Active Galaxies NewsletterAn electronic publication dedicated to the observations and theory of active galaxies
Below is a list of abstracts posted in the Newsletter in the past 6 months, along with relevant contact
Please note that the latest abstracts can be found in the recent issues of the newsletter.
Outflows launched near the central supermassive black holes (SMBHs) are a common and important component of active galactic nuclei (AGNs). Outflows in luminous AGNs (i.e., quasars) play a key role in mass accretion onto SMBH as well as in the feedback into host galaxies. The most prominent signature of such outflows appears as broad absorption lines (BALs) that are blueshifted from the emission line with a few thousands km s-1 velocities. In this dissertation, I place further constrains upon the size scale, internal structure, dynamics, and evolution of the outflows investigating profiles, properties, and variation characteristics of BAL troughs. I present observational results on BAL troughs in a large quasar sample utilizing spectroscopic observations from the Sloan Digital Sky Survey spanning on multi-year timescales. The results presented here, for the first time, provide a large and well-defined variability data base capable of discriminating between time-dependent hydrodynamic wind calculations in a statistically powerful manner.
In a study of 582 quasars, I present 21 examples of BAL trough disappearance. Approximately 3.3% of BAL quasars show disappearing CIV trough on rest-frame timescales of 1.1-3.9 yr. BAL disappearance appears to occur mainly for shallow and weak or moderate-strength absorption troughs but not the strongest ones. When one BAL trough in a quasar spectrum disappears, the other present troughs usually weaken. Possible causes of such coordinated variations could be disk-wind rotation or variations of shielding gas that lead to variations of ionizing-continuum radiation.
I present a detailed study on the variability of 428 CIV and 235 SiIV BAL troughs using a systematically observed sample of 291 BAL quasars. BAL variation distributions indicate that BAL disappearance is an extreme type of general BAL variability, rather than a qualitatively distinct phenomenon. The high observed frequency of BAL variability on multi-year timescales is generally supportive of models where most BAL absorption arises at radii of 10-1000 light days. Average lifetime for a BAL trough along our line-of-sight is a few thousand years which is long compared to the orbital time of the accretion disk at the wind-launching radius. We have examined if BAL variations on several timescales depend upon quasar properties, including quasar luminosity, Eddington luminosity ratio, black hole mass, redshift, and radio loudness. Within the ranges of these properties spanned by our sample, we do not find any strong dependences. The coordinated trough variability of BAL quasars with multiple troughs suggests that changes in "shielding gas" may play a significant role in driving general BAL variability.
I present a study investigating the dependence of CIV BAL properties and variation characteristics on accompanying SiIV and AlIII absorption. Results of this study show that CIV BAL trough shapes, depths, velocity widths and strengths show a strong dependence on the presence of SiIV and AlIII BAL troughs at corresponding velocities. Similarly, the variation characteristics and depth variation profiles of CIV BAL troughs also show a strong connection to BAL troughs in these transitions. Using these ions as a basic tracer of ionization level of the absorbing gas, systematic measurements of variability and profiles for a large sample of CIV, SiIV, and AlIII BAL troughs present observational evidences of the relation between ionization level, column density and kinematics of outflows.
Utilizing observational investigations on a large BAL quasar sample, we show that ionization level, column density and kinematics of outflows show correlated object-to-object differences. We present a detailed comparison between the observational results of this study and the well studied disk-wind model of quasar outflows, which suggests that the wind is launched from the accretion disk at ∼ 1016 - 1017 cm and radiatively driven by UV line pressure. Results of this study show that lines-of-sight with different viewing inclinations successfully explain the characteristics and the differences between those three CIV trough groups with a good agreement to our observational findings.