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|Gas stripping in galaxy groups - the case of the starburst spiral NGC 2276|
Ram-pressure stripping of galactic gas is generally assumed to beinefficient in galaxy groups due to the relatively low density of theintragroup medium (IGM) and the small velocity dispersions of groups. Totest this assumption, we obtained Chandra X-ray data of the starburstingspiral NGC 2276 in the NGC 2300 group of galaxies, a candidate for astrong galaxy interaction with hot intragroup gas. The data reveal ashock-like feature along the western edge of the galaxy and a lowsurface brightness tail extending to the east, similar to the morphologyseen in other wavebands. Spatially resolved spectroscopy shows that thedata are consistent with intragroup gas being pressurized at the leadingwestern edge of NGC 2276 due to the galaxy moving supersonically throughthe IGM at a velocity ~850 km s-1. Detailed modelling of thegravitational potential of NGC 2276 shows that the resulting rampressure could significantly affect the morphology of the outer gas discbut is probably insufficient to strip large amounts of cold gas from thedisc. We estimate the mass-loss rates due to turbulent viscous strippingand starburst outflows being swept back by ram pressure, showing thatboth mechanisms could plausibly explain the presence of the X-ray tail.Comparison to existing HI measurements shows that most of the gasescaping the galaxy is in a hot phase. With a total mass-loss rate of~5Msolaryr-1, the galaxy could be losing itsentire present HI supply within a Gyr. This demonstrates that theremoval of galactic gas through interactions with a hot IGM can occurrapidly enough to transform the morphology of galaxies in groups.Implications of this for galaxy evolution in groups and clusters arebriefly discussed.
|The hot, warm and cold gas in Arp 227 - an evolving poor group|
Arp 227 represents a prototypical example of an interacting mixed pairof galaxies located in a low-density environment. We investigate the gasproperties of the pair in the X-ray, Hα, HI and CO bands. We alsodetect two additional members of the group in HI which indicates thatthe pair constitutes the dominant members of a loose group.The HI distribution shows a tail of gas that connects the spiral member,NGC 470, to the lenticular, NGC 474, showing that the two main membersare currently undergoing interaction. The Hα emission reveals thepresence of secondary components at the centre of NGC 470, superposed onthe main component tracing the rotation of the galaxy. This latter mapsa nearly unperturbed velocity field. The dominant, nearly unperturbedtrend of the kinematics is confirmed by CO observations, althoughrestricted to the centre of the galaxy. The X-ray luminosity of NGC 470is comparable with that of a `normal' spiral galaxy. NGC 474 on theother hand is very gas-poor and has not been detected in Hα. ItsX-ray luminosity is consistent with the low end of the expected emissionfrom discrete sources.Arp 227 as a loose group shows several signatures of galaxy-galaxyinteraction. Our observations suggest the presence of signatures ofinteraction in the overall kinematics of the spiral companion. Theongoing interaction is clearly visible only in the outer HI halo of NGC470. While the large shell system of NGC 474 could be associated with anaccretion event, the secondary components in the Hα profile in thecentre of NGC 470 could be due to the interaction with the companion.The low X-ray luminosity of NGC 470 seems to be a characteristic ofdynamically young systems. All the above evidence suggest that Arp 227is an evolving group in the early phase of its evolution and that itsdrivers are the accretion of faint galaxies and the ongoing large-scaleinteraction between NGC 470 and 474.
|Gas in early-type galaxies: cross-fuelling in late-type-early-type pairs?|
We present 12CO (J= 1-0) and 12CO (J= 2-1)observations of eight early-type galaxies, forming part of a sample ofinteracting galaxies, each consisting of one late- and one early-typesystem. All of the early-type galaxies observed are undetected in CO tolow levels, allowing us to place tight constraints on their moleculargas content. Additionally, we present HI absorption data for one system.The implications for possible gas transfer from the late- to theearly-type galaxy during the interaction are discussed.
