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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.
| A Virgo high-resolution Hα kinematical survey - II. The Atlas
A catalogue of ionized gas velocity fields for a sample of 30 spiral andirregular galaxies of the Virgo cluster has been obtained by using 3Doptical data. The aim of this survey is to study the influence ofhigh-density environments on the gaseous kinematics of local clustergalaxies. Observations of the Hα line by means of Fabry-Perotinterferometry have been performed at the Canada-France-HawaiiTelescope, European Southern Observatory 3.6-m telescope, Observatoirede Haute-Provence 1.93-m telescope and Observatoire du montMégantic telescope at angular and spectral samplings from 0.4 to1.6arcsec and 7 to 16kms-1. A recently developed, automaticand adaptive spatial binning technique is used to reach a nearlyconstant signal-to-noise ratio (S/N) over the whole field of view,allowing us to keep a high spatial resolution in high-S/N regions andextend the detection of signal in low-S/N regions. This paper is part ofa series and presents the integrated emission-line and velocity maps ofthe galaxies. Both Hα morphologies and kinematics exhibit signs ofperturbations in the form of, for example, external filaments, inner andnuclear spiral- and ring-like structures, inner kinematical twists,kinematical decoupling of a nuclear spiral, streaming motions alongspiral arms and misalignment between kinematical and photometricorientation axes.
| The Stellar Population of Stripped Cluster Spiral NGC 4522: A Local Analog to K+A Galaxies?
We present observations of the stripped Virgo Cluster spiral galaxy NGC4522, a clear, nearby example of a galaxy currently undergoinginterstellar medium-intracluster medium (ISM-ICM) stripping. UtilizingSparsePak integral field spectroscopy on the Wisconsin-Indiana-Yale-NOAO(WIYN) 3.5 m telescope and Galaxy Evolution Explorer (GALEX) UVphotometry, we present an analysis of the outer disk (r>3 kpc)stellar population of this galaxy, beyond the H I and Hαtruncation radius. We find that the star formation in the gas-strippedouter disk ceased very recently, ~100 Myr ago, in agreement withprevious claims that this galaxy is currently being stripped. At thetime of this stripping, data and models suggest that the galaxyexperienced a modest starburst. The stripping is occurring in a regionof the cluster well outside the cluster core, likely because this galaxyis experiencing extreme conditions from a dynamic ICM due to an ongoingsubcluster merger. The outer disk has a spectrum of a K+A galaxy,traditionally observed in high-redshift cluster galaxies. In the case ofNGC 4522, a K+A spectrum is formed by simple stripping of theinterstellar gas by the hot intracluster medium. These data show K+Aspectra can be created by cluster processes and that these processeslikely extend beyond the cluster core.
| The Nature of the Peculiar Virgo Cluster Galaxies NGC 4064 and NGC 4424
Using extensive kinematical and morphological data on two Virgo Clustergalaxies undergoing strong nuclear star formation, we show that rampressure stripping and gravitational interactions can act together ongalaxies that have recently fallen into clusters. We present a detailedstudy of the peculiar H I-deficient Virgo Cluster spiral galaxies NGC4064 and NGC 4424 using 12CO 1-0 interferometry, opticalimaging, and integral field spectroscopic observations in order to learnwhat type of environmental interactions have affected these galaxies.Optical imaging reveals that NGC 4424 has a strongly disturbed stellardisk, with banana-shaped isophotes and shells. NGC 4064, which lies inthe cluster outskirts, possesses a relatively undisturbed outer stellardisk and a central bar. In both galaxies Hα emission is confinedto the central kiloparsec and originates in barlike strings of luminousstar-forming complexes surrounded by fainter filaments. Complexes ofyoung blue stars exist beyond the present location of ongoing starformation, indicating rapidly shrinking star-forming disks. Disturbeddust lanes extend out to a radius of 2-3 kpc, much farther than theHα and CO emission detected by us but similar to the blue stellarcomplexes. CO observations reveal bilobal molecular gas morphologies,with Hα emission peaking inside the CO lobes, implying a timesequence in the star formation process. Gas kinematics reveals strongbarlike noncircular motions in the molecular gas in both galaxies,suggesting that the material is radially infalling. In NGC 4064 thestellar kinematics reveals strong barlike noncircular motions in thecentral 1 kpc and stars supported by rotation with V/σ>1 beyonda radius of 15" (1.2 kpc). On the other hand, NGC 4424 has extremelymodest stellar rotation velocities (Vmax~30 kms-1), and stars are supported by random motions as far out aswe can measure, with V/σ=0.6 at r=18'' (1.4 kpc). Theionized gas kinematics in the core are disturbed and possiblycounterrotating. The observations suggest that the peculiarities of NGC4424 are the result of an intermediate-mass merger plus ram pressurestripping. In the case of NGC 4064, the evidence suggests an alreadystripped ``truncated/normal'' galaxy that recently suffered a minormerger or tidal interaction with another galaxy. Observations of thepresent star formation rate and gas content suggest that these galaxieswill become small-bulge S0s within the next 3 Gyr. We propose thatgalaxies with ``truncated/compact'' Hα morphologies such as theseare the result of the independent effects of ram pressure stripping,which removes gas from the outer disk, and gravitational interactionssuch as mergers, which heat stellar disks, drive gas to the centralkiloparsec, and increase the central mass concentrations. Together theseeffects transform the morphology of these galaxies.
