Search

Studying the orbital motion of stars around Sagittarius A* in the Galactic Center provides a unique opportunity to probe the gravitational potential near the supermassive black hole at the heart of our Galaxy. Interferometric data obtained with the GRAVITY instrument at the Very Large Telescope Interferometer (VLTI) since 2016 has allowed us to achieve unprecedented precision in tracking the orbits of these stars. GRAVITY data have been key to detecting the in-plane, prograde Schwarzschild precession of the orbit of the star S2, as predicted by General Relativity. By combining astrometric and spectroscopic data from multiple stars, including S2, S29, S38, and S55 - for which we have data around their time of pericenter passage with GRAVITY - we can now strengthen the significance of this detection to an approximately $10 \sigma$ confidence level. The prograde precession of S2's orbit provides valuable insights into the potential presence of an extended mass distribution surrounding Sagittarius A*, which could consist of a dynamically relaxed stellar cusp comprised of old stars and stellar remnants, along with a possible dark matter spike. Our analysis, based on two plausible density profiles - a power-law and a Plummer profile - constrains the enclosed mass within the orbit of S2 to be consistent with zero, establishing an upper limit of approximately $1200 \, M_\odot$ with a $1 \sigma$ confidence level. This significantly improves our constraints on the mass distribution in the Galactic Center. Our upper limit is very close to the expected value from numerical simulations for a stellar cusp in the Galactic Center, leaving little room for a significant enhancement of dark matter density near Sagittarius A*., Comment: Submitted to A&A on September 17, 2024

After decades of experimental projects and fast-paced technical advances, optical / infrared (O/IR) interferometry has seen a revolution in the last years. The GRAVITY instrument at the VLTI with four 8 meter telescopes reaches thousand times fainter objects than possible with earlier interferometers, and the CHARA array routinely offers up to 330 meter baselines and aperture-synthesis with six 1 meter telescopes. The observed objects are fainter than 19 magnitude, the images have sub-milliarcsecond resolution, and the astrometry reaches micro-arcsecond precision. We give an overview of breakthrough results from the past 15 years in O/IR interferometry on the Galactic Center, exo-planets and their atmospheres, active galactic nuclei, young stellar objects, and stellar physics. Following a primer in interferometry, we summarize the technical and conceptual advances which led to the boosts in sensitivity, precision, and imaging of modern interferometers. Single-mode beam combiners now combine all available telescopes of the major interferometers for imaging, and specialized image reconstruction software advances over earlier developments for radio interferometry. With a combination of large telescopes, adaptive optics, fringe-tracking, and especially dual-beam interferometry, GRAVITY has boosted the sensitivity by many orders of magnitudes. Another order of magnitude improvement will come from upgrades with laser guide star adaptive optics. In combination with large separation fringe-tracking, O/IR interferometry will then provide complete sky coverage for observations in the Galactic plane, and substantial coverage for extragalactic targets. VLTI and CHARA will remain unique in the era of upcoming 30-40m extremely large telescopes (ELTs)., Comment: 50 pages, 14 figures, to be published in Annual Review of Astronomy and Astrophysics

The GRAVITY instrument has been revolutionary for near-infrared interferometry by pushing sensitivity and precision to previously unknown limits. With the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) in GRAVITY+, these limits will be pushed even further, with vastly improved sky coverage, as well as faint-science and high-contrast capabilities. This upgrade includes the implementation of wide-field off-axis fringe-tracking, new adaptive optics systems on all Unit Telescopes, and laser guide stars in an upgraded facility. GRAVITY+ will open up the sky to the measurement of black hole masses across cosmic time in hundreds of active galactic nuclei, use the faint stars in the Galactic centre to probe General Relativity, and enable the characterisation of dozens of young exoplanets to study their formation, bearing the promise of another scientific revolution to come at the VLTI., Comment: Published in the ESO Messenger

