Pc Of Sgr Research Articles

An Updated Reference Frame for the Galactic Inner Parsec

Infrared observations of stellar orbits about Sgr A* probe the mass distribution in the inner parsec of the Galaxy and provide definitive evidence for the existence of a massive black hole. However, the infrared astrometry is relative and is tied to the radio emission from Sgr A* using stellar SiO masers that coincide with infrared-bright stars. To support and improve this two-step astrometry, we present new astrometric observations of 15 stellar SiO masers within 2 pc of Sgr A*. Combined with legacy observations spanning 25.8 yr, we reanalyze the relative offsets of these masers from Sgr A* and measure positions and proper motions that are significantly improved compared to the previously published reference frame. Maser positions are corrected for epoch-specific differential aberration, precession, nutation, and solar gravitational deflection. Omitting the supergiant IRS 7, the mean position uncertainties are 0.46 mas and 0.84 mas in R.A. and decl., and the mean proper motion uncertainties are 0.07 mas yr−1 and 0.12 mas yr−1, respectively. At a distance of 8.2 kpc, these correspond to position uncertainties of 3.7 and 6.9 au and proper motion uncertainties of 2.7 and 4.6 km s−1. The reference frame stability, the uncertainty in the variance-weighted mean proper motion of the maser ensemble, is 8 μas yr−1 (0.30 km s−1) in R.A. and 11 μas yr−1 (0.44 km s−1) in decl., which represents a 2.3-fold improvement over previous work and a new benchmark for the maser-based reference frame.

Kinematics in the Galactic Center with SiO masers

AbstractStellar SiO masers are found in the atmospheres of asymptotic giant branch (AGB) stars with several maser transitions observed around 43 and 86 GHz. At least 28 SiO maser stars have been detected within ∼2 pc projected distance from Sgr A* by the Very Large Array (VLA) and Atacama Millimeter/submillimeter Array (ALMA). A subset of these masers have been studied for several decades and form the basis of the radio reference frame that anchors the reference frame for infrared stars in the Galactic Center (GC). We present new observations of the GC masers from VLA and ALMA. These new data combined with extant maser astrometry provide 3D positions, velocities, and acceleration limits. The proper motions and Doppler velocities are measured with unprecedented precision for these masers. We further demonstrate how these measurements may be used to trace the stellar and dark matter mass distributions within a few pc of Sgr A*.

Depletion of Bright Red Giants in the Galactic Center during Its Active Phases

Abstract Observations in the near-infrared domain showed the presence of the flat core of bright late-type stars inside ∼0.5 pc from the Galactic center supermassive black hole (Sgr A*), while young massive OB/Wolf–Rayet stars form a cusp. Several dynamical processes were proposed to explain this apparent paradox of the distribution of the Galactic center stellar populations. Given the mounting evidence on the significantly increased activity of Sgr A* during the past million years, we propose a scenario based on the interaction between the late-type giants and a nuclear jet, whose past existence and energetics can be inferred from the presence of γ-ray Fermi bubbles and bipolar radio bubbles. Extended, loose envelopes of red giant stars can be ablated by the jet with kinetic luminosity in the range of L j ≈ 1041– within the inner ∼0.04 pc of Sgr A* (S-cluster region), which would lead to their infrared luminosity decrease after several thousand jet–star interactions. The ablation of the atmospheres of red giants is complemented by the process of tidal stripping that operates at distances of ≲1 mpc, and by the direct mechanical interaction of stars with a clumpy disk at ≳0.04 pc, which can explain the flat density profile of bright late-type stars inside the inner half parsec from Sgr A*.

NuSTAR and Chandra Observations of New X-Ray Transients in the Central Parsec of the Galaxy

Abstract We report NuSTAR and Chandra observations of two X-ray transients, SWIFT J174540.7−290015 (T15) and SWIFT J174540.2−290037 (T37), which were discovered by the Neil Gehrels Swift Observatory in 2016 within r ∼ 1 pc of Sgr A*. NuSTAR detected bright X-ray outbursts from T15 and T37, likely in the soft and hard states, with 3–79 keV luminosities of 8 × 1036 and 3 × 1037 erg s−1, respectively. No X-ray outbursts have previously been detected from the two transients and our Chandra ACIS analysis puts an upper limit of L X ≲ 2 × 1031 erg s−1 on their quiescent 2–8 keV luminosities. No pulsations, significant quasi-periodic oscillations, or type I X-ray bursts were detected in the NuSTAR data. While T15 exhibited no significant red noise, the T37 power density spectra are well characterized by three Lorentzian components. The declining variability of T37 above ν ∼ 10 Hz is typical of black hole (BH) transients in the hard state. NuSTAR spectra of both transients exhibit a thermal disk blackbody, X-ray reflection with broadened Fe atomic features, and a continuum component well described by Comptonization models. Their X-ray reflection spectra are most consistent with high BH spin (a * ≳ 0.9) and large disk density (n e ∼ 1021 cm−3). Based on the best-fit ionization parameters and disk densities, we found that X-ray reflection occurred near the inner-disk radius, which was derived from the relativistic broadening and thermal disk component. These X-ray characteristics suggest the outbursting BH-low-mass X-ray binary scenario for both transients and yield the first BH spin measurements from X-ray transients in the central 100 pc region.