| A Comparison of Hα and Stellar Scale Lengths in Virgo and Field Spirals
The scale lengths of the old stars and ionized gas distributions arecompared for similar samples of Virgo Cluster members and field spiralgalaxies via Hα and broad R-band surface photometry. While theR-band and Hα scale lengths are, on average, comparable for thecombined sample, we find significant differences between the field andcluster samples. While the Hα scale lengths of the field galaxiesare a factor of 1.14+/-0.07 longer, on average, than their R-band scalelengths, the Hα scale lengths of Virgo Cluster members are, onaverage, 20% smaller than their R-band scale lengths. Furthermore, inVirgo, the scale length ratios are correlated with the size of thestar-forming disk: galaxies with smaller overall Hα extents alsoshow steeper radial falloff of star formation activity. At the sametime, we find no strong trends in scale length ratio as a function ofother galaxy properties, including galaxy luminosity, inclination,morphological type, central R-band light concentration, or bar type. Ourresults for Hα emission are similar to other results for dustemission, suggesting that Hα and dust have similar distributions.The environmental dependence of the Hα scale length placesadditional constraints on the evolutionary process(es) that cause gasdepletion and a suppression of the star formation rate in clusters ofgalaxies.
| A dynamical model for the heavily ram pressure stripped Virgo spiral galaxy NGC 4522
Context: .Recent Hi and Hα observations of NGC 4522 have revealedthat this spiral galaxy represents one of the best cases of ongoing rampressure stripping in the Virgo cluster. Aims: .We determined theparameters of the interaction between the interstellar medium of NGC4522 and the intracluster medium of the Virgo cluster. Methods: .Adynamical model including ram pressure stripping is applied to thestrongly Hi deficient Virgo spiral galaxy NGC 4522. A carefully chosenmodel snapshot is compared with existing VLA Hi observations.Results: .The model successfully reproduces the large-scale gasdistribution and the velocity field. However it fails to reproduce thelarge observed Hi linewidths in the extraplanar component, for which wegive possible explanations. In a second step, we solve the inductionequation on the velocity fields of the dynamical model and calculate thelarge scale magnetic field. Assuming a Gaussian distribution ofrelativistic electrons we obtain the distribution of polarized radiocontinuum emission which is also compared with our VLA observations at 6cm. The observed maximum of the polarized radio continuum emission issuccessfully reproduced. Our model suggests that the ram pressuremaximum occurred only 50 Myr ago. Conclusions: .Since NGC4522 is located far away from the cluster center ( 1 Mpc) where theintracluster medium density is too low to cause the observed strippingif the intracluster medium is static and smooth, two scenarios areenvisaged: (i) the galaxy moves very rapidly within the intraclustermedium and is not even bound to the cluster; in this case the galaxy hasjust passed the region of highest intracluster medium density; (ii) theintracluster medium is not static but moving due to the infall of the M49 group of galaxies. In this case the galaxy has just passed the regionof highest intracluster medium velocity. This study shows the strengthof combining high resolution Hi and polarized radio continuum emissionwith detailed numerical modeling of the evolution of the gas and thelarge-scale magnetic field.
| The evolution of substructure - III. The outskirts of clusters
We present an investigation of satellite galaxies in the outskirts ofgalaxy clusters taken from a series of high-resolution N-bodysimulations. We focus on the so-called backsplash population, i.e.satellite galaxies that once were inside the virial radius of the hostbut now reside beyond it. We find that this population is significant innumber and needs to be appreciated when interpreting the various galaxymorphology environmental relationships and decoupling the degeneracybetween nature and nurture. Specifically, we find that approximatelyhalf of the galaxies with current cluster-centric distance in theinterval 1-2 virial radii of the host are backsplash galaxies that oncepenetrated deep into the cluster potential, with 90 per cent of theseentering to within 50 per cent of the virial radius. These galaxies haveundergone significant tidal disruption, losing on average 40 per cent oftheir mass. This results in a mass function for the backsplashpopulation different from those galaxies infalling for the first time.We further show that these two populations are kinematically distinctand should be observable within existent spectroscopic surveys.
