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NGC 4676A (Mice Galaxies)



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Water-Vapor Maser Survey for Active Galactic Nuclei: A Megamaser in NGC 6926
We made a survey of water-vapor maser emission for 93 AGNs with theNobeyama 45-m and Mopra 22-m telescopes from 1999 to 2002. A megamaserwas detected in a Seyfert 2 galaxy, NGC 6926, at a distance of 80Mpc, in2002 June. [Greenhill et al. (2003a) have also reported a detection ofthe megamaser at the close date.] The peak flux density was 110mJy, andthe total isotropic luminosity was 340 Lȯ. The masershows triply peaked spectrum, suggesting an edge-on disk. A narrow-linefeature of the maser components at VLSR = 6001 kms-1 was strongly variable with a time scale of a few tens ofdays, and the variation should be of intrinsic origin. We also showed apossibility of variability of water-vapor maser emission of a megamaserpreviously detected in a Seyfert/ultraluminous FIR galaxy, NGC 6240.

Chandra observations of the interacting galaxies NGC 3395/3396 (Arp 270)
In this paper we present the results of a 20-ks high-resolution ChandraX-ray observation of the peculiar galaxy pair NGC 3395/3396, a system ata very early stage of merging, and less evolved than the famous Antennaeand Mice merging systems. Previously unpublished ROSAT High-ResolutionImager data are also presented. The point-source population and the hotdiffuse gas in this system are investigated and compared with othermerging galaxy pairs.16 X-ray point sources are detected in Arp 270, seven of which areclassified as ultraluminous X-ray sources (ULXs, LX>=1039 erg s-1). From spectral fits and the age ofthe system it seems likely that these are predominantly high-mass X-raybinaries. The diffuse gas emits at a global temperature of ~0.5 keV,consistent with temperatures observed in other interacting systems, andwe see no evidence of the starburst-driven hot gaseous outflows seen inmore evolved systems such as The Mice and The Antennae. It is likelythat these features are absent from Arp 270 as the gas has hadinsufficient time to break out of the galaxy discs. 32 per cent of theluminosity of Arp 270 arises from the diffuse gas in the system, this islow when compared with later stage merging systems and gives furthercredence that this is an early-stage merger.Comparing the ULX population of Arp 270 to other merging systems, wederive a relationship between the star formation rate of the system,indicated by LFIR, and the number [N(ULX)] and luminosity(LULX) of its ULX population. We find N(ULX)~L0.18FIR andLULX~L0.54FIR. These relationships,coupled with the relation of the point-source X-ray luminosity(LXP) to LK and LFIR+UV (Colbert et al.2003), indicate that the ULX sources in an interacting system havecontributions from both the old and young stellar populations.

A Chandra view of the anomalous half-merger NGC 520
High spatial and spectral resolution Chandra X-ray observations of theanomalous merging galaxy NGC 520, a similarly evolved system to thewell-known Antennae galaxies, are presented here. Of great interest isthe fact that NGC 520, on account of it being supposedly due (as seen invarious multiwavelength studies) to the result of an encounter betweenone gas-rich disc and one gas-poor disc, appears in X-rays to be only`half a merger' whereas an ultraluminous X-ray source (ULX) lies at theprimary (south-eastern), more-massive nucleus, no sources are seen atthe secondary nucleus. Whereas what appears to be a starburst-drivengalactic wind is seen outflowing perpendicular to the molecular discsurrounding the primary nucleus, no such diffuse structure is seenanywhere near the secondary nucleus. Comparing the X-ray properties withthose of other merging galaxies, including famous gas-rich-gas-richmergers such as the Mice and the Antennae, one sees that, relative toits star formation rate, the number of ULXs seen within the system israther small. Similarly, the total X-ray luminosity and the fraction ofthis emission that appears diffuse are both a factor of ~2 less thanthat expected based on NGC 520's evolutionary merger stage.Although only half of NGC 520 appears in X-rays as other mergers do,there is still a wealth of structure and detail: 15 X-ray sources aredetected within the system, many of them showing long-term variability,including a small number of bright ULXs that flatten the source X-rayluminosity function to a level similar to that of the Antennae and othermergers. Lastly, to see what appears to be a starburst-driven diffusegalactic wind, with a spectrum entirely consistent with that of otherknown galactic winds, although unusually, emanating from only one of thenuclei, is a surprise, given that one might have expected suchstructures to have distorted very quickly in such a rapidly evolvingenvironment. The wind is larger and more massive than structures seen inevolutionarily earlier systems (e.g. the Mice), but smaller and lessmassive than as seen in later systems (e.g. the Antennae) or classicstarbursts. Perhaps these structures can survive for longer than waspreviously thought.

