Chemical abundances in low mass young galaxies

by Calabro', Antonello

Abstract (Summary)
Low mass (dwarf) galaxies are the most abundant systems of the Universe at all cosmic epochs and they are considered the building blocks from which more massive galaxies assemble. It is now well established that cosmic star formation rate density peaks about 3.5 Gyr after the Big Bang (at redshift 2) and declines exponentially at later times. Nearly 25% of the stellar mass observed today has been assembled over the last eight billion years of cosmic evolution, and a significant fraction of this mass has been formed in young low mass galaxies through vigorous, short-lived starburst episodes. Tracing the galaxy-averaged properties of large, representative samples of star forming dwarf galaxies at intermediate redshifts is therefore a necessary first step for a complete understanding of the mechanisms regulating galaxy assembly. Despite their relevance in the cosmic scenery, the properties of low mass galaxies are still not completely characterized beyond the local universe. Historically, their small sizes and intrinsic faintness have limited detailed studies to small samples of the brightest, extreme cases. Recently, the emergence of large, deep spectroscopic surveys of fields imaged by the Hubble Space Telescope has opened an avenue to study dwarf galaxies with unprecedented detail and statistical significance. In this thesis, we present a comprehensive study of the chemical abundances and physical properties of a large and representative sample of 168 emission-line dwarf galaxies at 0.2 < z < 0.9 in the VIMOS Ultra-Deep Survey (VUDS, Le Fevre et al. 2015). Being designed for high redshift galaxies, VUDS has two main advantages compared to previous surveys: (i) an unprecedented sensitivity, probing dwarf galaxies of faint apparent magnitudes iAB ∼ 23-25, and (ii) a large coverage of three deep fields, COSMOS, ECDFS, and VVDS, for which a wealth of ancillary multiwavelength data is available. This makes our VUDS sample a unique probe for the detailed characterization of their physical properties from their emission line spectra. Its unprecedented sensitivity allows us to detect the faint emission lines needed for the derivation of the electron temperature Te for a large fraction of them. The advantages described here are particularly important for the main goal of this thesis: to investigate the metallicity and ionization of galaxies with stellar masses as low as 10^7 M(solar), and the relation with their ongoing star formation activity. Using a robust Chi-square minimization routine (HCM, Pérez-Montero 2014), based on the comparison of observed line ratios with detailed photoionization models, we derive Te-consistent galaxy-averaged metallicities and ionization parameters for the entire sample. We explore then the low-mass end of the mass-metallicity relation and other scaling laws involving also SFR and size. Overall, we find that our mass-metallicity relation is flatter than derived from previous studies in the same mass range at lower redshift, while the scatter increases at low masses. Comparing our results with the predictions of different chemical evolution models, we find that most dwarfs in our sample are better fit by an open model (i.e. allowing gas in/out-flows) rather than a model where galaxies evolve as a closed box. This suggests a scenario where massive gas flows with the external environment play a key role in the stellar mass growth of these small galaxies. In particular, younger, more metal-poor dwarfs may have recently accreted large amounts of fresh, metal-poor gas which dilutes metallicity and feed the current starburst. On the other hand, stellar feedback may produce large-scale outflows in these small, low-mass systems, favoring both the dispersion and eventual escape of metals and the cesation of violent star formation.
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Bibliographical Information:

Advisor:Ricardo Amorin and Adriano Fontana

School:Universitr degli studi di Roma La Sapienza

School Location:Italy

Source Type:Master's Thesis

Keywords:metallicity, dwarf galaxies, emission line galaxies, mass-metallicity relation


Date of Publication:11/11/2015

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