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The ultraluminous X-ray sources in the high-velocity system of NGC1275
We report the results of a study of X-ray point sources coincident withthe high-velocity system (HVS) projected in front of NGC1275. A verydeep X-ray image of the core of the Perseus cluster, made with theChandra X-ray Observatory, has been used. We find a population ofultraluminous X-ray sources [ULXs seven sources with LX(0.5 -7.0 keV) > 7 × 1039ergs-1]. As with theULX populations in the Antennae and Cartwheel galaxies, those in the HVSare associated with a region of very active star formation. Severalsources have possible optical counterparts found on the Hubble SpaceTelescope (HST) images, although the X-ray brightest one does not.Absorbed power-law models fit the X-ray spectra, with most having aphoton index between 2 and 3.
| Spitzer MIPS Infrared Imaging of M31: Further Evidence for a Spiral-Ring Composite Structure
New images of M31 at 24, 70, and 160 μm taken with the MultibandImaging Photometer for Spitzer (MIPS) reveal the morphology of the dustin this galaxy. This morphology is well represented by a composite oftwo logarithmic spiral arms and a circular ring (radius ~10 kpc) of starformation offset from the nucleus. The two spiral arms appear to startat the ends of a bar in the nuclear region and extend beyond thestar-forming ring. As has been found in previous work, the spiral armsare not continuous, but composed of spiral segments. The star-formingring is very circular except for a region near M32 where it splits. Thelack of well-defined spiral arms and the prominence of the nearlycircular ring suggest that M31 has been distorted by interactions withits satellite galaxies. Using new dynamical simulations of M31interacting with M32 and NGC 205, we find that, qualitatively, suchinteractions can produce an offset, split ring like that seen in theMIPS images.
| Dynamical evolution of intermediate-mass black holes and their observable signatures in the nearby Universe
We investigate the consequences of a model of the assembly and growth ofmassive black holes (MBHs) from primordial seeds, remnants of the firstgeneration of stars in a hierarchical structure formation scenario. Ourmodel traces the build-up of MBHs from an early epoch, and follows themerger history of dark matter haloes and their associated holes viaMonte Carlo realizations of the merger hierarchy, from early times tothe present time, in a Λ cold dark matter cosmology. The sequenceof minor and major mergers experienced by galactic haloes in theirhierarchical growth affects the merger history of MBHs embedded in theirnuclei. So, if the formation route for the assembly of supermassiveblack holes dates back to the early universe, a large number of blackhole (BH) interactions is inevitable. The coalescence time-scales ofbinary black holes can be long enough for a third BH to fall in andinteract with the central binary. These BH triple interactions leadtypically to the final expulsion of one of the three bodies and to therecoil of the binary. Also, asymmetric emission of gravitational wavesin the last stages of the BH merging can give a recoil velocity to thecentre of mass of the coalescing binary. This scenario leads to theprediction of a population of intermediate-mass black holes (IMBHs)wandering in galaxy haloes at the present epoch. We compute theluminosity distribution produced by these IMBHs accreting from theircircumstellar medium. We find that in a Milky Way-sized galaxy they areunable to account for sources with luminosities >~1039ergs-1, unless they carry a baryonic remnant from which theyare able to accrete for a long time. We also find that, for typicalspiral galaxies, the bright end of the point-source distributioncorrelates with the mass of the galaxy and the most luminous sources areexpected to be found in the disc.
| Hierarchical merging, ultraluminous and hyperluminous X-ray sources
Various arguments strongly suggest that the population of ultraluminousX-ray sources (ULXs, apparent X-ray luminosity > Eddington limit for10 Msolar~= 1039erg s-1) in nearbygalaxies are mostly stellar-mass X-ray binaries in unusual evolutionarystages. However, there are indications that the very brightest systemsmay be difficult to explain this way. Accordingly, we consider the classof hyperluminous X-ray sources (HLXs; i.e. those with apparentbolometric luminosities >~1041erg s-1). Becausethis class is small (currently only the M82 object is a secure member)we do not need to invoke a new formation mechanism for its black holes.We explore instead the idea that HLXs may be the nuclei of satellitegalaxies captured during hierarchical merging. The observed correlationbetween active galactic nuclei and tidal interactions implies that HLXactivity would switch on during passage through the host galaxy, closeto the pericentre. This suggests that HLXs should appear near the hostgalaxy, be associated with star formation and thus possibly with ULXs.
