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|The orbital period of the ultraluminous X-ray source in M 82.|
The ultraluminous x-ray source (ULX) in the galaxy M82 has beenidentified as a possible intermediate-mass black hole. We have foundthat the x-ray flux from M82 is modulated with a peak-to-peak amplitudecorresponding to an isotropic luminosity of 2.4x10^40 erg/s in M82 and aperiod of 62.0 +/- 2.5 days, which we interpret as the orbital period ofthe ULX binary. This orbital period implies that the mass-donor starmust be a giant or supergiant. Large mass-transfer rates, sufficient tofuel the ULX, are expected for a giant-phase mass donor in an x-raybinary. The giant phase has a short lifetime, indicating that we see theULX in M82 in a brief and unusual period of its evolution.
|Mid-Infrared All-Sky Survey with the Infrared Camera (IRC) on Board the ASTRO-F Satellite|
An all-sky survey in two mid-infrared bands covering wavelengths from 6to 12 and 14 to 26 μm, with a spatial resolution of ~9.4"-10", willbe performed with the Infrared Camera (IRC) on board the ASTRO-Finfrared astronomical satellite. The expected detection limit for pointsources is 80-130 mJy (5 σ). The all-sky survey will provide datawith a detection limit and a spatial resolution an order of magnitudedeeper and higher, respectively, than those of the Infrared AstronomicalSatellite survey. The IRC is optimally designed for deep imaging instaring observations. It employs 256 × 256 Si:As IBC infraredfocal plane arrays for the two mid-infrared channels. In order to makeobservations with the IRC during the scanning observations for theall-sky survey, a new method of operation for the arrays has beendeveloped-``scan mode'' operation. In the scan mode, only 256 pixels ina single row aligned in the cross-scan direction on the array are usedas the scan detector, and they are sampled every 44 ms. Special care hasbeen taken to stabilize the temperature of the array in scan mode, whichenables the user to achieve a low readout noise, comparable to that inthe imaging mode (20-30 e-). The accuracy of the positiondetermination and the flux measurement for point sources is examinedboth in computer simulations and laboratory tests with the flight modelcamera and moving artificial point sources. In this paper we present thescan mode operation of the array, the results of the computer simulationand the laboratory performance test, and the expected performance of theIRC all-sky survey observations.
|A Multitransition CO Study in the 30 Doradus Complex in the Large Magellanic Cloud|
We have made a multitransition CO study of the largest H II complex, the30 Doradus nebula, in the Large Magellanic Cloud (LMC). This is the mostluminous example of a starburst region in the Local Group. We havesearched for 12CO J=7-->6 emission toward the 30 Doraduscomplex with the Antarctic Submillimeter Telescope and RemoteObservatory (AST/RO), located at 2847 m altitude at the Amundsen-ScottSouth Pole Station. As a result, we have detected a 12COJ=7-->6 emitting cloud near the 30 Doradus complex. The12CO J=7-->6/12CO J=4-->3 line temperatureratio in this region is approximately a factor of 2 higher than thatobserved near the Sgr B2 complex. A radiative transfer calculation usingthe line ratios shows that the core of massive star formation in the LMCis much warmer and denser than that of the Milky Way.
|Programs for laser-AO assisted integral-field spectrometers on ionized flows|
An AO-assisted integral-field spectrograph is becoming the mostefficient tool with which to explore ionized gas outflows. It maps faintspectral lines that diagnose cloud dust content, gas pressure,excitation mechanism, and chemical abundances. Coupled with recentimprovements in photoionization models, the total mass hence flowenergetics can be estimated. Establishing a consistent dynamicalframework requires linking multi-frequency datasets to track the energyflow through its optimal-contrast emission in the various ISM phases. Ishow HST results on AGN, starburst nuclei, and Galactic Herbig-HaroObjects that need complementary 3D spectra at comparable spatialresolution to come soon from laser-guided AO + integral-fieldspectrographs at the William-Herschel and SOAR telescopes.
|Near-infrared integral-field spectroscopy of violent starburst environments|
Near-infrared (NIR) integral-field spectroscopy (IFS) of violentstarburst environments at high spatial (and spectral) resolution has thepotential to revolutionise our ideas regarding the local interactionsbetween the newly formed massive stars and the interstellar medium (ISM)of their host galaxies. To illustrate this point, I present NIR IFSanalysis of the central starburst region of NGC 1140, obtained withCIRPASS on Gemini-South. While strong [FeII] emission is foundthroughout the galaxy, higher-order Brackett emission is predominantlyassociated with the northern starburst region. Based on the spatialdistributions of the [FeII] versus Brackett line emission, I concludethat a galaxy-wide starburst was induced several ×107yr ago, with more recent starburst activity concentrated around thenorthern starburst region. I look forward and discuss the excitingprospects that IFS at higher spatial (and spectral) resolution willallow us trace (i) the massive outflows (“superwinds”)expected to originate in the dense, young massive star clusters commonlyfound in intense starburst environments, and (ii) their impact on thegalaxy’s ISM.