XMM-Newton observations of the interacting galaxy pairs NGC 7771/0 and NGC 2342/1
We present XMM-Newton X-ray observations of the interacting galaxy pairsNGC 7771/7770 and NGC 2342/2341. In NGC 7771, for the first time we areable to resolve the X-ray emission into a bright central source plus twobright (LX > 1040 erg s-1)ultraluminous X-ray sources (ULXs) located either end of the bar. In thebright central source (LX~ 1041 ergs-1), the soft emission is well-modelled by a two-temperaturethermal plasma with kT= 0.4/0.7 keV. The hard emission is modelled witha flat absorbed power-law (Γ~ 1.7, NH~ 1022cm-2), and this together with a low-significance (1.7σ)~ 300 eV equivalent width emission line at ~6 keV are the firstindications that NGC 7771 may host a low-luminosity AGN. For the barULXs, a power-law fit to X-1 is improved at the 2.5σ level withthe addition of a thermal plasma component (kT~ 0.3 keV), while X-2 isimproved only at the 1.3σ level with the addition of a discblackbody component with Tin~ 0.2 keV. Both sources arevariable on short time-scales implying that their emission is dominatedby single accreting X-ray binaries (XRBs). The three remaining galaxies,NGC 7770, NGC 2342 and NGC 2341, have observed X-ray luminosities of0.2, 1.8 and 0.9 × 1041 erg s-1,respectively (0.3-10 keV). Their integrated spectra are alsowell-modelled by multi-temperature thermal plasma components with kT=0.2-0.7 keV, plus power-law continua with slopes of Γ= 1.8-2.3that are likely to represent the integrated emission of populations ofXRBs as observed in other nearby merger systems. A comparison with otherisolated, interacting and merging systems shows that all four galaxiesfollow the established correlations for starburst galaxies betweenX-ray, far-infrared and radio luminosities, demonstrating that theirX-ray outputs are dominated by their starburst components.

Dynamics and star formation activity of CG J1720-67.8 unveiled through integral field spectroscopy and radio observations
CG J1720-67.8 is an ultra compact group of several galaxies with alow-velocity dispersion, and displaying the hallmarks of mutualinteraction and possible tidal dwarf galaxy formation. In hierarchicalmodels, the system is a possible precursor to a massive ellipticalgalaxy. In this paper, we use new optical integral field spectroscopicand radio observations to investigate the evolutionary status of thegroup in more detail: global star formation rates are estimated usingHα and 1.4-GHz radio continuum measurements; HI observationsprovide an upper limit to the global neutral gas content; opticalbroadband colours and spectra provide ages and stellar mass estimatesfor the tidal dwarf candidates; the bi-dimensional Hα velocityfield is used to trace the kinematics of the group and its members,which are compared with numerical simulations of galaxy encounters. Theobservations suggest a model in which multiple interactions haveoccurred, with the latest strong encounter involving at least two majorcomponents within the last 200Myr. Debris from the encounter fuelsongoing star formation at the global level of~20Msolaryr-1, with self-gravity within the tidaltail possibly providing a mechanism to enhance the star formation rateof the tidal dwarf candidates, with bursts of star formation in clumpsof mass ~2 × 107 Msolar appearing within thelast 10Myr. The amount of time required for final merging of all groupcomponents remains uncertain.

The star-forming environment of an ultraluminous X-ray source in NGC4559: an optical study
We have studied the candidate optical counterparts and the stellarpopulation in the star-forming complex around the bright ultraluminousX-ray source (ULX) in the western part of the spiral galaxy NGC4559,using the HST Wide Field Planetary Camera 2 (WFPC2), XMM-Newton/OpticalMonitor and ground-based data. We find that the ULX is located near asmall group of OB stars, but is not associated with any massive youngclusters nor with any extraordinary massive stars. The brightest pointsource in the Chandra error circle is consistent with a single bluesupergiant (BSG) of mass ~20Msolar and age ~10 Myr. A fewother stars are resolved inside the error circle: mostly BSGs and redsupergiants (RSGs) with inferred masses ~10-15Msolar and ages~20 Myr. This is consistent with the interpretation of this ULX as ablack hole (BH) accreting from a high-mass donor star in its supergiantphase, with mass transfer occurring via Roche-lobe overflow. Theobserved optical colours and the blue-to-red supergiant ratio suggest alow metal abundance for the stellar population: 0.2<~Z/Zsolar<~ 0.4 (using the Padua tracks), or 0.05<~Z/Zsolar<~ 0.2 (using the Geneva tracks). The age ofthe star-forming complex is <~30 Myr. Hα images show that thisstar-forming region has a ring-like appearance. We propose that it is anexpanding wave of star formation, triggered by an initial densityperturbation, in a region where the gas was only marginally stable togravitational collapse. We also suggest that the most likely trigger wasa collision with a satellite dwarf galaxy going through the gas-richouter disc of NGC4559 less than 30 Myr ago. The culprit could be thedwarf galaxy visible a few arcsec north-west of the complex. If this isthe case, this system is a scaled-down version of the Cartwheel galaxy.The X-ray data favour a BH more massive (M > 50Msolar)than typical Milky Way BH candidates. The optical data favour a young BHoriginating in the recent episode of massive star formation; however,they also rule out an association with young massive star clusters (noneare present in the X7 field). We speculate that other mechanisms maylead to the formation of relatively massive BHs (perhaps M~50-100Msolar) from stellar evolution processes inlow-metallicity environments, or when star formation is triggered bygalactic collisions.

