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Environmental Effects on Late-Type Galaxies in Nearby Clusters
The transformations that take place in late-type galaxies in theenvironment of rich clusters of galaxies at z=0 are reviewed. From thehandful of late-type galaxies that inhabit local clusters, whether theywere formed in situ and survived as such, avoiding transformation oreven destruction, or if they are newcomers that have recently fallen infrom outside, we can learn an important lesson on the latest stages ofgalaxy evolution. We start by reviewing the observational scenario,covering the broadest possible stretch of the electromagnetic spectrum,from the gas tracers (radio and optical) to the star formation tracers(UV and optical), the old star tracers (near-IR), and the dust (far-IR).Strong emphasis is given to the three nearby, well-studied clustersVirgo, A1367, and Coma, which are representative of differentevolutionary stages, from unrelaxed and spiral-rich (Virgo) to relaxedand spiral-poor (Coma). We continue by providing a review of models ofgalaxy interactions that are relevant to clusters of galaxies.Prototypes of various mechanisms and processes are discussed, and theirtypical timescales are given in an appendix. Observations indicate thepresence of healthy late-type galaxies falling into nearby clustersindividually or as part of massive groups. More rare are infallinggalaxies belonging to compact groups, where significant preprocessingmight take place. Once they have entered the cluster, they lose theirgas and quench their star formation activity, becoming anemic.Observations and theory agree in indicating that the interaction withthe intergalactic medium is responsible for the gas depletion. However,this process cannot be the origin of the cluster lenticular galaxypopulation. Physical and statistical properties of S0 galaxies in nearbyclusters and at higher redshift indicate that they originate from spiralgalaxies that have been transformed by gravitational interactions.
| Ram pressure stripping of disc galaxies: the role of the inclination angle
We present three-dimensional (3D) hydrodynamical simulations of rampressure stripping of massive disc galaxies in clusters. Studies ofgalaxies that move face-on have predicted that in such a geometry thegalaxy can lose a substantial amount of its interstellar medium. Butonly a small fraction of galaxies is moving face-on. In this work wefocus on a systematic study of the effect of the inclination anglebetween the direction of motion and the galaxy's rotation axis.In agreement with some previous works, we find that the inclinationangle does not play a major role for the mass loss as long as the galaxyis not moving close to edge-on (inclination angle <~60°). Weexplain this behaviour by extending Gunn & Gott's estimate of thestripping radius, which is valid for face-on geometries, to moderateinclinations.The inclination plays a role as long as the ram pressure is comparableto pressures in the galactic plane, which can span two orders ofmagnitude. For very strong ram pressures, the disc will be strippedcompletely, and for very weak ram pressures, mass loss is negligibleindependent of inclination. We show that in non-edge-on geometries thestripping proceeds remarkably similar. A major difference betweendifferent inclinations is the degree of asymmetry introduced in theremaining gas disc.We demonstrate that the tail of gas stripped from the galaxy does notnecessarily point in a direction opposite to the galaxy's direction ofmotion. Therefore, the observation of a galaxy's gas tail may bemisleading about the galaxy's direction of motion.
| Molecular gas in the galaxy cluster Abell 262. CO observations of UGC 1347 and other galaxies of the cluster
We present millimeter CO line emission observations of 12 galaxieswithin the Abell 262 cluster, together with L_FIRdata, in the context of a possible molecular gas deficiency within theregion of the cluster center. Several indications of the presence ofsuch a deficiency are highlighted and connected to a model ofcirrus-like cloud stripping. The model predicts a drop in the average100 μm flux density of galaxies in the core of the cluster comparedto the average 100 μm flux density in the outer regions, which isactually indicated in the IRAS data of the cluster members. This drop isexplained by the decrease in the total hydrogen column density N(H) and,therefore, also includes a decrease in the molecular gas content. Inaddition to results for the global CO content of the galaxy sample,high-resolution interferometric CO(1-0) observations of one of thecluster members, UGC 1347, exemplify the spatial distribution of themolecular gas in a galaxy of the cluster. With these observations, itwas possible to confirm the existence of a bright off-nuclearCO-emission source and to derive molecular masses and line ratios forthis source and the nucleus.
| An Hα survey of cluster galaxies - V. Cluster-field comparison for early-type galaxies
We have extended our Hα objective prism survey of eightlow-redshift clusters (viz. Abell 262, 347, 400, 426, 569, 779, 1367 and1656) to include a complete sample of early-type galaxies within 1.5Abell radii of the cluster centres. Of the 379 galaxies surveyed, 3 percent of E, E-S0 galaxies, 6 per cent of S0 galaxies, and 9 per cent ofS0/a galaxies were detected in emission. From a comparison of clusterand supercluster field galaxies, we conclude that the frequency ofemission-line galaxies (ELGs; Wλ>= 20 Å) issimilar for field and cluster early-type galaxies. A similar result haspreviously been obtained for galaxies of types Sa and later. Together,these results confirm the inference of Biviano et al. that the relativefrequency of ELGs in clusters and the field can be entirely accountedfor by the different mix of morphological types between the differingenvironments, and that, for galaxies of a given morphological type, thefraction of ELGs is independent of environment. Detected emission isclassified as `compact' or `diffuse', identified as circumnuclearstarburst or active galactic nucleus (AGN) emission and disc emission,respectively. By comparing spectroscopic data for cluster early-typeELGs with data for field galaxies from the Palomar spectroscopic surveyof nearby galactic nuclei, we demonstrate that there is modest evidencefor an enhancement of compact HII emission relative to AGN emission inthe early-type cluster ELGs as compared to the field. For the clusterearly-type galaxies, compact HII emission correlates strongly with adisturbed morphology. This suggests that, as for later-type clustergalaxies, this enhanced compact HII emission can readily be explained asan enhancement of circumnuclear starburst emission due to gravitationaltidal interactions, most likely caused by subcluster merging and otheron-going processes of cluster virialization.
