Only the Lonely: H I Imaging of Void Galaxies

Abstract

Void galaxies, residing within the deepest underdensities of the Cosmic Web, present an ideal population for the study of galaxy formation and evolution in an environment undisturbed by the complex processes modifying galaxies in clusters and groups, as well as provide an observational test for theories of cosmological structure formation. We have completed a pilot survey for the H i imaging aspects of a new Void Galaxy Survey (VGS), imaging 15 void galaxies in H i in local (d < 100 Mpc) voids. H i masses range from 3.5 × 10 8 to 3.8 × 10 9 M , with one nondetection with an upper limit of 2.1 × 10 8 M . Our galaxies were selected using a structural and geometric technique to produce a sample that is purely environmentally selected and uniformly represents the void galaxy population. In addition, we use a powerful new backend of the Westerbork Synthesis Radio Telescope that allows us to probe a large volume around each targeted galaxy, simultaneously providing an environmentally constrained sample of fore-and background control sample of galaxies while still resolving individual galaxy kinematics and detecting faint companions in H i. This small sample makes up a surprisingly interesting collection of perturbed and interacting galaxies, all with small stellar disks. Four galaxies have significantly perturbed H i disks, five have previously unidentified companions at distances ranging from 50 to 200 kpc, two are in interacting systems, and one was found to have a polar H i disk. Our initial findings suggest void galaxies are a gas-rich, dynamic population which present evidence of ongoing gas accretion, major and minor interactions, and filamentary alignment despite the surrounding underdense environment.

The Void Galaxy Survey

“…The void galaxies have a higher star formation rate per hydrogen mass. From Kreckel et al 2011. One particular aspect in which we find a systematic deviation of our void galaxies, with respect to the norm for similar galaxies, is their size. They have stellar disks that are smaller than average, with the r-band r 90 radius of our late type void galaxies systematically lower than the median for late type galaxies ( fig.…”

“…The first fully analyzed sample of 15 void galaxies demonstrated the success of our strategy [19]. With 14 detections out of 15, the HI detection rate is very high.…”

“…Using the WSRT we have thus far mapped the HI structure of 55 of the 60 galaxies. Of the total VGS sample of 60 void galaxies, the pilot subsample of 15 galaxies has been fully analyzed [32,19]. A necessary sample of comparison galaxies is obtained through simultaneous coverage of regions in front and behind the targeted void galaxies, probing the higher density regions surrounding the targeted void.…”

H I Imaging Surveys: Gas and Galaxy Evolution in Different Environments

“…Although 42 J. van Gorkom voids appear to be mostly empty there are some galaxies in voids and these provide a unique opportunity to study galaxies in an earlier stage of their evolution, mostly undisturbed by the larger environment. Kreckel et al 2011a have carried out an H I imaging survey of galaxies in voids selected from the SDSS. They use a new geometric method to find the deepest underdensities of the voids, a method which makes no a priori assumption about the shapes of the voids, and a method which recovers the structure on small scales as traced out by the galaxies.…”

“…Figure 1 shows an example of the location of selected void galaxies and control galaxies within the large scale structure. The main results of a pilot for this survey are presented in Kreckel et al 2011a Void galaxies turn out to be small, mostly blue and relatively gas rich. They follow the usual scaling relations, such as the Tully Fisher relation and they have typical H I masses for their optical luminosity.…”