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Astron. Astrophys. 348, 768-782 (1999)

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2. Investigating the nuclear bulge: research program and scientific objectives

Observations of AGNs show that their central engine , i.e. a Black Hole of mass [FORMULA] surrounded by an accretion disk, is embedded in a Nuclear Bulge of size of a few 102 pc, which appears to play an important role in stabilizing the Disk and initiating the mass flow which feeds the Black Hole. This motivated us to investigate the physical state of the Nuclear Bulge in our Galaxy. Here we present the first in a series of papers which combine ground-based and space-born (ISO 3) NIR/MIR observations with submm and radio observations of the central [FORMULA]. An average extinction of [FORMULA] [FORMULA] mag prevents any direct observations of the central few tens of parsecs at [FORMULA]m. The transition from dust-dominated to star-dominated radiation of the Nuclear Bulge occurs at [FORMULA]m. Hence stars in the Galactic and Nuclear Bulge - specifically medium and low mass stars - are preferentially observed in the NIR. High mass stars - in addition to being strong NIR sources - interact with the ISM by heating the dust and ionizing the gas. Dust emission at MIR/FIR wavelengths together with free-free radio emission trace these massive stars and give information about both their total ([FORMULA]) and Lyc luminosity ([FORMULA]), respectively.

The K band ([FORMULA]m) is a good compromise between a dust absorption increasing with [FORMULA], where [FORMULA] for [FORMULA]m, and the stellar flux density increasing with [FORMULA]. Since the seminal papers by Becklin & Neugebauer (1968, 1975 and 1978) who mapped the central 100 pc, the K band has extensively been used to investigate the stellar content of the Central Region (MDZ96, Sects. 2.2.1 and 5.3). These observations yielded a surface brightness variation [FORMULA] which - after having been calibrated with the observed rotation curve - translates for an assumed [FORMULA] into a mass enclosed within the radius R of


which can be used as a first approximation of the true mass distribution within [FORMULA] pc (Sanders & Lowinger 1972, corrected for [FORMULA]kpc). However, ten years after their first K band map was published Becklin & Neugebauer (1978) cautioned that due to their beam-switching observing techniques an extended continuum background may have been suppressed. The K band observations presented in this paper not only have a high sensitivity for the detection of point sources. The observing method in addition is very effective in recovering an extended background continuum which accounts for low-luminosity stars with [FORMULA]Jy. We find that the K band emission from the central [FORMULA] pc consists of [FORMULA] individual sources with [FORMULA]Jy or 4 [FORMULA] mag with an average surface density of [FORMULA] 80 sources pc-2 which are superimposed on an unresolved background. This detection limit corresponds to reddened (for a visual extinction of [FORMULA] mag) Main Sequence (MS) stars earlier than B5 or Giants more luminous than spectral type A1 to G0 (luminosity class III; see Appendix D).

The detection limit is determined by the angular resolution and sensitivity of the NIR array. The completeness limit, in addition, also depends on the source crowding, i.e. the number of stars per pc2 and their intensity. In the central pc2 the crowding of stars attains a maximum. Eckart et al. (1993) using speckle techniques were able to resolve [FORMULA] 350 stars pc-2 with a detection limit of [FORMULA]Jy, corresponding to [FORMULA] mag. It is found that in the central [FORMULA] a very limited number of high and intermediate mass stars, most of which must have formed during the past 108 yr, account for [FORMULA] of the K band integrated flux density, [FORMULA] of the total luminosity and all of the Lyc-photon production rate of [FORMULA] and [FORMULA]. Low-luminosity stars, on the other hand, which together form the unresolved continuum background, account for [FORMULA] of the stellar mass 5. Preliminary observational results suggest that beyond the central [FORMULA] the relative contribution of luminous stars to the K band flux density decreases with increasing distance from Sgr A* (MDZ96, Sects. 4.3 and 5.3).

The scientific objective of this paper is to learn more about the stellar population in the central [FORMULA] 30 pc of the Nuclear Bulge. This information can only be obtained at wavelengths [FORMULA]m. We present and discuss the following observational results: i) A K band mosaic image of size [FORMULA] surrounding Sgr A* which covers [FORMULA] of the total area of the Nuclear Bulge. ii) A [FORMULA] mm mosaic map of the dust emission from a more extended region of the Nuclear Bulge obtained with the IRAM 30m-MRT. iii) The K band luminosity function (KLF) of the sources contained in the mosaic image and in three subareas. iv) The radio through NIR spectrum spatially integrated over the central [FORMULA] and its decomposition into contributions by dust, stellar, free-free and free-bound emission.

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© European Southern Observatory (ESO) 1999

Online publication: August 13, 199