ALMA Detection of Extreme Blueshifted Ionized Gas within 0.2 pc of Sgr A* from −480 to −300 km s−1

Abstract We have used the capabilities of the Atacama Large Millimeter/Submillimeter Array to probe the ionized gas toward the Galactic center with the H30α mm hydrogen recombination line within 30″ of the Galaxy’s dynamical center. The observations are made with spatial and spectral resolutions of 0.″46 × 0.″35 and 3 km s−1, respectively. Multiple compact and extended sources are detected in the minicavity region with extreme negative radial velocities ranging from −480 to −300 km s−1 2″–3″ (0.08–0.12 pc) from Sgr A*. These are the highest radial velocities of ionized gas detected beyond the inner 1″ of Sgr A*. This new component is kinematically isolated from the orbiting ionized gas. The detected gas has a velocity gradient ranging from −50 to −200 km s−1 arcsec−1 located to the southwest of Sgr A* at a position angle of ∼−160°. Previous proper-motion measurements in the immediate vicinity of these extreme high-velocity ionized components have been made, and they show transverse velocities that range from 313 to 865 km s−1. If we assume that they are associated with one another, then the total velocity implies these components are gravitationally unbound. In addition, we report the kinematics of cometary radio and infrared sources. These sources are diagonally distributed with a position angle of ∼50° within 14″ of Sgr A*. We interpret the highly blueshifted features to the southwest where the minicavity is located, and the redshifted cometary sources to the northeast in terms of the interaction of a collimated outflow with an opening angle of ∼30°. An expected mass outflow rate of 2 × 10−7 or 4 × 10−5 M ⊙ yr−1 is estimated by a relativistic jet-driven outflow or collimated stellar winds, respectively.

ALMA Detection of Bipolar Outflows: Evidence for Low-mass Star Formation within 1 pc of Sgr A*

Abstract We report the discovery of 11 bipolar outflows within a projected distance of 1 pc from Sgr A* based on deep ALMA observations of 13CO, H30α, and SiO (5−4) lines with subarcsecond and ∼1.3 km s−1 resolutions. These unambiguous signatures of young protostars manifest as approaching and receding lobes of dense gas swept up by the jets created during the formation and early evolution of stars. The lobe masses and momentum transfer rates are consistent with young protostellar outflows found throughout the disk of the Galaxy. The mean dynamical age of the outflow population is estimated to be years. The rate of star formation is ∼5 × 10−4 yr−1 assuming a mean stellar mass of ∼0.3 . This discovery provides evidence that star formation is taking place within clouds surprisingly close to Sgr A*, perhaps due to events that compress the host cloud, creating condensations with sufficient self-gravity to resist tidal disruption by Sgr A*. Low-mass star formation over the past few billion years at this level would contribute significantly to the stellar mass budget in the central few parsecs of the Galaxy. The presence of many dense clumps of molecular material within 1 pc of Sgr A* suggests that star formation could take place in the immediate vicinity of supermassive black holes in the nuclei of external galaxies.

ON THE DISTRIBUTION OF STELLAR REMNANTS AROUND MASSIVE BLACK HOLES: SLOW MASS SEGREGATION, STAR CLUSTER INSPIRALS, AND CORRELATED ORBITS

