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The Group Evolution Multiwavelength Study (GEMS): bimodal luminosity functions in galaxy groups
We present B- and R-band luminosity functions (LFs) for a sample of 25nearby groups of galaxies. We find that the LFs of the groups with lowX-ray luminosity (LX < 1041.7 ergs-1) are significantly different from those of the X-raybrighter groups, showing a prominent dip around MB=-18. Whileboth categories show lack of late-type galaxies in their centralregions, X-ray dim groups also show a more marked concentration ofoptical luminosity towards the centre. A toy simulation shows that inthe low velocity dispersion environment, as in the X-ray dim group,dynamical friction would facilitate more rapid merging, thus depletingintermediate-luminosity galaxies to form a few giant central galaxies,resulting in the prominent dip seen in our LFs. We suggest that X-raydim (or low velocity dispersion) groups are the present sites of rapiddynamical evolution rather than their X-ray bright counterparts, and maybe the modern precursors of fossil groups. We predict that these groupsof low velocity dispersion would harbour younger stellar populationsthan groups or clusters with higher velocity dispersion.
| Dissecting the Luminosity Function of the Coma Cluster of Galaxies Using Canada-France-Hawaii Telescope Wide-Field Images
We determined the relative spatial density of the Coma Cluster galaxies,selected by luminosity and by central brightness, i.e., the luminosityfunction bivariate in central brightness. The Coma Cluster and controlfields were imaged using the CFH12K(42'×28') and UH8K(28'×28') wide-field cameras at theCanada-France-Hawaii Telescope. Selected Hubble Space Telescope (HST)images were used for testing. Quantities were derived from measurementsin at least two colors, which have the following features: (1) galaxiesas faint as 3 times the luminosity of the brightest globular clustersare in the completeness region of our data. (2) We have a completecensus (in the explored region) of low surface brightness galaxies witha central surface brightness almost as low as the faintest ones so farcataloged. (3) The explored area is among the largest ever sampled withCCDs at comparable depth for any cluster of galaxies. (4) The errorbudget includes all sources of errors known to date. Using HST images,we also discovered that blends of globular clusters, not resolved intoindividual components due to seeing, look like dwarf galaxies whenobserved from the ground and are numerous and bright. When mistaken asextended sources, they increase the steepness of the luminosity functionat faint magnitudes. The derived Coma luminosity function is relativelysteep (α=-1.4) over the 11 magnitudes sampled, but the slope andshape depend on color. A large population of faint low surfacebrightness galaxies was discovered, representing the largest contributor(in number) to the luminosity function at faint magnitudes. We found aclear progression for a faintening of the luminosity function from highsurface brightness galaxies (μ~20 mag arcsec-2) togalaxies of very faint central brightness (μ~24.5 magarcsec-2), and some evidence for a steepening. Compactgalaxies, usually classified as stars and therefore not included in theluminosity function, are found to be a minor population in Coma. Basedon observations obtained at the Canada-France-Hawaii Telescope and inpart at the Hubble Space Telescope.
| Compact groups in the UZC galaxy sample
Applying an automatic neighbour search algorithm to the 3D UZC galaxycatalogue (Falco et al. \cite{Falco}) we have identified 291 compactgroups (CGs) with radial velocity between 1000 and 10 000 kms-1. The sample is analysed to investigate whether Tripletsdisplay kinematical and morphological characteristics similar to higherorder CGs (Multiplets). It is found that Triplets constitute lowvelocity dispersion structures, have a gas-rich galaxy population andare typically retrieved in sparse environments. Conversely Multipletsshow higher velocity dispersion, include few gas-rich members and aregenerally embedded structures. Evidence hence emerges indicating thatTriplets and Multiplets, though sharing a common scale, correspond todifferent galaxy systems. Triplets are typically field structures whilstMultiplets are mainly subclumps (either temporarily projected orcollapsing) within larger structures. Simulations show that selectioneffects can only partially account for differences, but significantcontamination of Triplets by field galaxy interlopers could eventuallyinduce the observed dependences on multiplicity. Tables 1 and 2 are onlyavailable in electronic at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/35
| Galaxy coordinates. II. Accurate equatorial coordinates for 17298 galaxies
Using images of the Digitized Sky Survey we measured coodinates for17298 galaxies having poorly defined coordinates. As a control, wemeasured with the same method 1522 galaxies having accurate coordinates.The comparison with our own measurements shows that the accuracy of themethod is about 6 arcsec on each axis (RA and DEC).
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