Mid-Infrared Spectra of Classical AGNs Observed with the Spitzer Space Telescope
Full low-resolution (65

Space Telescope Imaging Spectrograph Observations of Young Star Clusters in the Antennae Galaxies (NGC 4038/4039)
Long-slit spectra of several dozen young star clusters have beenobtained at three positions in the Antennae galaxies with the SpaceTelescope Imaging Spectrograph and its 52''×0.2" slit.Based on Hα emission-line measurements, the averagecluster-to-cluster velocity dispersion in seven different clusteraggregates (``knots'') is <10 km s-1. The fact that thisupper limit is similar to the velocity dispersion of gas in the disks oftypical spiral galaxies suggests that the triggering mechanism for theformation of young massive compact clusters (``super star clusters'') isunlikely to be high-velocity cloud-cloud collisions. On the other hand,models in which preexisting giant molecular clouds in the disks ofspiral galaxies are triggered into cluster formation are compatible withthe observed low-velocity dispersions. These conclusions are consistentwith those reached by Zhang and coworkers based on comparisons betweenthe positions of the clusters and the velocity and density structure ofthe nearby interstellar medium. We find evidence for systematicallylower values of the line ratios [N II]/Hα and [S II]/Hα inthe bright central regions of some of the knots relative to their outerregions. This suggests that the harder ionizing photons are used up inthe regions nearest the clusters, and the diffuse ionized gas fartherout is photoionized by ``leakage'' of the leftover low-energy photons.The low values of the [S II]/Hα line ratio, typically [SII]/Hα<0.4, indicate that the emission regions are photoionizedrather than shock heated. The absence of evidence for shock-heated gasis an additional indication that high-velocity cloud-cloud collisionsare not playing a major role in the formation of young clusters.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.

Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved H II Regions
We present high spatial resolution X-ray imaging data for theinteracting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-raytelescope. In addition to the unresolved starburst nucleus, a variablepoint source with LX~1040 ergs s-1 wasdetected 1.5" (270 pc) to the northwest of the nucleus, coincident witha blue, extremely optically luminous (MV~-14.1) point sourceon Hubble Space Telescope images. Eleven other candidate pointlikeultraluminous X-ray sources (ULXs) were also detected in the vicinity ofNGC 7714/7715, two of which exceed 1040 ergs s-1.Ten of these appear to be associated with interaction-induced features,but only two are associated with star formation regions. We also founddiffuse emission with LX~3×1040 ergss-1 extending 11" (1.9 kpc) to the north of the nucleus. Itsspectrum can be fitted with either a two-temperature MEKAL function(kT=0.59+0.05-0.06 and8+10-3 keV) or a 0.6 keV MEKAL function plus apower law (Γ=1.8+/-0.2). The hard component may be due tohigh-mass X-ray binaries (HMXBs) with possible contributions frominverse Compton radiation, while the soft component is likely from asuperwind. Superbubble models imply an expansion age of ~15 Myr,supporting previous assertions of an intermediate-age nuclear stellarpopulation in addition to a 5 Myr starburst. We also detected extendedX-ray emission associated with four extranuclear H II region complexes.The emission from these H II regions and the nuclear starburst could bedue to either an enhanced population of HMXBs relative to Local Groupgalactic averages or to diffuse gas heated by winds from supernovae, ifthe X-ray production efficiency LX/Lmech is high(~5%). To estimate LX/Lmech, we collectedpublished data for well-studied H II regions and superbubbles in nearbygalaxies. For H II regions with ages less than 3.5 Myr, the medianLX/Lmech~0.02%, while for older star formationregions, LX/Lmech~0.2%-7%. Thus, it is possiblethat gas heating by supernovae may be sufficient to account for theobserved X-rays from these H II regions. In galaxies much more distantthan NGC 7714, for example, the Cartwheel galaxy, H II region complexessimilar to those in NGC 7714 will be unresolved by Chandra and willmimic ULXs. No X-ray emission was detected from the Type Ib supernova SN1999dn, with an upper limit of ~2×1038 ergss-1.

XMM-Newton observations of the merger-remnant galaxies NGC 3921 and 7252
Using the high sensitivity of XMM-Newton, we have studied the X-rayemission of the two prototypical late-stage merger remnants, NGC 3921and 7252. In the case of NGC 7252, this is complemented by archivalChandra data. We investigate the nature of the discrete X-ray pointsource populations and the hot diffuse gas components in these twogalaxies, and compare them in the light of their different merger agesand histories.We detect three candidate ultraluminous X-ray point sources (ULXs) inNGC 3921 and at least six in NGC 7252, for which we have high spatialresolution Chandra data. These have luminosities ranging from ~1.4× 1039-1040 erg s-1 (forH0= 75 km s-1 Mpc-1). We expect theseULXs to be high-mass X-ray binaries, associated with the recent starformation in these two galaxies.Extended hot gas is observed in both galaxies. We have sufficient countsin the XMM-Newton data to fit two-component hot plasma models to theirX-ray spectra, and estimate the X-ray luminosities of the hot diffusegas components to be 2.75 × 1040 erg s-1 and2.09 × 1040 erg s-1 in NGC 3921 and 7252,respectively. These luminosities are low compared with the luminositiesobserved in typical mature elliptical galaxies (LX~1041-1042 erg s-1), into which thesemerger remnants are expected to evolve. We do not see evidence that theX-ray haloes of these galaxies are currently being regenerated to themasses and luminosities seen in typical elliptical galaxies. The mass ofatomic gas available to fall back into the main bodies of these galaxiesand shock-heat to X-ray temperatures is insufficient for this to be thesole halo regeneration mechanism. We conclude that halo regeneration ismost likely a long-term (>10 Gyr) process, occurring predominantlyvia mass loss from evolving stars, in a subsonic outflow stagecommencing ~2 Gyr after the merging event.

