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University of Oklahoma
Julianna Voelker is an undergraduate student studying astrophysics at the University of Oklahoma. They and undergraduate student Cora DeFrancesco conducted this research with Dr. Karen Leighly, with additional guidance from graduate student Hyunseop Choi. They presented their results at the 237th American Astronomical Society Meeting in January 2021.
Quasars are the most luminous class of Active Galactic Nuclei (AGNs), which lie at the centers of galaxies and contain actively accreting supermassive black holes. Quasar outflows can play a role in galaxy evolution. The light from the accretion disk is absorbed by gas in our line of sight, producing blue-shifted absorption lines in Broad Absorption Line Quasar (BALQ) spectra. By modeling these spectra, we can derive the physical parameters (eg. density and thickness) of the absorbing gas and the velocities of quasar outflows, which tell us how powerful the outflow is. Powerful outflows impact the star-forming gas in the galaxy and quench star formation and can thus provide information about galaxy evolution and feedback. FeLoBALQs contain high ionization and low ionization (Lo) broad absorption lines as well as iron (Fe) lines, and they can contain the most powerful outflows due to the thickness of the absorbing gas.
We modeled a sample of high luminosity (46.5 < logLbol < 47.5 [erg s-1]), high-redshift (2.1 < z < 2.6) FeLoBALQs using the spectral synthesis code SimBAL. Simbal uses Markov Chain Monte Carlo methods to forward model the spectra and to extract the physical parameters of the absorbing gas from the results. We compared our sample to a sample of low-redshift (0.5 < z < 1.5) quasars with a luminosity range of 45 < logLbol < 47.5 [erg s-1] previously studied by our group. We found that the outflows in the high-redshift objects in our sample are found preferentially at larger distances from the central black holes than the outflows in the low-redshift objects (Figure 1). Additionally, we observed systematically higher outflow velocities in the high-redshift objects, possibly due to the higher luminosities of this sample, which corresponded to a greater percentage of powerful outflows than in the low-redshift sample. Because higher redshifts correspond to younger galaxies, our results indicate that quasar outflows may have played a more important role in galaxy feedback at earlier times than at later times.
Figure 1: Left: Ionization vs. distance of outflow from the galaxy’s central black hole. Powerful outflows are located at higher logR values and lower logU (ionization) values for the high redshift objects than for the low redshift objects. Right: Selection of two quasar spectra modeled by SimBAL. Panels include the full model with absorption (solid red) and a model without absorption (dashed green).
Astrobite edited by: Jamie Sullivan