| Study of the Structure of the Coma Cluster Based on a Hierarchical Force Clustering Method
Six subclusters in the Coma cluster have been selected on the basis of ahierarchical clustering method that takes the gravitational interactionamong galaxies into account. Of these, 3 central subclusters around thegalaxies NGC 4889, NGC 4874, and NGC 4839 have been singled out. We haveused the objective statistical criterion applied by Vennik and Anosovain studies of close groups of galaxies to evaluate each member includedin a subcluster with a high probability. Galaxies with a significantdeficit of hydrogen HI, including objects from the Bravo-Alfaro list,have been identified with members of the subclusters, with the greatestnumber of them in the subclusters around NGC 4874 and NGC 4839. Aquantitative estimate of the hydrogen deficit using the HI index in theRCG3 catalog reveals a statistically significant excess value for thosegalaxies that are members of the subclusters compared to galaxies with ahydrogen deficit in the overall Coma cluster field. A substantial numberof the spiral galaxies with a hydrogen deficit in the subclusters turnedout to be radio galaxies as well.
| Ram pressure stripping of disk galaxies. From high to low density environments
Galaxies in clusters and groups moving through the intracluster orintragroup medium (abbreviated ICM for both) are expected to lose atleast a part of their interstellar medium (ISM) by the ram pressure theyexperience. We perform high resolution 2D hydrodynamical simulations offace-on ram pressure stripping (RPS) of disk galaxies to compile acomprehensive parameter study varying galaxy properties (mass, verticalstructure of the gas disk) and covering a large range of ICM conditions,reaching from high density environments like in cluster centres to lowdensity environments typical for cluster outskirts or groups. We findthat the ICM-ISM interaction proceeds in three phases: firstly theinstantaneous stripping phase, secondly the dynamic intermediate phase,thirdly the quasi-stable continuous viscous stripping phase. In thefirst phase (time scale 20 to 200~Myr) the outer part of the gas disk isdisplaced but only partially unbound. In the second phase (10 times aslong as the first phase) a part of the displaced gas falls back (about10% of the initial gas mass) despite the constant ICM wind, but mostdisplaced gas is now unbound. In the third phase the galaxy continues tolose gas at a rate of about 1~Mȯ~~yr-1 byturbulent viscous stripping. We find that the stripping efficiencydepends slightly on the Mach number of the flow, however, the mainparameter is the ram pressure. The stripping efficiency does not dependon the vertical structure and thickness of the gas disk. We discussuncertainties in the classic estimate of the stripping radius of Grunn& Gott (1972, ApJ, 176, 1), which compares the ram pressure to thegravitational restoring force. In addition, we adapt the estimate usedby Mori & Burkert (2000, ApJ, 538, 559) for spherical galaxies,namely the comparison of the central pressure with ram pressure. We findthat the latter estimate predicts the radius and mass of the gas diskremaining at the end of the second phase very well, and better than theGrunn & Gott (1972, ApJ, 176, 1) criterion. From our simulations weconclude that gas disks of galaxies in high density environments areheavily truncated or even completely stripped, but also the gas disks ofgalaxies in low density environments are disturbed by the flow andback-falling material, so that they should also be pre-processed.
| The Compact Group of Galaxies HCG 31 in an Early Phase of Merging
We have obtained high spectral resolution (R=45,900) Fabry-Perotvelocity maps of the Hickson compact group HCG 31 in order to revisitthe important problem of the merger nature of the central object A+C andto derive the internal kinematics of the candidate tidal dwarf galaxiesin this group. Our main findings are as follows: (1) double kinematiccomponents are present throughout the main body of A+C, which stronglysuggests that this complex is an ongoing merger; (2) regions A2 and E,to the east and south of complex A+C, present rotation patterns withvelocity amplitudes of ~25 km s-1, and they counterrotatewith respect to A+C; and (3) region F, which was previously thought tobe the best example of a tidal dwarf galaxy in HCG 31, presents norotation and negligible internal velocity dispersion, as is also thecase for region A1. HCG 31 presents an undergoing merger in its center(A+C), and it is likely that it has suffered additional perturbationsdue to interactions with the nearby galaxies B, G, and Q.Based on observations collected at the European Southern Observatory andGemini North Telescope (project GN-2003-Q-12).
