Journey to the Heart of Darkness - Exploring Void Galaxies

Void Galaxies are isolated single galaxies or interacting pairs or triplets located in highly under-dense void regions far from their nearest neighbors. They are an test case for models of of galaxy formation and evolution. Their isolation from interactions with the filaments, walls and clusters of denser regions of large scale structure allows for the internal dynamical processes to dominate their evolution. Standard ΛCDM Cosmological models also make predictions about the number of dwarf galaxies that should be present in voids as a function of the matter density parameter Ωm. Peeples described this as the void phenomenon, and it has two aspects, the expected number of dwarf galaxies in voids and their statistical properties which could potentially diverge from galaxies in denser fields. An earlier survey conducted by Kreckel using the SDSS found that void galaxies tended to be luminous blue star forming dwarfs but the result was limited by the magnitude cutoff of the SDSS at ∼ 17 in the r band. For this observational program we conducted a dedicated search for low surface brightness (LSB) galaxies that would fall below the SDSS magnitude cutoff by searching for Hydrogen-α emission from star forming regions using the narrow- band Hα filters on the 4-m Mayall telescope at the Kitt Peak National Observatory (KPNO). Two voids were chosen from the SDSS Void Catalog whose barycenters in redshift were in the center of one of the filters, allowing for candidates to be identified by the residuals from subtraction. Candidates identified this way first had their estimated distances supported by their SDSS photometric redshifts because at z=0.3 the O[III] 5007 Angstrom line would be redshifted into the same bandpass as the Hydrogen-α emission from z=0.03 void galaxies. Then these void galaxy candidates were observed spectroscopically with the LBT MODS spectrograph and with Gemini North to search for emission lines that would allow for measures of the metallicity, star formation rate and gas fraction to be calculated. Unfortunately for this study the combination of two different photometric estimates was not as accurate as it had been hoped; only one of the eleven new galaxy candidates was actually in the void, in large part because the odds of a catastrophic failure of photometric redshift is more likely at low redshifts, but the follow-up observations of known void galaxy VGS9 unexpectedly found previously unresolved structures that could be either star formation regions or possibly satellite galaxies.