Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Comment regarding the functional form of the Schmidt law
Star formation rates on the galactic scale are describedphenomenologically by two distinct relationships, as emphasized recentlyby Elmegreen [Elmegreen, B.G., 2002. ApJ, 577, 206. astro-ph/0207114.].The first of these is the Schmidt law, which is a power-law relationbetween the star formation rate SFR and the column density Σ. Theother relationship is that there is a cutoff in the gas density belowwhich star formation shuts off. The purpose of this paper is to arguethat: (1) these two relationships can be accommodated by a singlefunctional form of the Schmidt law, (2) this functional form ismotivated by the hypothesis that star formation is a criticalphenomenon, and that as a corollary, (3) the existence of a sharp cutoffmay thus be an emergent property of galaxies, as was argued by Seiden[Seiden, P.E.,1983. ApJ, 266, 555.], as opposed to the classical viewthat this cutoff is due to an instability criterion.
| 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.
| Late-type galaxies observed with SAURON: two-dimensional stellar and emission-line kinematics of 18 spirals
We present the stellar and gas kinematics of a sample of 18 nearbylate-type spiral galaxies (Hubble types ranging from Sb to Sd), observedwith the integral-field spectrograph SAURON at the 4.2-m WilliamHerschel Telescope. SAURON covers the spectral range 4800-5380Å,allowing us to measure the Hβ, Fe, Mgb absorption features and theemission in the Hβ line and the [OIII]λλ4959,5007Å and [NI]λλ5198, 5200Å doublets over a 33× 41-arcsec2 field of view. The maps cover the nuclearregion of these late-type galaxies and in all cases include the entirebulge. In many cases the stellar kinematics suggests the presence of acold inner region, as visible from a central drop in the stellarvelocity dispersion. The ionized gas is almost ubiquitous and behaves ina complicated fashion: the gas velocity fields often display morefeatures than the stellar ones, including wiggles in the zero-velocitylines, irregular distributions, ring-like structures. The line ratio[OIII]/Hβ often takes on low values over most of the field,probably indicating a wide-spread star formation.
| A dynamical model for the extraplanar gas in spiral galaxies
Recent HI observations reveal that the discs of spiral galaxies aresurrounded by extended gaseous haloes. This extraplanar gas reacheslarge distances (several kiloparsecs) from the disc and shows peculiarkinematics (low rotation and inflow). We have modelled the extraplanargas as a continuous flow of material from the disc of a spiral galaxyinto its halo region. The output of our models is pseudo data cubes thatcan be directly compared to the HI data. We have applied these models totwo spiral galaxies (NGC 891 and NGC 2403) known to have a substantialamount of extraplanar gas. Our models are able to reproduce accuratelythe vertical distribution of extraplanar gas for an energy inputcorresponding to a small fraction (<4 per cent) of the energyreleased by supernovae. However, they fail in two important aspects: (1)they do not reproduce the right gradient in rotation velocity; (2) theypredict a general outflow of the extraplanar gas, contrary to what isobserved. We show that neither of these difficulties can be removed ifclouds are ionized and invisible at 21cm as they leave the disc butbecome visible at some point on their orbits. We speculate that thesefailures indicate the need for accreted material from the intergalacticmedium that could provide the low angular momentum and inflow required.
| The metallicity dependence of the Cepheid period-luminosity relation .