| 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.
| Stellar-mass black hole binaries as ultraluminous X-ray sources
Ultraluminous X-ray sources (ULXs) with Lx >1039 erg s-1 have been discovered in great numbersin external galaxies with ROSAT, Chandra and XMM-Newton. The centralquestion regarding this important class of sources is whether theyrepresent an extension in the luminosity function of binary X-raysources containing neutron stars and stellar-mass black holes (BHs), ora new class of objects, e.g. systems containing intermediate-mass BHs(100-1000 Msolar). We have carried out a theoretical study totest whether a large fraction of the ULXs, especially those in galaxieswith recent star formation activity, can be explained with binarysystems containing stellar-mass BHs. To this end, we have applied aunique set of binary evolution models for BH X-ray binaries, coupled toa binary population synthesis code, to model the ULXs observed inexternal galaxies. We find that for donor stars with initial masses>~10 Msolar the mass transfer driven by the normal nuclearevolution of the donor star is sufficient to potentially power mostULXs. This is the case during core hydrogen burning and, to an even morepronounced degree, while the donor star ascends the giant branch,although the latter phases last only ~5 per cent of the main-sequencephase. We show that with only a modest violation of the Eddington limit,e.g. a factor of ~10, both the numbers and properties of the majority ofthe ULXs can be reproduced. One of our conclusions is that ifstellar-mass BH binaries account for a significant fraction of ULXs instar-forming galaxies, then the rate of formation of such systems is ~3× 10-7 yr-1 normalized to a core-collapsesupernova rate of 0.01 yr-1.
| 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.
| X-Ray and Optical Eclipses in Ultraluminous X-Ray Sources as Possible Indicators of Black Hole Mass
Ultraluminous X-ray sources (ULXs) with 1039 ergss-1 <~ LX < 1041 ergss-1 have been discovered in great numbers in externalgalaxies with ROSAT, Chandra, and XMM-Newton. The central questionregarding this important class of sources is whether they represent anextension to the luminosity function of binary X-ray sources containingneutron stars and stellar-mass black holes (BHs), or a new class ofobjects, e.g., systems containing intermediate-mass black holes(100-1000 Msolar). We suggest searching for X-ray and opticaleclipses in these systems to provide another diagnostic to helpdistinguish between these two possibilities. The sense of the effect isthat ULXs with stellar-mass black hole accretors should be at leasttwice as likely to exhibit eclipses as intermediate-mass black holesystems-and perhaps much more than a factor of 2. Among other systemparameters, the orbital period would follow. This would provideconsiderable insight as to the nature of the binary.
| Faint Fuzzies and the Formation of Lenticular Galaxies
We investigate the dynamical state of a new class of extended starclusters known as ``faint fuzzies,'' which were discovered in two nearbyS0 galaxies, NGC 1023 and NGC 3384. It is shown that the faint fuzziesof NGC 1023 lie in a fast-rotating ringlike structure within thegalactic disk with mean radius of 5 kpc, rotational velocity of 200 kms-1, and velocity dispersion of 115 km s-1. Wepropose a scenario for the origin of faint fuzzies that is connected tothe origin of S0 galaxies as a result of galaxy-galaxy interactions indense environments. As is apparent in the Cartwheel galaxy and confirmedby numerical simulations, the passage of a small galaxy through, orclose to, the center of a disk galaxy can form a ring of clumpy starformation with a radius comparable to the faint fuzzy ring radius in NGC1023. In this case, the faint fuzzies are signposts for thetransformation of spiral galaxies into lenticulars via suchinteractions.
| The Detection of Nonthermal Radio Continuum Spokes and the Study of Star Formation in the Cartwheel
New sensitive Very Large Array 20 cm continuum observations of theCartwheel, the prototypical collisional ring galaxy, were carried outwith the principal aim of tracing supernova remnants that are expectedto lie in the wake of the expanding ring and in the ring itself. Wedetect predominantly nonthermal radio continuum emission from regionsassociated with 13 ring H II complexes. The emission interior to thering is confined to structures that resemble spokes of the wheel. Thespokes start near bright H II complexes and extend to around 6" (4 kpc)inward in the direction of the geometrical center of the ring. There isno apparent positional coincidence between the radio continuum andoptical spokes. Radial distribution of intensity along the spokessuggests that the past star formation rate (SFR) in the Cartwheel wasmuch lower than the current SFR. New Hα observations were used torevise the current SFR in the Cartwheel. The revised value is 18Msolar yr-1 , which is a factor of 4lower than the value reported previously but is in good agreement withthe SFR estimated from far-infrared luminosity. About 30% of theobserved 20 cm continuum nonthermal emission seems to originate inprocesses that are not related to star formation. Revised SFR in theCartwheel is comparable to that in the rest of the ring galaxies.