We present a large ${\sim 30" \times 30"}$ spectroscopic survey of the Galactic Center using the SINFONI IFU at the VLT. Combining observations of the last two decades we compile spectra of over $2800$ stars. Using the Bracket-$\gamma$ absorption lines we identify $195$ young stars, extending the list of known young stars by $79$. In order to explore the angular momentum distribution of the young stars, we introduce an isotropic cluster prior. This prior reproduces an isotropic cluster in a mathematically exact way, which we test through numerical simulations. We calculate the posterior angular momentum space as function of projected separation from Sgr~A*. We find that the observed young star distribution is substantially different from an isotropic cluster. We identify the previously reported feature of the clockwise disk and find that its angular momentum changes as function of separation from the black hole, and thus confirm a warp of the clockwise disk ($p \sim 99.2\%$). At large separations, we discover three prominent overdensities of angular momentum. One overdensity has been reported previously, the counter-clockwise disk. The other two are new. Determining the likely members of these structures, we find that as many as $75\%$ of stars can be associated with one of these features. Stars belonging to the warped clockwise-disk show a top heavy K-band luminosity function, while stars belonging to the larger separation features do not. Our observations are in good agreement with the predictions of simulations of in-situ star formation, and argue for common formation of these structures., Comment: Accepted for Publication in APJL

Design and construction of the instruments for ESO's Extremely Large Telescope (ELT) began in 2015. We present here a brief overview of the status of the ELT Instrumentation Plan. Dedicated articles on each instrument are presented elsewhere this volume., Comment: 4 pages, 3 figures

GRAVITY acquisition camera implements four optical functions to track multiple beams of Very Large Telescope Interferometer (VLTI): a) pupil tracker: a $2 \times 2$ lenslet images four pupil reference lasers mounted on the spiders of telescope secondary mirror; b) field tracker: images science object; c) pupil imager: reimages telescope pupil; d) aberration tracker: images a Shack-Hartmann. The estimation of beam stabilization parameters from the acquisition camera detector image is carried out, for every 0.7 s, with a dedicated data reduction software. The measured parameters are used in: a) alignment of GRAVITY with the VLTI; b) active pupil and field stabilization; c) defocus correction and engineering purposes. The instrument is now successfully operational on-sky in closed loop. The relevant data reduction and on-sky characterization results are reported., Comment: SPIE, Optical and Infrared Interferometry and Imaging V, Proceedings Volume 9907, 990727, 2016, "See, https://doi.org/10.1117/12.2233315"

Infrared interferometry is a new frontier for precision ground based observing, with new instrumentation achieving milliarcsecond (mas) spatial resolutions for faint sources, along with astrometry on the order of 10 microarcseconds. This technique has already led to breakthroughs in the observations of the supermassive black hole at the Galactic centre and its orbiting stars, AGN, and exo-planets, and can be employed for studying X-ray binaries (XRBs), microquasars in particular. Beyond constraining the orbital parameters of the system using the centroid wobble and spatially resolving jet discrete ejections on mas scales, we also propose a novel method to discern between the various components contributing to the infrared bands: accretion disk, jets and companion star. We demonstrate that the GRAVITY instrument on the Very Large Telescope Interferometer (VLTI) should be able to detect a centroid shift in a number of sources, opening a new avenue of exploration for the myriad of transients expected to be discovered in the coming decade of radio all-sky surveys. We also present the first proof-of-concept GRAVITY observation of a low-mass X-ray binary transient, MAXI J1820+070, to search for extended jets on mas scales. We place the tightest constraints yet via direct imaging on the size of the infrared emitting region of the compact jet in a hard state XRB., Comment: 12 Pages, 3 figures, accepted for publication in MNRAS

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., Comment: 17 pages, 6 figures. v2: added some references and fixed minor typos to match version in press in MNRAS

