Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
The effect of gravitational recoil on black holes forming in a hierarchical universe
Galactic bulges are known to harbour central black holes whose mass istightly correlated with the stellar mass and velocity dispersion of thebulge. In a hierarchical universe, galaxies are built up throughsuccessive mergers of subgalactic units, a process that is accompaniedby the amalgamation of bulges and the likely coalescence ofgalactocentric black holes. In these mergers, the beaming ofgravitational radiation during the plunge phase of the black holecollision can impart a linear momentum kick or `gravitational recoil' tothe remnant. If large enough, this kick will eject the remnant from thegalaxy entirely, and populate intergalactic space with wandering blackholes. Using a semi-analytic model of galaxy formation, we investigatethe effect of black hole ejections on the scatter of the relationbetween black hole and bulge mass. We find that while not being thedominant source of the measured scatter, they do provide a significantcontribution and may be used to set a constraint, vkick<~500kms-1, on the typical kick velocity, in agreement withvalues found from general relativistic calculations. Even for the moremodest kick velocities implied by these calculations, we find that asubstantial number of central black holes are ejected from theprogenitors of present-day galaxies, giving rise to a population ofwandering intrahalo and intergalactic black holes whose distribution weinvestigate in high-resolution N-body simulations of the Milk Way masshaloes. We find that intergalactic black holes make up only ~2-3 percent of the total galactic black hole mass but, within a halo, wanderingblack holes can contribute up to about half of the total black hole massorbiting the central galaxy. Intrahalo black holes offer a naturalexplanation for the compact X-ray sources often seen near the centres ofgalaxies and for the hyperluminous non-central X-ray source in M82.
| 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.
| Mining for normal galaxies in the first XMM-Newton Serendipitous Source Catalog
This paper uses the first XMM-Newton Serendipitous Source Catalogcompiled by the XMM-Newton Science Centre to identify low-z X-rayselected normal galaxy candidates. Our sample covers a total area of~6deg2 to the 0.5-2keV limit~10-15ergs-1cm-2. A total of 23 sourcesare selected on the basis of low X-ray to optical flux ratiologfX/fopt < -2, soft X-ray spectral propertiesand optical spectra, when available, consistent with stellar formationrather than active galactic nucleus (AGN) processes. This sample iscombined with similarly selected systems from the Needles in theHaystack Survey to provide a total of 46 unique (z<~ 0.2) X-raydetected normal galaxies, the largest low-z sample yet available. Thisis first used to constrain the normal galaxy logN-logS at bright fluxes(10-15-10-13ergs-1cm-2). Weestimate a slope of -1.46 +/- 0.13 for the cumulative number countsconsistent with the Euclidean prediction. We further combine our samplewith 23 local (z<~ 0.2) galaxies from the Chandra Deep Field-Northand -South surveys to construct the local X-ray luminosity function ofnormal galaxies. A Schechter form provides a good fit to the data with abreak at logL*=41.02+0.14-0.12ergs-1 and a slope ofα=-1.76 +/- 0.10. Finally, for the sample of 46 systems, weexplore the association between X-ray luminosity and host galaxyproperties, such as star formation rate (SFR) and stellar mass. We findthat the LX of the emission-line systems correlates withHα luminosity and 1.4-GHz radio power, both providing an estimateof the current SFR. In the case of early-type galaxies withabsorption-line optical spectra, we use the K band as an approximationof stellar mass and find a correlation of the formLX~L1.5K. This is flatter than theLX-LB relation for local ellipticals. This may bedue to either LK providing a better approximation of galaxymass or selection effects biasing our sample against very luminousearly-type galaxies, LX >1042ergs-1.
| Emission-line properties of Seyfert 2 nuclei
This is the third paper of a series devoted to the study of the globalproperties of Joguet's sample of 79 nearby galaxies observable from thesouthern hemisphere, of which 65 are Seyfert 2 galaxies. We use thepopulation synthesis models of Paper II to derive `pure' emission-linespectra for the Seyfert 2 galaxies in the sample, and thus explore thestatistical properties of the nuclear nebular components and theirrelation to the stellar populations. We find that the emission-lineclouds suffer substantially more extinction than the starlight, and weconfirm the correlations between stellar and nebular velocitydispersions and between emission-line luminosity and velocitydispersions, although with substantial scatter. Nuclear luminositiescorrelate with stellar velocity dispersions, but Seyferts withconspicuous star-forming activity deviate systematically towards higherluminosities. Removing the contribution of young stars to the opticalcontinuum produces a tighter and steeper relation,L~σ4*, consistent with the Faber-Jacksonlaw.Emission-line ratios indicative of the gas excitation such as[OIII]/Hβ and [OIII]/[OII] are statistically smaller for Seyfertswith significant star formation, implying that ionization by massivestars is responsible for a substantial and sometimes even a dominantfraction of the Hβ and [OII] fluxes. We use our models to constrainthe maximum fraction of the ionizing power that can be generated by ahidden active galactic nucleus (AGN). We correlate this fraction withclassical indicators of AGN photoionization (i.e. X-ray luminosity andnebular excitation), but find no significant correlations. Thus, whilethere is a strong contribution of starbursts to the excitation of thenuclear nebular emission in low-luminosity Seyferts, the contribution ofthe hidden AGN remains elusive even in hard X-rays.
