Searching for the Brightest Distant Galaxies in the COSMOS Field.
What do we really understand about the very early Universe? How do the first galaxies compare to present-day galaxies? Why is observing distant galaxies important?
Welcome to the Y-NBS project page, where you can learn all about the XGAL group’s newest research. Y-NBS is a Very Large Telescope (VLT) Y-band and Narrow Band emission line Survey which aims to find some of the brightest and most distant galaxies in the Universe.
In particular, a large chunk of this research will be carried out by me – Heather Wade. I am a PhD student working with David Sobral and I started in October 2019. (Contact me: email@example.com, @heatherphoenixx on Twitter) Y-NBS and the search for distant galaxies will make up a large part of my PhD project, as I hunt for new galaxies and study them in detail to better understand the very early Universe.
The official title of Y-NBS is “HAWK-I into the epoch of reionisation: a pilot for the first z=7.7 Lyman-alpha survey to unveil early ionisation bubbles and the nature of their luminous hosts”. Hopefully, the next few paragraphs will make all of these terms much clearer. This work continues on from SC4K – a very large survey for Lyman-alpha emitters in the COSMOS field – and following analysis produced by the XGAL team which found 4000 distant galaxies in 16 different redshift slices, from z=2.2 to z=6.6. You can read more about that work here: Sobral et al. 2018 and also check out SC4K youtube videos! Y-NBS adds an additional redshift slice at z=7.7, extending the survey further and continuing the great analysis and understanding of distant galaxies.
Firstly, more about the survey itself, which is PI-ed by my PhD supervisor. Y-NBS is a survey carried out using ESO’s Very Large Telescope (VLT). This is a set of four 8-m telescopes in the Atacama Desert in Chile which had their first light is 1998. This facility has produced the most scientific papers for any ground-based telescope, meaning it collects a lot of exciting, cutting-edge astrophysical data! Unfortunately, it is beaten overall by the Hubble Space Telescope (HST), but that’s cheating because the HST has been active for longer and doesn’t have to wait for sunset to observe! The main part of the VLT consists of 4 individual telescopes, all with mirrors of 8.2m in diameter – hence the name, the Very Large Telescope. These 4 telescopes can image the sky in many wavelengths, from visible through to infra-red, and can even work together!
For this particular survey, an instrument on one of the 4 telescopes was used – HAWK-I. This stands for High Acuity Wide field K-band Imager. HAWK-I observes the sky through a variety of filters (or bands). These filters only collect light in a specific wavelength range. Narrow band filters, as you might have guessed, only allow a very narrow range of wavelengths through, whereas a wide band filter is less picky and will allow a wider range of wavelengths through. Y-NBS uses two filters, the “ultra-wide” (1 micron) Y band and the NB1060 narrow band. See Figure 2 for the profiles of these two filters.
We used the VLT (Yepun, UT4) for a total of 43.9 hours of observing time to complete Y-NBS, from December 2016 to March 2018. We did this by using HAWK-I and imaging 69 different patches of the sky, resulting in approximately 700GB of raw data for our survey!
These 69 patches of sky that make up Y-NBS are all within the COSMOS field, which is a 2 square degree area of the sky – the equivalent area of 8 full moons. This piece of sky is thought to contain over 2 million galaxies, some of which are yet to be discovered, and might even be found in Y-NBS. Almost every professional telescope has observed the COSMOS field and this focussing of telescope time and resources has meant that important discoveries have been made faster.
This is not the first time that David and the XGAL Team have worked in the COSMOS field – this is where the SC4K galaxies were observed, and where CR7 was discovered. CR7, or COSMOS Redshift 7, is a galaxy at z=6.6 and it is a really important discovery because it is the brightest known galaxy in the early Universe! Having a redshift value of 6.6 means that we are looking back in time to when the Universe was only 0.8 billion years old. This corresponds to a lookback time of approximately 12.9 billion years. This means that CR7 is 12.9 billion light years away, which is 12,200,000,000,000,000,000,000,000 kilometres! For a lot more information about CR7 and its discoveries and properties, check out these two papers: Sobral et al. 2015, Sobral et al. 2019.
