dvsg 0.1.0

Calculate the kinematic disturbance parameter ΔV⋆−g

DVSG

A Python module to calculate the kinematic disturbance parameter $\Delta V_{\star-g}$ (pronounced 'DVSG').

Background

Following Powley et al. (2026), a galaxy's $\Delta V_{\star-g}$ value is defined as:

$$ \Delta V_{\star-g} = \frac{1}{N} \sum_{j} \left| {V_{\star,\text{norm}}^{j} - V^{j}_{g,\text{norm}}} \right| $$

where:

• $V_{\star,\text{norm}}$ is the normalised stellar velocity map
• $V_{g,\text{norm}}$ is the normalised gas velocity map
• $\sum_{j} \left| {V_{\star,\text{norm}}^{j} - V^{j}_{g,\text{norm}}} \right|$ is the sum over all spaxels/bins, $j$, of the absolute difference between the normalised stellar and gas velocity maps
• $N$ is the number of bins/spaxels contributing towards the sum

A lower $\Delta V_{\star-g}$ value implies greater similarity between a galaxy's stellar and gas kinematics, whereas a higher $\Delta V_{\star-g}$ implies a greater degree of kinematic disturbance. For more detailed information about the steps to calculate $\Delta V_{\star-g}$, please refer to Powley et al. (2026).

Overview

The calculations module contains functions to calculate the $\Delta V_{\star-g}$ value of a galaxy.

The modelling module contains the MapModel class, which can create basic mock data and test how $\Delta V_{\star-g}$ changes when velocity maps are artificially rotated.

The plotting module contains functions that can be used to quickly produce science plots from a $\Delta V_{\star-g}$ calculation.

The helpers module contains MaNGA-specific loading and bin utilities used by the plateifu-based workflows.

A demonstration of how to use the MaNGA workflow to calculate $\Delta V_{\star-g}$ and visualise the results is provided in /demo/dvsg_demo.ipynb.

Development

If you want to install the development version:

# Clone the repository
git clone https://github.com/jmpowley/dvsg.git
cd dvsg

# Editable install in a pinned conda environment
python -m pip install -e . --no-deps

# Editable install in a plain pip environment
python -m pip install -e ".[manga]"

For local MaNGA development, some packages (e.g. sdss-marvin) can require stricter pins when setting up notebook environments. To install dvsg with strict pins (e.g. packaging=20.9, setuptools<81, wheel<0.46), prefer:

python -m pip install -e /path/to/dvsg --no-deps

Environment variables

Some functions in helpers rely on a local installation of MaNGA Data Analysis Pipeline (DAP) products. If you would like to use these functions, you will need to define the environment variables outlined in the DAP documentation.

Citation

If you use this code in your research, please cite Powley et al. (2026).

@article{Powley2026,
  author  = {Powley, Jonah M. and Smethurst, Rebecca J. and Lintott, Chris J. and G{\'e}ron, Tobias},
  title   = {Introducing $\Delta V_{\star-g}$: a new universal kinematic disturbance parameter},
  journal = {Monthly Notices of the Royal Astronomical Society},
  year    = {2026},
  doi     = {10.1093/mnras/stag747},
  eid     = {stag747}
}