| Abstract: Elements heavier than hydrogen and helium are the building blocks of life. These elements are forged in stars that ultimately met their end, either in explosive supernovae or by gradually releasing their enriched material into space. This stellar recycling enriches the interstellar medium (ISM) with "metals," shaping how galaxies evolve, how stars are born, and how planets eventually form. Metallicity, the abundance of such heavy elements, is therefore a key ingredient governing the physical and chemical behavior of the ISM. Yet, nearly half of all stars in today’s Universe formed at a time when the ISM was much poorer in metals. To understand how star and planet formation operated under such primitive conditions, this thesis studies a unique nearby analogue: the outer Milky Way (MW), refers to the region of the Galactic disk located beyond the Solar Circle. Its low-metallicity environment and relatively simplified interstellar conditions provide a powerful laboratory for addressing these gaps in our knowledge. Using HCN and HCO+ observations from the TRAO 14-m radio telescope combined with archival data, we show that commonly used dense gas tracers do not exclusively trace the densest regions of molecular clouds. Instead, much of the emission arises from diffuse gas, questioning the traditional uses of these tracers. We study that dense gas (HCN, HCO+) mass-luminosity conversion factors vary across the MW, challenging universal scaling relations and highlighting the need for metallicity-dependent interpretations in Galactic and extragalactic studies. Using uniform dataset from UKIDSS, we present the first robust comparison of disk fractions in low and solar-metallicity MW clusters, showing that metal-poor clusters host fewer protoplanetary disks without showing faster disk dispersal, challenging the idea that low metallicity universally accelerates disk loss. This thesis establishes the outer MW as a rich laboratory for low-metallicity studies and opens new avenues for future research. |