|The Two-dimensional XMM-Newton Group Survey: z < 0.012 Groups|
We present the results of the two-dimensional XMM-Newton Group Survey(2dXGS), an archival study of nearby galaxy groups. In this paper weconsider 11 nearby systems (z<0.012) in Mulchaey et al., which span abroad range in X-ray luminosity from 1040 to 1043ergs s-1. We measure the iron abundance and temperaturedistribution in these systems and derive pressure and entropy maps. Wefind statistically significant evidence for structure in the entropy andpressure of the gas component of seven groups on the 10%-20% level. TheXMM-Newton data for the three groups with best statistics also suggestpatchy metallicity distributions within the central 20-50 kpc of thebrightest group galaxy, probed with 2-10 kpc resolution. This providesinsights into the processes associated with thermalization of thestellar mass loss. Analysis of the global properties of the groupsreveals a subclass of X-ray-faint groups, which are characterized byboth higher entropy and lower pressure. We suggest that the mergerhistory of the central elliptical is responsible for both the source andthe observed thermodynamical properties of the hot gas of theX-ray-faint groups.
|Hubble Space Telescope Identification of the Optical Counterparts of Ultraluminous X-Ray Sources in M51|
We present the results of a search for optical identifications ofultraluminous X-ray sources (ULXs) in M51 by using mosaic images takenwith the Hubble Space Telescope(HST) Advanced Camera for Surveys (ASC)in filters F435W (B), F555W (V), F814W (I), and F658N (Hα). Oursample, consisting of nine ULXs, is defined by analyzing the threeChandra observations of M51 performed in 2000 June, 2001 June, and 2003August. We found that four ULXs have one or two candidates forcounterparts, while two have multiple stars within their error circles.The other three have no candidate counterparts. Four ULXs are locatednear or in a star cluster, while others have no association with acluster. These results indicate that the companion stars, environments,and origins of ULXs are probably heterogeneous.
|Low-Luminosity Active Galaxies and Their Central Black Holes|
Central black hole masses for 117 spiral galaxies representingmorphological stages S0/a through Sc and taken from the largespectroscopic survey of Ho et al. are derived using Ks-banddata from the Two Micron All Sky Survey. Black hole masses are foundusing a calibrated black hole-Ks bulge luminosity relation,while bulge luminosities are measured by means of a two-dimensionalbulge-disk decomposition routine. The black hole masses are correlatedagainst a variety of parameters representing properties of the nucleusand host galaxy. Nuclear properties such as line width (FWHM [N II]), aswell as emission-line ratios (e.g., [O III]/Hβ, [O I]/Hα, [NII]/Hα, and [S II]/Hα), show a very high degree ofcorrelation with black hole mass. The excellent correlation with linewidth supports the view that the emission-line gas is in virialequilibrium with either the black hole or bulge potential. The very goodemission-line ratio correlations may indicate a change in ionizingcontinuum shape with black hole mass in the sense that more massiveblack holes generate harder spectra. Apart from theinclination-corrected rotational velocity, no excellent correlations arefound between black hole mass and host galaxy properties. Significantdifferences are found between the distributions of black hole masses inearly-, mid-, and late-type spiral galaxies (subsamples A, B, and C) inthe sense that early-type galaxies have preferentially larger centralblack holes, consistent with observations that Seyfert galaxies arefound preferentially in early-type systems. The line width distributionsshow a marked difference among subsamples A, B, and C in the sense thatearlier type galaxies have larger line widths. There are also cleardifferences in line ratios between subsamples A+B and C that likely arerelated to the level of ionization in the gas. Finally, aKs-band Simien & de Vaucouleurs diagram shows excellentagreement with the original B-band relation, although there is a largedispersion at a given morphological stage.