|The ultraluminous X-ray source in M82: an intermediate-mass black hole with a giant companion|
The starburst galaxy M82, at a distance of 12 million light years, isthe host of an unusually bright 2.4-16 × 1040ergs-1 X-ray point source, which is best explained by anaccreting black hole 102 to 104 times more massivethan the Sun. Though the strongest candidate for a so-calledintermediate-mass black hole, the only support stems from the observedluminosity and the 0.05-0.1Hz quasi-periodicity in its signal.Interestingly, the 7-12Myr old star cluster MGG-11 which has beenassociated with the X-ray source is sufficiently dense that anintermediate mass black hole could have been produced in the clustercore via collision runaway. The recently discovered 62.0 +/- 2.5 dperiodicity in the X-ray source X-1 further supports the hypothesis thatthis source is powered by a black hole several hundred times moremassive than the Sun. We perform detailed binary evolution simulationswith an accreting compact object of 10-5000Msolar and findthat the X-ray luminosity, the age of the cluster, the observedquasi-periodic oscillations and the now observed orbital period areexplained best by a black hole of 200-5000Msolar thataccretes material from a 22-25Msolar giant companion in astate of Roche-lobe contact. Interestingly, such a companion star isconsistent with the expectation based on the tidal capture in a youngand dense star cluster such as MGG-11, making the pictureself-consistent.
|HST/STIS optical spectroscopy of five super star clusters in the starburst galaxy M82|
We present optical spectroscopy obtained with the Space TelescopeImaging Spectrograph of five young massive star clusters in thestarburst galaxy M82. A detailed analysis is performed for one cluster`M82-A1' and its immediate environment in the starburst core. FromHubble Space Telescope archive images, we find that it is ellipticalwith an effective radius of 3.0 +/- 0.5pc and is surrounded by a compact(r = 4.5 +/- 0.5pc) HII region. We determine the age and reddening ofM82-A1 using synthetic spectra from population synthesis models byfitting both the continuum energy distribution and the depth of theBalmer jump. We find an age of 6.4 +/- 0.5Myr and a photometric massestimate of M = 7-13 × 105 Msolar. Weassociate its formation with the most recent starburst event 4-6Myr ago.We find that the oxygen abundance of the HII region surrounding M82-A1is solar or slightly higher. The HII region has a high pressure P/k =1-2 × 107 cm-3K. The diffuse gas in region Ahas a slightly lower pressure, which together with the broad Hαemission-line width, suggests that both the thermal and turbulentpressures in the M82 starburst core are unusually high. We discuss howthis environment has affected the evolution of the cluster wind forM82-A1. We find that the high pressure may have caused thepressure-driven bubble to stall. We also obtain spectroscopic ages forclusters B1-2 and B2-1 in the `fossil' starburst region and for theintermediate age clusters F and L. These are consistent with earlierstudies and demonstrate that star formation activity, sufficientlyintense to produce super star clusters, has been going on in M82 duringthe past Gyr, perhaps in discrete and localized episodes.
|Evidence for the strong effect of gas removal on the internal dynamics of young stellar clusters|
We present detailed luminosity profiles of the young massive clustersM82-F, NGC 1569-A and NGC 1705-1 which show significant departures fromequilibrium (King and Elson, Fall & Freeman) profiles. We comparethese profiles with those from N-body simulations of clusters that haveundergone the rapid removal of a significant fraction of their mass as aresult of gas expulsion. We show that the observations and simulationsagree very well with each other, suggesting that these young clustersare undergoing violent relaxation and are also losing a significantfraction of their stellar mass.That these clusters are not in equilibrium can explain the discrepantmass-to-light ratios observed in many young clusters with respect tosimple stellar population models without resorting to non-standardinitial stellar mass functions as claimed for M82-F and NGC 1705-1. Wealso discuss the effect of rapid gas removal on the complete disruptionof a large fraction of young massive clusters (`infant mortality').Finally, we note that even bound clusters may lose >50 per cent oftheir initial stellar mass as a result of rapid gas loss (`infantweight-loss').
