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I review current work on the Galactic bulge, with emphasis on issues that may connect to the environment of the Galactic Center. There is growing evidence that the field population of the bulge at RGC > 500pc is as old as the metal rich Galactic center globular clusters, and that field and clusters have the same spatial and metallicity distribution. We suggest that by analogy, extragalactic metal rich cluster systems, which also tend to follow the spheroid light, are old. On the other hand, there has been long standing evidence for an age gradient toward the Galactic center, and recent observations confirm without doubt that there is active star formation there. If a long-lived bar has been funneling gas (and inducing star formtion) in the central 100 pc, the star formation history there will be complicated and interesting.
We obtained WFPC2 high spatial resolution images, in filters F555W and F814W, of a few fields in the bulge of M31. Two are located SW along the major axis of M31, respectively at 6.1 and 3.2 arcmin of the galaxy nucleus; another is located NE along the major axis at 3.7 arcmin. Adopting 1 arcmin = 250 pc from Rich & Mighell (1995), these separations correspond to projected distances of about 1.55, 0.80, and 0.92 kpc, respectively. We present here some of our results after analysis of the I; V – I colour magnitude diagrams of the bulge stellar population, which are part of an extensive work to be published elsewhere (Jablonka et al. 1997).
The disk of the Milky Way galaxy shows evidence for gas-phase abundances which increase with decreasing radius (Simpson et al. 1995; Afflerbach et al. 1997). Sustained star formation in the center of the Milky Way Galaxy may be fueled by inflow of inner disk gas (Serabyn & Morris 1996), suggesting that Galactic Center (GC) stars may be metal-rich. Measurements of stellar abundances in the GC allow us to explore the chemical evolution of our Galaxy's nucleus and to infer its star formation history.
Mira variables span a wide range of stellar populations and have a well-defined K, log P relation (e.g. Feast et al 1989; Hughes & Wood 1990), so can be used to map out the 3-D distribution of the bulge. Using IRAS-selected Miras, Whitelock & Catchpole (1992) showed for the first time from individual stellar distances that the bulge had a bar-like structure, but there is some uncertainty as to whether they obey the same K, log P relation as optically selected Miras. Hence the need for confirmation from an optical Mira survey.
We are studying the kinematic structure of our Galaxy by detecting SiO maser lines around 43 GHz with the Nobeyama 45-m telescope. Till today, about 500 IRAS sources (AGB stars) in the central region of the Galaxy are observed and the radial velocities of about 300 detected sources are obtained. The longitude-velocity map of SiO sources clearly shows a presence of forbidden regions (holes) which are located at the same positions of HI and CO holes. The presence of holes is interpreted as a streaming motions of stars due to a bar structure of the Galactic bulge
An overview is given of the results obtained about Galactic Structure studies towards the Galactic Centre from star counts. The results indicate the presence of an old metal-rich population (Z = 0.005–0.08; t = 13–15 Gyr), presumably the old Galactic Bulge. In addition, a younger, less metal-rich population was found (Z = 0.005-0.03; t = 8–9 Gyr), likely related to the tri-axial bar found in other studies. In Baade's Window a ratio bulge/bar stars of approximately 1/2 was obtained from star counts.
We show that a recently discovered spiral-induced radial mass accretion process could account for the formation of the Galactic Bulge in a Hubble time. This process is thus expected to be important in the formation of bulges in spiral galaxies, and in the secular evolution of galaxies along the Hubble sequence from late to earlier types.
We compute the stellar population of nearby Seyfert host galaxies and non-active galaxies using population and evolutionary synthesis methods. We find indication of enhanced star formation rates in the host-galaxies of AGN - especially in the circumnuclear regions of Seyfert 2 galaxies.
The knowledge of age and spatial distribution of stars in the Galactic bulge require observational constraints to establish whether its stellar population is very old (Larson 1990) or is a younger, disk-like component (Raha et al. 1992), and if its shape is spherical or extended, or perhaps a bar (Blitz & Spergel 1991). Yet other possibilities are a flattened bulge or a disk-like system (Zinn 1985; Armandroff 1989; Ortolani et al. 1993; Minniti 1995).
After the classical description of bulges by a de Vaucouleurs profile was found to be inadequate, a generalized profile, Sersic's law, was used successfully to describe the surface brightness:
(Andredakis et al. 1994 (APB95)). The exponent n was found to vary systematically with the morphological type of the galaxy, from n = 4 for the bulges of S0s to n = 2 for intermediate type spirals and n = 1 (pure exponential) for the late types. (APB95, de Jong 1996). This has been confirmed also by the kinematics (Heraudeau et al 1996). This variation of n has been interpreted in two ways: (i) As the effect of the disk forming around an already developed bulge (APB95) and (ii) as evidence that the bulge originated from secular processes in the galaxy, after the disk was formed (Courteau et al. 1996). This needs to be resolved.
