Stability of galaxies across morphological sequence

Abstract

We investigate the stability of nearby disc galaxies and galaxies at redshift ($z$) equal to 4.5. We explore the connection between the stability parameter $(Q_{RW})$, star formation rate ($SFR$), gas fraction $(f^{Gas})$, and the time scale for growth of gravitational instabilities $(\tau)$. We find that, despite differences in morphology $91$ $\%$ of the nearby galaxies have a minimum value of stability parameter ($Q^{Min}_{RW}$) greater than $1$ indicating stability against the growth of axisymmetric instabilities. The spirals in our sample have higher median star formation rate, lower median $Q_{RW}$, a lower $f^{Gas}$ and small time scale for growth of gravitational instabilities than irregular galaxies. We find that the gravitational instabilities in spirals convert a large fraction of gas into stars quickly, depleting the gas reservoirs. On the other hand, star formation occurs more gradually over longer timescales in irregulars with a higher gas fraction. We then compare the stability of the nearby galaxies with galaxies at $z\,=\,4.5$. We find that net stability levels in the nearby galaxies and the galaxies at $z\,=\,4.5$ are primarily driven by the stellar disc suggesting the presence of an inherent mechanism that self-regulates the stability. Finally, upon removing the contribution of the dark matter to the total potential, the median $Q_{RW}$ for the nearby galaxies and galaxies at $z \,= \,4.5$ remains unchanged indicating that the baryons can self-regulate the stability levels, at least in a statistical sense.