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|RR Lyrae-based calibration of the Globular Cluster Luminosity Function|
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
|A quasar in every galaxy ?|
|The Light Echo around Supernova 2003gd in Messier 74|
We confirm the discovery of a light echo around the Type II-plateausupernova 2003gd in Messier 74 (NGC 628), seen in images obtained withthe High Resolution Channel of the Advanced Camera for Surveys on boardthe Hubble Space Telescope (HST) as part of a larger Snapshot program onthe late-time emission from supernovae. The analysis of the echo wepresent suggests that it is due to the SN light pulse scattered by asheet of dust grains located ~113 pc in front of the SN, and that thesegrains are not unlike those assumed to be in the diffuse Galacticinterstellar medium, both in composition and in size distribution. Theecho is less consistent with scattering off carbon-rich grains, and ifanything, the grains may be somewhat more silicate rich than theGalactic dust composition. The echo also appears to be more consistentwith a SN distance closer to 7 Mpc than to 9 Mpc. This further supportsthe conclusion we reached elsewhere that the initial mass for the SNprogenitor was relatively low (~8-9 Msolar). The HST shouldbe used to continue to monitor the echo in several bands, particularlyin the blue, to better constrain its origin.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute (STScI),which is operated by the Association of Universities for Research inAstronomy (AURA), Inc., under NASA contract NAS 5-26555.
|Spectral Energy Distributions of M81 Globular Clusters in the BATC Multicolor Survey|
In this paper, we give the spectral energy distributions (SEDs) of 42M81 globular clusters in 13 intermediate-band filters from 4000 to 10000Å using the CCD images of M81, observed as part of theBeijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey. TheBATC multicolor filter system is specifically designed to exclude mostof the bright and variable night-sky emission lines, including the OHforest. Hence, it can present accurate SEDs of the observed objects.These SEDs are low-resolution spectra and can reflect the stellarpopulations of the globular clusters. This paper confirms theconclusions of Schroder et al., that M81 contains clusters as young as afew Gyr, which were also observed in both M31 and M33.
|Why the fundamental plane of black hole activity is not simply a distance driven artifact|
The fundamental plane of black hole activity is a non-linear correlationamong radio core luminosity, X-ray luminosity and mass of all accretingblack holes, both of stellar mass and supermassive, found by Merloni etal. [Merloni, A., Heinz, S., di Matteo, T. 2003. MNRAS 345, 1057(MHD03)] and, independently, by Falcke et al. [Falcke, H., Körding,E., Markoff, S., 2004. A& A 414, 895]. Here we further examine anumber of statistical issues related to this correlation. In particular,we discuss the issue of sample selection and quantify the biasintroduced by the effect of distance in two of the correlatedquantities. We demonstrate that the fundamental plane relation cannot bea distance artifact, and that its non-linearity must represent anintrinsic characteristic of accreting black holes. We also discusspossible future observational strategies to improve our understanding ofthis correlation.
|Scalar potential model of redshift and discrete redshift|
On the galactic scale the universe is inhomogeneous and redshift z isoccasionally less than zero. A scalar potential model (SPM) that linksthe galaxy scale z to the cosmological scale z of the Hubble Law ispostulated. Several differences among galaxy types suggest that spiralgalaxies are Sources and that early type, lenticular, and irregulargalaxies are Sinks of a scalar potential field. The morphology-radiusand the intragalactic medium cluster observations support the movementof matter from Source galaxies to Sink galaxies. A cell structure ofgalaxy groups and clusters is proposed to resolve a paradox concerningthe scalar potential like the Olber’s paradox concerning light.For the sample galaxies, the ratio of the luminosity of Source galaxiesto the luminosity of Sink galaxies approaches 2.7 ± 0.1. Anequation is derived from sample data, which is anisotropic andinhomogeneous, relating z of and the distance D to galaxies. Thecalculated z has a correlation coefficient of 0.88 with the measured zfor a sample of 32 spiral galaxies with D calculated using Cepheidvariable stars. The equation is consistent with z < 0 observations ofclose galaxies. At low cosmological distances, the equation reduces to z≈ exp(KD)‑1 ≈ KD, where K is a constant, positive value. Theequation predicts z from galaxies over 18 Gpc distant approaches aconstant value on the order of 500. The SPM of z provides a physicalbasis for the z of particle photons. Further, the SPM qualitativelysuggests the discrete variations in z, which was reported by Tifft[Tifft, W.G., 1997. Astrophy. J. 485, 465] and confirmed by others, areconsistent with the SPM.
