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Scalar potential model of redshift and discrete redshift
On the galactic scale the universe is inhomogeneous and redshift z isoccasionally less than zero. A scalar potential model (SPM) that linksthe galaxy scale z to the cosmological scale z of the Hubble Law ispostulated. Several differences among galaxy types suggest that spiralgalaxies are Sources and that early type, lenticular, and irregulargalaxies are Sinks of a scalar potential field. The morphology-radiusand the intragalactic medium cluster observations support the movementof matter from Source galaxies to Sink galaxies. A cell structure ofgalaxy groups and clusters is proposed to resolve a paradox concerningthe scalar potential like the Olber’s paradox concerning light.For the sample galaxies, the ratio of the luminosity of Source galaxiesto the luminosity of Sink galaxies approaches 2.7 ± 0.1. Anequation is derived from sample data, which is anisotropic andinhomogeneous, relating z of and the distance D to galaxies. Thecalculated z has a correlation coefficient of 0.88 with the measured zfor a sample of 32 spiral galaxies with D calculated using Cepheidvariable stars. The equation is consistent with z < 0 observations ofclose galaxies. At low cosmological distances, the equation reduces to z≈ exp(KD)‑1 ≈ KD, where K is a constant, positive value. Theequation predicts z from galaxies over 18 Gpc distant approaches aconstant value on the order of 500. The SPM of z provides a physicalbasis for the z of particle photons. Further, the SPM qualitativelysuggests the discrete variations in z, which was reported by Tifft[Tifft, W.G., 1997. Astrophy. J. 485, 465] and confirmed by others, areconsistent with the SPM.
| Nuclear embedded star clusters in NGC 7582*
We report on the discovery of several compact regions of mid-infraredemission in the star-forming circumnuclear disc of the starburst/Seyfert2 galaxy NGC 7582. The compact sources do not have counterparts in theoptical and near-infrared, suggesting that they are deeply embedded indust. We use the [NeII] 12.8-μm line emission to estimate theemission measure of the ionized gas, which in turn is used to assess thenumber of ionizing photons. Two of the brighter sources are found tohave ionizing fluxes of ~2.5 × 1052, whereas thefainter ones have ~1 × 1052 photon s-1.Comparing with a 1-Myr-old starburst, we derive stellar masses in therange 3-5 × 105Msolar and find that thenumber of O stars in each compact source is typically 0.6-1.6 ×103. We conclude that the compact mid-infrared sources arelikely to be young, embedded star clusters, of which only a few areknown so far. Our observation highlights the need for high-resolutionmid-infrared imaging to discover and study embedded star clusters in theproximity of active galactic nuclei.
| Revisiting the infrared spectra of active galactic nuclei with a new torus emission model
We describe improved modelling of the emission by dust in atoroidal-like structure heated by a central illuminating source withinactive galactic nuclei (AGNs). We have chosen a simple but realistictorus geometry, a flared disc, and a dust grain distribution functionincluding a full range of grain sizes. The optical depth within thetorus is computed in detail taking into account the differentsublimation temperatures of the silicate and graphite grains, whichsolves previously reported inconsistencies in the silicate emissionfeature in type 1 AGNs. We exploit this model to study the spectralenergy distributions (SEDs) of 58 extragalactic (both type 1 and type 2)sources using archival optical and infrared data. We find that both AGNand starburst contributions are often required to reproduce the observedSEDs, although in a few cases they are very well fitted by a pure AGNcomponent. The AGN contribution to the far-infrared luminosity is foundto be higher in type 1 sources, with all the type 2 requiring asubstantial contribution from a circumnuclear starburst. Our resultsappear in agreement with the AGN unified scheme, because thedistributions of key parameters of the torus models turn out to becompatible for type 1 and type 2 AGNs. Further support to theunification concept comes from comparison with medium-resolutioninfrared spectra of type 1 AGNs by the Spitzer observatory, showingevidence for a moderate silicate emission around 10 μm, which ourcode reproduces. From our analysis we infer accretion flows in the innernucleus of local AGNs characterized by high equatorial optical depths(AV~= 100), moderate sizes (Rmax < 100 pc) andvery high covering factors (f~= 80 per cent) on average.