This work presents an interferometric study of the massive-binary fraction in the Orion Trapezium Cluster with the recently comissioned GRAVITY instrument. We observe a total of 16 stars of mainly OB spectral type. We find three previously unknown companions for $\theta ^1$ Ori B, $\theta ^2$ Ori B, and $\theta ^2$ Ori C. We determine a separation for the previously suspected companion of NU Ori. We confirm four companions for $\theta ^1$ Ori A, $\theta ^1$ Ori C, $\theta ^1$ Ori D, and $\theta ^2$ Ori A, all with substantially improved astrometry and photometric mass estimates. We refine the orbit of the eccentric high-mass binary $\theta ^1$ Ori C and we are able to derive a new orbit for $\theta ^1$ Ori D. We find a system mass of 21.7 $M_{\odot}$ and a period of $53$ days. Together with other previously detected companions seen in spectroscopy or direct imaging, eleven of the 16 high-mass stars are multiple systems. We obtain a total number of 22 companions with separations up to 600 AU. The companion fraction of the early B and O stars in our sample is about 2, significantly higher than in earlier studies of mostly OB associations. The separation distribution hints towards a bimodality. Such a bimodality has been previously found in A stars, but rarely in OB binaries, which up to this point have been assumed to be mostly compact with a tail of wider companions. We also do not find a substantial population of equal-mass binaries. The observed distribution of mass ratios declines steeply with mass, and like the direct star counts, indicates that our companions follow a standard power law initial mass function. Again, this is in contrast to earlier findings of flat mass ratio distributions in OB associations. We exclude collision as a dominant formation mechanism but find no clear preference for core accretion or competitive accretion., Comment: Accepted for publication in A&A

We report the first detection of the Galactic Centre massive black hole, Sgr~A*, in the far infrared. Our measurements were obtained with PACS on board the \emph{Herschel} satellite at $100~\mathrm{\mu m}$ and $160~\mathrm{\mu m}$. While the warm dust in the Galactic Centre is too bright to allow for a direct detection of Sgr~A*, we measure a significant and simultaneous variation of its flux of $\Delta F_{\nu\widehat{=}160 ~\mathrm{\mu m}} = (0.27\pm0.06)~\mathrm{Jy}$ and $\Delta F_{\nu\widehat{=}100 ~\mathrm{\mu m}}= (0.16\pm0.10)~\mathrm{Jy}$ during one observation. The significance level of the $160 ~\mathrm{\mu m}$ band variability is $4.5\sigma$ and the corresponding $100 ~\mathrm{\mu m}$ band variability is significant at $1.6\sigma$. We find no example of an equally significant false positive detection. Conservatively assuming a variability of $25\%$ in the FIR, we can provide upper limits to the flux. Comparing the latter with theoretical models we find that 1D RIAF models have difficulties explaining the observed faintness. However, the upper limits are consistent with modern ALMA and VLA observations. Our upper limits provide further evidence for a spectral peak at $\sim 10^{12} ~ \mathrm{Hz}$ and constrain the number density of $\gamma \sim 100$ electrons in the accretion disk and or outflow., Comment: accepted for publication in APJ

Astrometric and spectroscopic monitoring of individual stars orbiting the supermassive black hole in the Galactic Center offer a promising way to detect general relativistic effects. While low-order effects are expected to be detected following the periastron passage of S2 in Spring 2018, detecting higher-order effects due to black hole spin will require the discovery of closer stars. In this paper, we set out to determine the requirements such a star would have to satisfy to allow the detection of black hole spin. We focus on the instrument GRAVITY, which saw first light in 2016 and which is expected to achieve astrometric accuracies $10-100 \mu$as. For an observing campaign with duration $T$ years, $N_{obs}$ total observations, astrometric precision $\sigma_x$ and normalized black hole spin $\chi$, we find that $a_{orb}(1-e^2)^{3/4} \lesssim 300 R_S \sqrt{\frac{T}{4 \text{years}}} \left(\frac{N_{obs}}{120}\right)^{0.25} \sqrt{\frac{10 \mu as}{\sigma_x}} \sqrt{\frac{\chi}{0.9}}$ is needed. For $\chi=0.9$ and a potential observing campaign with $\sigma_x = 10 \mu$as, 30 observations/year and duration 4-10 years, we expect $\sim 0.1$ star with $K<19$ satisfying this constraint based on the current knowledge about the stellar population in the central 1". We also propose a method through which GRAVITY could potentially measure radial velocities with precision $\sim 50$ km/s. If the astrometric precision can be maintained, adding radial velocity information increases the expected number of stars by roughly a factor of two. While we focus on GRAVITY, the results can also be scaled to parameters relevant for future extremely large telescopes., Comment: Accepted to MNRAS

