PI: Dr. Kate Rowlands (krowlands@stsci.edu, https://www.stsci.edu/~krowlands/), co-Is & Dr. Katey Alatalo, Dr. Andreea Petric, Prof. Vivienne Wild, Prof. Omar Almaini
Project Duration: 1 year rotation with potential for thesis project. Funding available for three years.
Project motivation
One of the key unsolved problems in astronomy is to understand how galaxies evolve and transform over cosmic time. Over the last 10 billion years the demographics of the galaxy population has changed, with the number of star-forming galaxies staying roughly constant and quiescent galaxies increasing. To explain the bimodal galaxy population there must be processes that both disrupt the supply of gas that is fueling star formation and transforms the galaxies’ morphologies. Reproducing these trends is a key test for galaxy evolution models. There is much debate regarding the exact physical mechanisms and timescales involved in galaxy transformation. The general consensus until recently was that star formation stops because cold gas is consumed by star formation, or heated or removed by large-scale outflows driven by either supernovae or supermassive black holes. Recent discoveries of large amounts of gas in transitioning galaxies in the local Universe challenge this traditional picture. Over the past decade, while large-area galaxy surveys have revolutionized our knowledge of the demographics of the quiescent galaxy population and its build-up over cosmic time, the mechanisms that lead to the disruption of the gas supply and hence the shut-down of star formation are still poorly understood. This project seeks to understand the processes which trigger the shutdown in star formation and answer this major open question in astronomy.
Fig 1: Summary of the science goals. Post-starburst galaxies are thought to be caught in the act of transformation between star forming and quiescent and can be cleanly selected in large numbers using broadband photometry.
Project summary
There are several projects that can be undertaken by a graduate student on topics related to galaxy evolution, quenching and AGN at redshifts 0.5<z<3. These are part of a larger collaborative project to understand the processes which drive the shutdown of star formation in galaxies, using the wealth of photometric and spectroscopic data from ground-based and space-based observatories (e.g. HST, SOFIA, Chandra, JWST, Herschel, Spitzer, UKIRT, VLT) in deep extragalactic fields. The first part of the student-led project is to apply existing photometric selection techniques to deep fields to find the largest sample of high-redshift quenching galaxies to date. If expansion to a thesis project is desired, later parts of the project involve investigating the star-formation histories, morphologies, dust content, environment and AGN activity in the high redshift post-starburst galaxies.
These projects will be undertaken within the highly supportive and collaborative post-starburst galaxy group at STScI/JHU, and with collaborators in the UK. There are plenty of opportunities for travel to collaborators and conferences. All of the projects should result in a student-led refereed paper and presentations at conferences.
Fig 2: Summary of the existing photometric selection method (Wild et al. 2014, 2016), applied to the UDS field. Left: Super-color diagram for galaxies in the UDS (grey points) from Wilkinson et al. (2021), which cleanly selects post-starbursts (PSB) from star-forming and quiescent galaxies at 0.5 < z < 3. Colored points show spectroscopically classified galaxies. The first super-color (SC1) describes the SED shape and is correlated with specific SFR and light-weighted age. The second super-color (SC2) describes the strength of the 4000Å/Balmer break and correlates with the fraction of the stellar mass formed during the last Gyr and metallicity. The third super-color (not shown) describes the shape of the SED around 4000Å and is used to break the degeneracy betweenthe fraction of the stellar mass formed during the last Gyr and metallicity. Inset plots show stacked optical spectra of the different spectral classes identified using the super-color method from Wild et al. (2014, 2016). Middle: Optical spectra of post-starburst (PSB) and quiescent (red) galaxies at z ∼ 1 from Maltby et al. (2016), with post-starburst galaxies exhibiting characteristic deep Balmer absorption lines. Right: Stellar mass vs photometric redshift distribution, showing example bins and 90% mass completeness limits (dashed lines) from Wilkinson et al. (2021).
Student work
Planned work as part of photometry project:
Collect photometry for extragalactic deep fields, e.g. EGS. Assess usability of existing photometric catalogs, remeasure photometry as necessary.
Modify and apply existing photometric classification code to photometry to select post-starburst, quiescent and star-forming galaxies.
- Analyze galaxy population properties, e.g. number densities, stellar mass function and compare to literature.
Write a paper on the newly selected galaxies, detailing the selection method and galaxy properties, and present the findings at conferences.
Publish catalogs on public facing data repositories.
Skills (no prior experience necessary):
Coding in Python and/or IDL
Experience with photometry and/or spectroscopy, large datasets
Understanding of galaxy evolution
Academic/technical writing
