Though deep X-ray surveys have suggested that around 50% of high redshift (z > 2) quasars should be obscured (Treister & Urry 2012), previous optical surveys have been too shallow or to narrow to identify a significant population of luminous obscured quasars at the peak of galaxy formation. If we are to properly constrain the total number density of quasars at all redshifts, and their contribution to reionization at very high redshifts, it is critical that we uncover how the number of obscured quasars as a fraction of the total quasar population evolves with time. In turn, a sample of high redshift obscured quasars can lend insight in to this important period in galaxy evolution.

Using a systematic search of the BOSS survey of the SDSS, I identified 145 obscured quasar candidates, the largest sample of optically-selected obscured quasar candidates at 2< z< 4. Follow-up spectroscopy in the near-infrared conducted using the Apache Point Observatory, the Magellan Telescope, and Gemini Near-InfraRed Spectrograph indicates that these objects do not appear obscured in the rest-frame optical. This suggests either that this population represents quasars with grazing sightlines through the dusty torus, or perhaps quasars produced by dusty mergers with obscuration on galaxy-wide scales.

A co-added spectrum of these obscured quasar candidates is available here. If you use it please credit Alexandroff et al. 2013.


My research experience has provided me with extensive expertise in multi-wavelength observational astronomy with a particular interest in extragalactic science. I have experience in optical, near-infrared and ultraviolet spectroscopy as well as radio imaging and spectropolarimetry. To complete these projects I have been a co-Investigator on over a dozen successful observing proposals as well as the Principal Investigator of five successful proposals at a range of wavelengths. This has allowed me to tackle some of the most pressing problems in extragalactic astronomy, by tracing important physical processes across the electromagnetic spectrum. In particular, my work studying obscured and reddened quasars at the peak of galaxy formation not only identified the largest sample of optically-selected obscured quasar candidates at that redshift (2 < z < 4), but has begun to reveal how obscured quasars may play a role in galaxy evolution.

Identifying obscured quasars in the early universe

Alexandroff et al. 2013; Greene, Alexandroff et al. 2014

Quasar feedback in radio quiet quasars

Alexandroff et al. 2016; Hwang, Zakamska, Alexandroff et al. 2018

As currently the only all-sky radio survey is relatively shallow, the radio properties of all but the most radio-luminous high redshift objects remain unknown. In particular, what powers the radio emission in radio quiet quasars—star formation, radio coronae, radio jets or quasar winds— is not clear. Perhaps most intriguing, some theories suggest that quasar winds— our signpost of quasar feedback—could produce the observed radio emission. I investigated the radio properties of a bolometrically luminous radio quiet quasar test sample using the Very Large Array (VLA).

At a redshift of z~0.5, I found all of our sample of four radio intermediate/quiet quasars were resolved at a scale of a few kpc—this is a significant improvement over previous studies at a lower resolution (e.g. Hodge et al. 2011). I concluded that galaxy scales may be the significant scale at which to resolve radio emission in luminous quasars. Two resolved sources had an identifiable core and lobe structure. While this might be associated with a small-scale radio jet, the steep spectral index of the unresolved core emission implies, I argue, that the radio emission is being driven by a quasar wind that is less collimated than possible jet emission. This represents the first effort to directly distinguish radio jet and quasar wind feedback in individual radio observations