|The Hα Galaxy Survey . III. Constraints on supernova progenitors from spatial correlations with Hα emission|
Aims.We attempt to constrain progenitors of the different types ofsupernovae from their spatial distributions relative to star formationregions in their host galaxies, as traced by Hα + [Nii] lineemission. Methods: .We analyse 63 supernovae which have occurredwithin galaxies from our Hα survey of the local Universe. Threestatistical tests are used, based on pixel statistics, Hα radialgrowth curves, and total galaxy emission-line fluxes. Results:.Many type II supernovae come from regions of low or zero emission lineflux, and more than would be expected if the latter accurately traceshigh-mass star formation. We interpret this excess as a 40% "Runaway"fraction in the progenitor stars. Supernovae of types Ib and Ic doappear to trace star formation activity, with a much higher fractioncoming from the centres of bright star formation regions than is thecase for the type II supernovae. Type Ia supernovae overall show a weakcorrelation with locations of current star formation, but there isevidence that a significant minority, up to about 40%, may be linked tothe young stellar population. The radial distribution of allcore-collapse supernovae (types Ib, Ic and II) closely follows that ofthe line emission and hence star formation in their host galaxies, apartfrom a central deficiency which is less marked for supernovae of typesIb and Ic than for those of type II. Core-collapse supernova ratesoverall are consistent with being proportional to galaxy totalluminosities and star formation rates; however, within this total thetype Ib and Ic supernovae show a moderate bias towards more luminoushost galaxies, and type II supernovae a slight bias towardslower-luminosity hosts.
|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.
|Supernova 2005dl in NGC 2276|
IAUC 8594 available at Central Bureau for Astronomical Telegrams.
|Supernova 2005dl in NGC 2276|
IAUC 8588 available at Central Bureau for Astronomical Telegrams.
|Observations of an apparent SN in NGC 2276|
Federico Manzini (SAS obs., IAUC A12) report the observation byAlessandro Dimai, Cortina (Italy), and Marco Migliardi, Tour Tour(France), of an apparent SN in NGC 2276 (m. = +17.1), on four unfilteredCCD images taken on 2005 august 25.1 (lim. magn. about +19,5) with the0.5-m "Ullrich" telescope of the Col Druscié observatory (Cortinad'Ampezzo, Italy). The observation is confirmed by other threeunfiltered CCD images, taken on 2005 august 26.13 (m = +16.8 andlimiting magnitude about +19.8), with the same telescope on behalf ofthe CROSS (Col drusciè Remote Observatory Supernovae Search)program.
|Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis|
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
|Warm, Dense Molecular Gas in the ISM of Starbursts, LIRGs, and ULIRGs|
The role of star formation in luminous and ultraluminous infraredgalaxies (LIRGs, LIR>=1011 LsolarULIRGs, LIR>=1012 Lsolar) is a hotlydebated issue: while it is clear that starbursts play a large role inpowering the IR luminosity in these galaxies, the relative importance ofpossible enshrouded AGNs is unknown. It is therefore important to betterunderstand the role of star-forming gas in contributing to the infraredluminosity in IR-bright galaxies. The J=3 level of 12CO lies33 K above ground and has a critical density of~1.5×104 cm-3. The 12CO J=3-2line serves as an effective tracer for warm, dense molecular gas heatedby active star formation. Here we report on 12CO J=3-2observations of 17 starburst spiral galaxies, LIRGs, and ULIRGs, whichwe obtained with the Heinrich Hertz Submillimeter Telescope on MountGraham, Arizona. Our main results are as follows. (1) We find a nearlylinear relation between the infrared luminosity and warm, densemolecular gas such that the infrared luminosity increases as the warm,dense molecular gas to the power 0.92; we interpret this to be roughlyconsistent with the recent results of Gao & Solomon. (2) We findLIR/MH2warm,dense ratios ranging from~38 to ~482 Lsolar/Msolar using a modifiedCO-H2 conversion factor of 8.3×1019cm-2 (K km s-1)-1 derived in thispaper.
|EGRET Upper Limits and Stacking Searches of Gamma-Ray Observations of Luminous and Ultraluminous Infrared Galaxies|
We present a stacking analysis of EGRET γ-ray observations at thepositions of luminous and ultraluminous infrared galaxies. The latterwere selected from the recently presented HCN survey, which is thoughtto contain the most active star-forming regions of the universe.Different sorting criteria are used, and since there is no positivecollective detection of γ-ray emission from these objects, wedetermined both collective and individual upper limits. The uppermostexcess we find appears in the case of ULIRGs ordered by redshift, at avalue of 1.8 σ.