| The Interacting Galaxy Pair NGC 5394/95: Near-Infrared Photometry, Structure, and Morphology
We present near-infrared observations in the J, H, and K' passbands ofthe interacting pair of galaxies NGC 5394/95 (KPG 404). The totalmagnitudes, colors, surface brightnesses, and color profiles arecalculated. In addition, aperture magnitudes are compared againstprevious determinations. We also perform a structural (disk + bulge)analysis, as well as a two-dimensional Fourier analysis, to gain insightinto the morphology of the pair. The disk + bulge fit shows that NGC5394 (KPG 404A) is more compact than normal galaxies, while NGC 5395(KPG 404B) is less concentrated. The two-dimensional Fourier analysisshows that NGC 5394 is an H2β galaxy in the dust-penetrated (DP)classification by Block & Puerari. NGC 5395, in contrast, displays avery complex structure that needs a number of Fourier coefficients to beexplained. A tightly wound m=1 coefficient (DP class H1α) is themain structure, but other m=1 and m=2 coefficients (suggestingmodulation) are also present in the Fourier spectra. The m=1coefficients represent a pseudo-ring-type structure, indicative of acollision rather than a passage. Based on our results we are able toassert that the scenario of the interaction between the galaxy membersof KPG 404 should take into account a crossing of NGC 5394 through thedisk of NGC 5395 in a Cartwheel-like encounter rather than a passage asin M51-type pairs. Numerical simulations could help to unravel thestructural and morphological evolution of this interacting pair.
| 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.
| A ULX in NGC 4559: a ``mini-cartwheel'' scenario?
We have studied the peculiar environment around a ULX in NGC 4559 (withL[x] ≈ 2×1040 erg s-1 and M[BH] ≳50Mȯ). The X-ray source is located near the rim of ayoung (age < 30 Myr), large (diameter ≈ 700 pc) ring-like starforming complex possibly triggered by the impact of a dwarf satellitegalaxy through the gas-rich outer disk of NGC 4559. We speculate thatgalaxy interactions (including the infall of high-velocity clouds andsatellites on a galactic disk) and low-metallicity environments offerfavourable conditions for the formation of compact remnants more massivethan ``standard'' X-ray binaries, and accreting from a massiveRoche-lobe filling companion.
| Are Ultraluminous X-Ray Sources Intermediate-Mass Black Holes Accreting from Molecular Clouds?
The origin and nature of ultraluminous X-ray sources (ULXs) is acontentious and controversial topic. There are ongoing debates about themasses of the objects responsible, their sources of mass for accretion,and their relation to stellar populations in galaxies. A new picture ofthese objects is proposed in which they are intermediate-mass blackholes confined to the disks of their host galaxies and accreting fromthe interstellar medium. They are then preferentially found in or nearmolecular clouds. This model correctly predicts the shape of theobserved luminosity function and requires only a very small fraction ofthe baryonic mass of a galaxy to be in the form of intermediate-massblack holes. Because the X-rays they produce strongly heat nearbyinterstellar gas and because they move relatively rapidly in and out ofdense regions, ULXs are predicted to have brief episodes of highluminosity, perhaps ~105 yr in duration, but they may recurmany times.
| Probing the Evolution of the Galaxy Interaction/Merger Rate Using Collisional Ring Galaxies
We present the results from our program to determine the evolution ofthe galaxy interaction/merger rate with redshift using the uniquestar-forming characteristics of collisional ring galaxies. We haveidentified 25 distant collisional ring galaxy candidates (CRGCs) in atotal of 162 deep Hubble Space Telescope (HST) Wide Field PlanetaryCamera 2 images obtained from the HST archives. Based on measured andestimated redshifts, these 25 CRGCs all lie in the redshift interval of0.1<=z<=1. Using the local collisional ring galaxy volume densityand the new ``standard'' cosmology, we find that in order to account forthe number of identified CRGCs in our surveyed fields, the galaxyinteraction/merger rate, parameterized as (1+z)m, mustincrease steeply with redshift. We determine a minimum value ofm=5.2+/-0.7, although m could be as high as 7 or 8. We can rule out anonevolving (m=0) and weakly evolving (m=1-2) galaxy interaction/mergerrate at greater than the 4 σ level of confidence.
