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Formation and evolution of late-type dwarf galaxies - I. NGC1705 and NGC1569
We present one-zone chemical evolution models for two dwarf starburstgalaxies, NGC1705 and NGC1569. Though especially designed for the inner~1 kpc region, where numerous HII regions and most of the stars areobserved, the models also account for the presence of extended gaseousand dark matter haloes, and properly compute the binding energy of thegas heated by supernova explosions. Using information about the paststar formation history and initial mass function of the systemspreviously obtained from Hubble Space Telescope optical andnear-infrared colour-magnitude diagrams, we identify possible scenariosof chemical enrichment and development of galactic winds. We assume thatthe galactic winds are proportional to the Type II and Type Ia supernovarates. As a consequence, they do not necessarily go to zero when thestar formation stops. In order not to overestimate the currentmetallicity of the interstellar gas inferred from HII regionspectroscopy, we suggest that the winds efficiently remove from thegalaxies the metal-rich ejecta of dying stars. Conversely, requiring thefinal mass of neutral gas to match the value inferred from 21-cmobservations implies a relatively low efficiency of interstellar mediumentrainment in the outflow, thus confirming previous findings that thewinds driving the evolution of typical starbursts are differential.These conclusions could be different only if the galaxies accrete hugefractions of unprocessed gas at late times. By assuming standard stellaryields we obtain a good fit to the observed nitrogen-to-oxygen (N/O)ratio of NGC1569, while the mean N/O ratio in NGC1705 is overestimatedby the models. Reducing the extent of hot bottom burning inlow-metallicity intermediate-mass stars does not suffice to solve theproblem. Localized self-pollution from stars more massive than 60Msolar in NGC1705 and/or funnelling of larger fractions ofnitrogen through its winds are then left to explain the discrepancybetween model predictions and observations. Inspection of the log(N/O)versus log(O/H)+12 diagram for a large sample of dwarf irregular andblue compact dwarf galaxies in the literature favours the latterhypothesis, but the physical mechanisms responsible for such a selectiveloss of metals remain unclear.
| On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
We report 4.5 μm luminosities for 27 nearby (D<~5 Mpc) dwarfirregular galaxies measured with the Spitzer Infrared Array Camera. Wehave constructed the 4.5 μm luminosity-metallicity (L-Z) relation for25 dwarf galaxies with secure distance and interstellar medium oxygenabundance measurements. The 4.5 μm L-Z relation is12+log(O/H)=(5.78+/-0.21)+(-0.122+/-0.012)M[4.5], whereM[4.5] is the absolute magnitude at 4.5 μm. The dispersionin the near-infrared L-Z relation is smaller than the correspondingdispersion in the optical L-Z relation. The subsequently derived stellarmass-metallicity (M*-Z) relation is12+log(O/H)=(5.65+/-0.23)+(0.298+/-0.030)logM*, and extendsthe SDSS M*- Z relation to lower mass by about 2.5 dex. Wefind that the dispersion in the M*-Z relation is similar over5 orders of magnitude in stellar mass, and that the relationship betweenstellar mass and interstellar medium metallicity is similarly tight fromhigh-mass to low-mass systems. We find a larger scatter at low mass inthe relation between effective yield and total baryonic mass. In fact,there are a few dwarf galaxies with large yields, which is difficult toexplain if galactic winds are ubiquitous in dwarf galaxies. The lowscatter in the L-Z and M*-Z relationships are difficult tounderstand if galactic superwinds or blowout are responsible for the lowmetallicities at low mass or luminosity. Naively, one would expect anever increasing scatter at lower masses, which is not observed.
| Modeling the Pan-Spectral Energy Distribution of Starburst Galaxies. II. Control of the H II Region Parameters
We examine, from a theoretical viewpoint, how the physical parameters ofH II regions are controlled in both normal galaxies and in starburstenvironments. These parameters are the H II region luminosity function,the time-dependent size, the covering fraction of molecular clouds, thepressure in the ionized gas, and the ionization parameter. The factorsthat control them are the initial mass function (IMF) of the excitingstars, the cluster mass function, the metallicity, and the mean pressurein the surrounding interstellar medium. We investigate the sensitivityof the Hα luminosity to the IMF, and find that this can translateto more than a factor 2 variation in derived star formation rates. Themolecular cloud dissipation timescale is estimated from a case study ofM17 to be ~1 Myr for this object. Based on H II luminosity functionfitting for nearby galaxies, we suggest that the H II region clustermass function is fitted by a lognormal form peaking at ~100Msolar. The cluster mass function continues the stellar IMFto a higher mass regime. The pressure in the H II regions is controlledby the mechanical luminosity flux from the central cluster. Since thisis closely related to the ionizing photon flux, we show that theionization parameter is not a free variable, and that the diffuseionized medium may be composed of many large, faint, and old H IIregions. Finally, we derive theoretical probability distributions forthe ionization parameter as a function of metallicity and compare theseto those derived for SDSS galaxies.
