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|The SAURON project - VII. Integral-field absorption and emission-line kinematics of 24 spiral galaxy bulges|
We present observations of the stellar and gas kinematics for arepresentative sample of 24 Sa galaxies obtained with our custom-builtintegral-field spectrograph SAURON operating on the William HerschelTelescope. The data have been homogeneously reduced and analysed bymeans of a dedicated pipeline. All resulting data cubes were spatiallybinned to a minimum mean signal-to-noise ratio of 60 per spatial andspectral resolution element. Our maps typically cover thebulge-dominated region. We find a significant fraction of kinematicallydecoupled components (12/24), many of them displaying central velocitydispersion minima. They are mostly aligned and co-rotating with the mainbody of the galaxies, and are usually associated with dust discs andrings detected in unsharp-masked images. Almost all the galaxies in thesample (22/24) contain significant amounts of ionized gas which, ingeneral, is accompanied by the presence of dust. The kinematics of theionized gas are consistent with circular rotation in a disc co-rotatingwith respect to the stars. The distribution of mean misalignmentsbetween the stellar and gaseous angular momenta in the sample suggeststhat the gas has an internal origin. The [OIII]/Hβ ratio is usuallyvery low, indicative of current star formation, and shows variousmorphologies (ring-like structures, alignments with dust lanes oramorphous shapes). The star formation rates (SFRs) in the sample arecomparable with that of normal disc galaxies. Low gas velocitydispersion values appear to be linked to regions of intense starformation activity. We interpret this result as stars being formed fromdynamically cold gas in those regions. In the case of NGC5953, the datasuggest that we are witnessing the formation of a kinematicallydecoupled component from cold gas being acquired during the ongoinginteraction with NGC5954.
|Optical Counterparts of Ultraluminous X-Ray Sources Identified from Archival HST WFPC2 Images|
We present a systematic analysis of archival HST WFPC2 ``Association''data sets that correlate with the Chandra positions of a set of 44ultraluminous X-ray sources (ULXs) of nearby galaxies. The mainmotivation is to address the nature of ULXs by searching for opticalcounterparts. Sixteen of the ULXs are found in early-type galaxies (RC3Hubble type <3). We have improved the Chandra/HST relative astrometrywhenever possible, resulting in errors circles of 0.3"-1.7" in size.Disparate numbers of potential ULX counterparts are found, and in somecases none are found. The lack of or low number of counterparts in somecases may be due to insufficient depth in the WFPC2 images. Particularlyin late-type galaxies, the HST image in the ULX region was often complexor crowded, requiring source detection to be performed manually. Wetherefore address various scenarios for the nature of the ULX since itis not known which, if any, of the sources found are true counterparts.The optical luminosities of the sources are typically in the range104-106 Lsolar, with (effective) Vmagnitudes typically in the range 22-24. In several cases colorinformation is available, with the colors roughly tending to be more redin early-type galaxies. This suggests that, in general, the (potential)counterparts found in early-type galaxies are likely to be older stellarpopulations and are probably globular clusters. Several early-typegalaxy counterparts have blue colors, which may be due to youngerstellar populations in the host galaxies, however, these could also bebackground sources. In spiral galaxies the sources may also be due tolocalized structure in the disks rather than bound stellar systems.Alternatively, some of the counterparts in late-type galaxies may beisolated supergiant stars. The observed X-ray/optical flux ratio isdiluted by the optical emission of the cluster in cases where the systemis an X-ray binary in a cluster, particularly in the case of a low-massX-ray binaries in an old cluster. If any of the counterparts are boundsystems with ~104-106 stars and are the truecounterparts to the ULX sources, then the X-ray luminosities of the ULXare generally well below the Eddington limit for a black hole with mass~0.1% of the cluster mass. Finally, we find that the optical flux of thecounterparts is consistent with being dominated by emission from anaccretion disk around an intermediate-mass black hole if the black holehappens to have a mass >~102 Msolar and isaccreting at close to the Eddington rate, unless the accretion disk isirradiated (which would result in high optical disk luminosities atlower black hole masses).Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. This project isassociated with Archival proposal 9545.