| Radio and Far-Infrared Emission as Tracers of Star Formation and Active Galactic Nuclei in Nearby Cluster Galaxies
We have studied the radio and far-infrared (FIR) emission from 114galaxies in the seven nearest clusters (<100 Mpc) with prominentX-ray emission to investigate the impact of the cluster environment onthe star formation and active galactic nucleus (AGN) activity in themember galaxies. The X-ray selection criterion is adopted to focus onthe most massive and dynamically relaxed clusters. A large majority ofcluster galaxies show an excess in radio emission over that predictedfrom the radio-FIR correlation, the fraction of sources with radioexcess increases toward cluster cores, and the radial gradient in theFIR/radio flux ratio is a result of radio enhancement. Of theradio-excess sources, 70% are early-type galaxies, and the same fractionhost an AGN. The galaxy density drops by a factor of 10 from thecomposite cluster center out to 1.5 Mpc, yet galaxies show no change inFIR properties over this region and show no indication of masssegregation. We have examined in detail the physical mechanisms thatmight impact the FIR and radio emission of cluster galaxies. Whilecollisional heating of dust may be important for galaxies in clustercenters, it appears to have a negligible effect on the observed FIRemission for our sample galaxies. The correlations between radio and FIRluminosity and radius could be explained by magnetic compression fromthermal intracluster medium pressure. We also find that simple delayedharassment cannot fully account for the observed radio, FIR, and mid-IRproperties of cluster galaxies.
| M/L, Hα Rotation Curves, and H I Gas Measurements for 329 Nearby Cluster and Field Spirals. II. Evidence for Galaxy Infall
We have conducted a study of optical and H I properties of spiralgalaxies (size, luminosity, Hα flux distribution, circularvelocity, and H I gas mass) to explore the role of gas stripping as adriver of morphological evolution in clusters. We find a strongcorrelation between the spiral and S0 fractions within clusters, and thespiral fraction scales tightly with cluster X-ray gas luminosity. Weexplore young star formation and identify spirals that are (1)asymmetric, with truncated Hα emission and H I gas reservoirs onthe leading edge of the disk, on a first pass through the denseintracluster medium in the cores of rich clusters; (2) strongly H Ideficient and stripped, with star formation confined to the inner 5h-1 kpc and 3 disk scale lengths; or (3) reddened, extremelyH I deficient, and quenched, where star formation has been halted acrossthe entire disk. We propose that these spirals are in successive stagesof morphological transformation, between infalling field spirals andcluster S0's, and that the process that acts to remove the H I gasreservoir suppresses new star formation on a similarly fast timescale.These data suggest that gas stripping plays a significant role inmorphological transformation and rapid truncation of star formationacross the disk.
| M/L, Hα Rotation Curves, and H I Measurements for 329 Nearby Cluster and Field Spirals. I. Data
A survey of 329 nearby galaxies (redshift z<0.045) has been conductedto study the distribution of mass and light within spiral galaxies overa range of environments. The 18 observed clusters and groups span arange of richness, density, and X-ray temperature and are supplementedby a set of 30 isolated field galaxies. Optical spectroscopy taken withthe 200 inch (5 m) Hale Telescope provides separately resolved Hαand [N II] major-axis rotation curves for the complete set of galaxies,which are analyzed to yield velocity widths and profile shapes, extents,and gradients. H I line profiles provide an independent velocity widthmeasurement and a measure of H I gas mass and distribution. I-bandimages are used to deconvolve profiles into disk and bulge components,to determine global luminosities and ellipticities, and to checkmorphological classification. These data are combined to form a unifieddata set ideal for the study of the effects of environment upon galaxyevolution.
| The ISOPHOT 170 μm Serendipity Survey II. The catalog of optically identified galaxies%
The ISOPHOT Serendipity Sky Survey strip-scanning measurements covering≈15% of the far-infrared (FIR) sky at 170 μm were searched forcompact sources associated with optically identified galaxies. CompactSerendipity Survey sources with a high signal-to-noise ratio in at leasttwo ISOPHOT C200 detector pixels were selected that have a positionalassociation with a galaxy identification in the NED and/or Simbaddatabases and a galaxy counterpart visible on the Digitized Sky Surveyplates. A catalog with 170 μm fluxes for more than 1900 galaxies hasbeen established, 200 of which were measured several times. The faintest170 μm fluxes reach values just below 0.5 Jy, while the brightest,already somewhat extended galaxies have fluxes up to ≈600 Jy. For thevast majority of listed galaxies, the 170 μm fluxes were measured forthe first time. While most of the galaxies are spirals, about 70 of thesources are classified as ellipticals or lenticulars. This is the onlycurrently available large-scale galaxy catalog containing a sufficientnumber of sources with 170 μm fluxes to allow further statisticalstudies of various FIR properties.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Members of the Consortium on the ISOPHOT Serendipity Survey (CISS) areMPIA Heidelberg, ESA ISO SOC Villafranca, AIP Potsdam, IPAC Pasadena,Imperial College London.Full Table 4 and Table 6 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/39
| UV and FIR selected samples of galaxies in the local Universe. Dust extinction and star formation rates
We have built two samples of galaxies selected at 0.2 μm (hereafterUV) and 60 μm (hereafter FIR) covering a sky area of 35.36deg2. The UV selected sample contains 25 galaxies brighterthan AB0.2=17. All of them, but one elliptical, are detectedat 60 μm with a flux density larger or equal to 0.2 Jy. The UV countsare significantly lower than the Euclidean extrapolation towardsbrighter fluxes of previous determinations. The FIR selected samplecontains 42 galaxies brighter than f60=0.6 Jy. Except fourgalaxies, all of them have a UV counterpart at the limiting magnitudeAB0.2=20.3 mag. The mean extinction derived from the analysisof the FIR to UV flux ratio is 1 mag for the UV selected sample and2 mag for the FIR selected one. For each sample we compare severalindicators of the recent star formation rate (SFR) based on the FIRand/or the UV emissions. We find linear relationships with slopes closeto unity between the different SFR indicator, which means that, over thewhole converting offset. Various absolute calibrations for both samplesare discussed in this paper. A positive correlation between extinctionand SFR is found when both samples are considered together although witha considerable scatter. A similar result is obtained when using the SFRnormalized to the optical surface of the galaxies.Tables 3, 4 and Fig. 1 are only available in electronic form athttp://www.edpsciences.org
