Evolution of emission line properties and metallicities of star-forming galaxies up to z ~ 3
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Date
26/11/2015Author
Cullen, Fergus
Metadata
Abstract
Until recently, obtaining rest-frame optical spectra of galaxies at z > 1 was a
time consuming and challenging observation due to the difficult nature of near-infrared
(near-IR) spectroscopy. However, with the advent of second generation
ground-based near-IR spectrographs (e.g. KMOS, MOSFIRE), and the new low
resolution near-IR grisms on the Hubble Space Telescope (HST), we have entered
a new era in the study of high redshift galaxies. This thesis explores the physical
properties of star-forming galaxies in the redshift range 1 < z < 3 by utilising a
custom reduction of the 3D-HST near-IR grism spectroscopic survey.
One of the most important observational constraints on the evolution of galaxies is
the mass-metallicity relation (MZR), which is sensitive to both the star-formation
history and various inflow/outflow processes. I use the 3D-HST spectra to
provide a new constraint on the MZR at 2:0 < z < 2:3, and moreover measure
the O/H abundance directly from the oxygen and hydrogen emission lines ([OII],
[OIII] and Hβ) as opposed to the more common method at high redshift of
inferring O/H from the N/H ratio (via [NII] and Hα). I show that the traditional
form of the MZR is recovered from the 3D-HST data, with metallicity increasing
with the stellar mass of a galaxy. However, the absolute metallicity values I
derive are inconsistent with previous N/H-based measurements of metallicity at
these redshifts. Moreover, I show that the 3D-HST data is inconsistent with the
`fundamental metallicity relation' (FMR), and that, contrary to previous claims,
this local Universe relation may not hold out to z & 2.
To investigate this metallicity discrepancy further, I measure the evolution of the
[OIII]/Hβ nebular emission line ratio in the 3D-HST spectra over the redshift
range 1:3 < z < 2:3. I compare this observed line ratio evolution with state-of-the-
art theoretical models which take into account the independent evolution
of the ionization parameter, electron density and metallicity of star-forming
regions with redshift. The homogeneous 3D-HST dataset allows me to perform
a consistent analysis of this evolution which takes into account line luminosity
selection effects. I show that, according to models, the observed [OIII]/Hβ
evolution cannot be accounted for by pure metallicity evolution. Instead I am
able to infer that the line ratio evolution is more consistent with, at the very
least, an evolution to stronger ionizing conditions at high redshift, and perhaps
even denser star-forming regions. I explore how this result can also explain the
observed discrepancy between high redshift metallicity measurements.
In light of this finding, I revisit the MZR at z >~ 2 and employ a purely theoretical
approach to inferring metallicities from nebular lines, which is able to account for
an evolution in ionization conditions. I then use a selection of galaxies from the
local Universe, which mimic the properties of high redshift galaxies, to derive a
more robust ionization sensitive, conversion, between N/H and O/H. With this
new conversion which I am able to bring the previous inconsistent metallicity
measurements at z >~ 2 back into agreement. Finally, I am able to show that, in
this new formalism, the metallicity evolution between z = 2 and z = 3 is perhaps
not as large as previously reported.
To conclude I discuss ongoing work as part of the KMOS Deep Survey (KDS)
being undertaken with the near near-IR Multi-Object Spectrograph KMOS on the
VLT. I describe the observations and data reduction that has been completed to
date and describe how this instrument will allow me to extend the work presented
in this thesis to z > 3. I also introduce FIGS, a new HST near-IR grism survey
seeking to spectroscopically identify galaxies at 5:5 < z < 8:5 and work I have
begun in exploring this dataset.