No abstract

We identify new structures in the halo of the Milky Way Galaxy from positions, colors and magnitudes of five million stars detected in the Sloan Digital Sky Survey. Most of these stars are within 1.26 degrees of the celestial equator. We present color-magnitude diagrams (CMDs) for stars in two previously discovered, tidally disrupted structures. The CMDs and turnoff colors are consistent with those of the Sagittarius dwarf galaxy, as had been predicted. In one direction, we are even able to detect a clump of red stars, similar to that of the Sagittarius dwarf, from stars spread across 110 square degrees of sky. Focusing on stars with the colors of F turnoff objects, we identify at least five additional overdensities of stars. Four of these may be pieces of the same halo structure, which would cover a region of the sky at least 40 degrees in diameter, at a distance of 11 kpc from the Sun (18 kpc from the center of the Galaxy). The turnoff is significantly bluer than that of thick disk stars, and closer to the Galactic plane than a power-law spheroid. We suggest two models to explain this new structure. One possibility is that this new structure could be a new dwarf satellite of the Milky Way, hidden in the Galactic plane, and in the process of being tidally disrupted. The other possibility is that it could be part of a disk-like distribution of stars which is metal-poor, with a scale height of approximately 2 kpc and a scale length of approximately 10 kpc. The fifth overdensity, which is 20 kpc away, is some distance from the Sagittarius dwarf streamer orbit and is not associated with any known structure in the Galactic plane. It is likely that there are many smaller streams of stars in the Galactic halo.Comment: ApJ, in press; 26 figures including several in colo

We present a deep photometric survey of the Andromeda galaxy, conducted with the wide-field cameras of CFHT and INT, that covers the inner 50 kpc of the galaxy and the southern quadrant out to $150 kpc and includes an extension to M33 at >200 kpc. This is the first systematic panoramic study of this very outermost region of galaxies. We detect a multitude of large-scale structures of low surface brightness, including several streams, and two new relatively luminous (M V $ À9) dwarf galaxies: And XV and And XVI. Significant variations in stellar populations due to intervening streamYlike structures are detected in the inner halo, which is particularly important in shedding light on the mixed and sometimes conflicting results reported in previous studies. Underlying the many substructures lies a faint, smooth, and extremely extended halo component, reaching out to 150 kpc, whose stellar populations are predominantly metal-poor. We find that the smooth halo component in M31 has a radially decreasing profile that can be fitted with a Hernquist model of immense scale radius $55 kpc, almost 4 times larger than theoretical predictions. Alternatively a power law with AE V / R À1:91AE0:11 can be fitted to the projected profile, similar to the density profile in the Milky Way. If it is symmetric, the total luminosity of this structure is $10 9 L , again similar to the stellar halo of the Milky Way. This vast, smooth, underlying halo is reminiscent of a classical ''monolithic'' model and completely unexpected from modern galaxy formation models. M33 is also found to have an extended metal-poor halo component, which can be fitted with a Hernquist model also of scale radius $55 kpc. These extended slowly decreasing halos will provide a challenge and strong constraints for further modeling.