Shock-induced star formation in a model of the Mice
Star formation plays an important role in the fate of interactinggalaxies. To date, most galactic simulations including star formationhave used a density-dependent star formation rule designed toapproximate a Schmidt law. Here, I present a new star formation rulewhich is governed by the local rate of energy dissipation in shocks. Thenew and old rules are compared using self-consistent simulations of NGC4676; shock-induced star formation provides a better match to theobservations of this system.

Hα Kinematics of Tidal Tails in Interacting Systems: Projection Effects and Dark Matter in TDGs
Several interacting systems exhibit at the tip of their long tidal tailsmassive condensations of atomic hydrogen, which may be the progenitorsof Tidal Dwarf Galaxies. Because, quite often, these tails are observededge-on, projection effects have been claimed to account for the largeHI column densities measured there. Here we show that determining thevelocity field all along the tidal features, one may disentangleprojection effects along the line of view from real bound structures.Due to its large field of view, high spectral and 2D spatialresolutions, Fabry-Perot observations of the ionized gas are welladapted to detect a kinematical signature of either streaming motionsalong a bent tidal tail or of infalling/rotating material associatedwith a forming TDG. Spectroscopic observations also allow to measure thedynamical masses of the TDGs that are already relaxed and check theirdark matter content.

The Dynamical Masses of Tidal Dwarf Galaxies
A variety of substructures have been identified within the tidal debrisaround interacting galaxies. These structures range in scale fromGlobular Clusters to the so-called ``Tidal Dwarf Galaxies''. We reviewobservations of such objects, with particular emphasis on what can beinferred from dynamical mass estimates. We then present the results of adynamical analysis of structures which develop within the tidal tails ofa large-N numerical simulation (N˜1 million). We find that under thebest conditions, ``observations'' of this system recover the true massof the bound substructures to within a factor of two. Poor spatial andvelocity resolution (coarser than the true half-light radii and velocitydispersions) and more inclined viewing geometries lead the dynamicalmasses to be over-estimated by factors of ten or more. A combination ofpoor resolution and edge-on viewing geometries lead to the most dramaticdiscrepancies, with dynamical masses over-estimated by factors of up to1000. Furthermore, projection effects can lead to apparentconcentrations of material at the ends of tidal tails that is in realityspread over very large distances, with mass scales well beyond that ofany truly bound regions. Since many of the well studied tidal dwarfcandidates are found within edge-on tails, we conclude that their massand extent may have been greatly over-estimated.

A Galaxy Merging Sequence Traced by X-rays
We are studying a sample of nearly 20 nearby (cz13,000 kms-1) IR-luminous interacting/merging galaxies observed withthe ACIS in the Chandra archive (e.g., Gao et al. 2003; Ptak et al.2003). Most galaxies in the sample are luminous infrared galaxies (LIGs)and nearly half of them are ultraluminous infrared galaxies (ULIGs) withbolometric luminosities comparable to QSOs. Based on multiwavelengthdata and numerical simulations of gas-rich galaxy mergers, we haveattempted to arrange them in a merger sequence perspective and comparedtheir broad-band X-ray emission here with the optical images (mostlyDSS).

Faint Galaxies in Deep Advanced Camera for Surveys Observations
We present the analysis of the faint galaxy population in the AdvancedCamera for Surveys (ACS) Early Release Observation fields VV 29 (UGC10214) and NGC 4676. These observations cover a total area of 26.3arcmin2 and have depths close to that of the Hubble DeepFields in the deepest part of the VV 29 image, with 10 σ detectionlimits for point sources of 27.8, 27.6, and 27.2 AB magnitudes in thegF475W, VF606W, and IF814W bands,respectively.Measuring the faint galaxy number count distribution is a difficulttask, with different groups arriving at widely varying results even onthe same data set. Here we attempt to thoroughly consider all aspectsrelevant for faint galaxy counting and photometry, developing methodsthat are based on public software and that are easily reproducible byother astronomers. Using simulations we determine the best SExtractorparameters for the detection of faint galaxies in deep Hubble SpaceTelescope observations, paying special attention to the issue ofdeblending, which significantly affects the normalization and shape ofthe number count distribution. We confirm, as claimed by Bernstein,Freedman, & Madore, that Kron-like magnitudes, such as the onesgenerated by SExtractor, can miss more than half of the light of faintgalaxies, what dramatically affects the slope of the number counts. Weshow how to correct for this effect, which depends sensitively not onlyon the characteristics of the observations, but also on the choice ofSExtractor parameters.We present catalogs for the VV 29 and NGC 4676 fields with photometry inthe F475W, F606W, and F814W bands. We also show that combining theBayesian software BPZ with superb ACS data and new spectral templatesenables us to estimate reliable photometric redshifts for a significantfraction of galaxies with as few as three filters.After correcting for selection effects, we measure slopes of 0.32+/-0.01for 2225.5 can be well approximated in all our filters by apassive luminosity evolution model based on the COMBO-17 luminosityfunction (α=-1.5), with a strong merging rate following theprescription of Glazebrook et al., φ*~(1+Qz), with Q=4.

