I am particularly interested in Broad Absorption Line or BAL quasars, which show absorption from gas with blueshifted outflow velocities of typically <=0.1c (in C IV). About 30% of quasars exhibit BAL troughs, but this is usually attributed to an orientation effect. Most quasars probably have BAL outflows covering ~30% of the sky as seen from the quasar, with mass loss rates possibly comparable to the accretion rates required to power the quasar. 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.
One 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 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. We also show that the gas in this absorber must consist of individual subunits rather than being a single monolithic absorber.
The most extreme examples of BAL quasars may be a help in this endeavor, as they illustrate the full range of parameter space spanned by BAL outflows. The SDSS has confirmed that there exist populations of unusual BAL quasars (Hall et al. 2002). One of the most unusual BAL quasars found in the entire SDSS was described in my 2007 paper A Quasar with Broad Absorption in the Balmer Lines. Its absorption appears to originate in a partially-ionized region of sufficient optical depth that Lyman photons must random-walk their way out. That leads at any given time to a substantial population of H I excited to the n=2 shell from which Balmer absorption occurs.
I am an External Collaborator for the study of Broad Absorption Line quasars in the Sloan Digital Sky Survey III and IV, working with Niel Brandt (PSU) and others. During 2011, on sabbatical at the University of Cambridge, I discovered in the SDSS-III a number of Broad Absorption Line Quasars With Redshifted Troughs. We 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. We have been granted observing time on the Gemini-North telescope for follow-up near-infrared observations to help pin down these objects' redshifts and to look for any evidence that they are binary quasars.
Actually, though, the first discovery from the study of BAL quasars in the SDSS-III occurred 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 Fe II 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. The next study (Filiz Ak et al. 2013) showed, among other things, that changes in shielding gas may drive correlated variability between troughs separated by thousands of km/s or more.
In related research, my postdoc Dr. Paola Rodriguez Hidalgo, my graduate student 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 currently have Gemini spectroscopic observations of many these targets in the queue. Those third-epoch observations will constrain whether emerging BAL troughs rapidly or slowly reach a constant absorption strength, or whether they repeatedly strengthen and weaken before reaching the more or less steady state seen in most BAL outflows.
However, spectroscopic observations of BAL quasars in search of trough variability are time-consuming and do not always reveal the hoped-for variability. To circumvent this problem, my graduate student Jesse Rogerson is using g,r,i band imaging from the Canada-France-Hawaii telescope to search for colour changes in quasars indicative of changing or newly appeared absorption troughs. Follow-up Gemini spectroscopy is being used to verify and study the changed absorption in detail.
Dr. Rodriguez Hidalgo and I are also collaborating with Daniel Proga (UNLV) and Fred Hamann (Florida) to study the time variability of BAL outflows in Proga's computer models of accretion disk winds.
My group (myself, postdoc, graduate students, undergraduates) and occasional 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. Help in these endeavors is always welcome!
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 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 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).
With Laura Chajet and York undergraduates Emil Noordeh and Erik Weiss, I have 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 am helping to search for PHL 1811 analogues (quasars with very weak X-ray emission and 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 appear to have succeeded (Wu et al. 2011 & Wu et al. 2012).
I have always been interested in gravitational lensing, because it's cool. After joining the SDSS collaboration as a Princeton/Catolica postdoc in 2000, I was a part of the SDSS quasar lens collaboration which has produced many papers on lensed quasars. Most recently, in 2009, I discovered the most distant gravitationally lensed quasar currently known (z=4.8; McGreer et al. 2010). I have some ideas for studying gravitationally lensed arcs, as well, but they may not come to fruition until the era of 30-meter class telescopes...
Meanwhile, my graduate student Jesse Rogerson's Master's Thesis used quasar asterisms (binaries, pairs and lenses) to probe the spatial structure of Mg II absorption in intervening galaxy halos. Chen & Tinker (2008) used single-quasar observations to produce a model of Mg II halos; we used data from the literature on quasar asterisms as a new test of their model (Rogerson & Hall 2012).
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.
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, 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 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).