A. Eckart

A Black Hole in the Galactic Center Complex IRS 13E

The IRS 13E complex is an unusual concentration of massive, early-type stars at a projected distance of ~0.13 pc from the Milky Ways central supermassive black hole Sagittarius A* (Sgr A*). Because of their similar proper motion and their common nature as massive, young stars, it has recently been suggested that IRS 13E may be the remnant of a massive stellar cluster containing an intermediate-mass black hole (IMBH) that binds its members gravitationally in the tidal field of Sgr A*. Here, we present an analysis of the proper motions in the IRS 13E environment that combines the currently best available data with a time line of 10 years. We find that an IMBH in IRS 13E must have a minimum mass of ~104 M☉ in order to bind the source complex gravitationally. This high-mass limit in combination with the absence so far of compelling evidence for a nonthermal radio and X-ray source in IRS 13E make it appear unlikely that an IMBH exists in IRS 13E that is sufficiently massive to bind the system gravitationally.

A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way

Many galaxies are thought to have supermassive black holes at their centres—more than a million times the mass of the Sun. Measurements of stellar velocities and the discovery of variable X-ray emission have provided strong evidence in favour of such a black hole at the centre of the Milky Way, but have hitherto been unable to rule out conclusively the presence of alternative concentrations of mass. Here we report ten years of high-resolution astrometric imaging that allows us to trace two-thirds of the orbit of the star currently closest to the compact radio source (and massive black-hole candidate) Sagittarius A*. The observations, which include both pericentre and apocentre passages, show that the star is on a bound, highly elliptical keplerian orbit around Sgr A*, with an orbital period of 15.2 years and a pericentre distance of only 17 light hours. The orbit with the best fit to the observations requires a central point mass of (3.7 ± 1.5) × 106 solar masses (M[circdot]). The data no longer allow for a central mass composed of a dense cluster of dark stellar objects or a ball of massive, degenerate fermions.

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.

Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre

Recent measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* (refs 4, 5) at the Galactic Centre is a 3.6-million-solar-mass black hole. Sgr A* is remarkably faint in all wavebands other than the radio region, however, which challenges current theories of matter accretion and radiation surrounding black holes. The black holes rotation rate is not known, and therefore neither is the structure of space-time around it. Here we report high-resolution infrared observations of Sgr A* that reveal ‘quiescent’ emission and several flares. The infrared emission originates from within a few milliarcseconds of the black hole, and traces very energetic electrons or moderately hot gas within the innermost accretion region. Two flares exhibit a 17-minute quasi-periodic variability. If the periodicity arises from relativistic modulation of orbiting gas, the emission must come from just outside the event horizon, and the black hole must be rotating at about half of the maximum possible rate.

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.

BARS AND WARPS TRACED BY THE MOLECULAR GAS IN THE SEYFERT 2 GALAXY NGC 1068

We present new interferometer observations of the 12CO (1¨0) and 12CO (2¨1) line emission of NGC 1068 with a resolution of The molecular gas in the inner 5A is resolved into a ring with two bright 0A knots east and west of the nuclear continuum emission. For the —rst time in NGC 1068, we can trace molecular gas at (13 pc) from the nucleus. The high velocities in this region imply an enclosed B0A.18 mass of D108 This value is consistent with a black hole mass of 1.7 ) 107 as estimated from M _ . M _ , nuclear maser emission, plus a contribution from a compact nuclear stellar cluster. Perpendicular H 2 O to the kinematic major axis, optical images of NGC 1068 show a bright, stellar, oval structure of eccen- tricity 0.8 and a deprojected length of 17 kpc. Analysis of the rotation curve shows the CO spiral arms are at the inner Lindblad resonance of this barlike structure. Inside the molecular spiral arms, 10A from the nucleus, the CO kinematic axis changes direction probably in response to the 2.5 kpc (deprojected) long stellar bar seen in the near-infrared (NIR). The low velocity dispersion indicates the molecular gas is in a disk with a thickness of 10 pc in the nuclear region and 100 pc in the spiral arms. We constructed kinematic models for the molecular gas using elliptical orbits caused by a D1A (72 pc) nuclear bar and using tilted rings resulting in a warp. We —nd that the gas motions are consistent with either the warp or the bar models. However, because there is no evidence for a D1A nuclear bar in NIR images, we favor the warp model. A warped CO disk can also explain the obscuration of the active galactic nucleus (AGN), the extinction of light from the nuclear stellar cluster, and the observed NIR and mid-IR polarization. The model predicts that the warped CO disk should become edge-on at a radius of 70 pc, thereby creating a cavity for the ionization cone. Subject headings: galaxies: individual (NGC 1068) ¨ galaxies: ISMgalaxies: nuclei ¨ radio lines: ISM

A Multi-Transition HCN and HCO+ Study of 12 Nearby Active Galaxies: Active Galactic Nucleus versus Starburst Environments