|On the mass of dense star clusters in starburst galaxies from spectrophotometry|
The mass of unresolved young star clusters derived fromspectrophotometric data may well be off by a factor of 2 or more oncethe migration of massive stars driven by mass segregation is accountedfor. We quantify this effect for a large set of cluster parameters,including variations in the stellar initial mass function (IMF), theintrinsic cluster mass, and mean mass density. Gas-dynamical modelscoupled with the Cambridge stellar evolution tracks allow us to derive ascheme to recover the real cluster mass given measured half-lightradius, one-dimensional velocity dispersion and age. We monitor theevolution with time of the ratio of real to apparent mass through theparameter η. When we compute η for rich star clusters, we findnon-monotonic evolution in time when the IMF stretches beyond a criticalcut-off mass of 25.5Msolar. We also monitor the rise ofcolour gradients between the inner and outer volume of clusters: we findtrends in time of the stellar IMF power indices overlapping well withthose derived for the Large Magellanic Cloud cluster NGC 1818 at an ageof 30Myr. We argue that the core region of massive Antennae clustersshould have suffered from much segregation despite their low ages. Weapply these results to a cluster mass function, and find that the peakof the mass distribution would appear to observers shifted to lowermasses by as much as 0.2dex. The star formation rate derived for thecluster population is then underestimated by from 20 to 50 per cent.
|15 years of very long baseline interferometry observations of two compact radio sources in Messier 82|
We present the results of a second epoch of 18-cm global very longbaseline interferometry (VLBI) observations, taken on 2001 February 23,of the central kiloparsec of the nearby starburst galaxy Messier 82.These observations further investigate the structural and flux evolutionof the most compact radio sources in the central region of M82. The twomost compact radio objects in M82 have been investigated (41.95+575 and43.31+592). Using this recent epoch of data in comparison with ourprevious global VLBI observations and two earlier epochs of EuropeanVLBI network observations, we measure expansion velocities in the rangeof 1500-2000 kms-1 for 41.95+575 and 9000-11000kms-1 for 43.31+592 using various independent methods. Ineach case, the measured remnant expansion velocities are significantlylarger than the canonical expansion velocity (500 kms-1) ofsupernova remnants (SNRs) within M82 predicted from theoretical models.In this paper, we discuss the implications of these measured expansionvelocities with respect to the high-density environment that the SNRsare expected to reside in within the centre of the M82 starburst.
|Far-infrared detection limits - II. Probing confusion including source confusion|
We present a comprehensive analysis for the determination of theconfusion levels for the current and the next generation of far-infraredsurveys assuming three different cosmological evolutionary scenarios. Weinclude an extensive model for diffuse emission from infrared cirrus inorder to derive absolute sensitivity levels taking into account thesource confusion noise due to point sources, the sky confusion noise dueto the diffuse emission, and instrumental noise. We use our derivedsensitivities to suggest best survey strategies for the current and thefuture far-infrared space missions Spitzer, AKARI (ASTRO-F), Herscheland SPICA. We discuss whether the theoretical estimates are realisticand the competing necessities of reliability and completeness. We findthe best estimator for the representation of the source confusion andproduce predictions for the source confusion using far-infrared sourcecount models. From these confusion limits considering both source andsky confusions, we obtain the optimal, confusion limited redshiftdistribution for each mission. Finally, we predict the cosmicfar-infrared background (CFIRB), which includes information about thenumber and distribution of the contributing sources.
|An ultraluminous X-ray microquasar in NGC5408?|
We studied the radio source associated with the ultraluminous X-raysource in NGC5408 (LX ~ 1040ergs-1).The radio spectrum is steep (index ~ -1), consistent with optically thinsynchrotron emission, not with flat-spectrum core emission. Its fluxdensity (~0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same inthe March 2000 and December 2004 observations, suggesting steadyemission rather than a transient outburst. However, it is orders ofmagnitude higher than expected from steady jets in stellar-massmicroquasar. Based on its radio flux and spectral index, we suggest thatthe radio source is either an unusually bright supernova remnant, or,more likely, a radio lobe powered by a jet from the black hole (BH).Moreover, there is speculative evidence that the source is marginallyresolved with a radius ~30 pc. A faint HII region of similar sizeappears to coincide with the radio and X-ray sources, but its ionizationmechanism remains unclear. Using a self-similar solution for theexpansion of a jet-powered electron-positron plasma bubble, in theminimum-energy approximation, we show that the observed flux and(speculative) size are consistent with an average jet power ~ 7 ×1038ergs-1 ~ 0.1LX ~0.1LEdd, an age ~105 yr, a current velocity ofexpansion ~80 km s-1. We briefly discuss the importance ofthis source as a key to understand the balance between luminosity andjet power in accreting BHs.
