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The Calibration of the O/H Abundance Indicators for Extragalactic H II Regions based on O II Recombination Lines
Based on O II recombination lines we present a new calibration (called OII_{RL}) of Pagel's O_{23} indicator to determine the O/H abundanceratio in extragalactic H II regions and emission line galaxies. The OII_{RL} calibration produces O/H abundances about a factor of two higherthan those derived from the T(4363) method with t^2 = 0.00. The OII_{RL} calibration has implications for the study of differentproperties of emission line galaxies such as their metallicity, starformation rate, and initial mass function. The O II_{RL} calibrationalso affects the abundance determinations based on other O/H indicators,that include collisionally excited lines, like those known as O_3N_2,N_2, S_{23], Ar_3O_3, and S_3O_3. We argue that the controversy betweenthe T(4363) method and the photoionization models method to derive O/Hvalues is mainly due to temperature variations inside the observed H IIregions.
| Multiwavelength Study of Young Massive Star Clusters in the Interacting Galaxy Arp 24
We made a multiwavelength study of young massive star clusters (YSCs) inthe interacting galaxy Arp 24 using optical and ultraviolet images fromthe Hubble Space Telescope (HST), the Sloan Digital Sky Survey, and theGalaxy Evolution Explorer, mid-infrared images from the Spitzer SpaceTelescope, and narrowband Hα images and optical spectra from theNational Astronomical Observatories of the Chinese Academy of Sciences2.16 m telescope. Based on the HST images, we found that the brightestinfrared knot in Arp 24 is associated with a complex of five YSCs withina region of ~0.95" radius (127 pc) in size. The ages and masses of thestar clusters in this complex and other regions were estimated using HSTbroadband photometries and the Starburst99 synthesis models. The starclusters in this complex are very young (within ages of ~3-5 Myr) andmassive (masses of ~105 Msolar). The ionizationparameter and metallicity of the complex were estimated using theemission-line ratios, and the star formation rates were calculated usingmonochromatic 24 μm, far-ultraviolet, and Hα-line luminosities.We speculate that Arp 24 may have formed in a retrograde flyby encounterindicated by its one-armed appearance and fanlike structure, and theformation of the YSCs in this galaxy was triggered by the interaction.The clusters in the YSC complex may have formed in a single giantmolecular cloud simultaneously. From the ultraviolet to mid-infraredspectral energy distributions, we found that the region of the YSCcomplex is relatively bluer in the optical and has higher 24 μm dustemission relative to the starlight and 8 μm emission. This warminfrared color may due to a strong UV radiation field or othermechanisms (e.g., shocks) within this region that may destroy thepolycyclic aromatic hydrocarbons and enhance the small-grain emission at24 μm.
| Temperature fluctuations in H II regions: t{^2} for the two-phase model
Aims.We investigate temperature fluctuations in H II regions in terms ofa two-phase model, which assumes that the nebular gas consists of a hotand a cold phase. Methods: We derive general formulae for T([O III]),the [O III] forbidden line temperature, and T(H I), the hydrogen Balmerjump temperature, in terms of the temperatures of the hot and coldphases, Th and T_c. Results: For large temperaturedifferences, the values of t2 required to account for theobserved difference between T([O III]) and T(H I) are much lower thanthose deduced using the classical formulae that assume random and smallamplitude temperature fluctuations. One should therefore be cautiouswhen using a two-phase model to account for empirically derivedt2 values. We present a correction of a recent work byGiammanco & Beckman, who use a two-phase model to estimate theionization rate of H II regions by cosmic rays. We show that a verysmall amount of cold gas is sufficient to account for t2values typically inferred for H ii regions.
| The Oxygen Abundance in the Inner H II Regions of M101: Implications for the Calibration of Strong-Line Metallicity Indicators
I present deep spectroscopy of four H II regions in the inner,metal-rich zone of the spiral galaxy M101 obtained with the LRISspectrograph at the Keck telescope. From the analysis of thecollisionally excited lines in two of the target H II regions, H1013 andH493, I have obtained oxygen abundances 12+log(O/H)=8.52 and12+log(O/H)=8.74, respectively. These measurements extend thedetermination of the oxygen abundance gradient of M101 via the directmethod to only 3 kpc from the center. The intensity of the C IIλ4267 line in H1013 leads to a carbon abundance 12+log(C/H)=8.66,corresponding to nearly twice the solar value. From a comparison of thecontinuum temperature derived from the Balmer discontinuity, T(Bac)=5000K, and the line temperature derived from [O III]λ4363/λ5007, T[O III]=7700 K, an average temperatureT0=5500 K and a mean square temperature fluctuationt2=0.06 have been derived. Accounting for the spatialinhomogeneity in temperature raises the oxygen abundance obtained fromthe oxygen auroral lines to 12+log(O/H)=8.93. These findings arediscussed in the context of the calibration of strong-line metallicityindicators, in particular of the upper branch of R23. Thereis no evidence for the strong abundance biases arising from temperaturegradients predicted theoretically for metal-rich H II regions.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of 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.
