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Comment regarding the functional form of the Schmidt law
Star formation rates on the galactic scale are describedphenomenologically by two distinct relationships, as emphasized recentlyby Elmegreen [Elmegreen, B.G., 2002. ApJ, 577, 206. astro-ph/0207114.].The first of these is the Schmidt law, which is a power-law relationbetween the star formation rate SFR and the column density Σ. Theother relationship is that there is a cutoff in the gas density belowwhich star formation shuts off. The purpose of this paper is to arguethat: (1) these two relationships can be accommodated by a singlefunctional form of the Schmidt law, (2) this functional form ismotivated by the hypothesis that star formation is a criticalphenomenon, and that as a corollary, (3) the existence of a sharp cutoffmay thus be an emergent property of galaxies, as was argued by Seiden[Seiden, P.E.,1983. ApJ, 266, 555.], as opposed to the classical viewthat this cutoff is due to an instability criterion.
| Radial transport of dust in spiral galaxies
Motivated by recent observations which detect dust at largegalactocentric distances in the disks of spiral galaxies, we propose amechanism of outward radial transport of dust by spiral stellar densitywaves. We consider spiral galaxies in which most of dust formation islocalized inside the corotation radius. We show that in the disks ofsuch spiral galaxies, the dust grains can travel over radial distancesthat exceed the corotation radius by roughly 25%. A fraction of the dustgrains can be trapped on kidney-shaped stable orbits between the stellarspiral arms and thus can escape the destructive effect of supernovaexplosions. These grains form diffuse dusty spiral arms, which stretch 45 kpc from the sites of active star formation. About 10% of dust by massinjected inside corotation can be transported over radial distances 3 4kpc during ≈1.0 Gyr. This is roughly an order of magnitude moreefficient than can be provided by the turbulent motions.
| Near-infrared spectroscopy of a young super-star cluster in NGC 6946: chemical abundances and abundance patterns*
Using the NIRSPEC spectrograph at Keck II, we have obtained H- andK-band echelle spectra for a young (~10-15 Myr), luminous(MV~-13.2) super-star cluster in the nearby spiral galaxy NGC6946. From spectral synthesis and equivalent width measurements weobtain for the first time accurate abundances and abundance patterns inan extragalactic super-star cluster. We find [Fe/H]=-0.45 +/- 0.08 dex,an average α-enhancement of ~+0.22 +/- 0.1 dex, and a relativelylow 12 C/13 C ~ 8 +/- 2 isotopic ratio. We alsomeasure a velocity dispersion of ~9.1 km s-1, in agreementwith previous estimates. We conclude that integrated high-dispersionspectroscopy of massive star clusters is a promising alternative toother methods for abundance analysis in extragalactic young stellarpopulations.
| 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.
| Optical and infrared observations of the TypeIIP SN2002hh from days 3 to 397
We present optical and infrared (IR) observations of the TypeII SN2002hhfrom 3 to 397d after explosion. The optical spectroscopic (4-397d) andphotometric (3-278d) data are complemented by spectroscopic (137-381d)and photometric (137-314d) data acquired at IR wavelengths. This is thefirst time L-band spectra have ever been successfully obtained for asupernova (SN) at a distance beyond the Local Group. The VRI lightcurves in the first 40d reveal SN2002hh to be an SNIIP (plateau) - themost common of all core-collapse SNe. SN2002hh is one of the most highlyextinguished SNe ever investigated. To provide a match between itsearly-time spectrum and a coeval spectrum of the TypeIIP SN1999em, aswell as maintaining consistency with KI interstellar absorption, weinvoke a two-component extinction model. One component is due to thecombined effect of the interstellar medium (ISM) of our Milky Way Galaxyand the SN host galaxy, while the other component is due to a `dustpocket' where the grains have a mean size smaller than in the ISM. Theearly-time optical light curves of SNe1999em and 2002hh are generallywell matched, as are the radioactive tails of these two SNe and SN1987A.The late-time similarity of the SN2002hh optical light curves to thoseof SN1987A, together with measurements of the optical/IR