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

K. G. Noeske, N. G. Guseva+4 more|arXiv (Cornell University)|Jul 11, 2000

Stellar, planetary, and galactic studies3 citations

TL;DR

This study investigates the cometary blue compact dwarf galaxies Mkn 59 and Mkn 71 using deep spectrophotometry and HST imaging to explore their host galaxy structure and starburst properties. It finds that their low surface brightness hosts are significantly younger (~2 Gyr) than typical iE/nE BCDs, with efficient metal mixing and intermediate structural parameters, suggesting cometary BCDs may represent a distinct evolutionary stage between young i0 BCDs and classical iE/nE BCDs.

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.

Motivation & Objective

• To determine the age and structural properties of the low surface brightness (LSB) host galaxies in cometary blue compact dwarf galaxies (iI,C BCDs), which are rare systems with off-center starbursts.
• To assess whether the morphological peculiarity of iI,C BCDs arises from intrinsic physical properties or stochastic processes.
• To investigate the metallicity distribution and mixing efficiency in the starburst regions and along the major axes of the hosts.
• To evaluate whether iI,C BCDs represent a distinct class or an evolutionary stage connecting young i0 BCDs and classical iE/nE BCDs.

Proposed method

• Deep ground-based spectrophotometry and HST imaging were used to obtain high dynamic range surface brightness profiles (SBPs) of Mkn 59 and Mkn 71.
• A profile decomposition method was applied to separate the starburst and LSB host components, with radial extent verified using Hα and colour maps.
• Long-slit spectroscopy was used to measure ionized gas emission, which was then semi-empirically modeled and subtracted to isolate the underlying stellar continuum.
• Spectral population synthesis modeling was applied at multiple positions along the slits to derive the age and metallicity of the underlying stellar populations.
• Oxygen abundances were measured via emission-line analysis, and metallicity gradients were assessed to infer mixing efficiency.
• The results were compared with other known cometary BCDs to test for consistency and support for an evolutionary hypothesis.

Experimental results

Research questions

• RQ1 Are the low surface brightness host galaxies of cometary BCDs significantly younger than those of classical iE/nE BCDs?
• RQ2 Does the off-center starburst morphology in iI,C BCDs correlate with distinct structural and age properties of the host galaxy?
• RQ3 Is metal mixing efficient enough to produce small metallicity gradients across the host galaxies of iI,C BCDs?
• RQ4 Do the structural and age properties of iI,C BCD hosts place them between dI/dE and iE/nE BCDs in a potential evolutionary sequence?
• RQ5 Can the morphological peculiarity of iI,C BCDs be explained by intrinsic physical processes rather than stochastic events?

Key findings

• The azimuthally averaged radial intensity profiles of the LSB hosts in Mkn 59 and Mkn 71 are well approximated by an exponential law with central surface brightness and scale length intermediate between typical iE/nE BCDs and dwarf irregulars.
• Spectral population synthesis modeling indicates a most probable formation age of ~2 Gyr for the LSB host galaxies, with upper limits of ~4 Gyr for Mkn 59 and ~3 Gyr for Mkn 71.
• The average oxygen abundance is 12+log(O/H) = 8.0 (1/8 Z☉) for Mkn 59 and 12+log(O/H) = 7.8 (1/14 Z☉) for Mkn 71, typical for BCDs but with small scatter (~0.2 dex) across the starburst region and major axis.
• The small metallicity scatter suggests efficient mixing of heavy elements on scales of several kiloparsecs in both systems.
• Five additional cometary BCDs with available data show similar structural and age properties, supporting a common physical origin for the iI,C morphology.
• The findings suggest that iI,C BCDs may represent a distinct evolutionary stage between young i0 BCDs and classical iE/nE BCDs, rather than a stochastic phenomenon.