|A post-mortem investigation of the Type IIb supernova 2001ig|
We present images taken with the Gemini Multi-Object Spectrograph onGemini-South, in excellent (<0.5 arcsec) seeing, of supernova (SN)2001ig in NGC 7424, ~1000 d after explosion. A point source seen at thesite of the SN is shown to have colours inconsistent with being an HIIregion or a SN 1993J-like remnant, but can be matched to a late-Bthrough late-F supergiant with AV < 1. We believe thisobject is the massive binary companion responsible for periodicmodulation in mass-loss material around the Wolf-Rayet progenitor whichgave rise to significant structure in the SN radio light curve.
|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.
|Magnetic jets from swirling discs|
A broad swathe of astrophysical phenomena, ranging from tubularplanetary nebulae through Herbig-Haro objects, radio galaxy and quasaremissions to gamma-ray bursts and perhaps high-energy cosmic rays, maybe driven by magnetically dominated jets emanating from accretion discs.We give a self-contained account of the analytic theory ofnon-relativistic magnetically dominated jets wound up by a swirling discand making a magnetic cavity in a background medium of any prescribedpressure, p(z). We solve the time-dependent problem for any specifieddistribution of magnetic flux P(R, 0) emerging from the disc at z = 0,with any specified disc angular velocity Ωd(R). Thephysics required to do this involves only the freezing of the lines offorce to the conducting medium and the principle of minimum energy.In a constant pressure environment, the magnetically dominated cavity ishighly collimated and advances along the axis at a constant speedclosely related to the maximum circular velocity of the accretion disc.Even within the cavity the field is strongly concentrated towards theaxis. The twist in the jet field/<|Bz|> is close to and thewidth of the jet decreases upwards. By contrast, when the backgroundpressure falls off with height with powers approaching z-4,the head of the jet accelerates strongly and the twist of the jet ismuch smaller. The width increases to give an almost conical magneticcavity with apex at the source. Such a regime may be responsible forsome of the longest strongly collimated jets. When the backgroundpressure falls off faster than z-4, there are no quasi-staticconfigurations of well-twisted fields and the pressure confinement isreplaced by a dynamic effective pressure or a relativistic expansion. Inthe regimes with rapid acceleration, the outgoing and incoming fieldslinking the twist back to the source are almost anti-parallel so thereis a possibility that magnetic reconnections may break up the jet into aseries of magnetic `smoke-rings' travelling out along the axis.