Radial Gas Flows in Colliding Galaxies: Connecting Simulations and Observations
We investigate the detailed response of gas to the formation oftransient and long-lived dynamical structures induced in the earlystages of a disk-disk collision and identify observational signatures ofradial gas inflow through a detailed examination of the collisionsimulation of an equal-mass bulge-dominated galaxy. Our analysis anddiscussion mainly focuses on the evolution of the diffuse and dense gasin the early stages of the collision, when the two disks are interactingbut have not yet merged. Stars respond to the tidal interaction byforming both transient arms and long-lived m=2 bars, but the gasresponse is more transient, flowing directly toward the central regionswithin about 108 yr after the initial collision. The rate ofinflow declines when more than half of the total gas supply reaches theinner few kiloparsecs, where the gas forms a dense nuclear ring insidethe stellar bar. The average gas inflow rate to the central 1.8 kpc is~7 Msolar yr-1 with a peak rate of 17Msolar yr-1. Gas with high volume density is foundin the inner parts of the postcollision disks at size scales close tothe spatial resolution of the simulations, and this may be a directresult of shocks traced by the discontinuity in the gas velocity field.The evolution of gas in a bulgeless progenitor galaxy is also discussed,and a possible link to the ``chain galaxy'' population observed at highredshifts is inferred. The evolution of the structural parameters suchas asymmetry and concentration of both stars and gas are studied indetail. Further, a new structure parameter (the compactness parameter K)that traces the evolution of the size scale of the gas relative to thestellar disk is introduced, and this may be a useful tracer to determinethe merger chronology of colliding systems. Noncircular gas kinematicsdriven by the perturbation of the nonaxisymmetric structure can producedistinct emission features in the ``forbidden velocity quadrants'' ofthe position-velocity diagram (PVD). The dynamical mass calculated usingthe rotation curve derived from fitting the emission envelope of the PVDcan determine the true mass to within 20%-40%. The evolution of themolecular fraction(MH2/MH2+HI) is a potentialtracer to quantitatively assign the age of the interaction, but theapplication to real systems may require additional observationaldiagnostics to properly assess the exact chronology of the mergerevolution.

The Unusual Tidal Dwarf Candidate in the Merger System NGC 3227/3226: Star Formation in a Tidal Shock?
We report the discovery of active star formation in the H I cloudassociated with the interacting Seyfert system NGC 3227/3226 that wasoriginally identified as a candidate tidal dwarf galaxy (TDG) by Mundellet al. and that we name J1023+1952. We present broadband optical B, R, I(from the Isaac Newton Telescope), and ultraviolet images (fromXMM-Newton) that show that the H I cloud is associated with massiveongoing star formation seen as a cluster of blue knots(MB<~-15.5 mag) surrounded by a diffuse ultraviolet haloand cospatial with a ridge of high neutral hydrogen column density(NH~3.7×1021 cm-2) in thesouthern half of the cloud. We also detect Hα emission from theknots with a flux density ofFHα~2.55×10-14 ergs s-1cm-2 corresponding to a star formation rate ofSFR(Hα)~10.6×10-3 Msolaryr-1. J1023+1952 lies at the base of the northern tidal tail,and, although it spatially overlaps the edge of the disk of NGC 3227,Mundell et al. showed that the H I cloud is kinematically distinct withan H I mean velocity 150 km s-1 higher than that of NGC 3227.Comparison of ionized (Hα) and neutral (H I) gas kinematics of thecloud shows closely matched recessional velocities, providing strongevidence that the star-forming knots are embedded in J1023+1952 and arenot merely optical knots in the background disk of NGC 3227, thusconfirming J1023+1952 as a gas-rich (MH/LB>1.5)dwarf galaxy. No star formation is detected in the northern half of thecloud, despite similar H I column densities; instead, our newhigh-resolution H I image shows a ridge of high column densitycoincident with the reddest structures evident in our B-I image. Wesuggest that these structures are caused by the background stellarcontinuum from the disk of NGC 3227 being absorbed by dust intrinsic toJ1023+1952, thus placing J1023+1952 in front of NGC 3227 along the lineof sight. We discuss two scenarios for the origin of J1023+1952: as athird, preexisting dwarf galaxy involved in the interaction with NGC3227 and NGC 3226, or as a newly forming dwarf galaxy condensing out ofthe tidal debris removed from the gaseous disk of NGC 3227. The firstscenario is feasible given that NGC 3227 is the brightest member of agalaxy group, an environment in which preexisting dwarf galaxies areexpected to be common. However, the lack of a detectable old stellarpopulation in J1023+1952 makes a tidal origin more likely. If J1023+1952is a bound object forming from returning gaseous tidal tail material,its unusual location at the base of the northern tail implies adynamically young age similar to its star formation age, and suggests itis in the earliest stages of TDG evolution. Whatever the origin ofJ1023+1952, we suggest that its star formation is shock-triggered bycollapsing tidal debris.

The Luminosity Function of Early-Type Field Galaxies at z ~ 0.75
We measure the luminosity function of morphologically selected E/S0galaxies from z=0.5 to 1.0 using deep high-resolution Advanced Camerafor Surveys (ACS) imaging data. Our analysis covers an area of 48arcmin2 (8 times the area of the Hubble Deep Field North) andextends 2 mag deeper (I~24 mag) than was possible in the Deep GrothStrip Survey (DGSS). Our fields were observed as part of the ACSGuaranteed Time Observations. At 0.51.7, E/S0 galaxies at brighter luminosities(MB<-20.1), but are increasingly different at faintermagnitudes, where blue galaxies are both smaller and have lower Sersicparameters. We find differences in both the size-magnitude relation andthe photometric plane offset for red and blue E/S0s, although neitherred nor blue galaxies give a good fit to the size-magnitude relation.Fits of the colors to stellar population models suggest that most E/S0galaxies have short star formation timescales (τ<1 Gyr), and thatgalaxies have formed at an increasing rate from z~8 until z~2, afterwhich there has been a gradual decline.