| 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.
| Kinematics of Interstellar Gas in Nearby UV-selected Galaxies Measured with HST STIS Spectroscopy
We measure Doppler shifts of interstellar absorption lines in HST STISspectra of individual star clusters in nearby UV-selected galaxies.Values for systemic velocities, which are needed to quantify outflowspeeds, are taken from the literature and verified with stellar lines.We detect outflowing gas in 8 of 17 galaxies via low-ionization lines(e.g., C II, Si II, Al II), which trace cold and/or warm gas. Thestarbursts in our sample are intermediate in luminosity (and mass) todwarf galaxies and luminous infrared galaxies (LIRGs), and we confirmthat their outflow speeds (ranging from -100 to nearly -520 kms-1, with an accuracy of ~80 km s-1) areintermediate to those previously measured in dwarf starbursts and LIRGs.We do not detect the outflow in high-ionization lines (such as C IV orSi IV); higher quality data will be needed to empirically establish howvelocities vary with the ionization state of the outflow. We do verifythat the low-ionization UV lines and optical Na I doublet give roughlyconsistent outflow velocities, solidifying an important link betweenstudies of galactic winds at low and high redshift. To obtain a highersignal-to-noise ratio (S/N), we create a local average compositespectrum and compare it to the high-z Lyman break composite spectrum. Itis surprising that the low-ionization lines show similar outflowvelocities in the two samples. We attribute this to a combination ofweighting toward higher luminosities in the local composite, as well asboth samples being, on average, brighter than the ``turnover''luminosity in the v-SFR relation.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. These observations areassociated with program GO-9036.
| Masses of the local group and of the M81 group estimated from distortions in the local velocity field
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
| Massive Star Cluster Populations in Irregular Galaxies as Probable Younger Counterparts of Old Metal-rich Globular Cluster Populations in Spheroids
Peak metallicities of metal-rich populations of globular clusters(MRGCs) belonging to early-type galaxies and spheroidal subsystems ofspiral galaxies (spheroids) of different mass fall within the somewhatconservative -0.7<=[Fe/H]<=-0.3 range. Indeed, if possible ageeffects are taken into account, this metallicity range might becomesmaller. Irregular galaxies such as the Large Magellanic Cloud (LMC),with longer timescales of formation and lower star formation (SF)efficiency, do not contain old MRGCs with [Fe/H]>-1.0, but they areobserved to form populations of young/intermediate-age massive starclusters (MSCs) with masses exceeding 104 Msolar.Their formation is widely believed to be an accidental process fullydependent on external factors. From the analysis of available data onthe populations and their hosts, including intermediate-age populousstar clusters in the LMC, we find that their most probable meanmetallicities fall within -0.7<=[Fe/H]<=-0.3, as the peakmetallicities of MRGCs do, irrespective of signs of interaction.Moreover, both the disk giant metallicity distribution function (MDF) inthe LMC and the MDFs for old giants in the halos of massive spheroidsexhibit a significant increase toward [Fe/H]~-0.5. That is in agreementwith a correlation found between SF activity in galaxies and theirmetallicity. The formation of both the old MRGCs in spheroids and MSCpopulations in irregular galaxies probably occurs at approximately thesame stage of the host galaxies' chemical evolution and is related tothe essentially increased SF activity in the hosts around the samemetallicity that is achieved very early in massive spheroids, later inlower mass spheroids, and much later in irregular galaxies. Changes inthe interstellar dust, particularly in elemental abundances in dustgrains and in the mass distribution function of the grains, may be amongthe factors regulating star and MSC formation activity in galaxies.Strong interactions and mergers affecting the MSC formation presumablyplay an additional role, although they can substantially intensify theinternally regulated MSC formation process. Several implications of oursuggestions are briefly discussed.