| Soft gamma repeaters outside the Local Group
We propose that the best sites to search for soft gamma repeaters (SGRs)outside the Local Group are galaxies with active massive-star formation.Different possibilities to observe SGR activity from these sites arediscussed. In particular, we have searched for giant flares from thenearby galaxies (~2-4 Mpc away) M82, M83, NGC 253 and 4945 in the Burstand Transient Source Experiment (BATSE) data. No candidate giant SGRflares were found. The absence of such detections implies that the rateof giant flares with energy release in the initial spike above 0.5× 1044 erg is less than 1/30 yr-1 in ourGalaxy. However, hyperflares similar to that of 2004 December 27 can beobserved from larger distances. Nevertheless, we do not see anysignificant excess of short GRBs from the Virgo galaxy cluster or fromthe galaxies Arp 299 and NGC 3256 (both with extremely high starformation rates). This implies that the Galactic rate of hyperflareswith energy release ~1046 erg is less than ~10-3yr-1. With this constraint the fraction of possibleextragalactic SGR hyperflares among BATSE's short GRBs should not exceeda few per cent. We present the list of short GRBs coincident with thegalaxies mentioned above, and discuss the possibility that some of themare SGR giant flares. We propose that the best target for theobservations of extragalactic SGR flares with Swift is the Virgocluster.
| 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.
| Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Imaging Molecular Gas in the Luminous Merger NGC 3256: Detection of High-Velocity Gas and Twin Gas Peaks in the Double Nucleus
Molecular gas in the merging starburst galaxy NGC 3256 has been imagedwith the Submillimeter Array at a resolution of1''×2'' (170×340 pc at 35 Mpc). Thisis the first interferometric imaging of molecular gas in the mostluminous galaxy within z=0.01. There is a large disk of molecular gas(r>3 kpc) in the center of the merger with a strong gas concentrationtoward the double nucleus. The gas disk having a mass of~3×109 Msolar in the central 3 kpc rotatesaround a point between the two nuclei that are 850 pc apart on the sky.The molecular gas is warm and turbulent and shows spatial variation ofthe intensity ratio between CO isotopomers. High-velocity molecular gasis discovered at the galactic center. Its velocity in our line of sightis up to 420 km s-1 offset from the systemic velocity of thegalaxy; the terminal velocity is twice as large as that due to therotation of the main gas disk. The high-velocity gas is most likely dueto a molecular outflow from the gas disk, entrained by thestarburst-driven superwind in the galaxy. The molecular outflow isestimated to have a rate of ~10 Msolar yr-1 and toplay a significant role in the dispersal or depletion of molecular gasfrom the galactic center. A compact gas concentration and steep velocitygradient are also found around each of the twin nuclei. They aresuggestive of a small gas disk rotating around each nucleus. If theseare indeed minidisks, their dynamical masses are ~109Msolar within a radius of 170 pc.
| Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from Spitzer MIPS Observations
We use very deep Spitzer MIPS 24 μm observations to examine thebolometric luminosities (Lbol) and UV extinction propertiesof more than 200 spectroscopically identified, optically selected(UnGR) z~2 galaxies, supplemented with near-IR-selected(``BzK'' and ``DRG'') and submillimeter galaxies at similar redshifts,in the GOODS-N field. Focusing on redshifts 1.51012 Lsolar, with a mean~=2×1011 Lsolar. Using24 μm observations as an independent probe of dust extinction, wefind that, as in the local universe, the obscurationLIR/L1600 is strongly dependent on Lboland ranges in value from <1 to ~1000 within the sample considered.However, the obscuration is generally ~10 times smaller at a givenLbol at z~2 than at z~0. We show that the values ofLIR and obscuration inferred from the UV spectral slopeβ generally agree well with the values inferred fromL5-8.5μm for Lbol<1012Lsolar. Using the specific SFRs of galaxies as a proxy forcold gas fraction, we find a wide range in the evolutionary state ofgalaxies at z~2, from galaxies that have just begun to form stars tothose that have already accumulated most of their stellar mass and areabout to become, or already are, passively evolving.Based, in part, on data obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation. Alsobased in part on observations made with the Spitzer Space Telescope,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under a contract with NASA.