|Infall, the Butcher-Oemler Effect, and the Descendants of Blue Cluster Galaxies at z~0.6|
Using wide-field Hubble Space Telescope WFPC2 imaging and extensive KeckLRIS spectroscopy, we present a detailed study of the galaxy populationsin MS 2053-04, a massive, X-ray-luminous cluster at z=0.5866+/-0.0011.Analysis of 149 confirmed cluster members shows that MS 2053 is composedof two structures that are gravitationally bound to each other; theirrespective velocity dispersions are 865+/-71 km s-1 (113members) and 282+/-51 km s-1 (36 members). MS 2053's totaldynamical mass is 1.2×1015Msolar. MS 2053 isa classic Butcher-Oemler cluster with a high fraction of blue members(24%+/-5%) and an even higher fraction of star-forming members(44%+/-7%), as determined from their [O II] λ3727 emission. Thenumber fraction of blue/star-forming galaxies is much higher in theinfalling structure than in the main cluster. This result is the mostdirect evidence to date that the Butcher-Oemler effect is linked togalaxy infall. In terms of their colors, luminosities, estimatedinternal velocity dispersions, and [O II] λ3727 equivalentwidths, the infalling galaxies are indistinguishable from the fieldpopulation. MS 2053's deficit of S0 galaxies combined with itsoverabundance of blue spirals implies that many of these late-typegalaxies will evolve into S0 members. The properties of the blue clustermembers in both the main cluster and infalling structure indicate thatthey will evolve into low-mass, L3.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.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the California Institute of Technology and theUniversity of California.
|A catalogue of ultraluminous X-ray sources in external galaxies|
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1125
|Metallicity vs rotation velocity in late-type galaxies|
The studies on the chemical abundances in galaxies are very important tounderstand the physical processes in galaxy formation and evolution, andto provide strong constraints on its framework. With the advantage ofobservation techniques and theoretical researches, systematicinvestigations on the relationships between metallicity and macroscopicproperties in galaxies are possible, among which the strong correlationbetween metallicity and galaxy luminosity of galaxies is one of the mostsignificant phenomena. However, recent researches have shown that thecorrelation of metallicity vs rotation velocity for spiral and irregulargalaxies has no such noticeable phenomena. There seems to be a criticalvalue in rotation velocity, below which the metallicity increases withrotation velocity, and above which the metallicity is almost constant.In the present paper, we summarize in detail the observed correlationbetween metallicity and rotation velocity for spiral and irregulargalaxies, and the physical mechanisms are also discussed.
|The origin and evolution of cosmic magnetism|
Magnetism is one of the four fundamental forces. However, the origin ofmagnetic fields in stars, galaxies and clusters is an open problem inastrophysics and fundamental physics. When and how were the first fieldsgenerated? Are present-day magnetic fields a result of dynamo action, ordo they represent persistent primordial magnetism? What role do magneticfields play in turbulence, cosmic ray acceleration and galaxy formation?Here, we demonstrate how the Square Kilometer Array can deliver new datawhich will directly address these currently unanswered issues. Much ofwhat we present is based on an all-sky survey of rotation measures, inwhich Faraday rotation towards >107 background sourceswill provide a dense grid for probing magnetism in the Milky Way, innearby galaxies, and in distant galaxies, clusters and protogalaxies.Using these data, we can map out the evolution of magnetised structuresfrom redshifts z > 3 to the present, can distinguish betweendifferent origins for seed magnetic fields in galaxies, and can developa detailed model of the magnetic field geometry of the intergalacticmedium and of the overall Universe. With the unprecedented capabilitiesof the SKA, the window to the Magnetic Universe can finally be opened.
