The Cometary Blue Compact Dwarf Galaxies Mkn 59 and Mkn 71: Clues to Dwarf Galaxy Evolution?

TL;DR

This study investigates the properties and possible evolutionary links of cometary Blue Compact Dwarf Galaxies, analyzing their structure, metallicity, and age to understand their place in dwarf galaxy evolution.

Contribution

It provides the first detailed analysis of the physical properties, ages, and structure of the host galaxies of iI,C BCDs, suggesting they may be a transitional phase in dwarf galaxy evolution.

Findings

Host galaxies have formation ages around 2 Gyr, younger than typical BCDs.

Metallicity shows effective mixing over kpc scales.

Surface brightness profiles are intermediate between other BCD types.

Abstract

"Cometary" Blue Compact Dwarf Galaxies (iI,C BCDs) are characterized by off-center starbursts close to the end of an elongated dwarf irregular (dI)-like host galaxy. This may either represent randomly enhanced star-forming activity of a dI, or may be caused by a set of special properties of such systems or their environment. For a first investigation of this issue, we analyse the nearby iI,C BCDs Markarian 59 and Markarian 71. Using deep ground-based spectrophotometric data and HST images, we derive physical properties, structure and ages of the starburst regions and the underlying stellar host galaxies. The metallicities show small scatter in the vicinity of the star-forming regions and along the major axis of Mrk 59 which suggests effective mixing of heavy elements on kpc scales. The surface brightness profiles of the underlying host galaxies in either iI,C BCD show an exponential decay with a central surface brightness and scale length that are intermediate between typical iE/nE BCDs and dIs. Spectral population synthesis models in combination with colour magnitude diagrams and colour profiles yield most likely formation ages of ~2 Gyr for the host galaxies in both iI,C BCDs, with upper limits of ~4 Gyr for Mrk 59 and ~3 Gyr for Mrk 71, i.e. significantly lower than the typical age of several Gyr derived for the host galaxies of iE/nE BCDs. These findings raise the question whether iI,C systems form a distinct class within BCDs with respect to the age and structure of their hosts, or whether they represent an evolutionary stage connecting young i0 BCDs and "classical" iE/nE BCDs. Properties of analogous objects studied in the local universe and at medium redshifts provide some support for this evolutionary hypothesis.