| The Localized Chemical Pollution in NGC 5253 Revisited: Results from Deep Echelle Spectrophotometry
We present echelle spectrophotometry of the blue compact dwarf galaxyNGC 5253 obtained with the VLT UVES. We have measured the intensities ofa large number of permitted and forbidden emission lines in four zonesof the central part of the galaxy. We detect faint C II and O IIrecombination lines, the first time that these are unambiguouslydetected in a dwarf starburst galaxy. The physical conditions of theionized gas have been derived using a large number of different lineintensity ratios. Chemical abundances of He, N, O, Ne, S, Cl, Ar, and Fehave been determined following standard methods. C++ andO++ abundances have been derived from pure recombinationlines and are larger than those obtained from collisionally excitedlines (from 0.30 to 0.40 dex for C++ and from 0.19 to 0.28dex for O++). This result is consistent with a temperaturefluctuation parameter (t2) between 0.050 and 0.072. Weconfirm previous results that indicate the presence of a localized Nenrichment in certain zones of NGC 5253 and detect a possible slight Heoverabundance in the same zones. The enrichment pattern agrees with thatexpected for the pollution by the ejecta of Wolf-Rayet (W-R) stars. Theamount of enriched material needed to produce the observed overabundanceis consistent with the mass lost by the number of W-R stars estimated inthe starbursts. We discuss the possible origin of the difference betweenabundances derived from recombination and collisionally excited lines(the so-called abundance discrepancy problem) in H II regions, findingthat a recent hypothesis based on the delayed enrichment by SN ejectainclusions seems not to explain the observed features.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 70.C-0008(A).
| The oxygen abundances in HII regions of the spiral galaxy M101 from the Sloan Digital Sky Survey spectra.
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| The massive star population in the giant HII region Tol89 in NGC5398
We present new high spectral resolution Very Large Telescope(VLT)/UV-Visual Echelle Spectrograph (UVES) spectroscopy and archivalHubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS)imaging and spectroscopy of the giant HII region Tol89 in NGC5398. Fromoptical and ultraviolet (UV) HST images, we find that the star-formingcomplex as a whole contains at least seven young compact massiveclusters. We resolve the two brightest optical knots, A and B, into fiveindividual young massive clusters along our slit, A1-4 and B1,respectively. From UV spectral modelling using the STARBURST99 code ofLeitherer et al., and nebular Hβ equivalent widths in the optical,we derive ages that are consistent with the formation of two separateburst events, of ~4 +/- 1 Myr and <3 Myr for knots A (A1-4) and B(B1), respectively. A Large Magellanic Cloud (LMC) metallicity ismeasured for both knots from a nebular line analysis, while nebular HeII4686 is observed in knot B and perhaps in knot A. We detect underlyingbroad wings on the strongest nebular emission lines indicatingvelocities up to 600 km s-1. From UV and opticalspectroscopy, we estimate that there are ~95 early WN stars and ~35early WC stars in Tol89-A, using empirical template spectra of LMCWolf-Rayet (WR) stars from Crowther & Hadfield, with the WCpopulation confined to cluster A2. Remarkably, we also detect a smallnumber of approximately three mid WNs in the smallest (mass) cluster inTol89-A, A4, whose spectral energy output in the UV is entirelydominated by the WN stars. From the strength of nebular Hβ, weobtain N(O) ~ 690 and 2800 for knots A and B, respectively, whichimplies N(WR)/N(O) ~ 0.2 for knot A. We also employ a complementaryapproach using STARBURST99 models, in which the O star content isinferred from the stellar continuum, and the WR population is obtainedfrom spectral synthesis of optical WR features using the grids fromSmith et al. We find reasonable agreement between the two methods forthe O star content and the N(WR)/N(O) ratio but find that the WR subtypedistribution is in error in the STARBURST99 models, with far too few WNstars being predicted. We attribute this failure to the neglect ofrotational mixing in evolutionary models. Our various modellingapproaches allow us to measure the cluster masses. We identify A1 as asuper star cluster (SSC) candidate with a mass of ~1-2 ×105 Msolar. A total mass of ~6 ×105 Msolar is inferred for the ionizing sourceswithin Tol89-B.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 73.B-0238(A) and with the NASA/ESA HST, obtainedfrom the ESO/ST-ECF Science Archive Facility.E-mail: fs@star.ucl.ac.uk
| Hα line profiles for a sample of supergiant HII regions. II. Broad, low intensity components
We analyze the broad, low intensity, high velocity components that areseen in the Hα line profiles for a sample of HII regions. TheseHII regions are chosen from among the brightest and most isolated in asample of spiral galaxies for which we have photometric andspectroscopic data: NGC 157, NGC 3631, NGC 6764, NGC 3344, NGC 4321, NGC5364, NGC 5055, NGC 5985, and NGC 7479. We confirm that the lineprofiles of most of these bright, giant extragalactic HII regionscontain broad kinematic components of low intensity, but high velocity,that we denote as wings. We analyze these components, deriving emissionmeasures, central velocities, and velocity dispersions of the blue andred features, which are similar. We interpret these components asexpanding shells within the HII regions and produced by the stellarwinds from the ionizing stars. We compare the kinetic energies of theseexpanding shells with the kinetic energy available from the stellarwinds. If we allow for the hypothesis that the brightest HII regions aredensity bounded, we show that, for these HII regions, the stellar windmechanism can explain the observed shell kinetic energies.