| Spectroscopic Confirmation of a Large Population of Active Galactic Nuclei in Clusters of Galaxies
We have completed a spectroscopic survey of X-ray point sources in eightlow-redshift clusters of galaxies (0.051041)~5%.We stress that additional, lower luminosity AGNs are expected to bepresent in the MR<-20 mag cluster members. Our dataunambiguously demonstrate that cluster galaxies host AGNs morefrequently than previously expected. Only four of these galaxies haveobvious visible-wavelength AGN signatures, even though their X-rayluminosities are too high for their X-ray emission to be due topopulations of low-mass X-ray binaries or hot, gaseous halos. Weattribute the significant difference in visible and X-ray AGNidentification to dilution of low-luminosity AGN spectral signatures byhost galaxy starlight and/or obscuration of accretion onto the central,supermassive black hole.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT HRI Observations. II. Statistical Properties
The statistical properties of the nonnuclear X-ray point sources fromthe ROSAT HRI survey of nearby galaxies in Paper I are studied, withparticular attention to the contamination from background and/orforeground objects. This study reveals a statistical preference for theultraluminous X-ray sources (ULXs) to occur in late-type galaxies overearly-type galaxies, and in starburst/H II galaxies over nonstarburstgalaxies. There is a trend of greater occurrence frequencies and ULXrates for galaxies with increasing star formation rates, confirming theconnection between the ULX phenomenon and the star formation. Anonlinear correlation is found between the number of ULXs and the starformation rate, with significantly more ULXs at low star formation ratesthan the ULX population expected from the high-mass X-ray binaries(HMXBs) as an indicator of the star formation and the accompanying youngstellar population, suggestive of another population of ULXs associatedwith the low-mass X-ray binaries (LMXBs) and the old stellar population.There are no breaks around 1039 ergs s-1 in theluminosity functions of ULXs in all galaxies or in late-type galaxies,suggesting the regular ULXs below 1040 ergs s-1are a high-luminosity extension of the ordinary HMXB/LMXB populationsbelow 1039 ergs s-1. There is evidence that theextreme ULXs above 1040 ergs s-1 might be adifferent ULX class from the regular ULXs below 1040 ergss-1, although a larger sample with more ULXs is needed toestablish the statistical properties of the extreme ULXs as a class.
| Spitzer Number Counts of Active Galactic Nuclei in the GOODS Fields
We present mid-infrared observations of active galactic nuclei (AGNs) inthe GOODS fields, performed with the Spitzer Space Telescope. These arethe deepest infrared and X-ray fields to date and cover a total area of~0.1 deg2. AGNs are selected on the basis of their hard (2-8keV) X-ray emission. The median AGN infrared luminosity is at least 10times larger than the median for normal galaxies with the same redshiftdistribution, suggesting that the infrared emission is dominated by thecentral nucleus. The X-ray-to-infrared luminosity ratios of GOODS AGNs,most of which are at 0.5<~z<~1.5, are similar to the valuesobtained for AGNs in the local universe. The observed infrared fluxdistribution has an integral slope of ~1.5, and there are 1000 sourcesper square degree brighter than ~50 μJy at ~3-6 μm. The countsapproximately match the predictions of models based on AGN unification,in which the majority of AGNs are obscured. This agreement confirms thatthe faintest X-ray sources, which are dominated by the host galaxy lightin the optical, are obscured AGNs. Using these Spitzer data, the AGNcontribution to the extragalactic infrared background light iscalculated by correlating the X-ray and infrared catalogs. This islikely to be a lower limit given that the most obscured AGNs are missedin X-rays. We estimate the contribution of AGNs missed in X-rays, usinga population synthesis model, to be ~45% of the observed AGNcontribution, making the AGN contribution to the infrared background atmost ~2%-10% in the 3-24 μm range, depending on wavelength, lowerthan most previous estimates. The AGN contribution to the infraredbackground remains roughly constant with source flux in the IRAC bandsbut decreases with decreasing flux in the MIPS 24 μm band, where thegalaxy population becomes more important.
| A Survey of Merger Remnants. II. The Emerging Kinematic and Photometric Correlations
This paper is the second in a series exploring the properties of 51optically selected, single-nuclei merger remnants. Spectroscopic datahave been obtained for a subsample of 38 mergers and combined withpreviously obtained infrared photometry to test whether mergers exhibitthe same correlations as elliptical galaxies among parameters such asstellar luminosity and distribution, central stellar velocity dispersion(σ0), and metallicity. Paramount to the study is totest whether mergers lie on the fundamental plane. Measurements ofσ0 have been made using the Ca triplet absorption lineat 8500 Å for all 38 mergers in the subsample. Additionalmeasurements of σ0 were made for two of the mergers inthe subsample using the CO absorption line at 2.29 μm. The resultsindicate that mergers show a strong correlation among the parameters ofthe fundamental plane but fail to show a strong correlation betweenσ0 and metallicity (Mg2). In contrast toearlier studies, the σ0 values of the mergers areconsistent with objects that lie somewhere between intermediate-mass andluminous giant elliptical galaxies. However, the discrepancies withearlier studies appear to correlate with whether the Ca triplet or COabsorption lines are used to derive σ0, with the latteralmost always producing smaller values. Finally, the photometric andkinematic data are used to demonstrate for the first time that thecentral phase-space densities of mergers are equivalent to those inelliptical galaxies. This resolves a long-standing criticism of themerger hypothesis.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| High spatial resolution mid-infrared spectroscopy of the starburst galaxies NGC 3256, II Zw 40 and Henize 2-10
Aims.In order to show the importance of high spatial resolutionobservations of extra-galactic sources when compared to observationsobtained with larger apertures such as ISO, we present N-band spectra(8-13 μm) of some locations in three starburst galaxies. Inparticular, we show the two galactic nuclei of the spiral galaxy NGC3256, the compact IR supernebula in the dwarf galaxy II Zw 40 and thetwo brightest IR knots in the central starburst of the WR galaxy He2-10. Methods: .The spectra were obtained with TIMMI2 on the ESO3.6 m telescope. An inventory of the spectra in terms of atomicfine-structure lines and molecular bands is presented. Results:.We show the value of these high spatial resolution data in constrainingproperties such as the extinction in the mid-IR, metallicity or stellarcontent (age, IMF, etc.). We have constrained the stellar content of theIR compact knot in II Zw 40 by using the mid-IR fine-structure lines andsetting restrictions on the nebular geometry. We have constructed a newmid-/far-IR diagnostic diagram based on the 11.2 μm PAH andcontinuum, accessible to ground-based observations. We find thatextra-galactic nuclei and star clusters observed at high spatialresolution (as is the case of the TIMMI2 observations) are closer inPAH/far-IR to compact H II regions, while galaxies observed by largeapertures such as ISO are closer to exposed PDRs such as Orion. This islikely due to the aperture difference. We find a dependence between thepresence of PAHs and the hardness of the radiation field as measured bythe [S IV]/[Ne II] ratio that may be explained by the PAH-dustcompetition for FUV photons or the relative contribution of thedifferent phases of the interstellar medium.
