The Galactic Center
The nucleus of our Galaxy is located ~ 8.5 kpc from the Earth, more than 100 times closer than the nearest large external galaxy, or one thousandth of the distance to the nearest active Galaxy. Its relative proximity allows detailed views of the structure and physical conditions in the Galactic Nucleus, some of which almost certainly are present in the more distant galactic nuclei. The distribution of material in the central region of the Galaxy has been summarised by Genzel & Townes (1987), Genzel, Hollenbach & Townes (1994) and Zylka et al (1994, 1995). An unusual compact (~ 1 AU - Krichbaum et al 1993, Rogers et al 1994) variable radio source close to the dynamical centre of the Galaxy, SgrA*, has been proposed to be a quiescent black hole, or a stellar cluster.
This figure at the top of the page shows part of a submillimetre continuum survey of the Galactic Centre that we have made with SCUBA on the JCMT. SCUBA's on-the-fly scan-map mode has allowed us to make extremely wide-field maps of thermal dust emission with unprecedented speed and sensitivity.
The Galactic Centre is of great interest, being both an extreme region of our own galaxy - containing much active star formation - and the nearest example of a galactic nucleus. Our ultimate aim is to map as much as possible of the `Central Molecular Zone', which contains up to 10% of the Galaxy's molecular ISM. Thermal dust continuum emission allows us to make an unbiased map of the temperature weighted column-density of material, and hence derive the total masses of the molecular clouds, find regions of star formation, and investigate cloud structures.
This new survey covers a significantly greater area than previous maps (cf. the 800 micron map made at the CSO by Lis & Carlstrom 1994, ApJ, 424:189-199) and we are also able to make spectral index maps using the simultaneously observed 450 micron and 850 micron data. The current reduced map has a total size of approximately 100 x 30 arcmin. It covers the SgrA region - including SgrA*, the circumnuclear disc, and the 20km/s and 50km/s clouds; the area around the Pistol; SgrB2 - the brightest feature on the map; and at its Galactic Eastern and Western edges, the Sgr D and Sgr C regions. The images reveal many rich, striking features such as filaments and shell-like structures, as well as point sources such as SgrA* itself and a set of methanol maser sites. The survey has resolutions of 8 arcsec at 450 micron and 14 arcsec at 850 microns, corresponding to distances of 0.33 pc and 0.58 pc at a distance of 8.5 kpc. The sensitivities per beam are approximately 30 mJy and 300 mJy at 850 and 450 microns respectively, or ~3 Msuns per beam. The total mass detected in the current region is 1 x 10^7 Msun, assuming a dust temperature of 30 K. We have derived fluxes and mass estimates for the prominent features in the survey area, and have used the 450 and 850 micron data to produce a spectral index map, and hence a point-to-point map of the dust opacity index (assuming knowledge of the dust temperature). For example, assuming a dust temperature of 30K, we estimate the masses of the 20km/s and 50km/s clouds (M-0.13-0.08 and M-0.02-0.07, near SgrA) as 150,000 Msun and 80,000 Msun respectively.
The total mass in the vicinity of these two clouds is ~500,000 Msun. The maps have also been used to calculate sub-millimetre fluxes of the point-source SgrA*, complementing SCUBA polarimeter jiggle-map observations of the same source. The 450-2000 micron spectrum of SgrA*, together with its polarisation, is discussed by Aitken et al. (ApJL, submitted). We have also investigated many issues related to the reduction of SCUBA scan-map data.
Within 1 arc second of SgrA* is a bright infrared complex, IRS16, which contains at least 15 components, and is probably the source of the strong stellar wind which has velocities of up to ~ 700 km s-1. Surrounding this is a cavity, which is dominated by atomic and ionised gas, which contains a total mass ~ 1 - 3 106 Mo, and has a luminosity ~ 7 106 - 2.3 107 Lo. Almost all of this luminosity is absorbed by material lying close to the Centre, and then re-emitted as IR continuum, or via atomic and molecular line emission (Av to the Nucleus = 30 magnitudes). Zylka et al (1995) show that the column density of warm (T ~ 200 - 400 K) absorbing dust in the centre of the cavity is much lower from the surrounding Circum-Nuclear Disc (CND); a torus of dense and warm atomic and molecular gas and dust.
The CND is a ring of gas and dust, rotating around the central ionised cavity with a velocity ~ 110 km s-1 and having a mass ~ 10^4 Mo of clumpy material. First recognised from the far-infrared observations by Becklin, Gatley & Werner (1982) it extends from a radius of 1.5 - 2 pc from the centre out to ~ 6 - 8 pc, and may be connected to the gravitational potential of material in the Galactic Centre (Duschl 1989). Molecular line studies showed that the CND was composed predominantly of highly excited gas, co-rotating in the same general direction as the rest of the Galaxy (Fukui et al 1977, Genzel et al 1982, Harris et al 1985, Sandqvist & Loren 1985, Serabyn et al 1986, Gusten et al 1987 a,b). The neutral material in this ring is believed to be hot (T ~ 150 - 450 K), dense (nH2 ~ 10^3-7 cm-3), and highly fragmented (Genzel et al 1982, 1988 - filling factor ~ 10 per cent, Zylka et al 1995). This clumpy structure allows ultraviolet radiation from the central ionised region to penetrate into the neutral ring, heating and photo-ionising the gas.
The likely source of this radiation is the cluster of hot, luminous stars, some of which are the HeI / HI stars detected by Krabbe et al (1991).The presence of hot gas was independently inferred from the detection of near-IR H2 emission, which suggested that the material was shock excited at the edge of the ionised region (Gatley et al 1984). The neutral gas in the ring appears to be dynamically coupled to the gas in the central cavity of ionised and atomic gas, and has its rotational axis close to that of the larger scale Galactic rotation.
The sharp edge of the CND abuts directly against the ionised arc-like filaments (The Mini-spiral), which has led to speculation that material from the CND may be infalling towards the Centre along the ionised arms of the Mini-spiral. The submillimetre continuum data (Zylka et al 1995) show that the central ~ 30 arc second diameter cavity contains ~ 400 Mo of dust at T ~ 40 K, 4 Mo at ~ 170 K and ~ 0.01 Mo at ~ 400 K. The luminosity of the emitted radiation for these three phases is ~ 2 10^6 Lo, 4 10^6 Lo and 5 10^5 Lo respectively. The hottest gas lies close to Sgr A*, whilst the brightest far-infrared emission comes from the transition region between the neutral CND and surrounding HII regions such as Sgr A East (a synchrotron shell-like source lying behind the central HII region (Pedlar et al 1989), which has probably been formed as the result of and explosive event inside its core) and Sgr A West - the central HII region within which Sgr A* is embedded (Dent et al 1993).
Lying further out are two molecular clouds, M -0.02-0.07 (also known as the 50 km s-1 cloud) and M -0.13-0.08 (also known as the 20 km s-1 cloud). These two clouds are believed to be connected together with the core of Sgr A East (Mezger et al 1989, Okumura et al 1991, Ho 1994).
Molecular line observations can be used to examine a) where and how much hot gas lies close to the ring, b) what is the spatial variation of the chemistry - are we for instance preferentially observing highly excited material being stripped away from the inner edge of the ring, or is it excited by an high ultra field, c) how clumpy and fragmented is the ring, d) how do the relative distributions of atomic (CI) and molecular (CO) gas differ in this region to that of other molecular clouds, e) how is the material heated, and f) are there signatures of photoionisation at inner edge of the ring.