luminosity and[FeII]1.257μm emission indicate that 0.07 +/- 0.02Msolarof 56Ni was ejected by SN2002hh. However, during the nebularphase the HKL' luminosities of SN2002hh exhibit a growing excess withrespect to those of SN1987A. We attribute much of this excess to anIR-echo from a pre-existing, dusty circumstellar medium. Based on anIR-echo interpretation of the near-IR (NIR) excess, we deduce that theprogenitor of SN2002hh underwent recent mass-loss of~0.3Msolar. A detailed comparison of the late-time opticaland NIR spectra of SNe1987A and 2002hh is presented. While the overallimpression is one of similarity between the spectra of the two events,there are notable differences. The MgI1.503μm luminosity of SN2002hhis a factor of 2.5 greater than in SN1987A at similar epochs, yet coevalsilicon and calcium lines in SN2002hh are fainter. Interpreting thesedifferences as being due to abundance variations, the overall abundancetrend between SN1987A and 2002hh is not consistent with explosion modelpredictions. It appears that during the burning to intermediate-masselements, the nucleosynthesis did not progress as far as might have beenexpected given the mass of iron ejected. Evidence for mixing in theejecta is presented. Pronounced blueshifts seen in the more isolatedlines are attributed to asymmetry in the ejecta. However, during thetime-span of these observations (~1-yr post-explosion) we find noevidence of dust condensation in the ejecta such as might have beenrevealed by an increasing blueshift and/or attenuation of the red wingsof the emission lines. Nevertheless, the clear detection of firstovertone CO emission by 200d and the reddening trend in (K -L')0 suggest that dust formation in the ejecta may occur atlater epochs. From the [OI] λλ6300, 6364Å doubletluminosity we infer a 16-18Msolar main-sequence progenitorstar. The progenitor of SN2002hh was probably a red supergiant with asubstantial, dusty wind.
| XMM-Newton observations of the brightest ultraluminous X-ray sources
We present an analysis of 13 of the best quality ultraluminous X-raysource (ULX) data sets available from XMM-Newton European Photon ImagingCamera (EPIC) observations. We utilize the high signal-to-noise in theseULX spectra to investigate the best descriptions of their spectral shapein the 0.3-10keV range. Simple models of an absorbed power law ormulticolour disc blackbody prove inadequate at describing the spectra.Better fits are found using a combination of these two components, withboth variants of this model - a cool (~0.2keV) disc blackbody plus hardpower-law continuum, and a soft power-law continuum, dominant at lowenergies, plus a warm (~1.7keV) disc blackbody - providing good fits to8/13 ULX spectra. However, by examining the data above 2keV, we findevidence for curvature in the majority of data sets (8/13 with at leastmarginal detections), inconsistent with the dominance of a power law inthis regime. In fact, the most successful empirical description of thespectra proved to be a combination of a cool (~0.2keV) classic blackbodyspectrum, plus a warm disc blackbody that fits acceptably to 10/13 ULXs.The best overall fits are provided by a physically self-consistentaccretion disc plus Comptonized corona model (DISKPN + EQPAIR), whichfits acceptably to 11/13 ULXs. This model provides a physicalexplanation for the spectral curvature, namely that it originates in anoptically thick corona, though the accretion disc photons seeding thiscorona still originate in an apparently cool disc. We note similaritiesbetween this fit and models of Galactic black hole binaries at highaccretion rates, most notably the model of Done & Kubota. In thisscenario the inner disc and corona become energetically coupled at highaccretion rates, resulting in a cooled accretion disc and opticallythick corona. We conclude that this analysis of the best spectral datafor ULXs shows it to be plausible that the majority of the populationare high accretion rate stellar-mass (perhaps up to 80Msolar)black holes, though we cannot categorically rule out the presence oflarger, ~1000-Msolar intermediate-mass black holes (IMBHs) inindividual sources with the current X-ray data.
| 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.
| 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.
| 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.