| 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.
| Mid infrared properties of distant infrared luminous galaxies
We present evidence that the mid infrared (MIR, rest frame 5-30 μm)is a good tracer of the total infrared luminosity, L(IR)(=L[8{-}1000μm]), and star formation rate (SFR), of galaxies up to z 1.3. Weuse deep MIR images from the Infrared Space Observatory (ISO) and theSpitzer Space Telescope in the Northern field of the Great ObservatoriesOrigins Deep Survey (GOODS-N) together with VLA radio data to computethree independant estimates of L(IR). The L(IR, MIR) derived from theobserved 15 and/or 24 μm flux densities using a library of templateSEDs, and L(IR, radio), derived from the radio (1.4 and/or 8.5 GHz)using the radio-far infrared correlation, agree with a 1-σdispersion of 40%. We use the k-correction as a tool to probe differentparts of the MIR spectral energy distribution (SED) of galaxies as afunction of their redshift and find that on average distant galaxiespresent MIR SEDs very similar to local ones. However, in the redshiftrange z= 0.4-1.2, L(IR, 24 μm) is in better agreement with L(IR,radio) than L(IR, 15 μm) by 20%, suggesting that the warm dustcontinuum is a better tracer of the SFR than the broad emission featuresdue to polycyclic aromatic hydrocarbons (PAHs). We find marginalevidence for an evolution with redshift of the MIR SEDs: two thirds ofthe distant galaxies exhibit rest-frame MIR colors (L(12 μm)/L(7μm) and L(10 μm)/L(15 μm) luminosity ratios) below the medianvalue measured for local galaxies. Possible explanations are examinedbut these results are not sufficient to constrain the physics of theemitting regions. If confirmed through direct spectroscopy and if itgets amplified at higher redshifts, such an effect should be consideredwhen deriving cosmic star formation histories of dust-obscured galaxies.We compare three commonly used SED libraries which reproduce thecolor-luminosity correlations of local galaxies with our data anddiscuss possible refinements to the relative intensities of PAHs, warmdust continuum and silicate absorption.
| Young star cluster complexes in NGC 4038/39. Integral field spectroscopy using VIMOS-VLT
We present the first results of a survey to obtain Integral FieldSpectroscopy of merging galaxies along the Toomre Sequence. In thepresent work, we concentrate on the star cluster complexes in theAntennae galaxies (NGC 4038/39) in the overlap region as well as thenuclear region of NGC 4038. Using optical spectroscopy we derive theextinction, age, metallicity, velocity, velocity dispersion of the gasand star formation rate for each of the eight complexes detected. Wesupplement this study with archival HST-WFPC2 U, B, V, Hα, and Iband imaging. Correcting the observed colours of the star clusterswithin the complexes for extinction, measured through our opticalspectra, we compare the clusters with simple stellar population models,with which we find an excellent agreement, and hence proceed to derivethe ages and masses of the clusters from comparison with the models. Infive of the complexes we detect strong Wolf-Rayet emission features,indicating young ages (3-5 Myr). The ionized gas surrounding thecomplexes is expanding at speeds of 20{-}40 km s-1. This slowexpansion can be understood as a bubble, caused by the stellar winds andsupernovae within the complexes, expanding into the remnant of theprogenitor giant molecular cloud. We also find that the complexesthemselves are grouped, at about the largest scale of which young starclusters are correlated, representing the largest coherent star formingregion. We show that the area normalized star formation rates of thesecomplexes clearly place them in the regime of star forming regions instarburst galaxies, thereby justifying the label of localizedstarbursts. Finally, we estimate the stability of the complexes, andfind that they will probably loose a large fraction of their mass to thesurrounding environment, although the central regions may merge into asingle large star cluster.