|A `super' star cluster grown old: the most massive star cluster in the Local Group|
We independently redetermine the reddening and age of the globularcluster (GC) 037-B327 in M31 by comparing independently obtainedmulticolour photometry with theoretical stellar population synthesismodels. 037-B327 has long been known to have a very large reddeningvalue, which we confirm to be E(B - V) = 1.360 +/- 0.013, in goodagreement with the previous results. We redetermine its most likely ageat 12.4 +/- 3.2 Gyr.037-B327 is a prime example of an unusually bright early counterpart tothe ubiquitous `super' star clusters presently observed in mosthigh-intensity star-forming regions in the local Universe. In order tohave survived for a Hubble time, we conclude that its stellar initialmass function (IMF) cannot have been top-heavy. Using this constraint,and a variety of simple stellar population (SSP) models, we determine aphotometric mass of , somewhat depending on the SSP models used, themetallicity and age adopted and the IMF representation. This mass, andits relatively small uncertainties, makes this object the most massivestar cluster of any age in the Local Group. Assuming that thephotometric mass estimate thus derived is fairly close to its dynamicalmass, we predict that this GC has a (one-dimensional) velocitydispersion of the order of (72 +/- 13) km s-1. As a surviving`super' star cluster, this object is of prime importance for theoriesaimed at describing massive star cluster evolution.
|On the evolution of the black hole: spheroid mass ratio|
We present the results of a study which uses the 3C RR sample ofradio-loud active galactic nuclei to investigate the evolution of theblack hole:spheroid mass ratio in the most massive early-type galaxiesfrom 0 < z < 2. Radio-loud unification is exploited to obtainvirial (linewidth) black hole mass estimates from the 3C RR quasars, andstellar mass estimates from the 3C RR radio galaxies, thereby providingblack hole and stellar mass estimates for a single population ofearly-type galaxies. At low redshift (z<~ 1), the 3C RR sample isconsistent with a black hole:spheroid mass ratio ofMbh/Msph~= 0.002, in good agreement with thatobserved locally for quiescent galaxies of similar stellar mass(Msph~= 5 × 1011Msolar). However,over the redshift interval 0 < z < 2 the 3C RR black hole:spheroidmass ratio is found to evolve as Mbh/Msph~ (1+z)2.07+/-0.76, reaching Mbh/Msph~=0.008 by redshift z~= 2. This evolution is found to be inconsistent withthe local black hole:spheroid mass ratio remaining constant at amoderately significant level (98 per cent). If confirmed, the detectionof evolution in the 3C RR black hole:spheroid mass ratio furtherstrengthens the evidence that, at least for massive early-type galaxies,the growth of the central supermassive black hole may be completedbefore that of the host spheroid.
|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 hot, warm and cold gas in Arp 227 - an evolving poor group|
Arp 227 represents a prototypical example of an interacting mixed pairof galaxies located in a low-density environment. We investigate the gasproperties of the pair in the X-ray, Hα, HI and CO bands. We alsodetect two additional members of the group in HI which indicates thatthe pair constitutes the dominant members of a loose group.The HI distribution shows a tail of gas that connects the spiral member,NGC 470, to the lenticular, NGC 474, showing that the two main membersare currently undergoing interaction. The Hα emission reveals thepresence of secondary components at the centre of NGC 470, superposed onthe main component tracing the rotation of the galaxy. This latter mapsa nearly unperturbed velocity field. The dominant, nearly unperturbedtrend of the kinematics is confirmed by CO observations, althoughrestricted to the centre of the galaxy. The X-ray luminosity of NGC 470is comparable with that of a `normal' spiral galaxy. NGC 474 on theother hand is very gas-poor and has not been detected in Hα. ItsX-ray luminosity is consistent with the low end of the expected emissionfrom discrete sources.Arp 227 as a loose group shows several signatures of galaxy-galaxyinteraction. Our observations suggest the presence of signatures ofinteraction in the overall kinematics of the spiral companion. Theongoing interaction is clearly visible only in the outer HI halo of NGC470. While the large shell system of NGC 474 could be associated with anaccretion event, the secondary components in the Hα profile in thecentre of NGC 470 could be due to the interaction with the companion.The low X-ray luminosity of NGC 470 seems to be a characteristic ofdynamically young systems. All the above evidence suggest that Arp 227is an evolving group in the early phase of its evolution and that itsdrivers are the accretion of faint galaxies and the ongoing large-scaleinteraction between NGC 470 and 474.