| Near-Infrared [Fe II] Emission in Starburst Galaxies. I. Measured Properties
We used the near-infrared [Fe II] emission line signature to detectsupernova remnants (SNRs) in the nearby starburst galaxies NGC 1569, NGC3738, and NGC 5253. The near-infrared narrowband imaging program has ledto the detection of 10 SNR candidates in NGC 1569, 7 in NGC 5253, andnone in NGC 3738. The luminosity of the SNRs candidates varies from 72to 780 Lsolar and from 69 to 331 Lsolar for NGC1569 and NGC 5253, respectively. Also, a spatially extended component tothe [Fe II] line emission is observed in NGC 1569 and NGC 5253. Thiscomponent dominates the integrated [Fe II] luminosity in both galaxies,the compact sources accounting for 14% and 7% of the total [Fe II]luminosity of NGC 1569 and NGC 5253, respectively.
| The Survey for Ionization in Neutral Gas Galaxies. I. Description and Initial Results
We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG),a census of star formation in H I-selected galaxies. The survey consistsof Hα and R-band imaging of a sample of 468 galaxies selected fromthe H I Parkes All Sky Survey (HIPASS). The sample spans three decadesin H I mass and is free of many of the biases that affect otherstar-forming galaxy samples. We present the criteria for sampleselection, list the entire sample, discuss our observational techniques,and describe the data reduction and calibration methods. This paperfocuses on 93 SINGG targets whose observations have been fully reducedand analyzed to date. The majority of these show a single emission linegalaxy (ELG). We see multiple ELGs in 13 fields, with up to four ELGs ina single field. All of the targets in this sample are detected inHα, indicating that dormant (non-star-forming) galaxies withMHI>~3×107 Msolar are veryrare. A database of the measured global properties of the ELGs ispresented. The ELG sample spans 4 orders of magnitude in luminosity(Hα and R band), and Hα surface brightness, nearly 3 ordersof magnitude in R surface brightness and nearly 2 orders of magnitude inHα equivalent width (EW). The surface brightness distribution ofour sample is broader than that of the Sloan Digital Sky Survey (SDSS)spectroscopic sample, the EW distribution is broader than prism-selectedsamples, and the morphologies found include all common types ofstar-forming galaxies (e.g., irregular, spiral, blue compact dwarf,starbursts, merging and colliding systems, and even residual starformation in S0 and Sa spirals). Thus, SINGG presents a superior censusof star formation in the local universe suitable for further studiesranging from the analysis of H II regions to determination of the localcosmic star formation rate density.
| Cepheid Distances to SNe Ia Host Galaxies Based on a Revised Photometric Zero Point of the HST WFPC2 and New PL Relations and Metallicity Corrections
With this paper we continue the preparation for a forthcoming summaryreport of our experiment with the HST to determine the Hubble constantusing Type Ia supernovae as standard candles. Two problems areaddressed. (1) We examine the need for, and determine the value of, thecorrections to the apparent magnitudes of our program Cepheids in the 11previous calibration papers due to sensitivity drifts and chargetransfer effects of the HST WFPC2 camera over the life time of theexperiment from 1992 to 2001. (2) The corrected apparent magnitudes areapplied to all our previous photometric data from which revised distancemoduli are calculated for the eight program galaxies that are parents tothe calibrator Ia supernovae. Two different Cepheid P-L relations areused; one for the Galaxy and one for the LMC. These differ both in slopeand zero point at a fixed period. The procedures for determining theabsorption and reddening corrections for each Cepheid are discussed.Corrections for the effects of metallicity differences between theprogram galaxies and the two adopted P-L relations are derived andapplied. The distance moduli derived here for the eight supernovaeprogram galaxies, and for 29 others, average 0.20 mag fainter (moredistant) than those derived by Gibson et al. and Freedman et al. intheir 2000 and 2001 summary papers for reasons discussed in this paper.The effect on the Hubble constant is the subject of our forthcomingsummary paper.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| 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.
| The Opaque Nascent Starburst in NGC 1377: Spitzer SINGS Observations
We analyze extensive data on NGC 1377 from the Spitzer Infrared NearbyGalaxies Survey (SINGS). Within the category of nascent starbursts thatwe previously selected as having infrared-to-radio continuum ratios inlarge excess of the average and containing hot dust, NGC 1377 has thelargest infrared excess yet measured. Optical imaging reveals amorphological distortion suggestive of a recent accretion event.Infrared spectroscopy reveals a compact and opaque source dominated by ahot, self-absorbed continuum (τ~20 in the 10 μm silicate band).We provide physical evidence against nonstellar activity being theheating source. H II regions are detected through the single [Ne II]line, probing <1% of the ionizing radiation. Not only is the opticaldepth in different gas and dust phases very high, but >85% ofionizing photons are suppressed by dust. The only other detectedemission features are molecular hydrogen lines, arguably excited mainlyby shocks, besides photodissociation regions, and weak aromatic bands.The new observations support our interpretation in terms of an extremelyyoung starburst (<1 Myr). More generally, galaxies deficient in radiosynchrotron emission are likely observed within a few Myr of the onsetof a starburst and after a long quiescence, prior to the replenishmentof the interstellar medium with cosmic rays. The similar infrared-radioproperties of NGC 1377 and some infrared-luminous galaxies suggest thatNGC 1377 constitutes an archetype that will be useful to betterunderstand starburst evolution. Although rare locally because observedin a brief evolutionary stage, nascent starbursts may represent anonnegligible fraction of merger-induced starbursts that dominate deepinfrared counts. Since they differ dramatically from usual starbursttemplates, they have important consequences for the interpretation ofdeep surveys.