| Molecular Gas Dynamics in NGC 6946: A Bar-driven Nuclear Starburst ``Caught in the Act''
We present high angular resolution (~1" and 0.6") millimeterinterferometric observations of the 12CO(1-0) and12CO(2-1) line emission in the central 300 pc of thelate-type spiral galaxy NGC 6946. The data, obtained with the IRAMPlateau de Bure Interferometer (PdBI), allow the first detection of amolecular gas spiral in the inner ~10" (270 pc) with a largeconcentration of molecular gas(MH2~1.6×107 Msolar)within the inner 60 pc. This nuclear clump shows evidence for a ringlikegeometry with a radius of ~10 pc as inferred from the position-velocitydiagrams. Both the distribution of the molecular gas and its kinematicscan be well explained by the influence of an inner stellar bar of about400 pc length. A qualitative model of the expected gas flow shows thatstreaming motions along the leading sides of this bar are a plausibleexplanation for the high nuclear gas density. Thus, NGC 6946 is a primeexample of molecular gas kinematics being driven by a small-scale,secondary stellar bar.Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany),and IGN (Spain).
| Detectability of [C II] 158 μm Emission from High-Redshift Galaxies: Predictions for ALMA and SPICA
We discuss the detectability of high-redshift galaxies via [C II] 158μm line emission by coupling an analytic model with cosmologicalsmoothed particle hydrodynamics (SPH) simulations that are based on theconcordance Λ cold dark matter (CDM) model. Our analytic modeldescribes a multiphase interstellar medium (ISM) irradiated by thefar-ultraviolet (FUV) radiation from local star-forming regions, and itcalculates thermal and ionization equilibrium between cooling andheating. The model allows us to predict the mass fraction of a coldneutral medium (CNM) embedded in a warm neutral medium (WNM). Ourcosmological SPH simulations include a treatment of radiativecooling/heating, star formation, and feedback effects from supernovaeand galactic winds. Using our method, we make predictions for the [C II]luminosity from high-redshift galaxies that can be directly comparedwith upcoming observations by the Atacama Large Millimeter Array (ALMA)and the Space Infrared Telescope for Cosmology and Astrophysics (SPICA).We find that the number density of high-redshift galaxies detectable byALMA and SPICA via [C II] emission depends significantly on the amountof neutral gas, which is highly uncertain. Our calculations suggestthat, in a CDM universe, most [C II] sources at z=3 are faint objectswith Sν<0.01 mJy. Lyman break galaxies (LBGs) brighterthan RAB=23.5 mag are expected to have flux densitiesSν=1-3 mJy depending on the strength of galactic windfeedback. The recommended observing strategy for ALMA and SPICA is toaim at very bright LBGs or star-forming DRG/BzK galaxies.
| 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.
| The Extended H I Rotation Curve and Mass Distribution of M31
New H I observations of Messier 31 (M31) obtained with the Effelsbergand Green Bank 100 m telescopes make it possible to measure the rotationcurve of that galaxy out to ~35 kpc. Between 20 and 35 kpc, the rotationcurve is nearly flat at a velocity of ~226 km s-1. A model ofthe mass distribution shows that at the last observed velocity point,the minimum dark-to-luminous mass ratio is ~0.5 for a total mass of3.4×1011 Msolar at R<35 kpc. This can becompared to the estimated Milky Way mass of 4.9×1011Msolar for R<50 kpc.
| 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).
| Radio and X-Ray Emission as Probes of Type IIP Supernovae and Red Supergiant Mass Loss
Type IIP (plateau) supernovae are thought to come from stars withinitial mass ~8-25 Msolar that end their lives as redsupergiants. The expected stellar endpoints can be found fromevolutionary calculations, and the corresponding mass-loss properties atthese points can be estimated from typical values for Galactic stars.The mass-loss densities of observed supernovae can be estimated fromobservations of the thermal X-ray and radio synchrotron emission thatresult from the interaction of the supernova with the surrounding wind.Type IIP supernovae are expected to have energy-conserving interactionduring typical times of observation. Because Type IIP supernovae have anextended period of high optical luminosity, Compton cooling could affectthe radio-emitting electrons, giving rise to a relatively flat radiolight curve in the optically thin regime. Alternatively, a highefficiency of magnetic field production results in synchrotron coolingof the radio-emitting electrons. Both the X-ray and radio luminositiesare sensitive to the mass loss and initial masses of the progenitorstars, although the turn-on of radio emission is probably the bestestimator of circumstellar density. Both the mass-loss density and thevariation of density with stellar mass are consistent with expectationsfor the progenitor stars deduced from direct observations of recentsupernovae. Current observations are consistent with mass being the onlyparameter; observations of supernovae in metal-poor regions could showhow the mass loss depends on metallicity.