We have assessed the influence of the stellar iron content on theCepheid Period-Luminosity (PL) relation by relating the V band residualsfrom the \citet{fre01} PL relation to [Fe/H] for 68 Galactic andMagellanic Cloud Cepheids. The iron abundances were measured from FEROSand UVES high-resolution and high signal-to-noise optical spectra. Ourdata indicate that the stars become fainter as metallicity increases,until a plateau or turnover point is reached at about solar metallicity.This behavior appears at odds both with the PL relation beingindependent from iron abundance and with Cepheids becoming monotonicallybrighter as metallicity increases \citep[e.g.][]{ken98,sak04}.
| Scale Heights of Non-Edge-on Spiral Galaxies
We present a method of calculating the scale height of non-edge-onspiral galaxies, together with a formula for errors. The method is basedon solving Poisson's equation for a logarithmic disturbance of matterdensity in spiral galaxies. We show that the spiral arms can not extendto inside the ``forbidden radius'' r0, due to the effect ofthe finite thickness of the disk. The method is tested by re-calculatingthe scale heights of 71 northern spiral galaxies previously calculatedby Ma, Peng & Gu. Our results differ from theirs by less than 9%. Wealso present the scale heights of a further 23 non-edge-on spiralgalaxies.
| 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.
| Hubble Space Telescope Identification of the Optical Counterparts of Ultraluminous X-Ray Sources in M51
We present the results of a search for optical identifications ofultraluminous X-ray sources (ULXs) in M51 by using mosaic images takenwith the Hubble Space Telescope(HST) Advanced Camera for Surveys (ASC)in filters F435W (B), F555W (V), F814W (I), and F658N (Hα). Oursample, consisting of nine ULXs, is defined by analyzing the threeChandra observations of M51 performed in 2000 June, 2001 June, and 2003August. We found that four ULXs have one or two candidates forcounterparts, while two have multiple stars within their error circles.The other three have no candidate counterparts. Four ULXs are locatednear or in a star cluster, while others have no association with acluster. These results indicate that the companion stars, environments,and origins of ULXs are probably heterogeneous.
| Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Balmer and Paschen Jump Temperature Determinations in Low-Metallicity Emission-Line Galaxies
We have used the Balmer and Paschen jumps to determine the temperaturesof the H+ zones of a total sample of 47 H II regions. TheBalmer jump was used on MMT spectrophotometric data of 22low-metallicity H II regions in 18 blue compact dwarf (BCD) galaxies andof one H II region in the spiral galaxy M101. The Paschen jump was usedon spectra of 24 H II emission-line galaxies selected from the DataRelease 3 of the Sloan Digital Sky Survey (SDSS). To derive thetemperatures, we have used a Monte Carlo technique varying the electrontemperature in the H+ zone, the extinction of the ionized gasand that of the stellar population, the relative contribution of theionized gas to the total emission, and the star formation history to fitthe spectral energy distribution of the galaxies. For the MMT spectra,the fit was done in the wavelength range 3200-5200 Å, whichincludes the Balmer discontinuity, and for the SDSS spectra, in thewavelength range 3900-9200 Å, which includes the Paschendiscontinuity. We find for our sample of H II regions that thetemperatures of the O2+ zones determined from thenebular-to-auroral line intensity ratio of doubly ionized oxygen [O III]λλ(4959+5007)/λ4363 do not differ, in a statisticalsense, from the temperatures of the H+ zones determined fromfitting the Balmer and Paschen jumps and the spectral energydistributions (SEDs). We cannot rule out small temperature differencesof the order of 3%-5%.
| The Polar Regions of Cassiopeia A: The Aftermath of a Gamma-Ray Burst?
Are the polar regions of Cas A the aftermath of a gamma-ray burst?Probably not, but it is interesting nevertheless to investigate just howclose Cas A might have come to generating such an event. Focusing on thenortheast jet filaments, we analyze the polar regions of the recentlyacquired very deep 1 Ms Chandra X-ray observation. We infer that theso-called ``jet'' regions are indeed due to jets emanating from theexplosion center and not due to polar cavities in the circumstellarmedium at the time of explosion. We place limits on the equivalentisotropic explosion energy in the polar regions (around2.3×1052 ergs) and the opening angle of theX-ray-emitting ejecta (around 7°), which give a total energy in thenortheast jet of the order of 1050 ergs, an order ofmagnitude or more lower than inferred for ``typical'' GRBs. While theCas A progenitor and explosion exhibit many of the features associatedwith GRB hosts, e.g., extensive presupernova mass loss and rotation andjets associated with the explosion, we speculate that the recoil of thecompact central object, with a velocity of 330 km s-1, mayhave rendered the jet unstable. In such cases the jet rapidly becomesbaryon loaded, if not truncated altogether. Although unlikely to haveproduced a gamma-ray burst, the jets in Cas A suggest that such outflowsmay be common features of core-collapse SNe.
