Sardinia Radio Telescope observations of Local Group dwarf galaxies – I. The cases of NGC 6822, IC 1613, and WLM

ABSTRACT Almost all dwarf galaxies in the Local Group (LG) that are not satellites of the Milky Way or M 31 belong to either one of two highly symmetric planes. It is still a matter of debate whether these planar structures are dynamically stable or whether they only represent a transient alignment. Proper motions, if they could be measured, could help to discriminate between these scenarios. Such motions could be determined with multi-epoch very long baseline interferometry (VLBI) of sources that show emission from water and methanol at frequencies of 22 and 6.7 GHz, respectively. We report searches for such masers. We have mapped three LG galaxies, NGC 6822, IC 1613, and WLM, in the bands covering the water vapour and methanol lines. These systems are members of the two above-mentioned planes of galaxies. We have produced deep radio continuum (RC) maps and spectral line cubes. The former have been used to identify star-forming regions and to derive global galactic star formation rates (SFRs). These SFRs turn out to be lower than those determined at other wavelengths in two of our sources. This indicates that dwarf galaxies may follow predictions on the RC–SFR relation only in individual regions of enhanced RC emission, but not when considering the entire optical body of the sources. No methanol or water maser emission has been confidently detected, down to line luminosity limits of ∼4 × 10−3 and 10 × 10−3 L⊙, respectively. This finding is consistent with the small sizes, low SFRs, and metallicities of these galaxies.

The spindle and kinetochore–associated (Ska) complex enhances binding of the anaphase-promoting complex/cyclosome (APC/C) to chromosomes and promotes mitotic exit

The spindle and kinetochore–associated (Ska) protein complex is a heterotrimeric complex required for timely anaphase onset. The major phenotypes seen after small interfering RNA–mediated depletion of Ska are transient alignment defects followed by metaphase arrest that ultimately results in cohesion fatigue. We find that cells depleted of Ska3 arrest at metaphase with only partial degradation of cyclin B1 and securin. In cells arrested with microtubule drugs, Ska3-depleted cells exhibit slower mitotic exit when the spindle checkpoint is silenced by inhibition of the checkpoint kinase, Mps1, or when cells are forced to exit mitosis downstream of checkpoint silencing by inactivation of Cdk1. These results suggest that in addition to a role in fostering kinetochore–microtubule attachment and chromosome alignment, the Ska complex has functions in promoting anaphase onset. We find that both Ska3 and microtubules promote chromosome association of the anaphase-promoting complex/cyclosome (APC/C). Chromosome-bound APC/C shows significantly stronger ubiquitylation activity than cytoplasmic APC/C. Forced localization of Ska complex to kinetochores, independent of microtubules, results in enhanced accumulation of APC/C on chromosomes and accelerated cyclin B1 degradation during induced mitotic exit. We propose that a Ska-microtubule-kinetochore association promotes APC/C localization to chromosomes, thereby enhancing anaphase onset and mitotic exit.

Novel Kinetic Effects in Vkcoelasttc Surfactant Solutions Under Shear

ABSTRACTUsing small-angle neutron scattering (SANS), we have investigated the transient alignment and relaxation under Couette shear of viscoelastic aqueous micellar solutions of cetyltrimethylarmmonium 3,5-dichlorobenzoate (CTA3,5C1) and CTA3,5C1/CTAB mixtures at concentrations well above ф* (but below 1.0 wt. %). Time constants of the order of ten's of minutes are reported for alignment and relaxation, orders of magnitude slower than any previously observed in similar micellar systems. The collective properties of the network of entangled, threadlike micelles, rather than the individual micellar segments, dominate the alignment and relaxation behavior. At low micellar surface charge density (σ) (e.g., in pure CTA3,5C1), the first observation of alignment proceeding in two stages has been made. Increasing a decreases by an order of magnitude the shear rate required to reach full alignment and provides a comparable decrease in the rate constant for relaxation after cessation of shear.