|Runaway collisions in young star clusters - II. Numerical results|
We present a new study of the collisional runaway scenario to form anintermediate-mass black hole (IMBH, MBH>~100Msolar) at the centre of a young, compact stellar cluster.The first phase is the formation of a very dense central core of massivestars (M*~= 30-120Msolar) through mass segregationand gravothermal collapse. Previous work established the conditions forthis to happen before the massive stars evolve off the main sequence(MS). In this and a companion paper, we investigate the next stage byimplementing direct collisions between stars. Using a Monte Carlostellar dynamics code, we follow the core collapse and subsequentcollisional phase in more than 100 models with varying cluster mass,size, and initial concentration. Collisions are treated either as ideal,`sticky-sphere' mergers or using realistic prescriptions derived from 3Dhydrodynamics computations. In all cases for which the core collapsehappens in less than the MS lifetime of massive stars (~=3Myr), weobtain the growth of a single very massive star (VMS, M*~=400-4000Msolar) through a runaway sequence of mergers. Massloss from collisions, even for velocity dispersions as high asσv~ 1000kms-1, does not prevent the runaway.The region of cluster parameter space leading to runaway is even moreextended than predicted in previous work because, in clusters withσv > 300kms-1, collisions accelerate(and, in extreme cases, drive) core collapse. Although the VMS growsrapidly to >~1000Msolar in models exhibiting runaway, wecannot predict accurately its final mass. This is because thetermination of the runaway process must eventually be determined by acomplex interplay between stellar dynamics, hydrodynamics, and thestellar evolution of the VMS. In the vast majority of cases, we findthat the time between successive collisions becomes much shorter thanthe thermal time-scale of the VMS. Therefore, our assumption that allstars return quickly to the MS after a collision must eventually breakdown for the runaway product, and the stellar evolution of the VMSbecomes very uncertain. For the same reason, the final fate of the VMS,including its possible collapse to an IMBH, remains unclear.
|Runaway collisions in young star clusters - I. Methods and tests|
We present the methods and preparatory work for our study of thecollisional runaway scenario to form a very massive star (VMS,M* > 400Msolar) at the centre of a young,compact stellar cluster. In the first phase of the process, a very densecentral core of massive stars (M*~= 30-120Msolar)forms through mass segregation and gravothermal collapse. This leads toa collisional stage, likely to result in the formation of a VMS (itselfa possible progenitor for an intermediate-mass black hole) through arunaway sequence of mergers between the massive stars. In this paper, wepresent the runaway scenario in a general astrophysical context. We thenexplain the numerical method used to investigate it. Our approach isbased on a Monte Carlo code to simulate the stellar dynamics ofspherical star clusters, using a very large number of particles (a few105 to several 106). Finally, we report on testcomputations carried out to ensure that our implementation of theimportant physics is sound. In a second paper, we present results frommore than 100 cluster simulations realized to determine the conditionsleading to the collisional formation of a VMS and the characteristics ofthe runaway sequences.
|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.
|Modelling galaxy spectra in presence of interstellar dust - I. The model of interstellar medium and the library of dusty single stellar populations|
The advent of modern infrared astronomy has brought into evidence therole played by the interstellar dust in galaxy formation and evolution.Therefore, to fully exploit modern data, realistic spectrophotometricmodels of galaxies must include this important component of theinterstellar medium (ISM).In this paper, the first of a series of two devoted to modelling thespectra of galaxies of different morphological type in the presence ofdust, we present our description of the dust both in the diffuse ISM andin the molecular clouds (MCs).Our galaxy model contains three interacting components: the diffuse ISM,made of gas and dust, the large complexes of MCs in which active starformation occurs and, finally, the populations of stars that are nolonger embedded in the dusty environment of their parental MCs.Our model for the dust takes into account three components, i.e.graphite, silicates and polycyclic aromatic hydrocarbons (PAHs). Weconsider and adapt to our aims two prescriptions for the sizedistribution of the dust grains and two models for the emission of thedusty ISM. We cross-check the emission and extinction models of the ISMby calculating the extinction curves and the emission for the typicalenvironments of the Milky Way (MW) and the Large and Small MagellanicClouds (LMC and SMC) and by comparing the results with the observationaldata. The final model we have adopted is a hybrid one which stems fromcombining the analysis of Guhathakurta & Draine for the emission ofgraphite and silicates and Puget, Leger & Boulanger for the PAHemission, and using the distribution law of Weingartner & Draine andthe ionization model for PAHs of Weingartner & Draine.We apply the model to calculate the spectral energy distribution (SED)of single stellar populations (SSPs) of different age and chemicalcomposition, which may be severely affected by dust at least in twotypes of stars: the young, massive stars while they are still embeddedin their parental MCs and the intermediate- and low-mass asymptoticgiant branch (AGB) stars when they form their own dust shell around.We use the `ray-tracing' method to solve the problem of radiativetransfer and to calculate extended libraries of SSP SEDs. Particularcare is taken to model the contribution from PAHs, introducing differentabundances of C in the population of very small carbonaceous grains(VSGs) and different ionization states in PAHs. The SEDs of young SSPsare then compared with observational data of star-forming regions offour local galaxies successfully reproducing their SEDs from theultraviolet (UV)-optical regions to the mid- and far-infrared region(MIR and FIR, respectively).