| Hubble Space Telescope STIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass on Hubble Type
We study the nuclear star clusters (NCs) in spiral galaxies of variousHubble types using spectra obtained with the STIS on board the HubbleSpace Telescope (HST). We observed the nuclear clusters in 40 galaxies,selected from two previous HST WFPC2 imaging surveys. At a spatialresolution of ~0.2" the spectra provide a better separation of clusterlight from underlying galaxy light than is possible with ground-basedspectra. Approximately half of the spectra have a sufficiently highsignal-to-noise ratio for detailed stellar population analysis. For theother half we only measure the continuum slope, as quantified by the B-Vcolor. To infer the star formation history, metallicity, and dustextinction, we fit weighted superpositions of single-age stellarpopulation templates to the high signal-to-noise ratio spectra. We usethe results to determine the luminosity-weighted age, mass-to-lightratio, and masses of the clusters. Approximately half of the sampleclusters contain a population younger than 1 Gyr. Theluminosity-weighted ages range from 10 Myr to 10 Gyr. The stellarpopulations of NCs are generally best fit as a mixture of populations ofdifferent ages. This indicates that NCs did not form in a single event,but that instead they had additional star formation long after theoldest stars formed. On average, the sample clusters in late-typespirals have a younger luminosity-weighted mean age than those inearly-type spirals (L=8.37+/-0.25 vs.9.23+/-0.21). The average mass-weighted ages are older by ~0.7 dex,indicating that there often is an underlying older population that doesnot contribute much light but does contain most of the mass. The averagecluster masses are smaller in late-type spirals than in early-typespirals (logM=6.25+/-0.21 vs. 7.63+/-0.24) and exceed the masses typicalof globular clusters. The cluster mass correlates loosely with totalgalaxy luminosity. It correlates more strongly with both the Hubble typeof the host galaxy and the luminosity of its bulge. The lattercorrelation has the same slope as the well-known correlation betweensupermassive black hole mass and bulge luminosity. The properties ofboth nuclear clusters and black holes in the centers of spiral galaxiesare therefore intimately connected to the properties of the host galaxy,and in particular its bulge component. Plausible formation scenarioshave to account for this. We discuss various possible selection biasesin our results, but conclude that none of them can explain thedifferences seen between clusters in early- and late-type spirals. Theinability to infer spectroscopically the populations of faint clustersdoes introduce a bias toward younger ages, but not necessarily towardhigher 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 NAS5-26555. These observations areassociated with proposals 9070 and 9783.
| Mid-Infrared Images of Stars and Dust in Irregular Galaxies
We present mid-IR to optical properties of 22 representative irregulargalaxies: 18 irregular (Im) galaxies, 3 blue compact dwarfs, and 1Magellanic-type spiral galaxy. The mid-IR is based on images from theSpitzer Space Telescope archives. The 3.6 and 4.5 μm bands and theUBVJHK images are used to examine disk morphology and the integrated andazimuthally averaged magnitudes and colors of stars. The nonstellarcontribution to the 4.5 μm images is used to trace hot dust. The 5.8and 8.0 μm images reveal emission from hot dust and polycyclicaromatic hydrocarbons (PAHs), and both may contribute to thesepassbands, although we refer to the nonstellar emission as PAH emission.We compare the 8.0 μm images to Hα. Im galaxies have no hiddenbars, and those with double-exponential optical light profiles have thesame at mid-IR. Most galaxies have similar optical and mid-IR scalelengths. Four galaxies have super star clusters that are not visible atoptical bands. Galaxies with higher area-normalized star formation rateshave more dust and PAH emission relative to starlight. Hot dust and PAHemission is found mostly in high surface brightness H II regions,implying that massive stars are the primary source of heating. Galaxieswith intense, widespread star formation have more extended PAH emission.The ratio of PAH to Hα emission is not constant on small scales.PAHs are associated with shells and giant filaments, so they are notdestroyed during shell formation.This work is based in part on archival data obtained with the SpitzerSpace Telescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology, under a contract with NASA.
