Spectral Observations of Superthin Galaxies

TL;DR

This study uses spectral and photometric observations to analyze the structure, dynamics, and dark matter content of superthin galaxies, revealing differences between red and blue types and identifying a surface density threshold for thin disk stability.

Contribution

It provides a comprehensive modeling approach combining spectral data, photometry, and disk properties to estimate dark halo parameters in superthin galaxies, highlighting differences based on galaxy color.

Findings

Blue STGs are more dynamically under-evolved with lower velocity dispersions.

Dark halo-to-disk scale ratio is generally under 2, shorter in red STGs.

A surface density threshold of 50 M_sun/pc^2 is identified for thin disk stability.

Abstract

We conduct spectral observations of 138 superthin galaxies (STGs) with high radial-to-vertical stellar disk scales ratio with the Dual Imaging Spectrograph (DIS) on the 3.5m telescope at the Apache Point Observatory (APO) to obtain the ionized gas rotation curves with R ~ 5000 resolution. We also performed near infrared (NIR) H and Ks photometry for 18 galaxies with the NICFPS camera on the 3.5m telescope. The spectra, the NIR photometry and published optical and NIR photometry are used for modeling that utilizes the thickness of the stellar disk and rotation curves simultaneously. The projection and dust extinction effects are taken into account. We evaluate eight models that differ by their free parameters and constraints. As a result, we estimated masses and scale lengths of the galactic dark halos. We find systematic differences between the properties of our red and blue STGs. The blue STGs have a large fraction of dynamically under-evolved galaxies whose vertical velocity dispersion is low in both gas and stellar disks. The dark halo-to-disk scale ratio is shorter in the red STGs than in the blue ones, but in a majority of all STGs this ratio is under 2. The optical color $(r-i)$ of the superthin galaxies correlates with their rotation curve maximum, vertical velocity dispersion in stellar disks, and mass of the dark halo. We conclude that there is a threshold central surface density of 50 $M_{\odot}$\,pc$^{-2}$ below which we do not observe very thin, rotationally supported galactic disks.