Why galaxies?

My research interests are centred around understanding the evolution of galaxies across the lifetime of the Universe. We know that there are different types of galaxies in the local Universe. Some are elliptical and typically contain older stars with little to no star formation. As the massive, hot, blue stars with short life times have long since died, these galaxies are described as 'red and dead'. Others are much like our own galaxy, the Milky Way, with a disk-like spiral structure that is rich in dust and gas and still forming stars at a steady rate. I investigate how these different types of systems evolve to be the way they are by looking at everything from individual stars in our own and neighbouring galaxies, the fossil record in local elliptical galaxies, the wider environments galaxies live in, formation at great cosmological distances (i.e. at young ages), and even to the cosmic web of the Universe. All of these aspects hold key information to furthering our understanding of not only how galaxies evolve but also how the Universe came to exist in its current form.

The largest structures in the Universe

Galaxy clusters are big collections of galaxies and are the largest known gravitationally bound structures in the Universe. It is well established that galaxies within dense cluster environments differ from those in the field. For example, the proportion of red and dead elliptical galaxies is higher towards the centre of galaxy clusters (Dressler 1980). Something about these dense environments changes the evolutionary paths of these galaxies, and this can help us understand the evolution of all galaxies if we can figure out what these processes are. Given that galaxy clusters are so massive, it takes significant time from the very simple early Universe to build up sufficient mass to create these large structures. Therefore, as we look to greater distances (and earlier times), these systems are increasingly rare. However, the clusters that do exist at high redshifts provide valuable laboratories for catching these transformative evolutionary processes at play in the early Universe. (Image: A high density of red and dead ellipticals in the cluster Abell 1689. Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), J. Blakeslee (NRC Herzberg Astrophysics Program, Dominion Astrophysical Observatory), and H. Ford (JHU)).

The K-band Multi-Object Spectrograph (KMOS)

During my PhD I worked with an instrument on the Very Large Telescope in the Atacama Desert in Chile called KMOS. KMOS is an integral field spectroscopic instrument, meaning that for individual objects it can observe many spatially resolved spectra. KMOS is also a multi-object spectrograph; it has 24 robotic arms, each of which has an integral field unit (IFU), so it can simultaneously observe 24 targets in great detail. This multi-plexed nature makes KMOS unique at these valuable near-infrared wavelengths and makes a huge range of astrophysical research possible. (Image: The 24 arms of KMOS on the VLT. Credit: STFC/UKATC/ESO.)

The KMOS Cluster Survey (KCS)

I worked as part of the KMOS guaranteed observing program, the KMOS Cluster Survey (KCS; P.I.s Bender & Davies), during my PhD. The aim of KCS was to study the evolution of galaxies in dense environments i.e. clusters of galaxies, between redshifts 1 < z < 2. These distant clusters provide a valuable opportunity to observe evolving galaxies in the early Universe. The clusters are also sufficiently dense so that multiple targets can be observed efficiently with KMOS. To study the properties of the galaxies, we obtained deep absorption-line spectra, observing individual cluster galaxies for over 20 hours each with the 24 IFUs of KMOS. These spectra, when combined with Hubble Space Telescope (HST) images, then enabled us to derive the kinematics, masses, and ages of the stellar populations in the galaxies (Chan et al. 2016, 2018, Beifiori et al. 2017, Prichard et al. 2017b). (Images: Four clusters from the KCS sample spanning 1.39 < z < 1.80, with names, redshifts, and sources shown.)

My work as part of KCS has been focussed on the highest redshift cluster in the sample, JKCS 041 at z = 1.80. In Prichard et al. 2017b, we present the most detailed study of a high-redshift cluster performed to date. It was discovered from X-rays emitted by its hot intracluster medium by Andreon et al. (2009) and spectroscopically confirmed at z = 1.80 by Newman et al. (2014). Building on the work in Beifiori et al. 2017 of the analysis of three KCS clusters, in Prichard et al. 2017b, we investigated the properties of the galaxies and the dynamics of JKCS 041 to understand its galaxy and cluster formation. In order to determine stellar ages of the galaxies, we used the relation between the size, velocity dispersion, and the surface brightness of elliptical galaxies; this is called the fundamental plane (Djorgovski & Davis 1987, Dressler et al. 1987). Assuming that the size and dispersion of galaxies does not change largely over cosmological time, this plane will evolve with the age of the stellar population of the galaxy as the massive bluer stars die off. From the cluster dynamics, morphology, and galaxy stellar age results, we see a cluster in formation consisting of two merging groups. The infalling group comprises younger, more compact galaxies. This links the ages of galaxies to large-scale structure of the cosmic web for the first time at this redshift. The study provides significant insights into galaxy and cluster formation at early cosmological times.

