WSRT observations and surface photometry of two unusual spiral galaxies^⋆

¹ Moscow M.V. Lomonosov State University, Sternberg Astronomical Institute, Universitetskii pr. 13, 119992 Moscow, Russia
e-mail: saburovaann@gmail.com
² Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
e-mail: jozsa@astron.nl
³ Argelander – Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ New Mexico State University and Apache Point Observatory, Sunspot, NM 88349, USA
e-mail: dmbiz@apo.nmsu.edu

Received: 13 December 2012
Accepted: 30 April 2013

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 mass-to-light ratio. 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 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 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)_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)_disk ≈ 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.