| XMM-Newton Observation of an X-Ray Trail between the Spiral Galaxy NGC 6872 and the Central Elliptical Galaxy NGC 6876 in the Pavo Group
We present XMM-Newton observations of a trail of enhanced X-ray emissionextending along the full 8.7 arcmin ×4' region betweenthe large spiral galaxy NGC 6872 and the dominant elliptical galaxy NGC6876 in the Pavo group, the first known X-ray trail associated with aspiral galaxy in a poor galaxy group and, with a projected length of 90kpc, one of the longest known X-ray trails. The X-ray surface brightnessin the trail region is roughly constant beyond ~20 kpc of NGC 6876 inthe direction of the spiral galaxy. The trail is hotter (~1 keV) thanthe undisturbed Pavo IGM (~0.5 keV) and has low metal abundances (0.2Zsolar). The 0.5-2 keV luminosity of the trail, measuredusing a 67×90 kpc rectangular region, is 6.6×1040ergs s-1. We compare the properties of gas in the trail tothe spectral properties of gas in the spiral galaxy NGC 6872 and in theelliptical galaxy NGC 6876 to constrain its origin. We suggest that theX-ray trail is either IGM gas gravitationally focused into a Bondi-Hoylewake, a thermal mixture of ~60% Pavo IGM gas with ~40% galaxy gas thathas been removed from the spiral galaxy NGC 6872 by turbulent viscousstripping, or both, due to the spiral galaxy's supersonic motion atangle ξ~40deg with respect to the plane of the sky,through the densest region of the Pavo IGM. Assumingξ=40deg and a filling factor η in a cylindrical volumewith radius 33 kpc and projected length 90 kpc, the mean electrondensity and total hot gas mass in the trail are1×10-3η-1/2 cm-3 and1.1×1010η1/2 Msolar,respectively.
| Dense Cloud Ablation and Ram Pressure Stripping of the Virgo Spiral NGC 4402
We present optical, H I, and radio continuum observations of the highlyinclined Virgo Cluster Sc galaxy NGC 4402, which show evidence for rampressure stripping and dense cloud ablation. Very Large Array H I andradio continuum maps show a truncated gas disk and emission to thenorthwest of the main disk emission. In particular, the radio continuumemission is asymmetrically extended to the north and skewed to the west.The Hα image shows numerous H II complexes along the southern edgeof the gas disk, possibly indicating star formation triggered by theintracluster medium (ICM) pressure. Our BVR images at 0.5" resolutionobtained with the WIYN Tip-Tilt Imager show a remarkable dust lanemorphology: at half the optical radius, the dust lane of the galaxycurves up and out of the disk, matching the H I morphology. Large dustplumes extend upward for ~1.5 kpc from luminous young star clusters atthe southeast edge of the truncated gas disk. These star clusters arevery blue, indicating very little dust reddening, which suggests dustblown away by an ICM wind at the leading edge of the interaction. To thesouth of the main ridge of interstellar material, where the galaxy isrelatively clean of gas and dust, we have discovered 1 kpc long lineardust filaments with a position angle that matches the extraplanar radiocontinuum tail; we interpret this angle as the projected ICM winddirection. One of the observed dust filaments has an H II region at itshead. We interpret these dust filaments as large, dense clouds that wereinitially left behind as the low-density interstellar medium wasstripped but were then ablated by the ICM wind. These results providestriking new evidence on the fate of molecular clouds in strippedcluster galaxies.
| A Virgo high-resolution Hα kinematical survey. I. NGC 4438
New Hα emission-line observations of the Virgo cluster galaxy NGC4438 are presented. Fabry-Perot interferometry data at an effectiveangular resolution of ~2 arcsec are used to map the kinematics of theionized gas in the galaxy. For the first time we obtain a velocity fieldcovering a large area in NGC 4438, much larger than that deduced fromprevious Hi and CO maps. The kinematics of the extended, low surfacebrightness Hα filaments to the West of the galactic disk isdiscussed. We report on the discovery of a northern Hα structurewhich is clumpier than the other filaments. Evidence for multiplespectral components through the data-cube are presented in a nuclearshell and in the approaching half of the disk. The role of VCC 1040, adwarf elliptical galaxy located to the South of NGC 4438, is presentedto investigate the origin of a small-scale stellar tail of NGC 4438. Itcould be due to a minor tidal interaction between the two galaxies.