| Two Populations of Young Massive Star Clusters in Arp 220
We present new optical observations of young massive star clusters inArp 220, the nearest ultraluminous infrared galaxy, taken in UBVI withthe Hubble Space Telescope ACS HRC camera. We find a total of 206probable clusters whose spatial distribution is centrally concentratedtoward the nucleus of Arp 220. We use model star cluster tracks todetermine ages, luminosities, and masses for 14 clusters with completeUBVI indices or previously published near-infrared data. We estimaterough masses for 24 additional clusters with I<24 mag from BVIindices alone. The clusters with useful ages fall into two distinctgroups: a ``young'' population (<10 Myr) and an intermediate-agepopulation (~=300 Myr). There are many clusters with masses clearlyabove 106 Msolar and possibly even above107 Msolar in the most extreme instances. Thesemasses are high enough that the clusters being formed in the Arp 220starburst can be considered to be genuine young globular clusters. Inaddition, this study allows us to extend the observed correlationbetween global star formation rate and maximum cluster luminosity bymore than 1 order of magnitude in star formation rate.
| 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
| Evidence for Chimney Breakout in the Galactic Supershell GSH 242-03+37
We present new high-resolution neutral hydrogen (H I) images of theGalactic supershell GSH 242-03+37. These data were obtained with theParkes Radiotelescope as part of the Galactic All-Sky Survey (GASS). GSH242-03+37 is one of the largest and most energetic H I supershells inthe Galaxy, with a radius of 565+/-65 pc and an expansion energy of3×1053 ergs. Our images reveal a complicated shell withmultiple chimney structures on both sides of the Galactic plane. Thesechimneys appear capped by narrow filaments about 1.6 kpc above and belowthe Galactic midplane, confirming structures predicted in simulations ofexpanding supershells. The structure of GSH 242-03+37 is extremelysimilar to the only other Galactic supershell known to have blown out ofboth sides of the plane, GSH 277+00+36. We compare the GASS H I datawith X-ray and Hα images, finding no strong correlations.
| An Initial Look at the Far-Infrared-Radio Correlation within Nearby Star-forming Galaxies Using the Spitzer Space Telescope
We present an initial look at the far-infrared-radio correlation withinthe star-forming disks of four nearby, nearly face-on galaxies (NGC2403, NGC 3031, NGC 5194, and NGC 6946). Using Spitzer MIPS imaging,observed as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS),and Westerbork Synthesis Radio Telescope (WSRT) radio continuum data,taken for the WSRT SINGS radio continuum survey, we are able to probevariations in the logarithmic 24 μm/22 cm (q24) and 70μm/22 cm (q70) surface brightness ratios across each diskat subkiloparsec scales. We find general trends of decreasingq24 and q70 with declining surface brightness andwith increasing radius. The residual dispersion around the trend ofq24 and q70 versus surface brightness is smallerthan the residual dispersion around the trend of q24 andq70 versus radius, on average by ~0.1 dex, indicating thatthe distribution of star formation sites is more important indetermining the infrared/radio disk appearance than the exponentialprofiles of disks. We have also performed preliminary phenomenologicalmodeling of cosmic-ray electron (CR electron) diffusion using animage-smearing technique and find that smoothing the infrared mapsimproves their correlation with the radio maps. We find that exponentialsmoothing kernels work marginally better than Gaussian kernels,independent of projection for these nearly face-on galaxies. This resultsuggests that additional processes besides simple random walk diffusionin three dimensions must affect the evolution of CR electrons. Thebest-fit smoothing kernels for the two less active star-forming galaxies(NGC 2403 and NGC 3031) have much larger scale lengths than those of themore active star-forming galaxies (NGC 5194 and NGC 6946). Thisdifference may be due to the relative deficit of recent CR electroninjection into the interstellar medium for the galaxies that havelargely quiescent disks.
