Galaxy evolution across the Virgo cluster environment

Abstract

In this thesis, I present three studies of the properties of galaxies in the Virgo cluster with the goal of providing new insights into how we can measure the effects of the cluster environment on these member galaxies. Throughout this work, I move from analyzing integrated quantities of galaxies toward studying galaxy star formation on a spatially-resolved scale.

First, I analyze deep stellar mass functions of galaxies in the Virgo cluster. I split these stellar mass functions according to star formation rate and region within the cluster. I find that the quenched fractions remain high across the cluster, including in the infall region, and especially at low stellar masses. Additionally, the shapes of the mass functions are mostly independent of position within the cluster. Using a simple model of infalling and backsplash galaxy populations, I show that the quenched fractions seen in the cluster outskirts require unrealistically high backsplash fractions, implying that galaxies are being pre-processed outside of the main cluster and are often already quenched upon first infall.

The second section of this thesis focuses on quantifying the edges of star-forming disks as a means to analyze truncation and outside-in quenching. I develop a novel method for measuring disk truncation that takes advantage of the depth and spatially-resolved nature of the VESTIGE Hα data. This method involves constructing radial sSFR profiles and identifying the turn-off point where star formation drops off. I compare this edge of the star-forming disk with a measure of the expected size based on normal star-forming galaxies, which gives a measure of the degree of truncation. Ultimately, I find that moderately-severely truncated disks are ubiquitous across the cluster environment, and show only mild correlations with parameters such as stellar mass, distance from the SFMS, and HI-deficiency. I invoke toy models of RPS and starvation to show the effects that each of these mechanisms has on disk truncation. While RPS can rapidly truncate the star-forming disk, it is only particularly effective toward the cluster centre. Starvation can slowly truncate disks much earlier by cutting off a galaxy's supply of fresh gas -- as the gas density decreases, the outer part of the disk falls below the threshold necessary to sustain star formation first, causing a slow outside-in quenching. This again highlights the need for pre-processing to explain quenching in the Virgo environment, and indicates a ``slow-then-rapid'' quenching sequence.

Finally, I take advantage of the spatially-resolved and multi-wavelength data available for Virgo cluster galaxies to map signatures of quenching on more local scales. I do this by empirically calculating SFRs based on both Hα and FUV fluxes, thus giving me SFRs on both ~10 and ~100 Myr timescales. Comparing the two helps to identify regions where rapid quenching is occurring. I perform spatially resolved SED fitting on a sample of Virgo galaxies using CIGALE which allows me to map out galaxy parameters. Combining the SED fitting results with emission line ratios from MaNGA, I derive corrections based on dust, NII contamination (in the Hα) and pAGB contribution to the FUV before converting the respective fluxes to SFRs. Taking the ratio of SFRha and SFRfuv allows me to visually identify interesting features in these galaxies. I identify two key features: lopsidedness in the distribution of SFRha/SFRfuv, and outer disk truncation. I quantify these metrics, and find that while a few galaxies that have strong gas asymmetries and have been identified as RPS candidates have a high degree of lopsidedness, there are no strong correlations with other galaxy parameters. I discuss the possibility that these metrics are unique signatures of processes like RPS at certain stages, though drawing strong conclusions about their nature proves difficult.

I end off with future perspectives, based on my own work with the CASTOR and GIRMOS science teams, the broader context of new instrumentation and analysis tools, and my own goals for the coming years as I seek to continue work into the study of galaxy evolution. I look forward to pursuing studies with new data, exploring new redshift ranges and galaxy environments, and looking to continue developing novel techniques to help make inferences about galaxies in the broad context of understanding our Universe.