Kinematics of tidal tails in interacting galaxies: Tidal dwarf galaxies and projection effects
The kinematics of tidal tails in colliding galaxies has been studied viaFabry-Pérot observations of the Hα emission. With theirlarge field of view and high spatial resolution, the Fabry-Pérotdata allow us to probe simultaneously, in 2D, two kinematical featuresof the tidal ionized gas: large-scale velocity gradients due tostreaming motions along the tails, and small-scale motions related tothe internal dynamics of giant HII regions within the tails. In severalinteracting systems, massive (109 Mȯ)condensations of HI, CO and stars are observed in the outer regions oftails. Whether they are genuine accumulations of matter or not is stilldebated. Indeed a part of the tidal tail may be aligned with theline-of-sight, and the associated projection effect may result inapparent accumulations of matter that does not exist in the 3D space.Using numerical simulations, we show that studying the large-scalekinematics of tails, it is possible to know whether these accumulationsof matter are the result of projection effects or not. We conclude thatseveral ones (Arp 105-South, Arp 242, NGC 7252, and NGC 5291-North) aregenuine accumulations of matter. We also study the small-scale motionsinside these regions: several small-scale velocity gradients areidentified with projected values as large as 50-100 km s-1accross the observed HII regions. In the case of NGC 5291-North, thespatial resolution of our observations is sufficient to detail thevelocity field; we show that this system is rotating andself-gravitating, and discuss its dark matter content. TheFabry-Pérot observations have thus enabled us to prove that some109 Mȯ condensations of matter are realstructures, and are kinematically decoupled from the rest of the tail.Such massive and self-gravitating objects are the progenitors of theso-called ``Tidal Dwarf Galaxies''.Based on observations collected at the European Southern Observatory, LaSilla, Chile and at the Canada-France-Hawaii Observatory, Hawaii, USA.Appendix is only available in electronic form athttp://www.edpsciences.org

A variable ultra-luminous X-ray source in the colliding galaxy NGC 7714
We studied the colliding galaxy NGC 7714 with two XMM-Newtonobservations, six months apart. The galaxy contains two bright X-raysources: we show that they have different physical nature. Theoff-nuclear source is an accreting compact object, one of the brightestultraluminous X-ray sources (ULXs) found to date. It showed spectral andluminosity changes between the two observations, from a low/soft to ahigh/hard state; in the high state, it reached Lx ≈ 6× 1040 erg s-1. Its lightcurve in the highstate suggests variability on a ≈ 2 h timescale. Its peculiarlocation, where the tidal bridge between NGC 7714 and NGC 7715 joins theouter stellar ring of NGC 7714, makes it an interesting example of theconnection between gas flows in colliding galaxies and ULX formation.The nuclear source (Lx ≈ 1041 ergs-1) coincides with a starburst region, and is thecombination of thin thermal plasma emission and a point-sourcecontribution (with a power-law spectrum). Variability in the power-lawcomponent between the two observations hints at the presence of asingle, bright point source (Lx  3 ×1040 erg s-1): either a hidden AGN or another ULX.

Stellar populations and star cluster formation in interacting galaxies with the Advanced Camera for Surveys
Pixel-by-pixel colour-magnitude and colour-colour diagrams-based on asubset of the Hubble Space Telescope Advanced Camera for Surveys EarlyRelease Observations-provide a powerful technique to explore and deducethe star and star cluster formation histories of the Mice and the Tadpole interactinggalaxies. In each interacting system we find some 40 bright young starclusters (20<~F606W(mag)<~25, with a characteristic mass of~3×106 Msolar), which are spatiallycoincident with blue regions of active star formation in their tidaltails and spiral arms. We estimate that the main events triggering theformation of these clusters occurred ~(1.5-2.0)×108 yrago. We show that star cluster formation is a major mode of starformation in galaxy interactions, with >~35% of the active starformation in encounters occurring in star clusters. This is the firsttime that young star clusters have been detected along the tidal tailsin interacting galaxies. The tidal tail of the Tadpole system isdominated by blue star forming regions, which occupy some 60% of thetotal area covered by the tail and contribute ~70% of the total flux inthe F475W filter (decreasing to ~40% in F814W). The remaining pixels inthe tail have colours consistent with those of the main disk. Thetidally triggered burst of star formation in the Mice is of similarstrength in both interacting galaxies, but it has affected onlyrelatively small, spatially coherent areas.

Chandra observations of the Mice
Presented here are high spatial and spectral resolution Chandra X-rayobservations of the famous interacting galaxy pair, the Mice, a systemsimilar to, though less evolved than, the well-known Antennae galaxies.Previously unpublished ROSAT High Resolution Imager data of the systemare also presented.Starburst-driven galactic winds outflowing along the minor axis of bothgalaxies (but particularly the northern one) are observed, and spectraland spatial properties, and energetics are presented. That such aphenomenon can occur in such a rapidly evolving and turbulent system issurprising, and this is the first time that the very beginning - theonset, of starburst-driven hot gaseous outflow in a full-blown disc-discmerger has been seen.Point-source emission is seen at the galaxy nuclei, and within theinteraction-induced tidal tails. Further point-source emission isassociated with the galactic bar in the southern system. A comparison ofthe source X-ray luminosity function and of the diffuse emissionproperties is made with the Antennae and other galaxies, and evidence ofa more rapid evolution of the source population than the diffusecomponent is found. No evidence for variability is found between theChandra and previous observations.

