Studying the growth of galaxies with JWST
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Date
26/11/2019Author
Kemp, Thomas William
Metadata
Abstract
The James Webb Space Telescope (JWST or Webb) is a large (6.5m diameter
primary mirror) infrared (0.6 < λ < 30μm) space telescope. It impressive size and
wavelength coverage will revolutionise the field of galaxy formation and evolution,
enabling the community to push back the observational frontier to within a few
100 million years of the Big Bang. With JWST expected to launch in 2021,
a detailed understanding of its capabilities is essential to guarantee the success
of the mission. In this thesis, I present a new analysis of how best JWST can
be utilised to study the evolution and formation of galaxies across cosmic time.
Primarily this work uses simulated JWST and ancillary HST observations in
an attempt to assess JWST's capabilities, specifically its ability to study high-mass,
high-redshift, dusty star-forming galaxies in the Universe. Additionally, I
use currently available data from the UV/optical to sub-mm to perform a new
study of the prevalence of high-mass, high-redshift, dusty star-forming galaxies
accessible before the launch of JWST.
Firstly, I introduce a piece of software that I have built to simulate observations
that will be made using the Mid-infrared Instrument (MIRI) onboard JWST.
This piece of software is capable of simulating all of MIRI's available observing
modes: MIRI Imager, Low-Resolution Spectrograph (LRS), Medium Resolution
Spectrometer (MRS). I then use this software to show the potential for measurements
of the Hα emission line in high-redshift star-forming galaxies using the
MRS IFU. At redshifts z > 6.7, MIRI is the only instrument onboard JWST
that can directly observe Hα, a sensitive star-formation indicator, tracing very
recent star-formation. I show that with approximately ≃ 6-hour integration time
with the MIRI MRS, the Hα emission line in the brightest known z ≃ 7 galaxies
can be detected to a SNR of approximately ≃ 11. Therefore, I conclude that
the MIRI MRS could be an impressive tool for determining the star-formation
rates of high-redshift galaxies in the epoch of reionisation. Secondly, I present an
overview of the Public Release Imaging for Extragalactic Research (PRIMER)
proposal submitted as part of the Director's Discretionary Early Release Science
(DD-ERS) programme for Cycle-1 of JWST. PRIMER is a large (52hr), deep,
fully-sampled NIRCam and MIRI imaging programme, designed to efficiently
(≈ 75% observing efficiency) observe the faintest galaxies in the best-studied
available (Non-GTO Covered) equatorial HST CANDELS field, COSMOS. As
well as detailing the science goals and observational design of this programme,
I place it in the context of other planned JWST programmes (ERS & GTO)
focussed on observations of high-redshift galaxies.
Thirdly, I present a new analysis of the potential power of deep, near-infrared,
imaging surveys with the JWST to improve our knowledge of galaxy evolution.
In this work, I properly simulate what can be achieved with realistic survey
strategies, and utilise rigorous signal:noise calculations to calculate the resulting
posterior constraints on the physical properties of galaxies. I explore a broad
range of assumed input galaxy types (> 20,000 models, including extremely
dusty objects) across a wide redshift range (out to z ≃ 12), while at the
same time considering a realistic mix of galaxy properties based on our current
knowledge of the evolving population (as quantified through the Empirical Galaxy
Generator: EGG). While our main focus is on imaging surveys with NIRCam,
spanning λobs = 0:8 - 5:0 μm, an important goal of this work is to quantify
the impact/added-value of: i) parallel imaging observations with MIRI at longer
wavelengths and ii) deeper supporting optical/UV imaging with HST (potentially
prior to JWST launch) in maximising the power and robustness of a major
extragalactic NIRCam survey. I show that MIRI parallel 7.7-μm imaging is
of most value for better constraining the redshifts and stellar masses of the
dustiest (AV > 3) galaxies, while deep B-band imaging (reaching mAB ' 28:5mag)
with ACS on HST is vital for determining the redshifts of the large numbers of
faint/low-mass, z < 5 galaxies that will be detected in a deep JWST NIRCam
survey.
Finally, I attempt to assess the prevalence of the dusty subset of galaxies that
MIRI will observe, utilising the current deepest UV/optical/near-IR and far-
IR/sub-mm observations. The data collected from numerous telescopes covers a
total of 1.8 deg2 in the UDS and COSMOS fields. Combining the UV and IR data,
I assess the total number density as a function of cosmic time and compare the
methods of detecting/selecting high-redshift, high mass dusty galaxies. I calculate
the total star-formation rate (SFR) as a function of redshift and compare the
contribution from sub-mm galaxies and UV/optically-selected dusty galaxies. I
also calculate the star-formation rate density (SFRD) across cosmic history in
an attempt to extend the Madau & Dickinson (2014) plot to higher redshifts
for obscured star-formation. I show that both the UV/optical and (sub)-mm
approach to detecting high-redshift dusty galaxies both produce a consistent
estimation of their evolving comoving number density at high redshift. I find
clear evidence of a rapid decline in the total comoving number density of these
objects beyond z > 3, which results in a similarly steep decline in the SFRD
of the Universe contributed by obscured star-formation. Comparing to recent
work through in the literature, my results strengthen the existing evidence for a
transition between unobscured & obscured star formation as the dominant mode
of star-formation activity at z ≃ 2-3.