Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
CXOU J121538.2+361921 in the galaxy NGC 4214: a double neutron star in the making?
CXOU J121538.2+361921 is the brightest X-ray source in the galaxy NGC4214, with an X-ray luminosity of up to 0.7 ×1039ergs-1. The observed periodicity of 3.62 h,derived from eclipse timing, is interpreted as the orbital period of thesystem. It has been suggested that the system is a low-mass helium starwith a lower-mass compact companion. If this idea is correct, then CXOUJ121538.2+361921 will evolve into a double neutron star, a binaryconsisting of a radio pulsar and another neutron star. In this study weinvestigate this possibility further. We find that the X-ray luminosityis consistent with super-Eddington accretion in a helium star-neutronstar binary. The binary is in a state of mass transfer which isinitiated when the helium-star donor is at the helium shell burningstage. A donor star with a current mass in the range of around2.2-3.6Msolar is required to explain the observed orbitalperiod. Helium stars in this mass range are massive enough to collapsein a supernova explosion, making CXOU J121538.2+361921 the immediateprogenitor of a double neutron star.
| On the reliability of CIVλ1549 as an abundance indicator for high-redshift star-forming galaxies
We reconsider the use of the equivalent width of CIVλ1549,EW(CIV), as an indicator of the oxygen abundance in star-forminggalaxies, as proposed by Heckman et al. for nearby starbursts. We refinethe local calibration of EW(CIV) versus log(O/H) by using a restrictedwavelength window which minimizes blending with interstellar absorptionlines. When applied to the stellar component only of the complexCIVλ1549 features in two high-redshift galaxies with good qualityspectra, MS1512-cB58 (z = 2.7268) and Q1307-BM1163 (z = 1.4105), thelocal calibration gives values of the oxygen abundance which are in goodagreement with other metallicity determinations based on nebularemission and interstellar absorption lines. Our main conclusion is thatfor this method to give reliable results at high redshifts, it shouldonly be used on data of sufficiently high spectral resolution (R >~1000) for stellar and interstellar CIV components to be clearlyseparated. Oxygen abundances will be systematically overestimated if thelocal calibration is applied to spectra of high-redshift galaxiesobtained with the low resolving powers (R ~= 200-300) of many currentwide-field surveys. It will also be necessary to understand better thecauses of the scatter in the local relation, before we can be confidentof inferences from it at high z.
| Multiwavelength Star Formation Indicators: Observations
We present a compilation of multiwavelength data on different starformation indicators for a sample of nearby star forming galaxies. Herewe discuss the observations, reductions and measurements of ultravioletimages obtained with STIS on board the Hubble Space Telescope (HST),ground-based Hα, and VLA 8.46 GHz radio images. These observationsare complemented with infrared fluxes, as well as large-apertureoptical, radio, and ultraviolet data from the literature. This databasewill be used in a forthcoming paper to compare star formation rates atdifferent wave bands. We also present spectral energy distributions(SEDs) for those galaxies with at least one far-infrared measurementsfrom ISO, longward of 100 μm. These SEDs are divided in two groups,those that are dominated by the far-infrared emission, and those forwhich the contribution from the far-infrared and optical emission iscomparable. These SEDs are useful tools to study the properties ofhigh-redshift galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
We report 4.5 μm luminosities for 27 nearby (D<~5 Mpc) dwarfirregular galaxies measured with the Spitzer Infrared Array Camera. Wehave constructed the 4.5 μm luminosity-metallicity (L-Z) relation for25 dwarf galaxies with secure distance and interstellar medium oxygenabundance measurements. The 4.5 μm L-Z relation is12+log(O/H)=(5.78+/-0.21)+(-0.122+/-0.012)M[4.5], whereM[4.5] is the absolute magnitude at 4.5 μm. The dispersionin the near-infrared L-Z relation is smaller than the correspondingdispersion in the optical L-Z relation. The subsequently derived stellarmass-metallicity (M*-Z) relation is12+log(O/H)=(5.65+/-0.23)+(0.298+/-0.030)logM*, and extendsthe SDSS M*- Z relation to lower mass by about 2.5 dex. Wefind that the dispersion in the M*-Z relation is similar over5 orders of magnitude in stellar mass, and that the relationship betweenstellar mass and interstellar medium metallicity is similarly tight fromhigh-mass to low-mass systems. We find a larger scatter at low mass inthe relation between effective yield and total baryonic mass. In fact,there are a few dwarf galaxies with large yields, which is difficult toexplain if galactic winds are ubiquitous in dwarf galaxies. The lowscatter in the L-Z and M*-Z relationships are difficult tounderstand if galactic superwinds or blowout are responsible for the lowmetallicities at low mass or luminosity. Naively, one would expect anever increasing scatter at lower masses, which is not observed.