Why search for distant galaxies?
There are still many open questions regarding the early Universe, the stars and galaxies within it, and the processes that lead to what we observe today. Observing the Universe with cutting-edge telescopes is the best way to answer these open questions and better understand the Universe.
Y-NBS specifically looks towards the epoch of reionisation. The epoch of reionisation is a very exciting time in the Universe’s history that we still haven’t fully figured out. It is thought to have begun at z=15, when the very first stars formed, and it lasted to approximately z=6. These values of redshift correspond to when the Universe was 0.27 billion and 0.94 billion years old respectively. Considering the Universe is now approximately 13.7 billions years old, we are looking very far back in time. Before the epoch of reionisation, however, the Universe was formed from fundamental particles that was extremely energetic and hot. Eventually, these particles cooled down, and the electrons and protons could combine to form neutral hydrogen in a process called recombination, which marks the first major phase transition of hydrogen. It just so happens that this neutral hydrogen is a key ingredient for stars to be born, and it is with the formation of these first stars, at around z=15, that the epoch of reionisation began. These early stars and galaxies emitted large quantities of ionising radiation; high energy photons which have enough energy to tear apart molecules. These photons ionised (or reionised) the neutral hydrogen surrounding the first stars, causing bubbles of ionised hydrogen around the stars – this marks the last major phase transition of hydrogen in the Universe. It is these reionisation bubbles, and the bright sources which occupy them, that are of particular interest to us, hence Y-NBS searching for galaxies at z=7.7.
There is a specific type of galaxy that Y-NBS has been especially designed to find – Lyman-Alpha Emitters (LAEs). Lyman-alpha (Lya) is an emission line emitted by galaxies which contain lots of bright, hot, highly-ionising stars (classified as O and B stars). The high-energy photons emitted by these stars ionise the neutral hydrogen surrounding the star. When the protons and electrons recombine, they emit photons of many different wavelengths as the energetic electron returns to the low energy, ground state. Ly-a photons are emitted when the electron transitions from the n=2 energy level to the n=1 energy level. This energy transition translates to a wavelength of 1216A (or 121.6nm) which is in the UV wavelength range. However, for a galaxy above z=2, this emission line will be redshifted into the visible wavelength ranges, making it much easier to observe. It also happens to be intrinsically the strongest emission line, so Lya is great for finding distant galaxies!
Y-NBS has been specifically designed to find LAEs at z=7.7 due to the filters used. If a galaxy is at z=7.7 and emits a Ly-a emission line, it will be seen in the NB1060 filter. Figure 4 visualises this effect: there will be an excess emission (lots of light will be detected) in the filter if it covers the redshifted Ly-a emission line.
Aims and Motivation
So, to summarise, my analysis of the galaxies found in Y-NBS will help to track the evolution of LAEs as I will compare my data for z=7.7 to data from the literature for lower redshifts. This will help us to understand how these galaxies change as we move into the epoch of reionisation. Y-NBS marks the largest search for objects at this distance, as it reaches volumes of 10^6 Mpc^3. This is ground-breaking work and will hopefully give us lots of interesting results about a period of the Universe’s history which hasn’t been studied in this much detail before.
With the results from this current batch of data, larger and deeper surveys can be planned in order to find out even more about the galaxies found. Also, if new galaxies are found at z=7.7, these can be observed in much greater detail with different telescopes in order to better understand the structures and properties. For example, in this paper, CR7 is observed with 4 different telescopes (VLT, HST, ALMA and Subaru) in many different wavelengths meaning that the 3D structure, the mass and many other properties can be deciphered from the data.
Hopefully, once analysis is complete on the Y-NBS data, we will be closer to understanding the galaxies within the early Universe, the epoch of reionisation and how galaxies evolve into our present-day neighbours.
Keep your eyes peeled for results from Y-NBS and for blog posts discussing my work in more detail!
Heather Wade 03-03-20