Introduction
Imagine voyaging out of our solar system and beyond all the stars in our Milky Way galaxy. The faint dots of light visible to you then then wouldn't be stars, but entire distant galaxies. At the centre of any of those galaxies you'll find a swarm of stars and, if the galaxy is big enough, a black hole with a mass of millions or even billions of Suns.
Watch as one of the clouds of hydrogen and helium gas and trace elements in that galaxy ventures too close to the black hole. Gravity stretches the gas cloud into a disk of gas bigger than our solar system, orbiting at incredible speeds but slowly spiralling inwards to accrete onto the black hole. In the inner disk, friction makes the gas hotter than the Sun over an area millions of times larger than the Sun's surface. That enormous, glowing disk can be seen all the way across the universe, and we call it a quasar.
This turbulent accretion disk of hot, rotating gas often tosses up gas clumps which are pushed outward by the light from the inner disk. The clumps form spiralling streamers of gas which block the inner disk from view in some directions and which race out into the galaxy hosting the quasar. These fast-moving outflows can reverse the inflow of gas clouds supplying the accretion disk, shutting down the accretion process and turning off the quasar. They can even shock-heat and blow out almost all the gas in the galaxy, ending star formation in it and drastically changing its future appearance and environment.
My research team and collaborators and I are surveying quasars for outflows, studying their time variability, and comparing them to computer simulations. We are learning how quasars and their outflows work so that we can refine the picture outlined above and improve our models of how galaxies form and develop.
Research Pursuits
I am a member of the Sloan Digital Sky Survey Mark V (SDSS-V) Black Hole Mapper survey. I study the properties of quasars and their outflows. Of interest for potential students is the study of broad absorption line quasars in SDSS-V (e.g., Hemler et al. 2019) and the modeling of quasar spectra using machine learning.
- Intrinsic Absorption in Quasars
- Physical Properties of Quasars
- Unusual White Dwarfs
- Miscellaneous
Intrinsic Absorption in Quasars
Up to 70% of quasars and AGN show UV absorption from gas with blueshifted outflow velocities from a few hundreds of km/s to 20% of lightspeed. I am trying to understand these winds and outflows in all their gory glory.
I am particularly interested in Broad Absorption Line or BAL quasars. About 30% of quasars exhibit BAL troughs; this is probably due to a combination of orientation and evolution. Most quasars probably have BAL outflows covering on average ~30% of the sky as seen from the quasar, with mass loss rates comparable to the accretion rates required to power the quasar, and with the coverage and mass loss rates dependent on the emission spectrum from the inner accretion disk in the quasars. Also, some or perhaps even all young quasars may experience a phase of close to 100% covering by BAL outflows. Therefore an understanding of BAL outflows is required for an understanding of quasars as a whole.
The most extreme examples of BAL quasars may help in this endeavor, as they illustrate the full range of parameter space spanned by BAL outflows. The Sloan Digital Sky Survey (SDSS) has confirmed that there exist populations of unusual BAL quasars (Hall et al. 2002).
For example, I discovered in the SDSS-III a number of Broad Absorption Line Quasars With Redshifted Troughs. My collaborators and I are still digesting the implications of these objects, but they are likely to be examples of high-velocity infall or of rotationally dominated outflows, or both, with a dark horse explanation that they are binary quasars (where one quasar illuminates another quasar's outflow) still in the running for some objects. Further observations of these objects are called for, as is some assistance in finishing writing up the implications of some follow-up Gemini-North telescope near-infrared observations already in hand, intended to help pin down these objects' redshifts and to look for any evidence that they are binary quasars.
The first discovery from the study of BAL quasars in the SDSS-III occurred just in the target selection stage, when I discovered that the formerly heavily absorbed FeLoBAL quasar J1408+3054 exhibited much weaker absorption in a recent spectrum. Further observations showed that its iron absorption has vanished (animation), leaving it merely a LoBAL quasar, at least for now (Hall et al. 2011). The first data release from SDSS-III occurred in 2012, and PSU graduate student Nur Filiz Ak led the first BAL quasar paper utilizing this dataset (Filiz Ak et al. 2012). This study of the disappearance of BAL outflows suggests that a considerable fraction of such outflows have lifetimes along our line of sight of at most a century.
In related research, my former postdoc (now U. Washington Bothell Professor) Dr. Paola Rodriguez Hidalgo, graduate student (now York University Division of Natural Science Professor) Jesse Rogerson and I have identified a population of emergent BAL troughs which were not present in SDSS spectra taken in the first half of the 2000s but were present in SDSS-III spectra of the same quasars taken starting in 2010. We used Gemini spectroscopic follow-up observations to student how emerging BAL troughs behave (Rogerson et al. 2018). We observed coordinated variability among pairs of troughs at different velocities, likely due to clouds at different velocities responding to the same changes in illumination.