|Planetary nebulae as tracers of galaxy stellar populations|
We address the general problem of the luminosity-specific planetarynebula (PN) number, better known as the `α' ratio, given byα=NPN/Lgal, and its relationship with theage and metallicity of the parent stellar population. Our analysisrelies on population synthesis models that account for simple stellarpopulations (SSPs), and more elaborate galaxy models covering the fullstar formation range of the different Hubble morphological types. Thistheoretical framework is compared with the updated census of the PNpopulation in Local Group (LG) galaxies and external ellipticals in theLeo group, and the Virgo and Fornax clusters.The main conclusions of our study can be summarized as follows. (i)According to the post-asymptotic giant branch (AGB) stellar core mass,PN lifetime in a SSP is constrained by three relevant regimes, driven bythe nuclear (Mcore>~ 0.57Msolar), dynamical(0.57Msolar>~Mcore>~ 0.55Msolar)and transition (0.55Msolar>~Mcore>~0.52Msolar) time-scales. The lower limit for Mcorealso sets the minimum mass for stars to reach the AGB thermal-pulsingphase and experience the PN event. (ii) Mass loss is the crucialmechanism to constrain the value of α, through the definition ofthe initial-to-final mass relation (IFMR). The Reimers mass-lossparametrization, calibrated on Pop II stars of Galactic globularclusters, poorly reproduces the observed value of α in late-typegalaxies, while a better fit is obtained using the empirical IFMRderived from white dwarf observations in the Galaxy open clusters. (iii) The inferred PN lifetime for LG spirals and irregulars exceeds10000yr, which suggests that Mcore<~ 0.65Msolarcores dominate, throughout. (iv) The relative PN deficiency inelliptical galaxies, and the observed trend of α with galaxyoptical colours, support the presence of a prevailing fraction oflow-mass cores (Mcore<~ 0.55Msolar) in the PNdistribution and a reduced visibility time-scale for the nebulae as aconsequence of the increased AGB transition time. The stellar componentwith Mcore<~ 0.52Msolar, which overrides the PNphase, could provide an enhanced contribution to hotter HB and post-HBevolution, as directly observed in M 32 and the bulge of M 31. Thisimplies that the most UV-enhanced ellipticals should also display thelowest values of α, as confirmed by the Virgo cluster early-typegalaxy population. (v) Any blue-straggler population, invoked asprogenitor of the Mcore>~ 0.7Msolar PNe inorder to preserve the constancy of the bright luminosity-functioncut-off magnitude in ellipticals, must be confined to a small fraction(a few per cent at most) of the whole galaxy PN population.
|Spurs and feathering in spiral galaxies|
We present smoothed particle hydrodynamic (SPH) simulations of theresponse of gas discs to a spiral potential. These simulations show thatthe commonly observed spurs and feathering in spiral galaxies can beunderstood as being due to structures present in the spiral arms thatare sheared by the divergent orbits in a spiral potential. Thus, densemolecular cloud-like structures generate the perpendicular spurs as theyleave the spiral arms. Subsequent feathering occurs as spurs are furthersheared into weaker parallel structures as they approach the next spiralpassage. Self-gravity of the gas is not included in these simulations,stressing that these features are purely due to the hydrodynamics inspiral shocks. Instead, a necessary condition for this mechanism to workis that the gas need be relatively cold (1000 K or less) in order thatthe shock is sufficient to generate structure in the spiral arms, andsuch structure is not subsequently smoothed by the gas pressure.
|Hαkinematics of the SINGS nearby galaxies survey - I*|
This is the first part of an Hαkinematics follow-up survey of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample. The data for28galaxies are presented. The observations were done on three differenttelescopes with Fabry-Perot of New Technology for the Observatoire dumont Megantic (FaNTOmM), an integral field photon-counting spectrometer,installed in the respective focal reducer of each telescope. The datareduction was done through a newly built pipeline with the aim ofproducing the most homogenous data set possible. Adaptive spatialbinning was applied to the data cubes in order to get a constantsignal-to-noise ratio across the field of view. Radial velocity andmonochromatic maps were generated using a new algorithm, and thekinematical parameters were derived using tilted-ring models.
|The lifetime of grand design|
The lifetime of the structure in grand design spiral galaxies isobservationally ill-determined, but is essentially set by how accuratelythe rotation of the pattern can be characterized by a single angularpattern speed. This paper derives a generalized version of theTremaine-Weinberg method for observationally determining pattern speeds,in which the pattern speed is allowed to vary arbitrarily with radius.The departures of the derived pattern speed from a constant then providea simple metric of the lifetime of the spiral structure. Application ofthis method to CO observations of NGC 1068 reveals that the patternspeed of the spiral structure in this galaxy varies rapidly with radius,and that the lifetime of the spiral structure is correspondingly veryshort. If this result turns out to be common in grand-design spiralgalaxies, then these features will have to be viewed as highly transientphenomena.
|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).
|Scale Heights of Non-Edge-on Spiral Galaxies|
We present a method of calculating the scale height of non-edge-onspiral galaxies, together with a formula for errors. The method is basedon solving Poisson's equation for a logarithmic disturbance of matterdensity in spiral galaxies. We show that the spiral arms can not extendto inside the ``forbidden radius'' r0, due to the effect ofthe finite thickness of the disk. The method is tested by re-calculatingthe scale heights of 71 northern spiral galaxies previously calculatedby Ma, Peng & Gu. Our results differ from theirs by less than 9%. Wealso present the scale heights of a further 23 non-edge-on spiralgalaxies.