| Stellar Abundances and Molecular Hydrogen in High-Redshift Galaxies: The Far-Ultraviolet View
FUSE spectra of star-forming regions in nearby galaxies are compared tocomposite spectra of Lyman break galaxies (LBGs), binned by strength ofLyα emission and by mid-UV luminosity. Several far-UV spectralfeatures, including lines dominated by stellar wind and by photosphericcomponents, are very sensitive to stellar abundances. Their measurementin LBGs is compromised by the strong interstellar absorption features,allowing in some cases only upper limits to be determined. The derived Cand N abundances in the LBGs are no higher than half solar (scaled tooxygen abundance for comparison with emission-line analyses),independent of the strength of Lyα emission. P V absorptionindicates abundances as low as 0.1 solar, with an upper limit near 0.4solar in the reddest and weakest emission galaxies. Unresolvedinterstellar absorption components would further lower the derivedabundances. Trends of line strength and derived abundances are strongerwith mid-UV luminosity than with Lyα strength. H2absorption in the Lyman and Werner bands is very weak in the LBGs.Template H2 absorption spectra convolved to the appropriateresolution show that strict upper limitsN(H2)<1018 cm-2 apply in all cases,with more stringent values appropriate for the stronger emissioncomposites and for mixes of H2 level populations like thoseon Milky Way sight lines. Since the UV-bright regions are likely to bewidespread in these galaxies, these results rule out massive diffusereservoirs of primordial H2 and suggest that the dust-to-gasratio is already fairly large at z~3.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE). FUSE is operated for NASA by The JohnsHopkins University under NASA contract NAS5-32985.
| Massive Stellar Content of Giant H II Regions in M33 and M101
Far-ultraviolet (900-1200 Å) spectral synthesis of nine giantextragalactic H II regions in M33 and M101 is performed to study theirmassive stellar content. Several parameters are quantified, predicted,and compared to the literature: age, stellar mass, initial mass function(IMF) slope, number of O-type and Wolf-Rayet stars, and Hα and5500 Å continuum fluxes. The results of this particular techniqueare consistent with other methods and observations. This work shows thata total stellar mass of a few 103 Msolar is neededto populate the IMF bins well enough at high masses to obtain accurateresults from the spectral synthesis technique in the far-ultraviolet. Aflat IMF slope seems to characterize better the stellar line profiles ofthese objects, which is likely the first sign of a small numberstatistics effect on the IMF. Finally, the H II region NGC 5461 isidentified as a good candidate for hosting a second generation of stars,not yet seen at far-ultraviolet wavelengths.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| Temperature fluctuations in H II regions: Ionization by cosmic rays as a key mechanism
We present a detailed model capable of explaining quantitatively thetemperature fluctuations observed in luminous, large H II regions. Themodel is based on two assumptions which we justify on the basis ofobservations: that the major fraction of the hydrogen in the clouds thatform the H II regions is not photoionized and is essentially H I, thisH I is lightly ionized by fluxes of low energy cosmic rays (CR) producedby processes originating in the hot stars which illuminate the regions.
| Abundance gradients in a sample of barred spiral galaxies
We used photoionization models in order to reproduce the observedgradients of emission-line ratios for H II regions located in the normalspiral galaxy M 101 and in three barred spiralgalaxies, namely NGC 1365, NGC925, and NGC 1073. The behavior of thefollowing nebular parameters across the disk of these galaxies wasdetermined: temperature of the ionizing star (T_eff), ionizationparameter (U), and the abundance ratios O/H, N/O, and S/O. Our O/Hpredictions were found to be consistent with some empirical abundancedeterminations, but are overestimated by a factor of 0.1-0.4 dex whencompared to the direct abundance determinations. NGC1073 seems to be overabundant in nitrogen compared to otherspiral galaxies. No gradient of S/O was found in the studied galaxies,and we derived a positive T_eff gradient of triangle T_eff/triangle R =(400 ± 112) K kpc-1 and U ranging from -3.0 to -2.3.The N/O vs. O/H diagram in general is consistent with chemical evolutionmodels that assume that the nitrogen synthesis has both a primary and asecondary component compared to oxygen. However, a very strong N/Odependence on the O/H at high abundance was found.
| Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved H II Regions
We present high spatial resolution X-ray imaging data for theinteracting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-raytelescope. In addition to the unresolved starburst nucleus, a variablepoint source with LX~1040 ergs s-1 wasdetected 1.5" (270 pc) to the northwest of the nucleus, coincident witha blue, extremely optically luminous (MV~-14.1) point sourceon Hubble Space Telescope images. Eleven other candidate pointlikeultraluminous X-ray sources (ULXs) were also detected in the vicinity ofNGC 7714/7715, two of which exceed 1040 ergs s-1.Ten of these appear to be associated with interaction-induced features,but only two are associated with star formation regions. We also founddiffuse emission with LX~3×1040 ergss-1 extending 11" (1.9 kpc) to the north of the nucleus. Itsspectrum can be fitted with either a two-temperature MEKAL function(kT=0.59+0.05-0.06 and8+10-3 keV) or a 0.6 keV MEKAL function plus apower law (Γ=1.8+/-0.2). The hard component may be due tohigh-mass X-ray binaries (HMXBs) with possible contributions frominverse Compton radiation, while the soft component is likely from asuperwind. Superbubble models imply an expansion age of ~15 Myr,supporting previous assertions of an intermediate-age nuclear stellarpopulation in addition to a 5 Myr starburst. We also detected extendedX-ray emission associated with four extranuclear H II region complexes.The emission from these H II regions and the nuclear starburst could bedue to either an enhanced population of HMXBs relative to Local Groupgalactic averages or to diffuse gas heated by winds from supernovae, ifthe X-ray production efficiency LX/Lmech is high(~5%). To estimate LX/Lmech, we collectedpublished data for well-studied H II regions and superbubbles in nearbygalaxies. For H II regions with ages less than 3.5 Myr, the medianLX/Lmech~0.02%, while for older star formationregions, LX/Lmech~0.2%-7%. Thus, it is possiblethat gas heating by supernovae may be sufficient to account for theobserved X-rays from these H II regions. In galaxies much more distantthan NGC 7714, for example, the Cartwheel galaxy, H II region complexessimilar to those in NGC 7714 will be unresolved by Chandra and willmimic ULXs. No X-ray emission was detected from the Type Ib supernova SN1999dn, with an upper limit of ~2×1038 ergss-1.
| Clusters in the Luminous Giant H II Regions in M101
We have obtained HST WFPC2 observations of three very luminous butmorphologically different giant H II regions (GHRs) in M101, NGC 5461,NGC 5462, and NGC 5471, in order to study cluster formation in GHRs.Images obtained in the F547M and F675W bands are used to identifycluster candidates and for photometric measurements, and images in theF656N band are used to show ionized interstellar gas. The measuredcolors and magnitudes are compared with the evolutionary tracksgenerated by the Starburst99 and Bruzual & Charlot populationsynthesis models to determine the ages and masses of the clustercandidates that are more luminous than MF547M=-9.0. Thebrightest clusters detected in the PC images are measured and found tohave effective radii of 0.7-2.9 pc. NGC 5461 is dominated by a veryluminous core and has been suggested to host a super-star cluster (SSC).Our observations show that it contains three R136-class clusterssuperposed on a bright stellar background in a small region. This tightgroup of clusters may dynamically evolve into an SSC in the future, andmay appear unresolved and be identified as an SSC at large distances,but at present NGC 5461 contains no SSCs. NGC 5462 consists of looselydistributed H II regions and clusters without a prominent core. It hasthe largest number of cluster candidates among the three GHRs studied,but most of them are faint and older than 10 Myr. NGC 5471 has multiplebright H II regions and contains a large number of faint clustersyounger than 5 Myr. Two of the clusters in NGC 5471 are older than R136,but just as luminous; they may be the most massive clusters in the threeGHRs studied. The fraction of stars formed in massive clusters has beenestimated from the clusters' contribution to the total stellar continuumemission and from a comparison between the ionizing power of theclusters and the ionizing requirement of the associated H II regions.Both estimates show that <~50% of massive stars are formed in massiveclusters; consequently, the Hα luminosity of an H II region doesnot provide a sufficient condition for the existence of SSCs. Thecluster luminosity functions (LFs) of the three GHRs show differentslopes. NGC 5462 has the steepest cluster LF and the most looselydistributed interstellar gas, qualitatively consistent with thehypothesis that massive clusters are formed in high-pressureinterstellar environments. The combined cluster LF of the three GHRs hasa slope similar to the universal cluster LFs seen in starburst galaxiesand nonstarburst spiral galaxies.
