Shell galaxies make a class of tidally distorted galaxies, characterised bywide concentric arc(s), extending out to large galactocentric distances withsharp outer edges. Recent observations of young massive star clusters in theprominent outer shell of NGC 474 suggest that such systems host extremeconditions of star formation. In this paper, we present a hydrodynamicsimulation of a galaxy merger and its transformation into a shell galaxy. Weanalyse how the star formation activity evolves with time, location-wise withinthe system, and what are the physical conditions for star formation. During theinteraction, an excess of dense gas appears, triggering a starburst, i.e. anenhanced star formation rate and a reduced depletion time. Star formationcoincides with regions of high molecular gas fraction, such as the galacticnucleus, spiral arms, and occasionally the tidal debris during the early stagesof the merger. Tidal interactions scatter stars into a stellar spheroid, whilethe gas cools down and reforms a disc. The morphological transformation aftercoalescence stabilises the gas and thus quenches star formation, without theneed for feedback from an active galactic nucleus. This evolution showssimilarities with a compaction scenario for compact quenched spheroids athigh-redshift, yet without a long red nugget phase. Shells appear aftercoalescence, during the quenched phase, implying that they do not host theconditions necessary for in situ star formation. The results suggest thatshell-forming mergers might be part of the process of turning blue late-typegalaxies into red and dead early-types.

The interaction between Active Galactic Nuclei (AGN) and their host galaxiesis scarcely resolved. Narrow-line Seyfert 1 (NLS1) galaxies are believed torepresent AGN at early stages of their evolution and allow to observe AGNfeeding and feedback processes at high accretion rates. We apply aspectroastrometric analysis to VLT MUSE NFM-AO observations of Mrk 1044, anearby super-Eddington accreting NLS1. This allows us to map two ionised gasoutflows traced by [O$\,$III] which have velocities of $-560\pm20\,{\rmkm\:s}^{-1}$ and $-144 \pm 5 \,{\rm km\:s}^{-1}$. Both outflows are spatiallyunresolved and located close to the galaxy nucleus ($<1\,{\rm pc}$). They havegas densities higher than $10^5\,{\rm cm}^{-3}$, which implies that the BPTdiagnostic cannot be used to constrain the underlying ionisation mechanism. Weexplore whether an expanding shell model can describe the velocity structure ofMrk 1044's unresolved multi-phase outflow. A kinematic analysis suggests thatsignificant turbulence may be present in the ISM around the nucleus, which maylead to a condensation rain, potentially explaining the efficient feeding ofMrk 1044's AGN. We identify an additional ionised gas outflowing component thatis spatially resolved. It has a velocity of $-211 \pm 22 \,{\rm km\:s}^{-1}$and a projected size of $4.6 \pm 0.6 \,{\rm pc}$. Within the innermost 0.5"(160$\,{\rm pc}$) around the nucleus we detect modest star formation hidden bythe beam-smeared emission from the outflow, which suggests that Mrk 1044's AGNphase set on recently. We estimate that the multi-phase outflow has beenlaunched $<10^4 \,{\rm yrs}$ ago. It carries enough mass and energy to impactthe host galaxy star formation on different spatial scales, highlighting thecomplexity of the AGN feeding and feedback cycle in its early stages.

Galaxies broadly fall into two categories: star-forming (blue) galaxies andquiescent (red) galaxies. In between, one finds the less populated ``greenvalley". Some of these galaxies are suspected to be in the process of ceasingtheir star-formation through a gradual exhaustion of gas supply or already deadand are experiencing a rejuvenation of star-formation through fuel injection.We use the Galaxy And Mass Assembly database and the Galaxy Zoo citizen sciencemorphological estimates to compare the morphology of galaxies in the greenvalley against those in the red sequence and blue cloud. Our goal is to examine the structural differences within galaxies that fallin the green valley, and what brings them there. Previous results found discfeatures such as rings and lenses are more prominently represented in the greenvalley population. We revisit this with a similar sized data set of galaxieswith morphology labels provided by the Galaxy Zoo for the GAMA fields based onnew KiDS images. Our aim is to compare qualitatively the results from expertclassification to that of citizen science. We observe that ring structures are indeed found more commonly in greenvalley galaxies compared to their red and blue counterparts. We suggest thatring structures are a consequence of disc galaxies in the green valley activelyexhibiting characteristics of fading discs and evolving disc morphology ofgalaxies. We note that the progression from blue to red correlates withloosening spiral arm structure.