| 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.
| Ultraviolet-to-Far-Infrared Properties of Local Star-forming Galaxies
We present the results of a multiwavelength study of nearby galaxiesaimed at understanding the relation between the ultraviolet andfar-infrared emission in star-forming galaxies. The data set comprisesnew ultraviolet (from HST STIS), ground-based Hα, and radiocontinuum observations, together with archival infrared data (from IRASand ISO). The local galaxies are used as benchmarks for comparison ofthe infrared-to-ultraviolet properties with two populations ofhigh-redshift galaxies: the submillimeter star-forming galaxies detectedby SCUBA and the ultraviolet-selected Lyman break galaxies (LBGs). Inaddition, the long wavelength baseline covered by the present dataenables us to compare the star formation rates (SFRs) derived from theobserved ultraviolet, Hα, infrared, and radio luminosities and togauge the impact of dust opacity in the local galaxies. We also derive anew calibration for the nonthermal part of the radio SFR estimator,based on the comparison of 1.4 GHz measurements with a new estimator ofthe bolometric luminosity of the star-forming regions. We find that moreactively star-forming galaxies show higher dust opacities, which is inline with previous results. We find that the local star-forming galaxieshave a lower Fλ(205 μm)/Fλ(UV)ratio by 2-3 orders of magnitude than the submillimeter-selectedgalaxies and may have a similar or somewhat higherFλ(205 μm)/Fλ(UV) ratio thanLBGs. The Fλ(205 μm)/Fλ(UV) ratioof the local galaxy population may be influenced by the cool dustemission in the far-infrared heated by nonionizing stellar populations,which may be reduced or absent in the LBGs.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| 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.
| Masses of the local group and of the M81 group estimated from distortions in the local velocity field
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
| Massive Star Cluster Populations in Irregular Galaxies as Probable Younger Counterparts of Old Metal-rich Globular Cluster Populations in Spheroids
Peak metallicities of metal-rich populations of globular clusters(MRGCs) belonging to early-type galaxies and spheroidal subsystems ofspiral galaxies (spheroids) of different mass fall within the somewhatconservative -0.7<=[Fe/H]<=-0.3 range. Indeed, if possible ageeffects are taken into account, this metallicity range might becomesmaller. Irregular galaxies such as the Large Magellanic Cloud (LMC),with longer timescales of formation and lower star formation (SF)efficiency, do not contain old MRGCs with [Fe/H]>-1.0, but they areobserved to form populations of young/intermediate-age massive starclusters (MSCs) with masses exceeding 104 Msolar.Their formation is widely believed to be an accidental process fullydependent on external factors. From the analysis of available data onthe populations and their hosts, including intermediate-age populousstar clusters in the LMC, we find that their most probable meanmetallicities fall within -0.7<=[Fe/H]<=-0.3, as the peakmetallicities of MRGCs do, irrespective of signs of interaction.Moreover, both the disk giant metallicity distribution function (MDF) inthe LMC and the MDFs for old giants in the halos of massive spheroidsexhibit a significant increase toward [Fe/H]~-0.5. That is in agreementwith a correlation found between SF activity in galaxies and theirmetallicity. The formation of both the old MRGCs in spheroids and MSCpopulations in irregular galaxies probably occurs at approximately thesame stage of the host galaxies' chemical evolution and is related tothe essentially increased SF activity in the hosts around the samemetallicity that is achieved very early in massive spheroids, later inlower mass spheroids, and much later in irregular galaxies. Changes inthe interstellar dust, particularly in elemental abundances in dustgrains and in the mass distribution function of the grains, may be amongthe factors regulating star and MSC formation activity in galaxies.Strong interactions and mergers affecting the MSC formation presumablyplay an additional role, although they can substantially intensify theinternally regulated MSC formation process. Several implications of oursuggestions are briefly discussed.
| 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.