| Clusters in the Luminous Giant HII Regions in M101
Giant HII regions (GHRs), with high concentrations of massive stars, arean excellent laboratory to study modes of massive star formation and apotential host of super-star clusters (SSCs). We have used HST WFPC2images in the F547M and F675W bands to study cluster formation in threevery luminous but morphologically different GHRs in M101 - NGC5461,NGC5462, and NGC5471.NGC5461 has been suggested to host a SSC in its luminous core. Ourobservations show that it contains three R136-class clusters superposedon a small, bright stellar background. This tight group of clusters maydynamically evolve into an SSC in the future, and may appear unresolvedat large distances and be identified as an SSC. NGC5462 has looselydistributed HII regions and clusters without a prominent core. It hasthe largest number of clusters among the three GHRs, but most of themare faint and older than 10 Myr. NGC5471 has multiple bright HII regionsand many faint clusters younger than 5 Myr. It has two clusters olderthan R136, but just as luminous; they may be the most massive clustersin the three GHRs.The fraction of stars formed in massive clusters is estimated from theclusters' contribution to the total stellar continuum emission and acomparison of the clusters' ionizing power to the ionization requirementof the associated HII regions. Both estimates show that < ˜ 50%of stars are formed in massive clusters. The cluster luminosityfunctions (CLFs)of the three GHRs show different slopes. NGC5462 has thesteepest CLF and most loosely distributed interstellar gas,qualitatively consistent with the hypothesis that massive clusters areformed in high-pressure interstellar environments. The combined CLF ofthese three GHRs is similar to those in starburst galaxies and normalspiral galaxies.This research is supported by grants STScI GO-6829.01-95A andGO-9934.01-A.
| Far-Ultraviolet Spectroscopy of Star-forming Regions in Nearby Galaxies: Stellar Populations and Abundance Indicators
We present Far Ultraviolet Spectroscopic Explorer spectroscopy andsupporting data for star-forming regions in nearby galaxies, to examinetheir massive-star content and explore the use of abundance andpopulation indicators in this spectral range for high-redshift galaxies.New far-ultraviolet spectra are shown for four bright H II regions inM33 (NGC 588, 592, 595, and 604), the H II region NGC 5461 in M101, andthe starburst nucleus of NGC 7714, supplemented by the very lowmetallicity galaxy I Zw 18. In each case we see strong Milky Wayabsorption systems from H2, but intrinsic absorption withineach galaxy is weak or undetectable, perhaps because of the ``UV bias''in which reddened stars that lie behind molecular-rich areas are alsoheavily reddened. We see striking changes in the stellar wind lines fromthese populations with metallicity, suggesting that C II, C III, C IV, NII, N III, and P V lines are potential tracers of stellar metallicity instar-forming galaxies. Three of these relations-involving N IV, C III,and P V-are nearly linear over the range from O/H=0.05-0.8 solar. Themajor difference in continuum shapes among these systems is that thegiant H II complex NGC 604 has a stronger continuum shortward of 950Å than any other object in this sample. Small number statisticswould likely go in the other direction; we favor this as the result of adiscrete star-forming event ~3 Myr ago, as suggested by previous studiesof its stellar population.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE). FUSE is operated for NASA by JohnsHopkins University under NASA contract NAS 5-32985.
| An XMM-Newton view of M101 - I. The luminous X-ray source population
We present the first results of an XMM-Newton EPIC observation of theluminous X-ray source population in the face-on supergiant spiral galaxyM101. We have studied the spectral and temporal properties of the 14most luminous sources, all of which have intrinsic X-ray luminositiesexceeding the Eddington limit for a 1.4-Msolar neutron star,with a subset in the ultraluminous X-ray source (ULX) regime(LX>= 1039 erg s-1). Eleven sourcesshow evidence of short-term variability, and most vary by a factor of~2-4 over a baseline of 11-24 yr, providing strong evidence that thesesources are accreting X-ray binary (XRB) systems. Our resultsdemonstrate that these sources are a heterogeneous population, showing avariety of spectral shapes. Interestingly, there is no apparent spectraldistinction between those sources above and below the ULX luminositythreshold. Nine sources are well fitted with either simple absorbed discblackbody or power-law models. However, in three of the four sourcesbest fitted with power-law models, we cannot exclude the disc blackbodyfits and therefore conclude that, coupled with their high luminosities,eight out of nine single-component sources are possibly high-state XRBs.The nuclear source (XMM-10) has the only unambiguous power-law spectrum(Γ~ 2.3), which may be evidence for the presence of alow-luminosity active galactic nucleus (LLAGN). The remaining fivesources require at least two-component spectral fits, with an underlyinghard component that can be modelled by a power-law continuum or, inthree cases, a hot disc blackbody (Tin= 0.9-1.5 keV), plus asoft component modelled as a cool blackbody/disc blackbody/thermalplasma. We have compared the spectral shapes of nine sources covered byboth this observation and an archival 100-ks Chandra observation ofM101; eight show behaviour typical of Galactic XRBs (i.e. softening withincreasing luminosity), the only exception being a transient source(XMM-2) which shows little change in spectral hardness despite a factorof ~30 increase in luminosity. We find no definitive spectral signaturesto indicate that these sources contain neutron star primaries, andconclude that they are likely to be stellar-mass black hole XRBs(BHXBs), with black hole masses of ~2-23 Msolar if accretingat the Eddington limit.