|The Central Engines of 19 LINERs as Viewed by Chandra|
Using archival Chandra observations of 19 LINERs, we explore the X-rayproperties of their inner kiloparsec to determine the origin of theirnuclear X-ray emission, to investigate the presence of an AGN, and toidentify the power source of the optical emission lines. The relativenumbers of LINER types in our sample are similar to those in opticalspectroscopic surveys. We find that diffuse, thermal emission is verycommon and is concentrated within the central few hundred parsecs. Theaverage spectra of the hot gas in spiral and elliptical galaxies arevery similar to those of normal galaxies. They can be fitted with athermal plasma (kT~0.5 keV) plus a power-law (photon index of 1.3-1.5)model. There are on average three detected point sources in their innerkiloparsec with 1037 ergss-1
|Optical Star Formation Rate Indicators|
Using integrated optical spectrophotometry for 412 star-forming galaxiesat z~0, and fiber-aperture spectrophotometry for 120,846 SDSS galaxiesat z~0.1, we investigate the Hα λ6563, Hβλ4861, [O II] λ3727, and [O III] λ5007 nebularemission lines and the U-band luminosity as quantitative star formationrate (SFR) indicators. We demonstrate that the extinction-correctedHα λ6563 luminosity is a reliable SFR tracer even in highlyobscured star-forming galaxies. We find that variations in dustreddening dominate the systematic uncertainty in SFRs derived from theobserved Hβ, [O II], and U-band luminosities, producing a factor of~1.7, ~2.5, and ~2.1 scatter in the mean transformations, respectively.We show that [O II] depends weakly on variations in oxygen abundanceover a wide range in metallicity, 12+log(O/H)=8.15-8.7(Z/Zsolar=0.28-1.0), and that in this metallicity intervalgalaxies occupy a narrow range in ionization parameter(-3.8<~logU<~-2.9). We show that the scatter in [O III]λ5007 as a SFR indicator is a factor of 3-4 due to itssensitivity to metal abundance and ionization. We develop empirical SFRcalibrations for Hβ and [O II] parameterized in terms of the B-bandluminosity, which remove the systematic effects of reddening andmetallicity and reduce the SFR scatter to +/-40% and +/-90%,respectively, although individual galaxies may deviate substantiallyfrom the median relations. Finally, we compare the z~0 relations betweenblue luminosity and reddening, ionization, and [O II]/Hα ratioagainst measurements at z~1 and find broad agreement. We emphasize,however, that optical emission-line measurements including Hα forlarger samples of intermediate- and high-redshift galaxies are needed totest the applicability of our locally derived SFR calibrations todistant galaxies.
|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.
|Toward a clean sample of ultra-luminous X-ray sources|
Context: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided.
|A radio census of nuclear activity in nearby galaxies|
In order to determine the incidence of black hole accretion-drivennuclear activity in nearby galaxies, as manifested by their radioemission, we have carried out a high-resolution Multi-ElementRadio-Linked Interferometer Network (MERLIN) survey of LINERs andcomposite LINER/Hii galaxies from a complete magnitude-limited sample ofbright nearby galaxies (Palomar sample) with unknown arcsecond-scaleradio properties. There are fifteen radio detections, of which three arenew subarcsecond-scale radio core detections, all being candidate AGN.The detected galaxies supplement the already known low-luminosity AGN -low-luminosity Seyferts, LINERs and composite LINER/Hii galaxies - inthe Palomar sample. Combining all radio-detected Seyferts, LINERs andcomposite LINER/Hii galaxies (LTS sources), we obtain an overall radiodetection rate of 54% (22% of all bright nearby galaxies) and weestimate that at least ~50% (~20% of all bright nearby galaxies) aretrue AGN. The radio powers of the LTS galaxies allow the construction ofa local radio luminosity function. By comparing the luminosity functionwith those of selected moderate-redshift AGN, selected from the 2dF/NVSSsurvey, we find that LTS sources naturally extend the RLF of powerfulAGN down to powers of about 10 times that of Sgr A*.
