Study of DDO 68: nearest candidate for a young galaxy?

¹ Special Astrophysical Observatory RAS, Nizhnij Arkhyz, Karachai-Circassia, 369167 Russia e-mail: sap@sao.ru
² European Southern Observatory, Garching-bei-Munchen, Germany

Received: 21 March 2005
Accepted: 8 July 2005

Abstract

We present the results of optical spectroscopy and imaging with the SAO 6 m telescope for the dwarf galaxy DDO 68 (UGC 5340 = VV 542), falling into the region of very low density of luminous () galaxies (Lynx-Cancer void). Its deep images in bands and in the narrow Hα-filter show that this galaxy has the very irregular morphology, with a long curved tail on the South and a ring-like structure at the Northern edge. The latter consists of 5 separate regions, in three of which we could measure O/H by the classical T_e method. Their weighted mean oxygen abundance corresponds to , coincident within uncertainties with those for I Zw 18. The colour of DDO 68 is rather blue all over the galaxy, indicating the youth of its stellar populations. Comparing the colour of the underlying exponential disk of 0 with the PEGASE.2 models for the evolving stellar clusters, we give the first estimate of the ages of the oldest stellar population, which needs confirmation by the other colours and the photometry of resolved stars. These ages are in the range of 200–900 Myr for continuous star formation law, and ~100–115 Myr for the instantaneous starburst. We discuss the properties and the possible youth of this nearby object (~2.3 times closer than the famous young galaxy I Zw 18) in the context of its atypical environment.