My Research - Part 1. Quasar Outflows
Quasar Outflows - Variability
My PhD dissertation research focused on quasar outflows, in particular broad absorption line (BAL) outflows. These are high-velocity outflows that likely exist in all quasars and could play a major role in feedback to galaxy evolution. BAL refers to the broad absorption features, greater than 2000 km/s in width, that are seen in ~10-15% of quasar spectra. We published a series of three papers on the variability in BAL outflows in a sample of 24 luminous quasars. Studying the variability in these BALs can help us understand the structure, evolution, and basic physical properties of these outflows. We conducted a BAL monitoring program, which now includes 163 spectra of 24 luminous quasars at z=1.2-2.9, covering time-scales from ~1 week to 8 years in the quasar rest-frame (Paper 1 - Capellupo et al. 2011). We investigate changes in both the CIV λ1550 BALs and the SiIV λ1400 BALs, and we see a variety of phenomena, including some BALs with dramatic variability over a wide range in outflow velocities (Fig. 1) and other BALs that did not change at all over the entire observation period. Variability generally occurred in only portions of BAL troughs, as in Fig. 2. The fraction of quasars with CIV BAL variability increased as we added more observing epochs to our sample, with 88% of the quasars showing CIV BAL variability. When comparing CIV to SiIV BAL variability, we found that SiIV BALs are more likely to vary than CIV BALs (Paper 2 - Capellupo et al. 2012). When both the CIV and SiIV BALs vary within the same quasar, they always vary in the same sense (with both lines either getting stronger or weaker).
The most recent data in our monitoring campaign, taken in Spring 2010, specifically focuses on short-term monitoring on time-scales <1 month in the quasar rest-frame. Our data extends down to 1-week time-scales, and we detect variability in one of our quasars on this short time-scale (Fig. 2). We report this result in Paper 3 (Capellupo et al. 2013), where we discuss the likelihood that the variability is caused by the outflow, or some substructure in the flow, moving across our line-of-sight to the quasar. Adopting this scenario, typical variability times of the order of ~1 yr indicate crossing speeds of a few thousand km/s and radial distances of ~1 pc from the central black hole. However, the most rapid BAL changes occurring in 8-10 days require crossing speeds of 17,000 to 84,000 km/s and radial distances of only 0.001-0.02 pc. These speeds are similar to or greater than the observed radial outflow speeds, and the inferred locations are within the nominal radius of the broad emission-line region.
Quasar Outflows - Outflow Rates and Energetics
In order to determine the viability of quasar outflows as a feedback mechanism affecting galaxy evolution, we need estimates of their mass outflow rates and kinetic energy yields. These quantities depend on the column densities of the flows, which are difficult to obtain directly from spectra of the BALs. We turn to a low-abundance species, PV λ1118, 1128. Phosphorus is much less abundant than, for example, carbon (P/C ~ 0.001 in the Sun), so a detection of a PV BAL indicates that other lines, such as CIV, are saturated. We detect variability in a PV BAL in Q1413+1143, corresponding to variable SiIV and CIV BALs. The variability in the PV BAL confirms that the absorption is intrinsic to the quasar and provides a constraint on the location of the gas. Using the apparent optical depth of the PV BAL and photoionization models to constrain the true column density of the outflow, we estimate the kinetic energy yields and compare to simulations to find that this outflow could be a viable feedback mechanism (Capellupo et al. 2014).
Collaborators: Fred Hamann (my thesis adviser; UF), Joe Shields (OU), Tom Barlow (IPAC), Paola Rodríguez Hidalgo (Penn State), Jules Halpern (Columbia)
SDSS-III BOSS Project: The Energetics of Quasar Broad Absorption Line Outflows
This project significantly expands on the work on PV absorption described above from my dissertation. We have searched through the thousands of BAL quasars at z > 2.3 in the SDSS-III BOSS quasar catalog to look for PV absorption, and we compare properties of those quasars with and without PV absorption. The first paper from this project will be submitted for publication soon...