Andromeda

Abstract: We report here the discovery of an apparent excess of 2MASS M giant candidates with dereddened 0.85 b > -40° and covering most of the constellations of Triangulum and Andromeda. This structure does not seem to be preferentially distributed around a clear core, but rather lies in a tenuous, clumpy cloudlike structure tens of kiloparsecs away. The reduced proper-motion diagram, as well as spectroscopy of a subsample, shows these excess stars to be real giants, not contaminating dwarfs. Radial velocity measurements indicate among those M giants the presence of a coherent kinematical structure with a velocity dispersion σ < 17 km s-1. Our findings support the existence of a quite dispersed stellar structure around the Milky Way that, because of its coreless and sparse distribution, could be part of a tidal stream or a new kind of satellite galaxy.

Abstract: We present a systematic and homogeneous analysis of population gradients for the Local Group dwarf spheroidals (dSphs) Carina, Sculptor, Sextans, Tucana, Andromeda I-III, V, and VI. For all of the Milky Way companions studied here we find significant population gradients. The same is true for the remote dSph Tucana located at the outskirts of the LG. Among the M 31 dSph companions only Andromeda I and VI show obvious gradients. In all cases where a HB morphology gradient is visible, the red HB stars are more centrally concentrated. The occurence of a HB morphological gradient shows a correlation with a morphology gradient in the red giant branch. It seems likely that metallicity is the driver of the gradients in Sextans, Sculptor, Tucana, and Andromeda VI, while age is an important factor in Carina. We find no evidence that the vicinity of a nearby massive spiral galaxy influences the formation of the population gradients.

Abstract: There are two fundamentally different physical origins of faint satellite galaxies: cosmological sub-structures that contain shining baryons and the fragmentation of gas-rich tidal arms thrown out from interacting galaxies during hierarchical structure formation. The latter tidal-dwarf galaxies (TDG) may form populations with correlated orbital angular momenta about their host galaxies. The existence of TDGs is a stringent necessity because they arise as a result of fundamental physical principals. We determine the significance of the apparent disc-like distribution of Milky Way (MW) satellite galaxies. The distribution of the MW satellites is found to be inconsistent with an isotropic or prolate DM sub-structure distribution at a 99.5 per cent level including the recently discovered UMa and CVn dwarf spheroidal galaxies. The distribution is extremely oblate and inclined by about 88 ◦ with respect to the the MW disc. We also apply the methods to Andromeda’s (M31) satellite galaxies using two recently published data-sets. It can not be excluded that the whole population of M31 companions is drawn randomly from an isotropic parent distribution. However, two subsamples of Andromeda satellites are identified which have disc-like features. A kinematically motivated subsample of eight Andromeda satellites forms a pronounced disc-like distribution in both data-sets. The existence of this disc would be inconsistent with a CDM parent distribution of subhaloes if the disc is rotationally supported. The M31 satellite distribution is inclined by about 59 ◦ with respect to the M31 disc, and has virtually the same orientation as the disc derived for the whole M31 satellite sample. We present a new geometric method to set restrictions on possible locations of angular momentum vectors for Andromeda satellites. Our conclusion is that both, the MW and M31, may indeed have satellite galaxies derived from TDGs. Further, both host-discs and both identified discs-of-satellites are highly inclined relative to the supergalactic plane. The discs-of-satellites therefore cannot be created from individual accretion events from the supergalactic plane further supporting the possibility that they are of TDG origin.