|Cosmic magnetic fields - as observed in the Universe, in galactic dynamos, and in the Milky Way|
Cosmic magnetism has that exotic ``Je ne sais quoi''! Magnetism has beenobserved in various objects, located near the edge of the Universe andall the way down to the Milky Way's center. The observed magnetic fieldcan take the cell-type shape in randomly-oriented large blobs found inintracluster gas or outside of clusters of galaxies, the helix shape insynchrotron jets, the longitudinal shape in ram-pressured shocks inradio lobes near elliptical galaxies, the spiral shape of logarithmicarms in spiral galaxies, or the egg shape of an enlarged interstellarbubble. In strength, the magnetic field varies from 0.1 nG(cosmological), to 20 μG (galaxies, jets, superbubbles), and to 1 mGin the Milky Way filaments.Magnetism plays a small physical role in the formation of largestructures. It acts as a tracer of the dynamical histories ofcosmological and intracluster events, it guides the motion of theinterstellar ionised gas, and it aligns the charged dust particles.Batteries and dynamos are often employed in models to create and amplifyseed magnetic fields. Starting soon after the Big Bang (redshiftz>2000), this review covers the cosmological background surface(z~1100, distance ~4.3 Gpc), the epoch of first stars (z~20 distance~4.1 Gpc), the currently observable Universe (z~10, distance ~3.9 Gpc),superclusters of galaxies (size ~50 Mpc), intracluster gas (size ~10Mpc), galaxies (~30 kpc), spiral arms (~10 kpc), interstellarsuperbubbles (~100 pc), synchrotron filaments (~10 pc), and the MilkyWay's center.
|Spatial distribution of galaxies in the Puppis region|
We determine the spatial distribution of the galaxies located behind thepart of the zone of avoidance of the Milky Way defined by 220°
|HCN Survey of Normal Spiral, Infrared-luminous, and Ultraluminous Galaxies|
We report systematic HCN J=1-0 (and CO) observations of a sample of 53infrared (IR) and/or CO-bright and/or luminous galaxies, including sevenultraluminous infrared galaxies, nearly 20 luminous infrared galaxies,and more than a dozen of the nearest normal spiral galaxies. This is thelargest and most sensitive HCN survey of galaxies to date. All galaxiesobserved so far follow the tight correlation between the IR luminosityLIR and the HCN luminosity LHCN initially proposedby Solomon, Downes, & Radford, which is detailed in a companionpaper. We also address here the issue of HCN excitation. There is noparticularly strong correlation between LHCN and the 12 μmluminosity; in fact, of all the four IRAS bands, the 12 μm luminosityhas the weakest correlation with the HCN luminosity. There is also noevidence of stronger HCN emission or a higher ratio of HCN and COluminosities LHCN/LCO for galaxies with excess 12μm emission. This result implies that mid-IR radiative pumping, orpopulating, of the J=1 level of HCN by a mid-IR vibrational transitionis not important compared with the collisional excitation by densemolecular hydrogen. Furthermore, large velocity gradient calculationsjustify the use of HCN J=1-0 emission as a tracer of high-densitymolecular gas (>~3×104/τcm-3) andgive an estimate of the mass of dense molecular gas from HCNobservations. Therefore, LHCN may be used as a measure of thetotal mass of dense molecular gas, and the luminosity ratioLHCN/LCO may indicate the fraction of moleculargas that is dense.
|Theoretical Modeling of the Diffuse Emission of Gamma Rays from Extreme Regions of Star Formation: The Case of ARP 220|
Our current understanding of ultraluminous infrared galaxies suggeststhat they are recent galaxy mergers in which much of the gas in theformer spiral disks, particularly that located at distances less than 5kpc from each of the premerger nuclei, has fallen into a common center,triggering a huge starburst phenomenon. This large nuclear concentrationof molecular gas has been detected by many groups, and estimates ofmolecular mass and density have been made. Not surprisingly, theseestimates were found to be orders of magnitude larger than thecorresponding values found in our Galaxy. In this paper, aself-consistent model of the high-energy emission of the superstarburstgalaxy Arp 220 is presented. The model also provides an estimate of theradio emission from each of the components of the central region of thegalaxy (western and eastern extreme starbursts and molecular disk). Thepredicted radio spectrum is found as a result of the synchrotron andfree-free emission and absorption of the primary and secondary steadypopulation of electrons and positrons. The latter is the output ofcharged pion decay and knock-on leptonic production, subject to a fullset of losses in the interstellar medium. The resulting radio spectrumis in agreement with subarcsecond radio observations, which is whatallows us to estimate the magnetic field. In addition, the FIR emissionis modeled with dust emissivity, and the computed FIR photon density isused as a target for inverse Compton process as well as to give anaccount of losses in the γ-ray escape. Bremsstrahlung emission andneutral pion decay are also computed, and the γ-ray spectrum isfinally predicted. Future possible observations with GLAST and theground-based Cerenkov telescopes are discussed.
