The conditions within the nuclear bulge (NB, R ~ 300 pc) of our Galaxy are extreme: less than 0.1% of the volume of the galactic disc contains more than 10% of the total ISM and stars. There are indications that star formation in the NB is not a continuous process, but rather undergoes cyclic variations. Details of the current and past star formation processes and the stellar content of the central parsecs - especially the ratio of old to young population - are not very well known (Mezger et al., 1996).

We have used the 10-meter Keck I telescope and the camera mode of the long wavelength spectrometer to observe the central region of the Galaxy at 12.5 μm. The 96×70 As:Si array used had a scale of 0.114 arcsec per pixel. The filter was centered at 12.5 μm and had a bandwidth of about 1 μm. The array was flat-fielded using sky flats from the background. We observed the central 20×20 arcsec region (about 1pc × 1pc) by using a mosaic pattern of the 11×8 arcsec array in approximately half-array steps. The position of the array was determined after the fact by using structure in the flux in the overlap regions. The accuracy of the positioning was better than 0.1 arcsec. The resultant spatial resolution of the final map was about 0.7 arcsec FWHM based on the size of IRS 7 and IRS 3. The demonstrated diffraction limit of the phased Keck telescope at 12.5 microns is just over 0.3 arcsec FWHM, so that the final resolution is a result of atmospheric seeing and chopper smear. The final map is shown in Figure 1. The map is similar, but of much higher angular resolution, to the 12.5 μm map of Gezari (1992, The Center, Bulge and Disk of the Galaxy, ed. Blitz, Dordrecht: Kluwer, 23).

We have determined the position of Sgr A∗ with respect to the mid-infrared (5-25 m) sources in the central parsec by direct correlation of our 12.4 m array image (Gezari et al. 1994) and the new 2-cm continuum VLA map (Yusef-Zadeh 1997; private communication), without a priori knowledge of any other position determinations. Menten et al. (1997) recently succeeded in registering the radio and near-infrared (2.2 m) reference frames with high precision (+0.03 arcsec) using VLA observations of Sgr A∗, SiO masers and H2O masers. Unfortunately, registering the mid-infrared and radio reference frames with comparable accuracy cannot be done by applying the 2.2 m calibration. Most near-infrared sources have no detectable mid-infrared counterparts, and it is not obvious which of those that do are coincident (if any), since near-infrared and mid-infrared emission generally does not arise from the same physical source component. Dramatic examples of shifts between the brightest near- and mid-infrared peaks can be seen in Orion BN/KL and the Ney-Allen Nebula (Gezari and Backman 1994; Gezari, Backman and Werner 1997) corresponding to 0.1 - 0.5 arcsec if they were located at 8.5 kpc. Further, several Sgr A West IRS sources are displaced significantly in the infrared and radio, suggesting they may actually be compact clusters of objects.

The 3.4μm absorption feature, first detected in the Galactic center source IRS7 (Soifer et al., 1976), has been observed in other Galactic center sources IRS3, IRS12 (McFadzean et al., 1989), and IRS6E (Pendleton et al., 1994). This feature is regarded as characteristic of dust in the diffuse interstellar medium, and attributed to C-H stretching vibrations. Several objects in the lines of sight other than the Galactic center is now known to have the feature (Sandford et al. 1995 and references therein; Imanishi et al. 1996). The absorption depths per unit visual extinction τ3.4/AV for these sources are compared with those for the Galactic center sources, and they are thought to increase near the Galactic center. However, the “Galactic center” sources are all in the central parsec cluster, and the features observed in them may be only representative of interstellar medium local to the central parsec, not the general diffuse interstellar medium of the inner Galaxy. In this paper, we report the 3.4μm absorption feature detected in near-infrared sources within 1° of the Galactic center.

We report 36 to 50 μm observations of Arp 220 by the Goddard Cryogenic Grating Spectrometer on the Kuiper Airborne Observatory in May 1994. In this measurement, we find the galaxy to be four times brighter than in the measurements of Joy et al. (1986). If both of the observations are correct, this large far infrared luminosity increasing in a short time scale between the two observations suggests that the infrared emission in Arp 220 consists mostly of nonthermal synchrotron radiation which originates from the active nucleus.

Spectra at 16 - 45 μm of several regions within the central 80″ of the Galaxy have been obtained at 20″ resolution using the Goddard Cryogenic Grating Spectrometer No. 2 on the Kuiper Airborne Observatory. A broad band of excess emission extending from 24 to 45 μm is present in the spectra at positions covering the “tongue” and the inner edge of the circumnuclear disk. A similar dust emission feature has been observed in some carbon-rich evolved stars and in a nitrogen-rich evolved massive star. The observations reported here are the first detection of this dust emission feature in the interstellar medium. After considering several possible candidates of the carrier for this 30 μm dust feature, we find that MgS is the best owing to its good fit to the observed spectra. The origin of this ~ 30 μm feature in the Galactic center is unknown. Based on the theoretical results of dust condensation and elemental abundances in a supernova, we find that the supernovae in the central 500 pc could provide the amount of MgS dust, which we proposed as the carrier of the 30 μm dust feature, observed in the central 3 pc.

