Intrinsic shape of bulges in disk galaxies

Evidences suggesting that bulges of non-barred disk galaxies could be triaxial structures come from different aspects. From the photometry, the twist of the inner isophotes and the misaligment between the disk and the bulge major axis position angle are not possible under the hypothesis of symmetry for both components. From the kinematics, rotation curves with velocities higher than permitted by circular motions or velocity gradients along the galaxy minor axis can be understood as signs of triaxiality. If triaxiality is a common feature in bulges, it could have important implications on bulge formation and evolution. Asymmetries in the potential could provide a method of transporting gas into the galaxy center, and therefore must be consider when accounting for fuelling of central starbursts, for the formation of inner rings or for the formation and destruction of bars due to the build up of gas in the galactic center. Following the previous statistical works on bulges triaxiality (Zaritsky & Lo 1986; Bertola et al. 1991), we have used a photometric approach to derive the probability distribution function (PDF) of the intrinsic axis ratios of bulges. In our work, we improve the previous ones in several aspects. First, we use a more extensive (148 galaxies) and better defined sample of non-barred disk galaxies. Second, we use a fully bidimensional code to derive the photometric parameters of galaxies in the sample, using images in the J-band taken from the 2MASS archive. And third, we determine the PDF using a new method where the geometry of the system is inverted analytically. As a preliminary result, we have found for our sample of bulges that ~44% are close to be oblate ellipsoids, ~16% are close to prolate ellipsoids, ~5% are close to be spheres and the remaining ~34% are definitely triaxial ellipsoids.