| A thorough study of the intriguing X-ray emission from the Cartwheel ring
We present the results from the high resolution Chandra observation ofthe Cartwheel galaxy. Many individual sources areresolved in the image, mostly associated with the outer ring. Alldetected sources have a very high X-ray luminosity (≥1039 erg s-1) that classifies them as UltraLuminous X-ray sources (ULX). The brightest of them is possibly the mostluminous individual non-nuclear source observed so far, withLX 1041 erg s-1 (at D=122 Mpc).The spatial extent of this source is consistent with a point source atthe Chandra resolution. The luminosity function of individual X-raysources extends about an order of magnitude higher than previouslyreported in other galaxies. We discuss this in the context of the``universal'' luminosity function for High Mass X-ray Binaries and wederive a Star Formation Rate higher than in other starburst galaxiesstudied so far. A diffuse component, associated with hot gas, ispresent. However, deeper observations that we will obtain withXMM-Newton are needed to constrain its properties.Appendix is only available in electronic form athttp://www.edpsciences.org
| Numerical simulations of interacting gas-rich barred galaxies: vertical impact of small companions
We investigate the dynamical effects of an interaction between aninitially barred galaxy and a small spherical companion using anN-body/smoothed-particle-hydrodynamics algorithm. In the modelsdescribed here the small companion passes through the disc of the largergalaxy nearly perpendicular to its plane. The impact positions and timesare varied with respect to the phase of the bar and the dynamicalevolution of the disc.The interactions produce expanding ring structures, offset bars, spokesand other asymmetries in the stars and gas. These characteristicsignatures of the interaction are present in the disc for about 1 Gyr.We find that in some cases it is possible to destroy the bar whilekeeping the disc structure. In general, the central impacts cause largerdamage to the bar and the disc than the peripheral ones. The interactiontends to accelerate the transition from a strongly-barred galaxy to aweakly- or non-barred galaxy. The final disc morphology is determinedmore by the impact position relative to the bar rather than the impacttime.
| Nonnuclear Hyper/Ultraluminous X-Ray Sources in the Starbursting Cartwheel Ring Galaxy
We report the Chandra/ACIS-S detection of more than 20 ultraluminousX-ray sources (ULXs; L0.5-10keV>~3×1039ergs s-1) in the Cartwheel collisional ring galaxy system, ofwhich over a dozen are located in the outer active star-forming ring. Aremarkable hyperluminous X-ray source (HLX;L0.5-10keV>~1041 ergs s-1 assumingisotropic radiation), which dominates the X-ray emission from theCartwheel ring, is located in the same segment of the ring as most ULXs.These powerful H/ULXs appear to be coincident with giant H II regioncomplexes, young star clusters, and radio and mid-infrared hot spots:all strong indicators of recent massive star formation. The X-rayspectra show that H/ULXs have similar properties as those of the mostluminous ULXs found in the nearest starbursts and galaxy mergers such asthe Antennae galaxies and M82. The close association between the X-raysources and the starbursting ring strongly suggests that the H/ULXs areintimately associated with the production and rapid evolution ofshort-lived massive stars. The observations represent the most extremeX-ray luminosities discovered to date associated with star-formingregions-rivaling the X-ray luminosities usually associated with activegalactic nuclei.