| Hot Interstellar Gas and Stellar Energy Feedback in the Antennae Galaxies
We have analyzed Chandra archival observations of the Antennae galaxiesto study the distribution and physical properties of its hotinterstellar gas. Eleven distinct diffuse X-ray emission regions areselected according to their underlying interstellar structures and starformation activity. The X-ray spectra of these regions are used todetermine their thermal energy content and cooling timescales. Youngstar clusters in these regions are also identified and their photometricmeasurements are compared to evolutionary stellar population synthesismodels to assess their masses and ages. The cluster properties are thenused to determine the stellar wind and supernova energies injected intothe interstellar medium (ISM). Comparisons between the thermal energy inthe hot ISM and the expected stellar energy input show that young starclusters are sufficient to power the X-ray-emitting gas in some, but notall, active star formation regions. Super star clusters, with masses>=1×105 Msolar, heat the ISM, but theyield of hot interstellar gas is not directly proportional to thecluster mass. Finally, there exist diffuse X-ray emission regions thatdo not show active star formation or massive young star clusters. Theseregions may be powered by field stars or low-mass clusters formed withinthe last ~100 Myr.
| A Catalog of Neighboring Galaxies
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
| The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale
We use high signal-to-noise ratio spectra of 20 H II regions in thegiant spiral galaxy M101 to derive electron temperatures for the H IIregions and robust metal abundances over radii R=0.19-1.25R0(6-41 kpc). We compare the consistency of electron temperatures measuredfrom the [O III] λ4363, [N II] λ5755, [S III]λ6312, and [O II] λ7325 auroral lines. Temperatures from[O III], [S III], and [N II] are correlated with relative offsets thatare consistent with expectations from nebular photoionization models.However, the temperatures derived from the [O II] λ7325 line showa large scatter and are nearly uncorrelated with temperatures derivedfrom other ions. We tentatively attribute this result to observationaland physical effects, which may introduce large random and systematicerrors into abundances derived solely from [O II] temperatures. Ourderived oxygen abundances are well fitted by an exponential distributionover six disk scale lengths, from approximately 1.3(O/H)solar in the center to 1/15 (O/H)solar in theoutermost region studied [for solar 12+log(O/H)=8.7]. We measuresignificant radial gradients in N/O and He/H abundance ratios, butrelatively constant S/O and Ar/O. Our results are in approximateagreement with previously published abundances studies of M101 based ontemperature measurements of a few H II regions. However, our abundancesare systematically lower by 0.2-0.5 dex than those derived from the mostwidely used strong-line ``empirical'' abundance indicators, againconsistent with previous studies based on smaller H II region samples.Independent measurements of the Galactic interstellar oxygen abundancefrom ultraviolet absorption lines are in good agreement with theTe-based nebular abundances. We suspect that most of thedisagreement with the strong-line abundances arises from uncertaintiesin the nebular models that are used to calibrate the ``empirical''scale, and that strong-line abundances derived for H II regions andemission-line galaxies are as much as a factor of 2 higher than theactual oxygen abundances. However, other explanations, such as theeffects of temperature fluctuations on the auroral line basedabundances, cannot be completely ruled out. These results point to theneed for direct abundance determinations of a larger sample ofextragalactic H II regions, especially for objects more metal-rich thansolar.