| On the Determination of N and O Abundances in Low-Metallicity Systems
We show that in order to minimize the uncertainties in the N and Oabundances of low-mass, low-metallicity (O/H<=1/5 solar)emission-line galaxies, it is necessary to employ separateparameterizations for inferring Te(N+) andTe(O+) from Te(O+2). Inaddition, we show that for the above systems, the ionization correctionfactor (ICF) for obtaining N/O from N+/O+, wherethe latter is derived from optical emission-line flux ratios, is=1.08+/-0.09. These findings are based on state-of-the-art single-star HII region simulations, employing our own modeled stellar spectra asinput. Our models offer the advantage of having matching stellar andnebular abundances. In addition, they have O/H as low as 1/50 solar(lower than any past work), as well as log(N/O) and log(C/O) fixed atcharacteristic values of -1.46 and -0.7, respectively. The above resultswere used to rederive N and O abundances for a sample of 68 systems with12+log(O/H)<=8.1, whose dereddened emission-line strengths werecollected from the literature. The analysis of the log(N/O) versus12+log(O/H) diagram of the above systems shows that (1) the largestgroup of objects forms the well-known N/O plateau with a value for themean (and its statistical error) of-1.43+0.0084-0.0085, (2) the objects aredistributed within a range in log(N/O) of -1.54 to -1.27 in Gaussianfashion around the mean with a standard deviation ofσ=+0.071-0.084, and (3) a χ2analysis suggests that only a small amount of the observed scatter inlog(N/O) is intrinsic.
| Determination of the Hubble Constant, the Intrinsic Scatter of Luminosities of Type Ia Supernovae, and Evidence for Nonstandard Dust in Other Galaxies
A sample of 109 Type Ia supernovae (SNe Ia) with recession velocity<~30,000 km s-1 is compiled from published SN Ia lightcurves to explore the expansion rate of the local universe. Based on thecolor parameter ΔC12 and the decline rateΔm15, we found that the average absorption-to-reddeningratios for SN Ia host galaxies are RUBVI=4.37+/-0.25,3.33+/-0.11, 2.30+/-0.11, and 1.18+/-0.11, which are systematicallylower than the standard values in the Galaxy. We investigated thecorrelations of the intrinsic luminosity with light-curve decline rate,color index, and SN environmental parameters. In particular, we foundthat SNe Ia in E/S0 galaxies close to the central region are brighterthan those in the outer region, which may suggest a possible metallicityeffect on SN luminosity. The dependence of SN luminosity on galacticenvironment disappears after corrections for the extinction andΔC12. The Hubble diagrams constructed using 73 Hubbleflow SNe Ia yield a 1 σ scatter of <~0.12 mag in BVI bands and~0.16 mag in U band. The luminosity difference between normal SNe Ia andpeculiar objects (including SN 1991bg-like and SN 1991T-like events) hasnow been reduced to within 0.15 mag via ΔC12correction. We use the same precepts to correct the nearby SNe Ia withCepheid distances and found that the fully corrected absolute magnitudesof SNe Ia are MB=-19.33+/-0.06 andMV=-19.27+/-0.05. We deduced a value for the Hubble constantof H0=72+/-6 (total) km s-1 Mpc-1.
| Spatially Resolved Spitzer Spectroscopy of the Starburst Nucleus in NGC 5253
We present new Spitzer Space Telescope data on the nearby,low-metallicity starburst galaxy NGC 5253, from the Infrared ArrayCamera (IRAC) and the Infrared Spectrograph (IRS). The mid-IR luminosityprofile of NGC 5253 is clearly dominated by an unresolved cluster nearthe center, which outshines the rest of the galaxy at longerwavelengths. We find that the [Ne III]/[Ne II] ratio decreases from ~8.5at the center to ~2.5 at a distance of ~250 pc. The [S IV]/[S III]follows the [Ne III]/[Ne II] ratio remarkably well, being about 4-5times lower at all distances. Our spectra reveal for the first time aPAH emission feature at 11.3 μm, and its equivalent width increasessignificantly with distance from the center. The good anticorrelationbetween the PAH strength and the product between hardness and luminosityof the UV radiation field suggests photodestruction of the PAH moleculesin the central region. The high-excitation [O IV] 25.91 μm line wasdetected at 0.42×10-20 W cm-2. Our resultsdemonstrate the importance of spatially resolved mid-IR spectroscopy.
