Nonthermal Radio Source Sgr Research Articles

Hubble Space TelescopePaα and 1.9 Micron Imaging of Sagittarius A West

We present HST/NICMOS images at 0.2" resolution of the HI Paschen Alpha (PaA) emission line in a 70" x 90" region of the Galactic center centered on the non-thermal radio source Sgr A*. The majority of the emission arises from ionized gas in the mini-spiral in the central parsec. PaA emission is also seen from 26 stellar sources, presumably early-type stars with mass-loss winds. The new data reveal significant small-scale structure (<1"~0.04pc) in the ionized gas of the mini-spiral; low surface brightness emission features are also seen for the first time. Extinction, estimated from the ratio of observed PaA emission to 6-cm continuum emission, varies from 20 to 50 mag with a median Av=31.1 mag, in excellent agreement with earlier estimates for the stellar sources and indepedent measurements derived using H92alpha recombination line data. Large increases in extinction are seen along the periphery of the ionized gas, suggesting that the ionized gas is partially extincted by dust in the molecular clouds at the outside of the ionized regions. The small-scale, filamentary structures in the ionized gas have a free thermal expansion time of only ~ 3000 yrs; either magnetic fields or mass-loss winds from the hot emission line stars may contain the ionized filaments. For both the ionized gas and the stellar continuum, the centroids of the emission remain within ~+/- 1" from a radius of 2" out to 40", providing further evidence that Sgr A* is indeed at or extremely close to the dynamical center of the Galactic nucleus stellar distribution. The 1.9 micron surface brightness increases inwards to 0.9" and then decreases or levels off closer to Sgr A*, possibly indicating the core radius of the central stellar distribution or depletion of the late-type stars by stellar collisions near the central black hole.

Chandra ACIS Imaging Spectroscopy of Sgr A East

We report on the X-ray emission from the shell-like, non-thermal radio source Sgr A East located in the inner few parsecs of the Galaxy based on observations made with the ACIS detector on board the Chandra X-ray Observatory. The X-ray emission from Sgr A East is concentrated within the central ≃2 pc of the larger radio shell. The spectrum shows strong Kα lines from highly ionized ions of S, Ar, Ca, and Fe. A simple isothermal plasma model gives electron temperature ∼2 keV, absorption column ∼1 × 1023 H cm−2, luminosity ∼8 × 1034 ergs s−1 in the 2–10 keV band, and gas mass ∼2η½ M⊙ with a filling factor η. The plasma appears to be rich in heavy elements, over-abundant by roughly a factor of four with respect to solar abundances. Accompanied with filamentary or blob-like structures, the plasma shows a spatial gradient of elemental abundance: the spatial distribution of iron is more compact than that of the lighter elements. These Chandra results strongly support the long-standing hypothesis that Sgr A East is a supernova remnant (SNR). Since Sgr A East surrounds Sgr A* in projection, it is possible that the dust ridge compressed by the forward shock of Sgr A East hit Sgr A* in the past, and the passage of the ridge may have supplied material to accrete onto the black hole in the past, and may have removed material from the black hole vicinity, leading to its present quiescent state.

Diffuse X-Rays from a Distributed Component of Dark Matter Surrounding Sagittarius A*

There is now compelling evidence for the presence of a few million solar masses of dark matter within 0.015 pc of the nonthermal radio source Sgr A*. Recent Chandra observations suggest that the X-ray emission from this source is partially diffuse. This result provides an important clue that can be used to set some constraints on the mass distribution surrounding the black hole, and we here develop a simple model in which the diffuse emission is produced by a halo of neutron stars accreting from the gas falling toward the center. We discuss the various accretion mechanisms that are likely to contribute significantly to the X-ray flux and show that a highly magnetized fraction of old neutron stars may account for the diffuse high-energy source. If this picture is correct, the upper bound to the mass of the central black hole is ≈2.2 × 106 M☉. The core radius of the dark cluster must then be ≈0.06 pc. We also discuss the sensitivity of our results to the various assumptions made in our calculations.

AChandraStudy of Sagittarius A East: A Supernova Remnant Regulating the Activity of Our Galactic Center?

