Skip to main content

Joel Leja

Joel Leja — Assistant Professor — Faculty
Office: 515 Davey Laboratory
Address: 525 Davey Laboratory University Park, PA 16802 US
jql6565@psu.edu
+1 814 865 0418
Joel Leja
Astronomy ICDS PFS LSST Cosmic Surveys CTOC

Forming galaxies across cosmic time: My research interests focus on understanding the processes of the formation of galaxies (large collections of stars and dark matter) through the use of large surveys and statistics. I specialize in building and fitting flexible models to images and spectroscopy of galaxies. Galaxies are complex, so these models include the full range of things that live in the night sky: stars, black holes, gas, and cosmic dust. Understanding these components better often means writing down simple or more complex (numerical) models for how the full range of the galaxy population evolves as a whole, from the Big Bang to the present day. I've harnessed millions of hours of supercomputer time running specialized code to build a more complete picture of how galaxies form and evolve. I frequently use Bayesian statistics and machine learning (AI) to interpret the massive galaxy surveys of today and tomorrow; this simultaneously speeds up our models while also allowing them to become more sophisticated and complex.

Visit my website

ORCID: 0000-0001-6755-1315

Publications
  1. Joel Leja, Joshua Speagle, Yuan-Sen Ting, Benjamin Johnson, Charlie Conroy, Katherine Whitaker, Erica Nelson, Pieter Dokkum, Marijn Franx “A New Census of the 0.2< z <3.0 Universe, Part II: The Star-Forming Sequence.” arXiv e-prints (2021)
  2. Joel Leja, Joshua Speagle, Benjamin Johnson, Charlie Conroy, Pieter Dokkum, Marijn Franx “A New Census of the 0.2 < z < 3.0 Universe. I. The Stellar Mass Function.” 893 2 (2020) doi:10.3847/1538-4357/ab7e27
  3. Joel Leja, Adam Carnall, Benjamin Johnson, Charlie Conroy, Joshua Speagle “How to Measure Galaxy Star Formation Histories. II. Nonparametric Models.” 876 1 (2019) doi:10.3847/1538-4357/ab133c
  4. Joel Leja, Benjamin Johnson, Charlie Conroy, Pieter Dokkum, Joshua Speagle, Gabriel Brammer, Ivelina Momcheva, Rosalind Skelton, Katherine Whitaker, Marijn Franx, Erica Nelson “An Older, More Quiescent Universe from Panchromatic SED Fitting of the 3D-HST Survey.” 877 2 (2019) doi:10.3847/1538-4357/ab1d5a
  5. Joel Leja, Sandro Tacchella, Charlie Conroy “Beyond UVJ: More Efficient Selection of Quiescent Galaxies with Ultraviolet/Mid-infrared Fluxes.” 880 1 (2019) doi:10.3847/2041-8213/ab2f8c
  6. Joel Leja, Benjamin Johnson, Charlie Conroy, Pieter Dokkum “Hot Dust in Panchromatic SED Fitting: Identification of Active Galactic Nuclei and Improved Galaxy Properties.” 854 1 (2018) doi:10.3847/1538-4357/aaa8db
  7. Joel Leja, Benjamin Johnson, Charlie Conroy, Pieter Dokkum, Nell Byler “Deriving Physical Properties from Broadband Photometry with Prospector: Description of the Model and a Demonstration of its Accuracy Using 129 Galaxies in the Local Universe.” 837 2 (2017) doi:10.3847/1538-4357/aa5ffe
  8. Joel Leja, Pieter Dokkum, Marijn Franx, Katherine Whitaker “Reconciling the Observed Star-forming Sequence with the Observed Stellar Mass Function.” 798 2 (2015) doi:10.1088/0004-637X/798/2/115
  9. Joel Leja, Pieter Dokkum, Marijn Franx “Tracing Galaxies through Cosmic Time with Number Density Selection.” 766 1 (2013) doi:10.1088/0004-637X/766/1/33

Joel Leja's research group

Name Role Affiliations Email Phone Office Centers Research topics
Bingjie Wang Bingjie Wang Postdoc Astronomy, ICDS bwang@psu.edu +1 814 865 0418 520 Davey Laboratory IGC Cosmic Surveys
About our wordmark
Monica Rincon Ramirez

The IGC wordmark was created by Monica Rincon Ramirez while she was a graduate student at the Institute. Monica enjoys drawing new connections between fundamental theory and observations. Her graduate work included general relativity, loop quantum gravity, and quantum fields in cosmological backgrounds; her thesis focused on effective quantum corrections to gravitational phenomena from spinfoams and applications to cosmology. She received her PhD in 2024.

The wordmark symbolizes the scope of research at the IGC. The base represents quantum gravity, evoking the quantum geometrical picture from spinfoams and loop quantum gravity — approaches studied at the Center for Fundamental Theory. The middle shows galaxies embedded in a smooth surface characteristic of spacetime in general relativity and the larger physical scales studied at the Center for Theoretical and Observational Cosmology. The top curves to an extreme representing a supermassive black hole accompanied by an energetic jet, with a binary black hole pair nearby — the high-energy phenomena studied at the Center for Multimessenger Astrophysics.