| The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
| Far-Flung Filaments of Ejecta in the Young Supernova Remnant G292.0+1.8
New optical images of the young supernova remnant (SNR) G292.0+1.8,obtained from the 0.9 m telescope at Cerro Tololo Inter-AmericanObservatory, show a more extensive network of filaments than had beenknown previously. Filaments emitting in [O III] are distributedthroughout much of the 8' diameter shell seen in X-ray and radio images,including a few at the very outermost shell limits. In addition to theextensive [O III] filaments, we have detected four small complexes offilaments that show [S II] emission along with the oxygen lines. In asingle long-slit spectrum we find variations of almost an order ofmagnitude in the relative strengths of oxygen and sulfur lines, whichmust result from abundance variations. None of the filaments, with orwithout [S II], shows any evidence for hydrogen, so all appear to befragments of pure supernova ejecta. The [S II] filaments provide thefirst evidence for undiluted products of oxygen burning in the ejectafrom the supernova that gave rise to G292.0+1.8. Some oxygen burning,either hydrostatic or explosive, must have occurred, but the paucity of[S II]-emitting filaments suggests that either the oxygen burning wasnot extensive or that most of its products have yet to be excited. Mostof the outer filaments exhibit radial, pencil-like morphologies thatsuggest an origin as Rayleigh-Taylor fingers of ejecta, perhaps formedduring the explosion. Simulations of core-collapse supernovae predictthe development of such fingers, but these have never before been soclearly observed in a young SNR. Following careful subtraction of thestars in the field, we have measured the total flux in [O III]λ5007 as 5.4×10-12 ergs cm-2s-1. Using a distance of 6 kpc and an extinction correctioncorresponding to E(B-V)=0.6 mag (lower than previous values but moreconsistent both with our data and with X-ray and radio measurements ofthe hydrogen column density) leads to a luminosity of1.6×1035 ergs s-1 in the 5007 Å line.
| A catalogue of ultra-luminous X-ray source coincidences with FIRST radio sources
Aims.We search for ultra luminous X-ray source (ULXs) radio counterpartslocated in nearby galaxies in order to constrain their physicalnature. Methods: .Our work is based on a systematiccross-identification of the most recent and extensive available ULXcatalogues and archival radio data. Results: .A catalogue of 70positional coincidences is reported. Most of them are located within thegalaxy nucleus. Among them, we find 11 new cases of non-nuclear ULXsources with possibly associated radio emission.
| [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).
| Dynamical mass estimates for two luminous star clusters in galactic merger remnants
We present high-dispersion spectra of two extremely massive starclusters in galactic merger remnants, obtained using the UVESspectrograph mounted on the ESO Very Large Telescope. One cluster, W30,is located in the ~500 Myr old merger remnant NGC 7252 and has avelocity dispersion and effective radius of σ=27.5±2.5 kms-1 and Reff=9.3±1.7 pc, respectively. Theother cluster, G114, located in the ~3 Gyr old merger remnant NGC 1316,is much more compact, Reff=4.08±0.55 pc, and has avelocity dispersion of σ=42.1±2.8 km s-1. Thesemeasurements allow an estimate of the virial mass of the two clusters,yielding Mdyn(W30)=1.59(±0.26)× 10^7Mȯ and Mdyn(G114)=1.64(±0.13)×10^7 Mȯ. Both clusters are extremely massive, being morethan three times heavier than the most massive globular clusters in theGalaxy. For both clusters we measure light-to-mass ratios, which whencompared to simple stellar population (SSP) models of the appropriateage, are consistent with a Kroupa-type stellar mass function. Usingmeasurements from the literature we find a strong age dependence on howwell SSP models (with underlying Kroupa or Salpeter-type stellar massfunctions) fit the light-to-mass ratio of clusters. Based on this resultwe suggest that the large scatter in the light-to-mass ratio of theyoungest clusters is not due to variations in the underlying stellarmass function, but instead to the rapidly changing internal dynamics ofyoung clusters. Based on sampling statistics we argue that while W30 andG114 are extremely massive, they are consistent with being the mostmassive clusters formed in a continuous power-law cluster massdistribution. Finally, based on the positions of old globular clusters,young massive clusters (YMCs), ultra-compact dwarf galaxies (UCDs) anddwarf-globular transition objects (DGTOs) in κ-space we concludethat 1) UCDs and DGTOs are consistent with the high mass end of starclusters and 2) YMCs occupy a much larger parameter space than oldglobular clusters, consistent with the idea of preferential disruptionof star clusters.
| Conclusions of the workshop on the interferometric mode of OSIRIS
We present the conclusions of the workshop organized to define thescientific drivers and the derived main characteristics of high-orderscanning Fabry-Perot interferometers proposed to be coupled to OSIRIS atthe GTC.
| Some astronomical niches with 3D spectroscopy
An overview of some of the most interesting results obtained with theuse of 3D spectrometers working in 4m-class telescopes is given with thepurpose of taking advantage of those experiences in the definition ofscientific programs for telescopes of larger diameter as the GTC.
| Astrophysical magnetic fields and nonlinear dynamo theory
Electronic Article Available from Elsevier Science.
