MORPHOLOGY, NEAR-INFRARED LUMINOSITY, AND MASS OF THE GALACTIC BULGE FROM COBE DIRBE OBSERVATIONS

Near-infrared images of the Galactic bulge at 1.25, 2.2, 3.5, and 4.9 mu m obtained by the Diffuse Infrared Background Experiment (DIRBE) on board the Cosmic Background Explorer (COBE) satellite are used to char acterize its morphology and to determine its infrared luminosity and m ass. Earlier analysis of the DIRBE observations (Weiland et al. 1994) provided supporting evidence for the claim made by Blitz and Spergel ( 1991) that the bulge is bar-shaped with its near end in the first Gala ctic quadrant. Adopting various triaxial analytical functions to repre sent the volume emissivity of the source, we confirm the barlike natur e of the bulge and show that triaxial Gaussian-type functions provide a better fit to the data than other classes of functions, including an axisymmetric spheroid. The introduction of a ''boxy'' geometry, such as the one used by Kent, Dame, and Fazio (1991) improves the fit to th e data. Our results show that the bar is rotated in the plane with its near side in the first Galactic quadrant creating an angle of 20 degr ees + 10 degrees between its major axis and the line of sight to the G alactic center. Typical axis ratios of the bar are {1:0.33 +/- 0.11:0. 23 +/- 0.08}, resembling the geometry of prolate spheroids. There is n o statistically significant evidence for an out-of-plane tilt of the b ar at 2.2 mu m, and marginal evidence for a tilt of approximate to 2 d egrees at 4.9 mu m. The introduction of a roll around the intrinsic ma jor axis of the bulge improves the ''boxy'' appearance of some functio ns. A simple integration of the observed projected intensity of the bu lge gives a bulge luminosity of 1.2 x 10(9), 4.1 x 10(8), 2.3 x 10(8), and 4.3 x 10(7) L., respectively, at 1.25, 2.2, 3.5, and 4.9 mu m wav elength for a Galactocentric distance of 8.5 kpc. The 2.2 mu m luminos ity function of the bulge population in the direction of Baade's windo w yields a bolometric luminosity of L(bol) = 5.3 x 10(9) L.. Stellar e volutionary models relate this luminosity to the number of main-sequen ce progenitor stars that currently populate the red giant branch. Comb ined with the recent determination of the main-sequence turnoff mass f or the bulge by the Hubble Space Telescope (Hortzman et al. 1993) we d erive a photometrically determined bulge mass of approximate to 1.3 x 10(10) M. for a Salpeter initial mass function extended down to 0.1 M. .