# The Structure of Void Galaxies

## Contact

Peter Garnavich
University of Notre Dame
pgarnavi@nd.edu

## Summary

Most galaxies live in clusters, walls and filaments that make up the large scale structure of matter, while 80% of the universe’s volume is occupied by voids. Voids do contain some galaxies but they are difficult to identify hiding among the foreground and background galaxies that live in matter-dense regions. Recent research suggests that gas accretion and galaxy-galaxy interactions are important processes in the evolution of average galaxies, but void galaxies are likely to experience less outside influence and could be fundamentally different from their more gregarious counterparts. Their lower density environments may slow their evolution to the point that void galaxies are more primitive than similar mass galaxies in the structure. Kreckel et al. (2011) identified 60 void galaxies in the SDSS database and we aim to study the most massive subset of these galaxies using the resolved spectra provided by the MANGA project.

## Finding Targets

An object whose MANGA_TARGET3 or MNGTARG3 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Target Density (deg-2)
VOID 5 Void galaxies from Kreckel et al. (2011) 0.006

## Description

Lambda-CDM computer simulations suggest that there should be far more halo substructure than is observed in galaxies (e.g. Moore et al. 1999; Mateo 1998). As simulations have gotten more sophisticated, this “missing satellites problem” has become less acute, and very faint dwarf galaxies have been discovered, but prediction of the substructure remains at odds with the observations. One solution is that tidal stripping is more efficient in real life than in the simulations and substructure gets destroyed over time. Finding nearby galaxies that have been frozen in time so they retain their initial complement of satellites sounds far-fetched. But void galaxies may actually keep their primitive halos and provide a key test of the theory.
The target selection criteria for each class of transient objects is described below.

## Target Selection

Kreckel et al. 2011 (co-I on this program) has carefully selected a sample of void galaxies that reside in the deepest underdensities using topological techniques to define the large scale environment (Platen 2009). Previous work studied a sample of 60 targets with extensive multi-wavelength data as part of the Void Galaxy Survey (VGS; Kreckel et al. 2012) including HI imaging, medium deep GALEX NUV, Spitzer 3.6 and 4.5 micron, Hα narrowband and deep B- and R-band optical images.

We have selected 31 targets from this sample with masses above the 109 Msun threshold suitable for MaNGA and comparison with the MaNGA galaxies in average environments. 23 of these galaxies are already within the MaNGA target catalog and simply require prioritization. 8 additional galaxies meet the stellar mass threshold for MaNGA, and would be suitable for comparisons with other MaNGA galaxies.

We estimate that while void galaxies make up ~5% of the MaNGA target catalog, consistent with their rarity at this stellar mass range, they make up only 2% (120 galaxies) of the primary catalog, and thus will be under-represented in the final sample. This ancillary sample will provide a 25% increase in the number of void galaxies targeted by MaNGA, and will ensure that we target the void galaxies for which rich ancillary data is already available.

## REFERENCES

Kreckel, K., et al. 2011, AJ, 141, 4
Kreckel, K., et al., 2012, AJ, 144, 16
Mateo, M. 1998, ARAA, 36, 435
Moore, B. et al. 1999, ApJ, 524, L19
Platen, E., 2009, PhD Thesis, Kapteyn Institute, Groningen, The Netherlands