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Geha Research Group

Welcome to the Geha Research Group at Yale University in the Department of Astronomy. We are focused on understanding low mass galaxies around our Milky Way, in the Local Group and nearby Universe. Low mass galaxies provide significant constraints on galaxy formation processes, cosmology and the nature of dark matter. We study low mass galaxies ranging from gas-rich dwarf galaxies in isolation, to the least luminous and most dark matter-dominated galaxies in orbit around the Milky Way. The Geha group has weekly research meetings and welcomes postdocs, graduate and undergraduate students.

See some of the recent work from the group below.

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Research Interests

The SAGA Survey
The SAGA Survey

The SAGA Survey is a spectroscopic survey designed to better understand our Milky Way Galaxy by placing its satellite galaxy population into a broader cosmological context. The SAGA Survey goal is to measure the distribution of satellite galaxies around 100 Milky Way analogs well below the luminosity where predictions and observations diverge.
Isolated and Field Low Mass Galaxies
Isolated and Field Low Mass Galaxies

The properties of isolated low-mass galaxies provide insights into the internal processes regulating star formation.
Milky Way Dwarf Galaxies
Milky Way Dwarf Galaxies

The Local Group’s population of satellite galaxies and star clusters include the lowest mass and least chemically enriched stellar systems in the known Universe. Proximity enables spectroscopic observations of individual stars, providing exquisite measurements of their kinematics and chemistries.

Outreach & Teaching

Astro 330
Astro 330

ASTR 330 focuses on high level Python computing in the context of astrophysics research. The course goal is to provide students with the fundamentals of building astronomical pipelines and the general use of cutting edge tools and techniques currently used in the astronomical community.
Astro Research Preparation Series
Astro Research Preparation Series

The Astro-RPS workshop series covers an introduction to the Python programming language in the context of physics and astrophysics applications. This graduate-student led series is primarily intended for undergraduate students with little to no exposure to coding, and designed to teach the fundamental skills neccessary for astronomy research.
REVU
REVU

The Research Experience for Veteran Undergraduates (REVU) is a 9-week summer research experience for enlisted U.S. veteran undergraduate students. REVU is run by Professor Geha with a grant from HHMI.

Recent Work

  1. Asali, Yasmeen, Geha, Marla, Kado-Fong, Erin, Mao, Yao-Yuan, Wechsler, Risa H. et al.; 2025; The SAGA Survey. VI. The Size─Mass Relation for Low-mass Galaxies Across Environments; Astrophysical Journal; 10.3847/1538-4357/ae147d
  2. Kado-Fong, Erin, Mao, Yao-Yuan, Asali, Yasmeen, Geha, Marla, Wechsler, Risa H. et al.; 2025; SAGAbg. III. Environmental Stellar Mass Functions, Self-quenching, and the Stellar-to-halo Mass Relation in the Dwarf Galaxy Regime; Astrophysical Journal; 10.3847/1538-4357/ae102d
  3. Cerny, William, Bissonette, Daisy, Ji, Alexander P., Geha, Marla, Chiti, Anirudh et al.; 2025; No Observational Evidence for Dark Matter Nor a Large Metallicity Spread in the Extreme Milky Way Satellite Ursa Major III / UNIONS 1; arXiv e-prints; 10.48550/arXiv.2510.02431
  4. de los Reyes, Mithi A. C., Asali, Yasmeen, Wechsler, Risa H., Geha, Marla, Mao, Yao-Yuan et al.; (Includes Kado-Fong, Erin, Gandhi, Pratik J.); 2025; Stellar Mass Calibrations for Local Low-mass Galaxies; Astrophysical Journal; 10.3847/1538-4357/ade4c5
  5. Tan, C. Y., Cerny, W., Drlica-Wagner, A., Pace, A. B., Geha, M. et al.; 2025; A Pride of Satellites in the Constellation Leo? Discovery of the Leo VI Milky Way Satellite Ultra-faint Dwarf Galaxy with DELVE Early Data Release 3; Astrophysical Journal; 10.3847/1538-4357/ad9b0c
  6. Cerny, W., Chiti, A., Geha, M., Mutlu-Pakdil, B., Drlica-Wagner, A. et al.; 2025; Discovery and Spectroscopic Confirmation of Aquarius III: A Low-mass Milky Way Satellite Galaxy; Astrophysical Journal; 10.3847/1538-4357/ad8eba
  7. Burke, Colin J. and Natarajan, Priyamvada and Baldassare, Vivienne F. and Geha, Marla; 2025; Multiwavelength Constraints on the Local Black Hole Occupation Fraction; Astrophysical Journal; 10.3847/1538-4357/ad94d9
  8. Geha, Marla, Mao, Yao-Yuan, Wechsler, Risa H., Asali, Yasmeen, Kado-Fong, Erin et al.; 2024; The SAGA Survey. IV. The Star Formation Properties of 101 Satellite Systems around Milky Way–mass Galaxies; Astrophysical Journal; 10.3847/1538-4357/ad61e7
  9. Mao, Yao-Yuan, Geha, Marla, Wechsler, Risa H., Asali, Yasmeen, Wang, Yunchong et al.; (Includes Kado-Fong, Erin); 2024; The SAGA Survey. III. A Census of 101 Satellite Systems around Milky Way–mass Galaxies; Astrophysical Journal; 10.3847/1538-4357/ad64c4
  10. Kado-Fong, Erin, Geha, Marla, Mao, Yao-Yuan, de los Reyes, Mithi A. C., Wechsler, Risa H. et al.; (Includes Asali, Yasmeen); 2024; SAGAbg. II. The Low-mass Star-forming Sequence Evolves Significantly between 0.05 < z < 0.21; Astrophysical Journal; 10.3847/1538-4357/ad8137
  11. Kado-Fong, Erin, Geha, Marla, Mao, Yao-Yuan, de los Reyes, Mithi A. C., Wechsler, Risa H. et al.; (Includes Asali, Yasmeen); 2024; SAGAbg. I. A Near-unity Mass-loading Factor in Low-mass Galaxies via Their Low-redshift Evolution in Stellar Mass, Oxygen Abundance, and Star Formation Rate; Astrophysical Journal; 10.3847/1538-4357/ad3042

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