| Mid-Infrared Images of Stars and Dust in Irregular Galaxies
We present mid-IR to optical properties of 22 representative irregulargalaxies: 18 irregular (Im) galaxies, 3 blue compact dwarfs, and 1Magellanic-type spiral galaxy. The mid-IR is based on images from theSpitzer Space Telescope archives. The 3.6 and 4.5 μm bands and theUBVJHK images are used to examine disk morphology and the integrated andazimuthally averaged magnitudes and colors of stars. The nonstellarcontribution to the 4.5 μm images is used to trace hot dust. The 5.8and 8.0 μm images reveal emission from hot dust and polycyclicaromatic hydrocarbons (PAHs), and both may contribute to thesepassbands, although we refer to the nonstellar emission as PAH emission.We compare the 8.0 μm images to Hα. Im galaxies have no hiddenbars, and those with double-exponential optical light profiles have thesame at mid-IR. Most galaxies have similar optical and mid-IR scalelengths. Four galaxies have super star clusters that are not visible atoptical bands. Galaxies with higher area-normalized star formation rateshave more dust and PAH emission relative to starlight. Hot dust and PAHemission is found mostly in high surface brightness H II regions,implying that massive stars are the primary source of heating. Galaxieswith intense, widespread star formation have more extended PAH emission.The ratio of PAH to Hα emission is not constant on small scales.PAHs are associated with shells and giant filaments, so they are notdestroyed during shell formation.This work is based in part on archival data obtained with the SpitzerSpace Telescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology, under a contract with NASA.
| Associations of Dwarf Galaxies
The Hubble Space Telescope Advanced Camera for Surveys has been used todetermine accurate distances for 20 galaxies from measurements of theluminosity of the brightest red giant branch stars. Five associations ofdwarf galaxies that had originally been identified based on strongcorrelations on the plane of the sky and in velocity are shown to beequally well correlated in distance. Two more associations with similarproperties have been discovered. Another association is identified thatis suggested to be unbound through tidal disruption. The associationshave the spatial and kinematic properties expected of bound structureswith (1-10)×1011 Msolar. However, theseentities have little light, with the consequence that the mass-to-lightratios are in the range 100-1000 MsolarL-1solar. Within a well-surveyed volume extendingto a 3 Mpc radius, all but one known galaxy lie within one of the groupsor associations that have been identified.
| The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
| Absolute Magnitude Distributions and Light Curves of Stripped-Envelope Supernovae
The absolute visual magnitudes of three Type IIb, 11 Type Ib, and 13Type Ic supernovae (collectively known as stripped-envelope supernovae)are studied by collecting data on the apparent magnitude, distance, andinterstellar extinction of each event. Weighted and unweighted meanabsolute magnitudes of the combined sample, as well as various subsetsof the sample, are reported. The limited sample size and theconsiderable uncertainties, especially those associated with extinctionin the host galaxies, prevent firm conclusions regarding differencesbetween the absolute magnitudes of supernovae of Types Ib and Ic, andregarding the existence of separate groups of overluminous andnormal-luminosity stripped-envelope supernovae. The spectroscopiccharacteristics of the events of the sample are considered. Three of thefour overluminous events are known to have had unusual spectra. Most butnot all of the normal-luminosity events have had typical spectra. Thelight curves of stripped-envelope supernovae are collected and compared.Because SN 1994I in M51 was very well observed, it often is regarded asthe prototypical Type Ic supernova, but it has the fastest light curvein the sample. Light curves are modeled by means of a simple analyticaltechnique that, combined with a constraint on E/M from spectroscopy,yields internally consistent values of ejected mass, kinetic energy, andnickel mass.
| Dynamical mass estimates for two luminous star clusters in galactic merger remnants
We present high-dispersion spectra of two extremely massive starclusters in galactic merger remnants, obtained using the UVESspectrograph mounted on the ESO Very Large Telescope. One cluster, W30,is located in the ~500 Myr old merger remnant NGC 7252 and has avelocity dispersion and effective radius of σ=27.5±2.5 kms-1 and Reff=9.3±1.7 pc, respectively. Theother cluster, G114, located in the ~3 Gyr old merger remnant NGC 1316,is much more compact, Reff=4.08±0.55 pc, and has avelocity dispersion of σ=42.1±2.8 km s-1. Thesemeasurements allow an estimate of the virial mass of the two clusters,yielding Mdyn(W30)=1.59(±0.26)× 10^7Mȯ and Mdyn(G114)=1.64(±0.13)×10^7 Mȯ. Both clusters are extremely massive, being morethan three times heavier than the most massive globular clusters in theGalaxy. For both clusters we measure light-to-mass ratios, which whencompared to simple stellar population (SSP) models of the appropriateage, are consistent with a Kroupa-type stellar mass function. Usingmeasurements from the literature we find a strong age dependence on howwell SSP models (with underlying Kroupa or Salpeter-type stellar massfunctions) fit the light-to-mass ratio of clusters. Based on this resultwe suggest that the large scatter in the light-to-mass ratio of theyoungest clusters is not due to variations in the underlying stellarmass function, but instead to the rapidly changing internal dynamics ofyoung clusters. Based on sampling statistics we argue that while W30 andG114 are extremely massive, they are consistent with being the mostmassive clusters formed in a continuous power-law cluster massdistribution. Finally, based on the positions of old globular clusters,young massive clusters (YMCs), ultra-compact dwarf galaxies (UCDs) anddwarf-globular transition objects (DGTOs) in κ-space we concludethat 1) UCDs and DGTOs are consistent with the high mass end of starclusters and 2) YMCs occupy a much larger parameter space than oldglobular clusters, consistent with the idea of preferential disruptionof star clusters.