| First Detection of PAHs and Warm Molecular Hydrogen in Tidal Dwarf Galaxies
We observed two faint tidal dwarf galaxies (TDGs), NGC 5291 N and NGC5291 S, with the Infrared Spectrograph on the Spitzer Space Telescope.We detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2,7.7, 8.6, 11.3, 12.6, and 16.5 μm, which match models of groups of~100 carbon atoms with an equal mixture of neutral and ionized PAHs. TheTDGs have a dominant warm (~140 K) dust component in marked contrast tothe cooler (40-60 K) dust found in starburst galaxies. For the firsttime we detect the low-J rotational lines from molecular hydrogen.Adopting LTE, there is ~105 Msolar of ~400 K gas,which is <0.1% of the cold gas mass inferred from 12CO(1-0) measurements. The combination of one-third solar metallicity witha recent (<5 million year) episode of star formation is reflected inthe S and Ne ratios. The excitation is higher than typical values forstarburst galaxies and similar to that found in BCDs. Using the InfraredArray Camera, we identify an additional 13 PAH-rich candidate TDGs.These sources occupy a distinct region of IRAC color space with[3.6]-[4.5]<0.4 and [4.5]-[8.0]>3.2. Their disturbed morphologiessuggest past merger events between companions; for example, NGC 5291 Shas a projected 11 kpc tail. NGC 5291 N and S have stellar masses of(1.5 and 3.0)×108 Msolar, which iscomparable to BCDs, although still roughly 10% of the LMC's stellarmass. The candidate TDGs are an order of magnitude less massive. Thissystem appears to be a remarkable TDG nursery.
| Oxygen and Nitrogen in Leo A and GR 8
We present elemental abundances for multiple H II regions in Leo A andGR 8 obtained from long-slit optical spectroscopy of these two nearbylow-luminosity dwarf irregular galaxies. As expected from theirluminosities, and in agreement with previous observations, the derivedoxygen abundances are extremely low in both galaxies. Highsignal-to-noise ratio (S/N) observations of a planetary nebula in Leo Ayield 12+log(O/H)=7.30+/-0.05 semiempirical calculations of the oxygenabundance in four H II regions in Leo A indicate12+log(O/H)=7.38+/-0.10. These results confirm that Leo A has one of thelowest ISM metal abundances of known nearby galaxies. Based on resultsfrom two H II regions with high S/N measurements of the weak [O III]λ4363 line, the mean oxygen abundance of GR 8 is12+log(O/H)=7.65+/-0.06 using ``empirical'' and ``semiempirical''methods, similar abundances are derived for six other GR 8 H II regions.Similar to previous results in other low-metallicity galaxies, the meanlog(N/O)=-1.53+/-0.09 for Leo A and -1.51+/-0.07 for GR 8. There is noevidence of significant variations in either O/H or N/O in the H IIregions. The metallicity-luminosity relation for nearby (D<5 Mpc)dwarf irregular galaxies with measured oxygen abundances has a meancorrelation of 12+log(O/H)=5.67MB-0.151MB, with adispersion in oxygen about the relationship of σ=0.21. Theseobservations confirm that gas-rich, low-luminosity galaxies haveextremely low elemental abundances in the ionized gas phase of theirinterstellar media. Although Leo A has one of the lowest metalabundances of known nearby galaxies, detection of tracers of an olderstellar population (RR Lyrae variable stars, horizontal branch stars,and a well-populated red giant branch) indicate that it is not a newlyformed galaxy, as has been proposed for some other similarlow-metallicity star-forming galaxies.
