Ghassan Sarrouh

York University

Ghassan is a doctoral candidate at York University working with Dr. Adam Muzzin. His research focuses on how galaxies built up their stellar mass in the very early universe using the James Webb and Hubble Space Telescopes. Ghassan obtained his bachelor’s degree in Physics & Astronomy from York University in 2020, and was the first graduate student invited to join a James Webb GTO team – the CAnadian NIRISS Unbiased Cluster Survey (CANUCS) – where he works in the photometry group. Ghassan is currently working on leading the first CANUCS data release for Cycles 1 & 2.

Physical properties like stellar masses and star formation rates are inferred from redshift and spectral energy distribution fitting. Photometric data (i.e. images) are used to sample the galaxy spectrum, with Hubble in the optical and James Webb in the near infrared, but the type of data taken matters. Some filters are broad, letting in more light but losing information on where in the spectrum that light came from. James Webb however is also equipped with a suite of 12 medium band filters which sample the spectrum more densely, increasing spectral resolution by a factor of ~2. The increased precision of the medium bands is especially important when dealing with the strong emission lines in high redshift galaxies unveiled by James Webb. Using CANUCS data – one of the only surveys to include all of the broad and medium band imaging available on James Webb’s Near InfraRed Camera – Ghassan hopes to understand how galaxies acquired their stellar mass occurred shortly after the Big Bang, placing tighter constraints on derived stellar masses than ever before.

Two key observables to track stellar mass assembly are the galaxy stellar mass function, and its integral the cumulative stellar mass density. As a first look into the systematic effects medium band photometry has on stellar masses, Ghassan led a study comparing stellar masses derived using photometry with and without medium band data (see figure below). This paper, published in the Astrophysical Journal Letters, showed that stellar mass density may be overestimated by factors of ~2-3 at z > 5 with data using broadband photometry alone. This promising result will support the main scientific thrust of his Ph.D. work studying the evolution of the stellar mass function and stellar mass density in the high redshift universe.

You can find the paper here.

Top: stellar mass density as a function of redshift where SED fitting was done using different sets of photometry: one with only wide bands (“WB”, orange) and another with both wide and medium bands (“MB”, purple). Above z~5 wide band photometry underestimates emission line strength and overestimates the continuum flux, leading to systematically overestimated stellar masses derived with wide band photometry alone. This trend persists even when wide bands fit the same redshift as medium bands, a key finding of the paper; Bottom: The ratio of wide-to-medium band stellar mass density, where the rise is more directly seen at z > 5.