My authored papers and presentations can be found in my ADS Library.
The Diffuse Night Sky: Zodiacal Light, Diffuse Galactic Light, and the Cosmic Background
The total sky brightness, as observed with the Hubble Space Telescope in low-Earth orbit, is dominated by zodiacal light: sunlight scattered by microscopic dust that fills the space near the mid-plane of the Solar System. For context, zodiacal light supplies >90% of the photons that Hubble has ever received. As Earth moves around the Sun, we look through slightly different parts of this dust cloud, so the sky can brighten or fade significantly. Understanding zodiacal light is important to understanding the shape and composition of our Solar System's interplanetary dust cloud, but is also crucial for predicting the brightness of the night sky (e.g., to best understand sensitivity limits for space telescopes, or to measure faint background beyond).
Zodiacal light is considered a diffuse component of the night sky brightness, because it is not discrete like individual stars and galaxies. However, it is not the only diffuse component Hubble can see. Starlight from the Milky Way is reflected and re-emitted by interstellar dust, producing diffuse Galactic light. Diffuse Galactic light is strongest toward the Galactic center and can rival or exceed zodiacal light in those directions.
An even fainter component to the diffuse night sky brightness is the extragalactic background light (EBL), the sum of all photons being emitted by stars, dust, and growing black holes in galaxies outside of the Milky Way. At optical wavelengths, it consists mainly of light from stars within distant galaxies, with a smaller contribution from accretion onto supermassive black holes. The EBL is a remarkable thing to study, because it encodes information about all stars and galaxies across cosmic time. We resolve a large portion of it with space telescopes like Hubble. However, the unresolved portion is just as interesting, because this is where we can study objects that may otherwise be impossible to detect. Hidden within measurements of the diffuse EBL is the light from the faintest galaxies in the universe, for which we do not yet have the technology to see. It may also carry the light from the very first stars and galaxies that are more than 13 billion light-years away and otherwise very difficult to detect. Studying the EBL may even reveal contributions from never-before-seen exotic sources like dark matter powered stars or direct-collapse black holes.
SKYSURF is the first Hubble program designed to measure these three components (zodiacal light, diffuse Galactic light, and the EBL) across the entire Hubble archive. By analyzing the sky surface brightness (sky-SB) in more than 200,000 images (totaling >20 TB), SKYSURF aims to deliver the most accurate, wavelength-dependent measurements of each contributor. Ground telescopes cannot match this because atmospheric airglow and light pollution mask much of the absolute sky surface-brightness. A list of current SKYSURF papers can be found at skysurf.asu.edu.
O'Brien et al. 2023 presented measurements of the absolute sky-SB across the full wavelength range of Hubble, from 0.2 to 1.6 μm. This sky-SB is mostly zodiacal light, but includes diffuse Galactic light and EBL. Using over 150,000 Hubble images from the Hubble archive, we developed and applied a sky-SB measurement algorithm to extract robust sky-SB estimates from individual Hubble exposures.
O'Brien et al. 2026 addresses a critical challenge in interpreting sky-SB measurements: modeling zodiacal light. We presented a new zodiacal light model that is optimized for optical wavelengths by updating the scattering phase function and albedo. Compared to other models developed for infrared missions, our model performs significantly better in the UV and visible. Despite these improvements, we detect a residual diffuse light signal of ~0.01 MJy sr-1, which may point to a missing isotropic component of the interplanetary dust cloud. The final model will is made publicly available on the official SKYSURF GitHub.
SKYSURF-IR is a JWST archival program to extend Project SKYSURF to the James Webb Space Telescope (JWST).
Looking for Faint Variable AGN & Supernovae
In O'Brien et al. 2024, we utilized repeated Hubble imaging of the JWST North Ecliptic Pole Time-Domain Field. This field is uniquely suited for transient searches due to its continuous visibility by JWST, minimal zodiacal light contamination, and extensive multi-wavelength coverage. As part of the TREASUREHUNT program, we conducted a systematic search for time-variable sources using ACS/WFC imaging in two HST filters: F435W (~0.4 μm) and F606W (~0.6 μm). We identified 12 transient events, most likely supernovae, as well as three X-ray detected sources potentially associated with faint quasars or high-redshift transients. We also found that approximately 0.42% of field galaxies exhibit significant variability indicative of AGN activity, with 190 variable galaxy cores identified.