We present a comparative study of two metal-rich globular clusters in the Galactic bulge, NGC 6316 and NGC 6624, based on Washington CCD photometry. These two clusters are very close to the Galactic center. NGC 6624 is known to be dynamically in a post-core-collapse stage, while NGC 6316 is not. CMT1T2 CCD images of these clusters were obtained at the CTIO 0.9m telescope. We adopt the reddening values of E(B – V) = 0.61 for NGC 6316 and E(B – V) = 0.26 for NGC 6624 in this study.
Is there an age and/or a metallicity gradient in the Bulge? This is a notoriously difficult question because of the well known age-metallicity-distance degeneracy in colour magnitude diagrams (CMD) as well as the severe crowding and large reddening towards the Galactic Bulge. The current observational data on the bulge in our galaxy and bulges in other spiral galaxies point in disparate directions, that is evidence for both early (e.g. existence of very old halo and bulge globular clusters) and late formation (e.g. Sgr dSph and bar instabilities) can be found as well as the existence and non-existence of metallicity gradients (for a review of the observational status see Wyse, Gilmore & Franx 1997). We here present the CMDs for two fields, Baade's window and SGR-I, Fig.1c and d. Both these regions have low extinction. To determine the age and metallicity for these stars we compare the CMDs with CMDs of globular clusters, also observed with WFPC2, of known metallicity and age, Fig.1a and b. This method enables us to work entirely in the in-flight magnitude system of WFPC2 and there is no need for transformations to standard colours and magnitudes, something which is not straight forward for WFPC2 passbands.
Recently it has been suggested that some bulges are really disks (Kormendy 1993). He argues that some bulges are not spheroidal components of steep luminosity distribution like elliptical galaxies but actually parts of flat disks, whose central luminosity profiles are steeper than the inward extrapolation of an exponential fit to the outer parts. The existence of disk-like bulges strongly supports the idea of disk secular evolution.
ISOGAL is a survey at 15 and 7 μm with ISOCAM of the inner Galactic disk. The survey covers ∼ 18 deg2 in selected areas of the central l = ±30 deg of the Galactic plane, all complemented by 0.8-2.2 μm DENIS data. Combined with the near infrared IJK data of the DENIS survey, it is mainly aimed at the study of the cold stellar populations of the most obscured regions of the inner Galaxy and the corresponding Galactic structure.
Nine regions in the Galactic bulge were observed at J and H bands with a PtSi 1040×520 array camera which is named the PtSi Astronomical Near Infrared Camera (PANIC). These regions were centered at l=(–5°,0°,+5°) b=(–6°,0°,+6°) respectively. Each region was covered with nine frames each of which spanned 30′ by 30′ square arcmin. Observations were carried out at the South African Astronomical Observatory, Cape Town, from 1995 to 1997 using a 40cm f/5 Newtonian telescope. A bolometric correction (Frogel and Whitford 1987) was given to the observed stars by using J-H values of reference RGB stars (Frogel et al 1990).
Using the Mid-Infrared (MIRS) on board the Infrared Telescope in Space (IRTS) we obtained the 4.5 to 11.7 μm spectra of the stellar populations and diffuse interstellar medium in the Galactic bulge (l ≈ 8.7°, b ≈ 2.9, 4.0, 4.7, and 5.7°). Below galactic latitute of 4.0° the mid-infrared background spectra in the bulge are similar to the spectrum of M and K giants. The UIR bands (6.2, 7.7, 8.6, and 11.3 μm) are also detected in these regions and they are likely arising from the diffuse interstellar medium in the bulge. Above galactic latitude of 4.0°, the mid-infrared background spectra are similar to the spectrum of those evolved stars with high mass-loss rate detected by IRAS. One likely interpretation is that this background emission arises predominantly from these stars with very low luminosities that have not been detected by IRAS. The main-sequence life time for such low luminosity evolved stars is at least 10 Gyr, even in the metal poor cases. If these low luminosity evolved stars are metal-rich then the age would be much older. Thus, the existence of a large number (~ 75) of such low luminosity evolved stars in a small region (8′ × 8′) in the bulge would have significant impact on our understanding of the stellar content and the age of the Galactic bulge.
Some observational data indicate that galaxy subsystems, including their central areas, first of all are the result of their global nonstationary evolution. That is why we earlier built (Nuritdinov 1992) the exact non-linearly pulsing rotating models of disklike and spherical self-gravitating systems. Unlike other authors we want to research the stability problem of nonlinear nonstationary models. In the present report we want to give only those results of the instability studied, which have a direct attitude to the subject under discussion. We put a certain question: what initial conditions have to exist, for instance, for the value of the virial parameter (2T/|U|)0 and the parameter of anisotropy < Tr > / < T⊥ >, that the collapse of a disk should result in a bar, and the spherical collapse will result in a thick ellipsoidal bulge. To answer the question it is very important to study stability of the solvable nonlinear unequilibrium models. All models discussed below pulsate under the law R = II(ψ)R0, where (Nuritdinov 1985)