| A Close Look at the Population of Supersoft and Quasi-soft X-Ray Sources Observed in M31 with XMM-Newton
The deepest X-ray images of M31, obtained with XMM-Newton, are examinedto derive spectral and statistical properties of the population of thevery soft X-ray sources. When classifying supersoft X-ray sources (SSSs)with criteria based on the same hardness ratios defined for recentChandra observations, one-quarter of the selected SSSs turn out to besupernova remnants (SNRs). Another quarter of the SSSs are spatiallycoincident with recent classical novae, although they are less than 10%of the nova population observed in the last 25 yr. Only 3 out of the 15non-SNR SSSs show clear variability, with X-ray flux variation of morethan 1 order of magnitude within a few months. Two of these sourcesdisplay additional, smaller amplitude variability on timescales ofseveral minutes. Their broadband spectra and those of the novae areapproximately fitted with a blackbody or white dwarf atmospheric modelat near-Eddington luminosity for the distance of M31. Two SSSs appear toreach very large, perhaps super-Eddington luminosities for part of thetime and probably eject material in a wind until the luminositydecreases again after a few months. Most quasi-soft sources (QSSs) arerepeatedly detected. I discuss the possibility that most QSSs in M31 maybe SNRs or foreground neutron stars. Two X-ray sources with both a softand hard component are in the positions of a recurrent nova and apossible symbiotic nova, but they are probably black hole transients.
| An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X-Ray Sources
We have studied the stellar and interstellar environments of twoluminous X-ray sources and five ultraluminous X-ray sources (ULXs) inorder to gain insight into their nature. Archival Hubble Space Telescopeimages were used to identify the optical counterparts of the ULXs Ho IXX-1 and NGC 1313 X-2, and to make photometric measurements of the localstellar populations of these and the luminous source IC 10 X-1. Weobtained high-dispersion spectroscopic observations of the nebulaearound these seven sources to search for He II λ4686 emission andto estimate the expansion velocities and kinetic energies of thesenebulae. Our observations did not detect nebular He II emission from anysource, with the exception of LMC X-1 this is either because we missedthe He III regions or because the nebulae are too diffuse to produce HeII surface brightnesses that lie within our detection limit. We comparethe observed ionization and kinematics of the supershells around theULXs Ho IX X-1 and NGC 1313 X-2 with the energy feedback expected fromthe underlying stellar population to assess whether additional energycontributions from the ULXs are needed. In both cases, we findinsufficient UV fluxes or mechanical energies from the stellarpopulation; thus these ULXs may be partially responsible for theionization and energetics of their supershells. All seven sources thatwe studied are in young stellar environments, and six of them haveoptical counterparts with masses >~7 Msolar thus, thesesources are most likely high-mass X-ray binaries.
