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Observations of High-Redshift Quasars
JWST’s PEARLS: A z=6 Quasar in a Train-Wreck Galaxy Merger System
arxiv: 2502.20550
As part of the PEARLS GTO program, we observed the z=5.89 quasar NDWFS J1425+3254 with JWST NIRSpec integral field spectroscopy (IFS).
We find that the quasar host has significant ongoing obscured star formation, as well as possibly one of the most extreme quasar-driven outflows in the early Universe.
We find that two companion galaxies are merging with the quasar host, with a connecting bridge of gas between one companion and the quasar providing smoking-gun evidence for the ongoing interaction.
The system is a “train-wreck” merger of three galaxies, with star formation and extreme quasar activity that were likely triggered by these ongoing interactions.
[OIII] velocity map for the train-wreck merger system NDWFS J1425+3254. Credits: M.Marshall
GA-NIFS & EIGER: A merging quasar host at z=7 with an overmassive black hole
arxiv: 2402.17163
We combined JWST observations from the GA-NIFS (IFS) and EIGER (imaging) teams to study the z=7.08 quasar J1120+0641, giving the most comprehensive understanding of a high-z quasar host galaxy with JWST to date.
We found that the quasar host is undergoing a merger with a bright companion galaxy. This alters the stellar mass measurements, which were previously overestimated due to the presence of this companion and a lack of emission line measurements. Now, we measure an extreme black hole-stellar mass ratio of ~50% for J1120, the most overmassive black hole ever discovered.
Black hole and host galaxy properties of two z ≃ 6.8 quasars from the NIRSpec IFU
DOI: 10.1051/0004-6361/202346113 arxiv: 2302.04795
This work highlights the exceptional capabilities of the JWST NIRSpec IFS for observing quasars in the early Universe, presenting data for our first high-z quasars observed in the GA-NIFS programme, DELS J0411–0907 at z = 6.82 and VDES J0020–3653 at z = 6.86.
We measured black hole masses, discovered multple merging companion galaxies, studied ionized gas outflows, and more.
Limits to Rest-Frame Ultraviolet Emission From Far-Infrared-Luminous z~6 Quasar Hosts.
DOI:10.3847/1538-4357/abaa4c, arXiv:2007.13859
We used observations from HST to obtain deep upper limits on the rest-frame ultraviolet luminosities of six z=6 quasar host galaxies. We discover up to 9 potential
companion galaxies surrounding these quasars, which may be interacting with their host galaxies. The two potential companions around NDWFS J1425+3254 have since been spectroscopically confirmed with our JWST data.
The BlueTides Hydrodynamical Simulation
Using the BlueTides cosmological hydrodynamical simulation, I have studied a range of properties of high-z galaxies and quasars.
In “The host galaxies of z=7 quasars: predictions from the BlueTides simulation” DOI: 10.1093/mnras/staa2982, arXiv:1912.03428, we studied the physical properties of z=7 quasar host galaxies within the simulation, such as their stellar masses, morphologies, sizes, and their location on the black hole–stellar mass relation. We then made explicit predictions for whether JWST could successfully detect these host galaxies in “Observing the host galaxies of high-redshift quasars with JWST: predictions from the BlueTides simulation” DOI: 10.1093/mnras/stab1763 arXiv:2101.01219, by creating mock images and then performing observational quasar-subtraction techniques. We predicted that quasar subtraction will be challenging, however a reasonable fraction of quasar host galaxies should be detectable with JWST NIRCam, which has indeed been true.
I then began to focus on the properties of high-z galaxies. In “The Impact of Dust on the Sizes of Galaxies in the Epoch of Reionization” DOI: 10.1093/mnras/stac380 arXiv:2110.12075 we studied the sizes of galaxies in BlueTides. We found that dust is the predominant cause of the observed positive size–luminosity relation, and that this relation varies with wavelength across the rest-frame ultraviolet and optical. With my undergraduate student Katelyn Watts, we created mock images of each BlueTides galaxy with JWST, Hubble, Roman, Euclid, Subaru, and VISTA, across a range of near- and mid-infrared filters. We released these mock images as the BlueTides Mock Image Catalogue, publicly available via the Mikulski Archive for Space Telescopes (MAST) archive at DOI: 10.17909/er09-4527. Example codes for using this catalogue can be found on GitHub. From these mock images, we made predictions for the expected number counts of observed high-z galaxies from JWST Cycle 1 surveys, as published in “The BlueTides Mock Image Catalogue: Simulated observations of high-redshift galaxies and predictions for JWST imaging surveys” DOI: 10.1093/mnras/stac211 arxiv: 2206.08941.
A merger of a quasar host galaxy in the BlueTides simulation. Credits: Y. Ni (Carnegie Mellon University) and L. Hustak (STScI)
The Meraxes Semi-Analytic Model
Meraxes is a semi-analytic model designed to study galaxy formation and evolution. I updated the Meraxes model to study the sizes, angular momenta and morphologies of high-z galaxies in “Dark-ages Reionization and Galaxy Formation Simulation – XVII. Sizes, angular momenta and morphologies of high redshift galaxies” 10.1093/mnras/stz1810, arXiv:1904.01619. I also used Meraxes to study the evolution of black holes and their host galaxies from high redshift to the present day in “Dark-ages reionization and galaxy formation simulation - XVIII. The high-redshift evolution of black holes and their host galaxies” DOI:10.1093/mnras/staa936, arXiv:1910.08124. The model predicts no significant evolution in the black hole–host mass relations out to high redshift, with the growth of galaxies and black holes tightly related even in the early Universe. We also examined the growth mechanisms of black holes in Meraxes, finding that the majority of black hole growth is caused by internal disc instabilities, and not by galaxy mergers.
Other Projects
FORECASTOR - II. Simulating Galaxy Surveys with the Cosmological Advanced Survey Telescope for Optical and UV Research
DOI: 10.1093/mnras/staf065 arxiv: 2402.17163
Our undergraduate student Laurie Amen developed code to make mock images for the upcoming CASTOR telescope. We used this code to predict the performance of the planned CASTOR extragalactic surveys.
Triggering active galactic nuclei in galaxy clusters
DOI: 10.1093/mnras/stx2996, arXiv:1708.05519
For my honours project, I used the semi-analytic galaxy formation model SAGE to study the triggering of active galactic nuclei (AGN) in galaxy clusters. We predicted that ram pressure is likely to be an important mechanism for triggering star formation and AGN activity in clusters, that has since been seen observationally.
Methanol absorption in PKS B1830-211 at milliarcsecond scales
DOI: 10.1093/mnras/stw3295
As an undergraduate project, I published Australian Long Baseline Array observations of 12.2-GHz methanol absorption detected in the gravitational lens system PKS B1830−211.