| The X-ray luminosity function of galaxies in the Coma cluster
The XMM-Newton survey of the Coma cluster of galaxies covers an area of1.86 square degrees with a mosaic of 16 pointings and has a total usefulintegration time of 400 ks. Detected X-ray sources with extent less than10'' were correlated with cataloged galaxies in the Comacluster region. The redshift information, which is abundant in thisregion of the sky, allowed us to separate cluster members frombackground and foreground galaxies. For the background sources, werecover a typical Log N-Log S in the flux range10-15-10-13 ergs s-1 cm-2 inthe 0.5-2.0 keV band. The X-ray emission from the cluster galaxiesexhibits X-ray colors typical of thermal emission. The luminosities ofComa galaxies lie in the 1039-1041 ergss-1 interval in the 0.5-2.0 keV band. The luminosity functionof Coma galaxies reveals that their X-ray activity is suppressed withrespect to the field by a factor of 5.6, indicating a lower level ofX-ray emission for a given stellar mass.
| Spectrophotometry of galaxies in the Virgo cluster. II. The data
Drift-scan mode (3600-6800 Å) spectra with 500
| Radio-selected Galaxies in Very Rich Clusters at z <= 0.25. I. Multiwavelength Observations and Data Reduction Techniques
Radio observations were used to detect the ``active'' galaxy populationwithin rich clusters of galaxies in a nonbiased manner that is notplagued by dust extinction or the K-correction. We present wide-fieldradio, optical (imaging and spectroscopy), and ROSAT All-Sky Survey(RASS) X-ray data for a sample of 30 very rich Abell (R>=2) clusterswith z<=0.25. The VLA radio data samples the ultrafaint radio(L1.4>=2×1022 W Hz-1) galaxypopulation within these extremely rich clusters for galaxies withMR<=-21. This is the largest sample of low-luminosity 20cm radio galaxies within rich Abell clusters collected to date.The radio-selected galaxy sample represents the starburst (starformation rate >=5 Msolar yr-1) and activegalactic nuclei populations contained within each cluster. Archival andnewly acquired redshifts were used to verify cluster membership for most(~95%) of the optical identifications. Thus, we can identify all thestarbursting galaxies within these clusters, regardless of the level ofdust obscuration that would affect these galaxies being identified fromtheir optical signature. Cluster sample selection, observations, anddata reduction techniques for all wavelengths are discussed.
| Star formation rate in galaxies from UV, IR, and Hα estimators
Infrared (IR) luminosity of galaxies originating from dust thermalemission can be used as an indicator of the star formation rate (SFR).Inoue et al. (\cite{inoue00}, IHK) have derived a formula for theconversion from dust IR luminosity to SFR by using the following threequantities: the fraction of Lyman continuum luminosity absorbed by gas(f), the fraction of UV luminosity absorbed by dust (epsilon ), and thefraction of dust heating from old (ga 108 yr) stellarpopulations (eta ). We develop a method to estimate those threequantities based on the idea that the various way of SFR estimates fromultraviolet (UV) luminosity (2000 Å luminosity), Hαluminosity, and dust IR luminosity should return the same SFR. Afterapplying our method to samples of galaxies, the following results areobtained in our framework. First, our method is applied to a sample ofstar-forming galaxies, finding that f ~ 0.6, epsilon ~ 0.5, and eta ~0.4 as representative values. Next, we apply the method to a starburstsample, which shows larger extinction than the star-forming galaxysample. With the aid of f, epsilon , and eta , we are able to estimatereliable SFRs from UV and/or IR luminosities. Moreover, the Hαluminosity, if the Hα extinction is corrected by using the Balmerdecrement, is suitable for a statistical analysis of SFR, because thesame {correction factor for the Lyman continuum extinction (i.e. 1/f)}is applicable to both normal and starburst galaxies over all the rangeof SFR. The metallicity dependence of f and epsilon is also tested:Only the latter proves to have a correlation with metallicity. As anextension of our result, the local (z=0) comoving density of SFR can beestimated with our dust extinction corrections. We show that all UV,Hα , and IR comoving luminosity densities at z=0 give a consistentSFR per comoving volume ( ~ 3x 10-2h M_sun yr-1Mpc-3). Useful formulae for SFR estimate are listed.Tables 1 and 2, and Appendix A are only available in electronic form athttp://www.edpsciences.org
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| A New Empirical Method for Estimating the Far-Infrared Flux of Galaxies
We propose a new empirical method to estimate the total far-infraredflux of galaxies from the spectral energy distribution (SED) atwavelengths of λ <= 100 μm. It is difficult to derive thetotal far-infrared luminosity from only the IRAS data, though it is oneof the most important properties of galaxies. Observations by InfraredTelescope in Space (IRTS) indicate that the SED of the diffuse emissionfrom the Galactic plane in this wavelength region can be derived fromthe 60 μm to 100 μm color. This empirical SED relation wasimproved in order to obtain a better fit to the Galactic plane data forIν(60 μm) / Iν(100 μm) > 0.6, andapplied to 96 IRAS galaxies for which ISOPHOT and KAO data are availableat λ > 100 μm. As a result, the empirical relation welldescribes the far-infrared (FIR) SED for a majority of galaxies.Additionally, the total FIR flux for λ >= 40 μm was derivedfrom the flux densities at 60 and 100 μm by using this model. For the96 IRAS galaxies, the uncertainty in the total far-infrared flux of thepresent method is 26%. The present method is more accurate than theprevious one widely used to derive the total infrared flux from the IRAS60 and 100 μm data.