|Cepheid Distances to SNe Ia Host Galaxies Based on a Revised Photometric Zero Point of the HST WFPC2 and New PL Relations and Metallicity Corrections|
With this paper we continue the preparation for a forthcoming summaryreport of our experiment with the HST to determine the Hubble constantusing Type Ia supernovae as standard candles. Two problems areaddressed. (1) We examine the need for, and determine the value of, thecorrections to the apparent magnitudes of our program Cepheids in the 11previous calibration papers due to sensitivity drifts and chargetransfer effects of the HST WFPC2 camera over the life time of theexperiment from 1992 to 2001. (2) The corrected apparent magnitudes areapplied to all our previous photometric data from which revised distancemoduli are calculated for the eight program galaxies that are parents tothe calibrator Ia supernovae. Two different Cepheid P-L relations areused; one for the Galaxy and one for the LMC. These differ both in slopeand zero point at a fixed period. The procedures for determining theabsorption and reddening corrections for each Cepheid are discussed.Corrections for the effects of metallicity differences between theprogram galaxies and the two adopted P-L relations are derived andapplied. The distance moduli derived here for the eight supernovaeprogram galaxies, and for 29 others, average 0.20 mag fainter (moredistant) than those derived by Gibson et al. and Freedman et al. intheir 2000 and 2001 summary papers for reasons discussed in this paper.The effect on the Hubble constant is the subject of our forthcomingsummary paper.
|The Central Engines of 19 LINERs as Viewed by Chandra|
Using archival Chandra observations of 19 LINERs, we explore the X-rayproperties of their inner kiloparsec to determine the origin of theirnuclear X-ray emission, to investigate the presence of an AGN, and toidentify the power source of the optical emission lines. The relativenumbers of LINER types in our sample are similar to those in opticalspectroscopic surveys. We find that diffuse, thermal emission is verycommon and is concentrated within the central few hundred parsecs. Theaverage spectra of the hot gas in spiral and elliptical galaxies arevery similar to those of normal galaxies. They can be fitted with athermal plasma (kT~0.5 keV) plus a power-law (photon index of 1.3-1.5)model. There are on average three detected point sources in their innerkiloparsec with 1037 ergss-1
|Observational Constraints on the Ages of Molecular Clouds and the Star Formation Timescale: Ambipolar-Diffusion-controlled or Turbulence-induced Star Formation?|
We revisit the problem of the star formation timescale and the ages ofmolecular clouds. The apparent overabundance of star-forming molecularclouds over clouds without active star formation has been thought toindicate that molecular clouds are ``short-lived'' and that starformation is ``rapid.'' We show that this statistical argument lacksself-consistency and, even within the rapid star formation scenario,implies cloud lifetimes ~10 Myr. We discuss additional observationalevidence from external galaxies that indicate lifetimes of molecularclouds and a timescale of star formation of ~107. These longcloud lifetimes, in conjunction with the rapid (~1 Myr) decay ofsupersonic turbulence, present severe difficulties for the scenario ofturbulence-controlled star formation. By contrast, we show that all 31existing observations of objects for which the line width, the size, andthe magnetic field strength have been reliably measured are in excellentquantitative agreement with the predictions of the ambipolar-diffusiontheory. Within the ambipolar-diffusion-controlled star formation theory,the line widths may be attributed to large-scale nonradial cloudoscillations (essentially standing large-amplitude, long-wavelengthAlfvén waves), and the predicted relation between the line width,the size, and the magnetic field is a natural consequence of magneticsupport of self-gravitating clouds.
|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.