| The Far Ultraviolet Spectroscopic Explorer Survey of O VI Absorption in and near the Galaxy
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations ofthe O VI λλ1031.926, 1037.617 absorption lines associatedwith gas in and near the Milky Way, as detected in the spectra of asample of 100 extragalactic targets and two distant halo stars. Wecombine data from several FUSE Science Team programs with guest observerdata that were public before 2002 May 1. The sight lines cover most ofthe sky above Galactic latitude |b|>25deg-at lowerlatitude the ultraviolet extinction is usually too large forextragalactic observations. We describe the details of the calibration,alignment in velocity, continuum fitting, and manner in which severalcontaminants were removed-Galactic H2, absorption intrinsicto the background target and intergalactic Lyβ lines. Thisdecontamination was done very carefully, and in several sight lines verysubtle problems were found. We searched for O VI absorption in thevelocity range -1200 to 1200 km s-1. With a few exceptions,we only find O VI in the velocity range -400 to 400 km s-1the exceptions may be intergalactic O VI. In this paper we analyze the OVI associated with the Milky Way (and possibly with the Local Group). Wediscuss the separation of the observed O VI absorption into componentsassociated with the Milky Way halo and components at high velocity,which are probably located in the neighborhood of the Milky Way. Wedescribe the measurements of equivalent width and column density, and weanalyze the different contributions to the errors. We conclude thatlow-velocity Galactic O VI absorption occurs along all sight lines-thefew nondetections only occur in noisy spectra. We further show thathigh-velocity O VI is very common, having equivalent width >65 mÅin 50% of the sight lines and equivalent width >30 mÅ in 70% ofthe high-quality sight lines. The central velocities of high-velocity OVI components range from |vLSR|=100 to 330 km s-1there is no correlation between velocity and absorption strength. Wediscuss the possibilities for studying O VI absorption associated withLocal Group galaxies and conclude that O VI is probably detected in M31and M33. We limit the extent of an O VI halo around M33 to be <100kpc [at a 3 σ detection limit of log N(O VI)~14.0]. Using themeasured column densities, we present 50 km s-1 wide O VIchannel maps. These show evidence for the imprint of Galactic rotation.They also highlight two known H I high-velocity clouds (complex C andthe Magellanic Stream). The channel maps further show that O VI atvelocities <-200 km s-1 occurs along all sight lines inthe region l=20deg-150deg, b<-30deg,while O VI at velocities >200 km s-1 occurs along all sightlines in the region l=180deg-300deg,b>20deg.
| Diffuse X-Ray Emission from M101
The total 0.45-1.0 keV luminosity of M101 is 3.2×1039ergs s-1, of which 2.3×1039 ergss-1 is due to diffuse emission. (Chandra observes only ~60%of the diffuse emission observed by ROSAT because of its superiorpoint-source detection.) Of the diffuse emission seen by Chandra, nomore than 6% can be due to luminous unresolved point sources, such asX-ray binaries, and ~10% is likely due to dwarf stars. The diffuseemission traces the spiral arms and is roughly correlated with theHα and far-UV emission. The radial distribution closely followsthe optical profile. The bulk of the diffuse emission is characterizedby a two-thermal component spectrum with kT=0.20 and 0.75 keV, and theratio of the emission measures of the two components is roughly constantas a function of both radius and surface brightness. The softercomponent has a sufficiently large covering factor that the bulk of theemission is likely extraplanar. We find no evidence of an extendedaxisymmetric X-ray halo, suggesting that any such halo has a strengthmuch smaller than current predictions.
| Planetary nebula carbon yields and the chemical evolution of the Galactic disc
Two sets of observational carbon stellar yields for low-and-intermediate mass stars are computed based on planetary nebulaabundances derived from CIIλ4267 and CIIIλλ1906 +1909 lines, respectively. These observational yields are assumed inchemical evolution models for the solar vicinity and the Galactic disc.C/O values observed in stars in the solar vicinity and Galactic HIIregions are compared with those predicted by chemical evolution modelsfor the Galaxy. It is concluded that the C yields derived from permittedlines are in better agreement with the observational constraints thanare those derived from forbidden lines.
| A Far Ultraviolet Spectroscopic Explorer Survey of Molecular Hydrogen in Intermediate-Velocity Clouds in the Milky Way Halo
Far Ultraviolet Spectroscopic Explorer (FUSE) data are used toinvestigate the molecular hydrogen (H2) content ofintermediate-velocity clouds (IVCs) in the lower halo of the Milky Way.We analyze interstellar absorption toward 56 (mostly extragalactic)background sources to study H2 absorption in the Lyman andWerner bands in 61 IVC components at H I column densities>=1019 cm-2. For data with good signal-to-noiseratio (S/N) (~9 per resolution element and higher), H2 in IVCgas is convincingly detected in 14 cases at column densities varyingbetween ~1014 and ~1017 cm-2. We findan additional 17 possible H2 detections in IVCs in FUSEspectra with lower S/N. The molecular hydrogen fractions, f, varybetween 10-6 and 10-3, implying a dense, mostlyneutral gas phase that is probably related to the cold neutral medium(CNM) in these clouds. If the H2 stays information-dissociation equilibrium, the CNM in these clouds can becharacterized by compact (D~0.1 pc) filaments with volume densities onthe order of nH~30 cm-3. The relatively highdetection rate of H2 in IVC gas implies that the CNM in theseclouds is ubiquitous. More dense regions with much higher molecularfractions may exist, but it would be difficult to detect them inabsorption because of their small size.
| Chemical Abundances of Extragalactic H II regions
The determination of the heavy element abundances from giantextragalactic H II regions is based on collisionally excited lines. Weargue that in the presence of temperature variations the abundancesdetermined are lower limits to the real heavy element abundances. Todetermine the real abundances it is necessary to take into account thetemperature variations present in these nebulae. We discuss therelevance of obtaining accurate line intensities of recombination linesof H, He, C, and O to determine the chemical composition ofextragalactic H II regions. We suggest that Pagel's method to derive theO/H ratio should be calibrated by using recombination lines instead ofphotoionization models or abundances derived from collisionally excitedlines.
