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An Imaging and Spectroscopic Survey of Galaxies within Prominent Nearby Voids. II. Morphologies, Star Formation, and Faint Companions
We analyze the optical properties of ~300 galaxies within and aroundthree prominent voids of the Center for Astrophysics Redshift Survey. Wedetermine CCD morphologies and Hα equivalent widths from ourimaging and spectroscopic survey. We also describe a redshift survey of250 neighboring galaxies in the imaging survey fields. We assess themorphology-density relation, EW(Hα)-density relation, and theeffects of nearby companions for galaxies in low-density environmentsselected with a smoothed large-scale (5 h-1 Mpc) galaxynumber density n. Both the morphological mix and the Hα line widthdistribution of galaxies at modest underdensities, 0.5R=16.13, demonstrates that the incidence ofa close companion in redshift space is insensitive to global densityover the range we investigate (0.163 σ) fromΔcz>~200 km s-1 at 0.5-1 at n<=0.5n. In the lowest densityenvironments, galaxies with companions clearly (~4 σ) havestronger star formation than comparable galaxies at larger globaldensity (0.5-1 kpc and 1000 km s-1) varies little over theentire density range. These results, combined with the luminosity- andcolor-density relations of this sample (Paper I), suggest that theformation and evolution of field galaxies are insensitive to large-scaleunderdensity down to a threshold of roughly half the mean density. Thedifferences in galaxy properties at the lowest global densities we canexplore (n<=0.5n) may be explained by (1) a relative scarcity of thesmall-scale primordial density enhancements needed to form massiveearly-type/absorption-line galaxies and (2) present-day galaxyencounters that are relatively more effective because of the lowervelocity dispersion on small scales (<~200 h-1 kpc) weobserve in these regions. In the voids, where the luminous galaxiespresumably formed more recently, there should be more gas and dustpresent for active star formation triggered by nearby companions.
| An Imaging and Spectroscopic Survey of Galaxies within Prominent Nearby Voids. I. The Sample and Luminosity Distribution
We study the optical properties of a large sample of galaxies inlow-density regions of the nearby universe. We make a 5 h-1Mpc smoothed map of the galaxy density throughout the Center forAstrophysics Redshift Survey (CfA2) to identify galaxies within threeprominent nearby ``voids'' with diameter >~30 h-1 Mpc. Weaugment the CfA2 void galaxy sample with fainter galaxies found in thesame regions from the more recent and deeper Century and Redshiftsurveys . We obtain B and R CCD images and high signal-to-noiselong-slit spectra for the resulting sample of 149 void galaxies, as wellas for an additional 131 galaxies on the periphery of these voids. Herewe describe the photometry for the sample, including B isophotalmagnitudes and B-R colors. For the 149 galaxies that lie in regionsbelow the mean survey density, the luminosity functions in B and R arewell fit by Schechter functions with respective parameters(αB=-0.5+/-0.3, B*=-18.9+/-0.2) and(αR=-0.9+/-0.3, R*=-20.4+/-0.3). The Bluminosity function (LF) is consistent with typical survey LFs (e.g.,the Southern Sky Redshift Survey), and the R LF is consistent with theCentury Survey. The B and R LFs of 131 galaxies in the ``voidperiphery''-regions between the mean density and twice the mean-havesimilar Schechter parameters. The CfA2 LF is inconsistent with bothsamples at the 3.5 σ level. When we narrow our analysis to the 46galaxies in regions below half the mean density, the LF is significantlysteeper: α~-1.4+/-0.5. The typical survey LFs are inconsistentwith this subsample at the ~2 σ level. The B-R color distributionof galaxies in the lowest density regions is also shifted significantly(~3 σ) blueward of the higher density samples. The most luminousred galaxies (R<~-21) are absent from the lowest density regions at the2.5 σ level.
| Galaxy structures in the Hercules region
216 redshifts have been obtained in a region of 981 sq deg south of theHercules supercluster. 172 of these redshifts are of galaxies withmpg less than or equal to 15.1, 110 of which had no previousvelocity measurement. 44 new redshifts are of galaxies fainter thanmpg = 15.1. With these new data we have been able to define asample in a vast region (approximately 1700 sq deg) around Herculeslimited to mpg less than or equal to 15.1 with a velocitycompleteness of 81.5%. 189 galaxies have been morphologically classifiedso that all galaxies in the sample with known velocity now also haveknown morphology. The magnitude limited sample, including 556 galaxies,is then used to identify and describe galaxy structures in the region.We find that the overdense volume is small, that its overall appearanceis that of a coral branch floating in a sea of nothing and that earlyand late type galaxies defined different structures.
| H I observations of galaxies in the Hercules supercluster. II - Additional UGC galaxies and galaxies in selected CGCG fields
The second installment of a H I redshift survey in the region of theHercules supercluster is presented. This part of the survey presents 218H I spectra of galaxies selected from the UGC which extends thepreviously begun survey of UGC galaxies in that region. In addition,spectra of 136 galaxies from selected CGCG fields are given. The CGCGfields were chosen to cover the projected position of the Hercules void.Measured H I parameters from the spectra are given. This survey wasconducted for application of the Tully-Fisher relation to search forstreaming motion around the void in front of the supercluster. Theselection effects affecting such a search are discussed in detail.
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