| A Search for Candidate Radio Supernova Remnants in the Nearby Irregular Starburst Galaxies NGC 4214 and NGC 4395
We present the results of a search for new candidate radio supernovaremnants (SNRs) in the nearby starburst irregular galaxies NGC 4214 andNGC 4395 using archived radio observations made with the Very LargeArray (VLA) at the wavelengths of 3.5 cm, 6 cm and 20 cm for NGC 4214and 6 cm and 20 cm for NGC 4395. These observations were analyzed aspart of our ongoing search for candidate radio SNRs in nearby galaxies:the goal of this search is to prepare a large sample of candidate radioSNRs for the purpose of a robust statistical study of the properties ofthese sources. Based on our analysis, we have confirmed the non-thermalnature of the discrete radio sources alpha and beta in NGC 4214 andclassify these sources as candidate radio SNRs based on their positionalcoincidences with HII regions in that galaxy. We have measured the fluxdensities of the two candidate radio SNRs at each wavelength andcalculated corresponding spectral indices: we have also measured fluxdensities of two other discrete radio sources in these galaxies - rho inNGC 4214 and #3 in NGC 4395 - which we suspect to be additionalcandidate radio SNRs based on their positional coincidences with otherHII regions in these galaxies. However, the radio data presentlyavailable for these sources cannot confirm such a classification andadditional observations are needed. We have also calculated the radioluminosities Lradio at the wavelength of 20 cm for these twocandidate radio SNRs as well as the corresponding values for the minimumtotal energy Emin required to power these radio sources viasynchrotron emission and the corresponding magnetic field strengthBmin. We have compared our mean calculated values for theseproperties with the mean values for populations of candidate radio SNRsin other starburst galaxies: while the values for Lradio andBmin are roughly comparable to the values seen in otherstarburst galaxies, the mean value for Emin is higher thanthe mean value of any other starburst galaxy. Finally, we include thesetwo candidate radio SNRs in a discussion of the Sigma-D relation forextragalactic candidate radio SNRs and find that these sources arelocated on the shallower end of the master Sigma-D relation for allextragalactic SNRs as derived by Urosevic et al. (2005).
| 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).
| GHASP: an Hα kinematic survey of spiral and irregular galaxies - IV. 44 new velocity fields. Extension, shape and asymmetry of Hα rotation curves
We present Fabry-Perot observations obtained in the frame of the GHASPsurvey (Gassendi HAlpha survey of SPirals). We have derived the Hαmap, the velocity field and the rotation curve for a new set of 44galaxies. The data presented in this paper are combined with the datapublished in the three previous papers providing a total number of 85 ofthe 96 galaxies observed up to now. This sample of kinematical data hasbeen divided into two groups: isolated (ISO) and softly interacting(SOFT) galaxies. In this paper, the extension of the Hα discs, theshape of the rotation curves, the kinematical asymmetry and theTully-Fisher relation have been investigated for both ISO and SOFTgalaxies. The Hα extension is roughly proportional toR25 for ISO as well as for SOFT galaxies. The smallestextensions of the ionized disc are found for ISO galaxies. The innerslope of the rotation curves is found to be correlated with the centralconcentration of light more clearly than with the type or thekinematical asymmetry, for ISO as well as for SOFT galaxies. The outerslope of the rotation curves increases with the type and with thekinematical asymmetry for ISO galaxies but shows no special trend forSOFT galaxies. No decreasing rotation curve is found for SOFT galaxies.The asymmetry of the rotation curves is correlated with themorphological type, the luminosity, the (B-V) colour and the maximalrotational velocity of galaxies. Our results show that the brightest,the most massive and the reddest galaxies, which are fast rotators, arethe least asymmetric, meaning that they are the most efficient withwhich to average the mass distribution on the whole disc. Asymmetry inthe rotation curves seems to be linked with local star formation,betraying disturbances of the gravitational potential. The Tully-Fisherrelation has a smaller slope for ISO than for SOFT galaxies.
