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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.
| An image database. II. Catalogue between δ=-30deg and δ=70deg.
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
| 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.
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