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Enhanced mergers of galaxies in low-redshift clusters
An ensemble cluster has been formed from a data set comprising acomplete magnitude-limited sample of 680 giant galaxies(M0B <~ -19) in eight low-redshift clusters,normalized by the velocity dispersions and virial radii for theearly-type cluster populations. Distinct galaxy populations have beenidentified, including an infall population. A majority (50-70 per centor greater) of the infall population are found to be in interacting ormerging systems characterized by slow gravitational encounters. Theobserved enhancement of galaxy-galaxy encounters in the infallpopulation compared to the field can be explained by gravitationalshocking. It is shown that disc galaxy mergers in the infall populationintegrated over the estimated lifetime of the cluster (~10 Gyr) canreadily account for the present cluster S0 population.
| An Hα survey of eight Abell clusters: the dependence of tidally induced star formation on cluster density
We have undertaken a survey of Hα emission in a substantiallycomplete sample of CGCG galaxies of types Sa and later within 1.5 Abellradii of the centres of eight low-redshift Abell clusters (Abell 262,347, 400, 426, 569, 779, 1367 and 1656). Some 320 galaxies weresurveyed, of which 116 were detected in emission (39 per cent ofspirals, 75 per cent of peculiars). Here we present previouslyunpublished data for 243 galaxies in seven clusters. Detected emissionis classified as `compact' or `diffuse'. From an analysis of the fullsurvey sample, we confirm our previous identification of compact anddiffuse emission with circumnuclear starburst and disc emissionrespectively. The circumnuclear emission is associated either with thepresence of a bar, or with a disturbed galaxy morphology indicative ofongoing tidal interactions (whether galaxy-galaxy, galaxy-group, orgalaxy-cluster). The frequency of such tidally induced (circumnuclear)starburst emission in spirals increases from regions of lower to higherlocal galaxy surface density, and from clusters with lower to highercentral galaxy space density. The percentages of spirals classed asdisturbed and of galaxies classified as peculiar show a similar trend.These results suggest that tidal interactions for spirals are morefrequent in regions of higher local density and for clusters with highercentral galaxy density. The prevalence of such tidal interactions inclusters is expected from recent theoretical modelling of clusters witha non-static potential undergoing collapse and infall. Furthermore, inaccord with this picture, we suggest that peculiar galaxies arepredominantly ongoing mergers. We conclude that tidal interactions arelikely to be the main mechanism for the transformation of spirals to S0sin clusters. This mechanism operates more efficiently in higher densityenvironments, as is required by the morphological type-local surfacedensity (T-Σ) relation for galaxies in clusters. For regions ofcomparable local density, the frequency of tidally induced starburstemission is greater in clusters with higher central galaxy density. Thisimplies that, for a given local density, morphological transformation ofdisc galaxies proceeds more rapidly in clusters of higher central galaxydensity. This effect is considered to be the result of subclustermerging, and could account for the previously considered anomalousabsence of a significant T-Σ relation for irregular clusters atintermediate redshift.
| IRAS galaxies in the Perseus cluster
We cross-correlate the optical catalogue of galaxies in the field of thePerseus cluster (Abell 426) with IRAS PSC sources. The optical cataloguecomprises data on 660 galaxies brighter than B25 ~ 19.5(Brunzendorf & Meusinger 1999). A sample of 19 IRAS galaxies isfound. According to their redshifts, 17 galaxies are likely members ofthe Perseus cluster, two are background galaxies. The sample-averagedFIR excess is higher than expected for normal galaxies. The opticalmorphology of the IRAS galaxies is evaluated on CCD images taken in theB band at a seeing of about 1'', complemented by CCD images taken in theredshifted Hα band or in the R band. Individual descriptions arepresented along with the B band images for all of the IRAS galaxies. Asubstantial fraction of the galaxies in the IRAS sample exhibit signsfor morphological distortion. A correlation between the FIR activity andthe strength of distortion is indicated. On the other hand, there areapproximately as many disturbed/interacting galaxies in the Perseuscluster region without IRAS counterparts as IRAS galaxies. The IRASgalaxies are much less concentrated towards the cluster centre thantypical bright cluster galaxies. For the distorted non-IRAS galaxies,such a trend is less pronounced. These differences may be related torapid stripping of gas as galaxies enter the cluster.