| From young massive star cluster to old globular: the LV-σ0 relationship as a diagnostic tool
We present a new analysis of the properties of the young massive starclusters (YMCs) forming profusely in intense starburst environments,which demonstrates that these objects are plausible progenitors of theold globular clusters (GCs) seen abundantly in the Local Group. Themethod is based on the tight relationship for old GCs between theirV-band luminosities, LV, and (central) velocity dispersions,σ0. We improve the significance of the relationship byincreasing the GC sample size and find that its functional form,LV/Lsolar~σ1.57+/-0.100(km s-1), is fully consistent with previous determinationsfor smaller Galactic and M31 GC samples. The tightness of therelationship for a GC sample drawn from environments as diverse as thosefound in the Local Group implies that its origin must be sought inintrinsic properties of the GC formation process itself. We evolve theluminosities of those YMCs in the local Universe which have velocitydispersion measurements to an age of 12 Gyr, adopting a variety ofinitial mass function (IMF) descriptions, and find that most YMCs willevolve to loci close to, or to slightly fainter luminosities than theimproved GC relationship. In the absence of significant externaldisturbances, this implies that these objects may potentially survive tobecome old GC-type objects over a Hubble time. The main advantage of ournew method is its simplicity. Whereas alternative methods, based ondynamical mass estimates, require one to obtain accurate size estimatesand to make further assumptions, the only observables required here arethe system's velocity dispersion and luminosity. The most importantfactor affecting the robustness of our conclusions is the adopted formof the IMF. We use the results of N-body simulations to confirm thatdynamical evolution of the clusters does not significantly alter ourconclusions about the likelihood of individual clusters surviving tolate times. Finally, we find that our youngest observed clusters areconsistent with having evolved from a relation of the form . Thisrelation may actually correspond to the origin of the GC fundamentalplane.
| A Chandra X-ray survey of nearby dwarf starburst galaxies - II. Starburst properties and outflows
We present a comprehensive comparison of the X-ray properties of asample of eight dwarf starburst galaxies observed with Chandra (IZw18,VIIZw403, NGC1569, NGC3077, NGC4214, NGC4449, NGC5253 and He2-10). InPaperI, we presented in detail the data reduction and analysis of theindividual galaxies. For the unresolved X-ray sources, we find thefollowing: point sources are in general located close to bright HIIregions, rims of superbubbles or young stellar clusters. The number ofX-ray point sources appears to be a function of the current starformation (SF) rate and the blue luminosity of the hosts. UltraluminousX-ray sources (ULXs) are only found in those dwarf galaxies that arecurrently interacting. The power-law (PL) index of the combinedcumulative X-ray point-source luminosity function is α= 0.24 +/-0.06, shallower than that of more massive starburst galaxies (α=0.4 -0.8) and of non-starburst galaxies (α~ 1.2). For thosegalaxies showing extended X-ray emission (six out of the eightgalaxies), we derive the following: superwinds develop along thesteepest gradient of the HI distribution with volume densities of0.02-0.06cm-3, pressures of 1-3 ×105Kcm-3, thermal energies of 2-30 ×1054erg and hot gas masses of 2-20 ×106Msolar (~1 per cent of the HI masses). Onglobal scales, the distribution of the X-ray emission looks remarkablysimilar to that seen in Hα (comparing azimuthal averages);locally, however, their distribution is clearly distinct in many cases,which can be explained by the different emission mechanisms (forwardversus reverse shocks). Mass loading of order 1 to 5 is required toexplain the differences between the amount of hot gas and the modelledmass loss from massive stars. The metallicity of the dwarf galaxiescorrelates with the diffuse X-ray luminosity and anticorrelates with thecooling time of the hot gas. The diffuse X-ray luminosity is also afunction of the current star formation rate (SFR). The mechanicalluminosities of the developing superwinds are energetic enough toovercome the gravitational potentials of their host galaxies. Thisscenario is supported by the overpressures of the hot gas compared withthe ambient interstellar medium (ISM). Extended HI envelopes such astidal tails, however, may delay outflows on time-scales exceeding thoseof the cooling time of the hot gas.
