One of the main theories for explaining the formation of spiral arms ingalaxies is the stationary density wave theory. This theory predicts theexistence of an age gradient across the arms. We use the stellar clustercatalogues of the galaxies NGC 1566, M51a, and NGC 628 from the LegacyExtragalactic UV Survey (LEGUS) program. In order to test for the possibleexistence of an age sequence across the spiral arms, we quantified theazimuthal offset between star clusters of different ages in our targetgalaxies. We found that NGC 1566, a grand-design spiral galaxy with bisymmetricarms and a strong bar, shows a significant age gradient across the spiral armsthat appears to be consistent with the prediction of the stationary densitywave theory. In contrast, M51a with its two well-defined spiral arms and aweaker bar does not show an age gradient across the arms. In addition, acomparison with non LEGUS star cluster catalogues for M51a yields similarresults. We believe that the spiral structure of M51a is not the result of astationary density wave with a fixed pattern speed. Instead, tidal interactionscould be the dominant mechanism for the formation of spiral arms. We also foundno offset in the azimuthal distribution of star clusters with different agesacross the weak spiral arms of NGC 628.

We investigate the metallicity dependence of HI surface densities instar-forming regions along many lines of sight within 70 nearby galaxies,probing kpc to 50 pc scales. We employ HI, SFR, stellar mass, and metallicity(gradient) measurements from the literature, spanning a wide range (5 dex) instellar and gas mass and (1.6 dex) in metallicity. We consider metallicities asobserved, or rescaled to match the mass-metallicity relation determined forSDSS galaxies. At intermediate to high metallicities (0.3-2 times solar), wefind that the HI surface densities saturate at sufficiently large total gassurface density. The maximal HI columns vary approximately inversely withmetallicity, and show little variation with spatial resolution, galactocentricradius, or among galaxies. In the central parts of massive spiral galaxies theHI gas is depressed by factors of 2. The observed behavior is naturallyreproduced by metallicity dependent shielding theories for the HI-to-H2transitions in star-forming galaxies. We show that the inverse scaling of themaximal HI columns with metallicity suggests that the area filling fraction ofatomic-molecular complexes in galaxies is of order unity, and weakly dependenton metallicity.

Brightest Cluster Galaxies (BCGs) in massive dark matter halos are shaped bycomplex merging processes. We present a detailed stellar population analysis inthe central region of Abell 3827 at $z\sim0.1$, including five-nucleus galaxiesinvolved in a BCG assembly. Based on deep spectroscopy from Multi UnitSpectroscopic Explorer (MUSE), we fit the optical spectra of 13 early-typegalaxies (ETGs) in the central 70 kpc of the cluster. The stellar populationsin the central $R=1$ kpc of these ETGs are old (6-10 Gyr). Their [Fe/H]increases with $\sigma_{\star}$ and stellar mass. More importantly,[$\alpha$/Fe] of galaxies close to the cluster center do not seem to depend on$\sigma_{\star}$ or stellar mass, indicating that the cluster center shapes the[$\alpha$/Fe]-$\sigma_{\star}$ and [$\alpha$/Fe]-$M_{\star}$ relationsdifferently than other environments where [$\alpha$/Fe] is observed to increasewith increasing $\sigma_{\star}$ or $M_{\star}$. Our results reveal thecoordinated assembly of BCGs: their building blocks are different from thegeneral low mass populations by their high [$\alpha$/Fe]. Massive galaxies thusgrow by accreting preferentially high [$\alpha$/Fe] systems. The radialprofiles also bear the imprint of the coordinated assembly. Their declining[Fe/H] and flat [$\alpha$/Fe] radial profiles confirm that the accreted systemshave low metallicity and high [$\alpha$/Fe] stellar contents.

