X-ray binary (XRB) populations in high-redshift star-forming galaxies
There is growing evidence that star-forming galaxies in the early Universe drove the bulk of the reionisation of neutral hydrogen within the first billion years of the Universe's lifetime. However, it is still unclear how many ionising photons were produced by massive stars in these galaxies, and how many of these escaped into the intergalactic medium to drive reionisation. It may also be possible that a class of high-mass binary star systems, which includes a stellar mass black hole accreting material from a companion high mass star, shining brightly at X-ray wavelengths, may play a role in the production and escape of ionising photons.
My recent research has been focused on identifying and constraining the X-ray activity in very distant galaxies powered by these X-ray binary systems in a bid to constrain their overall contribution to the photon production budget of star-forming galaxies, and their eventual impact on the reionisation of the Universe. A preprint reporting the latest results from this work can be found here:
The VANDELS Survey: New constraints on the high-mass X-ray binary populations in normal star-forming galaxies at 3 < z < 5.5, 2021
Physical properties of Helium II emitting galaxies in VANDELS
Galaxies in the early Universe that emit copius amounts of Helium II are likely to be dominated by very massive stars that have very few metals. Such galaxies represent a population of the first galaxies that may have formed in the Universe, and studying large samples can reveal the physics that dominates within the earliest galaxies to appear in the Universe.
The paper reporting the Helium II emitting galaxies from VANDELS can be found here:
The properties of He II λ1640 emitters at z = 2.5-5 from the VANDELS survey, 2020
Our follow-up work has investigated whether the presence of exotic stellar systems, such as interacting black hole and massive star binaries, or X-ray binaries, can power the bulk of He II emission that is observed in distant galaxies. The paper reporting these results can be found here:
X-ray properties of He II λ1640 emitting galaxies in VANDELS, 2020
Discovery of the most distant radio galaxy to date
In June 2018, my research team discovered the most distant radio galaxy observed to date, named TGSS J1530+1049, at a distance of 12.7 billion light years away (redshift = 5.72)! In comparison, the age of the Universe is 13.6 billion years. Therefore, this is a galaxy that was formed within the first 7% of the Universe's lifetime! This discovery broke the distance record for a radio galaxy after almost 20 years. Radio galaxies are some of the most massive galaxies in the Universe and harbour a supermassive black hole in their centres that is actively eating up the gas and dust that surrounds it. Discovering such an object at such a large distance from us poses some interesting challenges about how and when the seeds of this galaxy and the supermassive black hole were sown, and how they evolved to be so massive in such a short period of time after the Big Bang.
The paper reporting this discovery has now been published in the Monthly Notices of the Royal Astronomical Society and can be found here:
Discovery of a radio galaxy at z = 5.72, 2018
In the media:
Astronomy Now: Most distant radio galaxy, host to a voracious black hole, is found
Phys.org: The most distant radio galaxy discovered
Astronomie.nl: Astronomen ontdekken verst verwijderde radiostelsel ooit (Astronomers discover the most distant radio galaxy ever)
BBC Brasil: Radiogaláxia mais distante da Terra é descoberta com participação de brasileiro - e dá mais pistas sobre o Big Bang (Most distant radio galaxy is discovered with Brazilian participation - and gives more clues about the Big Bang)
Media INAF: Scoperta la radiogalassia più lontana (The most distant radio galaxy discovered)
High-redshift extreme spectrum project (HiZESP)
We have started a large campaign to hunt for distant radio galaxies by taking advantage of the new all-sky surveys at low radio frequencies using telescopes such as the Giant Metrewave Radio Telescope (GMRT) in India, the Very Large Array (VLA) in USA and the Low Frequency Array (LOFAR) in the Netherlands and all of Europe. Early results from this project can be found here:
A search for faint high-redshift radio galaxy candidates at 150 MHz, 2018
In addition to studying these sources at radio wavelengths, it is essential to also observe them at optical and infrared wavelengths. As part of this project, we have been awarded observing time on telescopes all over the world to a) obtain spectra and determine redshifts of candidate high-redshift radio galaxies and b) observe our targets at near-infrared wavelengths to study the underlying stellar populations.
The telescopes being used for this project include the Hobby-Eberly Telescope (HET), Gemini North, William Herschel Telescope (WHT) and the Large Binocular Telescope (LBT). The paper reporting these observations has now been accepted for publication in the Monthly Notices of the Royal Astronomical Society and can be found here:
The nature of faint radio galaxies at high redshifts, 2019
Modelling the growth of radio AGN across cosmic time
One of the first projects of my PhD was to build a model capable of tracking the evolution of radio galaxies from first principles, and predicting the luminosity and size distribution of radio sources at any given epoch. This model included recipes for a host of physical phenomenon and the results can be found here:
Modelling the luminosities and sizes of radio sources: radio luminosity function at z = 6, 2017