| Chandra Observation of the Starburst Galaxy NGC 2146
We present six monitoring observations of the starburst galaxy NGC 2146using the Chandra X-ray Observatory. We detected 67 point sources in thefield of view of the ACIS-S detector. Six of these sources wereUltra-Luminous X-ray Sources, the brightest of which had a luminosity of5 × 1039 erg s-1. One of them, with a luminosity of 1× 1039 erg s-1, is coincident with the dynamical center location,which may be a low-luminosity active galactic nucleus. We have produceda table where the positions and main characteristics of the detectedsources are reported. A comparison between the positions of the X-raysources and those detected in NIR or radio indicates no definitecounterpart. We have derived a log N ‑ log S relation and aluminosity function. The luminosity function has a slope of 0.71 above adetection limit, which is similar to those found in other starburstgalaxies. Diffuse emissions were detected in both soft (0.5-2.0keV) andhard (2.0-10.0keV) energy bands. The spectra of the diffuse componentwere fitted with two (hard and soft) components. The hard power-lawcomponent, with a luminosity of 4 × 1039 erg s-1, is likelyto have originated by unresolved point sources.
| Chandra observations of the interacting galaxies NGC 3395/3396 (Arp 270)
In this paper we present the results of a 20-ks high-resolution ChandraX-ray observation of the peculiar galaxy pair NGC 3395/3396, a system ata very early stage of merging, and less evolved than the famous Antennaeand Mice merging systems. Previously unpublished ROSAT High-ResolutionImager data are also presented. The point-source population and the hotdiffuse gas in this system are investigated and compared with othermerging galaxy pairs.16 X-ray point sources are detected in Arp 270, seven of which areclassified as ultraluminous X-ray sources (ULXs, LX>=1039 erg s-1). From spectral fits and the age ofthe system it seems likely that these are predominantly high-mass X-raybinaries. The diffuse gas emits at a global temperature of ~0.5 keV,consistent with temperatures observed in other interacting systems, andwe see no evidence of the starburst-driven hot gaseous outflows seen inmore evolved systems such as The Mice and The Antennae. It is likelythat these features are absent from Arp 270 as the gas has hadinsufficient time to break out of the galaxy discs. 32 per cent of theluminosity of Arp 270 arises from the diffuse gas in the system, this islow when compared with later stage merging systems and gives furthercredence that this is an early-stage merger.Comparing the ULX population of Arp 270 to other merging systems, wederive a relationship between the star formation rate of the system,indicated by LFIR, and the number [N(ULX)] and luminosity(LULX) of its ULX population. We find N(ULX)~L0.18FIR andLULX~L0.54FIR. These relationships,coupled with the relation of the point-source X-ray luminosity(LXP) to LK and LFIR+UV (Colbert et al.2003), indicate that the ULX sources in an interacting system havecontributions from both the old and young stellar populations.
| The XMM-Newton Needles in the Haystack Survey: the local X-ray luminosity function of `normal' galaxies
In this paper we estimate the local (z < 0.22) X-ray luminosityfunction of `normal' galaxies derived from the XMM-Newton Needles in theHaystack Survey. This is an on-going project that aims to identifyX-ray-selected normal galaxies (i.e. non-AGN dominated) in the localUniverse. We are using a total of 70 XMM-Newton fields covering an areaof 11 deg2 which overlap with the Sloan Digital Sky SurveyData Release 2. Normal galaxies are selected on the basis of theirresolved optical light profile, their low X-ray-to-optical flux ratio[log(fx/fo) < - 2] and soft X-ray colours. Wefind a total of 28 candidate normal galaxies to the 0.5-8keV band fluxlimit of ~2 × 10-15ergcm-2s-1.Optical spectra are available for most sources in our sample (82 percent). These provide additional evidence that our sources are bona fidenormal galaxies with X-ray emission coming from diffuse hot gas emissionand/or X-ray binaries rather than a supermassive black hole. 16 of ourgalaxies have narrow emission lines or a late-type spectral energydistribution (SED) while the remaining 12 present only absorption linesor an early-type SED. Combining our XMM-Newton sample with 18 local (z< 0.22) galaxies from the Chandra Deep Field North and South surveys,we construct the local X-ray luminosity function of normal galaxies.This can be represented with a Schechter form with a break atL*~ 3+1.4-1.0×1041ergs-1 and a slope of α~ 1.78 +/- 0.12.Using this luminosity function and assuming pure luminosity evolution ofthe form ~(1 +z)3.3 we estimate a contribution to the X-raybackground from normal galaxies of ~10-20 per cent (0.5-8keV). Finally,we derive, for the first time, the luminosity functions for early- andlate-type systems separately.
