No abstract

We describe the structural and kinematic properties of the first compact stellar systems discovered by the AIMSS project. These spectroscopically confirmed objects have sizes (∼6 < R e [pc] < 500) and masses (∼2×10 6 < M * /M < 6×10 9 ) spanning the range of massive globular clusters (GCs), ultra compact dwarfs (UCDs) and compact elliptical galaxies (cEs), completely filling the gap between star clusters and galaxies.Several objects are close analogues to the prototypical cE, M32. These objects, which are more massive than previously discovered UCDs of the same size, further call into question the existence of a tight mass-size trend for compact stellar systems, while simultaneously strengthening the case for a universal "zone of avoidance" for dynamically hot stellar systems in the mass-size plane.Overall, we argue that there are two classes of compact stellar systems: 1) massive star clusters and 2) a population closely related to galaxies. Our data provide indications for a further division of the galaxy-type UCD/cE population into two groups, one population that we associate with objects formed by the stripping of nucleated dwarf galaxies, and a second population that formed through the stripping of bulged galaxies or are lower-mass analogues of classical ellipticals. We find compact stellar systems around galaxies in low to high density environments, demonstrating that the physical processes responsible for forming them do not only operate in the densest clusters.

We present the discovery of several ultra‐compact dwarfs (UCDs) located in field/group environments. Examination of these objects, plus literature UCDs, confirms the existence of two distinct formation channels for these compact stellar systems. We find that the UCDs we have discovered around the group elliptical NGC 3923 (and most UCDs in general) have properties consistent with their being the most luminous members of the host galaxy’s globular cluster (GC) system. As with GCs they are therefore likely to be the product of the most violent epochs of galaxy formation. We describe UCDs of this type as giant GCs (GGCs). In contrast, the UCD we have found associated with the isolated S0 NGC 4546 is clearly the result of the stripping of a nucleated companion galaxy. The young age (∼3.4 Gyr) of the UCD, the lack of a correspondingly young GC population, the apparently short dynamical friction decay time‐scale (∼0.5 Gyr) of the UCD and the presence of a counter‐rotating gas disc in the host galaxy (corotating with respect to the UCD) together suggest that this UCD is the liberated nucleus remaining after the recent stripping of a companion by NGC 4546. We infer that the presence of UCDs of either category (GGCs formed in major star‐forming events, or stripped nuclei formed in minor mergers) can provide a useful probe of the assembly history of the host galaxy. We suggest a general scheme that unifies the formation of GCs, UCDs and galaxy nuclei. In this picture, ‘normal’ GCs are a composite population, composed of GCs formed in situ, GCs acquired from accreted galaxies and a population of lower mass stripped dwarf nuclei masquerading as GCs. Above a ‘scaling onset mass’ of 2 × 106 M⊙ (MV∼−10), UCDs emerge together with a mass–size relation and a likely mass–metallicity relation (the ‘blue tilt’). In the mass range up to 7 × 107 M⊙ (MV∼−13) UCDs comprise a composite population of GGCs and stripped nuclei. Interestingly, dwarf nuclei have similar colours to blue GCs and UCDs across the scaling onset mass, smoothly extending the blue tilt, while nuclei of more massive galaxies and a prominent minority of UCDs extend the red locus of GCs. Above 7 × 107 M⊙, UCDs must be almost exclusively stripped nuclei, as no sufficiently rich GC systems exist to populate such an extreme of the GCLF.

We relate transitions in galaxy structure and gas content to refueling, here defined to include both the external gas accretion and the internal gas processing needed to renew reservoirs for star formation. We analyze two z = 0 data sets: a high-quality ∼200-galaxy sample (the Nearby Field Galaxy Survey, data release herein) and a volume-limited ∼3000-galaxy sample with reprocessed archival data. Both reach down to baryonic masses ∼10 9 M ⊙ and span void-to-cluster environments. Two mass-dependent transitions are evident: (i) below the "gas-richness threshold scale (V ∼ 125 km s −1 ), gas-dominated quasi-bulgeless Sd-Im galaxies become numerically dominant, while (ii) above the "bimodality scale (V ∼ 200 km s −1 ), gas-starved E/S0s become the norm. Notwithstanding these transitions, galaxy mass (or V as its proxy) is a poor predictor of gas-to-stellar mass ratio M gas /M * . Instead, M gas /M * correlates well with the ratio of a galaxys stellar mass formed in the last Gyr to its preexisting stellar mass, such that the two ratios have numerically similar values. This striking correspondence between past-averaged star formation and current gas richness implies routine refueling of star-forming galaxies on Gyr timescales. We argue that this refueling underlies the tight M gas /M * vs. color correlations often used to measure "photometric gas fractions. Furthermore, the threshold and bimodality scale transitions reflect mass-dependent demographic shifts between three refueling regimes -accretion dominated, processing dominated, and quenched. In this picture, gas-dominated dwarfs are explained not by inefficient star formation but by overwhelming gas accretion, which fuels stellar mass doubling in 1 Gyr. Moreover, moderately gas-rich bulged disks such as the Milky Way are transitional, becoming abundant only in the narrow range between the threshold and bimodality scales.