We study the long term dynamical evolution of stellar mass black holes (BHs) at the Galactic center (GC) and put constraints on their number and central mass distribution. Models of the GC are considered that have not yet achieved a steady state under the influence of random gravitational encounters. Contrary to some recent claims that mass-segregation can rapidly rebuild a density cusp in the stars, we find that time scales associated with cusp regrowth are longer than the Hubble time. These results cast doubts on standard models that postulate high densities of BHs near the GC and motivate studies that start from initial conditions which correspond to well-defined physical models. For the first time, we consider the distribution of BHs in a dissipationless formation model for the Milky Way nuclear cluster (NC), in which massive stellar clusters merge in the GC to form a nucleus. We simulate the successive inspiral of massive clusters containing an inner dense cluster of BHs. The pre-existing mass segregation is not completely erased as the clusters are disrupted by the massive black hole tidal field. As a result, after 12 inspiral events a NC forms in which the BHs have higher central densities than the stars. After evolving the model for 5-10 Gyr, the BHs do form a steep central cusp, while the stellar distribution maintains properties that resemble those of the Milky Way NC. Finally, we investigate the effect of BH perturbations on the motion of the GC S-stars, as a means of constraining the number of the perturbers. We find that reproducing the S-star orbital distribution requires >~1000 BHs within 0.1 pc of Sgr A*. A dissipationless formation scenario for the Milky Way NC is consistent with this lower limit and therefore could reconcile the need for high central densities of BHs (to explain the orbits of the S-stars), with the missing-cusp problem of the GC giant star population.

Ionized gas dynamics in the inner 2 pc of Sgr A West

AbstractWe present a data cube of the [NeII] (12.8 μm) emission from the inner 2 pc of Sgr A West with 1″ and 4 km s−1 resolution, and with substantially better SNR and velocity resolution than previous observations of the ionized gas. We compare the observations to two proposed models of the gas motions and distribution: flows along tidally stretched streamers, and more nearly circular motions with density wave compression. The density wave model provides a considerably better fit to the kinematics of the northern arm and western arc. Neither model fits the eastern arm and bar kinematics well.To help understand the origin of the spiral pattern we calculated orbits in the potential of a black hole in a star cluster and find that the orbits naturally evolve to set up a one-armed spiral wave very similar to that observed, both spatially and kinematically. Magnetic or other perturbing forces may influence the formation of the spiral wave, but self gravity is not required. Because a density wave evolves on the orbit precession timescale, rather than the orbital timescale, a wave pattern should persist for several 105 yr. No net inward motion of the gas is required by the model. If there is inflow, it is much smaller than is suggested by the infalling streamer model.

Young, massive star candidates detected throughout the nuclear star cluster of the Milky Way

Aims. Young, massive stars have been found at projected distances R < 0.5 pc from supermassive black hole, Sgr A* at the center of our Galay. In recent years, increasing evidence has been found for the presence of young, massive stars also at R > 0.5 pc. Our goal in this work is a systematic search for young, massive star candidates throughout the entire region within R ~ 2.5 pc of the black hole. Methods. The main criterion for the photometric identification of young, massive early-type stars is the lack of CO-absorption in the spectra. We used narrow-band imaging with VLT/ISAAC to search for young, massive stars within ~2.5 pc of Sgr A*. Results. We have found 63 early-type star candidates at R < 2.5 pc, with an estimated erroneous identification rate of only about 20%. Considering their K-band magnitudes and interstellar extinction, they are candidates for Wolf-Rayet stars, supergiants, or early O-type stars. Of these, 31 stars are so far unknown young, massive star candidates, all of which lie at R>0.5pc. The surface number density profile of the young, massive star candidates can be well fit by a single power-law, with Gamma = 1.6 +- 0.17 at R < 2.5 pc, which is significantly steeper than that of the late-type giants that make up the bulk of the observable stars in the NSC. Intriguingly, this power-law is consistent with the power-law that describes the surface density of young, massive stars in the same brightness range at R < 0.5 pc. Conclusions. The finding of a significant number of newly identified early-type star candidates at the Galactic center suggests that young, massive stars can be found throughout the entire cluster which may require us to modify existing theories for star formation at the Galactic center. Follow-up studies are needed to improve the existing data and lay the foundations for a unified theory of star formation in the Milky Way's NSC.

