11 - Faint dwarf irregular galaxies

Summary

IC 1613

Introduction

The dwarf irregular galaxy IC 1613 was discovered by Wolf (1906) and is described as “F,eeL” (i.e., faint and most extremely large) in the Second Index Catalogue of Nebulae (Dreyer 1908). The true nature of IC 1613 was first recognized by Baade (1935), who determined its distance by using the period–luminosity relation for Cepheids having periods ranging from 14 days to 42 days. Baade concluded that “it is without doubt a system of low luminosity.” Subsequently it was included as a bona fide member of the Local Group by Hubble (1936, p. 145). IC 1613 is a nonbarred irregular that serves as the prototype for DDO type Ir V. The fact that IC 1613 was already known almost a century ago, even though it is quite faint (M_V = –15.3), suggests that the inventory of all but the dimmest Local Group members is (excepting objects at low Galactic latitude) probably reasonably complete. A blue image taken with the Palomar 1.2-m Schmidt telescope is shown in Figure 11.1. A higher resolution photograph of IC 1613, which was obtained with the Palomar 5-m reflector, is shown in Volders & Högbom (1961). A beautiful photograph inthe light of Hα is reproduced in Sandage (1971).

Distance and reddening

The discovery of Cepheids in IC 1613 by Baade (1935) was followed up by a more detailed investigation (Baade 1963, pp. 218–226) that resulted in the identification of 25 Cepheids with periods ranging from 2.4 days to 146 days.