|Radio polarization and sub-millimeter observations of the Sombrero galaxy (NGC 4594). Large-scale magnetic field configuration and dust emission|
We observed the nearby early-type spiral galaxy NGC 4594 (M 104,Sombrero galaxy) with the Very Large Array at 4.86 GHz, with theEffelsberg 100-m telescope at 8.35 GHz as well as with the HeinrichHertz Telescope at 345 GHz in radio continuum. The 4.86 and 8.35 GHzdata contain polarization information and hence information about themagnetic fields: we detected a large-scale magnetic field which is toour knowledge the first detection of a large-scale magnetic field in anSa galaxy in the radio range. The magnetic field orientation in M 104 ispredominantly parallel to the disk but has also vertical components atlarger z-distances from the disk. This field configuration is typicalfor normal edge-on spiral galaxies. The 345 GHz data pertain to the colddust content of the galaxy. Despite the optical appearance of the objectwith the huge dust lane, its dust content is smaller than that of morelate-type spirals.
|The multi-phase gaseous halos of star forming late-type galaxies. I. XMM-Newton observations of the hot ionized medium|
This study presents first results from an X-ray mini-survey carried outwith XMM-Newton to investigate the diffuse Hot Ionized Medium in thehalos of nine nearby star-forming edge-on spiral galaxies. Diffusegaseous X-ray halos are detected in eight of our targets, covering awide range of star formation rates from quiescent to starburst cases.For four edge-on spiral galaxies, namely NGC 3044, NGC 3221, NGC 4634,and NGC 5775, we present the first published high resolution/sensitivitydetections of extended soft X-ray halos. EPIC X-ray contour mapsoverlaid onto Hα imaging data reveals that in all cases thepresence of X-ray halos is correlated with extraplanar Diffuse IonizedGas. Moreover, these halos are also associated with non-thermal cosmicray halos, as evidenced by radio continuum observations. SupplementalUV-data obtained with the OM-telescope at 210 nm show Diffuse IonizedGas to be well associated with UV emission originating in the underlyingdisk. Beside NGC 891, NGC 4634 is the second non-starburst galaxy with adiffuse soft X-ray halo (|z|≤ 4 kpc). In case of NGC 3877, for whichwe also present the first high resolution X-ray imaging data, no haloemission is detectable. EPIC pn spectra (0.3-12 keV) of the diffuseX-ray emission are extracted at different offset positions from thedisk, giving evidence to a significant decrease of gas temperatures,electron densities, and gas masses with increasing distance to theplane. A comparison between dynamical and radiative cooling time scalesimplies that the outflow in all targets is likely to be sustained. Wefind very strong indications that spatially correlated multi-phasegaseous halos are created by star forming activity in the disk plane. Ina forthcoming paper, we will present multi-frequency luminosityrelations and evaluate key parameters which might trigger the formationof multi-phase galaxy halos.
|Fallacies in astronomy and medicine.|
|X-ray observations of the edge-on star-forming galaxy NGC 891 and its supernova SN1986J|
We present XMM-Newton observations of NGC 891, a nearby edge-on spiralgalaxy. We analyse the extent of the diffuse emission emitted from thedisc of the galaxy, and find that it has a single-temperature profilewith best-fitting temperature of 0.26 keV, though the fit of adual-temperature plasma with temperatures of 0.08 and 0.30 keV is alsoacceptable. There is a considerable amount of diffuse X-ray emissionprotruding from the disc in the north-west direction out toapproximately 6 kpc. We analyse the point-source population using aChandra observation, using a maximum-likelihood method to find that theslope of the cumulative luminosity function of point sources in thegalaxy is -0.77+0.13-0.1. Using a sample of otherlocal galaxies, we compare the X-ray and infrared properties of NGC 891with those of `normal' and starburst spiral galaxies, and conclude thatNGC 891 is most likely a starburst galaxy in a quiescent state. Weestablish that the diffuse X-ray luminosity of spirals scales with thefar-infrared luminosity asLX~L0.87+/-0.07FIR, except for extremestarbursts, and NGC 891 does not fall in the latter category. We studythe supernova SN1986J in both XMM-Newton and Chandra observations, andfind that the X-ray luminosity has been declining with time more steeplythan expected (LX~t-3).