| Scaling Mass Profiles around Elliptical Galaxies Observed with Chandra and XMM-Newton
We investigated the dynamical structure of 53 elliptical galaxies usingthe Chandra archival X-ray data. In X-ray-luminous galaxies, temperatureincreases with radius and gas density is systematically higher at theoptical outskirts, indicating the presence of a significant amount ofthe group-scale hot gas. In contrast, X-ray-dim galaxies show a flat ordeclining temperature profile against radius and the gas density isrelatively lower at the optical outskirts. Thus, it is found thatX-ray-bright and faint elliptical galaxies are clearly distinguished bythe temperature and gas density profile. The mass profile is well scaledby a virial radius r200 rather than an optical half-radiusre, is quite similar at (0.001-0.03)r200 betweenX-ray-luminous and dim galaxies, and smoothly connects to those profilesof clusters of galaxies. At the inner region of(0.001-0.01)r200 or (0.1-1)re, the mass profilewell traces a stellar mass with a constant mass-to-light ratio ofM/LB=3-10 Msolar/Lsolar. TheM/LB ratio of X-ray-bright galaxies rises up steeply beyond0.01r200 and thus requires a presence of massive dark matterhalo. From the deprojection analysis combined with the XMM-Newton data,we found that X-ray-dim galaxies NGC 3923, NGC 720, and IC 1459 alsohave a high M/LB ratio of 20-30 at 20 kpc, comparable to thatof X-ray-luminous galaxies. Therefore, dark matter is indicated to becommon in elliptical galaxies; their dark matter distribution, as wellas that of galaxy clusters, almost follows the NFW profile.
| Oxygen and Nitrogen in Isolated Dwarf Irregular Galaxies
We present long-slit optical spectroscopy of 67 H II regions in 21 dwarfirregular galaxies to investigate the enrichment of oxygen, nitrogen,neon, sulfur, and argon in low-mass galaxies. Oxygen abundances areobtained via direct detection of the temperature-sensitive emissionlines for 25 H II regions; for the remainder of the sample, oxygenabundances are estimated from strong-line calibrations. The directabundance determinations are compared to the strong-line abundancecalibrations of both McGaugh and Pilyugin. While the McGaugh calibrationyields a statistical offset of 0.07 dex, the photoionization model gridtraces the appropriate isometallicity contour shape in theR23-O32 diagnostic diagram. In contrast, while thePilyugin calibration yields a negligible statistical offset, theresiduals in this strong-line calibration method are correlated withionization parameter. Thus, these observations indicate that oxygenabundances will be overestimated by the p-method for H II regions withlow-ionization parameters. Global oxygen and nitrogen abundances forthis sample of dwarf irregular galaxies are examined in the context ofopen- and closed-box chemical evolution models. While several galaxiesare consistent with closed-box chemical evolution, the majority of thissample has an effective yield ~1/4 of the expected yield for a constantstar formation rate and Salpeter IMF, indicating that either outflow ofenriched gas or inflow of pristine gas has occurred. The effective yieldstrongly correlates with MH/LB in the sense thatgas-rich galaxies are more likely to be closed systems. However, theeffective yield does not appear to correlate with other globalparameters such as dynamical mass, absolute magnitude, star formationrate, or surface brightness. In addition, open and closed systems arenot identified easily in other global abundance measures; for example,the observed correlation between luminosity and metallicity isconsistent with other recent results in the literature. A correlation isfound between the observed nitrogen-to-oxygen ratio and the color of theunderlying stellar population; redder dwarf irregular galaxies havehigher N/O ratios than blue dwarf irregular galaxies. The relativeabundance ratios are interpreted in the context of delayed release ofnitrogen and varied star formation histories.
| 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.
| The Starburst-Interstellar Medium Interaction in NGC 1569. II. Small-Scale Examination of Nebular Emission, H II Region Size Distribution, and H II Region Luminosity Function
As the nearest dramatic example of a poststarburst galaxy driving agalactic wind, NGC 1569 is an ideal test environment to understand theimpact of ``feedback'' from massive star lives and deaths on thesurrounding interstellar medium. We present Hubble Space Telescope WideField Planetary Camera 2 narrowband imagery of NGC 1569 in an attempt tounderstand the underlying ionizing emission mechanisms on a 3 pc scaleand to generate a H II region size distribution and luminosity function.We use [O III]/Hβ and [S II]/Hα ratio maps to find thatnonphotoionizing mechanisms (e.g., shocks) are responsible for 10%+/-3%of the Hα emission, 2.5-3 times larger than results from similargalaxies. Note that our method of determining this result is differentfrom these past results, a point that we discuss further in this paper.The area of the nonphotoionized region is 10%-23% of the total. Ourresults for NGC 1569 indicate that these nonphotoionized areas do notlie in low surface brightness regions exclusively. A comparison withmultiwavelength point-source catalogs of NGC 1569 indicates that thedominant nonphotoionizing mechanisms are shocks from supernovae or windsfrom massive stars. To explain this large percentage of nonphotoionizedemission, we suggest that NGC 1569 is, indeed, in a poststarburst phase,as previous authors have claimed. We also derive slopes for the H IIregion luminosity function (-1.00+/-0.08) and size distribution(-3.02+/-0.27). The luminosity slope, although shallow, is similar toprevious work on this galaxy and other irregular galaxies. The sizedistribution slope is shallower than previous slopes found for irregulargalaxies, but our slope value fits into their confidence intervals, andvice versa. Within 4 pc of the 10-20 Myr old super star clusters A1, A2,and B, no bright H II regions exist to a luminosity limit of2.95×1036 ergs s-1, suggesting that thewinds and shocks have effectively terminated star formation in thissmall cavity. In the three annular regions around the super starclusters, both the H II region luminosity function and H II region sizedistribution are consistent with respect to one another and the galaxyas a whole. The H II region surface densities within the annuli remainthe same as the annuli are moved away from the super star clusters.These results indicate that feedback effects in NGC 1569 are confined tothe immediate vicinity of the most recent massive star formation eventon scales of ~1 pc.