| Chemical Enrichment of the Complex Hot ISM of the Antennae Galaxies. II. Physical Properties of the Hot Gas and Supernova Feedback
We investigate the physical properties of the interstellar medium (ISM)in the merging pair of galaxies known as the Antennae (NGC 4038/4039),using the deep co-added ~411 ks Chandra ACIS-S data set. The method ofanalysis and some of the main results from the spectral analysis, suchas metal abundances and their variations from ~0.2 to ~20-30 timessolar, are described in Paper I (Baldi et al.). In the present paper weinvestigate in detail the physics of the hot emitting gas, derivingmeasures for the hot gas mass (~107 Msolar),cooling times (107-108 yr), and pressure(3.5×10-11-2.8×10-10 dynecm-2). In at least one of the two nuclei (NGC 4038), the hotgas pressure is significantly higher than the CO pressure, implying thatshock waves may be driven into the CO clouds. Comparison of the metalabundances with the average stellar yields predicted by theoreticalmodels of SN explosions points to SNe of Type II as the maincontributors of metals to the hot ISM. There is no evidence of anycorrelation between radio-optical star formation indicators and themeasured metal abundances. Although due to uncertainties in the averagegas density we cannot exclude that mixing may have played an importantrole, the short time required to produce the observed metal masses(<~2 Myr) suggests that the correlations are unlikely to have beendestroyed by efficient mixing. More likely, a significant fraction ofType II SN ejecta may be in a cool phase, in grains, or escaping in hotwinds. In each case, any such fraction of the ejecta would remainundetectable with soft X-ray observations.
| The origin of magnetic fields in galaxies: Observational tests with the Square Kilometre Array
The all-sky survey of Faraday rotation, a Key Science Project of theplanned Square Kilometre Array, will accumulate tens of millions ofrotation measure measurements toward background radio sources and willprovide a unique database for characterizing the overall magneticgeometry of magnetic fields in galaxies and in the intergalactic medium.Deep imaging of the polarized synchrotron emission from a large numberof nearby galaxies, combined with Faraday rotation data, will allow usto test primordial, gas flow, and dynamo models for field origin andamplification. The SKA will find the first magnetic fields in younggalaxies and determine the timescale for building up small-scaleturbulent and large-scale coherent fields. The spectrum of dynamo modes,if existing, will be resolved. The present-day coherent field may keepmemory of the direction of the seed field which can be used for mappingthe structure of the seed field before galaxy formation.
| Environment and luminosity of supernova remnants
The explosion of supernovae and the evolution of their remnants (SNRs)accelerate cosmic rays over a vast range of timescales. Magnetic fieldscan be investigated indirectly through one of the observationalsignatures of this acceleration, namely radio synchrotron emission. Withthe aim of better understanding the role of the magnetic field insupernova evolution, we explore the variation of SNR radio luminositieswith physical conditions in the surrounding interstellar medium. With adata set that comprises more than 90 individual SNRs in 10 galaxies, anda range of 3000 in ISM density and 104 in radio synchrotronluminosity, we find a significant correlation between the twoquantities. The observed trends support the hypothesis that adiabaticcompression of magnetic fields by itself is insufficient to explain theradio emission of the brighter and more luminous in SNRs.
| Hubble Space Telescope Observations of Star Clusters in M101
Hubble Space Telescope Advanced Camera for Surveys (ACS) images are usedto identify and study star cluster candidates in the nearby spiralgalaxy M101. About 3000 round, slightly resolved cluster candidates areidentified in 10 ACS pointings covering an area of 106arcmin2. The cluster candidates' color and size distributionsare consistent with those of star clusters in other nearby spirals. Themajority of the M101 candidates are blue and more likely to beassociated with the galaxy's spiral arms, implying that they are young.The galaxy-luminosity-normalized number of young massive clusters inM101 is similar to that found in other spirals, as is the clusterdensity at a fiducial absolute magnitude. We confirm a previous findingthat M101 has a large number of faint red star clusters: if these areold globular clusters, then this galaxy has a very large globularcluster population. More plausible is that the faint red clusters arereddened young clusters; their colors and luminosities are alsoconsistent with this explanation.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withprograms 8640 and 9490.
| 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.