| Infrared mergers and infrared quasi-stellar objects with galactic winds - III. Mrk 231: an exploding young quasi-stellar object with composite outflow/broad absorption lines (and multiple expanding superbubbles)
We present a study of outflow (OF) and broad absorption line (BAL)systems in Mrk 231, and in similar infrared (IR) quasi-stellar objects(QSOs). This study is based mainly on one-dimensional andtwo-dimensional spectroscopy (obtained at La Palma/William HerschelTelescope, Hubble Space Telescope, International Ultraviolet Explorer,European Southern Observatory/New Technology Telescope, Kitt PeakNational Observatory, Apache Point Observatory and Complejo AstronomicoEl Leoncito observatories) plus Hubble Space Telescope images. For Mrk231, we report evidence that the extreme nuclear OF process has at leastthree main components on different scales, which are probably associatedwith: (i) the radio jet, at parsec scale; (ii) the extreme starburst atparsec and kiloparsec scale. This OF has generated at least fourconcentric expanding superbubbles and the BAL systems.Specifically, inside and very close to the nucleus the two-dimensionalspectra show the presence of an OF emission bump in the blendHα+[NII], with a peak at the same velocity of the main BAL-Isystem (VEjectionBAL-I~-4700 km s-1). This bumpwas more clearly detected in the area located at 0.6-1.5 arcsec(490-1220 pc), to the south-west of the nucleus core, showing a strongand broad peak. In addition, in the same direction [at position angle(PA) ~-120°, i.e. close to the PA of the small-scale radio jet] at1.7-2.5 arcsec, we also detected multiple narrow emission-linecomponents, with `greatly' enhanced [NII]/Hα ratio (very similarto the spectra of jets bow shocks). These results suggest that the BAL-Isystem is generated in OF clouds associated with the parsec-scale jet.The Hubble Space Telescope images show four (or possibly five) nuclearsuperbubbles or shells with radii r~ 2.9, 1.5, 1.0, 0.6 and 0.2 kpc. Forthese bubbles, the two-dimensional Hα velocity field map andtwo-dimensional spectra show the following. (i) At the border of themore extended bubble (S1), a clear expansion of the shell withblueshifted velocities (with circular shape and at a radius r~ 5.0arcsec). This bubble shows a rupture arc - to the south - suggestingthat the bubble is in the blowout phase. The axis of this rupture orejection (at PA ~ 00°) is coincident with the axis of theintermediate and large-scale structures detected at radio wavelengths.(ii) In addition, in the three more external bubbles (S1, S2, S3), thetwo-dimensional William Herschel Telescope spectra show multipleemission-line components with OF velocities, of S1, S2 and S3 =[-(650 - 420) +/- 30], [-500+/- 30] and [-230 +/- 30] km s-1. (iii) In the wholecircumnuclear region (1.8 < r < 5 arcsec), the [NII]/Hα and[SII]/Hα narrow emission-line ratios show high values (>0.8),which are consistent with low-ionization nuclear emission-line region/OFprocesses associated with fast velocity shocks. Therefore, we suggestthat these giant bubbles are associated with the large-scale nuclear OFcomponent, which is generated - at least in part - by the extremenuclear starburst: giant supernova/hypernova explosions.The variability of the short-lived BAL-III NaI D system was studied,covering almost all the period in which this system appeared (between~1984 and 2004). We have found that the BAL-III light curve is clearlyasymmetric with a steep increase, a clear maximum and an exponentialfall (similar to the shape of a supernova light curve). The origin ofthis BAL-III system is discussed, mainly in the framework of an extremeexplosive event, probably associated with giant supernova/hypernovaexplosions.Finally, the IR colour diagram and the ultraviolet BAL systems of IR +GW/OF + FeII QSOs are analysed. This study shows two new BAL IR QSOs andsuggests/confirms that these objects could be nearby young BAL QSOs,similar to those detected recently at z~ 6.0. We propose that the phaseof young QSOs is associated with accretion of a large amount of gas (bythe supermassive black hole) + extreme starbursts + extreme compositeOFs/BALs.
| An atlas of calcium triplet spectra of active galaxies
We present a spectroscopic atlas of active galactic nuclei covering theregion around the λλ8498, 8542, 8662 calcium triplet(CaT). The sample comprises 78 objects, divided into 43 Seyfert 2s, 26Seyfert 1s, three starburst and six normal galaxies. The spectra pertainto the inner ~300 pc in radius, and thus sample the central kinematicsand stellar populations of active galaxies. The data are used to measurestellar velocity dispersions (σ*) with bothcross-correlation and direct fitting methods. These measurements arefound to be in good agreement with each other and with those in previousstudies for objects in common. The CaT equivalent width is alsomeasured. We find average values and sample dispersions ofWCaT of 4.6 +/- 2.0, 7.0 +/- 1.0 and 7.7 +/- 1.0 Å forSeyfert 1s, Seyfert 2s and normal galaxies, respectively. We furtherpresent an atlas of [SIII]λ9069 emission-line profiles for asubset of 40 galaxies. These data are analysed in a companion paperwhich addresses the connection between stellar and narrow-line regionkinematics, the behaviour of the CaT equivalent width as a function ofσ*, activity type and stellar population properties.
| 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.