|Dust in spiral galaxies: global properties|
We present and analyse high-quality Submillimetre Common-User BolometerArray (SCUBA) 850- and 450-μm images of 14 local spiral galaxies,including the detection of dust well out into the extended disc in manycases. We use these data in conjunction with published far-infrared fluxdensities from IRAS and ISO, and millimetre-wave measurements fromground-based facilities to deduce the global properties of the dust inthese galaxies, in particular temperature and mass. We find that simpletwo-temperature greybody models of fixed dust emissivity index β= 2and with typical temperatures of 25 < Twarm < 40 K and10 < Tcold < 20 K provide good fits to the overallspectral energy distributions. The dust mass in the cold componentcorrelates with the mass in atomic hydrogen and the mass in the warmcomponent correlates with the mass in molecular hydrogen. These resultsthus fit the simple picture in which the cold dust is heatedpredominantly by the interstellar radiation field, while the hot dust isheated predominantly by OB stars in more active regions, although weargue that there is some mixing. The mean gas-to-dust mass ratio is 120+/- 60, very similar to that found within our own galaxy and roughly afactor of 10 lower than that derived from IRAS data alone. Thegas-to-dust mass ratios in the warm, molecular component are on averagehigher than those in the cold, atomic component. We compare ourmodelling results with similar results for more luminous spiral galaxiesselected at far-infrared wavelengths by the SCUBA Local Universe GalaxySurvey. We find that whilst the total dust mass distributions of the twosamples are indistinguishable, they have significantly different dusttemperature distributions in both the warm and cold components. Wesuggest that this difference might be related to the level of starformation activity in these systems, with the more active galaxieshaving more intense interstellar radiation fields and higher dusttemperatures.
|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.
|XMM-Newton Observations of Ultraluminous X-Ray Sources in Nearby Galaxies|
We examined X-ray spectral and timing properties of ultraluminous X-raysources (ULXs) in nearby galaxies in XMM-Newton archival data. Thereappear to be three distinct classes of spectra. One class shows emissionfrom hot, diffuse plasma. This thermal emission is similar to that seenfrom recent supernovae; the temperatures are in the range 0.6-0.8 keV,and the luminosities are the lowest in our sample, near 1039ergs s-1. Three sources have spectra that are strongly curvedat high energies and have the highest temperatures in our sample,1.0-1.4 keV. These spectra are well fitted with a power-law plusmulticolor disk blackbody model with the power law dominant at lowenergies or a Comptonization model. The remainder of the sources arebest fitted with a power-law plus multicolor disk blackbody model, as iscommonly used to describe the spectra of accreting black holes. Thesesources have the lowest thermal component temperatures, 0.1-0.4 keV, andextend to the highest luminosities, above 1040 ergss-1. The temperature of the thermal component is in threedistinct ranges for the three source classes. This diversity of spectralshapes and the fact that the sources lie in three distinct temperatureranges suggests that the ULXs are a diverse population. Two ULXs thatshow state transitions stay within a single class over the course of thetransition. However, we cannot conclude with certainty that the classesrepresent distinct types of objects rather than spectral states of asingle population of objects. More monitoring observations of ULXs withXMM-Newton are required. We also searched for timing noise from thesources and report detection of noise above the Poisson level from fivesources. In three of the sources, the power density spectrum increaseswith decreasing frequency as a power law down to the lowest frequenciesobserved, below 10-4 Hz.
