X-ray source population in the polar ring galaxy NGC 660 as observed by Chandra

We present Chandra observations of the nearby polar ring galaxy NGC 660 to study its X-ray source population. Based on our analysis, we detected a total of 23 X-ray sources in the 0.5−8 keV band, with luminosities ranging from ∼1037 to ∼1039 erg s−1. Twenty-two of these sources are located off-nuclear and have luminosities below the ultraluminous X-ray source (ULX) threshold value of L 0.5−8 keV < 1039 erg s−1, suggesting that they are likely to be X-ray binary (XRB) candidates. The remaining source is located at the center of the galaxy, suggesting it is an active galactic nucleus (AGN). However, we estimated that four of the detected sources could be associated with background objects. Based on the source count rates in each of the Chandra observations, we found evidence for variability in nine of the 23 sources, including the AGN. However, further investigation with spectral analysis suggested no significant differences in the AGN luminosities between the observations. The X-ray luminosity distribution of the galaxy was found to be generally lower than that expected from previous studies on star forming and collisional ring galaxies. No ULX was also detected in the galaxy, in contrast with what was expected from the galaxyʼs SFR and metallicity (i.e., SFR = 14.43 ± 0.19 M ⊙ yr−1 and Z = 0.94 ± 0.01 Z ⊙, respectively). These results suggest a deficit in the X-ray sources detected. Based on source hardness ratio distribution, we found evidence that the fainter sources have a harder source spectrum, indicating higher absorption. This further suggests that there could be more X-ray sources that were not detected in the galaxy due to significant obscuration.

Revisiting the Role of Toxoplasma gondii ERK7 in the Maintenance and Stability of the Apical Complex

The conoid is an enigmatic, dynamic organelle positioned at the apical tip of the coccidian subgroup of the Apicomplexa, close to the apical polar ring (APR) from which the subpellicular microtubules (SPMTs) emerge and through which the secretory organelles (micronemes and rhoptries) reach the plasma membrane for exocytosis. In Toxoplasma gondii , the conoid protrudes concomitantly with microneme secretion, during egress, motility, and invasion.

Revisiting the role of Toxoplasma gondii ERK7 in the maintenance and stability of the apical complex

Toxoplasma gondii ERK7 is known to contribute to the integrity of the apical complex and to be involved only in the final step of the conoid biogenesis. In the absence of ERK7, mature parasites lose their conoid complex and are unable to glide, invade or egress from host cells. In contrast to a previous report, we show here that depletion of ERK7 phenocopies the depletion of the apical cap proteins AC9 or AC10. The absence of ERK7 leads to the loss of the apical polar ring, the disorganization of the basket of subpellicular microtubules and an impairment in micronemes secretion. Ultra-expansion microscopy (U-ExM) coupled to NHS-Ester staining on intracellular parasites offers an unprecedented level of resolution and highlights the disorganization of the rhoptries as well as the dilated plasma membrane at the apical pole in the absence of ERK7. Comparative proteomics analysis of wild-type and ERK7 or AC9 depleted parasites led to the disappearance of known, predicted, as well as putative novel components of the apical complex. In contrast, the absence of ERK7 led to an accumulation of microneme proteins, resulting from the defect in exocytosis of the organelles.

Active galactic nuclei among the polar-ring galaxies

Based on SDSS data the nuclear activity statistics of the polar-ring galaxies (PRGs) was investigated. BPT-diagrams for PRGs and comparison samples were plotted and analyzed. Obtained results show evidence of excess of active galactic nuclei among PRGs compared to regular objects. Possible interpretation of such effect is discussed.

Extreme kinematic misalignment in IllustrisTNG galaxies: the origin, structure, and internal dynamics of galaxies with a large-scale counterrotation

ABSTRACT We provide an in-deep analysis of 25 galaxies with substantial counterrotation from IllustrisTNG100 simulations in the stellar mass range 2×109−3×1010 M⊙. The counterrotation is a result of an external gas infall ≈2–8 Gyr ago. The infall leads to the removal of pre-existing gas, which is captured and mixed together with the infalling component. This mixture ends up in the counterrotating gaseous disc where ${\approx}90{{\ \rm per\ cent}}$ of counterrotating stars formed in-situ. During the early phases of the infall, gas can be found in extended structures which, in some galaxies, are similar to (nearly-) polar ring-like components. We suggest that the AGN activity does not cause the counterrotation, although it is efficiently triggered by the retrograde gas infall, and it correlates well with the misaligned component appearance. We also find the vertical-to-radial velocity dispersion ratio above unity implying the importance of misalignment in shaping the velocity ellipsoids.

Essential function of the alveolin network in the subpellicular microtubules and conoid assembly in Toxoplasma gondii

The coccidian subgroup of Apicomplexa possesses an apical complex harboring a conoid, made of unique tubulin polymer fibers. This enigmatic organelle extrudes in extracellular invasive parasites and is associated to the apical polar ring (APR). The APR serves as microtubule-organizing center for the 22 subpellicular microtubules (SPMTs) that are linked to a patchwork of flattened vesicles, via an intricate network composed of alveolins. Here, we capitalize on ultrastructure expansion microscopy (U-ExM) to localize the Toxoplasma gondii Apical Cap protein 9 (AC9) and its partner AC10, identified by BioID, to the alveolin network and intercalated between the SPMTs. Parasites conditionally depleted in AC9 or AC10 replicate normally but are defective in microneme secretion and fail to invade and egress from infected cells. Electron microscopy revealed that the mature parasite mutants are conoidless, while U-ExM highlighted the disorganization of the SPMTs which likely results in the catastrophic loss of APR and conoid.

Simulating the galactic system in interaction AM 2229-735 and the formation of its polar structure

We study the formation of polar ring galaxies via minor mergers. We used N-body+hydrodynamics simulations to reproduce the dynamics of the observed system AM 2229-735 that is a minor merger whose interaction signals are those of a progenitor for a polar ring galaxy. We used the observational information of the system to get initial conditions for the orbit and numerical realisations of the galaxies to run the simulations. Our simulations reproduce the global characteristics of interaction observed in the system such as arms and a material bridge connecting the galaxies. As a merger remnant, we found a quasi-stable and self gravitating planar tidal stream with dark matter, stars and gas orbiting in a plane approximately perpendicular to the main galaxy disk leading in the future to a polar ring galaxy. We studied the dynamical conditions of the polar structure and found evidence suggesting that this kind of merger remnant can settle down in a disk-like structure with isothermal support, providing inspiring evidence about the process of formation of galactic disks and providing a potentially independent scenario to study the presence of dark matter in this kind of galaxies.