The G2 object has recently passed its pericenter passage in our Galactic Center. While the $Br_\gamma$ emission shows clear signs of tidal interaction, the change in the observed luminosity is only of about a factor of 2, in contention with all previous predictions. We present high resolution simulations performed with the moving mesh code, RICH, together with simple analytical arguments that reproduce the observed $Br_\gamma$ emission. In our model, G2 is a gas cloud that undergoes tidal disruption in a dilute ambient medium. We find that during pericenter passage, the efficient cooling of the cloud results in a vertical collapse, compressing the cloud by a factor of $\sim5000$. By properly taking into account the ionization state of the gas, we find that the cloud is UV starved and are able to reproduce the observed $Br_\gamma$ luminosity. For densities larger than $\approx500\;\mathrm{cm}^{-3}$ at pericenter, the cloud fragments, due to cooling instabilities and the emitted radiation is inconsistent with observations. For lower densities, the cloud survives the pericenter passage intact and its emitted radiation matches the observed lightcurve. From the duration of $Br_\gamma$ emission which contains both redshifted and blueshifted components, we show that the cloud is not spherical but rather elongated with a size ratio of 4 at year 2001. The simulated cloud's elongation grows as it travels towards pericenter and is consistent with observations, due to viewing angles. The simulation is also consistent with having a spherical shape at apocenter.

Using 25 years of data from uninterrupted monitoring of stellar orbits in the Galactic Center, we present an update of the main results from this unique data set: A measurement of mass of and distance to SgrA*. Our progress is not only due to the eight year increase in time base, but also due to the improved definition of the coordinate system. The star S2 continues to yield the best constraints on the mass of and distance to SgrA*; the statistical errors of 0.13 x 10^6 M_sun and 0.12 kpc have halved compared to the previous study. The S2 orbit fit is robust and does not need any prior information. Using coordinate system priors, also the star S1 yields tight constraints on mass and distance. For a combined orbit fit, we use 17 stars, which yields our current best estimates for mass and distance: M = 4.28 +/- 0.10|stat. +/. 0.21|sys. x 10^6 M_sun and R_0 = 8.32 +/- 0.07|stat. +/- 0.14|sys. kpc. These numbers are in agreement with the recent determination of R_0 from the statistical cluster parallax. The positions of the mass, of the near-infrared flares from SgrA* and of the radio source SgrA* agree to within 1mas. In total, we have determined orbits for 40 stars so far, a sample which consists of 32 stars with randomly oriented orbits and a thermal eccentricity distribution, plus eight stars for which we can explicitly show that they are members of the clockwise disk of young stars, and which have lower eccentricity orbits., Comment: submitted to ApJ, 19 pages, 14 figures

The VLTI instrument GRAVITY combines the beams from four telescopes and provides phase-referenced imaging as well as precision-astrometry of order 10 microarcseconds by observing two celestial objects in dual-field mode. Their angular separation can be determined from their differential OPD (dOPD) when the internal dOPDs in the interferometer are known. Here, we present the general overview of the novel metrology system which performs these measurements. The metrology consists of a three-beam laser system and a homodyne detection scheme for three-beam interference using phase-shifting interferometry in combination with lock-in amplifiers. Via this approach the metrology system measures dOPDs on a nanometer-level., Comment: 9 pages, 5 figures

GRAVITY is the four-beam, near- infrared, AO-assisted, fringe tracking, astrometric and imaging instrument for the Very Large Telescope Interferometer (VLTI). It is requiring the development of one of the most complex instrument software systems ever built for an ESO instrument. Apart from its many interfaces and interdependencies, one of the most challenging aspects is the overall performance and stability of this complex system. The three infrared detectors and the fast reflective memory network (RMN) recorder contribute a total data rate of up to 20 MiB/s accumulating to a maximum of 250 GiB of data per night. The detectors, the two instrument Local Control Units (LCUs) as well as the five LCUs running applications under TAC (Tools for Advanced Control) architecture, are interconnected with fast Ethernet, RMN fibers and dedicated fiber connections as well as signals for the time synchronization. Here we give a simplified overview of all subsystems of GRAVITY and their interfaces and discuss two examples of high-level applications during observations: the acquisition procedure and the gathering and merging of data to the final FITS file., Comment: 8 pages, 7 figures, published in Proc. SPIE 9146, Optical and Infrared Interferometry IV, 91462B