| X-ray observations of the edge-on star-forming galaxy NGC 891 and its supernova SN1986J
We present XMM-Newton observations of NGC 891, a nearby edge-on spiralgalaxy. We analyse the extent of the diffuse emission emitted from thedisc of the galaxy, and find that it has a single-temperature profilewith best-fitting temperature of 0.26 keV, though the fit of adual-temperature plasma with temperatures of 0.08 and 0.30 keV is alsoacceptable. There is a considerable amount of diffuse X-ray emissionprotruding from the disc in the north-west direction out toapproximately 6 kpc. We analyse the point-source population using aChandra observation, using a maximum-likelihood method to find that theslope of the cumulative luminosity function of point sources in thegalaxy is -0.77+0.13-0.1. Using a sample of otherlocal galaxies, we compare the X-ray and infrared properties of NGC 891with those of `normal' and starburst spiral galaxies, and conclude thatNGC 891 is most likely a starburst galaxy in a quiescent state. Weestablish that the diffuse X-ray luminosity of spirals scales with thefar-infrared luminosity asLX~L0.87+/-0.07FIR, except for extremestarbursts, and NGC 891 does not fall in the latter category. We studythe supernova SN1986J in both XMM-Newton and Chandra observations, andfind that the X-ray luminosity has been declining with time more steeplythan expected (LX~t-3).
| GHASP: an Hα kinematic survey of spiral and irregular galaxies - IV. 44 new velocity fields. Extension, shape and asymmetry of Hα rotation curves
We present Fabry-Perot observations obtained in the frame of the GHASPsurvey (Gassendi HAlpha survey of SPirals). We have derived the Hαmap, the velocity field and the rotation curve for a new set of 44galaxies. The data presented in this paper are combined with the datapublished in the three previous papers providing a total number of 85 ofthe 96 galaxies observed up to now. This sample of kinematical data hasbeen divided into two groups: isolated (ISO) and softly interacting(SOFT) galaxies. In this paper, the extension of the Hα discs, theshape of the rotation curves, the kinematical asymmetry and theTully-Fisher relation have been investigated for both ISO and SOFTgalaxies. The Hα extension is roughly proportional toR25 for ISO as well as for SOFT galaxies. The smallestextensions of the ionized disc are found for ISO galaxies. The innerslope of the rotation curves is found to be correlated with the centralconcentration of light more clearly than with the type or thekinematical asymmetry, for ISO as well as for SOFT galaxies. The outerslope of the rotation curves increases with the type and with thekinematical asymmetry for ISO galaxies but shows no special trend forSOFT galaxies. No decreasing rotation curve is found for SOFT galaxies.The asymmetry of the rotation curves is correlated with themorphological type, the luminosity, the (B-V) colour and the maximalrotational velocity of galaxies. Our results show that the brightest,the most massive and the reddest galaxies, which are fast rotators, arethe least asymmetric, meaning that they are the most efficient withwhich to average the mass distribution on the whole disc. Asymmetry inthe rotation curves seems to be linked with local star formation,betraying disturbances of the gravitational potential. The Tully-Fisherrelation has a smaller slope for ISO than for SOFT galaxies.
| Broad-band colours and overall photometric properties of template galaxy models from stellar population synthesis
We present here a new set of evolutionary population synthesis modelsfor template galaxies along the Hubble morphological sequence. Themodels, which account for the individual evolution of the bulge, disc,and halo components, provide basic morphological features, along withbolometric luminosity and colour evolution (including Johnson/Cousins,Gunn g, r, i, and Washington C, M, T1, T2photometric systems) between 1 and 15 Gyr. The luminosity contributionfrom residual gas is also evaluated, both in terms of nebular continuumand Balmer-line enhancement.Our theoretical framework relies on the observed colours of present-daygalaxies, coupled with a minimal set of physical assumptions related tosimple stellar population (SSP) evolution theory, to constrain theoverall distinctive properties of galaxies at earlier epochs. Acomparison with more elaborate photometric models, and with empiricalsets of reference spectral energy distributions (SEDs) for early- andlate-type galaxies is accomplished, in order to test output reliabilityand investigate the internal uncertainty of the models.The match with observed colours of present-day galaxies tightlyconstrain the stellar birth rate, b, which smoothly increases from E toIm types. The comparison with the observed supernova (SN) rate inlow-redshift galaxies shows, as well, a pretty good agreement, andallows us to tune up the inferred star formation activity and the SN andhypernova rates among the different galaxy morphological types. Amongothers, these results could find useful application also in cosmologicalstudies, given for instance the claimed relationship between hypernovaevents and gamma-ray bursts.One outstanding feature of the back-in-time evolution model is theprevailing luminosity contribution of the bulge at early epochs. As aconsequence, the current morphological look of galaxies mightdrastically change when moving to larger distances, and we discuss herehow sensibly this bias could affect the observation (and theinterpretation) of high-redshift surveys.In addition to broad-band colours, the modelling of Balmer line emissionin disc-dominated systems shows that striking emission lines, likeHα, can very effectively track stellar birth rate in a galaxy. Forthese features to be useful age tracers as well, however, one shouldfirst assess the real change of b versus time on the basis ofsupplementary (and physically independent) arguments.