| Stellar populations in early-type Coma cluster galaxies - I. The data
We present a homogeneous and high signal-to-noise ratio data set (meanS/N ratio of ~60 Å-1) of Lick/IDS stellar populationline indices and central velocity dispersions for a sample of 132 bright(bj<= 18.0) galaxies within the central 1°(≡1.26h-1 Mpc) of the nearby rich Coma cluster (A1656). Ourobservations include 73 per cent (100 out of 137) of the totalearly-type galaxy population (bj<= 18.0). Observationswere made with the William Herschel 4.2-m telescope and theAUTOFIB2/WYFFOS multi-object spectroscopy instrument (resolution of~2.2-Å FWHM) using 2.7-arcsec diameter fibres (≡0.94h-1 kpc). The data in this paper have well-characterizederrors, calculated in a rigorous and statistical way. Data are comparedwith previous studies and are demonstrated to be of high quality andwell calibrated on to the Lick/IDS system. Our data have median errorsof ~0.1 Å for atomic line indices, ~0.008 mag for molecular lineindices and 0.015 dex for velocity dispersions. This work provides awell-defined, high-quality baseline at z~ 0 for studies of medium- tohigh-redshift clusters. Subsequent papers will use this data set toprobe the stellar populations (which act as fossil records of galaxyformation and evolution) and the spectrophotometric relations of thebright early-type galaxies within the core of the Coma cluster.
| The effects of ram pressure stripping on cluster galaxies
We investigate the effects of ram pressure stripping of spiral galaxiesusing a numerical model that is directly confronted with interferometricobservations. The current status of our research is presented.
| Evolution of Star-forming and Active Galaxies in Nearby Clusters
We have used optical spectroscopy to investigate the active galaxypopulations in a sample of 20 nearby Abell clusters. The targets wereidentified on the basis of 1.4 GHz radio emission, which identifies themas either active galactic nuclei (AGNs) or galaxies forming stars atrates comparable to or greater than that of the Milky Way. The spectrawere used to characterize the galaxies via their emission and absorptionfeatures. The spectroscopy results reveal a significant population ofstar-forming galaxies with large amounts of nuclear dust extinction.This extinction eliminates bluer emission lines such as [O II] from thespectra of these galaxies, meaning their star formation could easily beoverlooked in studies that focus on such features. Around 20% of thecluster star-forming galaxies have spectra of this type. The radialdistributions of active galaxies in clusters show a strong segregationbetween star-forming galaxies and AGNs, with star-forming galaxiesbroadly distributed and AGNs preferentially in the cluster cores. Theradial distribution of the dusty star-forming galaxies is more centrallyconcentrated than the star-forming galaxies in general, which arguesthat they are a consequence of some cluster environmental effect.Furthermore, we note that such galaxies may be identified using their4000 Å break strengths. We find that discrepancies in reportedradio luminosity functions for AGNs are likely the result ofclassification differences. There exists a large population of clustergalaxies whose radio fluxes, far-infrared fluxes, and optical magnitudessuggest their radio emission may be powered by stars yet whose spectralack emission lines. Understanding the nature of these galaxies iscritical to assessing the importance of AGNs in the radio luminosityfunction at low luminosities. We also find that regardless of thispopulation, the crossover point where the radio luminosity function iscomposed equally of star-forming galaxies and AGNs occurs at lowerluminosities in clusters than in the field. This is likely a simpleconsequence of the reduction in star formation in cluster galaxies andthe morphological mix in clusters compared with the field.Based in part on observations obtained with the Apache Point Observatory(APO) 3.5 m telescope, which is owned and operated by the AstrophysicalResearch Consortium.
