Mark R. Morris

MEASURING DISTANCE AND PROPERTIES OF THE MILKY WAY'S CENTRAL SUPERMASSIVE BLACK HOLE WITH STELLAR ORBITS

We report new precision measurements of the properties of our Galaxys supermassive black hole. Based on astrometric (1995-2007) and radial velocity (RV; 2000-2007) measurements from the W. M. Keck 10m telescopes, a fully unconstrained Keplerian orbit for the short-period star S0-2 provides values for the distance (R_0) of 8.0±0.6 kpc, the enclosed mass (M_(bh)) of 4.1±0.6x10^6 M☉ and the black holes RV, which is consistent with zero with 30 km/s uncertainty. If the black hole is assumed to be at rest with respect to the Galaxy (e. g., has no massive companion to induce motion), we can further constrain the fit, obtaining R_0 = 8.4±0.4kpc and M_(bh) 4.5±0.4x10^6 M☉. More complex models constrain the extended dark mass distribution to be less than 3-4x10^5 M☉ within 0.01 pc, ~100 times higher than predictions from stellar and stellar remnant models. For all models, we identify transient astrometric shifts from source confusion (up to 5 times the astrometric error) and the assumptions regarding the black holes radial motion as previously unrecognized limitations on orbital accuracy and the usefulness of fainter stars. Future astrometric and RV observations will remedy these effects. Our estimates of R_0 and the Galaxys local rotation speed, which it is derived from combining R_0 with the apparent proper motion of Sgr A*, (θ_0 = 229±18 km/s), are compatible with measurements made using other methods. The increased black hole mass found in this study, compared to that determined using projected mass estimators, implies a longer period for the innermost stable orbit, longer resonant relaxation timescales for stars in the vicinity of the black hole and a better agreement with the M_(bh)-σ relation.

Stellar Orbits around the Galactic Center Black Hole

Wepresentnew diffraction-limitedimagesoftheGalacticcenter, obtainedwith theW.M.KeckI10mtelescope. Within0B4oftheGalaxy’scentraldarkmass,17proper-motionstars,withKmagnitudesrangingfrom 14.0to16.8, areidentified,and10ofthesearenewdetections(sixwerealsoindependentlydiscoveredbyothers).Inthissample, three newly identified (S0-16, S0-19, and S0-20) and four previously known (S0-1, S0-2, S0-4, and S0-5) sources have measured proper motions that reveal orbital solutions. Orbits are derived simultaneously so that they jointly constrain the central dark object’s properties: its mass,its position, and, for the first time using orbits, its motion on the plane of the sky. This analysis pinpoints the Galaxy’s central dark mass to within 1.3 mas (10 AU) and limits its propermotionto1:5 � 0:5masyr � 1 (orequivalently60 � 20kms � 1 )withrespecttothecentralstellarcluster.This localizationofthecentraldarkmassisconsistentwithourderivationofthepositionoftheradiosourceSgrA*inthe infrared reference frame (� 10 mas) but with an uncertainty that is a factor of 8 times smaller, which greatly facilitates searches for near-infrared counterparts to the central black hole. Consequently, one previous claim for such a counterpart can now be ascribed to a close stellar passage in 1996. Furthermore, we can place a conservative upper limit of 15.5 mag on any steady state counterpart emission. The estimated central dark mass from orbital motions is 3:7(� 0:2) ;10 6 R0= 8k pc ðÞ ½� 3 M� ; this is a more direct measure of mass than those obtained from velocitydispersion measurements,which are asmuchasafactorof2 smaller.The Galactic center’sdistance,which adds an additional 19% uncertainty in the estimated mass, is now the limiting source of uncertainty in the absolute mass. For stars in this sample, the closest approach is achieved by S0-16, which came within a mere 45 AU (=0:0002 pc ¼ 600Rs) at a velocity of 12,000 km s � 1 . This increases the inferred dark mass density by 4 orders of magnitude compared to earlier analyses based on velocity and acceleration vectors, making the Milky Way the strongest existing case for a supermassive black hole at the center of a normal-type galaxy. Well-determined orbital parameters for these seven Sgr A* cluster stars also provide new constraints on how these apparently massive, young (<10 Myr) stars formed in a region that seems to be hostile to star formation. Unlike the more distant He i emission line stars—another population of young stars in the Galactic center—that appear to have coplanar orbits, the Sgr A* cluster stars have orbital properties (eccentricities, angular momentum vectors, and apoapse directions) that are consistent with an isotropic distribution. Therefore, many of the mechanisms proposed for the formation of the He i stars, such as formation from a preexisting disk, are unlikely solutions for the Sgr A* cluster stars. Unfortunately, alternative theories for producing young stars, or old stars that look young, in close proximity to a centralsupermassiveblackholeareallalsosomewhatproblematic.Understandingtheapparentyouthofstarsinthe Sgr A* cluster, as well as the more distant Hei emission line stars, has now become one of the major outstanding issues in the study of the Galactic center. Subject headingg black hole physics — Galaxy: center — Galaxy: kinematics and dynamics — infrared: stars — techniques: high angular resolution

High Proper Motion Stars in the Vicinity of Sgr A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy

Over a 2 year period we have conducted a di†raction-limited imaging study at 2.2 km of the inner 6A ) 6A of the central stellar cluster of the Galaxy using the W. M. Keck 10 m telescope. The K-band images obtained in 1995 June, 1996 June, and 1997 May have the highest angular resolution obtained at near-infrared wavelengths from ground or space pc) and reveal a large population of (h res \ 0A\0.002 faint stars. We use an unbiased approach for identifying and selecting stars to be included in this proper- motion study, which results in a sample of 90 stars with brightness ranging from K \ 9E17 mag and two-dimensional velocities as large as 1400 ^ 100 km s~1. Compared to earlier work et al. (Eckart 1997; et al. the source confusion is reduced by a factor of 9, the number of stars with proper- Genzel 1997), motion measurement in the central 25 arcsec2 of our Galaxy is doubled, and the accuracy of the velocity measurements in the central 1 arcsec2 is improved by a factor of 4. The peaks of both the stellar surface density and the velocity dispersion are consistent with the position of the unusual radio source and black hole candidate Sgr A*, which suggests that Sgr A* is coincident with the dynamical center (^0A.1) of the Galaxy. As a function of distance from Sgr A*, the velocity dispersion displays a fallo† well--tted by Keplerian motion about a central dark mass of 2.6 ^ 0.2 ) 106 con-ned to a (p v D r~0.5B0.1) M _ volume of at most 10~6 pc3, which is consistent with earlier results. Although uncertainties in the mea- surements mathematically allow for the matter to be distributed over this volume as a cluster, no realis- tic cluster is physically tenable. Thus, independent of the presence of Sgr A*, the large inferred central density of at least 1012 pc~3, which exceeds the volume-averaged mass densities found at the center M _ of any other galaxy, leads us to the conclusion that our Galaxy harbors a massive central black hole. Subject headings: black hole physics E Galaxy: center E Galaxy: kinematics and dynamics E infrared: stars E stars: kinematics E techniques: image processing

Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre

The nuclei of most galaxies are now believed to harbour supermassive black holes. The motions of stars in the central few light years of our Milky Way Galaxy indicate the presence of a dark object with a mass of about 2.6 × 106 solar masses (refs 2, 3). This object is spatially coincident with the compact radio source Sagittarius A* (Sgr A*) at the dynamical centre of the Galaxy, and the radio emission is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole. Sgr A* is, however, much fainter than expected at all wavelengths, especially in X-rays, which has cast some doubt on this model. The first strong evidence for X-ray emission was found only recently. Here we report the discovery of rapid X-ray flaring from the direction of Sgr A*, which, together with the previously reported steady X-ray emission, provides compelling evidence that the emission is coming from the accretion of gas onto a supermassive black hole at the Galactic Centre.

Hubble Space Telescope/NICMOS Observations of Massive Stellar Clusters near the Galactic Center

We report Hubble Space Telescope (HST) Near-Infrared Camera and Multiobject Spectrometer (NICMOS) observations of the Arches and Quintuplet clusters, two extraordinary young clusters near the Galactic center. For the first time, we have identified main-sequence stars in the Galactic center with initial masses well below 10 M?. We present the first determination of the initial mass function (IMF) for any population in the Galactic center, finding an IMF slope that is significantly more positive (? ? -0.65) than the average for young clusters elsewhere in the Galaxy (? ? -1.4). The apparent turnoffs in the color-magnitude diagrams suggest cluster ages that are consistent with the ages implied by the mixture of spectral types in the clusters; we find ?age ~ 2 ? 1 Myr for the Arches cluster and ?age ~ 4 ? 1 Myr for the Quintuplet. We estimate total cluster masses by adding the masses of observed stars down to the 50% completeness limit and then extrapolating down to a lower mass cutoff of 1 M?. Using this method, we find 104 M? for the total mass of the Arches cluster. Such a determination for the Quintuplet cluster is complicated by the double-valued mass-magnitude relationship for clusters with ages 3?Myr. We find a lower limit of 6300 M? for the total cluster mass and suggest a best estimate of twice this value, which accounts for the outlying members of the cluster. Both clusters have masses that place them as the two most massive young clusters in the Galaxy.

The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth

We have obtained the first detection of spectral absorption lines in one of the high-velocity stars in the vicinity of the Galaxys central supermassive black hole. Both Brγ (2.1661 μm) and He I (2.1126 μm) are seen in absorption in S0-2 with equivalent widths (2.8 ± 0.3 and 1.7 ± 0.4 A) and an inferred stellar rotational velocity (220 ± 40 km s^(-1)) that are consistent with that of an O8-B0 dwarf, which suggests that it is a massive (~15 M_☉) young (less than 10 Myr) main-sequence star. This presents a major challenge to star formation theories, given the strong tidal forces that prevail over all distances reached by S0-2 in its current orbit (130-1900 AU) and the difficulty in migrating this star inward during its lifetime from farther out where tidal forces should no longer preclude star formation. The radial velocity measurements (v_z) = -510 ± 40 km s^(-1)) and our reported proper motions for S0-2 strongly constrain its orbit, providing a direct measure of the black hole mass of 4.1(±0.6) × 10^6 (Ro/8kpc)^3 M_☉. The Keplerian orbit parameters have uncertainties that are reduced by a factor of 2-3 compared to previously reported values and include, for the first time, an independent solution for the dynamical center; this location, while consistent with the nominal infrared position of Sgr A^*, is localized to a factor of 5 more precisely (±2 mas). Furthermore, the ambiguity in the inclination of the orbit is resolved with the addition of the radial velocity measurement, indicating that the star is behind the black hole at the time of closest approach and counterrevolving against the Galaxy. With further radial velocity measurements in the next few years, the orbit of S0-2 will provide the most robust estimate of the distance to the Galactic center.