|Mid-Infrared Spectra of Classical AGNs Observed with the Spitzer Space Telescope|
Full low-resolution (65
|A Chandra X-Ray Investigation of the Violent Interstellar Medium: From Dwarf Starbursts to Ultraluminous Infrared Galaxies|
We have analyzed observations with the Chandra X-Ray Observatory of thediffuse emission by hot gas in seven dwarf starburst galaxies, sixedge-on starburst galaxies, and nine ultraluminous infrared galaxies.These systems cover ranges of ~104 in X-ray luminosity, andseveral thousand in star formation rate and K-band luminosity (a proxyfor stellar mass). Despite this range in fundamental parameters, we findthat the properties of the diffuse X-ray emission are very similar inall three classes of starburst galaxies. The spectrum of the diffuseemission is well fitted by thermal emission from gas with kT~0.25-0.8keV and with several times solar abundance ratios of α-elements toFe. The ratio of the thermal X-ray to far-infrared luminosity is roughlyconstant, as is the characteristic surface brightness of the diffuseX-ray emission. The size of the diffuse X-ray source increasessystematically with both far-infrared and K-band luminosity. All threeclasses show strong morphological relationships between the regions ofhot gas probed by the diffuse X-ray emission and the warm gas probed byoptical line emission. These findings suggest that the same physicalmechanism is producing the diffuse X-ray emission in the three types ofstarbursts. These results are consistent with that mechanism beingshocks driven by a galactic ``superwind,'' which is powered by thekinetic energy collectively supplied by stellar winds and supernovae inthe starburst.
|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.
|A Chandra Snapshot Survey of Infrared-bright LINERs: A Possible Link Between Star Formation, Active Galactic Nucleus Fueling, and Mass Accretion|
We present results from a high-resolution X-ray imaging study of nearbyLINERs observed by ACIS on board Chandra. This study complements andextends previous X-ray studies of LINERs, focusing on the underexploredpopulation of nearby dust-enshrouded infrared-bright LINERs. The sampleconsists of 15 IR-bright LINERs (LFIR/LB>3),with distances that range from 11 to 26 Mpc. Combining our sample withprevious Chandra studies, we find that ~51% (28/55) of the LINERsdisplay compact hard X-ray cores. The nuclear 2-10 keV luminosities ofthe galaxies in this expanded sample range from ~2×1038to ~2×1044 ergs s-1. We find that the mostextreme IR-faint LINERs are exclusively active galactic nuclei (AGNs).The fraction of LINERs containing AGNs appears to decrease with IRbrightness and increase again at the highest values ofLFIR/LB. We find that of the 24 LINERs showingcompact nuclear hard X-ray cores in the expanded sample that wereobserved at Hα wavelengths, only eight actually show evidence of abroad line. Similarly, of the 14 LINERs showing compact nuclear hardX-ray cores with corresponding radio observations, only eight display acompact flat spectrum radio core. These findings emphasize the need forhigh-resolution X-ray imaging observations in the study of IR-brightLINERs. Finally, we find an intriguing trend in the Eddington ratioversus LFIR and LFIR/LB for theAGN-LINERs in the expanded sample that extends over 7 orders ofmagnitude in L/LEdd. This correlation may imply a linkbetween black hole growth, as measured by the Eddington ratio, and thestar formation rate, as measured by the far-IR luminosity andIR-brightness ratio. If the far-IR luminosity is an indicator of themolecular gas content in our sample of LINERs, our results may furtherindicate that the mass accretion rate scales with the host galaxy's fuelsupply. We discuss the potential implications of our results in theframework of black hole growth and AGN fueling in low-luminosity AGNs.
