within 1 pc of Sgr A*, using the adaptive optics integral field spectrometer SINFONI on the VLT. These observations represent the deepest spectroscopic data set so far obtained for the Galactic center, reaching a 50% completeness threshold at the approximate magnitude of the helium-burning red clump (KS ~ 15.5 mag). Combining our spectroscopic results with NaCo H and KS photometry, we construct an observed Hertzsprung-Russell diagram, which we quantitatively compare to theoretical distributions of various star formation histories of the inner Galaxy, using a χ2 analysis. Our best-fit model corresponds to continuous star formation over the last 12 Gyr with a top-heavy initial mass function (IMF). The similarity of this IMF to the IMF observed for the most recent epoch of star formation is intriguing and perhaps suggests a connection between recent star formation and the stars formed throughout the history of the Galactic center.

The effect of mass-segregation on gravitational wave sources near massive black holes

Gravitational waves (GWs) from the inspiral of compact remnants (CRs) into massive black holes (MBHs) will be observable to cosmological distances. While a CR spirals in, two-body scattering by field stars may cause it to fall into the central MBH before reaching a short-period orbit that would give an observable signal. As a result, only CRs very near (~0.01 pc) the MBH can spiral in successfully. In a multimass stellar population, the heaviest objects sink to the center, where they are more likely to slowly spiral into the MBH without being swallowed prematurely. We study how mass segregation modifies the stellar distribution and the rate of GW events. We find that the inspiral rate per galaxy is 30 Gyr-1 for white dwarfs, 6 Gyr-1 for neutron stars, and 250 Gyr-1 for 10 M? stellar black holes (SBHs). The high rate for SBHs is due to their extremely steep density profile, nBH(r) r-2. The GW detection rate will be dominated by SBHs.

The Distribution of Stars near the Supermassive Black Hole in the Galactic Center

We analyze three sets of infrared star counts in the inner ~0.5 pc of the Galactic center. We perform statistical tests on the star counts and model in detail the extinction field and the effects of dwarf-giant collisions on the luminosity function. We find that both the star counts and the depletion of the brightest stars in the inner ~0.05 pc can be explained by an n ∝ r-α stellar cusp with α in the range 3/2-7/4, in which the envelopes of the brightest giants are destroyed by stellar collisions. Such a cusp is consistent with the Bahcall-Wolf solution for the distribution of stars that have undergone two-body relaxation around a black hole. We show that systematic uncertainties due to variable extinction and unrelaxed stars are probably small, but deeper star counts are required to confirm these results. We estimate that the tidal disruption rate of cusp stars by the black hole is a few × 10-5 yr-1.

Beaming Binaries: A New Observational Category of Photometric Binary Stars

The new photometric spaceborne survey missions COROT and Kepler will be able to detect minute flux variations in binary stars due to relativistic beaming caused by the line-of-sight motion of their components. In all but very short period binaries (P > 10 days), these variations will dominate over the ellipsoidal and reflection periodic variability. Thus, COROT and Kepler will discover a new observational class: photometric beaming binary stars. We examine this new category and the information that the photometric variations can provide. The variations that result from the observatory heliocentric velocity can be used to extract some spectral information even for single stars.