| The QDOT all-sky IRAS galaxy redshift survey
We describe the construction of the QDOT survey, which is publiclyavailable from an anonymous FTP account. The catalogue consists ofinfrared properties and redshifts of an all-sky sample of 2387 IRASgalaxies brighter than the IRAS PSC 60-μm completeness limit(S_60>0.6Jy), sparsely sampled at a rate of one-in-six. At |b|>10deg, after removing a small number of Galactic sources, the redshiftcompleteness is better than 98per cent (2086/2127). New redshifts for1401 IRAS sources were obtained to complete the catalogue; themeasurement and reduction of these are described, and the new redshiftstabulated here. We also tabulate all sources at |b|>10 deg with noredshift so far, and sources with conflicting alternative redshiftseither from our own work, or from published velocities. A list of 95ultraluminous galaxies (i.e. with L_60μm>10^12 L_solar) is alsoprovided. Of these, ~20per cent are AGN of some kind; the broad-lineobjects typically show strong Feii emission. Since the publication ofthe first QDOT papers, there have been several hundred velocity changes:some velocities are new, some QDOT velocities have been replaced by moreaccurate values, and some errors have been corrected. We also present anew analysis of the accuracy and linearity of IRAS 60-μm fluxes. Wefind that the flux uncertainties are well described by a combination of0.05-Jy fixed size uncertainty and 8per cent fractional uncertainty.This is not enough to cause the large Malmquist-type errors in the rateof evolution postulated by Fisher et al. We do, however, find marginalevidence for non-linearity in the PSC 60-μm flux scale, in the sensethat faint sources may have fluxes overestimated by about 5per centcompared with bright sources. We update some of the previous scientificanalyses to assess the changes. The main new results are as follows. (1)The luminosity function is very well determined overall but is uncertainby a factor of several at the very highest luminosities(L_60μm>5x10^12L_solar), as this is where the remainingunidentified objects are almost certainly concentrated. (2) Thebest-fitting rate of evolution is somewhat lower than our previousestimate; expressed as pure density evolution with density varying as(1+z)^p, we find p=5.6+/-2.3. Making a rough correction for the possible(but very uncertain) non-linearity of fluxes, we find p=4.5+/-2.3. (3)The dipole amplitude decreases a little, and the implied value of thedensity parameter, assuming that IRAS galaxies trace the mass, isΩ=0.9(+0.45, -0.25). (4) Finally, the estimate of density varianceon large scales changes negligibly, still indicating a significantdiscrepancy from the predictions of simple cold dark matter cosmogonies.
| 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.
| Search and redshift survey for IRAS galaxies behind the Northern Milky Way
We made a search for IRAS galaxies behind the Northern Milky Way byinfrared selection using IRAS Point Source Catalog and visual inspectionon POSS (Palomar Observatory Sky Survey) paper prints, and carried out aredshift survey of the identified objects. This paper presents a catalogof 649 IRAS galaxies with f_60_>=0.6 Jy between l=150deg and 240degat |b|<=15deg , which contains 254 newly identified galaxies and 188newly measured radial velocities. Due to galactic extinction, our sampleis a lower limit sample of the flux limited sample of IRAS galaxies, butit can give some information on the distribution of galaxies in theregion perpendicular to the Supergalactic Plane. We confirm two regionswith enhanced density at l=~160deg , cz=~5000 km/s and l=~190deg ,cz=~5000 km/s and at least two possible voids.
| Far-infrared emission from the intracluster medium
IRAS images of 56 clusters of galaxies are examined for evidence ofdiffuse, extended FIR emission at 60 and 100 microns. Of these 56clusters, five show evidence of FIR excess, but at low levels ofsignificance. For A496 and MKW 1, this excess is attributable to thepresence of a strong point source at or near the position of the centralcD galaxy. In A2344, the observed 100-micron excess is the result of thepresence of a foreground cirrus concentration which coincides with thecluster. Examination of the sample as a whole indicates that clustersexhibit excess FIR emission at the 2-sigma level for apertures 4 and 10arcmin in diameter. FIR luminosities range from about 1-0 exp 9 solarluminosities for the smallest aperture to about 10 exp 14 solarluminosities for the entire cluster.
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