| Star-forming knots and density wave in NGC 2997
Context.Many grand design spiral galaxies show strings of bright knotsalong their arms on near-infrared K-band images. The alignment of suchknots suggests a relation to the spiral pattern and possibly to alarge-scale, star-forming front associated with a density wave. Aims.Bright knots in the southern arm of NGC 2997 were studied todetermine their nature and evolutionary state.Methods.Lowresolution near-infrared K- and J-band spectra of the knots wereobserved with ISAAC on the VLT. Results.Most of the knots showstrong H I Brγ emission with some also having He I and H2emission. A few knots show indications of 12CO absorption.This suggests that the knots are very young stellar clusters with massesup to 5×104 Mȯ.Conclusions.Theknots azimuthal distance from the K-band spiral correlates well withtheir Brγ strength, indicating that they are located inside theco-rotation of the density wave, which triggered them through alarge-scale, star-forming front. These relative azimuthal distancessuggest an age spread of more than 1.6 Myr, which is incompatible withstandard models for an instantaneous star burst. This indicates a morecomplex star-formation history, such as several bursts or continuousformation.
| [CII] emission and star formation in the spiral arms of M 31
Context: .The [Cii] 158 μm line is the most important coolant of theinterstellar medium in galaxies but substantial variations are seen fromobject to object. The main source of the emission at a galactic scale isstill poorly understood and candidates range from photodissociationregions (PDRs) to the cold neutral or diffuse warm ionized medium.Previous studies of the [Cii] emission in galaxies have a resolution ofseveral kpc or more so the observed emission is an average of differentISM components. Aims: .The aim of this work is to study, for thefirst time, the [Cii] emission at the scale of a spiral arm. We want toinvestigate the origin of this line and its use as a tracer of starformation. Methods: . We present [Cii] and [Oi] observations of asegment of a spiral arm of M 31 using the Infrared Space Observatory.The [Cii] emission is compared with tracers of neutral gas (CO, Hi) andstar formation (Hα, Spitzer 24 μm). Results: . Thesimilarity of the [Cii] emission with the Hα and 24 μm imagesis striking when smoothed to the same resolution, whereas thecorrelation with the neutral gas is much weaker. The [Cii] cooling rateper H atom increases dramatically from 2.7 ×10-26 erg s-1 atom-1 in the border ofthe map to 1.4 × 10-25 erg s-1atom-1 in the regions of star formation. The[Cii]/FIR{42-122} ratio is almost constant at 2%, a factor 3 higher thantypically quoted. However, we do not believe that M 31 is unusual.Rather, the whole-galaxy fluxes used for the comparisons include thecentral regions where the [Cii]/FIR ratio is known to be lower and theresolved observations neither isolate a spiral arm nor include data asfar out in the galactic disk as the observations presented here. A fitto published PDR models yields a plausible average solution ofG0 100 and n 3000 for the PDR emission in theregions of star formation in the arm of M 31.
| [CII] 158 μm emission and metallicity in photon dominated regions
We study the effects of a metallicity variation on the thermal balanceand [CII] fine-structure line strengths in interstellar photon dominatedregions (PDRs). We find that a reduction in the dust-to-gas ratio andthe abundance of heavy elements in the gas phase changes the heatbalance of the gas in PDRs. The surface temperature of PDRs decreases asthe metallicity decreases except for high density (n>106cm-3) clouds exposed to weak (χ< 100) FUV fields wherevibrational H2-deexcitation heating dominates over photoelectric heatingof the gas. We incorporate the metallicity dependence in our KOSMA-τPDR model to study the metallicity dependence of [CII]/CO line ratios inlow metallicity galaxies. We find that the main trend in the variationof the observed CII/CO ratio with metallicity is well reproduced by asingle spherical clump, and does not necessarily require an ensemble ofclumps as in the semi-analytical model presented by Bolatto et al.(1999).