| Chemical abundances of the H II regions NGC 5461 and NGC 5471 in M 101, derived from echelle spectrophotometry
We present high resolution spectroscopic data of the two giantextragalactic HII regions NGC 5461 and NGC 5471 in M 101. We calculatethe physical conditions in the two nebulae with a large number ofdiagnostics, and determine their chemical abundances by applyingionization correction factors (ICFs) to the observed ionic abundances. The comparison of the ICFs based on photo-ionization models (Luridiana& Peimbert 2001; Luridiana et al. 1999) to those computed followingthe prescriptions by Mathis & Rosa (1991) shows large discrepanciesfor several elements, including nitrogen, neon and chlorine.
| Optical Recombination Lines of Heavy Elements in Giant Extragalactic H II Regions
We present high-resolution observations of the giant extragalactic H IIregions NGC 604, NGC 2363, NGC 5461, and NGC 5471, based on observationstaken with the ISIS spectrograph on the William Herschel Telescope. Wehave detected, for the first time, C II and O II recombination lines inthese objects. We find that recombination lines give largerC++ and O++ abundances than collisionally excitedlines, suggesting that temperature variations may be present in theobjects. We detect [Fe IV] lines in NGC 2363 and NGC 5471, the mostconfident detection of optical lines of this kind in H II regions.Considering the temperature structure, we derive their H, He, C, N, O,Ne, S, Ar, and Fe abundances. From the recombination lines of NGC 5461and NGC 5471, we determine the presence of C/H and O/H gradients inM101. We calculate the ΔY/ΔO and ΔY/ΔZ valuesconsidering the presence of temperature variations and under theassumption of constant temperature. We obtain a better agreement withmodels of galactic chemical evolution by considering the presence oftemperature variations than by assuming that the temperature is constantin these nebulae. Based on observations made with William HerschelTelescope operated on the island of La Palma by the Isaac Newton Groupof Telescopes in the Spanish Observatorio del Roque de Los Muchachos ofthe Instituto de Astrofísica de Canarias.
| Observations of Galaxies with the Midcourse Space Experiment
We have imaged eight nearby spiral galaxies with the SPIRIT III infraredtelescope on the Midcourse Space Experiment (MSX) satellite in themid-infrared at 18" resolution at 8.3, 12.1, 14.7, and 21.3 μm. Eachof the eight shows interesting structure not previously detected witholder, lower resolution infrared data sets, such as a resolved nucleusor spiral structure. The MSX data are compared with existing data setsat ultraviolet, optical, and infrared wavelengths, including recentobservations from the Infrared Space Observatory. The infraredstructures in M83 and NGC 5055 show a striking similarity to theultraviolet emission but are less similar to the optical emission.Several point sources with no identified counterparts at otherwavelengths are found near M31, NGC 4945, M83, and M101. Over 200previously known objects are also detected at 8 μm.
| Hgamma and Hdelta absorption lines and the Initial Mass Function in extragalactic star forming regions. I. Data
High-dispersion, long-slit spectroscopic observations of 13extragalactic star-forming regions were obtained for two Blue CompactDwarf galaxies (IZw18, IZw36) and for bright star-formation regions inthe spiral galaxies M 101 and M 51 and in the irregular starburstgalaxies Mrk 171 and NGC 4449. The Hgamma and/orHdelta absorption lines have been observed in 11 star formingregions and will be used to constrain the Initial Mass Function and ageof the young stellar populations. In this paper we present the resultsof the observations and the measurements of the absorption and emissionlines. The astrophysical discussion is deferred to a forthcoming paper.
| Chemical Abundances of NGC 5461 and NGC 5471 Derived from Echelle Spectrophotometry
We present high resolution spectroscopic data of the two giantextragalactic H II regions NGC 5461 and NGC 5471 in M101, which havebeen obtained with the 2.1-m telescope of the Observatorio AstronómicoNacional in San Pedro Mártir, Baja California. We measured theintensities of several H and He recombination lines, and of forbiddenlines of a large number of ions. We calculate the physical conditions inthe two nebulae with a large number of diagnostics and determine theirchemical abundances by applying ionization correction factors (icf's) to the observed ionic abundances. For NGC 5461, theicf's are based on a tailored photoionization model of the region(Luridiana & Peimbert 2001), while for NGC 5471 they are computedfrom those predicted by a photoionization model of NGC 2363 (Luridiana,Peimbert, & Leitherer 1999), a region which is similar to NGC 5471in the ionization structure. For both regions, the icf's arecompared to those computed following the prescriptions by Mathis &Rosa (1991). Such comparison shows large discrepancies for severalelements, including nitrogen, neon, and chlorine.
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