Emission-line stars in the Andromeda Galaxy

The stellar content of a galaxy can reveal information about the way it has formed. The properties of stars, their colours, how bright they are, and whether they show an excess or lack of light at different wavelengths (i.e. emission or absorption lines in their spectrum), can tell us how massive a star is, its chemical composition, age, environment, and even the evolution of the wider galaxy in which it resides. One particularly useful element, due to its abundance in the Universe, is hydrogen. If a star shows a hydrogen emission line, this can reveal some key properties about the star and the physics driving the emission. The best way to study the stellar content of galaxies is to look at the individual stars themselves, however due to the vast distances of the cosmos this is only possible in the very local neighbourhood of our own galaxy. The Milky Way is in a gravitationally bound group of galaxies called the Local Group along with our similar but more massive spiral neighbour, the Andromeda Galaxy or Messier 31 (M31). In a project of unprecedented proportions aimed at characterising the stellar populations and properties of M31, the Panchromatic Hubble Andromeda Treasury (PHAT) survey resolved around a third of the disk of M31 in to more than 100 million stars (Dalcanton et al. 2012). Spectra of ~15,000 stars from the PHAT survey were observed as part of the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey (Dorman et al. 2012) to investigate the properties of stars in PHAT and kinematics of M31 in vast detail.

In Prichard et al. 2017a, we utilised this immense catalogue of six-filter HST data from PHAT and SPLASH spectroscopic data from one of the world's largest optical telescopes (the Keck Observatory on Mauna Kea in Hawaii), to understand the properties of hydrogen emission-line stars in Andromeda. We discovered five distinct categories of emission-line stars which gave insights into stellar evolution, their local environments, and physical processes in the stars. Some of these stars could also be used to measure distances due to their use as standard candles. One subset of these stars revealed potential clues as to the evolution of the whole of M31 and we found that it could have had a more violent accretion and formation history than our own galaxy. This study showed the importance of understanding galaxy evolution down to the stellar level as a means of unravelling their histories. (Image: PHAT and SPLASH sample in M31 with the different categories of hydrogen emission-line stars identified. Credit: Taken from Prichard et al. 2017, background image composed by Dustin Lang.)

The initial mass function (IMF) of IC 1459

How a galaxy forms in the early Universe is still not well understood. Whether it collapses from a huge cloud of dust and gas and fragments into its first stars, or whether smaller galaxies combine together to become the massive galaxies we see today, or whether there are alternate mechanisms altogether is still not clearly known. A key way to understand how galaxies assemble is to probe the light from the stars within it. Beyond the reaches of the Local Group, this becomes harder as we cannot resolve individual stars but instead look at their combined light. Understanding the stellar content can then become difficult as the most massive stars (~1% of the mass) can dominate the light from the system as a whole. We need to understand the stellar composition of the galaxy not only to shed light on how it formed, but also in order to derive its stellar mass — most basic of properties that we know heavily influences or helps us understand its evolutionary path. The number of stars of each mass that make up a galaxy when it was first formed is called the initial mass function (IMF). What this function looks like for galaxies of all shapes, sizes, ages, and masses is one of the most important yet still disputed aspects of astronomy today. Where we struggle is constraining the very faintest end of the IMF for low-mass stars. Fortunately for us, these low-mass stars have certain elements that are sensitive to surface gravity, so that some absorption features in their spectrum actually appear stronger in the lowest mass stars. We can therefore utilise these features to weigh the contribution of low-mass stars in the galaxies.

To investigate the IMF and to test whether it is universal (i.e. the same for all different types of systems at different formation epochs), one can look in detail at the relics left from formation or 'fossil record' of elliptical galaxies in the local Universe. These galaxies with very little dust and gas mean you can probe the stellar light of the galaxy. To measure the IMF across the surface of one of these galaxies might be able to show us if different parts of the galaxies have varying proportions of low-mass stars, and thus reveal information about its formation history. In Prichard, Davies & Vaughan 2018, submitted, I worked with KMOS data in the little utilised mosaic mode and a large optical cube from the Multi Unit Spectroscopic Explorer (MUSE) instrument on the VLT to study a bright, local elliptical galaxy, IC 1459 to test these ideas. This galaxy is a famous example of one with a counter-rotating core (Franx & Illingworth 1988); the stars in the inner part of the galaxy are rapidly counter rotating to the ionised gas and the stars in the outer region. This interesting system is therefore an ideal laboratory in which to test the universality of the IMF across its kinematically distinct components, and to understand its formation history. (Images: Right — optical image of IC 1459. Credit: The Carnegie-Irvine Galaxy Survey. Below — Kinematics of the counter-rotating core in IC 1459 from the KMOS mosaic.)


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