|The Star Formation Rate and Dense Molecular Gas in Galaxies|
HCN luminosity is a tracer of dense molecular gas,n(H2)>~3×104cm-3, associatedwith star-forming giant molecular cloud (GMC) cores. We present theresults and analysis of our survey of HCN emission from 65 infraredgalaxies, including nine ultraluminous infrared galaxies (ULIGs,LIR>~1012Lsolar), 22 luminousinfrared galaxies (LIGs,1011Lsolar0.06 are LIGs or ULIGs. Normal spiralsall have similar and low dense gas fractionsLHCN/LCO=0.02-0.05. The global star formationefficiency depends on the fraction of the molecular gas in a densephase.
|XMM-Newton Observations of an Intermediate X-Ray Object in NGC 2276|
We present the results from a ~53 ks XMM-Newton observation of NGC 2276.This galaxy has an unusual optical morphology with the disk of thisspiral appearing to be truncated along the western edge. This XMM-Newtonobservation shows that the X-ray source at the western edge is a brightintermediate X-ray object (IXO). Its spectrum is well fitted by amulticolor disk blackbody model used to fit optically thick standardaccretion disks around black holes. The luminosity derived for this IXOis 1.1×1041 ergs s-1 in the 0.5-10 keV band,making it one of the most luminous discovered to date. The large sourceluminosity implies a large-mass black hole if the source is radiating atthe Eddington rate. On the other hand, the inner-disk temperaturedetermined here is too high for such a massive object given the standardaccretion disk model. In addition to the IXO, we find that the nuclearsource in this galaxy has dimmed by at least a factor of severalthousand in the 8 years since the ROSAT HRI observations.
|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.
|An IRAS High Resolution Image Restoration (HIRES) Atlas of All Interacting Galaxies in the IRAS Revised Bright Galaxy Sample|
The importance of far-infrared observations for our understanding ofextreme activity in interacting and merging galaxies has beenillustrated by many studies. Even though two decades have passed sinceits launch, the most complete all-sky survey to date from which far-IRselected galaxy samples can be chosen is still that of the InfraredAstronomical Satellite (IRAS). However, the spatial resolution of theIRAS all-sky survey is insufficient to resolve the emission fromindividual galaxies in most interacting galaxy pairs, and hence previousstudies of their far-IR properties have had to concentrate either onglobal system properties or on the properties of very widely separatedand weakly interacting pairs. Using the HIRES image reconstructiontechnique, it is possible to achieve a spatial resolution ranging from30" to 1.5m (depending on wavelength and detector coverage), whichis a fourfold improvement over the normal resolution of IRAS. This issufficient to resolve the far-IR emission from the individual galaxiesin many interacting systems detected by IRAS, which is very importantfor meaningful comparisons with single, isolated galaxies. We presenthigh-resolution 12, 25, 60, and 100 μm images of 106 interactinggalaxy systems contained in the IRAS Revised Bright Galaxy Sample (RBGS,Sanders et al.), a complete sample of all galaxies having a 60 μmflux density greater than 5.24 Jy. These systems were selected to haveat least two distinguishable galaxies separated by less than threeaverage galactic diameters, and thus we have excluded very widelyseparated systems and very advanced mergers. Additionally, some systemshave been included that are more than three galactic diameters apart,yet have separations less than 4' and are thus likely to suffer fromconfusion in the RBGS. The new complete survey has the same propertiesas the prototype survey of Surace et al. We find no increased tendencyfor infrared-bright galaxies to be associated with other infrared-brightgalaxies among the widely separated pairs studied here. We find smallenhancements in far-IR activity in multiple galaxy systems relative toRBGS noninteracting galaxies with the same blue luminosity distribution.We also find no differences in infrared activity (as measured byinfrared color and luminosity) between late- and early-type spiralgalaxies.