| 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.
| Completing H I observations of galaxies in the Virgo cluster
High sensitivity (rms noise 0.5 mJy) 21-cm H I line observationswere made of 33 galaxies in the Virgo cluster, using the refurbishedArecibo telescope, which resulted in the detection of 12 objects. Thesedata, combined with the measurements available from the literature,provide the first set of H I data that is complete for all 355 late-type(Sa-Im-BCD) galaxies in the Virgo cluster with mp ≤ 18.0mag. The Virgo cluster H I mass function (HIMF) that was derived forthis optically selected galaxy sample is in agreement with the HIMFderived for the Virgo cluster from the blind HIJASS H I survey and isinconsistent with the Field HIMF. This indicates that both in this richcluster and in the general field, neutral hydrogen is primarilyassociated with late-type galaxies, with marginal contributions fromearly-type galaxies and isolated H I clouds. The inconsistency betweenthe cluster and the field HIMF derives primarily from the difference inthe optical luminosity function of late-type galaxies in the twoenvironments, combined with the HI deficiency that is known to occur ingalaxies in rich clusters.Tables \ref{t1, \ref{sample_dat} and Appendix A are only available inelectronic form at http://www.edpsciences.org
| From gas to galaxies
The unsurpassed sensitivity and resolution of the Square Kilometer Array(SKA) will make it possible for the first time to probe the continuumemission of normal star forming galaxies out to the edges of theuniverse. This opens the possibility for routinely using the radiocontinuum emission from galaxies for cosmological research as it offersan independent probe of the evolution of the star formation density inthe universe. In addition it offers the possibility to detect the firststar forming objects and massive black holes. In deep surveys SKA willbe able to detect HI in emission out to redshifts of z ≈ 2.5 andhence be able to trace the conversion of gas into stars over an erawhere considerable evolution is taking place. Such surveys will be ableto uniquely determine the respective importance of merging and accretinggas flows for galaxy formation over this redshift range (i.e. out towhen the universe was only one third its present age). It is obviousthat only SKA will able to see literally where and how gas is turnedinto stars. These and other aspects of SKA imaging of galaxies will bediscussed.
| Spiral Galaxy - ICM Interactions in the Virgo Cluster
We discuss HI and optical evidence for ongoing ICM-ISM interactions in 6HI-deficient Virgo cluster spiral galaxies. One of the clearest cases isthe highly inclined Virgo galaxy NGC 4522, which has a normal stellardisk but a truncated gas disk, and lots of extraplanar gas right next tothe gas truncation radius in the disk. Unusually strong HI, Hα andradio continuum emission are all detected from the extraplanar gas. Theradio continuum polarized flux and spectral index peak on the sideopposite the extraplanar gas, suggesting ongoing pressure by the ICM.Four other HI-deficient edge-on Virgo spirals show evidence ofextraplanar ISM gas or exhibit asymmetries in their disk HIdistributions, but contain much less extraplanar HI than NGC 4522.Comparison with recent simulations suggests this difference may beevolutionary, with large surface densities of extraplanar gas observedonly in early phases of an ICM-ISM interaction. In NGC 4569, theHα image shows 2 effects of ICM pressure on the galaxy ISM. Ananomalous arm of HII regions, possibly extraplanar, emerges from theedge of a truncated Hα disk. This resembles the arms seen insimulations which are formed by the combined effects of wind pressureplus rotation. An extended nebulosity near the minor axis, also in theNW, is interpreted as a starburst outflow bubble disturbed by ICM windpressure.