We present new, deep near-infrared SINFONI @ VLT integral field spectroscopy of the gas cloud G2 in the Galactic Center, from late summer 2013 and spring 2014. G2 is visible in recombination line emission. The spatially resolved kinematic data track the ongoing tidal disruption. As expected for an observation near pericenter passage, roughly half of the gas in 2014 is found at the redshifted, pre-pericenter side of the orbit, while the other half is at the post-pericenter, blueshifted side. We also present an orbital solution for the gas cloud G1, which was discovered a decade ago in L'-band images when it was spatially almost coincident with Sgr A*. The orientation of the G1 orbit in the three angles is almost identical to the one of G2, but it has a lower eccentricity and smaller semi-major axis. We show that the observed astrometric positions and radial velocities of G1 are compatible with the G2 orbit, assuming that (i) G1 was originally on the G2 orbit preceding G2 by 13 years and (ii) a simple drag force acted on it during pericenter passage. Taken together with the previously described tail of G2, which we detect in recombination line emission and thermal broadband emission, we propose that G2 may be a bright knot in a much more extensive gas streamer. This matches purely gaseous models for G2, such as a stellar wind clump or the tidal debris from a partial disruption of a star., Comment: 28 pages, paper as accepted by ApJ

Like many other late-type galaxies, the Milky Way contains a nuclear star cluster. In this work we obtain the basic properties of its dominant old stellar population. Firstly, we derive its structural properties by constructing a stellar surface density map of the central 1000" using extinction corrected star counts from VISTA, WFC3/IR and VLT/NACO data. We can describe the profile with a two-component model. The inner, slightly flattened (q=0.80+/-0.04) component is the nuclear cluster, while the outer component corresponds to the stellar component of the circumnuclear zone. We measure for the nuclear cluster a half-light radius of 178+/-51"~7+/-2 pc and a luminosity of M_Ks=-16.0+/-0.5. Secondly, we enlarge the range over which detailed dynamics are available from 1 pc to 4 pc. We obtain more than 10000 individual proper motions from NACO data, and more than 2500 radial velocities from VLT/SINFONI data. We determine the cluster mass by means of isotropic spherical Jeans modeling. We fix the distance to the Galactic Center and the mass of the supermassive black hole. We model the cluster either with a constant mass to light ratio or with a power law mass model. For the latter we obtain a slope of delta_M=1.18+/-0.06. Assuming spherical symmetry, we get a nuclear cluster mass within 100" of M_100"=6.09+/- 0.53_(fix_R_0) +/- 0.97_(R_0) * 10^6 M_sun for both modeling approaches. A model which includes the observed flattening gives a 47% larger mass, see Chatzopoulos et al. 2015. Our results slightly favor a core over a cusp in the mass profile. By minimizing the number of unbound stars within 8" in our sample we obtain a distance estimate of R$_0=8.53^{+0.21}_{-0.15}$ kpc, where an a priori relation between R_0 and SMBH mass from stellar orbits is used. Combining our mass and flux we obtain M/L=0.51+/-0.12 M_sun/L_(sun Ks). This is roughly consistent with a Chabrier IMF., Comment: 47 pages, 32 figures (3 compressed), 9 tables, table 9 in full length available on request; ApJ accepted

