Spiral structures are important drivers of the secular evolution of discgalaxies, however, the origin of spiral arms and their effects on thedevelopment of galaxies remain mysterious. In this work, we present twothree-armed spiral galaxies at z~0.3 in the Middle Age Galaxy Properties withIntegral Field Spectroscopy (MAGPI) survey. Taking advantage of the highspatial resolution (~0.6'') of the Multi-Unit Spectroscopic Unit (MUSE), weinvestigate the two-dimensional distributions of different spectral parameters:Halpha, gas-phase metallicity, and D4000. We notice significant offsets inHalpha (~0.2 dex) as well as gas-phase metallicities (~0.05 dex) among thespiral arms, downstream and upstream of MAGPI1202197197 (SG1202). Thisobservational signature suggests the spiral structure in SG1202 is consistentwith arising from density wave theory. No azimuthal variation in Halpha orgas-phase metallicities is observed in MAGPI1204198199 (SG1204), which can beattributed to the tighter spiral arms in SG1204 than SG1202, coming withstronger mixing effects in the disc. The absence of azimuthal D4000 variationin both galaxies suggests the stars at different ages are well-mixed betweenthe spiral arms and distributed around the disc regions. The differentazimuthal distributions in Halpha and D4000 highlight the importance of timescales traced by various spectral parameters when studying 2D distributions inspiral galaxies. This work demonstrates the feasibility of constraining spiralstructures by tracing interstellar medium (ISM) and stellar population atz~0.3, with a plan to expand the study to the full MAGPI survey.

We present here estimates of the average rates of accretion of neutral gasonto main-sequence galaxies and the conversion of atomic gas to molecular gasin these galaxies at two key epochs in galaxy evolution: (i) $z\approx1.3-1.0$,towards the end of the epoch of peak star-formation activity in the Universe,and (ii) $z\approx1-0$, when the star-formation activity declines by an orderof magnitude. We determine the net gas accretion rate $\rm{R_{Acc}}$ and themolecular gas formation rate $\rm{R_{Mol}}$ by combining the relations betweenthe stellar mass and the atomic gas mass, the molecular gas mass, and thestar-formation rate (SFR) at three epochs, $z=1.3$, $z=1.0$, and $z=0$, withthe assumption that galaxies evolve continuously on the star-formingmain-sequence. We find that, for all galaxies, $\rm{R_{Acc}}$ is far lower thanthe average SFR $\rm{R_{SFR}}$ at $z\approx1.3-1.0$; however, $\rm{R_{Mol}}$ issimilar to $\rm{R_{SFR}}$ during this interval. Conversely, both $\rm{R_{Mol}}$and $\rm{R_{Acc}}$ are significantly lower than $\rm{R_{SFR}}$ over the laterinterval, $z\approx1-0$. We find that massive main-sequence galaxies hadalready acquired most of their present-day baryonic mass by $z\approx1.3$. At$z\approx1.3-1.0$, the rapid conversion of the existing atomic gas to moleculargas was sufficient to maintain a high average SFR, despite the low net gasaccretion rate. However, at later times, the combination of the lower net gasaccretion rate and the lower molecular gas formation rate leads to a decline inthe fuel available for star-formation, and results in the observed decrease inthe SFR density of the Universe over the last 8 Gyr.