|The Discovery of a High-Redshift X-Ray-Emitting QSO Very Close to the Nucleus of NGC 7319|
A strong X-ray source only 8" from the nucleus of the Seyfert 2 galaxyNGC 7319 in Stephan's Quintet has been discovered by Chandra. We haveidentified the optical counterpart and show that it is a QSO withze=2.114. It is also an ultraluminous X-ray source withLX=1.5×1040 ergs s-1. From theoptical spectra of the QSO and the interstellar gas of NGC 7319together, we show that it is very likely that the QSO is interactingwith the interstellar gas.
|Dense Cloud Ablation and Ram Pressure Stripping of the Virgo Spiral NGC 4402|
We present optical, H I, and radio continuum observations of the highlyinclined Virgo Cluster Sc galaxy NGC 4402, which show evidence for rampressure stripping and dense cloud ablation. Very Large Array H I andradio continuum maps show a truncated gas disk and emission to thenorthwest of the main disk emission. In particular, the radio continuumemission is asymmetrically extended to the north and skewed to the west.The Hα image shows numerous H II complexes along the southern edgeof the gas disk, possibly indicating star formation triggered by theintracluster medium (ICM) pressure. Our BVR images at 0.5" resolutionobtained with the WIYN Tip-Tilt Imager show a remarkable dust lanemorphology: at half the optical radius, the dust lane of the galaxycurves up and out of the disk, matching the H I morphology. Large dustplumes extend upward for ~1.5 kpc from luminous young star clusters atthe southeast edge of the truncated gas disk. These star clusters arevery blue, indicating very little dust reddening, which suggests dustblown away by an ICM wind at the leading edge of the interaction. To thesouth of the main ridge of interstellar material, where the galaxy isrelatively clean of gas and dust, we have discovered 1 kpc long lineardust filaments with a position angle that matches the extraplanar radiocontinuum tail; we interpret this angle as the projected ICM winddirection. One of the observed dust filaments has an H II region at itshead. We interpret these dust filaments as large, dense clouds that wereinitially left behind as the low-density interstellar medium wasstripped but were then ablated by the ICM wind. These results providestriking new evidence on the fate of molecular clouds in strippedcluster galaxies.
|Gas and Stars in an H I-Selected Galaxy Sample|
We present the results of a J-band study of the H I-selected AreciboDual-Beam Survey and Arecibo Slice Survey galaxy samples using TwoMicron All Sky Survey data. We find that these galaxies span a widerange of stellar and gas properties. However, despite the diversitywithin the samples, we find a very tight correlation between luminosityand size in the J band, similar to that found in a previous paper byRosenberg & Schneider between the H I mass and size. We also findthat the correlation between the baryonic mass and the J-band diameteris even tighter than that between the baryonic mass and the rotationalvelocity.
|New H2O masers in Seyfert and FIR bright galaxies|
Using the Effelsberg 100-m telescope, detections of four extragalacticwater vapor masers are reported. Isotropic luminosities are ~50, 1000, 1and 230 Lȯ for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 andArp 299, respectively. Mrk 34 contains by far the most distant and oneof the most luminous water vapor megamasers so far reported in a Seyfertgalaxy. The interacting system Arp 299 appears to show two maserhotspots separated by approximately 20´´. With these newresults and even more recent data from Braatz et al. (2004, ApJ, 617,L29), the detection rate in our sample of Seyferts with known jet-NarrowLine Region interactions becomes 50% (7/14), while in star forminggalaxies with high (S100~μ m>50 Jy) far infrared fluxesthe detection rate is 22% (10/45). The jet-NLR interaction sample maynot only contain “jet-masers” but also a significant numberof accretion “disk-masers” like those seen in NGC 4258. Astatistical analysis of 53 extragalactic H2O sources (excluding theGalaxy and the Magellanic Clouds) indicates (1) that the correlationbetween IRAS Point Source and H2O luminosities, established forindividual star forming regions in the galactic disk, also holds forAGN-dominated megamaser galaxies; (2) that maser luminosities are notcorrelated with 60 μm/100 μm color temperatures; and (3) that onlya small fraction of the luminous megamasers (L_H_2O > 100Lȯ) detectable with 100-m sized telescopes have so farbeen identified. The H2O luminosity function (LF) suggests that thenumber of galaxies with 1 Lȯ < L_H_2O < 10Lȯ, the transition range between“kilomasers” (mostly star formation) and“megamasers” (active galactic nuclei), is small. The overallslope of the LF, ~-1.5, indicates that the number of detectable masersis almost independent of their luminosity. If the LF is not steepeningat very high maser luminosities and if it is possible to find suitablecandidate sources, H2O megamasers at significant redshifts should bedetectable even with present day state-of-the-art facilities.