We derive new constraints on the mass, rotation, orbit structure and statistical parallax of the Galactic old nuclear star cluster (NSC) and the mass of the supermassive black hole. We combine star counts and kinematic data from Fritz et al (2014), including 2'500 line-of-sight velocities and 10'000 proper motions. We show that the difference between the proper motion dispersions sigma_l and sigma_b cannot be explained by rotation, but is a consequence of the flattening of the NSC. We fit the surface density distribution of stars in the central 1000" by a spheroidal cluster with scale ~100" and a much larger nuclear disk component. We compute the two-integral distribution function f(E,Lz) for this density model, and add rotation self-consistently. We find that: (i) The orbit structure of the f(E,Lz) gives an excellent match to the observed velocity dispersion profiles as well as the proper motion and line-of-sight velocity histograms, including the double-peak in the v_l-histograms. (ii) This requires an axial ratio of q= 0.73+-0.04 for r<70" consistent with our determination from star counts. (iii) The NSC is approximately described by an isotropic rotator model. (iv) Using the corresponding Jeans equations to fit the proper motion and line-of-sight velocity dispersions, we obtain best estimates for the NSC mass, black hole mass, and distance M*(r<100")=(8.94+-0.31|stat+-0.9|syst)x10^6Msun, Mbh=(3.86+-0.14|stat+-0.4|syst)x10^6Msun, and R0=8.27+-0.09|stat+-0.1|syst kpc, where the systematic errors estimate additional uncertainties in the dynamical modeling. (v) The combination of the cluster dynamics with the S-star orbits around Sgr A* strongly reduces the degeneracy between black hole mass and Galactic centre distance present in previous S-star studies. A joint statistical analysis with the results of Gillessen et al (2009) gives Mbh=(4.23+-0.14)x10^6Msun and R0=8.33+-0.11kpc., Comment: Accepted for publication in MNRAS. Best values for M*, Mbh and R0 changed within errors due to improved star count model

The center of our galaxy is home to a massive black hole, SgrA*, and a nuclear star cluster containing stellar populations of various ages. While the late type stars may be too old to have retained memory of their initial orbital configuration, and hence formation mechanism, the kinematics of the early type stars should reflect their original distribution. In this contribution we present a new statistic which uses directly-observable kinematical stellar data to infer orbital parameters for stellar populations, and is capable of distinguishing between different origin scenarios. We use it on a population of B-stars in the Galactic center that extends out to large radii (0.5 pc) from the massive black hole. We find that the high K-magnitude population form an eccentric distribution, suggestive of a Hills binary-disruption origin., Comment: 4 pages, 2 figures, Proceedings of IAU Symposium No. 303: The Galactic Center: Feeding and Feedback in a Normal Galactic Nucleus

In 2011, we discovered a compact gas cloud ("G2") with roughly three Earth masses that is falling on a near-radial orbit toward the massive black hole in the Galactic Center. The orbit is well constrained and pericenter passage is predicted for early 2014. Our data beautifully show that G2 gets tidally sheared apart due to the massive black hole's force. During the next months, we expect that in addition to the tidal effects, hydrodynamics get important, when G2 collides with the hot ambient gas around Sgr A*. Simulations show that ultimately, the cloud's material might fall into the massive black hole. Predictions for the accretion rate and luminosity evolution, however, are very difficult due to the many unknowns. Nevertheless, this might be a unique opportunity in the next years to observe how gas feeds a massive black hole in a galactic nucleus., Comment: Proceedings of IAU Symposium #303, "The Galactic Center: Feeding and Feedback in a Normal Galactic Nucleus"; 30 Sep - 4 Oct 2013, Santa Fe / NM (USA)

