Within the hierarchical framework for galaxy formation, minor merging and tidal interactions are expected to shape all large galaxies to the present day. As a consequence, most seemingly normal disk galaxies should be surrounded by spatially extended stellar 'tidal features' of low surface brightness. As part of a pilot survey for such interaction signatures, we have carried out ultra deep, wide field imaging of 8 isolated spiral galaxies in the Local Volume, with data taken at

A candidate diffuse stellar substructure was previously reported in the halo of the nearby dwarf starburst galaxy NGC 4449 by Karachentsev et al. We map and analyze this feature using a unique combination of deep integrated-light images from the Black Bird 0.5-meter telescope, and high-resolution wide-field images from the 8-meter Subaru telescope, which resolve the nebulosity into a stream of red giant branch stars, and confirm its physical association with NGC 4449. The properties of the stream imply a massive dwarf spheroidal progenitor, which after complete disruption will deposit an amount of stellar mass that is comparable to the existing stellar halo of the main galaxy. The ratio between luminosity or stellar-mass between the two galaxies is ∼ 1:50, while the indirectly measured dynamical mass-ratio, when including dark matter, may be ∼ 1:10-1:5. This system may thus represent a "stealth" merger, where an infalling satellite galaxy is nearly undetectable by conventional means, yet has a substantial dynamical influence on its host galaxy. This singular discovery also suggests that satellite accretion can play a significant role in building up the stellar halos of low-mass galaxies, and possibly in triggering their starbursts.

Using deep photometric data from WFC@INT and WFI@ESO2.2m we measure the outer number density profiles of 19 stellar clusters located in the inner region of the Milky Way halo (within a Galactocentric distance range of 10-30 kpc) in order to assess the impact of internal and external dynamical processes on the spatial distribution of stars. Adopting power-law fitting templates, with index $-\gamma$ in the outer region, we find that the clusters in our sample can be divided in two groups: a group of massive clusters ($ \ge 10^5 $ M_sun) that has relatively flat profiles with $2.5 < \gamma < 4$ and a group of low-mass clusters ($ \le 10^5 $ M_sun), with steep profiles ($\gamma > 4$) and clear signatures of interaction with the Galactic tidal field. We refer to these two groups as 'tidally unaffected' and 'tidally affected', respectively. Our results also show a clear trend between the slope of the outer parts and the half-mass density of these systems, which suggests that the outer density profiles may retain key information on the dominant processes driving the dynamical evolution of Globular Clusters

Aims. We aim to measure the proper motion along the Sagittarius stream, which is the missing piece in determining its full 6D phase space coordinates. Methods. We conduct a blind search of over-densities in proper motion from the Gaia second data release in a broad region around the Sagittarius stream by applying wavelet transform techniques. Results. We find that for most of the sky patches, the highest intensity peaks delineate the path of the Sagittarius stream. The 1500 peaks identified depict a continuous sequence spanning almost 2π in the sky, only obscured when the stream crosses the Galactic disk. Altogether, around 100 000 stars potentially belong to the stream as indicated by a coarse inspection of the color-magnitude diagrams. From these stars, we determine the proper motion along the Sagittarius stream, making it the proper-motion sequence with the largest span and continuity ever measured for a stream. A first comparison with existing N-body models of the stream reveals some discrepancies, especially near the pericenter of the trailing arm and an underestimation of the total proper motion for the leading arm. Conclusions. Our study provides a starting point for determining the variation of the population of stars along the stream, the distance to the stream from the red clump stars, and the solar motion. It also permits much more accurate measurement of the Milky Way potential.

We used deep wide-field photometric observations to derive the fraction of binary systems in a sample of five high-latitude Galactic open clusters. By analysing the colour distribution of main-sequence stars, we derived the minimum fraction of binary systems required to reproduce the observed colour-magnitude diagram morphologies. We found that all the analysed clusters contain a minimum binary fraction larger than 11 per cent within the core radius. The estimated global fractions of binary systems range from 35 to 70 per cent depending on the cluster. The comparison with homogeneous estimates performed in globular clusters indicates that open clusters hold a significantly higher fraction of binary systems, as predicted by theoretical models and N-body simulations. A dependence of the relative fraction of binary systems on the cluster mass has been detected, suggesting that the binary disruption within the cluster core is the dominant process that drives the fraction of binaries in stellar systems.