| Detection of Wolf-Rayet stars in host galaxies of gamma-ray bursts (GRBs): are GRBs produced by runaway massive stars ejected from high stellar density regions?
We have obtained deep spectroscopic observations of several nearbygamma-ray burst (GRB) host galaxies revealing for the first time thepresence of Wolf-Rayet (WR) stars and numerous O stars located in richand compact clusters or star forming regions. Surprisingly, high spatialresolution imaging shows that the GRBs and the associated supernovae didnot occur in these regions, but several hundreds of parsec away.Considering various scenarios for GRB progenitors, we do not find anysimple explanation of why they should be preferentially born in regionswith low stellar densities. All the examined GRBs and associated SNehave occurred 400 to 800 pc from very high density stellar environmentsincluding large numbers of WR stars. Such distances can be travelledthrough at velocities of 100 km s-1 or larger, assuming thetravel time to be the typical life time of WR stars. It leads us tosuggest that GRB progenitors may be runaway massive stars ejected fromcompact massive star clusters. The ejection from such super starclusters may lead to a spin-up of these stars, producing the loss of thehydrogen and/or helium envelopes leading to the origin of the type Ibcsupernovae associated with GRBs. If this scenario applies to all GRBs,it provides a natural explanation of the very small fraction of massivestars that emit a GRB at the end of their life. An alternative to thisscenario could be a binary origin for GRBs, but this still requires anexplanation of why it would preferentially occur in low stellar densityregions.
| Star-forming knots and density wave in NGC 2997
Context.Many grand design spiral galaxies show strings of bright knotsalong their arms on near-infrared K-band images. The alignment of suchknots suggests a relation to the spiral pattern and possibly to alarge-scale, star-forming front associated with a density wave. Aims.Bright knots in the southern arm of NGC 2997 were studied todetermine their nature and evolutionary state.Methods.Lowresolution near-infrared K- and J-band spectra of the knots wereobserved with ISAAC on the VLT. Results.Most of the knots showstrong H I Brγ emission with some also having He I and H2emission. A few knots show indications of 12CO absorption.This suggests that the knots are very young stellar clusters with massesup to 5×104 Mȯ.Conclusions.Theknots azimuthal distance from the K-band spiral correlates well withtheir Brγ strength, indicating that they are located inside theco-rotation of the density wave, which triggered them through alarge-scale, star-forming front. These relative azimuthal distancessuggest an age spread of more than 1.6 Myr, which is incompatible withstandard models for an instantaneous star burst. This indicates a morecomplex star-formation history, such as several bursts or continuousformation.
| The extragalactic Cepheid bias: a new test using the period-luminosity-color relation
We use the Period-Luminosity-Color relation (PLC) for Cepheids to testfor the existence of a bias in extragalactic distances derived from theclassical Period-Luminosity (PL) relation. We calculate the parametersof the PLC using several galaxies observed with the Hubble SpaceTelescope and show that this calculation must be conducted with a PLCwritten in a form where the parameters are independent. The coefficientsthus obtained are similar to those derived from theoretical models.Calibrating with a few unbiased galaxies, we apply this PLC to allgalaxies of the Hubble Space Telescope Key Program (HSTKP) and comparethe distance moduli with those published by the HSTKP team. The newdistance moduli are larger (more exactly, the larger the distance thelarger the difference), consistent with a bias. Further, the bias trendthat is observed is the same previously obtained from two independentmethods based either on the local Hubble law or on a theoretical modelof the bias. The results are quite stable but when we force the PLCrelation closer to the classical PL relation by using unrealisticparameters, the agreement with HSTKP distance moduli is retrieved. Thisalso suggests that the PL relation leads to biased distance moduli. Thenew distance moduli reduce the scatter in the calibration of theabsolute magnitude of supernovae SNIa at their maximum. This may alsosuggest that the relation between the amplitude at maximum and the decayof the light curve Δ m15 may not be as strong asbelieved.