We report on the X-ray emission from the shell-like, nonthermal radio source Sgr A East (SNR 000.0+00.0), located in the inner few parsecs of the Galaxy based on observations made with the ACIS detector on board the Chandra X-Ray Observatory. This is the first time Sgr A East has been clearly resolved from other complex structures in the region. The X-ray-emitting region is concentrated within the central 2 pc of the larger radio shell. The spectrum shows strong Kα lines from highly ionized ions of S, Ar, Ca, and Fe. A simple isothermal plasma model gives electron temperature ~2 keV, absorption column ~1 × 1023 H cm-2, luminosity ~8 × 1034 ergs s-1 in the 2-10 keV band, and gas mass ~2η1/2 M☉ with a filling factor η. The plasma appears to be rich in heavy elements, overabundant by roughly a factor of 4 with respect to solar abundances, and shows a spatial gradient of elemental abundance; the spatial distribution of iron is more compact than that of the lighter elements. The gas mass and elemental abundance of the X-ray emission support the long-standing hypothesis that Sgr A East is a supernova remnant (SNR), perhaps produced by the Type II supernova explosion of a massive star with a main-sequence mass of 13-20 M☉. The combination of the radio and X-ray morphologies classifies Sgr A East as a new metal-rich mixed morphology (MM) SNR. The size of the Sgr A East radio shell is the smallest of the known MM SNRs, which strongly suggests that the ejecta have expanded into a very dense interstellar medium. The ejecta-dominated chemical compositions of the plasma indicate that the ambient materials should be highly homogeneous. We thus evaluate a simplified dynamical evolution model where an SNR was formed about 10,000 yr ago and expanded into an ambient medium with a homogeneous density of 103 cm-3. The model roughly reproduces most of the observed properties in the X-ray and radio wavelengths. A comparison with the radio observations requires the dense ambient medium to be ionized, but a luminous X-ray irradiator with an expected X-ray luminosity of ~1040 ergs s-1 is not currently present. The presence of the ionized gas may be explained if the massive black hole (MBH) associated with the compact, nonthermal radio source Sgr A* was bright in X-rays about 300 yr ago but is presently dim. It is possible that the dust/molecular ridge compressed by the forward shock of Sgr A East hit Sgr A* in the past, and the passage of the ridge may have supplied material to accrete onto the black hole in the past and may have removed material from the black hole vicinity, leading to its present quiescent state. This may be a specific example of the intimate relationship between an SNR and MBH accretion activity in galactic nuclei.

The Role of Magnetic Field Dissipation in the Black Hole Candidate Sagittarius A*

The compact, nonthermal radio source Sgr A* at the Galactic center appears to be coincident with a ~2.6 × 106 M☉ pointlike object. Its energy source may be the release of gravitational energy as gas from the interstellar medium descends into its deep potential well. However, simple attempts at calculating the radiative spectrum and flux based on this picture have come tantalizingly close to the observations, yet have had difficulty in accounting for the unusually low efficiency in this source. Regardless of whether the radiating particles in the accretion flow are thermal or nonthermal, there now appear to be two principal reasons for this low conversion rate of dissipated energy into radiation: (1) the plasma separates into two temperatures, with the protons attaining a significantly higher temperature than that of the radiating electrons; and (2) the magnetic field B is subequipartition, which reduces the magnetic bremsstrahlung emissivity, and therefore the overall power of Sgr A*. In this paper, we investigate the latter with a considerable improvement over what has been attempted before. In particular, rather than calculating B based on some presumed model (e.g., equipartition with the thermal energy of the gas), we instead infer its distribution with radius empirically with the requirement that the resulting spectrum matches the observations. Our assumed Ansatz for B(r) is motivated in part by earlier calculations of the expected magnetic dissipation rate due to reconnection in a compressed flow. We find reasonable agreement with the observed spectrum of Sgr A* as long as its distribution consists of three primary components: an outer equipartition field, a roughly constant field at intermediate radii (~103 Schwarzschild radii), and an inner dynamo (more or less within the last stable orbit for a nonrotating black hole), which increases B to about 100 G. The latter component accounts very well for the observed submillimiter hump in this source.

Hydrodynamical Accretion onto Sagittarius A* from Distributed Point Sources

Spectral and kinematic studies suggest that the nonthermal radio source Sgr A*, located at the center of the Milky Way, is a supermassive compact object with a mass ~(2-3) × 106 M☉. Winds from nearby stars, located ≈ 0.06 pc to the east of Sgr A*, should, in the absence of any outflow from the putative black hole itself, be accreting onto this object. We report the results of the first three-dimensional Bondi-Hoyle hydrodynamical numerical simulations of this process under the assumption that the Galactic center wind is generated by several different point sources (here assumed to be 10 pseudorandomly placed stars). Our results show that the accretion rate onto the central object can be higher than in the case of a uniform flow since wind-wind shocks dissipate some of the bulk kinetic energy and lead to a higher capture rate for the gas. However, even for this highly nonuniform medium, most of the accreting gas carries with it a relatively low level of specific angular momentum, although large transient fluctuations can occur. Additionally, the post-bow shock focusing of the gas can be substantially different than that for a uniform flow, but it depends strongly on the stellar spatial distribution. We discuss how this affects the morphology of the gas in the inner 0.15 pc of the Galaxy and the consequences for accretion disk models of Sgr A*.

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.