| A Chandra X-ray survey of nearby dwarf starburst galaxies - I. Data reduction and results
We present an analysis of Chandra X-ray observations of a sample ofeight dwarf starburst galaxies (IZw18, VIIZw403, NGC1569, NGC3077,NGC4214, NGC4449, NGC5253 and He2-10). Extended, diffuse X-ray emissionis detected in all but two of the objects. Unresolved sources were foundwithin all dwarf galaxies (total: 55 sources). These point sources arewell fitted by power-law (PL), thermal plasma (TP) or blackbody (BB)models. 10 of the point sources exceed an X-ray luminosity of1039 erg s-1 (ultraluminous X-ray sources, ULXs).In those galaxies where diffuse X-ray emission is detected, thisemission (with X-ray luminosities ranging from 4 × 1038to 2 × 1040 erg s-1) contains most (60-80per cent) of the X-ray photons. This diffuse emission can be well fittedby MEKAL one-temperature TP models once the contribution from theunresolved point sources is subtracted properly. The diffuse X-raycomponent is significantly extended, reaching as far as 0.5-5kpc intothe outskirts of their hosts. Azimuthally averaged X-ray surfacebrightness profiles are well approximated by exponential functions.Temperatures of various regions within the galaxies range from 1.6 to7.6 × 106K. With few exceptions, temperatures of thehot gas are remarkably uniform, hovering around 2-3 ×106K. Temperatures of the coronal gas in the outer regionsare in general ~2-3 times lower than those found in the central regions.Fits to the diffuse emission do not allow strong constraints to be puton the metallicities of the emitting plasmas. However, the derivedmetallicities are compatible with those determined from their HIIregions. An α/Fe ratio of ~2 is indicated for the hot gas withinat least three objects (NGC1569, NGC4449 and He2-10). Shadowing of thediffuse X-ray emission by the cooler disc gas is used to constrain theorientation of the galaxies.
| First Results from THINGS: The HI Nearby Galaxy Survey
We describe The HI Nearby Galaxy Survey (THINGS), the largestprogramever undertaken at the Very Large Array to perform 21-cm HIobservations of thehighest quality ( 7'', ≤ 5 km s^{-1}resolution) ofnearby galaxies. The goal of THINGS is to investigatekeycharacteristics related to galaxy morphology, star formation andmassdistribution across the Hubble sequence. A sample of 34 objectswithdistances between 3 and 10 Mpc will be observed, covering a widerangeof evolutionary stages and properties. Data from THINGSwillcomplement SINGS, the Spitzer Infrared Nearby Galaxy Survey. Forthe THINGS sample, high-quality observations at comparable resolutionwillthus be available from the X-ray regime through to the radio partofthe spectrum. THINGS data can be used to investigate issues such asthesmall-scale structure of the ISM, its three-dimensional structure,the(dark) matter distribution and processes leading to starformation. Todemonstrate the quality of the THINGS data products, wepresent someprelimary HI maps here of four galaxies from the THINGSsample.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| Comparison of Star Clusters With and Without Wolf-Rayet Stars in Wolf-Rayet Galaxies
We compare the properties of young star clusters with and withoutWolf-Rayet (W-R) stars in W-R galaxies using optical, near-infraredimagery and optical spectroscopy. Our work identifies the clusters withW-R stars in these galaxies for the first time. With this information,comparisons of clusters with and without W-R stars are now possible,enabling us to understand the chemical and morphological impact ofmassive stars on their environment and to constrain the parameters formodeling these systems. We find that clusters with W-R stars (W-Rclusters) are systematically younger, bluer clusters. Knowing this agedifference between the two cluster sets, we use an evolutionary scenarioto interpret their other properties. Young clusters, typically W-Rclusters, have a Strömgren sphere-like gas configuration. They alsotend to have H-K colors redder than those of theoretical models. Weinterpret the H-K excess as a combination of thermal emission from hotdust, nebular emission, and molecular emission. Older clusters,typically clusters without W-R stars, have ionized gas in a superbubbleconfiguration caused by the prolonged influence of stellar winds andsupernovae. The H-K excess is generally absent for these clusters. Thenitrogen-to-oxygen abundance ratio (N/O) does not appear to increase asa function of age over the first 10 Myr. Systems without W-R stars doappear to have a significant, elevated N/O over systems with W-R starsin the metallicity range 12+log(O/H)=7.7-7.9. For the entire metallicityrange in our sample, this finding is only marginally significant. Weconcur with previous studies, which find no correlation between thesulfur-to-oxygen abundance ratio and metallicity.