|Radiatively Inefficient Accretion Flow in the Nucleus of NGC 1097|
We present a model for the accretion flow around the supermassive blackhole in the LINER nucleus of NGC 1097 that fits the optical to X-rayspectral energy distribution (SED). The X-ray segment of the SED isbased on observations with the Chandra X-Ray Observatory, which arereported here for the first time. The inner part of the flow is modeledas a radiatively inefficient accretion flow (RIAF), and the outer partas a standard thin disk. The value of the transition radius(rtr~225RS, where RS=2GM/c2)between the RIAF and the outer thin disk was obtained from our previousfitting of the double-peaked Balmer emission line profile, whichoriginates in the thin disk. The black hole mass was inferred frommeasurements of the stellar velocity dispersion in the host galaxy. Whenthese parameters are used in the accretion flow model, the SED can besuccessfully reproduced, which shows that the line profile model and theaccretion flow model are consistent with each other. A small remainingexcess in the near-UV is accounted for by the contribution of anobscured starburst located within 9 pc from the nucleus, as we reportedin an earlier paper. The radio flux is consistent with synchrotronemission of a relativistic jet modeled by means of the internal shockscenario. In an appendix we also analyze the Chandra X-ray observationsof the ~1 kpc circumnuclear star-forming ring and of an ultraluminouscompact X-ray source located outside the ring.
|Extended Mid-Infrared Aromatic Feature Emission in M82|
We present new images (ground-based optical and mid-infrared [MIR] fromthe Spitzer Space Telescope) and spectra (from Spitzer) of thearchetypal starburst galaxy M82. The Spitzer data show that the MIRemission extends at least 6 kpc along the minor axis of the galaxy. Weuse the optical and infrared data to demonstrate that the extendedemission is dominated by emission from dust. The colors of the MIRemission and the spectra indicate that there is a strong component ofaromatic feature emission (the MIR features commonly attributed topolycyclic aromatic hydrocarbons). The dust continuum and aromaticfeature emission are both strong in the well-known superwind region ofthis galaxy; clearly, the carrier of the aromatic features can survivein close proximity to the wind, far from the plane of the galaxy. Wealso see significant emission by dust well outside the superwind region,providing the clearest picture to date of the dust distribution in thehalo of this galaxy.
|Black Hole Masses of Active Galaxies with Double-peaked Balmer Emission Lines|
We have obtained near-IR spectra of five AGNs that exhibit double-peakedBalmer emission lines (NGC 1097, Pictor A, PKS 0921-213, 1E0450.30-1817, and IRAS 0236.6-3101). The stellar velocity dispersions ofthe host galaxies were measured from the Ca II λλ8494,8542, 8662 absorption lines and were found to range from 140 to 200 kms-1. Using the well-known correlation between the black holemass and the stellar velocity dispersion, the black hole masses in thesegalaxies were estimated to range from 4×107 to1.2×108 Msolar. We supplement theobservations presented here with estimates of the black holes masses forfive additional double-peaked emitters (Arp 102B, 3C 390.3, NGC 4579,NGC 4203, and M81) obtained by other authors using similar methods.Using these black hole masses, we infer the ratio of the bolometricluminosity to the Eddington luminosity,(Lbol/LEdd). We find that two objects (Pictor Aand PKS 0921-213) have Lbol/LEdd~0.2, whereas theother objects have Lbol/LEdd<~10-2(nearby, low-luminosity double-peaked emitters are the most extreme,with Lbol/LEdd<~10-4). The physicaltimescales in the outer regions of the accretion disks (atr~103GM/c2) in these objects were also estimatedand range from a few months for the dynamical timescale to severaldecades for the sound crossing timescale. The profile variability inthese objects is typically an order of magnitude longer than thedynamical time, but we note that variability occurring on the dynamicaltimescale has not been ruled out by the observations.Based on observations carried out at Cerro Tololo Inter-AmericanObservatory, which is operated by AURA, Inc., under a cooperativeagreement with the National Science Foundation.