| Mid infrared properties of distant infrared luminous galaxies
We present evidence that the mid infrared (MIR, rest frame 5-30 μm)is a good tracer of the total infrared luminosity, L(IR)(=L[8{-}1000μm]), and star formation rate (SFR), of galaxies up to z 1.3. Weuse deep MIR images from the Infrared Space Observatory (ISO) and theSpitzer Space Telescope in the Northern field of the Great ObservatoriesOrigins Deep Survey (GOODS-N) together with VLA radio data to computethree independant estimates of L(IR). The L(IR, MIR) derived from theobserved 15 and/or 24 μm flux densities using a library of templateSEDs, and L(IR, radio), derived from the radio (1.4 and/or 8.5 GHz)using the radio-far infrared correlation, agree with a 1-σdispersion of 40%. We use the k-correction as a tool to probe differentparts of the MIR spectral energy distribution (SED) of galaxies as afunction of their redshift and find that on average distant galaxiespresent MIR SEDs very similar to local ones. However, in the redshiftrange z= 0.4-1.2, L(IR, 24 μm) is in better agreement with L(IR,radio) than L(IR, 15 μm) by 20%, suggesting that the warm dustcontinuum is a better tracer of the SFR than the broad emission featuresdue to polycyclic aromatic hydrocarbons (PAHs). We find marginalevidence for an evolution with redshift of the MIR SEDs: two thirds ofthe distant galaxies exhibit rest-frame MIR colors (L(12 μm)/L(7μm) and L(10 μm)/L(15 μm) luminosity ratios) below the medianvalue measured for local galaxies. Possible explanations are examinedbut these results are not sufficient to constrain the physics of theemitting regions. If confirmed through direct spectroscopy and if itgets amplified at higher redshifts, such an effect should be consideredwhen deriving cosmic star formation histories of dust-obscured galaxies.We compare three commonly used SED libraries which reproduce thecolor-luminosity correlations of local galaxies with our data anddiscuss possible refinements to the relative intensities of PAHs, warmdust continuum and silicate absorption.
| Stellar clusters in dwarf galaxies
We present new observations in the Ks (2.2 μm) and L' (3.7μm) infrared bands of a sample of blue dwarf galaxies with the largeraim of studying the population of massive stellar clusters, theoccurrence of dust-embedded stellar clusters, and their properties. AllKs images show a rich population of clusters, but only asmall fraction of them is bright in L'. Most L' sources have radiocounterparts. We derived the luminosity function in Ks forthe galaxies IC 4661 and NGC 5408, finding both to be consistent withthose of similar galaxies. We also compared the number of clusters andtheir luminosities with the star-formation rate of the host galaxies andfound no compelling evidence of correlation. We conclude that youngclusters and embedded clusters are a common feature of blue dwarfgalaxies and possibly of galaxies in general, we suggest that theiroccurrence is due to purely statistical effects rather than a phenomenonrelated to specific physical conditions. In this sense we expect theseobjects to be abundant at high red-shift.
| ISM properties in low-metallicity environments
We present new ISOCAM mid-infrared spectra of three starbursting nearbydwarf galaxies, NGC 1569, NGC 1140 and II Zw 40 and the 30 Dor region ofthe LMC and explore the properties of the ISM in low-metallicityenvironments, also using additional sources from the literature. Weanalyse the various components of the ISM probed by the mid-infraredobservations and compare them with other Galactic and extragalacticobjects. The MIR spectra of the low-metallicity starburst sources aredominated by the [ Ne iii] λ 15.56~μ m and [ S iv] λ10.51~μ m lines, as well as a steeply rising dust continuum. PAHbands are generaly faint, both locally and averaged over the fullgalaxy, in stark contrast to dustier starburst galaxies, where the PAHfeatures are very prominant and even dominate on global scales. Thehardness of the modeled interstellar radiation fields for the dwarfgalaxies increases as the presence of PAH band emission becomes lesspronounced. The [ Ne iii] /[ Ne ii] ratios averaged over the full galaxyare strikingly high, often >10. Thus, the hard radiation fields arepronounced and pervasive. We find a prominent correlation between thePAHs/VSGs and the [ Ne iii] /[ Ne ii] ratios for a wide range ofobjects, including the low metallicity galaxies as well as Galactic H iiregions and other metal-rich galaxies. This effect is consistent withthe hardness of the interstellar radiation field playing a major role inthe destruction of PAHs in the low metallicity ISM. We see a PAHs/VSGsand metallicity correlation, also found by Engelbracht et al. (2005,ApJ, 628, 29) for a larger survey. Combined effects of metallicity andradiation field seem to be playing important roles in the observedbehavior of PAHs in the low metallicity systems.