| The ACS Virgo Cluster Survey. XI. The Nature of Diffuse Star Clusters in Early-Type Galaxies
We use HST ACS imaging of 100 early-type galaxies in the ACS VirgoCluster Survey to investigate the nature of diffuse star clusters(DSCs). Compared to globular clusters (GCs), these star clusters havelow luminosities (MV>-8) and a broad distribution of sizes(320 magarcsec-2). The median colors of diffuse star cluster systems(1.1
| Neon and Oxygen Abundances in M33
We present new spectroscopic observations of 13 H II regions in theLocal Group spiral galaxy M33. The regions observed range from 1 to 7kpc in distance from the nucleus. Of the 13 H II regions observed, the[O III] λ4363 line was detected in six regions. Electrontemperatures were thus able to be determined directly from the spectrausing the [O III] λλ4959, 5007/λ4363 line ratio.Based on these temperature measurements, oxygen and neon abundances andtheir radial gradients were calculated. For neon, a gradient of-0.016+/-0.017 dex kpc-1 was computed, which agrees with theNe/H gradient derived previously from ISO spectra. A gradient of-0.012+/-0.011 dex kpc-1 was computed for O/H, much shallowerthan was derived in previous studies. The newly calculated O/H and Ne/Hgradients are in much better agreement with each other, as expected frompredictions of stellar nucleosynthesis. We examine the correlationbetween the WC/WN ratio and metallicity, and find that the new M33abundances do not impact the observed correlation significantly. We alsoidentify two new He II-emitting H II regions in M33, the first to bediscovered in a spiral galaxy other than the Milky Way. In both casesthe nebular He II emission is not associated with Wolf-Rayet stars.Therefore, caution is warranted in interpreting the relationship betweennebular He II emission and Wolf-Rayet stars when both are observed inthe integrated spectrum of an H II region.
| Observations of Chemically Enriched QSO Absorbers near z~2.3 Galaxies: Galaxy Formation Feedback Signatures in the Intergalactic Medium
We present a comparative study of galaxies and intergalactic gas towardthe z=2.73 quasar HS 1700+6416, to explore the effects of galaxyformation feedback on the IGM. Our observations and ionizationsimulations indicate that the volume within 100-200h-171 physical kpc of high-redshift galaxies ispopulated by very small (ΔL<~1 kpc), dense(ρ/ρ¯~1000), and metal-rich (Z>~1/10-1/3Zsolar) absorption-line regions. These systems often containshock-heated gas seen in O VI and may exhibit [Si/C] abundanceenhancements suggestive of preferential enrichment by Type IIsupernovae. We argue that the absorber geometries resemble thin sheetsor bubbles and that their unusual physical properties can be explainedusing a simple model of radiatively efficient shocks plowing throughmoderately overdense intergalactic filaments. The high metallicitiessuggest that these shocks are being expelled from, rather than fallinginto, star-forming galaxies. There is a drop-off in the intergalacticgas density at galaxy impact parameters of >~300 physical kpc (>~1comoving Mpc) that may represent boundaries of the gas structures wheregalaxies reside. The heavy-element enhancement near galaxies coverssmaller distances: at galactocentric radii between 100 and 200h-171 kpc the observed abundances blend into thegeneral metallicity field of the IGM. Our results suggest that eithersupernova-driven winds or dynamical stripping of interstellar gas altersthe IGM near massive galaxies, even at R>~100 kpc. However, only afew percent of the total mass in the Lyα forest is encompassed bythis active feedback at z~2.5. The effects could be more widespread ifthe more numerous metal-poor C IV systems at impact parameters >~200h-171 kpc are the tepid remnants of very powerfullate-time winds. However, based on present observations it is not clearthat this scenario is to be favored over one involving preenrichment bysmaller galaxies at z>~6.Includes observations made at the W. M. Keck Observatory, which isoperated as a scientific partnership between the California Institute ofTechnology and the University of California; it was made possible by thegenerous support of the W. M. Keck Foundation.
| 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.
| Hubble Space Telescope Observations of Star Clusters in M101
Hubble Space Telescope Advanced Camera for Surveys (ACS) images are usedto identify and study star cluster candidates in the nearby spiralgalaxy M101. About 3000 round, slightly resolved cluster candidates areidentified in 10 ACS pointings covering an area of 106arcmin2. The cluster candidates' color and size distributionsare consistent with those of star clusters in other nearby spirals. Themajority of the M101 candidates are blue and more likely to beassociated with the galaxy's spiral arms, implying that they are young.The galaxy-luminosity-normalized number of young massive clusters inM101 is similar to that found in other spirals, as is the clusterdensity at a fiducial absolute magnitude. We confirm a previous findingthat M101 has a large number of faint red star clusters: if these areold globular clusters, then this galaxy has a very large globularcluster population. More plausible is that the faint red clusters arereddened young clusters; their colors and luminosities are alsoconsistent with this explanation.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withprograms 8640 and 9490.