| A radio study of the superwind galaxy NGC 1482
We present multifrequency radio continuum as well as HI observations ofthe superwind galaxy NGC 1482, with both the Giant Metrewave RadioTelescope (GMRT) and the Very Large Array (VLA). This galaxy has aremarkable hourglass-shaped optical emission-line outflow as well asbipolar soft X-ray bubbles on opposite sides of the galactic disc. Thelow-frequency, lower-resolution radio observations show a smoothstructure. From the non-thermal emission, we estimate the availableenergy in supernovae, and examine whether this would be adequate todrive the observed superwind outflow. The high-frequency,high-resolution radio image of the central starburst region located atthe base of the superwind bi-cone shows one prominent peak and moreextended emission with substructure. This image has been compared withthe infrared, optical red continuum, Hα, and soft and hard X-rayimages from Chandra to understand the nature and relationship of thevarious features seen at different wavelengths. The peak of the infraredemission is the only feature that is coincident with the prominent radiopeak, and possibly defines the centre of the galaxy.The HI observations with the GMRT show two blobs of emission on oppositesides of the central region. These are rotating about the centre of thegalaxy and are located at ~2.4 kpc from it. In addition, theseobservations also reveal a multicomponent HI absorption profile againstthe central region of the radio source, with a total width of ~250 kms-1. The extreme blue- and redshifted absorption componentsare at 1688 and 1942 km s-1, respectively, while the peakabsorption is at 1836 km s-1. This is consistent with theheliocentric systemic velocity of 1850 +/- 20 km s-1,estimated from a variety of observations. We discuss possibleimplications of these results.
| Dynamics and star formation activity of CG J1720-67.8 unveiled through integral field spectroscopy and radio observations
CG J1720-67.8 is an ultra compact group of several galaxies with alow-velocity dispersion, and displaying the hallmarks of mutualinteraction and possible tidal dwarf galaxy formation. In hierarchicalmodels, the system is a possible precursor to a massive ellipticalgalaxy. In this paper, we use new optical integral field spectroscopicand radio observations to investigate the evolutionary status of thegroup in more detail: global star formation rates are estimated usingHα and 1.4-GHz radio continuum measurements; HI observationsprovide an upper limit to the global neutral gas content; opticalbroadband colours and spectra provide ages and stellar mass estimatesfor the tidal dwarf candidates; the bi-dimensional Hα velocityfield is used to trace the kinematics of the group and its members,which are compared with numerical simulations of galaxy encounters. Theobservations suggest a model in which multiple interactions haveoccurred, with the latest strong encounter involving at least two majorcomponents within the last 200Myr. Debris from the encounter fuelsongoing star formation at the global level of~20Msolaryr-1, with self-gravity within the tidaltail possibly providing a mechanism to enhance the star formation rateof the tidal dwarf candidates, with bursts of star formation in clumpsof mass ~2 × 107 Msolar appearing within thelast 10Myr. The amount of time required for final merging of all groupcomponents remains uncertain.
| ChaMPlane Optical Survey: Mosaic Photometry
The Chandra Multiwavelength Plane (ChaMPlane) survey to identify andanalyze the serendipitous X-ray sources in deep Galactic plane fieldsincorporates the ChaMPlane Optical Survey, which is one of NOAO'sLong-term Survey Programs. We started this optical imaging survey in2000 March and completed it in 2005 June. Using the NOAO 4 m telescopeswith the Mosaic cameras at CTIO and KPNO, deep images of the ChaMPlanefields are obtained in V, R, I, and Hα bands. This paper describesthe process of observation, data reduction, and analysis of fieldsincluded in the ChaMPlane Optical Survey and describes the search forHα emission objects and Chandra optical counterparts. Weillustrate these procedures using the ChaMPlane field for the black holeX-ray binary GRO J0422+32 as an example.
| XMM-Newton View of the Ultraluminous X-Ray Sources in M51
We present results based on XMM-Newton observations of the nearby spiralgalaxy M51 (NGC 5194 and NGC 5195). We confirm the presence of the sevenknown ultraluminous X-ray sources (ULXs) with luminosities exceeding theEddington luminosity for a 10 Msolar black hole, alow-luminosity active galactic nucleus (LLAGN) with 2-10 keV luminosityof 1.6×1039 ergs s-1, and soft thermalextended emission from NGC 5194 detected with Chandra. In addition, wealso detected a new ULX with luminosity of ~1039 ergss-1. We have studied the spectral and temporal properties ofthe LLAGN and eight ULXs in NGC 5194 and an ULX in NGC 5195. Two ULXs inNGC 5194 show evidence for short-term variability, and all but two ULXsvary on long timescales (over a baseline of ~2.5 yr), providing strongevidence that these are accreting sources. One ULX in NGC 5194, source69, shows possible periodic behavior in its X-ray flux. We derive aperiod of 5925+/-200 s at a confidence level of 95% on the basis ofthree cycles. This period is lower than the period of 7620+/-500 sderived from a Chandra observation in 2000. The higher effective area ofXMM-Newton enables us to identify multiple components in the spectra ofULXs. Most ULXs require at least two components, a power law and a softX-ray excess component that is modeled by an optically thin plasma or amulticolor disk blackbody (MCD). However, the soft excess emissionsinferred from all ULXs except source 69 are unlikely to be physicallyassociated with the ULXs, as their strengths are comparable to that ofthe surrounding diffuse emission. The soft excess emission of source 69is well described either by a two-temperature MEKAL plasma or asingle-temperature MEKAL plasma (kT~690 eV) and an MCD (kT~170 eV). TheMCD component suggests a cooler accretion disk compared to those inGalactic X-ray binaries, consistent with those expected forintermediate-mass black holes (IMBHs). An iron Kα line (EW~700 eV)or K absorption edge at ~7.1 keV is present in the EPIC pn spectrum ofsource 26. The spectrum of the ULX in NGC 5195, source 12, is consistentwith a simple power law. The LLAGN in NGC 5194 shows an extremely flathard X-ray power law (Γ~0.7), a narrow iron Kα line at 6.4keV (EW~3 keV), and strong soft X-ray excess emission. The full-bandspectrum is well described by a two-component MEKAL plasma andreflection from cold material such as a putative torus.
