Extremely metal-poor galaxy DDO 68: the LBV, H-alpha shells and the most luminous stars

TL;DR

This study investigates the extremely metal-poor galaxy DDO 68, focusing on its luminous blue variable star, H-alpha shells, and luminous stars, revealing new data on star formation, stellar variability, and galaxy structure.

Contribution

The paper provides new spectroscopic and photometric data on DDO 68's LBV, analyzes its H-alpha shells and luminous stars, and discusses star formation processes in an ultra-metal-poor galaxy.

Findings

LBV experienced a giant eruption with magnitude M_V = -10.5.

Significant variability observed in the luminous metal-poor stars.

Star formation likely triggered by shell gas instabilities and gravitational collapse.

Abstract

The paper presents new results of the ongoing study of the unusual Lynx-Cancer void galaxy DDO 68 with record-low-metallicity regions (12+log(O/H) ~7.14) of the current star formation (SF). They include: a) a new spectrum and photometry with the 6-m SAO RAS telescope (BTA) for the Luminous Blue Variable (LBV = DDO68-V1). Photometric data sets are complemented with those based on the Sloan Digital Sky Survey (SDSS) and the Hubble Space Telescope (HST) archive images; b) the analysis of the DDO~68 supergiant shell (SGS) and the prominent smaller H-alpha arcs/shells visible at the HST image coupled with kinematics maps in H-alpha obtained with the Fabry-Perot interferometer (FPI) at the BTA; c) the list of identified at the HST images of about 50 most luminous stars (-9.1 < M_V < -6.0 mag) related to star-forming regions with the known extremely low O/H. This is intended to pave the path for the actual science with the next generation of giant telescopes. We confirm the earlier hints on significant variations of the LBV optical light deriving its amplitude of dV > 3.7~mag for the first time. New data suggest that in 2008--2010 the LBV reached M_V = --10.5 and probably underwent a giant eruption. We argue that the structure of star-forming complexes along the SGS (`Northern Ring') perimeter provides evidence for the sequential induced SF episodes caused by the shell gas instabilities and gravitational collapse. The variability of some DDO~68 luminous extremely metal-poor stars can be monitored with medium-size telescopes at sites with superb seeing.