A Hydrodynamic Model for the Formation of the Galactic Center “Minicavity”

The minicavity in the streamer to the southwest of the dynamical center of the Galaxy and the unique, nonthermal radio source Sgr A* appear to be physically connected by a chain of plasma that may be in transit from the latter to the former. Recent observations at radio and infrared wavelengths have provided clear evidence that the cavity has an opening radius of ~1'' ≈ 0.04 pc and radiates with a power Lcav ≈ 6 × 1037 ergs s-1. Here, we model the formation of the cavity as the result of an impact between the streamer gas and the post-bow shock gas collimated by a massive (~106 M☉) black hole coincident with Sgr A*. Multiwavelength observations, including those of He I, Brα, and Brγ line emission in the inner ~0.3 pc region of the Galaxy provide strong evidence for the presence of an ambient Galactic center wind with velocity vw ≈ 500-700 km s-1 and mass-loss rate ~3-4 × 10-3 M☉ yr-1. We show that the Bondi-Hoyle process responsible for the accretion of ~1022 g s-1 by Sgr A* also produces a downstream, focused flow with a radius very similar to that of the minicavity and a mechanical luminosity about 2.5 larger than Lcav. In addition, the size and density of the blobs appear to be consistent with the gas characteristics in this flow.

Mini-Spiral at the Galactic Center: A Link between its Structure and the Value of a Central Point Mass

The innermost 2 pc contain a rotating ring (“circumnuclear disk”) of molecular gas, neutral hydrogen, and dust with an embedded H II region called Sgr A West; a dense stellar cluster; and a compact nonthermal radio source Sgr A∗ (for a recent review, see Blitz et al. 1993). The clumped, spiral-shaped morphology of Sgr A West, sometimes called lthe mini-spiral”, has been a subject of numerous speculations concerning its origin (for a review, see Genzel &amp; Townes 1987). Lacy et al. (1991) demonstrated that both the kinematics and shape of a part of Sgr A West can be fairly well approximated using an one-armed density-wave model.

The Population I Core of the Galaxy

Abstract We demonstrate the presence of a cluster of hot, population I stars at the very centre of the Galaxy, using the depth of the first overtone band of CO and the presence of emission in He I 2 ·058 μm and [Fell] 1·644μm to identify stars. The cluster is very compact and comprises at least several hundred stars. They lie close to the nonthermal radio source Sgr A* and dominate the luminosity and mass loss of the Galactic core. Their presence suggests that a starburst occurred at the Galactic centre.

The nucleus of M31

view Abstract Citations (25) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Nucleus of M31 Melia, Fulvio Abstract High-resolution measurements of the rapid rotation and large velocity dispersion in the nucleus of the Andromeda nebula (M31) hint at the possible influence of a massive black hole in that region. The recent detection of an unresolved radio source coincident with the dynamical center of the galaxy has evoked a direct comparison of M31's nucleus with the compact, nonthermal radio source Sgr A^*^ at the center of our own Galaxy. We calculate the broad-band spectrum expected of a 10^7^ M_sun_ black hole accreting from an ambient galactic wind in the nucleus of M31, and we discuss several testable predictions of this model: (1) when resolved, this source will probably be elongated, perhaps in the direction of the semistellar nucleus BD + 40^deg^148; (2) its radio spectral index is expected to be ~o.4 (compared with ~0.25 for Sgr A*); (3) the radio component should steepen sharply between ~10^10^ and 10^11^ HZ; (4) if an association between this radio source and a nearby Einstein X-ray source is confirmed, its radio luminosity should be variable on a time of <~1 yr; and (5) the variability of its radio and X-ray fluxes should be anticorrelated. Publication: The Astrophysical Journal Pub Date: October 1992 DOI: 10.1086/186585 Bibcode: 1992ApJ...398L..95M Keywords: Black Holes (Astronomy); Galactic Nuclei; Milky Way Galaxy; Radio Sources (Astronomy); Stellar Mass Ejection; Astronomical Models; Mass To Light Ratios; Very Long Base Interferometry; X Ray Sources; Astrophysics; BLACK HOLE PHYSICS; GALAXIES: INDIVIDUAL MESSIER NUMBER: M31; GALAXIES: NUCLEI; STARS: MASS LOSS; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (2) NED (1)

High-resolution IRCCD images of the Galactic center at 2.2 and 3.5 microns

An IRCCD camera was used to obtain high-resolution images at 2.2 and 3.5 microns of the Galactic center at subarcsecond resolution. These images show that the compact nonthermal radio source Sgr A-asterisk is not coincident with any part of IRS 16; Sgr A-asterisk is found to be 1.1 + or - 0.3 arcsec west of IRS 16C. At 3.5 microns, IRS 16 is a complex object. IRS 16SW is resolved into two sources, and IRS 16C may be double or extended. From the colors and sizes of the various components, it is difficult to explain these objects as stellar clusters; the required stellar density would be too high. Finally, there is evidence that there is a blue and a red source, separated by about 1 arcsec, for each of the northern arm sources, IRS 1W, IRS 5, and IRS 10.

Is the galactic centre black hole a dwarf?

A 2.2-µm image of IRS 16, the relatively blue infrared source near the galactic centre, is presented at the highest spatial resolution yet reported. It comprises several discrete, extended sources. By intercomparison of radio and infrared images of the region, the non-thermal radio source Sgr A* can be located relative to IRS 16 without the uncertainty of intermediate astrometric frames. No component of IRS 16 is coincident with Sgr A*. These data preclude the presence of a massive (>106 M⊙) black hole at the galactic centre. Any black hole present has a mass nearer to 100 M⊙.