|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 Space Telescope IRAC and MIPS Observations of the Interacting Galaxies IC 2163 and NGC 2207: Clumpy Emission|
IC 2163 and NGC 2207 are interacting galaxies that have been wellstudied at optical and radio wavelengths and simulated in numericalmodels to reproduce the observed kinematics and morphological features.Spitzer IRAC and MIPS observations reported here show over 200 brightclumps from young star complexes. The brightest IR clump is amorphologically peculiar region of star formation in the western arm ofNGC 2207. This clump, which dominates the Hα and radio continuumemission from both galaxies, accounts for ~12% of the total 24 μmflux. Nearly half of the clumps are regularly spaced along somefilamentary structure, whether in the starburst oval of IC 2163 or inthe thin spiral arms of NGC 2207. This regularity appears to influencethe clump luminosity function, making it peaked at a value nearly afactor of 10 above the completeness limit, particularly in the starburstoval. This is unlike the optical clusters inside the clumps, which havea luminosity function consistent with the usual power-law form. Thegiant IR clumps presumably formed by gravitational instabilities in thecompressed gas of the oval and the spiral arms, whereas the individualclusters formed by more chaotic processes, such as turbulencecompression, inside these larger scale structures.
|Identification of the Red Supergiant Progenitor of Supernova 2005cs: Do the Progenitors of Type II-P Supernovae Have Low Mass?|
The stars that end their lives as supernovae (SNe) have been directlyobserved in only a handful of cases, mainly because of the extremedifficulty of identifying them in images obtained prior to the SNexplosions. Here we report the identification of the progenitor for therecent Type II-plateau (core collapse) SN 2005cs in pre-explosionarchival images of the Whirlpool Galaxy (M51) obtained with the HubbleSpace Telescope (HST) Advanced Camera for Surveys (ACS). Fromhigh-quality ground-based images of the SN obtained with theCanada-France-Hawaii Telescope, we precisely determine the position ofthe SN and are able to isolate the SN progenitor to within 0.04" in theHST ACS optical images. We further pinpoint the SN location to within0.005" from HST ACS ultraviolet images of the SN, confirming ourprogenitor identification. From photometry of the SN progenitor obtainedwith the pre-SN ACS images, and also from limits to its brightness inpre-SN HST NICMOS images, we infer that the progenitor is a redsupergiant star of spectral type K3-M4 with initial mass 10+/-3Msolar. We also discuss the implications of the SN 2005csprogenitor identification and its mass estimate. There is an emergingtrend that the most common Type II-plateau SNe originate from low-masssupergiants (8-20 Msolar).
|An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X-Ray Sources|
We have studied the stellar and interstellar environments of twoluminous X-ray sources and five ultraluminous X-ray sources (ULXs) inorder to gain insight into their nature. Archival Hubble Space Telescopeimages were used to identify the optical counterparts of the ULXs Ho IXX-1 and NGC 1313 X-2, and to make photometric measurements of the localstellar populations of these and the luminous source IC 10 X-1. Weobtained high-dispersion spectroscopic observations of the nebulaearound these seven sources to search for He II λ4686 emission andto estimate the expansion velocities and kinetic energies of thesenebulae. Our observations did not detect nebular He II emission from anysource, with the exception of LMC X-1 this is either because we missedthe He III regions or because the nebulae are too diffuse to produce HeII surface brightnesses that lie within our detection limit. We comparethe observed ionization and kinematics of the supershells around theULXs Ho IX X-1 and NGC 1313 X-2 with the energy feedback expected fromthe underlying stellar population to assess whether additional energycontributions from the ULXs are needed. In both cases, we findinsufficient UV fluxes or mechanical energies from the stellarpopulation; thus these ULXs may be partially responsible for theionization and energetics of their supershells. All seven sources thatwe studied are in young stellar environments, and six of them haveoptical counterparts with masses >~7 Msolar thus, thesesources are most likely high-mass X-ray binaries.
|The ACS Virgo Cluster Survey. XI. The Nature of Diffuse Star Clusters in Early-Type Galaxies|
We use HST ACS imaging of 100 early-type galaxies in the ACS VirgoCluster Survey to investigate the nature of diffuse star clusters(DSCs). Compared to globular clusters (GCs), these star clusters havelow luminosities (MV>-8) and a broad distribution of sizes(320 magarcsec-2). The median colors of diffuse star cluster systems(1.1
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