As part of the H I Arecibo Galaxy Environments Survey (AGES) we have observed 5×4 degrees of sky centred on M33, reaching a limiting column density of ∼ 1.5 × 10 17 cm −2 (line width of 10 km s −1 and resolution 3.5 ). We particularly investigate the absence of optically detected dwarf galaxies around M33, something that is contrary to galaxy formation models. We identify 22 discrete H I clouds, 11 of which are new detections. The number of objects detected and their internal velocity dispersion distribution is consistent with expectations from standard galaxy formation models. However, the issue remains open as to whether the observed velocity dispersions can be used as a measure of the H I clouds total mass i.e. are the velocities indicative of virialised structures or have they been influenced by tidal interactions with other structures in the Local Group? We identify one particularly interesting H I cloud, AGESM33-31, that has many of the characteristics of H I distributed in the disc of a galaxy, yet there is no known optical counterpart associated with it. This object has a total H I mass of 1.22 × 10 7 M and a diameter of 18 kpc if at the distance of M33 (D M33 = 840 kpc). However, we also find that there are numerous other H I clouds in this region of sky that have very similar velocities and so it is plausible that all these clouds are actually associated with debris from the Magellanic stream.

We present new Hi observations from the Australia Telescope Compact Array and deep optical imaging from OmegaCam on the VLT Survey Telescope of NGC 1427A, an arrow-shaped dwarf irregular galaxy located in the Fornax cluster. The data reveal a star-less Hi tail that contains ∼10% of the atomic gas of NGC 1427A as well as extended stellar emission that shed new light on the recent history of this galaxy. Rather than being the result of ram pressure induced star-formation, as previously suggested in the literature, the disturbed optical appearance of NGC 1427A has tidal origins. The galaxy itself likely consists of two individual objects in an advanced stage of merging. The Hi tail may be made of gas expelled to large radii during the same tidal interaction. It is possible that some of this gas is subject to ram pressure, which would be considered a secondary effect and imply a northwest trajectory of NGC 1427A within the Fornax cluster.

We have used public data from the Next Generation Virgo Survey (NGVS) to investigate the dwarf galaxy population of the Virgo cluster beyond what has previously been discovered. We initially mask and smooth the data, and then use the object detection algorithm S extractor to make our initial dwarf galaxy selection. All candidates are then visually inspected to remove artefacts and duplicates. We derive S extractor parameters to best select low surface brightness galaxies using central surface brightness values of 22.5 µ g 0 26.0 µg and exponential scale lengths of 3.0 h 10.0 arc sec to identify 443 cluster dwarf galaxies -303 of which are new detections, with a surface density that decreases with radius from the cluster centre. We also apply our selection algorithm to 'background', non-cluster, fields and find zero detections. In combination, this leads us to believe that we have isolated a cluster dwarf galaxy population. The range of objects we are able to detect is limited because smaller scale sized galaxies are confused with the background, while larger galaxies are split into numerous smaller objects by the detection algorithm. Using data from previous surveys combined with our data, we find a faint end slope to the luminosity function of −1.35 ± 0.03, which does not significantly differ to what has previously been found for the Virgo cluster, but is a little steeper than the slope for field galaxies. There is no evidence for a faint end slope steep enough to correspond with galaxy formation models, unless those models invoke either strong feedback processes or use warm dark matter.

We report the automatic detection of a new sample of very low surface brightness (LSB) galaxies, likely members of the Virgo cluster. We introduce our new software, DeepScan, that has been designed specifically to detect extended LSB features automatically using the DBSCAN algorithm. We demonstrate the technique by applying it over a 5 degree 2 portion of the Next-Generation Virgo Survey (NGVS) data to reveal 53 low surface brightness galaxies that are candidate cluster members based on their sizes and colours. 30 of these sources are new detections despite the region being searched specifically for LSB galaxies previously. Our final sample contains galaxies with 26.0 ≤ µ e ≤ 28.5 and 19 ≤ m g ≤ 21, making them some of the faintest known in Virgo. The majority of them have colours consistent with the red sequence, and have a mean stellar mass of 10 6.3±0.5 M assuming cluster membership. After using ProFit to fit Sérsic profiles to our detections, none of the new sources have effective radii larger than 1.5 Kpc and do not meet the criteria for ultra-diffuse galaxy (UDG) classification, so we classify them as ultra-faint dwarfs.

Isolated Hi clouds with no optical counterparts are often taken as evidence for galaxygalaxy interactions, though an alternative hypothesis is that these are primordial 'dark galaxies' which have not formed stars. Similarly, certain kinematic features in Hi streams are also controversial, sometimes taken as evidence of dark galaxies but also perhaps explicable as the result of harassment. We numerically model the passage of a galaxy through the gravitational field of cluster. The galaxy consists of SPH particles for the gas and n-bodies for the stars and dark matter, while the cluster includes the gravitational effects of substructure using 400 subhalos (the effects of the intracluster medium are ignored). We find that harassment can indeed produce long Hi streams and these streams can include kinematic features resembling dark galaxy candidates such as VIRGOHI21. We also show that apparent clouds with diameter <20 kpc and velocity widths <50 km s −1 are almost invariably produced in these simulations, making tidal debris a highly probable explanation. In contrast, we show that the frequency of isolated clouds of the same size but velocity width >100 km s −1 is negligible -making this a very unlikely explanation for the observed clouds in the Virgo cluster with these properties.