Starburst galaxies are powered by newly formed massive stars, and are heavily obscured by dust, prohibiting the UV and optical studies. A fine-structure line from Ne+ ion ([Ne II] 12.8 μm) is usually the brightest mid-infrared(MIR) line emitted from starburst galaxies. The [Ne II] line suffers from far less dust extinction than the optical line does, and hence it is often used as a probe for starburst properties of galaxies(Achtermann & Lacy, 1995; Roche et al., 1991). However, the [Ne II] line should not be used as the physical probe unless the relationship between Ne+ abundance and spectral type of the ionizing star is established for very dusty H II regions. To study this, we started the [Ne II] line survey toward dusty ultracompact(UC) H II regions in our Galaxy. The UC H II regions are powered by newly formed early-type stars whose spectral type can be inferred from their radio and far-infrared(FIR) properties.

The recent detection of a γ-ray flux from the direction of the Galactic center by EGRET on the Compton GRO raises the question of whether this is a point source (possibly coincident with the massive black hole candidate Sgr A∗) or a diffuse emitter. Using the latest experimental particle physics data and theoretical models, we have examined in detail the γ-ray spectrum produced by synchrotron, inverse Compton scattering and mesonic decay resulting from the interaction of relativistic protons with hydrogen accreting onto a point-like object. Such a distribution of high-energy baryons may be expected to form within an accretion shock as the inflowing gas becomes supersonic. This scenario is motivated by hydrodynamic studies of Bondi-Hoyle accretion onto Sgr A∗, which indicate that many of its radiative characteristics may ultimately be associated with energy liberated as this plasma descends down into the deep potential well. Earlier attempts at analyzing this process concluded that the EGRET data are inconsistent with a massive point-like object (Mastichiadis & Ozernoy, 1994). Our results demonstrate that a more careful treatment of the physics of p-p scattering suggests that a ~ 106M⊙ black hole may be contributing to this high-energy emission.

We study the radial profiles of radio comtinuum (RC) and far-infrared (FIR) emissions and also the gas to dust ratio for a sample of 22 spiral galaxies. For this, we use the data from IRAS HiRES FIR maps, available VLA 20 cm maps and radial profile data available on neutral hydrogen (H) and molecular hydrogen (H2, derived from velocity integrated CO intensity). In order to have enough resolution with the 1-1.5 arcminute IRAS beam, the galaxies chosen were large sized and had a 60 μm flux density of more than 5 Jy. The sample consists of: NGC 628, NGC 2403, NGC 2841, NGC 2903, NGC 3079, NGC 3198, NGC 3627, NGC 3628, NGC 4192, NGC 4254, NGC 4303, NGC 4321, NGC 4501, NGC 4535, NGC 4569, NGC 4656, NGC 4736, NGC 5033, NGC 5055, NGC 6503, NGC 6946 and NGC 7331. For 13 of these galaxies information is also available on metallicity (z) and V band CCD maps.

The nature of the nonthermal source Sgr A∗ has been the subject of extensive debate, with evidence mounting that it is in fact a ~ 2 × 106 M⊙ black hole (e.g. Eckart & Genzel 1997). Although previous Mid-Infrared (MIR, ~5-40 μm) observations (Gezari 1992; Telesco, Davidson & Werner 1996) have estabished upper limits on the flux of a potential source at Sgr A∗, they were unable to differentiate a point source at the location of Sgr A∗ from the background. Stolovy et al. (1996) reported the first detection of a compact source coincident with Sgr A∗ to ±0.3″, which they measured to have a flux value of 25±5 mJy. The existence of this source, and a better understanding of its flux and spectral characteristics are needed. We present here confirmation of a MIR source near Sgr A∗.

The Circum-Nuclear Disk (CND) is probably a by-product of the explosive event which created Sgr A East. Its inner edge of radius R ~ 1.7pc is well developed and comprises the Central Cavity (CC) which is filled with ~ 260 M⊙ of ionized gas. ISM appears to flow through the CND into the Central Cavity at a rate of M ~ 0.01 M⊙yr−1. Dust and Lyman continuum luminosities of the central 1.25 parsec, corrected for photons not absorbed by dust and gas inside the CC are ~ 108L⊙ and ~ 1051 Lyc photons s−1, most of which are provided by a cluster of 24 hot and massive stars, which also provide ~ 1/3 of the integrated K-band flux density (see the recent review by Mezger et al., 1996). A larger number of cool M- and K-giants and more than a million low-mass MS stars provide the remaining flux density, which comes from a diffuse background emission first seen in our NIR mosaic maps (Zylka et al., 1997).

In this short note, we summarize recent developments of the understanding of the physics of the Galactic Center. For a much more detailled compilation, we refer to our review paper (Mezger, Duschl & Zylka 1996, A&AR 7, 289) and to Beckert & Duschl (1997, A&A, in press).

