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

We present an extragalactic perspective of an extended stellar tidal stream wrapping around the edge-on spiral galaxy NGC 5907. Our deep images reveal for the first time a large-scale complex of arcing loops that is an excellent example of how low-mass satellite accretion can produce an interwoven, rosette-like structure of debris dispersed in the halo of its host galaxy. The existence of this structure, which has probably formed and survived for several gigayears, confirms that halos of spiral galaxies in the local universe may still contain a significant number of galactic fossils from their hierarchical formation. To examine the validity of the external accretion scenario, we present N-body simulations of the tidal disruption of a dwarf galaxyYlike system in a disk galaxy plus dark halo potential that demonstrate that most of the tidal features observed in NGC 5907 can be explained by a single accretion event. Unfortunately, with no kinematic data and only the projected geometry of the stream as a constraint, the parameters of our model are considerably degenerate and, for now, must be considered illustrative only. Interestingly, NGC 5907 has long been considered a prototypical example of a warped spiral in relative isolation. The presence of an extended tidal stream challenges this picture and suggests that the gravitational perturbations induced by the stream progenitor must be considered as a possible cause for the warp. The detection of an old, complex tidal stream in a nearby galaxy with rather modest instrumentation points to the viability of surveys to find extragalactic tidal substructures around spiral galaxies in the Local Volume (<15 Mpc), with the prospect of obtaining a census with enough statistical significance to be compared with cosmological simulations.

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.

Context. We introduce the Dwarf Galaxy Survey with Amateur Telescopes (DGSAT) project and report the discovery of eleven low surface brightness (LSB) galaxies in the fields of the nearby galaxies NGC 2683, NGC 3628, NGC 4594 (M 104), NGC 4631, NGC 5457 (M 101), and NGC 7814. Aims. The DGSAT project aims to use the potential of small-sized telescopes to probe LSB features around large galaxies and to increase the sample size of the dwarf satellite galaxies in the Local Volume. Methods. Using long exposure images, fields of the target spiral galaxies are explored for extended LSB objects. After identifying dwarf galaxy candidates, their observed properties are extracted by fitting models to their light profiles. Results. We find three, one, three, one, one, and two new LSB galaxies in the fields of NGC 2683, 3628, 4594, 4631, 5457, and 7814, respectively. In addition to the newly found galaxies, we analyse the structural properties of nine already known galaxies. All of these 20 dwarf galaxy candidates have effective surface brightnesses in the range 25.3 < ∼ µ e < ∼ 28.8 mag arcsec −2 and are fit with Sersic profiles with indices n < ∼ 1. Assuming that they are in the vicinity of the above mentioned massive galaxies, their r-band absolute magnitudes, their effective radii, and their luminosities are in the ranges −15.6 < ∼ M r < ∼ −7.8, 160 pc < ∼ R e < ∼ 4.1 kpc, and 0.1 × 10 6 < ∼ L L r < ∼ 127 × 10 6 , respectively. To determine whether these LSB galaxies are indeed satellites of the above mentioned massive galaxies, their distances need to be determined via further observations. Conclusions. Using small telescopes, we are readily able to detect LSB galaxies with similar properties to the known dwarf galaxies of the Local Group.

We report the discovery of a giant, loop-like stellar structure around the edgeon spiral galaxy NGC 4013. This arcing feature extends 6 ′ (∼ 26 kpc in projected distance) northeast from the center and 3 ′ (≃ 12 kpc) from the disk plane; likely related features are also apparent on the southwest side of the disk, extending to 4 ′ (∼ 17 kpc). The detection of this low surface-brightness (µ R = 27.0 +0.3 −0.2 mag/ ✷ ′′ ) structure is independently confirmed in three separate datasets from three different telescopes.Although its true three dimensional geometry is unknown, the sky-projected morphology of this structure displays a match with the theoretical predictions for the edge-on, projected view of a stellar tidal streams of a dwarf satellite moving in a low inclined (≃ 25 • ), nearly circular orbit. Using the recent model of the Monoceros tidal stream in the Milky Way by Peñarrubia et al. as template, we find that the progenitor system may have been a galaxy with an initial mass -2 -6 × 10 8 M ⊙ , of which current position and final fate is unknown. According to this simulation, the tidal stream may be approximately ∼ 2.8 Gyr of age.Our results demonstrate that NGC 4013, previously considered a prototypical isolated disk galaxy in spite of having one of the most prominent H i warps detected thus far, may have in fact suffered a recent minor merger. This discovery highlights that undisturbed disks at high surface brightness levels in the optical but warped in H i maps may in fact reveal complex signatures of recent accretion events in deep photometric surveys.