|Revisiting the infrared spectra of active galactic nuclei with a new torus emission model|
We describe improved modelling of the emission by dust in atoroidal-like structure heated by a central illuminating source withinactive galactic nuclei (AGNs). We have chosen a simple but realistictorus geometry, a flared disc, and a dust grain distribution functionincluding a full range of grain sizes. The optical depth within thetorus is computed in detail taking into account the differentsublimation temperatures of the silicate and graphite grains, whichsolves previously reported inconsistencies in the silicate emissionfeature in type 1 AGNs. We exploit this model to study the spectralenergy distributions (SEDs) of 58 extragalactic (both type 1 and type 2)sources using archival optical and infrared data. We find that both AGNand starburst contributions are often required to reproduce the observedSEDs, although in a few cases they are very well fitted by a pure AGNcomponent. The AGN contribution to the far-infrared luminosity is foundto be higher in type 1 sources, with all the type 2 requiring asubstantial contribution from a circumnuclear starburst. Our resultsappear in agreement with the AGN unified scheme, because thedistributions of key parameters of the torus models turn out to becompatible for type 1 and type 2 AGNs. Further support to theunification concept comes from comparison with medium-resolutioninfrared spectra of type 1 AGNs by the Spitzer observatory, showingevidence for a moderate silicate emission around 10 μm, which ourcode reproduces. From our analysis we infer accretion flows in the innernucleus of local AGNs characterized by high equatorial optical depths(AV~= 100), moderate sizes (Rmax < 100 pc) andvery high covering factors (f~= 80 per cent) on average.
|How well do we know the age and mass distributions of the star cluster system in the Large Magellanic Cloud?|
The Large Magellanic Cloud (LMC) star cluster system offers the uniqueopportunity to independently check the accuracy of age (and thecorresponding mass) determinations based on a number of complementarytechniques. Using our sophisticated tool for star cluster analysis basedon broad-band spectral energy distributions (SEDs), `AnalySED', were-analyse the Hunter et al. LMC cluster photometry. Our main aim is toset the tightest limits yet on the accuracy of absolute agedeterminations based on broad-band SEDs, and therefore on the usefulnessof such an approach. Our broad-band SED fits yield reliable ages, withstatistical absolute uncertainties within Δlog(Age/yr) ~= 0.4overall. The systematic differences we find with respect to previous agedeterminations are caused by conversions of the observational photometryto a different filter system, thus leading to systematically inaccurateresults.The LMC's cluster formation rate (CFR) has been roughly constant outsideof the well-known age gap between ~3 and 13Gyr, when the CFR was afactor of ~5 lower. Using a simple approach to derive the characteristiccluster disruption time-scale, we find thatlog(tdis4/yr) = 9.9 +/- 0.1, wheretdis=tdis4(Mcl/104Msolar)0.62.This long characteristic disruption time-scale implies that we areobserving the initial cluster mass function (CMF). We conclude thatwhile the older cluster (sub)samples show CMF slopes that are fullyconsistent with the α~=-2 slopes generally observed in young starcluster systems, the youngest mass and luminosity-limited LMC clustersubsets show shallower slopes (at least below masses of a few×103Msolar), which is contrary to dynamicalexpectations. This may imply that the initial CMF slope of the LMCcluster system as a whole is not well represented by a power law,although we cannot disentangle the unbound from the bound clusters atthe youngest ages.