| The Stellar Content of Nearby Star-forming Galaxies. III. Unravelling the Nature of the Diffuse Ultraviolet Light
We investigate the nature of the diffuse intracluster ultraviolet lightseen in 12 local starburst galaxies, using long-slit ultravioletspectroscopy obtained with the Space Telescope Imaging Spectrograph(STIS) aboard the Hubble Space Telescope (HST). We take this faintintracluster light to be the field in each galaxy and compare itsspectroscopic signature with Starburst99 evolutionary synthesis modelsand with neighboring star clusters. Our main result is that the diffuseultraviolet light in 11 of the 12 starbursts lacks the strong O starwind features that are clearly visible in spectra of luminous clustersin the same galaxies. The difference in stellar features dominatingcluster and field spectra indicates that the field light comes primarilyfrom a different stellar population and not from scattering of UVphotons originating in the massive clusters. A cut along the spatialdirection of the UV spectra establishes that the field light is notsmooth but rather shows numerous ``bumps and wiggles.'' Roughly 30%-60%of these faint peaks seen in field regions of the closest (<4 Mpc)starbursts appear to be resolved, suggesting a contribution fromsuperpositions of stars and/or faint star clusters. Complementary WFPC2UVI imaging for the three nearest target galaxies, NGC 4214, NGC 4449,and NGC 5253, is used to obtain a broader picture and establish that allthree galaxies have a dispersed population of unresolved, luminous bluesources. Because the field spectra are dominated by B stars, we suggestthat the individual sources observed in the WFPC2 images are individualB stars (rather than O stars) or small star clusters. We considerseveral scenarios to understand the lack of observed massive stars inthe field and their implications for the origin of the field stellarpopulation. If the field stellar populations formed in situ, the fieldmust have either an IMF that is steeper than Salpeter (α~-3.0 to-3.5) or a Salpeter slope with an upper mass cutoff of 30-50Msolar. If star formation occurs primarily in star clusters,the field could be composed of older, faded clusters and/or a populationthat is coeval with the luminous clusters but lower in mass. We usethese benchmark populations to place constraints on the field stellarpopulation origin. Although the field probably includes stars ofdifferent ages, the UV light is dominated by the youngest stellarpopulations in the field. If the field is composed of older, dissolvingclusters, we estimate that star clusters (regardless of mass) need todissolve on timescales 7-10 Myr to create the field. If the field iscomposed of young clusters that fall below the detection limit ofindividual sources in our spectroscopy, they would have to be severalhundred solar masses or less, in order to be deficient in O stars,despite their extreme youth.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| A Chandra X-Ray Investigation of the Violent Interstellar Medium: From Dwarf Starbursts to Ultraluminous Infrared Galaxies
We have analyzed observations with the Chandra X-Ray Observatory of thediffuse emission by hot gas in seven dwarf starburst galaxies, sixedge-on starburst galaxies, and nine ultraluminous infrared galaxies.These systems cover ranges of ~104 in X-ray luminosity, andseveral thousand in star formation rate and K-band luminosity (a proxyfor stellar mass). Despite this range in fundamental parameters, we findthat the properties of the diffuse X-ray emission are very similar inall three classes of starburst galaxies. The spectrum of the diffuseemission is well fitted by thermal emission from gas with kT~0.25-0.8keV and with several times solar abundance ratios of α-elements toFe. The ratio of the thermal X-ray to far-infrared luminosity is roughlyconstant, as is the characteristic surface brightness of the diffuseX-ray emission. The size of the diffuse X-ray source increasessystematically with both far-infrared and K-band luminosity. All threeclasses show strong morphological relationships between the regions ofhot gas probed by the diffuse X-ray emission and the warm gas probed byoptical line emission. These findings suggest that the same physicalmechanism is producing the diffuse X-ray emission in the three types ofstarbursts. These results are consistent with that mechanism beingshocks driven by a galactic ``superwind,'' which is powered by thekinetic energy collectively supplied by stellar winds and supernovae inthe starburst.