| From young massive star cluster to old globular: the LV-σ0 relationship as a diagnostic tool
We present a new analysis of the properties of the young massive starclusters (YMCs) forming profusely in intense starburst environments,which demonstrates that these objects are plausible progenitors of theold globular clusters (GCs) seen abundantly in the Local Group. Themethod is based on the tight relationship for old GCs between theirV-band luminosities, LV, and (central) velocity dispersions,σ0. We improve the significance of the relationship byincreasing the GC sample size and find that its functional form,LV/Lsolar~σ1.57+/-0.100(km s-1), is fully consistent with previous determinationsfor smaller Galactic and M31 GC samples. The tightness of therelationship for a GC sample drawn from environments as diverse as thosefound in the Local Group implies that its origin must be sought inintrinsic properties of the GC formation process itself. We evolve theluminosities of those YMCs in the local Universe which have velocitydispersion measurements to an age of 12 Gyr, adopting a variety ofinitial mass function (IMF) descriptions, and find that most YMCs willevolve to loci close to, or to slightly fainter luminosities than theimproved GC relationship. In the absence of significant externaldisturbances, this implies that these objects may potentially survive tobecome old GC-type objects over a Hubble time. The main advantage of ournew method is its simplicity. Whereas alternative methods, based ondynamical mass estimates, require one to obtain accurate size estimatesand to make further assumptions, the only observables required here arethe system's velocity dispersion and luminosity. The most importantfactor affecting the robustness of our conclusions is the adopted formof the IMF. We use the results of N-body simulations to confirm thatdynamical evolution of the clusters does not significantly alter ourconclusions about the likelihood of individual clusters surviving tolate times. Finally, we find that our youngest observed clusters areconsistent with having evolved from a relation of the form . Thisrelation may actually correspond to the origin of the GC fundamentalplane.
| Σ-D relation for supernova remnants and its dependence on the density of the interstellar medium
During the last couple of decades of work on the Σ-D (radiosurface brightness to diameter) relation for supernova remnants (SNRs),it has been generally accepted that no single Σ-D relation can beconstructed for all SNRs. However, it may still be possible to constructthe relations for some classes of SNRs. In our previous paper weanalysed Σ-D relation(s) for remnants in the dense environments ofmolecular clouds. The aim of this paper is to examine, in the samecontext, a class of oxygen-rich SNRs, and to extend the analysis toremnants evolving in lower-density interstellar media, namelyBalmer-dominated SNRs. We have obtained good relations with certainsimilarities to our previous findings - similarities that emphasize,again, the role of ambient density in the evolution of SNRs.