Recent studies have indicated that the HCN-to-CO(J = 1–0) and HCO+-to-HCN(J = 1–0) ratios are significantly different between galaxies with AGN (active galactic nucleus) and SB (starburst) signatures. In order to study the molecular gas properties in active galaxies and search for differences between AGN and SB environments, we observed the HCN(J = 1–0), (J = 2–1), (J = 3–2), HCO+(J = 1–0), and HCO+(J = 3–2) emission with the IRAM 30 m in the center of 12 nearby active galaxies which either exhibit nuclear SB and/or AGN signatures. Consistent with previous results, we find a significant difference of the HCN(J = 2–1)-to-HCN(J = 1–0), HCN(J = 3–2)-to-HCN(J = 1–0), HCO+(J = 3–2)-to-HCO+(J = 1–0), and HCO+-to-HCN intensity ratios between the sources dominated by an AGN and those with an additional or pure central SB: the HCN, HCO+, and HCO+-to-HCN intensity ratios tend to be higher in the galaxies of our sample with a central SB as opposed to the pure AGN cases, which show rather low intensity ratios. Based on an LVG analysis of these data, i.e., assuming purely collisional excitation, the (average) molecular gas densities in the SB-dominated sources of our sample seem to be systematically higher than in the AGN sources. The LVG analysis seems to further support systematically higher HCN and/or lower HCO+ abundances as well as similar or higher gas temperatures in AGNs compared to the SB sources of our sample. In addition, we find that the HCN-to-CO ratios decrease with increasing rotational number J for the AGNs while they stay mostly constant for the SB sources.

The flare activity of Sagittarius A

Context. We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the Sgr A* counterpart associated with the massive 3−4 × 10 6 Mblack hole at the Galactic Center. Aims. We investigate the physical processes responsible for the variable emission from Sgr A*. Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatorys Very Large Telescopeand the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA �� on Mauna Kea, Hawaii, and the Very Large Array ��� in New Mexico. Results. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. The X-ray flare had an excess 2−8 keV luminosity of about 33 × 10 33 erg/s. The duration of this flare was completely covered in the infrared and it was detected as a simultaneous NIR event with a time lag of ≤10 min. Simultaneous infrared/X-ray observations are available for 4 flares. All simultaneously covered flares, combined with the flare covered in 2003, indicate that the time-lag between the NIR and X-ray flare emission is very small and in agreement with a synchronous evolution. There are no simultaneous flare detections between the NIR/X-ray data and the VLA and SMA data. The excess flux densities detected in the radio and sub-millimeter domain may be linked with the flare activity observed at shorter wavelengths. Conclusions. We find that the flaring state can be explained with a synchrotron self-Compton (SSC) model involving up-scattered sub- millimeter photons from a compact source component. This model allows for NIR flux density contributions from both the synchrotron and SSC mechanisms. Indications for an exponential cutoff of the NIR/MIR synchrotron spectrum allow for a straightforward explanation of the variable and red spectral indices of NIR flares.

The Position of Sagittarius A*: Accurate Alignment of the Radio and Infrared Reference Frames at the Galactic Center

We present a novel approach to the long-standing problem of locating the position of the compact nonthermal radio source Sgr A* on infrared images of the Galactic center region. Using the Very Large Array, we have detected SiO and H2O maser emission toward several sources within the central parsec of our Galaxy. These masers arise from the innermost parts of circumstellar envelopes of giant and supergiant stars that are members of the nuclear star cluster and appear as compact infrared sources in a diffraction-limited 2.2 μm infrared image. One of the SiO masers is associated with the M-type supergiant IRS 7, the most prominent 2.2 μm point source in the Galactic center region. The radio data allow measurements of the maser positions relative to the compact nonthermal radio continuum source Sgr A* with milliarcsecond accuracy. Because stellar SiO masers near the Galactic center trace their host stars to within a few milliarcseconds, these relative positions can be used to calibrate the plate scale and rotation of the infrared image. Our method allows registration of the radio relative to the infrared reference frame with an estimated accuracy of 003. Using the improved position accuracy we put a stringent upper limit on the 2.2 μm flux density of Sgr A* that is significantly lower than values predicted by recent theoretical model calculations.

The flare activity of Sagittarius A* New coordinated mm to X-ray observations

Context. We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the Sgr A* counterpart associated with the massive 3−4 × 10 6 Mblack hole at the Galactic Center. Aims. We investigate the physical processes responsible for the variable emission from Sgr A*. Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatorys Very Large Telescopeand the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA �� on Mauna Kea, Hawaii, and the Very Large Array ��� in New Mexico. Results. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. The X-ray flare had an excess 2−8 keV luminosity of about 33 × 10 33 erg/s. The duration of this flare was completely covered in the infrared and it was detected as a simultaneous NIR event with a time lag of ≤10 min. Simultaneous infrared/X-ray observations are available for 4 flares. All simultaneously covered flares, combined with the flare covered in 2003, indicate that the time-lag between the NIR and X-ray flare emission is very small and in agreement with a synchronous evolution. There are no simultaneous flare detections between the NIR/X-ray data and the VLA and SMA data. The excess flux densities detected in the radio and sub-millimeter domain may be linked with the flare activity observed at shorter wavelengths. Conclusions. We find that the flaring state can be explained with a synchrotron self-Compton (SSC) model involving up-scattered sub- millimeter photons from a compact source component. This model allows for NIR flux density contributions from both the synchrotron and SSC mechanisms. Indications for an exponential cutoff of the NIR/MIR synchrotron spectrum allow for a straightforward explanation of the variable and red spectral indices of NIR flares.