Tol 2240–384 – a new low-metallicity AGN candidate^*

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: izotov@mao.kiev.ua
² Main Astronomical Observatory, Ukrainian National Academy of Sciences, Zabolotnoho 27, Kyiv 03680, Ukraine
³ LUTH, Observatoire de Paris, CNRS, Universite Paris Diderot, Place Jules Janssen, 92190 Meudon, France
⁴ Institut für Astrophysik, Göttingen Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁵ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Department of Astronomy, Oskar Klein Centre, Stockholm University, 106 91 Stockholm, Sweden

Received: 9 March 2010
Accepted: 23 April 2010

Abstract

Context. Active galactic nuclei (AGNs) have typically been discovered in massive galaxies of high metallicity.

Aims. We attempt to increase the number of AGN candidates in low metallicity galaxies. We present VLT/UVES and archival VLT/FORS1 spectroscopic and NTT/SUSI2 photometric observations of the low-metallicity emission-line galaxy Tol 2240–384 and perform a detailed study of its morphology, chemical composition, and emission-line profiles.

Methods. The profiles of emission lines in the UVES and FORS1 spectra are decomposed into several components with different kinematical properties by performing multicomponent fitting with Gaussians. We determine abundances of nitrogen, oxygen, neon, sulfur, chlorine, argon, and iron by analyzing the fluxes of narrow components of the emission lines using empirical methods. We verify with a photoionisation model that the physics of the narrow-line component gas is similar to that in common metal-poor galaxies.

Results. Image deconvolution reveals two high-surface brightness regions in Tol 2240–384 separated by 2.4 kpc. The brightest southwestern region is surrounded by intense ionised gas emission that strongly affects the observed B–R colour on a spatial scale of ~5 kpc. The profiles of the strong emission lines in the UVES spectrum are asymmetric and all these lines apart from Hα and Hβ can be fitted by two Gaussians of FWHM ~ 75–92 km s^-1 separated by ~80 km s^-1 implying that there are two regions of ionised gas emitting narrow lines. The oxygen abundances in both regions are equal within the errors and in the range 12 + O/H = 7.83–7.89. The shapes of the Hα and Hβ lines are more complex. In particular, the Hα emission line consists of two broad components of FWHM ~ 700 km s^-1 and 2300 km s^-1, in addition to narrow components of two regions revealed from profiles of other lines. This broad emission in Hα and Hβ associated with the high-excitation, brighter southwestern H ii region of the galaxy is also present in the archival low- and medium-resolution VLT/FORS1 spectra. The extraordinarily high luminosity of the broad Hα line of 3 × 10⁴¹ erg s^-1 cannot be accounted for by massive stars at different stages of their evolution. The broad Hα emission persists over a period of 7 years, which excludes supernovae as a powering mechanism of this emission. This emission most likely arises from an accretion disc around a black hole of mass ~10⁷ .