| Mid-Infrared Spectra of Classical AGNs Observed with the Spitzer Space Telescope
Full low-resolution (65
| The X-Ray Spectral Properties of SCUBA Galaxies
Deep SCUBA surveys have uncovered a large population of massivesubmillimeter-emitting galaxies (SMGs; f850μm>~4 mJy)at z>~1. Although it is generally believed that these galaxies hostintense star formation activity, there is growing evidence that asubstantial fraction also harbor an active galactic nucleus (AGN; i.e.,an accreting super-massive black hole [SMBH]). We present here possiblythe strongest evidence for this viewpoint to date: the combination ofultradeep X-ray observations (the 2 Ms Chandra Deep Field-North) anddeep Keck spectroscopic data of SMGs with radio counterparts. We findthat the majority (~75%) of these radio-selected spectroscopicallyidentified SMGs host AGN activity; the other ~25% have X-ray propertiesconsistent with star formation (X-ray-derived star formation rates of~1300-2700 Msolar yr-1). The AGNs have propertiesgenerally consistent with those of nearby luminous AGNs(Γ~1.8+/-0.5, NH~1020-1024cm-2, and LX~1043-1044.5ergs s-1), and the majority (~80%) are heavily obscured(NH>~1023 cm-2). We constructcomposite rest-frame 2-20 keV spectra for three different obscurationclasses [NH<1023 cm-2,NH=(1-5)×1023 cm-2, andNH>5×1023 cm-2], which revealfeatures not seen in the individual X-ray spectra. An ~1 keV equivalentwidth Fe Kα emission line is seen in the composite X-ray spectrumof the most heavily obscured AGNs, suggesting Compton-thick or nearCompton-thick absorption. Even taking into account the effects ofabsorption, we find that the average X-ray to far-IR luminosity ratio ofthe AGN-classified SMGs (LX/LFIR=0.004) isapproximately 1 order of magnitude below that found for typical quasars.This result suggests that intense star formation activity (of order~1000 Msolar yr-1) dominates the bolometric outputof these SMGs. However, we also explore the possibility that the X-rayto far-IR luminosity ratio of the AGN components is intrinsically lessthan that found for typical quasars and postulate that some SMGs may beAGN dominated. We investigate the implications of our results for thegrowth of massive black holes, discuss the prospects for deeper X-rayobservations, and explore the scientific potential offered by the nextgeneration of X-ray observatories.
| The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies
LINER nuclei, which are present in many nearby galactic bulges, may bethe manifestation of low-rate or low-radiative-efficiency accretion ontosupermassive central black holes. However, it has been unclear whetherthe compact UV nuclear sources present in many LINERs are clusters ofmassive stars, rather than being directly related to the accretionprocess. We have used the Hubble Space Telescope to monitor the UVvariability of a sample of 17 galaxies with LINER nuclei and compactnuclear UV sources. Fifteen of the 17 galaxies were observed more thanonce, with two to five epochs per galaxy, spanning up to a year. Wedetect significant variability in most of the sample, with peak-to-peakamplitudes from a few percent to 50%. In most cases, correlatedvariations are seen in two independent bands (F250W and F330W).Comparison to previous UV measurements indicates, for many objects,long-term variations by factors of a few over decade timescales.Variability is detected in LINERs with and without detected compactradio cores, in LINERs that have broad Hα wings detected in theiroptical spectra (``LINER 1s''), and in those that do not (``LINER 2s'').This variability demonstrates the existence of a nonstellar component inthe UV continuum of all types and sets a lower limit to the luminosityof this component. Interestingly, all the LINERs that have detectedradio cores have variable UV nuclei, as one would expect from bona fideactive galactic nuclei. We note a trend in the UV color (F250W/F330W)with spectral type-LINER 1s tend to be bluer than LINER 2s. This trendmay indicate a link between the shape of the nonstellar continuum andthe presence or the visibility of a broad-line region. In one target,the poststarburst galaxy NGC 4736, we detect variability in a previouslynoted UV source that is offset by 2.5" (~60 pc in projection) from thenucleus. This may be the nearest example of a binary active nucleus andof the process leading to black hole merging.Based on observations with the Hubble Space Telescope, which is operatedby AURA, Inc., under NASA contract NAS 5-26555.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