| A Chandra X-ray survey of nearby dwarf starburst galaxies - II. Starburst properties and outflows
We present a comprehensive comparison of the X-ray properties of asample of eight dwarf starburst galaxies observed with Chandra (IZw18,VIIZw403, NGC1569, NGC3077, NGC4214, NGC4449, NGC5253 and He2-10). InPaperI, we presented in detail the data reduction and analysis of theindividual galaxies. For the unresolved X-ray sources, we find thefollowing: point sources are in general located close to bright HIIregions, rims of superbubbles or young stellar clusters. The number ofX-ray point sources appears to be a function of the current starformation (SF) rate and the blue luminosity of the hosts. UltraluminousX-ray sources (ULXs) are only found in those dwarf galaxies that arecurrently interacting. The power-law (PL) index of the combinedcumulative X-ray point-source luminosity function is α= 0.24 +/-0.06, shallower than that of more massive starburst galaxies (α=0.4 -0.8) and of non-starburst galaxies (α~ 1.2). For thosegalaxies showing extended X-ray emission (six out of the eightgalaxies), we derive the following: superwinds develop along thesteepest gradient of the HI distribution with volume densities of0.02-0.06cm-3, pressures of 1-3 ×105Kcm-3, thermal energies of 2-30 ×1054erg and hot gas masses of 2-20 ×106Msolar (~1 per cent of the HI masses). Onglobal scales, the distribution of the X-ray emission looks remarkablysimilar to that seen in Hα (comparing azimuthal averages);locally, however, their distribution is clearly distinct in many cases,which can be explained by the different emission mechanisms (forwardversus reverse shocks). Mass loading of order 1 to 5 is required toexplain the differences between the amount of hot gas and the modelledmass loss from massive stars. The metallicity of the dwarf galaxiescorrelates with the diffuse X-ray luminosity and anticorrelates with thecooling time of the hot gas. The diffuse X-ray luminosity is also afunction of the current star formation rate (SFR). The mechanicalluminosities of the developing superwinds are energetic enough toovercome the gravitational potentials of their host galaxies. Thisscenario is supported by the overpressures of the hot gas compared withthe ambient interstellar medium (ISM). Extended HI envelopes such astidal tails, however, may delay outflows on time-scales exceeding thoseof the cooling time of the hot gas.
| A Chandra X-ray survey of nearby dwarf starburst galaxies - I. Data reduction and results
We present an analysis of Chandra X-ray observations of a sample ofeight dwarf starburst galaxies (IZw18, VIIZw403, NGC1569, NGC3077,NGC4214, NGC4449, NGC5253 and He2-10). Extended, diffuse X-ray emissionis detected in all but two of the objects. Unresolved sources were foundwithin all dwarf galaxies (total: 55 sources). These point sources arewell fitted by power-law (PL), thermal plasma (TP) or blackbody (BB)models. 10 of the point sources exceed an X-ray luminosity of1039 erg s-1 (ultraluminous X-ray sources, ULXs).In those galaxies where diffuse X-ray emission is detected, thisemission (with X-ray luminosities ranging from 4 × 1038to 2 × 1040 erg s-1) contains most (60-80per cent) of the X-ray photons. This diffuse emission can be well fittedby MEKAL one-temperature TP models once the contribution from theunresolved point sources is subtracted properly. The diffuse X-raycomponent is significantly extended, reaching as far as 0.5-5kpc intothe outskirts of their hosts. Azimuthally averaged X-ray surfacebrightness profiles are well approximated by exponential functions.Temperatures of various regions within the galaxies range from 1.6 to7.6 × 106K. With few exceptions, temperatures of thehot gas are remarkably uniform, hovering around 2-3 ×106K. Temperatures of the coronal gas in the outer regionsare in general ~2-3 times lower than those found in the central regions.Fits to the diffuse emission do not allow strong constraints to be puton the metallicities of the emitting plasmas. However, the derivedmetallicities are compatible with those determined from their HIIregions. An α/Fe ratio of ~2 is indicated for the hot gas withinat least three objects (NGC1569, NGC4449 and He2-10). Shadowing of thediffuse X-ray emission by the cooler disc gas is used to constrain theorientation of the galaxies.
| MRC B1221-423: a compact steep-spectrum radio source in a merging galaxy
We present BVRIK images and spectroscopic observations of the z= 0.17host galaxy of the compact steep-spectrum (CSS) radio source MRCB1221-423. This is a young (~105 yr) radio source with doublelobes lying well within the visible galaxy. The host galaxy isundergoing tidal interaction with a nearby companion, with shells, tidaltails and knotty star-forming regions all visible. We analyse the imagesof the galaxy and its companion pixel-by-pixel, first usingcolour-magnitude diagrams and then fitting stellar population models tothe spectral energy distributions (SEDs) of each pixel. We also presentmedium-resolution spectroscopy of the system.The pixels separate cleanly in colour-magnitude diagrams, with pixels ofdifferent colours occupying distinct regions of the host galaxy and itscompanion. Fitting stellar population models to these colours, we haveestimated the age of each population. We find three distinct groups ofages: an old population (τ~ 15 Gyr) in the outskirts of the hostgalaxy; an intermediate-age population (τ~ 300 Myr) around thenucleus and tidal tail, and a young population (τ<~ 10 Myr) inthe nucleus and blue knots.The spectrum of the nucleus shows numerous strong emission lines,including [OI]λ6300, [O]λ3727, [SII]λλ6716,6731, Hα and [NII]λλ6548, 6583, characteristic of alow-ionization nuclear emission-line region (LINER) spectrum. Thecompanion galaxy shows much narrower emission lines with very differentline ratios, characteristic of a starburst galaxy.We have evidence for three distinct episodes of star formation inB1221-423. The correlation of age with position suggests the two mostrecent episodes were triggered by tidal interactions with the companiongalaxy. The evidence points to the active galactic nucleus (AGN) in thecentre of B1221-423 having been caught in the act of ignition. However,none of the components we have identified is as young as the radiosource, implying that the delay between the interaction and thetriggering of the AGN is at least 3 × 108 yr.
