Master's thesis defense by Daniel Lawther – Niels Bohr Institutet - Københavns Universitet

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Master's thesis defense by Daniel Lawther

Daniel Lawther will defend his master's thesis Friday 10 May 2013 at 15:00 in the DARK lounge.

Title: Disentangling the light : an imaging study of FeLoBAL quasar host galaxies

Advisor: Marianne Vestergaard

Link to thesis >>

Iron low-ionization broad absorption quasars (FeLoBALs) are a subset of quasars that
display broad (v>2000 km s-1) absorption troughs in one or more of their low-ionization broad emission lines, along with broad (or overlapping) absorption lines due to metastable Feii transitions. These objects are potentially an important clue as to the morphology and / or evolution of quasars in general: perhaps FeLoBAL objects are ordinary quasars seen at a certain viewing angle, or perhaps they are quasars seen at a certain evolutionary phase. A statistical comparison of FeLoBAL host galaxies to those of ordinary quasars may allow us to di erentiate between these scenarios.
This thesis is a study of four FeLoBAL host galaxies using Hubble Space Telescope
HST imaging data in the rest-frame ultraviolet (UV) and optical. Measuring the host galaxy
brightness in these two bands can constrain the presence of a young stellar population. To reduce the contrast between host galaxy and quasar in the rest-frame UV, the FeLoBALs were observed in a lter that overlaps with their BAL troughs - direct light from the nucleus should therefore be strongly absorbed in this lter. Nevertheless, all four quasars appeared as point sources in these observations. Simulations were performed to determine limiting host galaxy brightnesses in the rest-frame ultraviolet. The quasars were also imaged in an infrared (rest-frame optical) lter; in these images, extended ux was detected for 3 of 4 objects, and Sersic pro les were tted to the host galaxy, yielding host galaxy brightnesses and scale sizes. Detection and measurement of the FeLoBAL host galaxies is strongly sensitive to the accuracy of a point spread function (PSF) subtraction, especially in the rest-frame ultraviolet where the four quasars are strongly nucleus-dominated. I have performed tests to compare the reliability of empirical PSF templates (stacked stellar templates for HST ACS/WFC, single stellar templates for NICMOS) and analytical PSF models (Tiny Tim).
Three of the FeLoBAL host galaxies were detected in the rest-frame optical imaging; their
brightnesses and scale sizes are consistent with the ndings of non-BAL quasar host galaxy studies.
The upper limits on the FeLoBAL host galaxy brightnesses in the rest-frame ultraviolet, combined with the host galaxy brightnesses obtained in the rest-frame optical, suggest that the FeLoBALs do not possess a young stellar population - or, at least, constrain a young stellar population to a region hidden from the ACS observations (an unresolved nuclear region, or possibly a region hidden by the BAL out ow).
These results depend on the method used to t the host galaxies in the NICMOS images being robust. The host galaxy ts were hampered by the lack of a high signal-to-noise PSF star observation, and relied on tting Sersic pro les to the wings of the host galaxy, where PSF mismatch was less important. Tests of this tting method using simulated quasar images are necessary to ascertain the robustness of the measured host galaxy brightnesses. The robustness of the conclusion that FeLoBAL host galaxies lack a young stellar population is dependent on the outcome of such tests.