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Peer-reviewedCC BY 4.0Monthly Notices of the Royal Astronomical Society

geko : A tool for modelling galaxy kinematics and morphology in JWST /NIRCam slitless spectroscopic observations

  • A Lola Danhaive1,2
  • Sandro Tacchella1,2
  1. 1Kavli Institute for Cosmology, University of Cambridge , Madingley Road, Cambridge, CB3 0HA ,
  2. 2Cavendish Laboratory, University of Cambridge , 19 JJ Thomson Avenue, Cambridge, CB3 0HE ,
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Abstract

Wide-field slitless spectroscopy (WFSS) is a powerful tool for studying large samples of galaxies across cosmic time. With the arrival of JWST, and its NIRCAM grism mode, slitless spectroscopy can reach a medium spectral resolution of (R ∼ 1600), allowing it to spatially resolve the ionised-gas kinematics out to z ∼ 9. However, the kinematic information is convolved with morphology along the dispersion axis, a degeneracy that must be modelled to recover intrinsic properties. We present the Grism Emission-line Kinematics tOol (geko), a Python package that forward-models NIRCam grism observations and infers emission-line morphologies and kinematics within a Bayesian framework. geko combines Sérsic surface-brightness models with arctangent rotation curves, includes full point-spread function (PSF) convolution and forward-modelling of the grism dispersion, and leverages gradient-based sampling via jax/numpyro for efficient inference. It recovers parameters such as effective radius, velocity dispersion, rotational velocity, rotational support, and dynamical mass, with typical run times of ∼20 minutes per galaxy on GPUs. We validate performance using extensive mock data spanning position angle, S/N, and morphology, quantifying where degeneracies limit recovery. Finally, we demonstrate applications to real FRESCO Hα emitters at z ≈ 4 − 6, recovering both rotation- and dispersion-dominated systems. geko opens the way to statistical studies of galaxy dynamics in the early Universe and is publicly available at https://github.com/angelicalola-danhaive/geko.

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Cite

BibTeXRIS
BibTeX
@article{danhaive2026geko,
  title = {{geko : A tool for modelling galaxy kinematics and morphology in JWST /NIRCam slitless spectroscopic observations}},
  author = {A Lola Danhaive and Sandro Tacchella},
  journal = {Monthly Notices of the Royal Astronomical Society},
  year = {2026},
  month = oct,
  doi = {10.1093/mnras/stag1965},
  url = {https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag1965/8931747},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - geko : A tool for modelling galaxy kinematics and morphology in JWST /NIRCam slitless spectroscopic observations
AU  - A Lola Danhaive
AU  - Sandro Tacchella
PY  - 2026
DA  - 2026/10/16
JO  - Monthly Notices of the Royal Astronomical Society
DO  - 10.1093/mnras/stag1965
UR  - https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag1965/8931747
AB  - Wide-field slitless spectroscopy (WFSS) is a powerful tool for studying large samples of galaxies across cosmic time. With the arrival of JWST, and its NIRCAM grism mode, slitless spectroscopy can reach a medium spectral resolution of (R ∼ 1600), allowing it to spatially resolve the ionised-gas kinematics out to z ∼ 9. However, the kinematic information is convolved with morphology along the dispersion axis, a degeneracy that must be modelled to recover intrinsic properties. We present the Grism Emission-line Kinematics tOol (geko), a Python package that forward-models NIRCam grism observations and infers emission-line morphologies and kinematics within a Bayesian framework. geko combines Sérsic surface-brightness models with arctangent rotation curves, includes full point-spread function (PSF) convolution and forward-modelling of the grism dispersion, and leverages gradient-based sampling via jax/numpyro for efficient inference. It recovers parameters such as effective radius, velocity dispersion, rotational velocity, rotational support, and dynamical mass, with typical run times of ∼20 minutes per galaxy on GPUs. We validate performance using extensive mock data spanning position angle, S/N, and morphology, quantifying where degeneracies limit recovery. Finally, we demonstrate applications to real FRESCO Hα emitters at z ≈ 4 − 6, recovering both rotation- and dispersion-dominated systems. geko opens the way to statistical studies of galaxy dynamics in the early Universe and is publicly available at https://github.com/angelicalola-danhaive/geko.
SN  - 0035-8711
SN  - 1365-2966
ER  -