My researches at the infrared doors
As a historical and biographical introduction to this Conference, I givehere a brief review about my studies in infrared astronomy. I begunmaking regular observations in this unexplored (at that time) fieldmoving from the wavelengths just beyond the visible, where I discovered,for example, the galaxies then named Maffei 1 and Maffei 2, located inthe Zone of Avoidance. The analysis of the material thus collected,mainly aimed at studies on long period variables (LPVs), produced aseries of new and hardly predicted results. Further importantdevelopments of my researches in infrared are now expected from theongoing development of an Antarctica telescope for the mid-infraredbands. These bands were an almost unexplored range of the spectrum onlya few years ago: now the researches discussed in this meeting show howmany new fields of study have become active in them.

Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214
We present the first Advanced Camera for Surveys (ACS) observations ofyoung star clusters in the colliding/merging galaxy UGC 10214. Theobservations were made as part of the Early Release Observation (ERO)program for the newly installed ACS during service mission SM3B for theHubble Space Telescope (HST). Many young star clusters can be identifiedin the tails of UGC 10214, with ages ranging from ~3 to 10 Myr. Theextreme blue V-I (F606W-F814W) colors of the star clusters found in thetail of UGC 10214 can only be explained if strong emission lines areincluded with a young stellar population. This has been confirmed by ourKeck spectroscopy of some of these bright blue stellar knots. The mostluminous and largest of these blue knots has an absolute magnitude ofMV=-14.45, with a half-light radius of 161 pc, and if it is asingle star cluster, it would qualify as a super star cluster (SSC).Alternatively, it could be a superposition of multiple scaled OBassociations or clusters. With an estimated age of ~4-5 Myr, its derivedmass is less than 1.3×106Msolar. Thus, theyoung stellar knot is unbound and will not evolve into a normal globularcluster. The bright blue clusters and associations are much younger thanthe dynamical age of the tail, providing strong evidence that starformation occurs in the tail long after it was ejected. UGC 10214provides a nearby example of processes that contributed to the formationof halos and intracluster media in the distant and younger universe.

A Minor-Merger Interpretation for NGC 1097's ``Jets''
We have conducted a deep search for neutral hydrogen gas associated withthe faint optical ``jets'' of NGC 1097 using the Very Large Array.Measurable H I would have been expected if the jets were tidal in origingiven their moderately blue optical and near-infrared colors. The jetsare free of H I emission to a limiting surface density(ΣHI) of 0.06 Msolar pc-2 (3σ) over a 1102 km s-1 velocity range. We also rule outextended H I emission down to 0.02 Msolar pc-2 (3σ, ΔV=45 km s-1) within a 4' FWHM aperturecentered on the right-angle turn in jet R1. We have detected an H Isource [MHI=(5.1+/-1.0)×106Msolar] coincident with a small edge-on spiral or irregulargalaxy (NGC 1097B) 12' southwest of NGC 1097, situated between two jets.Two other ~106 Msolar H I point sources in thefield are considered marginal detections. Neither are associated withthe optical jets.The jets' radio-X-ray spectral energy distribution is most consistentwith starlight. However, from their morphology, optical/near-infraredcolors, and lack of H I, we argue that the jets are not tidal tailsdrawn out of NGC 1097's disk or stars stripped from the ellipticalcompanion NGC 1097A. We also reject in situ star formation in ancientradio jets as this requires essentially 100% conversion of gas intostars on large scales. Instead, we conclude that the jets represent thecaptured remains of a disrupted dwarf galaxy that passed through theinner few kiloparsecs of NGC 1097's disk.We present N-body simulations of such an encounter that reproduce theessential features of NGC 1097's jets: A long and narrow ``X''-shapedmorphology centered near the spiral's nucleus, right-angle bends, and nodiscernible dwarf galaxy remnant. A series of jetlike distributions areformed, with the earliest appearing ~1.4 Gyr after impact. Well-definedX shapes form only when the more massive galaxy has a strong diskcomponent. Ram-pressure stripping of the dwarf's interstellar mediumwould be expected to occur while passing through NGC 1097's disk,accounting for the jets' lack of H I and H II. The remnants' (B-V) colorwould still agree with observations even after ~3 Gyr of passiveevolution, provided the cannibalized dwarf was low-metallicity anddominated by young stars at impact.