| Outer structure of the Galactic warp and flare: explaining the Canis Major over-density
Aims.In this paper we derive the structure of the Galactic stellar warpand flare. Methods: .We use 2MASS red clump and red giant stars,selected at mean and fixed heliocentric distances ofRȯ≃3, 7 and 17 kpc. Results: .Our resultscan be summarized as follows: (i) a clear stellar warp signature isderived for the 3 selected rings, proving that the warp starts alreadywithin the solar circle; (ii) the derived stellar warp is consistent(both in amplitude and phase-angle) with that for the Galacticinterstellar dust and neutral atomic hydrogen; (iii) the consistency andregularity of the stellar-gaseous warp is traced out to aboutR_GC20 kpc; (iv) the Sun seems not to fall on the line of nodes.The stellar warp phase-angle orientation (φ15°) is closeto the orientation angle of the Galactic bar and this, most importantly,produces an asymmetric warp for the inner Rȯ≃3 and7 kpc rings; (v) a Northern/Southern warp symmetry is observed only forthe ring at Rȯ≃17 kpc, at which the dependency onφ is weakened; (vi) treating a mixture of thin and thick diskstellar populations, we trace the variation with R_GC of the diskthickness (flaring) and derive an almost constant scale-height (~0.65kpc) within R_GC15 kpc. Further out, the disk flaring increasegradually reaching a mean scale-height of ~1.5 kpc at R_GC23 kpc;(vii) the derived outer disk warping and flaring provide further robustevidence that there is no disk radial truncation at R_GC14 kpc. Conclusions: .In the particular case of the Canis Major (CMa)over-density we confirm its coincidence with the Southern stellarmaximum warp occurring near l240° (forRȯ≃7 kpc) which brings down the Milky Waymid-plane by ~3° in this direction. The regularity and consistencyof the stellar, gaseous and dust warp argues strongly against a recentmerger scenario for Canis Major. We present evidence to conclude thatall observed parameters (e.g. number density, radial velocities, propermotion etc) of CMa are consistent with it being a normal Milky Wayouter-disk population, thereby leaving no justification for more complexinterpretations of its origin. The present analysis or outer diskstructure does not provide a conclusive test of the structure or originof the Monoceros Ring. Nevertheless, we show that a warped flared MilkyWay contributes significantly at the locations of the Monoceros Ring.Comparison of outer Milky Way H I and CO properties with those of othergalaxies favors the suggestion that complex structures close to planarin outer disks are common, and are a natural aspect of warped andflaring disks.
| 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.
| The luminosity function of young star clusters: implications for the maximum mass and luminosity of clusters
We introduce a method to relate a possible truncation of the starcluster mass function at the high mass end to the shape of the clusterluminosity function (LF). We compare the observed LFs of five galaxiescontaining young star clusters with synthetic cluster population modelswith varying initial conditions. The LF of the SMC, the LMC and NGC 5236are characterized by a power-law behavior N d L ∝L-α d L, with a mean exponent of < α> = 2.0± 0.2. This can be explained by a cluster population formed witha constant cluster formation rate, in which the maximum cluster mass perlogarithmic age bin is determined by the size-of-sample effect andtherefore increases with log (age/yr). The LFs of NGC 6946 and M 51 arebetter described by a double power-law distribution or a Schechterfunction. When a cluster population has a mass function that istruncated below the limit given by the size-of-sample effect, the totalLF shows a bend at the magnitude of the maximum mass, with the age ofthe oldest cluster in the population, typically a few Gyr due todisruption. For NGC 6946 and M 51 this suggests a maximum mass of M_max= 0.5-1×10^6 Mȯ, although the bend is only a 1-2σ detection. Faint-ward of the bend the LF has the same slope asthe underlying initial cluster mass function and bright-ward of the bendit is steeper. This behavior can be well explained by our populationmodel. We compare our results with the only other galaxy for which abend in the LF has been observed, the "Antennae" galaxies (NGC4038/4039). There the bend occurs brighter than in NGC 6946 and M 51,corresponding to a maximum cluster mass of M_max =1.3-2.5×106 Mȯ. Hence, if the maximumcluster mass has a physical limit, then it can vary between differentgalaxies. The fact that we only observe this bend in the LF in the"Antennae" galaxies, NGC 6946 and M 51 is because there are enoughclusters available to reach the limit. In other galaxies there might bea physical limit as well, but the number of clusters formed or observedis so low, that the LF is not sampled up to the luminosity of the bend.The LF can then be approximated with a single power-law distribution,with an index similar to the initial mass function index.
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