Sanders, Ryan

Research Activities

Dr. Sanders's research group works on problems in the astronomy field of galaxy formation and evolution using multiwavelength observations across the electromagnetic spectrum, with a particular focus on spectroscopy at ultraviolet, optical, near-infrared, and sub-millimeter/millimeter wavelengths. The group uses imaging and spectroscopic observations from state-of-the-art observatories and telescopes including the James Webb Space Telescope (JWST), Hubble Space Telescope (HST), Atacama Large Millimeter/sub-millimeter Array (ALMA), the twin 10-meter Keck telescopes, and others. Physical interpreting these data sets involves the use of computationally intensive spectral modeling codes to translate the measured properties into physical properties of the stellar population and interstellar medium such as stellar mass, star formation history, star-formation rate, metallicity, chemical abundance pattern, ionization parameter, etc.

Among various projects, a major focus of the research program is to understand the physical mechanisms that govern the growth of galaxy mass over time. Understanding how the stellar and gaseous components of galaxies form and grow over time is one of the foremost goals of astronomy. While the hierarchical formation of dark matter halos is well understood, the physics governing the formation of the baryonic component of galaxies is poorly constrained due to the complex nature of baryonic interactions. Over the past few decades, a theoretical framework has emerged describing galaxy growth through interconnected baryon transport and processing mechanisms. Gas is accreted from the intergalactic medium (IGM) through the circumgalactic medium (CGM), fueling star formation after cooling in the interstellar medium (ISM). Feedback from stellar evolution processes (supernovae, photoionization, winds) and accreting supermassive black holes injects energy into the ISM, regulating star formation and ejecting gas in large-scale outflows. This process is known as the “cycle of baryons”. The group seeks answers to questions such as: How do gas, metals, and dust flow into, out of, and through galaxies? What physical mechanisms control galaxy growth and evolve high-redshift populations into the diverse set of pre- sent-day galaxies? What is the role of feedback and outflows in galaxy evolution? How do gas inflow and outflow vary as a function of redshift and galaxy properties? How and when did the first galaxies form? What is the chemical evolution history of the Universe and the origin of different chemical elements?

List of Projects

The AURORA Survey

The Assembly of Ultradeep Rest-optical Observations Revealing Astrophysics (AURORA) survey obtained ultra-deep and continuous R=1000 NIRSpec MSA spectroscopy over the wavelength range 1-5 microns using the G140M/F100LP, 235M/F170LP, G395M/F290LP gratings with JWST. Target selection was optimized to obtain the first statistical sample of ~45 z=1.5- 4.0 galaxies with detectable auroral emission lines enabling direct oxygen abundance estimates. With these measurements, representing an order-of-magnitude increase in the sample of direct oxygen abundances at z>=1.5, we will construct the first empirical strong-line metallicity calibrations at high redshift. Such calibrations are essential for exploiting the transformative rest-frame optical emission-line datasets to be collected with JWST/NIRspec out to z~10, and measuring actual oxygen abundances rather than simply the ratios of strong nebular emission lines. In turn, measurements of galaxy chemical abundances over cosmic time, in concert with their stellar masses, provide critical constraints on galaxy formation models. Using the proposed ultra-deep near-IR spectroscopic observations, we will also explore the physical properties of gas and stars in z>6 star-forming galaxies during the epoch of reionization, calibrate the nebular attenuation curve during the epoch of peak star formation (z=1.5-4.0), and constrain the formation histories of massive, quiescent galaxies.

Group members working on this project:
Ryan Sanders (PI)
Ali Ahmad Khostovan (staff scientist)

Establishing the Physical Picture for Rest-optical Emission Lines in Star-forming Galaxies from Cosmic Noon to Cosmic Dawn

There is an emerging consensus on the physical picture underlying the rest-optical emission-line spectra of distant star-forming galaxies. This consensus includes a radiation field dominated by massive stars with alpha-enhanced chemical abundance patterns, and lacking emission from diffuse ionized gas (DIG) – both in contrast to local star-forming galaxies. We have identified several outstanding directions required to complete this picture and carried out several corresponding follow-up spectroscopic programs with Keck instruments (MOSFIRE, LRIS, OSIRIS, and DEIMOS). With these Keck data already in hand, we will perform: (1) the first analysis of neutral oxygen emission in the ISM of z ∼ 2 star-forming galaxies; (2) the first joint in- depth analysis of a z ∼ 2 galaxies covered simultaneously by rest-UV and rest- optical spectra and integral field unit spectral maps of the ionized ISM; (3) a first joint analysis of doubly- and singly-ionized sulfur and oxygen emission at z ∼ 1.5; (4) analysis of the largest sample of new and existing direct oxygen abundance measurements at z ∼ 1.5 − 2.5. In addition, we will publicly release all new Keck spectroscopic datasets described here, and will also build the public software tool, Spectral Modeler of Integrated Line Emission (SMILE). This tool will incorporate all results from MOSDEF about the nature of the ionizing spectrum, the physical density, and ISM architecture at high redshift, and enable users to model their rest-optical emission-line spectra to obtain galaxy physical properties.

Group members working on this project:
Ryan Sanders (PI)
Shweta Jain (graduate student)

Computational Methods

Data analysis techniques primarily in Python. Bayesian analysis, Markov-Chain Monte Carlo (MCMC) sampling, dynamic nested sampling, machine learning techniques. All of these resources are publicly available.

List of Software

emcee and dynesty (Python packages)
Spectral energy distribution modeling codes: BAGPIPES, Cigale, Prospector
Cloudy photoionization modeling code

Collaborators

Ryan Sanders (UK, faculty, PI)
Ali Ahmad Khostovan (UK, staff scientist)
Shweta Jain (UK, graduate student)