A Hubble Space Telescope WFPC2 Investigation of the Nuclear Morphology in the Toomre Sequence of Merging Galaxies
We report on the properties of nuclear regions in the Toomre sequence ofmerging galaxies, based on imaging data gathered with the Hubble SpaceTelescope WFPC2 camera. We have imaged the 11 systems in the proposedevolutionary merger sequence in the F555W and F814W broadband filters,and in Hα+[N II] narrowband filters. The broadband morphology ofthe nuclear regions varies from nonnucleated starburst clumps throughdust-covered nuclei to a nucleated morphology. There is no unambiguoustrend in the morphology with merger stage. The emission-line morphologyis extended beyond the nucleus in most cases, but centrally concentrated(within 1 kpc) emission-line gas can be seen in the four latest-stagemerger systems. We have quantified the intrinsic luminosity densitiesand colors within the inner 100 pc and 1 kpc of each identified nucleus.We find little evidence for a clear trend in nuclear properties alongthe merger sequence other than a suggestive rise in the nuclearluminosity density in the most evolved members of the sequence. The lackof clear trends in nuclear properties is likely due both to the effectsof obscuration and geometry, as well as the physical variety of galaxiesincluded in the Toomre sequence.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. These observations are associated with proposal8669.

Massive Star Clusters in Ongoing Galaxy Interactions: Clues to Cluster Formation
We present HST WFPC2 observations, supplemented by ground-based Hαdata, of the star-cluster populations in two pairs of interactinggalaxies selected for being in very different kinds of encounters seenat different stages. Dynamical information and n-body simulationsprovide the details of encounter geometry, mass ratio, and timing. InNGC 5752/4 we are seeing a weak encounter, well past closest approach,after about 2.5×108 yr. The large spiral NGC 5754 has anormal population of disk clusters, while the fainter companion NGC 5752exhibits a rich population of luminous clusters with a flatterluminosity function. The strong, ongoing encounter in NGC 6621/2, seenabout 1.0×108 yr past closest approach between roughlyequal-mass galaxies, has produced an extensive population of luminousclusters, particularly young and luminous in a small region between thetwo nuclei. This region is dynamically interesting, with such a strongperturbation in the velocity field that the rotation curve reversessign. From these results, in comparison with other strongly interactingsystems discussed in the literature, cluster formation requires athreshold level of perturbation, with stage of the interaction a lessimportant factor. The location of the most active star formation in NGC6621/2 draws attention to a possible role for the Toomre stabilitythreshold in shaping star formation in interacting galaxies. The richcluster populations in NGC 5752 and NGC 6621 show that direct contactbetween gas-rich galaxy disks is not a requirement to form luminousclusters and that they can be triggered by processes happening within asingle galaxy disk (albeit triggered by external perturbations).Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.

From Globular Clusters to Tidal Dwarfs: Structure Formation in the Tidal Tails of Merging Galaxies
Using V and I images obtained with the Wide Field Planetary Camera 2(WFPC2) of the Hubble Space Telescope, we investigate compact stellarstructures within tidal tails. Six regions of tidal debris in the fourclassic ``Toomre sequence'' mergers: NGC 4038/39 (``Antennae''), NGC3256, NGC 3921, and NGC 7252 (``Atoms for Peace'') have been studied inorder to explore how the star formation depends on the local and globalphysical conditions. These mergers sample a range of stages in theevolutionary sequence and tails with and without embedded tidal dwarfgalaxies. The six tails are found to contain a variety of stellarstructures, with sizes ranging from those of globular clusters up tothose of dwarf galaxies. From V and I WFPC2 images, we measure theluminosities and colors of the star clusters. NGC 3256 is found to havea large population of blue clusters (0.2<~V-I<~0.9), particularlyin its western tail, similar to those found in the inner region of themerger. In contrast, NGC 4038/39 has no clusters in the observed regionof the tail, only less luminous point sources likely to be individualstars. NGC 3921 and NGC 7252 have small populations of clusters alongtheir tails. A significant cluster population is clearly associated withthe prominent tidal dwarf candidates in the eastern and western tails ofNGC 7252. The cluster-rich western tail of NGC 3256 is not distinguishedfrom the others by its dynamical age or by its total H I mass. However,the mergers that have few clusters in the tail all have tidal dwarfgalaxies, while NGC 3256 does not have prominent tidal dwarfs. Wespeculate that star formation in tidal tails may manifest itself eitherin small structures like clusters along the tail or in large structuressuch as dwarf galaxies, but not in both. Also, NGC 3256 has the higheststar formation rate of the four mergers studied, which may contribute tothe high number of star clusters in its tidal tails.Based in part on observations obtained with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS 5-26555.

Dynamical Evolution Driven by Bars and Interactions: Input from Numerical Simulations
We discuss the evolution of a disc galaxy due to the formation of a barand, subsequently, a peanut. After the formation stage there is stillconsiderable evolution, albeit slower. In purely stellar cases thepattern speed of the bar decreases with time, while its amplitude grows.However, if a considerable gaseous component is present in the disc, thepattern speed may increase with time, while the bar strength maydecrease. In some cases the gas can be brought sufficiently close to thecenter to create a strong central concentration, which, in turn, maymodify the properties of the bar. More violent evolution can take placeduring interactions, so that some disc substructures can be eitherformed or destroyed in a time scale which is small compared to a Hubbletime. These include spirals, bars, bridges, tails, rings, thick discsand bulges. In some cases interactions may lead to mergings. We brieflyreview comparisons of the properties of merger remnants with those ofelliptical galaxies, both for the case of pairwise mergings and the caseof multiple mergings.

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Observation and Astrometry data

Constellation:Coma Berenices
Right ascension:12h46m10.10s
Aparent dimensions:2.239′ × 1.318′

Catalogs and designations:
Proper NamesMice Galaxies
NGC 2000.0NGC 4676A
ICIC 819

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