# The kinematics of {\sigma}-drop bulges from spectral synthesis modelling of a hydrodynamical simulation

## TL;DR

This study uses spectral synthesis modeling of a hydrodynamical simulation to investigate the origin of { extsigma}-drops in galaxy centers, revealing young stars' kinematic effects cause the observed velocity dispersion minima.

## Contribution

It introduces a new spectral synthesis code, SYNTRA, to analyze simulated galaxy kinematics and demonstrates how young stars influence the { extsigma}-drop phenomenon.

## Key findings

- Luminosity-weighted kinematics are biased by young stars.
- Young stars cause higher rotation velocities and lower dispersions.
- { extsigma}-drop is linked to kinematically cool young stellar populations.

## Abstract

A minimum in stellar velocity dispersion is often observed in the central regions of disc galaxies. To investigate the origin of this feature, known as a {\sigma}-drop, we analyse the stellar kinematics of a high-resolution N-body + smooth particle hydrodynamical simulation, which models the secular evolution of an unbarred disc galaxy. We compared the intrinsic mass-weighted kinematics to the recovered luminosity-weighted ones. The latter were obtained by analysing synthetic spectra produced by a new code, SYNTRA, that generates synthetic spectra by assigning a stellar population synthesis model to each star particle based on its age and metallicity. The kinematics were derived from the synthetic spectra as in real spectra to mimic the kinematic analysis of real galaxies. We found that the recovered luminosity-weighted kinematics in the centre of the simulated galaxy are biased to higher rotation velocities and lower velocity dispersions due to the presence of young stars in a thin and kinematically cool disc, and are ultimately responsible for the {\sigma}-drop.