| Hα Morphologies and Environmental Effects in Virgo Cluster Spiral Galaxies
We describe the various Hα morphologies of Virgo Cluster andisolated spiral galaxies and associate the Hα morphologies withthe types of environmental interactions that have altered the clustergalaxies. The spatial distributions of Hα and R-band emission areused to divide the star formation morphologies of the 52 Virgo Clusterspiral galaxies into several categories: normal (37%), anemic (6%),enhanced (6%), and (spatially) truncated (52%). Truncated galaxies arefurther subdivided on the basis of their inner star formation rates intotruncated/normal (37%), truncated/compact (6%), truncated/anemic (8%),and truncated/enhanced (2%). The fraction of anemic galaxies isrelatively small (6%-13%) in both environments, suggesting thatstarvation is not a major factor in the reduced star formation rates ofVirgo spiral galaxies. The majority of Virgo spiral galaxies have theirHα disks truncated (52%), whereas truncated Hα disks arerarer in isolated galaxies (12%). Most of the Hα-truncatedgalaxies have relatively undisturbed stellar disks and normal toslightly enhanced inner disk star formation rates, suggesting thatintracluster medium-interstellar medium (ICM-ISM) stripping is the mainmechanism causing the reduced star formation rates of Virgo spiralgalaxies. Several of the truncated galaxies are peculiar, with enhancedcentral star formation rates, disturbed stellar disks, and barlikedistributions of luminous H II complexes inside the central 1 kpc but nostar formation beyond, suggesting that recent tidal interactions orminor mergers have also influenced their morphology. Two highly inclinedHα-truncated spiral galaxies have numerous extraplanar H IIregions and are likely in an active phase of ICM-ISM stripping. Severalspiral galaxies have one-sided Hα enhancements at the outer edgeof their truncated Hα disks, suggesting modest local enhancementsin their star formation rates due to ICM-ISM interactions. Low-velocitytidal interactions and perhaps outer cluster H I accretion seem to bethe triggers for enhanced global star formation in four Virgo galaxies.These results indicate that most Virgo spiral galaxies experienceICM-ISM stripping, many experience significant tidal effects, and manyexperience both.
| Stellar Populations of Dwarf Elliptical Galaxies: UBVRI Photometry of Dwarf Elliptical Galaxies in the Virgo Cluster
We present UBVRI surface photometry for 16 dwarf elliptical galaxies inthe Virgo Cluster with previously measured kinematic properties. Theglobal optical colors are red, with median values for the sample of0.24+/-0.03 in U-B, 0.77+/-0.02 in B-V, and 1.02+/-0.03 in V-I. Werecover the well-known color-magnitude relation for cluster galaxies butfind no significant difference in dominant stellar population betweenrotating and nonrotating dwarf elliptical galaxies; the average age ofthe dominant stellar population is 5-7 Gyr in all 16 galaxies in thissample. Analysis of optical spectra confirm these age estimates andindicate Fe and Mg abundances in the range of 1/20 to one-third ofsolar, as expected for low-luminosity galaxies. Based on Lick indicesand simple stellar population models, the derived [α/Fe] ratiosare subsolar to solar, indicating a more gradual chemical enrichmenthistory for dE's as compared with giant elliptical galaxies in the VirgoCluster. These observations confirm the marked difference in stellarpopulation and stellar distribution between dwarf and giant ellipticalgalaxies and further substantiate the need for alternative evolutionaryscenarios for the lowest mass cluster galaxies. We argue that it islikely that several different physical mechanisms played a significantrole in the production of the Virgo Cluster dE galaxies including insitu formation, infall of dE's that were once part of Local Groupanalogs, and transformation of dwarf irregular galaxies by the clusterenvironment. The observations support the hypothesis that a largefraction of the Virgo Cluster dE's are formed by ram pressure strippingof gas from infalling dI's.Based on observations with the VATT: the Alice P. Lennon Telescope andthe Thomas J. Bannan Astrophysics Facility.
| Radio Continuum Observations of the Virgo Cluster Spiral NGC 4522: The Signature of Ram Pressure
Radio continuum observations at 20 and 6 cm of the highly inclined Virgospiral galaxy NGC 4522 are presented. Both 20 and 6 cm total emissiondistributions are asymmetric with an extended component to the west,where extraplanar atomic gas and Hα emission are found. The 6 cmpolarized emission is located at the eastern edge of the galactic disk.Its peak is located about 1 kpc to the east of the total emission peak.We argue that this phenomena is a characteristic feature for clustergalaxies that are experiencing significant pressure from theintracluster medium. Polarized radio continuum emission is thus apowerful tool to detect interactions of spiral galaxies with the clusterICM. The degree of polarization decreases from the east to the west. Theflattest spectral index between 20 and 6 cm coincides with the peak ofthe 6 cm polarized emission. These findings are consistent with apicture of a large-scale shock due to ram pressure located at the eastof the galaxy where cosmic rays are accelerated. We conclude that it islikely that the galaxy experiences active ram pressure.