| Hα surface photometry of galaxies in the Virgo cluster. IV. The current star formation in nearby clusters of galaxies
Hα +[NII] imaging observations of 369 late-type (spiral) galaxiesin the Virgo cluster and in the Coma/A1367 supercluster are analyzed,covering 3 rich nearby clusters (A1367, Coma and Virgo) and nearlyisolated galaxies in the Great-Wall. They constitute an opticallyselected sample (mp<16.0) observed with ~ 60 %completeness. These observations provide us with the current(T<107 yrs) star formation properties of galaxies that westudy as a function of the clustercentric projected distances (Theta ).The expected decrease of the star formation rate (SFR), as traced by theHα EW, with decreasing Theta is found only when galaxies brighterthan Mp ~ -19.5 are considered. Fainter objects show no orreverse trends. We also include in our analysis Near Infrared data,providing information on the old (T>109 yrs) stars. Puttogether, the young and the old stellar indicators give the ratio ofcurrently formed stars over the stars formed in the past, or``birthrate'' parameter b. For the considered galaxies we also determinethe ``global gas content'' combining HI with CO observations. We definethe ``gas deficiency'' parameter as the logarithmic difference betweenthe gas content of isolated galaxies of a given Hubble type and themeasured gas content. For the isolated objects we find that b decreaseswith increasing NIR luminosity. In other words less massive galaxies arecurrently forming stars at a higher rate than their giant counterpartswhich experienced most of their star formation activity at earliercosmological epochs. The gas-deficient objects, primarily members of theVirgo cluster, have a birthrate significantly lower than the isolatedobjects with normal gas content and of similar NIR luminosity. Thisindicates that the current star formation is regulated by the gaseouscontent of spirals. Whatever mechanism (most plausibly ram-pressurestripping) is responsible for the pattern of gas deficiency observed inspiral galaxies members of rich clusters, it also produces the observedquenching of the current star formation. A significant fraction of gas``healthy'' (i.e. with a gas deficiency parameter less than 0.4) andcurrently star forming galaxies is unexpectedly found projected near thecenter of the Virgo cluster. Their average Tully-Fisher distance isfound approximately one magnitude further away (muo = 31.77)than the distance of their gas-deficient counterparts (muo =30.85), suggesting that the gas healthy objects belong to a cloudprojected onto the cluster center, but in fact lying a few Mpc behindVirgo, thus unaffected by the dense IGM of the cluster. Based onobservations taken at the Observatorio Astronómico Nacional(Mexico), the OHP (France), Calar Alto and NOT (Spain) observatories.Table \ref{tab4} is only available in electronic form athttp://www.edpsciences.org
| A deep Hα survey of galaxies in the two nearby clusters Abell 1367 and Coma. The Hα luminosity functions
We present a deep wide field Hα imaging survey of the centralregions of the two nearby clusters of galaxies Coma and Abell 1367,taken with the WFC at the INT 2.5 m telescope. We determine for thefirst time the Schechter parameters of the Hα luminosity function(LF) of cluster galaxies. The Hα LFs of Abell 1367 and Coma arecompared with each other and with that of Virgo, estimated using the Bband LF by Sandage et al. (\cite{Sandage85}) and a L(Hα ) vs.MB relation. Typical parameters of phi * ~100.00+/-0.07 Mpc-3, L* ~1041.25+/-0.05 erg s-1 and alpha ~ -0.70+/-0.10are found for the three clusters. The best fitting parameters of thecluster LFs differ from those found for field galaxies, showing flatterslopes and lower scaling luminosities L*. Since, however, ourHα survey is significantly deeper than those of field galaxies,this result must be confirmed on similarly deep measurements of fieldgalaxies. By computing the total SFR per unit volume of clustergalaxies, and taking into account the cluster density in the localUniverse, we estimate that the contribution of clusters like Coma andAbell 1367 is approximately 0.25% of the SFR per unit volume of thelocal Universe. Appendix a is only available in electronic form athttp://www.edpsciences.org
| Star formation and dust extinction in nearby star-forming and starburst galaxies
We study the star formation rate and dust extinction properties of asample of nearby star-forming galaxies as derived from Hα and UV (~ 2000 Å) observations and we compare them to those of a sample ofstarburst galaxies. The dust extinction in Hα is estimated fromthe Balmer decrement and the extinction in UV using the FIR to UV fluxratio or the attenuation law for starburst galaxies of Calzetti et al.(\cite{calzetti5}). The Hα and UV emissions are stronglycorrelated with a very low scatter for the star-forming objects and witha much higher scatter for the starburst galaxies. The Hα to UVflux ratio is found to be larger by a factor ~ 2 for the starburstgalaxies. We compare both samples with a purely UV selected sample ofgalaxies and we conclude that the mean Hα and UV properties ofnearby star-forming galaxies are more representative of UV-selectedgalaxies than starburst galaxies. We emphasize that the Hα to UVflux ratio is strongly dependent on the dust extinction: the positivecorrelation found between FHα/FUV andFFIR/FUV vanishes when the Hα and UV fluxare corrected for dust extinction. The Hα to UV flux ratiosconverted into star formation rate and combined with the Balmerdecrement measurements are tentatively used to estimate the dustextinction in UV.
