Team Members

Get to know us!

Galaxy with a purple circumgalactic medium, blue accretion, and white outflows

Circumgalactic Medium Group

We are a growing group in the Homer L. Dodge Department of Physics and Astronomy at the University of Oklahoma led by Asst. Prof. Nikki Nielsen. We currently consist of 4 graduate students, 5 undergraduate students, and 2 postdoctoral research associates.

We collaborate closely with astronomers around the world, including at Swinburne University of Technology in Australia, and US universities New Mexico State University and Pennsylvania State University.

The team regularly uses some of the largest telescopes in the world, including the twin Keck Observatory telescopes in Hawaii, the Very Large Telescope in Chile, and the Hubble Space Telescope.

Asst Prof Nikki Nielsen

Assistant Professor Nikki Nielsen

Dr Nielsen joined OU in August 2024. After obtaining her PhD at New Mexico State University in 2015, she was a postdoctoral resesarch associate in the Centre for Astrophysics and Supercomputing at Swinburne University of Technology in Melbourne, Australia. She then became the Swinburne ASTRO 3D Fellow in 2021, which was an independent fellowship funded by the Australian Research Council Centre of Excellence for All-Sky Astrophysics in 3 Dimensions.

Nikki's hobbies include knitting, gardening, baking, and consuming sci-fi and fantasy stories.

Postdocs

Dr Sameer

Dr Sameer

Dr Sameer recently joined the group after a postdoctoral stint at the University of Notre Dame. He completed his PhD at Penn State in 2022 with a thesis on unveiling the circumgalactic medium through Bayesian ionization modeling. He has established himself as a specialist in using absorption spectra to investigate the properties of multiphase circumgalactic gas across most of cosmic history.

When he’s not unraveling the complexities of galactic halos, Sameer enjoys playing basketball, solving cryptic crosswords, and working through puzzle games in the New York Times.

Website

Dr Grecco Oyarzun

Dr Grecco Oyarzún

Dr Oyarzún joined the group in October 2025 after spending a few years at Johns Hopkins University as a postdoc. He completed his PhD at the University of California, Santa Cruz in 2022.

Graduate Students

Kaitlyn Linder

Kaitlyn Linder

Kaitlyn is a 2nd year astrophysics PhD student. She is currently working with Keck/MOSFIRE spectroscopy to identify the host galaxies of cool CGM absorption at z=1-2.

Kaitlyn enjoys knitting, cooking, reading and unicycling.

Amisha Rane

Amisha Rane

Amisha just wrapped up her first year as a PhD student! She's broadly interested in how gas flows through the universe. During her Master's program, Amisha worked on analyzing and modeling methanol spectral lines toward ionized jet candidates in high-mass star-forming regions. Now she's expanding that passion to larger scales, studying the CGM to understand how galaxies acquire, recycle, and lose their gas.

Amisha genuinely loves working with telescopes – that hands-on experience is a big part of why she jumped at the chance to host star parties this past year. Outside of research, Amisha makes it a point to swim every day or at least try to! She absolutely loves vibing to music and dancing – it's her go-to way to unwind and reset!

Jacob Callebs

Jacob Callebs

Jacob joined the group in Spring 2026.

Haikun Xue

Haikun Xue

Haikun joined the group in Spring 2026.

Undergraduate Students

Jaden Cooper

Jaden Cooper

Jaden is a junior majoring in astrophysics, with interests in extragalactic astronomy and cosmology.

Jaden loves to stargaze, taking photos of the Moon and planets with her Celestron refractor telescope. She currently serves as the Treasurer for OU Women in Physics.

Isabella Hildinger

Isabella Hildinger

Isabella is a senior astrophysics major, who is currently doing her capstone research.

She loves all things astronomy, and in her free time composes and plays a little piano.

Emily Thrun

Emily Thrun

Emily is a second year studying astrophysics and Japanese. She was awarded the OU Provost's First-Generation UReCA Fellowship to do CGM research in Spring 2026!

Outside of research, Emily works as a consultant at the OU Writing Center. She is passionate about music, and loves collecting vinyls and going to concerts.

Yesenia Sanchez

Yesenia Sanchez

Yesenia recently joined the group and is getting up to speed with research!

Ethan Dyer

Ethan Dyer

Ethan is working primarily with Dr Sameer and the CMBM method for analyzing CGM absorption systems.

Former Members

Graduate Students

Imran Hossain