HUBBLE SPACE TELESCOPEOBSERVATIONS OF THE STELLAR DISTRIBUTION NEAR Sgr A*

We present HST/NICMOS data to study the surface brightness distribution of stellar light within the inner 10" of Sgr A* at 1.4, 1.7 and 1.9 microns. We use these data to independently examine the surface brightness distribution that had been measured previously with NICMOS and to determine whether there is a drop in the surface density of stars very near Sgr A*. Our analysis confirms that a previously reported drop in the surface brightness within 0.8" of Sgr A* is an artifact of bright and massive stars near that radius. We also show that the surface brightness profile within 5" or ~0.2 pc of Sgr A* can be fitted with broken power laws. The power laws are consistent with previous measurements, in that the profile becomes shallower at small radii. For radii > 0.7" the slope is beta=-0.34\pm0.04 where Sigma is proportional to r^beta and becomes flatter at smaller radii with beta=-0.13\pm0.04. Modeling of the surface brightness profile gives a stellar density that increases roughly as r^-1 within the inner 1" of Sgr A*. This slope confirms earlier measurements in that it is not consistent with that expected from an old, dynamically-relaxed stellar cluster with a central supermassive black hole. Assuming that the diffuse emission is not contaminated by a faint population of young stars down to the 17.1 magnitude limit of our imaging data at 1.70$\mu$, the shallow cusp profile is not consistent with a decline in stellar density in the inner arcsecond. In addition, converting our measured diffuse light profile to a stellar mass profile, with the assumption that the light is dominated by K0 dwarfs, the enclosed stellar mass within radius r < 0.1 pc of Sgr A* is ~ 3.2x10^4 M_solar (r/0.1 {pc})^2.1.

Pulsar-black hole binaries in the Galactic Centre

Binaries consisting of a pulsar and a black hole (BH) are a holy grail of astrophysics, both for their significance for stellar evolution and for their potential application as probes of strong gravity. In spite of extensive surveys of our Galaxy and its system of globular clusters, no pulsar-black hole (PSR-BH) binary has been found to date. Clues as to where such systems might exist are therefore highly desirable. We show that if the central parsec around Sgr A* harbors a cluster of ~25,000 stellar BHs (as predicted by mass segregation arguments) and if it is also rich in recycled pulsar binaries (by analogy with globular clusters), then 3-body exchange interactions should produce PSR-BHs in the Galactic center. Simple estimates of the formation rate and survival time of these binaries suggest that a few PSR-BHs should be present in the central parsec today. The proposed formation mechanism makes unique predictions for the PSR-BH properties: 1) the binary would reside within ~1 pc of Sgr A*; 2) the pulsar would be recycled, with a period of ~1 to a few tens of milliseconds, and a low magnetic field B<~10^10 G; 3) the binary would have high eccentricity, e~0.8, but with a large scatter; and 4) the binary would be relatively wide, with semi-major axis a_b~0.1 - >~3 AU. The potential discovery of a PSR-BH binary therefore provides a strong motivation for deep, high-frequency radio searches for recycled pulsars toward the Galactic center.

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.

An Overabundance of Transient X-Ray Binaries within 1 Parsec of the Galactic Center

During 5 years of Chandra observations, we have identified seven X-ray transients located within 23 pc of Sgr A*. These sources each vary in luminosity by more than a factor of 10 and have peak X-ray luminosities greater than 5 × 1033 ergs s-1, which strongly suggests that they are accreting black holes or neutron stars. The peak luminosities of the transients are intermediate between those typically considered outburst and quiescence for X-ray binaries. Remarkably, four of these transients lie within only 1 pc of Sgr A*. This implies that, compared to the numbers of similar systems located between 1 and 23 pc, transients are overabundant by a factor of 20 per unit stellar mass within 1 pc of Sgr A*. It is likely that the excess transient X-ray sources are low-mass X-ray binaries that were produced, as in the cores of globular clusters, by three-body interactions between binary star systems and either black holes or neutron stars that have been concentrated in the central parsec through dynamical friction. Alternatively, they could be high-mass X-ray binaries that formed among the young stars that are present in the central parsec.

Hot Molecular Gas in the Galactic Center

Using the new 23 GHz receivers at the Very Large Array (VLA), we have detected NH3 (6, 6) emission (ν = 25.056025 GHz) from hot (>150 K) clouds in the central 10 pc of the Galaxy. This is the first successful detection of NH3 (6, 6) with the VLA. The brightest emission comes from a region interior to the circumnuclear disk (CND), less than 1.5 pc in projected distance from Sagittarius A*. This region does not show emission from lower energy transitions such as NH3 (1, 1) and (2, 2), HCN (1-0), and HCO+ (1-0). Line ratios of NH3 (6, 6) and (3, 3) emission as well as NH3 (6, 6) line widths have peak values within 1.5 pc of Sgr A*, indicating that the gas is physically close to the nucleus. NH3 (6, 6) is also detected toward many features outside the CND observed in NH3 (1, 1), (2, 2), and (3, 3). These features tend to lie along ridges of gas associated with Sgr A East or the massive molecular ridge that connects the 20 km s-1 and 50 km s-1 giant clouds.