| Star Formation Across the Taffy Bridge: UGC 12914/15
We present Berkeley-Illinois-Maryland Association (BIMA) two-fieldmosaic CO (1-0) images of the Taffy galaxies (UGC 12914/15), which showthe distinct taffy-like radio continuum emission bridging the two spiraldisks. Large amounts of molecular gas (1.4×1010Msolar, using the standard Galactic CO-to-H2conversion applicable to Galactic disk giant molecular clouds) wereclearly detected throughout the taffy bridge between the two galaxies,which, as in the more extreme case of H I, presumably results from ahead-on collision between the two galaxies. The highest CO concentrationbetween the two galaxies corresponds to the Hα source in the taffybridge near the intruder galaxy UGC 12915. This H II region is alsoassociated with the strongest source of radio continuum in the bridgeand shows both morphological and kinematic connections to UGC 12915. Theoverall CO distribution of the entire system agrees well with that ofthe radio continuum emission, particularly in the taffy bridge. Thisargues for the star formation origin of a significant portion of theradio continuum emission. Compared with the H I morphology andkinematics, which are strongly distorted owing to the high-speedcollision, CO better defines the orbital geometry and impact parameterof the interaction, as well as the disk properties (e.g., rotation,orientation) of the progenitor galaxies. Based on the 20 cm-to-CO ratiomaps, we conclude that the starburst sites are primarily located in UGC12915 and the Hα source in the bridge and show that the moleculargas in the taffy bridge is forming into stars with star formationefficiency comparable to that of the target galaxy UGC 12914 and similarto that in the Galactic disk.
| Chandra Observations of the Interacting NGC 4410 Galaxy Group
We present high-resolution X-ray imaging data from the ACIS-S instrumenton the Chandra telescope of the nearby interacting galaxy group NGC4410. Four galaxies in the inner portion of this group are clearlydetected by Chandra, including the peculiar low-luminosity radio galaxyNGC 4410A. In addition to a nuclear point source, NGC 4410A containsdiffuse X-ray emission, including an X-ray ridge extending out to about12" (6 kpc) to the northwest of the nucleus. This ridge is coincidentwith an arc of optical emission-line gas, which has previously beenshown to have optical line ratios consistent with shock ionization. Thisstructure may be due to an expanding superbubble of hot gas caused bysupernovae and stellar winds or by the active nucleus. The Chandraobservations also show four or five possible compact ultraluminous X-ray(ULX) sources (LX>=1039 ergs s-1)associated with NGC 4410A. At least one of these candidate ULXs appearsto have a radio counterpart, suggesting that it may be due to an X-raybinary with a stellar-mass black hole, rather than an intermediate-massblack hole. In addition, a faint diffuse intragroup X-ray component hasbeen detected between the galaxies (LX~1041 ergss-1). This supports the hypothesis that the NGC 4410 group isin the process of evolving via mergers from a spiral-dominated group(which typically has no X-ray-emitting intragroup gas) to anelliptical-dominated group (which often has a substantial intragroupmedium).
| Applicability of a Schmidt law to star formation in ring galaxies
The gas hydrodynamics modeling of the Cartwheel ring galaxy is performedwith the purpose to reproduce the measured intensity and radialdistribution of Hα emission in the Cartwheel's disk. The starformation efficiency of epsilon =0.09-0.18 is derived for an assumedSchmidt law with power index N=1.5 and a full thickness of theCartwheel's gas disk of z0=250-1000 pc, respectively.Simulations with a Schmidt law of star formation yield a higher Hαsurface brightness (SigmaHα ) in the Cartwheel's innerring as compared to the observed values. The Toomre criterion for starformation fails to account for the observed sharp drop of massive starformation (MSF) in the inner ring. Numerical simulations indicate that alarge shear near/at the position of the inner ring can raise the gasthreshold for star formation and suppress MSF in the Cartwheel's innerregions. Hence, the Schmidt law has to be supplemented with a shearcriterion for star formation in order to reproduce the observed radialdistribution of SigmaH alpha in the Cartwheel's innerregions.
| 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.
| The SuperCOSMOS Sky Survey - I. Introduction and description
In this, the first in a series of three papers concerning theSuperCOSMOS Sky Survey (SSS), we give an introduction and user guide tothe survey programme. We briefly describe other wide-field surveys andcompare them with our own. We give examples of the data, and make acomparison of the accuracies of the various image parameters availablewith those from the other surveys providing similar data; we show thatthe SSS data base and interface offer advantages over these surveys.Some science applications of the data are also described and somelimitations discussed. The series of three papers constitutes acomprehensive description and user guide for the SSS.
