THE ENIGMATIC PAIR OF DWARF GALAXIES LEO IV AND LEO V: COINCIDENCE OR COMMON ORIGIN?

We have obtained deep photometry in two 1 degrees x 1 degrees fields covering the close pair of dwarf spheroidal galaxies Leo IV and Leo V and part of the area in between. From the distribution of likely red giant branch (RGB) and horizontal-branch (HB) stars in the data set, we find that both Leo IV and Leo V are significantly larger than indicated by previous measurements based on shallower data. With a half-light radius of r(h) = 4.'6 +/- 0.'8 (206 +/- 36 pc) and r(h) = 2.'6 +/- 0.'6 (133 +/- 31 pc), respectively, both systems are now well within the physical size bracket of typical dwarf spheroidal Milky Way satellites. Both are also found to be significantly elongated with an ellipticity of epsilon similar or equal to 0.5, a characteristic shared by many of the fainter (M-V > -8) Milky Way dwarf spheroidals. The large spatial extent of our survey allows us to search for extra-tidal features in the area between the two dwarf galaxies with unprecedented sensitivity. The spatial distribution of candidate RGB and HB stars in this region is found to be non-uniform at the similar to 3 sigma level. Interestingly, this substructure is aligned along the direction connecting the two systems, indicative of a possible bridge of extra-tidal material. Fitting the stellar distribution with a linear Gaussian model yields a significance of 4 sigma for this overdensity, a most likely FWHM of similar to 16 arcmin, and a central surface brightness of similar or equal to 32 mag arcsec(-2). We investigate different scenarios to explain the close proximity of Leo IV and Leo V, and the possible tidal bridge between them. Orbit calculations demonstrate that the two systems cannot share the exact same orbit, while a compromise orbit does not approach the Galactic center more than similar to 160 kpc, rendering it unlikely that they are remnants of a single disrupted progenitor. A comparison with cosmological simulations shows that a chance collision between unrelated subhalos is negligibly small. Given their relative distance and velocity, Leo IV and Leo V could be a bound tumbling pair, if their combined mass exceeds (8 +/- 4) x 10(9) M-circle dot. The scenario of an internally interacting pair that fell into the Milky Way together appears to be the most viable explanation for this close celestial companionship.