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 am currently completing a project using spectropolarimetry data. 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
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.
Quasar Feedback in Radio Quiet Quasars
Alexandroff et al. 2016
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
Constraining the Escape Fraction of Local Galaxies Using Indirect Indicators
Alexandroff et al. 2015; Heckman, Alexandroff et al. 2015
In addition to my work on high redshift quasar feedback, I have also published on the UV indicators of escaping Lyman continuum radiation. In order for early galaxies to have accomplished the reionization of the universe, sufficient energetic photons must not only be created but must also escape from their host galaxy to reach the circumgalactic medium. Directly measuring the escape fraction at the redshift of reionization is impossible and remains difficult at intermediate redshifts because of intervening atomic gas (Inoue et al. 2014) so I calibrated a set of indirect indicators of the escape fraction using a sample of local galaxies that most closely resemble high redshift Lyman Break Galaxies. I found that dominant, compact starbursts with strong wind signatures show the lowest covering fractions of neutral gas in the low ionization interstellar medium (ISM) lines. This makes sense as strong starburst-driven winds could clear channels in the ISM through with Lyman continuum radiation could escape. In addition, the equivalent width of Lyman alpha was positively correlated with other indirect indicators of high escape fraction, forming a strong case that the identification and detection of strong Lyman alpha emitters at high redshift will be an important tool for identify galaxies in the early universe that are most likely to be leaking Lyman continuum radiation in to the surrounding ISM.