Here we present the fundamental properties of the nuclear cluster of the Milky Way. First, we derive its structural properties by constructing a density map of the central 1000'' using extinction-corrected star counts. We can describe the data with a two-component model built from Sersic profiles. The inner nearly spherical component is the nuclear cluster. The outer, strongly flattened component can be identified with the stellar component of the circumnuclear zone. Second, we enlarge the radius inside which detailed dynamics are available from $1\,$pc to $4\,$pc. We use more than 10000 individual proper motions and more than 2700 radial velocities. We determine the cluster mass by means of isotropic spherical Jeans modeling. We get a nuclear cluster mass within 100'' of $M_{100''}=(6.11 \pm 0.52|_{\mathrm{fix} R_0}\pm 0.97|_{R_0}) \times 10^6 $ M$_{\odot}$, which corresponds to a total cluster mass of M$_{\mathrm{NC}}=(13.08 \pm 2.51|_{\mathrm{fix} R_0}\pm 2.08|_{R_0}) \times 10^6 $ M$_{\odot}$. By combination of our mass with the flux we calculate $M/L=0.50 \pm 0.12 M_{\odot}/L_{\odot,\mathrm{Ks}}$ for the central 100''. That is broadly consistent with a Chabrier IMF. With its mass and a luminosity of M$_{\mathrm{Ks}}=-15.30\pm0.26$ the nuclear cluster is a bright and massive specimen with a typical size., Comment: 4 pages, conference proceedings, IAU Symposium 303 "The GC: Feeding and Feedback in a Normal Galactic Nucleus", 2013 Sep. 30 - Oct. 4, Santa Fe New Mexico (USA)

Massive black holes (MBHs) in galactic nuclei are believed to be surrounded by a high density stellar cluster, whose mass is mostly in hard-to-detect faint stars and compact remnants. Such dark cusps dominate the dynamics near the MBH: a dark cusp in the Galactic center (GC) of the Milky Way would strongly affect orbital tests of General Relativity there; on cosmic scales, dark cusps set the rates of gravitational wave emission events from compact remnants that spiral into MBHs, and they modify the rates of tidal disruption events, to list only some implications. A recently discovered long-period massive young binary (P_12 <~ 1 yr, M_12 ~ O(100 M_sun), T_12 ~ 6x10^6 yr), only ~0.1 pc from the Galactic MBH (Pfuhl et al 2013), sets a lower bound on the 2-body relaxation timescale there, min t_rlx ~ (P_12/M_12)^(2/3)T_12 ~ 10^7 yr, and correspondingly, an upper bound on the stellar number density, max n ~ few x 10^8/ 1/pc^3, based on the binary's survival against evaporation by the dark cusp. However, a conservative dynamical estimate, the drain limit, implies t_rlx > O(10^8) yr. Such massive binaries are thus too short-lived and tightly bound to constrain a dense relaxed dark cusp. We explore here in detail the use of longer-period, less massive and longer-lived binaries (P_12 ~ few yr, M_12 ~ 2-4 M_sun, T_12 ~ 10^8-10^10 yr), presently just below the detection threshold, for probing the dark cusp, and develop the framework for translating their future detections among the giants in the GC into dynamical constraints., Comment: 13 pp. Submitted to ApJ

We have further followed the evolution of the orbital and physical properties of G2, the object currently falling toward the massive black hole in the Galactic Center on a near-radial orbit. New, very sensitive data were taken in April 2013 with NACO and SINFONI at the ESO VLT . The 'head' of G2 continues to be stretched ever further along the orbit in position-velocity space. A fraction of its emission appears to be already emerging on the blue-shifted side of the orbit, past pericenter approach. Ionized gas in the head is now stretched over more than 15,000 Schwarzschild radii RS around the pericenter of the orbit, at ~ 2000 RS ~ 20 light hours from the black hole. The pericenter passage of G2 will be a process stretching over a period of at least one year. The Brackett-{\gamma} luminosity of the head has been constant over the past 9 years, to within +- 25%, as have the line ratios Brackett-{\gamma} / Paschen-{\alpha} and Brackett-{\gamma} / Helium-I. We do not see any significant evidence for deviations of G2's dynamical evolution, due to hydrodynamical interactions with the hot gas around the black hole, from a ballistic orbit of an initially compact cloud with moderate velocity dispersion. The constant luminosity and the increasingly stretched appearance of the head of G2 in the position-velocity plane, without a central peak, is not consistent with several proposed models with continuous gas release from an initially bound zone around a faint star on the same orbit as G2., Comment: 10 figures, submitted to ApJ

