摘要:

I review our present knowledge of the stellar distribution within the inner parsec of the Galaxy.

ISSN:0094-243X    

DOI:10.1063/1.36434

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Dust obscures our view of the Galactic center, and complicates enormously the search for a ‘‘central engine.’’ One straightforward but indirect method is to study the thermal emission from dust in the nucleus. A very simplistic assumption, that the nucleus is choked with dust, leads to the prediction that a central engine, if present, will produce a single bright infrared source. Observations made more than a decade ago excluded this naive possibility. Instead, 10 &mgr;m images of the Galactic center are complicated, with multiple peaks in the emission.Existing radio observations of Sgr A had already suggested the presence of ultraviolet radiation in the nucleus, and so it was that some workers saw the 10&mgr;m image as evidence for a burst of star formation. The demonstration of the existence of late type supergiants within the field of the 10&mgr;m map encouraged that interpretation.The possibility that the dust density in the inner Galaxy is actually very low was not taken seriously until it was directly demonstrated by far infrared observations. These observations showed that a ring of neutral material encircles the nucleus at a radius of 2 parsecs, that within the central cavity of this ring the dust density is low, and that this inner region is transparent to optical and ultraviolet radiation; the structure observed at 10&mgr;m is located within the cavity in the ring.Maps of the infrared color temperature distribution are symmetric and peak centrally in the vicinity of the nuclear source IRS16. There are no temperature peaks at the 10&mgr;m brightness peaks. The energetics of the inner few parsecs of the Galaxy are dominated by a strong source of luminosity resident at the Galactic center.Wisps and streamers of material falling inward are exposed to the radiation field of the central object, which ionizes the gas and heats the dust. The clumpy density distribution is responsible for the complicated appearance of the 10&mgr;m map.There is no direct or compelling evidence for star formation in the very center of the Galaxy. The appearance of the interstellar medium in the inner Galaxy is dictated by the presence of a nuclear source. We cannot presently tell if the nuclear source is powered by star formation or by some more exotic object, such as a black hole.

ISSN:0094-243X    

DOI:10.1063/1.36406

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The Sgr A West HII‐region is surrounded by an extended (R≤8–9pc) massive (Md≊2–5 104M0) disk of atomic and molecular gas. The structure’s overall inclination to the line‐of‐sight is ≊70°, and the major axis is tilted against the galactic plane. The thickness of the disk seems supported by turbulent pressure. The dominant large‐scale motion is rotation (vrot≊110 kms−1) about the nucleus. Recent high spatial resolution data reveal that the circumnuclear disk is not a planar equilibrium configuration, but is warped and kinematically perturbed on short timescales (≊105yr−1).The atomic and molecular line emission arises from an unusually warm (T≊300 K), dense (n≊105cm−3) and highly turbulent medium that occupies only a small volume of the emission region (fv≤0.1). The excitation of the gas is due to a combination of UV heating, for the atomic phase, and dissipation of turbulent energy in small‐scale low velocity shocks for the bulk of the molecular material.The disk‐like gas distribution may represent a parsec sized circumnuclear accretion disk, which in order to maintain the turbulence of the gas, must be in slow overall contraction towards the nucleus. The existence of the central cavity may require a recent (≊105yr ago) energetic nuclear event.

ISSN:0094-243X    

DOI:10.1063/1.36433

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The current observational status of Sgr A*, the compact nonthermal radio source at the galactic center, is reviewed. It is a unique radio source at a unique position of the Galaxy. It is unlike any compact radio sources associated with known stellar objects, but it is similar to extragalactic nuclear compact radio sources. The positional offset between Sgr A* and IRS16 places little constraint on the nature of the underlying energy source, since the nature of IRS16 itself is not well understood and may not be the core of the central star cluster. With its unique properties in the Galaxy and being the only unsual object at the center with dimensions approaching the gravitational radius of a ∼106M⊙black‐hole, Sgr A* is still the best candidate for marking the location of a massive collapsed object.

ISSN:0094-243X    

DOI:10.1063/1.36435

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

This paper reviews recent observations and interpretations of the structure and kinematics of the ionized gas within 2 parsecs of the galactic center. The ionized gas exists largely within several arm‐like features, each of which contains a great deal of internal structure. Two of the arms lie on the inside edge of the rotating ring of neutral material at a radius of 2 pc and take part in that rotation. Two other features, the northern arm and the bar, pass much closer to the center. Their kinematics are not as well understood, although the northern arm appears to define a trajectory of infall towards th IRS 16/Sgr A* complex. A small, but spatially extended region of high velocity gas is found at the location of the above complex. This gas is probably undergoing radial motion and apparently is distant (∼0.05 pc) from the source of its motion; it may be the interface between gas in the bar and an as yet undetected outflowing wind from an active source in the nucleus.

ISSN:0094-243X    

DOI:10.1063/1.36438

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Recent observations suggest that galactic line emission at 511 keV results from the superposition of contributions from a variable, compact source and an interstellar distribution of positrons resulting from the decay of radionuclei produced by thermonuclear burning supernovae. The compact point source could have turned on as recently as 1977 and has not been seen since 1979. Photon‐photon pair production in the vicinity of a relatively small black hole (<103M⊙) could be the source of the annihilating positrons in the point source. It is not known whether this compact object lies exactly at the Galactic Center.

ISSN:0094-243X    

DOI:10.1063/1.36436

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

We review the history of the controversies surrounding the interpretation of the velocity dispersion and light profile in the center of M87. The most recent theoretical work suggests that the highly anisotropic velocity ellipsoid, which had been proposed as an alternative to a central massive object, is unstable to the formation of a bar. The current observations are just consistent with either a central, essentially point, mass or with a massive star cluster of the kind proposed by Dressler. However, the most recent observations of the center of M31 and M32 are difficult to reconcile with anything but a central point mass.

ISSN:0094-243X    

DOI:10.1063/1.36401

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

There is strong circumstantial evidence for a ∼106M⊙black hole at the Galactic Center. Such an object could account naturally for the unusual compact radio source. Capture and destruction of stars by the hole could lead to directional ejetion of some of the debris; this phenomenon might be relevant to the energetics and morphology within the central   cavity.’’

ISSN:0094-243X    

DOI:10.1063/1.36402

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Past studies have indicated that the stellar population at the Galactic Center includes a relatively young component with Source 7 being perhaps no older than 40 million years1. The extent of the young population could not be easily judged on the basis of older work. Preliminary results from a 2 &mgr;m survey covering an area of 400 square parsecs (5’×5’) of the Galactic Center are presented here. That a large amount of star formation has occurred recently is confirmed.

ISSN:0094-243X    

DOI:10.1063/1.36403

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

We have obtained high‐resolution 2‐&mgr;m spectra of six late‐type stars within 2 pc of the Galactic Center. We have derived spectral types, reddenings, and radial velocities for these stars. We find the supergiant density in the Galactic Center is lower than in previous determinations. No correspondence is found between stellar velocities and Ne II velocities observed along the same line of sight. No systematic rotation or ordered motion is seen in the stellar velocity distribution. The mass distribution derived from the stellar velocities is compared to those derived from gas velocities and from the 2‐&mgr;m light.

ISSN:0094-243X    

DOI:10.1063/1.36404

出版商:AIP    

年代:1987

数据来源: AIP