| Multi stage three-dimensional sweeping and annealing of disc galaxies in clusters
We present new three-dimensional, hydrodynamic simulations of the rampressure stripping of disc galaxies via interaction with a hotintracluster medium (ICM). The simulations were carried with thesmoothed-particle hydrodynamics, adaptive mesh `hydra' code(SPH-AP3M), with model galaxies consisting of gas and stellardisc components and dark haloes. The simulations also include radiativecooling, which is important for keeping the warm, diffuse gas ofmoderate density from being unrealistically heated by the ICM. Weexamine the role that wind velocity, density and galaxy tilt play in gasstripping. We include cases with lower ram pressures than other recentstudies. In accord with previous studies, we find that low columndensity gas is promptly removed from the outer disc. However, we alsofind that not all of the gas stripped from the disc escapes immediatelyfrom the halo, some of material can linger for times of order108yr. We use a simple analytic model to demonstrate that gaselements in the ICM wind feel an effective potential with a minimumdisplaced downstream from the halo centre. The onset of the ICM wind hasa profound effect on the disc gas that is not immediately stripped. Thisremnant disc is displaced relative to the halo centre and compressed.This can trigger gravitational instability and the formation of numerousflocculent spirals. These waves transport angular momentum outward,resulting in further compression of the inner disc and the formation ofa prominent gas ring. This `annealing' process makes the inner disc,which contains much of the total gas mass, resistant to furtherstripping, but presumably susceptible to global starbursts. Spirals inthe outer disc stretch, shear and are eventually stripped on time-scalesof a few times 108yr, after which time, mass and angularmomentum loss effectly cease. For inclined galaxies, these effects areconsiderably modified over the same time-scale. The amount of mass lossis reduced. In addition, we find that a higher galaxy tilt couples thewind and the rotating disc, and produces a higher degree of angularmomentum removal. Temperature and line-of-sight velocity maps fromseveral of the simulations are presented for comparison withobservation. When the mass loss and annealing processes go tocompletion, we find that the total amount of mass lost from a fixedtarget galaxy is well-fitted by a simple power-law function of adimensionless parameter that combines the ram pressure and internalproperties of the galaxy. Ramifications for the cluster galaxy evolutionare discussed.
| The Radio Galaxy Populations of Nearby Northern Abell Clusters
We report on the use of the NRAO VLA Sky Survey (NVSS) to identify radiogalaxies in 18 nearby Abell clusters. The listings extend from the coresof the clusters out to radii of 3 h-175 Mpc, whichcorresponds to 1.5 Abell radii and approximately 4 orders of magnitudein galaxy density. To create a truly useful catalog, we have collectedoptical spectra for nearly all of the galaxies lacking public velocitymeasurements. Consequently, we are able to discriminate between thoseradio galaxies seen in projection on the cluster and those that are inactuality cluster members. The resulting catalog consists of 329 clusterradio galaxies plus 138 galaxies deemed foreground or backgroundobjects, and new velocity measurements are reported for 273 of theseradio galaxies. The motivation for the catalog is the study of galaxyevolution in the cluster environment. The radio luminosity function is apowerful tool in the identification of active galaxies, as it isdominated by star-forming galaxies at intermediate luminosities andactive galactic nuclei (AGNs) at higher luminosities. The flux limit ofthe NVSS allows us to identify AGNs and star-forming galaxies down tostar formation rates less than 1 Msolar yr-1. Thissensitivity, coupled with the all-sky nature of the NVSS, allows us toproduce a catalog of considerable depth and breadth. In addition tothese data, we report detected infrared fluxes and upper limits obtainedfrom IRAS data. It is hoped that this database will prove useful in anumber of potential studies of the effect of environment on galaxyevolution. Based in part on observations obtained with the Apache PointObservatory 3.5 m telescope, which is owned and operated by theAstrophysical Research Consortium (ARC).
| Ram Pressure Stripping and Galaxy Orbits: The Case of the Virgo Cluster
We investigate the role of ram pressure stripping in the Virgo Clusterusing N-body simulations. Radial orbits within the Virgo Cluster'sgravitational potential are modeled and analyzed with respect to rampressure stripping. The N-body model consists of 10,000 gas cloudcomplexes that can have inelastic collisions. Ram pressure is modeled asan additional acceleration on the clouds located at the surface of thegas distribution in the direction of the galaxy's motion within thecluster. We made several simulations, changing the orbital parameters inorder to recover different stripping scenarios using realistic temporalram pressure profiles. We investigate systematically the influence ofthe inclination angle between the disk and the orbital plane of thegalaxy on the gasdynamics. We show that ram pressure can lead to atemporary increase of the central gas surface density. In some cases aconsiderable part of the total atomic gas mass (several 108Msolar) can fall back onto the galactic disk after thestripping event. A quantitative relation between the orbit parametersand the resulting H I deficiency is derived containing explicitly theinclination angle between the disk and the orbital plane. The comparisonbetween existing H I observations and the results of our simulationsshows that the H I deficiency depends strongly on galaxy orbits. It isconcluded that the scenario in which ram pressure stripping isresponsible for the observed H I deficiency is consistent with all H I21 cm observations in the Virgo Cluster.
