WSRT observations and surface photometry of two unusual spiral galaxies

TL;DR

This study uses WSRT and optical photometry to analyze the mass distribution of two spiral galaxies, revealing unusual stellar mass-to-light ratios that suggest a non-standard initial mass function.

Contribution

It provides the first detailed mass decomposition of NGC 6824 and UGC 11919, confirming non-standard stellar initial mass functions in these galaxies.

Findings

NGC 6824 has a typical stellar mass-to-light ratio.

UGC 11919 has a very low stellar mass-to-light ratio.

A bottom-light IMF is needed to explain UGC 11919's properties.

Abstract

We discuss the results of a mass decomposition of two spiral galaxies, NGC 6824 and UGC 11919. In a previous analysis of the Hyperleda catalog, the galaxies were identified as having a peculiar dynamical $M/L$. The aim of this study is to confirm or disprove the preliminary findings, indicating a non-standard stellar initial mass function (IMF) for the galaxies. The surface photometry in B, V, and R bands was carried out with the Apache Point 0.5-m telescope and the \ion{H}{I} data cubes were obtained with the Westerbork Synthesis Radio Telescope (WSRT). Photometric profiles were decomposed into bulge and exponential disk components. Using the obtained \ion{H}{I} data cubes, rotation curves of both galaxies were constructed. Employing the photometric profiles, the mass distribution of the galaxies was decomposed into mass components: bulge, stellar disk, gas, and pseudo-isothermal dark halo. We conclude that NGC 6824 possesses a stellar disk with mass-to-light ratio $(M/L_B)_{\rm disk} = 2.5$, in agreement with its color $(B-V)_0$. On the contrary, UGC 11919 appears to have a very lightweight disk. Its dynamically estimated mass corresponds to a low stellar disk mass-to-light ratio $(M/L_B)_{\rm disk} \approx 0.5$. Under standard assumptions, this ratio does not agree with the relatively red color of the disk, while a bottom light stellar initial mass function is needed to explain the observations.