Some post-merger galaxies are known to undergo a starburst phase that quicklydepletes the gas reservoir and turns it into a red-sequence galaxy, though thedetails are still unclear. Here we explore the pattern of recent star formationin the central region of the post-merger galaxy NGC7252 using high resolutionUV images from the UVIT on ASTROSAT. The UVIT images with 1.2 and 1.4 arcsecresolution in the FUV and NUV are used to construct a FUV-NUV colour map of thecentral region. The FUV-NUV pixel colour map for this canonical post-mergergalaxy reveals a blue circumnuclear ring of diameter $\sim$ 10 " (3.2 kpc) withbluer patches located over the ring. Based on a comparison to single stellarpopulation models, we show that the ring is comprised of stellar populationswith ages $\lesssim$ 300 Myr, with embedded star-forming clumps of younger age($\lesssim$ 150Myr). The suppressed star formation in the central region, alongwith the recent finding of a large amount of ionised gas, leads us to speculatethat this ring may be connected to past feedback from a central super-massiveblack hole that has ionised the hydrogen gas in the central $\sim$ 4" $\sim$1.3 kpc.

Superclusters form from the largest enhancements in the primordial densityperturbation field and extend for tens of Mpc, tracing the large-scalestructure of the Universe. We characterise XLSSsCN01, a rich supercluster atz~0.3 detected in the XXL Survey, composed of X-ray clusters of differentvirial masses and luminosities. As one of the first studies on this topic, weinvestigate the stellar populations of galaxies in different environments inthe supercluster region. We study a magnitude-limited (r<=20) and amass-limited sample (log(M*/M_sun)>10.8) of galaxies in the virialised regionand in the outskirts of 11 XLSSsCN01 clusters, in high- and low-density field.We compute the stellar population properties of galaxies using spectral energydistribution and spectral fitting techniques, and study the dependence of starformation rates (SFR), colours, and stellar ages on environment. For r<20, thefraction of SFing/blue galaxies, computed either from the specific-SFR (sSFR)or rest-frame(rf) colour, shows depletion within the cluster virial radii,where the number of galaxies with log(sSFR/yr^-1)>-12 and with (g-r)_rf<0.6 islower than in the field. For log(M*/M_sun)>10.8, no trends with environmentemerge, as massive galaxies are mostly already passive in all environments. Nodifferences among low- and high-density field members and cluster membersemerge in the sSFR-mass relation. The luminosity-weighted age-mass relation ofthe passive populations within cluster virial radii show signatures of recentenvironmental quenching. The study of luminous and massive galaxies in thissupercluster shows that while environment has a prominent role in determiningthe fractions of SFing/blue galaxies, its effects on the star formationactivity in SFing galaxies are negligible.

The Galactic halo contains a complex ecosystem of multiphaseintermediate-velocity and high-velocity gas clouds whose origin has defiedclear explanation. They are generally believed to be involved in a Galaxy-widerecycling process, either through an accretion flow or a large-scale fountainflow, or both. Here we examine the evolution of these clouds in light of recentclaims that they may trigger condensation of gas from the Galactic corona asthey move through it. Specifically, we measure gas condensation along a cloud'swake, with and without the presence of an ambient magnetic field, using two-and three-dimensional, high-resolution Adaptive Mesh Refinement (AMR)simulations. We find that three-dimensional simulations are essential tocapture the condensation even when no magnetic field is included. Magneticfields significantly inhibit condensation in the wake of clouds at $t \gtrsim25$ Myr, preventing the sharp upturn in cold gas mass seen in previousnon-magnetic studies. The magnetic field suppresses the onset of theKelvin-Helmholtz instability which is responsible for the ablation andconsequent mixing of cloud and halo gas that drives the condensation. Thiseffect is universal across different properties of the cloud (density,metallicity, and velocity) and of the magnetic field (strength andorientation), with exception of weak fields parallel to the cloud's motion. Ourresults show that an ambient magnetic field drastically lowers the efficiencyof fountain-driven accretion and the efficiency of recycling and accretion fromcondensation around high-velocity clouds.