Massive star formation near the Galactic center and the fate of the stellar remnants

Several points are made regarding massive stars, star formation, and stellar remnants in the Galactic center region, particularly the inner 1-10 parsecs. First, in light of the processes which act to inhibit or suppress star formation there, it is argued that those stars which do form tend to be formed by an externally caused compression of their parent clouds rather than by spontaneous cloud collapse. As a result of this, and because of the particular characteristics of the interstellar medium near the Galactic center, it is likely that the initial mass function (IMF) favors more massive stars than that in the Galactic disk, or at least that the lower mass cutoff of the Galactic center IMF is relatively large

Bipolar Pre-Planetary Nebulae: Hydrodynamics of Dusty Winds in Binary Systems. II. Morphology of the Circumstellar Envelopes

We have constructed three-dimensional smoothed particle hydrodynamics models to examine the influence of a detached binary companion on the dusty winds from red giants and asymptotic giant branch (AGB) stars and the degree to which this model can reproduce some of the observable characteristics of axisymmetric or bipolar pre-planetary nebulae. In this second paper in the series, we focus our attention on the morphology of the circumstellar envelopes. The parameter space of our models includes wind outflow velocities in the range 10-26 km s-1, circular orbits with binary separations 3.6-50 AU, and binary companions having masses in the range 0.25-2 M☉. By varying these parameters, we find a continuous range of envelope geometries and density contrasts that correspond well to observational classifications of planetary and pre-planetary nebulae: bipolar, with density contrasts from 10 to >200 between the equatorial plane and the polar direction; elliptical, with intermediate contrasts of 5-10; and quasi-spherical, with very low density contrasts. This last category manifests a hitherto unknown type of hydrodynamic solution, in which a spiral shock is formed, covering most of the solid angle around the binary. The cross sections of these shocks, and to a lesser extent the two-dimensional projections of the quasi-spherical envelopes, appear as a series of rings in the wind. We discuss the observational implications of that type of wind solution. The quasi-spherical geometry is the prevailing type in parameter space. From binary statistics we estimate that ~34%-40% of detached binaries will give rise to bipolars for a 10 km s-1 outflow. We present a classification scheme of the envelope geometries based on a combination of binary and wind parameters. We also find that the mass accretion rates onto the secondary are systematically lower than is predicted by the Bondi-Hoyle theory.

The large system of molecular clouds in Orion and Monoceros

Emission is noted over about one-eighth of an 850-sq deg region centered on Orion and Monoceros that has been surveyed in the J = 1 to 0 line of CO; most of the emission arises from giant molecular clouds associated with Orion A and B, and Mon R2. A much smaller area was surveyed for C-13O emission. A comparison of cloud masses obtained by three independent methods indicates that CO luminosity is as accurate a measure of cloud mass as other indicators. The possible relationships among clouds in the survey are discussed, including the conjecture that the overall Orion complex of clouds is a much larger system than previously considered, incorporating most of the clouds in the present survey.

A Deep Chandra Catalog of X-Ray Point Sources toward the Galactic Center

(abridged) We present a catalog of 2357 point sources detected during 590 ks of Chandra observations of the 17-by-17 arcminute field around Sgr A*. This field encompasses a physical area of 40 by 40 pc at a distance of 8 kpc. The completeness limit of the sample at the Galactic center is 10^{31} erg s^{-1} (2.0--8.0 keV), while the detection limit is an order of magnitude lower. The 281 sources detected below 1.5 keV are mainly in the foreground of the Galactic center, while comparisons to the Chandra deep fields at high Galactic latitudes suggest that only about 100 of the observed sources are background AGN. The surface density of absorbed sources (not detected below 1.5 keV) falls off as 1/theta away from Sgr A*, in agreement with the distribution of stars in infrared surveys. Point sources brighter than our completeness limit produce 10% of the flux previously attributed to diffuse emission. The log(N)-log(S) distribution of the Galactic center sources is extremely steep (power-law slope alpha = 1.7). If this distribution extends down to a flux of 10^{-17} erg cm^{-1} s^{-1} (10^{29} erg s^{-1} at 8 kpc, 2.0--8.0 keV) with the same slope, then point sources would account for all of the previously reported diffuse emission. Therefore, the 2.0--8.0 keV luminosity distribution must flatten between 10^{29} - 10^{31} erg s^{-1}. Finally, the spectra of more than half of the Galactic center sources are very hard, and can be described by a power law (