We present a new directly-observable statistic which uses sky position and proper motion of stars near the Galactic center massive black hole to identify populations with high orbital eccentricities. It is most useful for stars with large orbital periods for which dynamical accelerations are difficult to determine. We apply this statistic to a data set of B-stars with projected radii 0."1 < p < 25" (~0.004 - 1 pc) from the massive black hole in the Galactic center. We compare the results with those from N-body simulations to distinguish between scenarios for their formation. We find that the scenarios favored by the data correlate strongly with particular K-magnitude intervals, corresponding to different zero-age main-sequence (MS) masses and lifetimes. Stars with 14 < mK < 15 (15 - 20 solar masses, t_{MS} = 8-13 Myr) match well to a disk formation origin, while those with mK > 15 (<15 solar masses, t_{MS} >13 Myr), if isotropically distributed, form a population that is more eccentric than thermal, which suggests a Hills binary-disruption origin., Comment: Updated paper. 21 pages, 28 figures, 6 tables, ApJ accepted

We derive the extinction curve towards the Galactic Center from 1 to 19 micron. We use hydrogen emission lines of the minispiral observed by ISO-SWS and SINFONI. The extinction free flux reference is the 2 cm continuum emission observed by the VLA. Towards the inner 14" * 20" we find an extinction of A(2.166 micron)=2.62 +/- 0.11, with a power-law slope of alpha=-2.11 +/- 0.06 shortward of 2.8 micron, consistent with the average near infrared slope from the recent literature. At longer wavelengths, however, we find that the extinction is grayer than shortward of 2.8 micron. We find it is not possible to fit the observed extinction curve with a dust model consisting of pure carbonaceous and silicate grains only, and the addition of composite particles, including ices, is needed to explain the observations. Combining a distance dependent extinction with our distance independent extinction we derive the distance to the GC to be R_0=7.94 +/- 0.65 kpc. Towards Sgr A* (r<0.5") we obtain A_H=4.21 +/- 0.10, A_Ks=2.42 +/- 0.10 and A_L'=1.09 +/- 0.13., Comment: 23 pages, 12 figures, 9 tables, ApJ accepted

We present a detailed analysis of high resolution near-infrared imaging and spectroscopy of the potential star cluster IRS13E very close to the massive black hole in the Galactic Center. We detect 19 objects in IRS13E from Ks-band images, 15 of which are also detected reliably in H-band. We derive consistent proper motions for these objects from the two bands. Most objects share a similar westward proper motion. We characterize the objects using spectroscopy (1.45 to 2.45 micrometer) and (narrow-band) imaging from H- (1.66 mircrometer) to L'-band (3.80 micrometer). Nine of the objects detected in both Ks- and H-band are very red, and we find that they are all consistent with being warm dust clumps. The dust emission may be caused by the colliding winds of the two Wolf-Rayet stars in the cluster. Three of the six detected stars do not share the motion or spectral properties of the three bright stars. This leaves only the three bright, early-type stars as potential cluster members. It is unlikely that these stars are a chance configuration. Assuming the presence of an IMBH, a mass of about 14000 solar masses follows from the velocities and positions of these three stars. However, our acceleration limits make such an IMBH nearly as unlikely as a chance occurrence of such a star association. Furthermore, there is no variable X-ray source in IRS13E despite the high density of dust and gas. Therefore, we conclude that is unlikely that IRS13E hosts a black hole massive enough to bind the three stars., Comment: 19 pages, 14 figures, 2 tables, accepted for publication in Astrophysical Journal

We systematically investigate the error sources for high-precision astrometry from adaptive optics based near-infrared imaging data. We focus on the application in the crowded stellar field in the Galactic Center. We show that at the level of <=100 micro-arcseconds a number of effects are limiting the accuracy. Most important are the imperfectly subtracted seeing halos of neighboring 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 adaptive optics differential tilt jitter and anisoplanatism. For stars as bright as mK=14, residual image distortions limit the astrometry, for fainter stars the limitation is set by the seeing halos of the surrounding stars. In order to improve the astrometry substantially at the current generation of telescopes, an adaptive optics system with high performance and weak seeing halos over a relatively small field (r<=3") is suited best. Furthermore, techniques to estimate or reconstruct the seeing halo could be promising., Comment: accepted by MNRAS, 13 pages, 14 figures