| Mapping Large-Scale Gaseous Outflows in Ultraluminous Galaxies with Keck II ESI Spectra: Variations in Outflow Velocity with Galactic Mass
Measurements of interstellar Na I λλ5890, 5896 absorptionlines in 18 ultraluminous infrared galaxies (ULIGs) have been combinedwith published Na I data, to reassess the dependence of galactic outflowspeeds on starburst luminosity and galactic mass. The Doppler shiftsreveal outflows of relatively cool gas in 15 of 18 ULIGs with an averageoutflow speed at the line center of 330+/-100 km s-1. Therelation between outflow speed and star formation rate (SFR), defined bythe distribution's upper envelope over 4 orders of magnitude in SFR,demonstrates that winds from more luminous starbursts accelerateinterstellar gas to higher speeds roughly as v~SFR0.35. Thisresult is surprising since, in the traditional model forstarburst-driven winds, these relatively cool gas clouds are acceleratedby the ram pressure of a hot, supernova-heated wind that exhibits weak(if any) X-ray temperature variation with increasing galactic mass. Thelack of evidence for much hotter winds is partly a sensitivity issue,but the Na I velocities in ultraluminous starbursts actually areconsistent with acceleration by the tepid wind, indicating that a hottercomponent is unlikely to dominate the momentum flux. The Na I velocitiesin the dwarf starburst winds do not reach the terminal velocity of a hotwind at the measured temperature of kT~0.73 keV, a result that could beinterpreted simply as evidence that the hot superbubbles are tooconfined in dwarf starbursts to generate a free-flowing wind. Adynamically motivated scenario, however, is that the dwarf starburstwinds simply lack enough momentum to accelerate the clouds to thevelocity of the hot wind. Among the subsample of starbursts withwell-constrained dynamical masses, the terminal outflow velocities arealways found to approach the galactic escape velocity. Motivated by asimilar scaling relation for stellar winds, the galactic Eddingtonluminosity for dusty starbursts is shown to be within the range measuredfor ULIGs. If radiation pressure on dust grains, coupled to the coolwind, is indeed important for galactic wind dynamics, then feedback isstronger in massive galaxies than previously thought, helping shape thehigh-mass end of the galaxy luminosity function. Regardless of thenature of the acceleration mechanism in ULIGs, the mass flux of cool gasestimated from these data demonstrates that starburst-driven windstransport significant gas during the assembly stage of field ellipticalgalaxies, a factor that helps explain the rapid decline in SFR in thesesystems inferred from elemental abundance ratios.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Probing the Multiphase Interstellar Medium of the Dwarf Starburst Galaxy NGC 625 with Far Ultraviolet Spectroscopic Explorer Spectroscopy
We present new Far Ultraviolet Spectroscopic Explorer (FUSE)spectroscopy of the dwarf starburst galaxy NGC 625. These observationsprobe multiple phases of the interstellar medium (ISM), including thecoronal, ionized, neutral, and molecular gas. This nearby (D=3.9+/-0.2Mpc) system shows a clear detection of outflowing coronal gas as tracedby O VI λ1032 absorption. The centroid of the O VI profile isblueshifted with respect to the galaxy systemic velocity by ~30 kms-1, suggesting a low-velocity outflow. The implied O VIvelocity extent is found to be 100+/-20 km s-1, which isfully consistent with the detected H I outflow velocity found in radiosynthesis observations. We detect multiple lines of diffuseH2 absorption from the ISM of NGC 625; this is one of only afew extragalactic systems with FUSE detections of H2 lines inthe Lyman and Werner bands. We find a potential abundance offset betweenthe neutral and nebular gas that exceeds the errors on the derivedcolumn densities. Since such an offset has been found in multiple dwarfgalaxies, we discuss the implications of a lower-metallicity halosurrounding the central star-forming regions of dwarf galaxies. Theapparent offset may be due to saturation of the observed O I line, andhigher signal-to-noise ratio (S/N) observations are required to resolvethis issue.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985.
| Low-Mass Star Formation in the Gum Nebula: The CG 30/31/38 Complex
We present photometric and spectroscopic results for the low-masspre-main-sequence (PMS) stars with spectral types K-M in the cometaryglobule (CG) 30/31/38 complex. We obtained multiobject high-resolutionspectra for the targets selected as possible PMS stars frommultiwavelength photometry. We identified 11 PMS stars brighter thanV=16.5 with ages <~5 Myr at a distance of approximately 200 pc. Thespatial distribution of the PMS stars, CG clouds, and ionizing sources(O stars and supernova remnants) suggests a possible triggered origin ofthe star formation in this region. We confirm the youth of thephotometrically selected PMS stars using the lithium abundances. Theradial velocities of the low-mass PMS stars are consistent with those ofthe cometary globules. Most of the PMS stars show weak Hα emissionwith Wλ(Hα)<10 Å. Only one out of the11 PMS stars shows a moderate near-IR excess, which suggests a shortsurvival time (t<5 Myr) of circumstellar disks in this star-formingenvironment. In addition, we find five young late-type stars and one Aestar that have no obvious relation to the CG 30/31/38 complex. We alsodiscuss a possible scenario of the star formation history in the CG30/31/38 region.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
