On the morphology of sigma-drop galaxies

TL;DR

This study investigates the origins of sigma-drops in galaxy centers by comparing 20 sigma-drop galaxies with control galaxies, analyzing nuclear morphology, dust, and emission features to understand their connection to galaxy dynamics and nuclear activity.

Contribution

It provides new evidence linking sigma-drops to inflow-induced star formation and nuclear gas structures, highlighting their association with Seyfert activity and specific morphological features.

Findings

Sigma-drops are associated with nuclear dust spirals and Halpha rings.

Higher fraction of Seyfert galaxies in sigma-drop sample.

Sigma-drops likely caused by inflow-induced star formation and gas dynamics.

Abstract

Context: Local reductions of the stellar velocity dispersion in the central regions of galaxies are known as sigma-drops. Knowing the origin of these features can lead to better understanding of inner galactic dynamics. Aims: We present a sample of 20 sigma-drop galaxies matched with a control sample of galaxies without sigma-drop in order to search for correlations between sigma-drops and the properties, primarily morphological, of the nuclear zones and discs of their host galaxies. Methods: We study the dust and Halpha distribution at 0.1 arcsec scale, using Hubble Space Telescope imaging, in the circumnuclear zones of the two samples of galaxies, searching for differences and trying to establish a link between the nuclear kinematics and the host morphology. We have also considered the CO and HI emission of the galaxies and their luminosity profiles. Results: We classify the two samples following both morphological parameters and the luminosity profiles. We find a larger fraction of nuclear dust spirals and Halpha rings in the sigma-drop sample. We also find that the fraction of Seyfert galaxies in the sigma-drop sample is bigger than that of LINERs and that the reverse is true for the control sample. Conclusions: Our findings are evidence that a $\sigma$-drop is very probably due to inflow-induced star formation in a dynamically cool disc, or in a gas ring, shock focused by an inner Lindblad resonance above a certain critical density level. The same mechanism that feeds the nuclear ring or the nuclear disc is probably reponsible for the higher rate of Seyfert galaxies among the $\sigma$-drop hosts.