| Age Dating Ultraluminous Infrared Galaxies along the Merger Sequence
Imaging spectroscopy using the new Palomar Integral Field Spectrographis presented for the Paα line in four ultraluminous infraredgalaxies. The resulting integral field data cubes reveal line emissionpossessing a wide range of complex spatial morphologies, often quitedifferent from the appearance of the continuum. The velocity fields areequally diverse in nature, often failing to resemble typical modes ofgalactic motion. We see a variety of interesting phenomena in theindividual mergers including star formation rates of 2-5Msolar yr-1 in young tidal tails; a postencounterdisk that obeys the Tully-Fisher relation; a large-scale emission-linenebula possibly associated with a massive outflow; an apparently singlemerging system possessing two distinct kinematical axes belying thepresence of a second galaxy, mostly obscured by its merging companion;and possible formation of tidal dwarf galaxies. In most cases, we areable to establish the geometry of the merger and thus estimate the timein the merger process at which we are viewing the system. The resultingrange in estimated ages, some of which are very young encounters(~5×107 yr), is not predicted by merger models, whichproduce high rates of star formation either 1-2×108 yrafter the first encounter or very late (~109 yr) in themerger process. Even in the very young mergers, despite a sampleselection based on extended line emission, the ultraluminous activityappears to be centrally concentrated on the nucleus of one of theprogenitor galaxies-namely, the galaxy with a prograde orbital geometry.The inferred extinction to these concentrations is high, usually atleast 1 mag at the wavelength of Paα. The presence of asignificant population of very young ultraluminous mergers, togetherwith the majority of ultraluminous infrared galaxies existing in thefinal stages of merger activity, indicates that the ultraluminous galaxyphase is at least bimodal in time. An evolutionary scenario is proposedfor ultraluminous infrared galaxies, wherein the far-infrared luminositymay undergo multiple ultraluminous bursts during the course of theencounter. A substantial fraction of the merger lifetime may be spent ina phase identified with the less powerful luminous infrared galaxyclass.
| Optical Color Gradients in Star-forming Ring Galaxies
We compute radial color gradients produced by an outwardly propagatingcircular wave of star formation and compare our results with colorgradients observed in the classical ring galaxy, the ``Cartwheel.'' Weinvoke two independent models of star formation in the ring galaxies.The first one is the conventional density wave scenario, in which anintruder galaxy creates a radially propagating density wave accompaniedby an enhanced star formation following the Schmidt's law. The secondscenario is a pure self-propagating star formation model, in which theintruder sets off only the first burst of stars at the point of impact.Both models give essentially the same results. Systematic reddening ofB-V, V-K colors toward the center, such as that observed in theCartwheel, can be obtained only if the abundance of heavy elements inthe star-forming gas is a few times below solar. The B-V and V-K colorgradients observed in the Cartwheel can be explained as a result ofmixing of stellar populations born in a star-forming wave propagatingthrough a low-metallicity gaseous disk, and a preexisting stellar diskof the size of the gaseous disk with color properties typical to thoseobserved in nearby disk galaxies.
| A Multiwavelength Study of Stephan's Quintet
Stephan's Quintet (SQ) is a compact group that we find in an atypicalmoment when a high-velocity intruder is passing through it. Theintrusion is particularly interesting because a previous intruder hadstripped most of the gas from the group members. This debris field wasshocked in the ongoing collision with the new intruder. Thisevolutionary history agrees well with observations and explains how astrongly interacting system can show low levels of star formation. Wepresent new multiwavelength data including previously unpublished ROSATX-ray, Hα interference filter/Fabry-Pérot, ISO MIR/FIR, andradio line and continuum images. These observations and previouslypublished data provide new insights, as well as support for someprevious hypotheses. (1) Fabry-Pérot and H I velocities allow usto unambiguously distinguish between gas associated with SQ and the newintruder. (2) Most detected emission regions are found in the remnantinterstellar medium (ISM) of the new intruder, which allows us to inferits size and present physical state. (3) The few emission regionsassociated with the stripped ISM of SQ include the best candidate tidaldwarf galaxy. (4) Multiwavelength data suggest that strong MIR/FIRemission from the Seyfert 2 nucleus of NGC 7319 comes from dust heateddirectly by a power-law continuum rather than a starburst. (5) Thecorrespondence between extended X-ray/radio continuum/forbidden opticalemission confirms the existence of a large scale shock in SQ. (6) Weconfirm the presence of two stripped spiral members in the process oftransformation into E/S0 morphology. Finally (7) observations areconsistent with the idea that the collision in SQ is ongoing withpossible detection of H II region ablation and Rayleigh-Taylorinstabilities.