|Radio recombination lines from the starburst galaxy NGC 3256|
We have detected the radio recombination lines H91α and H92αwith rest frequencies of 8.6 GHz and 8.3 GHz from the starburst nucleusNGC 3256 at an angular resolution of 16.4'' × 9.6'' using theAustralia Telescope Compact Array and at an angular resolution of 12.0''× 2.9'' using the VLA. The line was detected at ~1 mJybeam-1 peak with a width of 160 km s-1 with theATCA and at ~0.5 mJy beam-1 peak with a width of 114 kms-1 with the VLA. Modelling the line emitting region as acollection of H II regions, we derive constraints on the required numberof H II regions, their temperature, density, and distribution. We findthat a collection of 10 to 300 H II regions with temperatures of 5000 K,densities of 1000 cm-3 to 5000 cm-3 and diametersof 15 pc produced good matches to the line and continuum emmission. TheLyman continuum production rate required to maintain the ionization is 2× 1052~s-1 to 6 ×1053~s-1, which requires 600 to 17 000 O5 stars tobe produced in the starburst.
|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
|The Leo Triplet: Common origin or late encounter?|
The kinematics, structure, and stellar population properties in thecenters of two early-type spiral galaxies of the Leo Triplet, NGC 3627and NGC 3623, are studied by means of integral-field spectroscopy.Unlike our previous targets, NGC 3384/NGC 3368 in the Leo I Group andNGC 5574/NGC 5576 in LGG 379, NGC 3623 and NGC 3627 do not appear toexperience a synchronous evolution. The mean ages of their circumnuclearstellar populations are quite different, and the magnesium overabundanceof the nucleus in NGC 3627 is evidence for a very brief last starformation event 1 Gyr ago whereas the evolution of the central part ofNGC 3623 looks more quiescent. In the center of NGC 3627 we observenoticeable gas radial motions, and the stars and the ionized gas in thecenter of NGC 3623 demonstrate more or less stable rotation. However,NGC 3623 has a chemically distinct core - a relic of a past starformation burst - which is shaped as a compact, dynamically cold stellardisk with a radius of 250-350 pc which has been formed not laterthan 5 Gyr ago.Based on observations collected with the 6 m telescope of the SpecialAstrophysical Observatory (SAO) of the Russian Academy of Sciences (RAS)which is operated under the financial support of Science Department ofRussia (registration number 01-43) and on data from the Isaac NewtonGroup (ING) and Hubble Space Telescope (HST) Archives.
|Chandra Observation of Luminous Sources in the Nearby Irregular Galaxy NGC 4449|
Using archival Chandra ACIS-S data, 0.5 8.0 keV X-ray spectra of twoluminous X-ray sources in the nearby dwarf irregular galaxy NGC 4449were studied. One, with an extremely high luminosity of 1.3 ×1039 erg s-1 in the 0.5 8.0 keV band, shows aspectrum that is well described with a power-law model of photon index 2. Its properties are consistent with those of ultraluminouscompact X-ray sources observed in nearby galaxies. The spectrum of theother, with a luminosity of 2.7 × 1038 ergs-1 in the same band, is successfully represented with aso-called multi-color disk blackbody emission model with an inner-mostdisk temperature of 0.59 keV. Its spectral parameters are typicalof ordinary black hole binaries observed in our Galaxy and the LargeMagellanic Cloud. These young population objects, together with a brightsupernova remnant and diffuse hot gas already reported, suggest that theX-ray emission from irregular galaxies is generally enhanced by theirrecent star-forming activities.