Another BAL quasar of particular interest is that studied in the paper Acceleration and Substructure Constraints in a Quasar Outflow by myself and former York undergrad (now Queen's University Professor) Sarah Sadavoy. It appears to show acceleration of a BAL outflow along our line of sight, which is one of only 3 or 4 observed cases of such acceleration. This quasar has continued to be observed by the Sloan Digital Sky Survey, and further analysis of it swould make for a nice student project.
My group (myself, graduate students, and undergraduates) and various collaborators are also thinking about what we can infer about BAL quasar variability and quasar accretion disk structure from existing observations, and what future observations would be most insightful.
Physical Properties Of Quasars
I am interested in how quasars work. Currently I am a member of the SDSS-V Black Hole Mapper project, a successor to the SDSS-RM project to infer the masses of distant quasars by measuring the sizes of their emission-line regions. (Given a size R and a characteristic emission-line velocity V, the mass M of the black hole is found essentially by using the Keplerian orbital velocity formula V=sqrt{G*M/R}.)
Quasar accretion disks do not always seem to behave as expected in the simplest theoretical models. I am interested in investigating possible contributors to those differences. One such possibility is that quasar accretion discs might not emit as blackbody radiators. I have also done some modeling of temperature spikes in quasar accretion disc temperature profiles to see what would be required to explain the larger-than-expected and roughly constant-temperature inner accretion disks found by the microlensing study of Blackburne et al. (2011). We find that the observations could be explained by sub-Keplerian discs partially supported by magnetic pressure and with a temperature spike at the radius where the disc material is slowed down from Keplerian to sub-Keplerian.
With Niel Brandt (PSU) and others, I have helped to search for and study PHL 1811 analogues (quasars with very weak X-ray emission, weak UV emission lines and unusual line ratios). Karen Leighly (Oklahoma) has argued that the weak emission lines of PHL 1811 are the result of its weak X-ray emission. We have turned this around and have obtained X-ray data on quasars with weak UV emission line fluxes. Our hope was to find a population of X-ray weak quasars which we can study further, to understand the origin of the X-ray weakness, and we succeeded (Wu et al. 2011 & Wu et al. 2012). The general subpopulation of "weak-line quasars" continues to be an area of study (Ni et al. 2018, Ni et al. 2020).
For his PhD thesis, my former graduate student Dr. Alireza Rafiee (now a lecturer at York University) has investigated the subtleties of deriving black hole masses from single-epoch spectroscopy. The first scientific use of the resulting black hole masses was presented in Rapidly Spinning Black Holes: An Open Question (Rafiee & Hall 2009), followed by a catalog paper (Rafiee & Hall 2011a) and a paper on the so-called sub-Eddington boundary (Rafiee & Hall 2011b).
With former York undergraduate Rachel Ward (now at the Ontario Science Centre) and my former graduate student Laura Chajet, I have also extended the Murray et al. (1995, 1998) model for producing single-peaked emission lines from rotating disk winds, relaxing some of the assumptions made by Murray et al. Our goal is to investigate what combinations of parameters can reproduce the large (800 km/s on average) blueshifts seen in the C IV emission lines of quasars, which are not matched by the original Murray et al. model. My former graduate student Laura Chajet worked in parallel to investigate the range of emission line profiles that can be produced in magnetohydrodynamic disk winds (Chajet & Hall 2013; Chajet & Hall 2017).
Unusual White Dwarfs
If you show me an astronomical spectrum and say, "No one understands this object," that's it, I'm hooked. So I have authored two papers on unusual white dwarfs.
In A Nearby Old Halo White Dwarf Candidate from the Sloan Digital Sky Survey (Hall et al 2008), high school student and York summer intern Akshay Awal helped me to discover one of the closest cool white dwarfs to the Earth. It's close enough that it can be seen zipping across the sky in this animation.
In C_2 in Peculiar DQ White Dwarfs (Hall & Maxwell 2008), former York undergrad Aaron Maxwell helped me rule out all explanations for the molecular bands seen in the subclass of "peculiar DQ" white dwarfs except for that of the C_2 molecule under extremely high pressures.
Miscellaneous
In The Naked-eye Optical Transient OT 120926 (Zhao, Hall, Delaney & Sandal 2013), published in the Journal of the American Association of Variable Star Observers, we report a magnitude 4.7 optical transient imaged by an undergraduate student with a hand-held camera. It was probably due to a flare on an M dwarf star with a record-setting amplitude of just over 11 magnitudes.
A recent theory for the formation of Earth's Moon proposes that it formed in a close orbit around the Earth and for a time had a very elliptical orbit, phase-locked so that perigee and apogee occurred at the quarter-moon phases. In an article in The Physics Teacher, former York undergraduate Emil Noordeh and I produced animations of the phases of Moon in this scenario, to illustrate the varying sizes of the Moon and the varying lengths of each phase due to the elliptical orbit.
With former York undergraduate Patrik Pirkola, I came up with a method of simulating the lower gravity of Mars in the classroom, using an empty vinyl storage bag (Pirkola & Hall 2015).