|Inner-truncated Disks in Galaxies|
We present an analysis of the disk brightness profiles of 218 spiral andlenticular galaxies. At least 28% of disk galaxies exhibit innertruncations in these profiles. There are no significant trends oftruncation incidence with Hubble type, but the incidence among barredsystems is 49%, more than 4 times that for nonbarred galaxies. However,not all barred systems have inner truncations, and not allinner-truncated systems are currently barred. Truncations represent areal dearth of disk stars in the inner regions and are not an artifactof our selection or fitting procedures nor the result of obscuration bydust. Disk surface brightness profiles in the outer regions are wellrepresented by simple exponentials for both truncated and nontruncateddisks. However, truncated and nontruncated systems have systematicallydifferent slopes and central surface brightness parameters for theirdisk brightness distributions. Truncation radii do not appear tocorrelate well with the sizes or brightnesses of the bulges. Thissuggests that the low angular momentum material apparently missing fromthe inner disk was not simply consumed in forming the bulge population.Disk parameters and the statistics of bar orientations in our sampleindicate that the missing stars of the inner disk have not simply beenredistributed azimuthally into bar structures. The sharpness of thebrightness truncations and their locations with respect to othergalactic structures suggest that resonances associated with diskkinematics, or tidal interactions with the mass of bulge stars, might beresponsible for this phenomenon.
|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
|Magnetic fields in barred galaxies. III. The southern peculiar galaxy NGC 2442|
Observations of the southern peculiar galaxy NGC 2442with the Australia Telescope Compact Array in total and linearlypolarized radio continuum at λ6 cm are presented and comparedwith previously obtained Hα data. The distribution of polarizedemission, a signature of regular magnetic fields, reveals some physicalphenomena which are unusual among spiral galaxies. We find evidence fortidal interaction and/or ram pressure from the intergalactic mediumcompressing the magnetic field at the northern and western edges of thegalaxy. The radial component of the regular magnetic field in thenorthern arm is directed away from the centre of the galaxy, a findingwhich is in contrast to the majority of galaxies studied to date. Theoval distortion caused by the interaction generates a sudden jump of themagnetic field pattern upstream of the inner northern spiral arm,similar to galaxies with long bars. An unusual ``island'' of strongregular magnetic field east of the galaxy is probably the brightest partof a magnetic arm similar to those seen in some normal spiral galaxies,which appear to be phase-shifted images of the preceding optical arm.The strong magnetic field of the ``island'' may indicate a past phase ofactive star formation when the preceding optical arm was exposed to rampressure.
|2-10 keV luminosity of high-mass binaries as a gauge of ongoing star-formation rate|
Based on recent work on spectral decomposition of the emission ofstar-forming galaxies, we assess whether the integrated 2-10 keVemission from high-mass X-ray binaries (HMXBs),L2-10HMXB, can be used as a reliable estimator ofongoing star formation rate (SFR). Using a sample of 46 local (z 0.1) star-forming galaxies, and spectral modeling of ASCA, BeppoSAX, andXMM-Newton data, we demonstrate the existence of a linear SFR -L2-10^ HMXB relation which holds over 5 decades in X-rayluminosity and SFR. The total 2-10 keV luminosity is not a precise SFRindicator because at low SFR (i.e., in normal andmoderately-starbursting galaxies) it is substantially affected by theemission of low-mass X-ray binaries, which do not trace the current SFRdue to their long evolution lifetimes, while at very high SFR (i.e., forvery luminous FIR-selected galaxies) it is frequently affected by thepresence of strongly obscured AGNs. The availability of purelySB-powered galaxies - whose 2-10 keV emission is mainly due to HMXBs -allows us to properly calibrate the SFR -L2-10HMXB relation. The SFR -L2-10HMXB relation holds also for distant (z 1) galaxies in the Hubble Deep Field North sample, for which we lackspectral information, but whose SFR can be estimated from deep radiodata. If confirmed by more detailed observations, it may be possible touse the deduced relation to identify distant galaxies that are X-rayoverluminous for their (independently estimated) SFR, and are thereforelikely to hide strongly absorbed AGNs.Appendix A is only available in electronic form athttp://www.edpsciences.org
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