|Neutral hydrogen absorption at the centre of NGC 2146|
We present 1.4-GHz HI absorption line observations towards thestarburst in NGC 2146, made with the Very Large Array and theMulti-Element Radio-Linked Interferometer Network. The HI gas has arotating disc/ring structure with column densities between 6 and 18× 1021 atom cm-2. The HI absorption has auniform spatial and velocity distribution, and does not reveal anyanomalous material concentration or velocity in the central region ofthe galaxy which might indicate an encounter with another galaxy or afar-evolved merger. We conclude that the signs of an encounter causingthe starburst should be searched for in the outer regions of the galaxy.
|Chandra observations of five ultraluminous X-ray sources in nearby galaxies|
We report the results of a programme of dual-epoch Chandra ACIS-Sobservations of five ultraluminous X-ray sources (ULXs) in nearby spiralgalaxies. All five ULXs are detected as unresolved, point-like X-raysources by Chandra, though two have faded below the 1039 ergs-1 luminosity threshold used to first designate thesesources as ULXs. Using this same criterion, we detect three further ULXswithin the imaged regions of the galaxies. The ULXs appear to be relatedto the star-forming regions of the galaxies, indicating that even innormal spiral galaxies the ULX population is predominantly associatedwith young stellar populations. A detailed study of the Chandra ACIS-Sspectra of six of the ULXs shows that five are better described by apower-law continuum than a multicolour disc blackbody model, thoughthere is evidence for additional very soft components to two of thepower-law continua. The measured photon indices in four out of fivecases are consistent with the low/hard state in black hole binaries,contrary to the suggestion that power-law-dominated spectra of ULXsoriginate in the very high state. A simple interpretation of this isthat we are observing accretion on to intermediate-mass black holes,though we might also be observing a spectral state unique to very highmass accretion rates in stellar-mass black hole systems. Short-term fluxvariability is only detected in one of two epochs for two of the ULXs,with the lack of this characteristic arguing that the X-ray emission ofthis sample of ULXs is not dominated by relativistically beamed jets.The observational characteristics of this small sample suggest that ULXsare a distinctly heterogeneous source class.
|LX-SFR relation in star-forming galaxies|
We compare the results of Grimm, Gilfanov & Sunyaev and Ranalli,Comastri & Seti on the LX-SFR (X-ray luminosity-starformation rate) relation in normal galaxies. Based on theLX-stellar mass dependence for low-mass X-ray binaries(LMXBs), we show that low-SFR (<~1 Msolar yr-1)galaxies in the Ranalli et al. sample are contaminated by the X-rayemission from LMXBs, unrelated to the current star formation activity.However, the most important conclusion from our comparison is that,after the data are corrected for the `LMXB contamination', the two datasets become consistent with each other, despite differences in theircontent, variability effects, adopted source distances, X-ray fluxes andSFR determinations, and also in the cosmological parameters used ininterpreting the Hubble Deep Field North (HDF-N) data. They also agreewell, both in the low- and high-SFR regimes, with the predictedLX-SFR dependence derived from the parameters of the`universal' high-mass X-ray binary (HMXB) luminosity function. Thisencouraging result emphasizes the potential of the X-ray luminosity asan independent SFR indicator for normal galaxies.
|XMM-Newton Observations of Starburst Galaxies|
We report on the results of XMM-Newton observations of nearbystarburst galaxies that form part of a multi-wavelength study of allphases of the extraplanar interstellar medium (ISM) in externalgalaxies. This study is conducted in order to assess the importance ofhalos as repositories of a metal-enriched medium and their significancein terms of galactic chemical evolution and possible metal enrichment ofthe intergalactic medium (IGM). Here we shortly summarize our findingsbased on XMM-Newton observations of NGC 1511 and NGC 1808 andpresent preliminary results for NGC 4666 and NGC 3628.
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