Chemical evolution of star-forming galaxies in the overdense environment

Abstract

As the largest gravitationally bound systems in the Universe, galaxy clusters are a unique laboratory for studying the extremes of galaxy evolution. The general consensus is that the large cluster-scale halo and the hot intracluster medium plays a significant role in the morphological and colour transformation of galaxies. However, its effect on details of baryonic processes in cluster galaxies remains unknown. I use the gas-phase chemical abundance to trace the cumulative effect of the gas inflow/outflow modulated star formation history of a galaxy. Current observations of the gas-phase chemical abundance of star-forming galaxies in the cluster environment are highly uncertain. This thesis aims to answer whether, when and how environment affect the chemical evolution star-forming galaxies.

present the first observation of cluster-scale radial metallicity gradients from star-forming galaxies in two CLASH clusters at $z\sim0.35$: MACS1115+0129 and RXJ1532+3021, using observations with the DEIMOS spectrograph on the Keck II telescope. I use observations for RXJ1532+3021 to quantify the systematic uncertainties in the flux calibration of DEIMOS spectrograph. We find that the gas-phase metallicity of galaxies decreases as a function of projected cluster-centric distance for MACSJ1115+0129, i.e., galaxies near the cluster core are metal-rich compared to galaxies in the cluster outskirts. Star-forming galaxies in MACSJ1115+0129 are +0.20 dex metal-rich compared to counterpart field galaxies. The negative cluster-scale metallicity gradient in our observations is not driven by the stellar mass of galaxies.

Star-forming galaxies in RXJ1532+3021 shows a bimodal radial metallicity distribution and their mass-metallicity distribution is consistent with local field galaxies. We suspect that either interloper galaxies or an in-plane merger causes the bimodality of the radial metallicity distribution in RXJ1532+30, indicating that the cluster-scale abundance gradient can probe the dynamical state of a cluster.

We speculate that the negative cluster-scale metallicity gradient originates from ram-pressure stripping and/or strangulation processes in the cluster environments. The truncation of star formation in galactic outskirts due to ram pressure stripping (disk truncation) can observationally bias the aperture-based integrated metallicity measurements towards the central metal-rich part of the galaxy. To quantify this observational bias, I use a semi-analytic model of ram pressure stripping and conduct mock observations of disk truncated galaxies from the IFU survey of local galaxies.

Our ram pressure stripping model predicts a typical cluster-scale metallicity gradient of -0.03 dex/Mpc and a minimal metallicity enhancement of +0.02 dex at a fixed stellar mass. The gas removal and subsequent quenching preferentially removes the low stellar mass galaxies from the cluster core. The small cluster-scale metallicity gradient predicted by our ram pressure stripping model is driven by the removal of the low mass galaxies from the cluster cores, which introduces a negative stellar mass gradient for the surviving population of star-forming cluster galaxies. Thus, our model shows that observational bias introduced by ram pressure stripping is not sufficient to explain the observed stellar mass independent negative cluster-scale abundance gradient and the metallicity enhancement on the mass-metallicity relation.

To identify the physical processes driving environment dependent chemical evolution, I use the new generation of cosmological simulation, in particular IllustrisTNG simulation. I investigate the evolution of the mass-metallicity relation for the star-forming cluster galaxies at z=0, by tracking them back in cosmic time. The simulation predicts that star-forming cluster galaxies have higher gas-phase metallicity compared to field galaxies at z<1.0. The metallicity enhancement predicted by simulation is qualitatively consistent with observations.

IllustrisTNG simulation predict the first systematic signature of ``chemical pre-processing'' of infalling cluster galaxies, i.e., the metallicity enhancement of cluster galaxies appears prior to their infall into the central cluster potential. In fact, infalling cluster galaxies show a ~0.05 dex higher metallicity compared to field galaxies at z<0.5 at a given stellar mass. By estimating the gas mass inflow rate and the metallicity of inflowing gas, we identify that the accretion of pre-enriched gas and the reduced gas mass inflow rate are key drivers of chemical evolution in the overdense environment, particularly in the stellar mass range 10^9< M_*<10^10 Msun. The signature of environment-dependent changes in properties of inflowing gas extends to infalling galaxies, i.e., for galaxies well outside the virial radius of clusters. Our work with IllustrisTNG motivates future observations of the chemical pre-processing and the pre-enrichment of inflowing gas in dense environments. Show more