| VLA H I Observations of Gas Stripping in the Virgo Cluster Spiral NGC 4522
We present VLA H I observations at ~20"~=1.5 kpc resolution of thehighly inclined, H I-deficient Virgo Cluster spiral galaxy NGC 4522,which is one of the clearest and nearest cases of ongoing intraclustermedium-interstellar medium (ICM-ISM) stripping. H I is abundant andspatially coincident with the stellar disk in the center, but beyond R=3kpc the H I distribution in the disk is sharply truncated, and the onlyH I is extraplanar and all on the northwest side. Forty percent of thetotal H I, corresponding to 1.5×108 Msolar,is extraplanar and has likely been removed from the galaxy disk by anICM-ISM interaction. The kinematics and the morphology of the H I appearmore consistent with ongoing stripping and less consistent with gasfall-back, which may occur long after peak pressure. Some of theextraplanar gas has line widths (FWZI) of 150 km s-1,including a blueshifted tail of weaker emission, and much of theextraplanar gas exhibits a modest net blueshift with respect to thegalaxy's disk rotational velocities, consistent with gas acceleratedtoward the mean cluster velocity. The southwest side of the galaxy hasless H I in the disk but more H I in the halo, suggesting more effectivegas removal on the side of the galaxy that is rotating into the ICMwind. In recent simulations of ICM-ISM interactions large surfacedensities of extraplanar gas like that in NGC 4522 are seen atrelatively early stages of active stripping and not during later gasfall-back stages. The galaxy is 3.3d~=800 kpc from M87, somewhat outsidethe region of strongest cluster X-ray emission. The ram pressure at thislocation, assuming a static smooth ICM and standard values for ICMdensity and galaxy velocity, appears inadequate to cause the observedstripping. We consider the possibility that the ram pressure issignificantly stronger than standard values, because of large bulkmotions and local density enhancements of the ICM gas, which may occurin a dynamic, shock-filled ICM experiencing subcluster merging. The H Iand Hα distributions are similar, with both truncated in the diskat the same radius and H II regions located throughout much of theextraplanar H I. This implies that the star-forming molecular ISM hasbeen effectively stripped from the outer disk of the galaxy along withthe H I. The inferred peak stripping rate of ~10 Msolaryr-1 is much larger than the galaxy's total star formationrate of ~0.1 Msolar yr-1, implying that the rateof triggered star formation due to ICM pressure is presently minorcompared with the rate of gas lost as a result of stripping.
| H I Observations of Barred Magellanic Spirals. II. The Frequency and Impact of Companions
The results of an H I 21 cm line survey of a sample of Magellanic spiralgalaxies with apparent optical companions reveal that only four of 13systems have confirmed H I-detected neighbors. The current interactionsare affecting the morphology of the main galaxy in only two cases, NGC3664 and NGC 3995. The presence of companions near NGC 2537 and UGC 5391appears to have no effect on the morphology of those galaxies. Overall,there is little difference between the asymmetry of the H I profiles ofthose galaxies with and without companions, and on average, theseMagellanic spirals have H I profiles that are no more asymmetric than arandom sample of spirals in the field. We conclude that currentinteractions cannot be responsible for the lopsided morphology of mostof the galaxies in this sample and that, whatever its original cause,lopsidedness must be a long-lived characteristic of these galaxies.
| Tracing the star formation history of cluster galaxies using the Hα/UV flux ratio
Since the Hα and UV fluxes from galaxies are sensitive to stellarpopulations of ages <107 and ≈ 108 yrrespectively, their ratio f(Hα)/f(UV) provides us with a tool tostudy the recent t ≤ 108 yr star formation history ofgalaxies, an exercise that we present here applied to 98 galaxies in 4nearby clusters (Virgo, Coma, Abell 1367 and Cancer). The observedf(Hα)/f(UV) ratio is a factor of two smaller than theexpected one as determined from population synthesis models assuming arealistic delayed, exponentially declining star formation history. Wediscuss various mechanisms that may have affected the observedf(Hα)/f(UV) ratio and we propose that the above discrepancy arisesfrom either the absorption of Lyman continuum photons by dust within thestar formation regions or from the occurrence of star formationepisodes. After splitting our sample into different subsamples accordingto evolutionary criteria we find that our reference sample of galaxiesunaffected by the cluster environment show an average value off(Hα)/f(UV) two times lower than the expected one. We argue thatthis difference must be mostly due to absorption of ≈45% of the Lymancontinuum photons within star forming regions. Galaxies with clear signsof an ongoing interaction show average values of f(Hα)/f(UV)slightly higher than the reference value, as expected if those objectshad SFR increased by a factor of ≃4. The accuracy of the currentUV and Hα photometry is not yet sufficient to clearly disentanglethe effect of interactions on the f(Hα)/f(UV) ratio, butsignificant observational improvements are shortly expected to resultfrom the GALEX mission.Tables 1-3 are only available in electronic form athttp://www.edpsciences.org
| NGC 4569: Recent evidence for a past ram pressure stripping event