|A variable Quasi-Periodic Oscillation in M82 X-1. Timing and spectral analysis of XMM-Newton and RossiXTE observations|
We report results from a spectral and timing analysis of M82 X-1, one ofthe brightest known ultraluminous X-ray sources. Data from a new 105-ksXMM-Newton observation of M82 X-1, performed in 2004 April, and ofarchival RossiXTE observations are presented. A very soft thermalcomponent is present in the XMM spectrum. Although it is not possible torule out a residual contamination from the host galaxy, modelling itwith a standard accretion disc would imply a black hole (BH) mass of~103Msolar. An emission line was also detected atan energy typical for fluorescent Fe emission. The power densityspectrum of the XMM observation shows a variable Quasi-PeriodicOscillation (QPO) at frequency of 113 mHz with properties similar tothose discovered by Strohmayer and Mushotzky. The QPO was also found inseven archival RXTE observations, that include those analysed byStrohmayer and Mushotzky, and Fiorito and Titarchuk. A comparison of theproperties of this QPO with those of the various types of QPOs observedin Galactic black hole candidates strongly suggests an association withthe type-C, low-frequency QPOs. Scaling the frequency inversely to theBH mass, the observed QPO frequency range (from 50 to 166 mHz) wouldyield a BH mass anywhere in the interval few tens to 1000Msolar.
|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.
|Unveiling the nature of Ultraluminous Infrared Galaxies with 3-4μm spectroscopy*|
We present the results of L-band spectroscopical observations of localbright Ultraluminous Infrared Galaxies (ULIRGs), performed with theInfrared Spectrometer And Array Camera (ISAAC) at the Very LargeTelescope. The excellent sensitivity of the telescope and of theinstrument provided spectra of unprecedented quality for this class ofobjects, which allowed a detailed study of the active galactic nucleus(AGN)/starburst contribution to the energy output, and of thecomposition of the circumnuclear absorber. We discuss the L-bandspectral features of seven single sources, and the statisticalproperties of a complete sample of 15 sources obtained combining ourobservations with other published 3-4 μm spectra. Our main resultsare as follows. (i) When a spectral indicator suggesting the presenceof an AGN (low equivalent width of the 3.3-μm emission line, steepλ-fλ spectrum, presence of an absorptionfeature at 3.4 μm) is found, the AGN is always confirmed byindependent analysis at other wavelengths. Conversely, in all known AGNsat least one of the above indicators is present. (ii) Two newdiagnostic diagrams are proposed combining the above indicators, inwhich starbursts and AGNs are clearly and completely separated. (iii)The above diagnostic techniques are possible with spectra of relativelylow quality, which can be obtained for several tens of ULIRGs withcurrently available telescopes. This makes L-band spectroscopy thecurrent best tool to disentangle AGN and starburst contributions inULIRGs. (iv) The L-band properties of ULIRGs are heterogeneous.However, we show that all the spectral differences among ULIRGs can bereproduced starting from pure intrinsic AGN and starburst spectra andtwo varying parameters: the amount of dust extinction of the AGNcomponent, and the relative AGN/starburst contribution to the bolometricluminosity. (v) Using the above decomposition model, we show that AGNsin ULIRGs have a low dust-to-gas ratio and a dust extinction curvedifferent from Galactic. (vi) The estimate of the presence andcontribution of AGNs in a complete sample shows that AGNs are hosted byapproximately two-thirds of ULIRGs, but their energetic contribution isrelevant (>30 per cent of the bolometric luminosity) only in ~20 percent of the sample.
|Near-Infrared [Fe II] Emission in Starburst Galaxies. I. Measured Properties|
We used the near-infrared [Fe II] emission line signature to detectsupernova remnants (SNRs) in the nearby starburst galaxies NGC 1569, NGC3738, and NGC 5253. The near-infrared narrowband imaging program has ledto the detection of 10 SNR candidates in NGC 1569, 7 in NGC 5253, andnone in NGC 3738. The luminosity of the SNRs candidates varies from 72to 780 Lsolar and from 69 to 331 Lsolar for NGC1569 and NGC 5253, respectively. Also, a spatially extended component tothe [Fe II] line emission is observed in NGC 1569 and NGC 5253. Thiscomponent dominates the integrated [Fe II] luminosity in both galaxies,the compact sources accounting for 14% and 7% of the total [Fe II]luminosity of NGC 1569 and NGC 5253, respectively.
|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.