| An Analytic Model of Galactic Winds and Mass Outflows
Galactic winds and mass outflows are observed both in nearby starburstgalaxies and in high-redshift star-forming galaxies. We develop a simpleanalytic model to understand the observed superwind phenomenon with adiscussion of the model uncertainties. Our model is built upon the modelof McKee & Ostriker for the interstellar medium. It allows one topredict how properties of a superwind, such as wind velocity and massoutflow rate, are related to properties of its star-forming host galaxy,such as size, gas density and star formation rate. The model predicts athreshold of star formation rate density for the generation ofobservable galactic winds. Galaxies with more concentrated starformation activities produce superwinds with higher velocities. Thepredicted mass outflow rates are comparable to (or slightly larger than)the corresponding star formation rates. We apply our model to both localstarburst galaxies and high-redshift Lyman break galaxies, and find itspredictions to be in good agreement with current observations. Our modelis simple and so can be easily incorporated into numerical simulationsand semi-analytical models of galaxy formation.
| First Results from THINGS: The HI Nearby Galaxy Survey
We describe The HI Nearby Galaxy Survey (THINGS), the largestprogramever undertaken at the Very Large Array to perform 21-cm HIobservations of thehighest quality ( 7'', ≤ 5 km s^{-1}resolution) ofnearby galaxies. The goal of THINGS is to investigatekeycharacteristics related to galaxy morphology, star formation andmassdistribution across the Hubble sequence. A sample of 34 objectswithdistances between 3 and 10 Mpc will be observed, covering a widerangeof evolutionary stages and properties. Data from THINGSwillcomplement SINGS, the Spitzer Infrared Nearby Galaxy Survey. Forthe THINGS sample, high-quality observations at comparable resolutionwillthus be available from the X-ray regime through to the radio partofthe spectrum. THINGS data can be used to investigate issues such asthesmall-scale structure of the ISM, its three-dimensional structure,the(dark) matter distribution and processes leading to starformation. Todemonstrate the quality of the THINGS data products, wepresent someprelimary HI maps here of four galaxies from the THINGSsample.
| The Classification of Galaxies: Early History and Ongoing Developments
"You ask what is the use of classification, arrangement,systematization. I answer you; order and simplification are the firststeps toward the mastery of a subject the actual enemy is the unknown."
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| Comparison of Star Clusters With and Without Wolf-Rayet Stars in Wolf-Rayet Galaxies
We compare the properties of young star clusters with and withoutWolf-Rayet (W-R) stars in W-R galaxies using optical, near-infraredimagery and optical spectroscopy. Our work identifies the clusters withW-R stars in these galaxies for the first time. With this information,comparisons of clusters with and without W-R stars are now possible,enabling us to understand the chemical and morphological impact ofmassive stars on their environment and to constrain the parameters formodeling these systems. We find that clusters with W-R stars (W-Rclusters) are systematically younger, bluer clusters. Knowing this agedifference between the two cluster sets, we use an evolutionary scenarioto interpret their other properties. Young clusters, typically W-Rclusters, have a Strömgren sphere-like gas configuration. They alsotend to have H-K colors redder than those of theoretical models. Weinterpret the H-K excess as a combination of thermal emission from hotdust, nebular emission, and molecular emission. Older clusters,typically clusters without W-R stars, have ionized gas in a superbubbleconfiguration caused by the prolonged influence of stellar winds andsupernovae. The H-K excess is generally absent for these clusters. Thenitrogen-to-oxygen abundance ratio (N/O) does not appear to increase asa function of age over the first 10 Myr. Systems without W-R stars doappear to have a significant, elevated N/O over systems with W-R starsin the metallicity range 12+log(O/H)=7.7-7.9. For the entire metallicityrange in our sample, this finding is only marginally significant. Weconcur with previous studies, which find no correlation between thesulfur-to-oxygen abundance ratio and metallicity.
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