Deep 21-cm H I line observations of the Virgo cluster spiral galaxy NGC4569 have been obtained with the VLA in its D configuration and with theEffelsberg 100-m telescope. A low surface density arm was discovered inthe west of the galaxy, whose velocity field is distinct from that ofthe overall disk rotation. The observed gas distribution, velocity fieldand velocity dispersion are compared to snapshots of dynamicalsimulations that include the effects of ram pressure. Two differentscenarios were explored: (i) ongoing stripping and (ii) a majorstripping event that took place about 300 Myr ago. It is concluded thatonly the post-stripping scenario can reproduce the main observedcharacteristics of NGC 4569. It is not possible to determine if the gasdisk of NGC 4569 had already been truncated before it underwent the rampressure event that lead to its observed H I deficiency.Movies are available in electronic form at http://www.edpsciences.org
| Radio continuum spectra of galaxies in the Virgo cluster region
New radio continuum observations of galaxies in the Virgo cluster regionat 4.85, 8.6, and 10.55 GHz are presented. These observations arecombined with existing measurements at 1.4 and 0.325 GHz. The sampleincludes 81 galaxies where spectra with more than two frequencies couldbe derived. Galaxies that show a radio-FIR excess exhibit centralactivity (HII, LINER, AGN). The four Virgo galaxies with the highestabsolute radio excess are found within 2° of the centerof the cluster. Galaxies showing flat radio spectra also host activecenters. There is no clear trend between the spectral index and thegalaxy's distance to the cluster center.Figure 3 is only available in electronic form athttp://www.edpsciences.orgTable 3 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/1
| Spectrophotometry of galaxies in the Virgo cluster. II. The data
Drift-scan mode (3600-6800 Å) spectra with 500
| A Survey for H2O Megamasers. III. Monitoring Water Vapor Masers in Active Galaxies
We present single-dish monitoring of the spectra of 13 extragalacticwater megamasers taken over a period of 9 years and a single epoch ofsensitive spectra for seven others. The primary motivation is a searchfor drifting line velocities analogous to those of the systemic featuresin NGC 4258, which are known to result from centripetal acceleration ofgas in an edge-on, subparsec molecular disk. We detect a velocity driftanalogous to that in NGC 4258 in only one source, NGC 2639. Another, themaser source in NGC 1052, exhibits erratic changes in its broad maserprofile over time. Narrow maser features in all of the other diskgalaxies discussed here either remain essentially constant in velocityover the monitoring period or are sufficiently weak or variable inintensity that individual features cannot be traced reliably from oneepoch to the next. In the context of a circumnuclear, molecular diskmodel, our results suggest that either (a) the maser lines seen aresystemic features subject to a much smaller acceleration than present inNGC 4258, presumably because the gas is farther from the nuclear blackhole, or (b) we are detecting ``satellite'' lines for which theacceleration is in the plane of the sky.Our data include the first K-band science observations taken with thenew 100 m Green Bank Telescope (GBT). The GBT data were taken duringtesting and commissioning of several new components and so are subjectto some limitations; nevertheless, they are in most cases the mostsensitive H2O spectra ever taken for each source and cover800 MHz (~=10,800 km s-1) of bandwidth. Many new maserfeatures are detected in these observations. Our data also include atentative and a clear detection of the megamaser in NGC 6240 at epochs ayear and a few months, respectively, prior to the detections reported byHagiwara et al. and Nakai et al.We also report a search for water vapor masers toward the nuclei of 58highly inclined (i>80deg), nearby galaxies. These sourceswere selected to investigate the tendency that H2O megamasersfavor inclined galaxies. None were detected, confirming that megamasersare associated exclusively with active galactic nuclei.
| Companions of Bright Barred Shapley-Ames Galaxies
Companion galaxy environment for a subset of 78 bright and nearby barredgalaxies from the Shapley-Ames Catalog is presented. Among the spiralbarred galaxies, there are Seyfert galaxies, galaxies with circumnuclearstructures, galaxies not associated with any large-scale galaxy cloudstructure, galaxies with peculiar disk morphology (crooked arms), andgalaxies with normal disk morphology; the list includes all Hubbletypes. The companion galaxy list includes the number of companiongalaxies within 20 diameters, their Hubble type, and projectedseparation distance. In addition, the companion environment was searchedfor four known active spiral galaxies, three of them are Seyfertgalaxies, namely, NGC 1068, NGC 1097, and NGC 5548, and one is astarburst galaxy, M82. Among the results obtained, it is noted that theonly spiral barred galaxy classified as Seyfert 1 in our list has nocompanions within a projected distance of 20 diameters; six out of 10Seyfert 2 bar galaxies have no companions within 10 diameters, six outof 10 Seyfert 2 galaxies have one or more companions at projectedseparation distances between 10 and 20 diameters; six out of 12 galaxieswith circumnuclear structures have two or more companions within 20diameters.
| An Hα survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies. I. How common are gaseous halos among non-starburst galaxies?