| AGN Activity and Circumnuclear Star Formation in Nearby Galaxies
We have selected a sample of nearby galaxies where an AGN (Seyfert orLINER) coexists with circumnuclear star formation. We use the highestspatial resolution data available now (HST UV, optical, andnear-infrared [IR]; radio) and in the future (mid-IR with Spitzer andCanariCam) with spatial resolutions of 3-25 pc for our sample. The maingoal of this project is to determine the bolometric importance of bothphenomena in nearby galaxies by careful modelling of the UV to radiospectral energy distributions. In this paper we present results forNGC469 and NGC253, and discuss possible observations with CanariCam onthe GTC.
| Chandra Observation of the Starburst Galaxy NGC 2146
We present six monitoring observations of the starburst galaxy NGC 2146using the Chandra X-ray Observatory. We detected 67 point sources in thefield of view of the ACIS-S detector. Six of these sources wereUltra-Luminous X-ray Sources, the brightest of which had a luminosity of5 × 1039 erg s-1. One of them, with a luminosity of 1× 1039 erg s-1, is coincident with the dynamical center location,which may be a low-luminosity active galactic nucleus. We have produceda table where the positions and main characteristics of the detectedsources are reported. A comparison between the positions of the X-raysources and those detected in NIR or radio indicates no definitecounterpart. We have derived a log N ‑ log S relation and aluminosity function. The luminosity function has a slope of 0.71 above adetection limit, which is similar to those found in other starburstgalaxies. Diffuse emissions were detected in both soft (0.5-2.0keV) andhard (2.0-10.0keV) energy bands. The spectra of the diffuse componentwere fitted with two (hard and soft) components. The hard power-lawcomponent, with a luminosity of 4 × 1039 erg s-1, is likelyto have originated by unresolved point sources.
| How large are the bars in barred galaxies?
I present a study of the sizes (semimajor axes) of bars in discgalaxies, combining a detailed R-band study of 65 S0-Sb galaxies withthe B-band measurements of 70 Sb-Sd galaxies from Martin (1995). As hasbeen noted before with smaller samples, bars in early-type (S0-Sb)galaxies are clearly larger than bars in late-type (Sc-Sd) galaxies;this is true both for relative sizes (bar length as fraction ofisophotal radius R25 or exponential disc scalelength h) andabsolute sizes (kpc). S0-Sab bars extend to ~1-10 kpc (mean ~ 3.3 kpc),~0.2-0.8R25 (mean ~ 0.38R25) and ~0.5-2.5h (mean ~1.4h). Late-type bars extend to only ~0.5-3.5 kpc,~0.05-0.35R25 and 0.2-1.5h their mean sizes are ~1.5 kpc, ~0.14R25 and ~0.6h. Sb galaxies resemble earlier-type galaxiesin terms of bar size relative to h; their smallerR25-relative sizes may be a side effect of higher starformation, which increases R25 but not h. Sbc galaxies form atransition between the early- and late-type regimes. For S0-Sbcgalaxies, bar size correlates well with disc size (both R25and h); these correlations are stronger than the known correlation withMB. All correlations appear to be weaker or absent forlate-type galaxies; in particular, there seems to be no correlationbetween bar size and either h or MB for Sc-Sd galaxies.Because bar size scales with disc size and galaxy magnitude for mostHubble types, studies of bar evolution with redshift should selectsamples with similar distributions of disc size or magnitude(extrapolated to present-day values); otherwise, bar frequencies andsizes could be mis-estimated. Because early-type galaxies tend to havelarger bars, resolution-limited studies will preferentially find bars inearly-type galaxies (assuming no significant differential evolution inbar sizes). I show that the bars detected in Hubble Space Telescope(HST) near-infrared(IR) images at z~ 1 by Sheth et al. have absolutesizes consistent with those in bright, nearby S0-Sb galaxies. I alsocompare the sizes of real bars with those produced in simulations anddiscuss some possible implications for scenarios of secular evolutionalong the Hubble sequence. Simulations often produce bars as large as(or larger than) those seen in S0-Sb galaxies, but rarely any as smallas those in Sc-Sd galaxies.
| 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).
| 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.
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