The Sgr A East H ii complex consists of 4 compact H ii regions situated just east of and following, in an arc pattern, the edge of the Sgr A East nonthermal shell. Located between the arc of H ii regions and the nonthermal shell is a dense molecular ridge – presumably compressed – known as the “50 km/s cloud”. The hypothesis that these H ii regions delineate massive star formation provoked by the rapid expansion of Sgr A East into the molecular cloud is problematical because of the mismatch of the shell expansion and star formation time scales. We therefore examine the alternative hypothesis that Sgr A East is a quasi-static or slowly expanding structure fed from within by the release of relativistic particles from sources at or near the nucleus. The elongation of SgrA East along the Galactic plane is ascribed to the shear inherent in the velocity field this close to the Galactic center (GC). In this proceeding we discuss our ongoing efforts to model the effects of shear in detail, using the elongation of Sgr A East to constrain its expansion time scale.

Many characteristics of galactic nuclei may be associated with the accretion of ambient gas by a central concentration of mass. Using a 3D hydrodynamical code, we have been simulating this accretion process for Sgr A∗, the compact nonthermal source at the center of the Milky Way, in order to realistically model the gaseous flows in the inner parsec of our Galaxy. In the most recent simulations, we have taken into account the multi-point-like distribution of wind sources and we find that the structure of the flow can be significantly different from that due to a uniform medium. We here present our results concerning the mass and angular momentum accretion rates and discuss how these may be used to set constraints on our Galaxy's central engine.

The Fourier analysis of the observed velocity field of ionized gas in the inner 1.5 pc of the Galactic Center (obtained by Roberts and Goss, 1993) is made. As follows from the analysis, the observed field of residual velocities is dominated by the second Fourier harmonic. This fact can be treated as a consequence of the presence of an one-armed density wave with the density maximum along the Northern Arm plus the Western Arc structure. The wave nature of this structure is proved on the base of the behaviour of the phase of the second harmonic of line-of-sight velocity field in the whole region. The Fourier analysis shows also the presence of systematic radial velocity. We consider this flow as a quasi-stationary radial drift caused by one-armed nonlinear density wave (‘mini-spiral’).

Line-of-sight and proper motions data for stars near the Galactic center have evolved in recent years (see, e.g. Eckart & Genzel). The data were used for determining the star velocity dispersions in different directions (with a result that the velocity distribution is isotropic to within the observational error). Here we note that there exists a possibility for determining the star phase density itself from such data. The method is based on the solution of the corrensponding integral equations with their left sides being calculated from radial velocities and proper motions. Remind that discussion of the problems connected with reconstruction of the phase density from the radial velocity data was started by Merritt. There is some difference in applying this approach to self-gravitating systems (open and globular clusters) and clusters within external gravitational field. Among the latter is, for example, the inner parts of the Galactic nuclear cluster: they are governed by the field of the central black hole. It follows from the proper motions data estimates for a mass within a radius of 0.16 pc and within 0.083 pc given by Eckart & Genzel (cited above). In the case of the cluster in the field of the black hole we have no need of information about the densities σ(s) (s is the projected radius) and ρ(r) (since the method uses only the gravitational potential Φ(r), which here is foregone: Φ(r) = −GMh/r). Unfortunately, the data on the stars around the central black hole are yet inadequate to derive the distribution function with satisfactory accuracy. (Obviously the necessary data will be available before long.) So we restrict ourselves to only testing the computer algorithms used on the example of the simplest (Plummer) model, for which f(E) ∞ (–E)7/2.

National Astronomical Observatory, Japan has a plan to construct a new VLBI system named VERA (VLBI Exploration of Radio Astrometry). The system can measure precise positions of Galactic radio sources (H2O masers and SiO masers) referring to adjacent radio sources (QSO etc.) by using a differential VLBI method (Sasao and Morimoto, 1991).

The magnetic environment of the Galactic nucleus contrasts sharply with that of the Galactic disk. The inner few hundred parsecs of our Galaxy appear to be dominated by a strong (~milligauss) and uniform dipole field which dominates the pressure within the central intercloud medium. An attractive hypothesis for the origin of the central vertical field is that it results from the concentration of protogalactic field by radial inflow of gas throughout the Galaxy's lifetime. The predominant orientation of the magnetic field within dense molecular clouds is parallel to the galactic plane, which can be understood in terms of the strong tidal shear to which these clouds are subjected. The contrasting geometries of the cloud and intercloud fields allow for magnetic field line reconnection at cloud surfaces, which, under the right circumstances, could produce the relativistic electrons which delineate the nonthermal radio filaments near the Galactic center with their synchrotron emission. The characteristics of the Galactic center “magnetosphere” should be generalizable to all gas-rich spiral galaxies. Inadequate spatial resolution currently prevents us from exploring magnetic fields in other galactic nuclei to the same depth as in the Galactic center, but existing evidence is consistent with similar magnetic geometries elsewhere.

Recent observations have strengthened the dynamical arguments for the presence of black holes in X-ray binary systems, the Galactic center and the nuclei of the majority of bright galaxies. They have also drawn attention to the fact that accretion does not always proceed in the simple, stationary manner envisaged in early theories. I wish to discuss four related topics that are relevant to understanding the behavior of accreting black holes. (In view of the length constraints imposed by conference proceedings I shall make no attempt to give adequate bibliographic references for what follows.)

We present the first submillimeter polarimetric observations of Sagittarius A.