\HI{} 21cm observations and dynamical modelling of the thinnest galaxy: FGC 2366

TL;DR

This study presents detailed HI observations and dynamical modeling of FGC 2366, the thinnest known galaxy, revealing the structure, kinematics, dark matter profile, and key physical factors behind its superthin stellar disc.

Contribution

It provides the first comprehensive dynamical analysis of the thinnest galaxy, combining HI data, stellar photometry, and advanced modeling to understand its unique structure.

Findings

FGC 2366 has an asymptotic rotational velocity of 100 km/s.

The galaxy's HI mass is approximately 10^9 solar masses.

A steeply-rising rotation curve is best fitted by an NFW dark matter halo.

Abstract

Superthin galaxies are bulgeless low surface brightness galaxies with unusually high major-to-minor axes ratio of the stellar disc, i.e.,$10<a/b<20$. We present Giant Metrewave Radio Telescope (GMRT) \HI{} 21cm radio-synthesis observations of FGC 2366, the thinnest galaxy known with $a/b=21.6$. Employing the 3-D tilted-ring modelling using Fully Automated TiRiFiC (FAT), we determine the structure and kinematics of the \HI{} gas disc, obtaining an asymptotic rotational velocity equal to 100 \kms and a total \HI{} mass equal to 10$^9 M_{\odot}$. Using $z$-band stellar photometry, we obtain a central surface brightness of 22.8 mag ${\rm{arcsec}}^{-2}$, a disc scale length of 2.6 kpc, and a scaleheight of 260 pc. Next, we determine the dark matter density profile by constructing a mass model and find that an NFW dark matter halo best fits the steeply-rising rotation curve. With the above mass inventory in place, we finally construct the dynamical model of the stellar disc of FGC 2366 using the stellar dynamical code "AGAMA". To identify the key physical mechanisms responsible for the superthin vertical structure, we carry out a Principal Component Analysis of the data corresponding to all the relevant dynamical parameters and $a/b$ for a sample of superthin and extremely thin galaxies studied so far. We note that the first two principal components explain 80$\%$ of the variation in the data, and the significant contribution is from the compactness of the mass distribution, which is fundamentally responsible for the existence of superthin stellar discs.