|A 2 Millimeter Spectral Line Survey of the Starburst Galaxy NGC 253|
We present the first unbiased molecular line survey toward anextragalactic source, namely the nuclear region of the starburst galaxyNGC 253. The scan covers the frequency band from 129.1 to 175.2 GHz,i.e., most of the 2 mm atmospheric window. We identify 111 spectralfeatures as transitions from 25 different molecular species. Eight ofwhich (three tentatively) are detected for the first time in theextragalactic interstellar medium. Among these newly detected species,we detected the rare isotopomers 34SO andHC18O+. Tentative detections of two deuteratedspecies, DNC and N2D+, are reported for the firsttime from a target beyond the Magellanic Clouds. In addition, threehydrogen recombination lines are identified, while no organic moleculeslarger than methanol are detected. Column densities and rotationtemperatures are calculated for all the species, including an upperlimit to the ethanol abundance. A comparison of the chemical compositionof the nuclear environment of NGC 253 with those of selected nearbygalaxies demonstrates the chemical resemblance of IC 342 and NGC 4945 tothat of NGC 253. On the other hand, the chemistries characterizing NGC253 and M82 are clearly different. We also present a comparison of thechemical composition of NGC 253 with those observed in Galacticprototypical sources. The chemistry of NGC 253 shows a strikingsimilarity with the chemistry observed toward the Galactic centermolecular clouds, which are thought to be dominated by low-velocityshocks. This resemblance strongly suggests that the heating in thenuclear environment of NGC 253 is dominated by the same mechanism asthat in the central region of the Milky Way.
|The Star Formation History of the Disk of the Starburst Galaxy M82|
Spectroscopic, photometric, and dynamical data of the inner 3 kpc of thestarburst galaxy M82 are analyzed in order to investigate the starformation history of its disk. The long-slit spectra along the majoraxis are dominated by Balmer absorption lines in the region outside thenuclear starburst all the way up to ~3.5 scale-lengths(μB=22 mag arcsec-2). Single stellar population(SSP) spectra of age 0.4-1.0 Gyr match the observed spectra in the 1-3kpc zone well, with a marginally higher mean age of the stellarpopulation in the outer parts. The mass in these populations, along withthat in the gas component, makes up for the inferred dynamical mass inthe same annular zone for a Kroupa initial mass function, with alow-mass cutoff of ml=0.4 Msolar. The observedratio of the abundances of α-elements with respect to Fe is alsoconsistent with the idea that almost all the stars in the M82 diskformed in a burst of short duration (0.3 Gyr) around 0.8 Gyr ago. Wefind that the optical/near-infrared colors and their gradients in thedisk are determined by the reddening, with visual extinction exceeding 1mag even in the outer parts of the disk, where there is apparently nocurrent star formation. The disk-wide starburst activity was most likelytriggered by the interaction of M82 with its massive neighbor M81,around 1 Gyr ago. The properties of the disk of M82 very much resemblethe properties of the disks of luminous compact blue galaxies seen atz=0.2-1.0.
|Mapping Large-Scale Gaseous Outflows in Ultraluminous Infrared Galaxies with Keck II ESI Spectra: Spatial Extent of the Outflow|
The kinematics of neutral gas and warm ionized gas have been mappedacross ultraluminous starburst galaxies using the Na Iλλ5890, 5896 absorption-line and Hα emission-lineprofiles, respectively, in Keck II ESI spectra. Blueshifted,interstellar absorption is found over extended regions, exceeding 15 kpcin several systems. An outflow diverging from the nuclear starburstwould have to reach large heights to cover this area in projection. Thescale height of the absorbing material could be lower, however, if theoutflow emanates from a larger region of the galaxy. The large velocitygradient discovered across several outflows is inconsistent with a flowdiverging from the nuclear starburst. Widespread star formation,triggered by the merger, probably drives these extended outflows viamechanical feedback from supernovae, although shocks generated by thegalaxy-galaxy merger may also contribute to the formation of a hot wind.In a typical ULIG, the mass carried by the cool phase of the outflow is~108 Msolar i.e., a few percent of the dynamicalmass in the starburst region. Assuming the starburst activity haspersisted for 10 Myr, the kinetic energy of the cool outflows is a fewpercent of the supernova energy. The cool wind is expected to beaccelerated by momentum deposition, possibly from radiation pressure aswell as the ram pressure of the hot, supernova-induced wind. Theturnaround radii of the cool outflows are at least ~30-90 kpc, whichpresents a significant Na I absorption cross section. If mostL>0.1L* galaxies pass through a luminous starburst phase,then relics of cool outflows will create a significant redshift-pathdensity. Galaxy formation models should include this cool phase of theoutflow in addition to a hot wind in feedback models.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
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