In a series of two papers we present results of a new Hα imagingsurvey, aiming at the detection of extraplanar diffuse ionized gas inhalos of late-type spiral galaxies. We have investigated a sample of 74nearby edge-on spirals, covering the northern and southern hemisphere.In 30 galaxies we detected extraplanar diffuse emission at meandistances of |z| ~ 1-2 kpc. Individual filaments can be traced out to|z|<=6 kpc in a few cases. We find a good correlation between the FIRflux ratio (S60/S100) and the SFR per unit area(LFIR/D225), based on thedetections/non-detections. This is actually valid for starburst, normaland for quiescent galaxies. A minimal SFR per unit area for the lowestS60/S100 values, at which extended emission hasbeen detected, was derived, which amounts to dotEA25thres = (3.2+/-0.5)*E40ergs-1 kpc-2. There are galaxies where extraplanaremission was detected at smaller values ofLFIR/D225, however, only in combinationwith a significantly enhanced dust temperature. The results corroboratethe general view that the gaseous halos are a direct consequence of SFactivity in the underlying galactic disk.Based on observations collected at the European Southern Observatory,Chile (ESO No. 63.N-0070, ESO No. 64.N-0034, ESO No. 65.N.-0002).
| 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
| Magnetic field evolution in galaxies interacting with the intracluster medium. 3D numerical simulations
A fully three-dimensional (3D) magnetohydrodynamical (MHD) model isapplied to simulate the evolution of the large-scale magnetic field incluster galaxies interacting with the intra-cluster medium (ICM). As themodel input we use time-dependent gas velocity fields resulting from 3DN-body sticky-particle simulations of a galaxy. The modeled clouds areaffected by the ram pressure due to their rapid motion through the ICMin the central part of a cluster. Numerical simulations have shown thatafter the initial compression phase due to ram pressure, a process ofgas re-accretion onto the galactic disk takes place. We find that thegas re-accretion leads to an increase of the total magnetic energywithout any dynamo action. The simulated magnetic fields are used toconstruct the model maps of high-frequency (Faraday rotation-free)polarized radio emission. We show that the evolution of the polarizedintensity shows features that are characteristic of differentevolutionary stages of an ICM-ISM interaction. The comparison ofpolarized radio continuum emission maps with our model permits us todetermine whether the galaxy is in the compression or in there-accretion phase. It also provides an important constraint upon thedynamical modeling of ICM-ISM interactions.
| [C II] emission and star formation in late-type galaxies. II. A model
We study the relationship between gas cooling via the [C II] (lambda =158 μm) line emission and dust cooling via the far-IR continuumemission on the global scale of a galaxy in normal (i.e. non-AGNdominated and non-starburst) late-type systems. It is known that theluminosity ratio of total gas and dust cooling, LC II/LFIR, shows a non-linear behaviour with the equivalent widthof the H alpha (lambda = 6563 Å) line emission, the ratiodecreasing in galaxies of lower massive star-formation activity. Thisresult holds despite the fact that known individual Galactic andextragalactic sources of the [C II] line emission show different [C II]line-to-far-IR continuum emission ratios. This non-linear behaviour isreproduced by a simple quantitative theoretical model of gas and dustheating from different stellar populations, assuming that thephotoelectric effect on dust, induced by far-UV photons, is the dominantmechanism of gas heating in the general diffuse interstellar medium ofthe galaxies under investigation. According to the model, the globalLC II/LFIR provides a direct measure of thefractional amount of non-ionizing UV light in the interstellar radiationfield and not of the efficiency of the photoelectric heating. The theoryalso defines a method to constrain the stellar initial mass functionfrom measurements of LC II and LFIR. A sample of20 Virgo cluster galaxies observed in the [C II] line with the LongWavelength Spectrometer on board the Infrared Space Observatory is usedto illustrate the model. The limited statistics and the necessaryassumptions behind the determination of the global [C II] luminositiesfrom the spatially limited data do not allow us to establish definitiveconclusions but data-sets available in the future will allow tests ofboth the reliability of the assumptions behind our model and thestatistical significance of our results.Based on observations with the Infrared Space Observatory (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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