A New Bar in the Center of the Milky Way

The Milky Way is a spiral galaxy, and contains as two thirds of them a stellar bar in its center. The presence of this bar is suggested by Doppler velocities of the central gas since about thirty years, and was confirmed by the infra-red images of the COBE satellite in the Nineties. The recent near infra-red all sky survey by the american project 2MASS brings today more resolution. An astronomer of Paris Observatory, Christophe Alard, proposes the existence of a second bar embedded in the first one. This second bar would bring more insight in the central matter dynamics, and for instance provide a mechanism to fuel the central black hole.

Our Galaxy, the Milky Way, cannot be observed as a whole at visible wavelengths, because dust hides from us the remote parts. An optical photograph, therefore, only reveals the foreground areas.
The low sensitivity of the infra-red light to the interstellar extinction makes it possible to reveal the structure of the Galaxy with an astonishing quality. The NASA COBE satellite carried out an infra-red survey of our Galaxy, as can be seen in Figure 1.

Figure 1: A view of the Galaxy obtained by the COBE satellite in the near infra-red

However the spatial resolution of COBE is low, and this can be largely improved now thanks to the new near infra-red all sky survey, recently made available, by 2MASS (cf Figure 2).

Figure 2: A view at high resolution of the center (bulge) of the Galaxy obtained from the stellar catalogue of the 2MASS survey

The strong and variable interstellar extinction which dominates in the area of the Galactic bulge makes particularly difficult to obtain a true map of the center, even in the K-band (at 2 microns wavelength). It is however possible to reduce considerably the effects of extinction by combining 2 photometric bands in order to correct for obscuration. After applying several adaptive smoothings and using the symmetry properties, star counts in this bands corrected for extinction lead to the map below (Figure 3). The 3 last isocontours of this map illustrate perfectly the asymmetry in longitude (left-right asymmetry) which reveals the central bar structure of the Galactic disc. Indeed, the bar is tilted with respect to the line of sight, and the side coming towards us (on the right), since it is nearer appears larger than the more distant side (on the left).

Figure 3: Detailed image of the central area after subtraction of the density associated with the first Galactic bar

Thus is highlighted a new component: a small bar close to the center of the principal bar. It is a secondary bar, embedded in the first larger bar, already suspected since the years 1970, and confirmed by the COBE satellite. The image of Figure 3 represents the residual density obtained after subtraction of the primary bar. This density is very significant and dominant within less than 1 degree of the Galactic center. The subtraction of the primary bar could be carried out without restrictive assumptions about its shape. The only required assumption for this subtraction is that the surface density of the bar decreases exponentially with radius. This assumption is well verified in numerical simulations.
Let us note the strong asymmetry in longitude of the secondary bar. It is interesting to notice that this asymmetry is opposite in direction to that observed for the primary bar. This difference in orientation cannot be due to a residual effect of extinction, because the extinction is higher in the region of maximum star counts. This means therefore that a secondary component clearly exists. Moreover the fact that this component is flattened and asymmetrical indicates that it most likely has a bar shape. The presence of a secondary bar, inside the primary bar, is a common phenomenon for barred spiral galaxies like the Milky Way (figure 4). It will certainly be confirmed by the GAIA satellite.

Figure 4 Examples of secondary bars embedded in primary bars, in the barred spiral galaxies NGC 1433 (left), and NGC 5566 (right), from Jungwiert, Combes, Axon (1997, A&AS 125, 479)

Reference:
Alard, C. "Another Bar in the Bulge", accepted in Astronomy & Astrophysics astro-ph/0110491

Contact:
C. Alard (DASGAL, Observatoire de Paris, and IAP)


Figure 4 Examples of secondary bars embedded in primary bars, in the barred spiral galaxies NGC 1433 (left), and NGC 5566 (right), from Jungwiert, Combes, Axon (1997, A&AS 125, 479)