| On the Neutral Gas Content and Environment of NGC 3109 and the Antlia Dwarf Galaxy
As part of a continuing survey of nearby galaxies, we have mapped theneutral gas content of the low surface brightness Magellanic-type galaxyNGC 3109 and its environment, including the Antlia dwarf galaxy, atunprecedented velocity resolution and brightness sensitivity. The H Imass of NGC 3109 is measured to be 3.8+/-0.5×108Msolar. A substantial warp in the disk of NGC 3109 isdetected in the H I emission image in the form of an extended lowsurface brightness feature. We report a positive detection in H I of thenearby Antlia dwarf galaxy and measure its total neutral gas mass to be6.8+/-1.4×105 Msolar. We show the warp inNGC 3109 to lie at exactly the same radial velocity as the gas in theAntlia dwarf galaxy and speculate that Antlia disturbed the disk of NGC3109 during a mild encounter ~1 Gyr in the past. H I data for a furthereight galaxies detected in the background are presented.
| The History of Star Formation in the Cartwheel Ring Galaxy
We model and analyze the B-V/V-K radial color gradients observed in theCartwheel ring galaxy. Along with the color-color diagrams, we use theQBVK combined color indices, which minimize the uncertaintiesin the observed B-V and V-K colors introduced by dust extinction. Tomodel the optical and near-infrared color properties of the Cartwheelgalaxy, we assume that an intruder galaxy generates an expanding ringdensity wave in the Cartwheel's disk, which in its turn triggers massivestar formation along the wave's perimeter according to the Schmidt law.The results of our modeling suggest that the pre-collision Cartwheel wasa late-type spiral, embedded in an extensive gaseous disk ofsub-critical surface density. The properties of the old stellar disk aretypical for the late-type Freeman disks, with the central surfacebrightness in V-band and the scale length being 21.0 magarcsec-2 and 2 kpc respectively. The pre-collision gaseousdisk has a metallicity gradient ranging from one fifth of the solar atthe outer regions to the solar at the central regions. At present, thewave of star formation has passed the extent of the pre-collision, oldstellar disk and is currently moving in the predominantly gaseous,low-metallicity disk at the radius of 16 kpc. We find that an additional10-Myr-old burst of star formation in the nuclear regions, along withthe visual extinction of 1.3 mag, is required to reproduce the peculiarcolors of the Cartwheel's nucleus.
| Radial B-V/V-K color gradients, extinction-free QBVK combined color indices, and the history of star formation of the Cartwheel ring galaxy
In this paper we model and analyse the B-V/V-K radial color gradientsobserved in the Cartwheel ring galaxy. Along with the color-colordiagrams, we use the QBVK combined color indices, whichminimise the uncertainties in the observed B-V and V-K colors introducedby dust extinction. To model the optical and near-infrared colorproperties of the Cartwheel galaxy, we assume that an intruder galaxygenerates an expanding ring density wave in the Cartwheel's disk, whichin its turn triggers massive star formation along the wave's perimeteraccording to the Schmidt law. We use the population synthesis tocalculate the color properties of stellar populations formed in theexpanding density wave. The results of color modelling suggest that thepre-collision Cartwheel was a late-type spiral, embedded in an extensivegaseous disk of sub-critical surface density. The properties of the oldstellar disk are typical for the late-type Freeman disks, with thecentral surface brightness in V-band and the scale length being muV0=21.0 ;mag ; arcsec-2 and R_0=2 kpcrespectively. The pre-collision gaseous disk has a metallicity gradientranging from z=zsun/3.5 at the outer regions toz=zsun in the central regions. At present, the wave of starformation has passed the initial extent of the pre-collision, oldstellar disk and is currently moving in the predominantly gaseous,low-metallicity disk at the radius of 16 kpc. Neither young stellarpopulations formed in an expanding density wave, nor their mixture withthe old, pre-collision stellar populations can reproduce the B-V and V-Kcolors of the Cartwheel's nucleus+inner ring. We find that an additional10-Myr-old burst of star formation in the nuclear regions, along withthe visual extinction of AV=1 fm 3, might be responsible forthe peculiar colors of the Cartwheel's nucleus.
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