My graduate student Laura Chajet and I are working with Nahum Arav (Virginia Tech) and his group to derive distances to the outflows in quasars observed at high spectroscopic resolution by Damien Hutsemekers (Liege) and myself. Knowing the distances to these outflows enables us to derive their kinetic luminosities (the amount of kinetic energy produced every second in the form of a wind by the quasar)
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 10% 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 seem to 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 Acceleration and Substructure Constraints in a Quasar Outflow by myself and former York undergrad Sarah Sadavoy. It appears to show acceleration of a BAL outfow along our line of sight, which is one of only 3 or 4 observed cases of such acceleration. (About time to get another spectrum to see if the absorption is continuing....) 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 A Quasar with Broad Absorption in the Balmer Lines (Hall 2007). 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.
My graduate student Jesse Rogerson has first-authored a paper on two of the most unusual BAL quasars known: Chandra Observations of Two Unusual BAL Quasars. We use the X-ray non-detections of these two bright Iron Low-Ionization BAL quasars (FeLoBALs) to constrain the parameters of their absorbing regions.
I am an External Collaborator for the study of Broad Absorption Line quasars in the Sloan Digital Sky Survey III (SDSS-III), working with Niel Brandt (PSU) and others. The first discovery from this program 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). During 2011, on sabbatical at the University of Cambridge, I discovered broad absorption line quasars with redshifted troughs in the SDSS-III. We are still digesting the implications of these objects; a paper will be submitted early in 2012.
For his PhD thesis, my former graduate student Alireza Rafiee 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 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 is working in parallel to investigate the range of emission line profiles that can be produced in magnetohydrodynamic disk winds. Both papers will be submitted in 2012.
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 is completing a Master's Thesis which uses 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 are using data from the literature on quasar asterisms as a new test of their model (Rogerson & Hall 2012).
I am still collaborating with Paul Smith, Gary Schmidt, Dean Hines and Damien Hutsemekers on polarimetry of samples of unusual and `normal' BALs from the SDSS.
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.