Red giant collisions in the Galactic Centre

We simulate collisions involving red giant stars in the centre of our Galaxy. Such encounters may explain the observed paucity of highly luminous red giants within ∼0.2 pc of Sgr A*. The masses of the missing stars are likely to be in the range ∼2–8 M⊙. Recent models of the density distribution and velocity dispersion of the Galactic Centre cluster are used to calculate two-body collision rates. In particular, we use stellar-evolution models to calculate the number of collisions a star will have during different evolutionary phases. We find that the number of two-body collisions per star is ≲1 in the central 0.1–0.2 pc, depending strongly on the Galactocentric radius, with some uncertainty from the assumed cluster models and stellar-evolution models. Using a 3D numerical hydrodynamics code (SPH), we simulate encounters involving cluster stars of various masses with a 2-M⊙ red giant and an 8-M⊙ red giant. The instantaneous mass loss in such collisions is rarely enough to destroy either giant. A fraction of the collisions do, however, lead to the formation of common-envelope systems where the impactor and core of the giant are enshrouded by the envelope of the giant. Such systems may evolve to expel the envelope, leaving a tight binary; the original giant is destroyed. The fraction of collisions that produce common-envelope systems is sensitive to the local velocity dispersion and hence Galactocentric radius. Whereas most of our collisions lead to common-envelope formation at a few parsecs from Sgr A*, very few collisions do so within the central 0.2 pc. Using our collision-rate calculations, we then compute the time-scales for a giant star to suffer such a collision within the Galactic Centre. These time-scales are ≳109–10 yr, and so are longer than the lifetimes of stars more massive than ∼2 M⊙. Thus the observed paucity of luminous giants is unlikely to be caused by the formation of common-envelope systems as a result of two-body encounters involving giant stars.

Stellar proper motions in the central 0.1 pc of the Galaxy

We report the first results of a programme to measure proper motions of stars in the innermost core of the Galaxy. From high-resolution near-infrared imaging over the last four years we have determined proper motions for 39 stars between 0.03 and 0.3 pc from the compact radio source Sgr A *. For 19 of these the derived motions are more significant than 40' in at least one coordinate. Proper motion and radial velocity dispersions are in very good agreement, indicating that the stellar velocity field on average is close to isotropic. Taking radial and proper motion data together the dynamic evidence is now strong that there is a 2.45 ( ± 0.4) x 106-Mo central dark mass located within ::;; 0.015 pc of Sgr A *. Its mass density is at least 6.5 x 109 Mo pc-3 , excluding the fact that the central mass concentration is in form of a compact white dwarf or neutron star cluster. In addition, we have detected significant changes in the structure of the innermost complex of stars in the immediate vicinity of Sgr A *, implying in at least one case stellar motions of z 1500 km s -1 within ~ 0.01 pc of the compact radio source. Including this preliminary evidence, the inferred density of the central dark mass would then have to be in excess of 10 12 Mo pc-3, implying that the central mass concentration is probably a single massive black hole.

Explaining the spectrum of Sagittarius A* with a model of an accreting black hole

THE radio source at the centre of our Galaxy1, Sagittarius A* (Sgr A*), seems to be a low-luminosity version of active galactic nuclei—a massive black hole that is accreting gas from the surrounding region1,2. This idea is supported by observations of the gas and stars within 1 pc of Sgr A*, which appear to move under the influence of a large central mass1,3,4. A recent determination of the upper limit5,6 to the hard X-ray emission from the Galactic Centre has posed a problem for this picture, however, as the mass accretion rate implied by applying a standard accretion model to the X-ray data is far below that estimated from the observations of gas flows. Here we present a new model of accretion onto Sgr A*, in which most of the energy released is carried along with the gas and lost into the black hole of mass ∼7 x 105 solar masses, rather than appearing as radiation. The model fits the observed spectrum of Sgr A* from radio to hard X-ray wavelengths, and reconciles the low observed luminosity with a high mass-accretion rate.

Stellar velocities and the mass distribution in the Galactic center

Radial velocities have been measured for 43 stars within 6.5 pc of the Galactic center. The stellar velocity dispersion of 75 km/s is virtually independent of radius from 0.36 pc to 6 pc, indicating that an extended mass distribution is being sampled over this entire range. The stellar velocities do not show the increase within 2 pc of Sgr A(asterisk) found for the gas, suggesting that the gas motions may be affected by nongravitational forces. If the core radius of the stellar distribution is about 0.1 pc, these data require no unseen mass within the Galactic center; however, if the core radius is about 0.6 pc, an unseen mass of 2,000,000 solar masses may still exist. 28 references.