| VLA HI Imaging of the brightest spiral galaxies in Coma. II. The HI Atlas and deep continuum imaging of selected early type galaxies
In the first paper of this series we used HI observations of the 19brightest spirals in Coma to analyze the dynamical state of the cluster.In this paper we present the detailed HI distribution and kinematics ofthe spirals that were detected in HI, and radio continuum data for asample of star forming and post starburst galaxies in Coma. We discussthe importance of ICM-ISM interactions to explain the observed HImorphology. A rough comparison of observed HI sizes with predicted HIsizes from simulations by Abadi et al. (\cite{Abadi99}) gives reasonableagreement. We use the results on radio continuum emission to estimatethe star formation rate in the PSB galaxies we pointed at. The radiocontinuum emission in the 11 so called post starburst galaxies,identified by Caldwell et al. (\cite{Caldwell93}) in the cluster, isweak. Eight of the 11 were not detected down to a 3 sigma upper limit of0.6 mJy. This sets an upper limit to the star formation rate in thesegalaxies of less than 0.2 Msun,yr-1. The threedetected post starburst galaxies have a star formation rate of less thanone solar mass per year. Thus none of the post starburst galaxies inComa are dust enshrouded starbursts. Appendix is only available inelectronic form at http://www.edpsciences.org
| 12CO(1-0) observations of NGC 4848: A Coma galaxy after stripping
We study the molecular gas content and distribution in the Coma clusterspiral galaxy NGC 4848 which is HI deficient and where the remainingatomic gas is on one side of the galaxy, presumably because of thestrong ram-pressure in the rich Coma cluster. Plateau de Bureinterferometric CO(1-0) observations reveal a lopsided H2distribution with an off-center secondary maximum coincident with theinner part of the HI. NGC 4848 is not at all deficient in molecular gasas it contains MH_2 ~ 4 x 109 Msun inthe central and inner disk regions. As predicted by earliercalculations, ram-pressure has little influence on the dense moleculargas in the inner disk, which appears dynamically normal at our 2 kpcresolution. At the interface between the CO and HI emission regions,about 8 kpc NW of the center, however, strong star formation is presentas witnessed by Hα and radio continuum emission. From the radioand Hα fluxes we estimate the star formation rate in the northernemission region to be close to 0.3 Msun yr-1,nearly that of an average quiescent spiral. This is the region in whichearlier Fabry-Pérot observations revealed a double-peakedHα line, indicating gas at two different velocities at the samesky position. In order to understand these observations, we present theresults of numerical simulations of the galaxy-cluster ICM interaction.We suggest that NGC 4848 already passed through the center of thecluster about 4x 108 years ago. At the observed stage rampressure has no more direct dynamical influence on the galaxy's ISM. Weobserve the galaxy when a fraction of the stripped gas is falling backonto the galaxy. Ram pressure is thus a short-lived event withlonger-lasting consequences. The combination of ram-pressure androtation results in gas at different velocities colliding where thedouble-peaked Hα line is observed. Ram-pressure can thus result,after re-accretion, in displaced molecular gas without the H2itself being pushed efficiently by the intracluster medium. Thisprocess, however, requires strong stripping, such that only galaxieswith radial orbits can be affected as much as NGC 4848. A scenario wheretwo interactions take place simultaneously is also consistent with theavailable data but less probable on the basis of our numericalsimulations.
| Mid and Far IR properties of late-type galaxies in the Coma and A1367 clusters: ISOCAM and ISOPHOT observations
We present Mid (MIR) and Far (FIR) Infrared observations of 18spiral/irregular galaxies belonging to the Coma and A1367 clusters,carried out with the CAM and PHOT instruments on board the ISOsatellite. Complementary photometry from the UV to the Near Infrared(NIR) together with Hα imaging, HI and 12CO linemeasurements allow us to study the relationships between the IR emissionand the star formation properties of these galaxies. Most of theresolved galaxies show extended MIR emission throughout their disks evenwhere no Hα emission is present. This suggests that the Aromaticcarriers can be excited by the general interstellar radiation field(ISRF), i.e. by visible photons. Only close to HII regions the UVphotons are the principal sources of Aromatic carrier excitation.However, when the UV radiation field becomes intense enough thesecarriers can be destroyed. The average integrated 15/6.75 mu m ratio ofthe observed galaxies is ~ 1, i.e. the typical value for thephotodissociation regions (PDRs). This suggests that, despite the highstar formation rate (SFR) and the very luminous HII regions of thesegalaxies, their integrated MIR emission is dominated by PDR-like regionsrather than HII-like regions. A cold dust component with averagetemperature ~ 22 K exists in most of the target galaxies, probablyarising from big dust grains (BGs) in thermal equilibrium with the ISRF.The contribution to the BGs heating from the ionizing stars decreaseswith increasing wavelength. A warmer dust component whose emissiondominates the spectrum between 20 and 100 mu m is likely to exist. Thisis probably due to both Very Small Grains (VSGs) and warm BGs emission.The dust to gas ratio of the target galaxies is comparable to that ofthe solar neighborhood. There is a weak trend between the dust totalmass and both the atomic and molecular gas content. The MIR and FIRproperties of the analyzed galaxies do not seem to be affected by theenvironment despite the fact that most of the targets are interactingwith the Intra-Cluster-Medium. Based on observations with ISO, an ESAproject with instruments funded by ESA member states (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA.
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