| Stellar Abundances and Molecular Hydrogen in High-Redshift Galaxies: The Far-Ultraviolet View
FUSE spectra of star-forming regions in nearby galaxies are compared tocomposite spectra of Lyman break galaxies (LBGs), binned by strength ofLyα emission and by mid-UV luminosity. Several far-UV spectralfeatures, including lines dominated by stellar wind and by photosphericcomponents, are very sensitive to stellar abundances. Their measurementin LBGs is compromised by the strong interstellar absorption features,allowing in some cases only upper limits to be determined. The derived Cand N abundances in the LBGs are no higher than half solar (scaled tooxygen abundance for comparison with emission-line analyses),independent of the strength of Lyα emission. P V absorptionindicates abundances as low as 0.1 solar, with an upper limit near 0.4solar in the reddest and weakest emission galaxies. Unresolvedinterstellar absorption components would further lower the derivedabundances. Trends of line strength and derived abundances are strongerwith mid-UV luminosity than with Lyα strength. H2absorption in the Lyman and Werner bands is very weak in the LBGs.Template H2 absorption spectra convolved to the appropriateresolution show that strict upper limitsN(H2)<1018 cm-2 apply in all cases,with more stringent values appropriate for the stronger emissioncomposites and for mixes of H2 level populations like thoseon Milky Way sight lines. Since the UV-bright regions are likely to bewidespread in these galaxies, these results rule out massive diffusereservoirs of primordial H2 and suggest that the dust-to-gasratio is already fairly large at z~3.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE). FUSE is operated for NASA by The JohnsHopkins University under NASA contract NAS5-32985.
| A Chandra ACIS Study of 30 Doradus. I. Superbubbles and Supernova Remnants
We present an X-ray tour of diffuse emission in the 30 Doradusstar-forming complex in the Large Magellanic Cloud using high spatialresolution X-ray images and spatially resolved spectra obtained with theAdvanced CCD Imaging Spectrometer on board the Chandra X-RayObservatory. The dominant X-ray feature of the 30 Doradus nebula is theintricate network of diffuse emission generated by interacting stellarwinds and supernovae working together to create vast superbubbles filledwith hot plasma. We construct maps of the region showing variations inplasma temperature (T=3-9 million degrees), absorption[NH=(1-6)×1021 cm-2], andabsorption-corrected X-ray surface brightness[SX=(3-126)×1031 ergs s-1pc-2]. Enhanced images reveal the pulsar wind nebula in thecomposite supernova remnant N157B, and the Chandra data show spectralevolution from nonthermal synchrotron emission in the N157B core to athermal plasma in its outer regions. In a companion paper we show thatR136, the central massive star cluster, is resolved at the arcsecondlevel into almost 100 X-ray sources. Through X-ray studies of 30 Doradusthe complete life cycle of such a massive stellar cluster can berevealed.
| Metallicity and Nuclear Star Formation in Nearby Galaxy Pairs: Evidence for Tidally Induced Gas Flows
We derive the first luminosity-metallicity (L-Z) relation for a largeobjectively selected sample of local galaxy pairs, and we compare theL-Z relation of the pairs with the relation for the Nearby Field GalaxySurvey. Galaxy pair members with small projected separations (s<20kpc h-1) have systematically lower metallicities (~0.2 dex onaverage) than either the field galaxies or more widely separated pairsat the same luminosity. There is a strong correlation betweenmetallicity and central burst strength in the galaxy pairs. All fivegalaxies in the pairs sample with strong central bursts have closecompanions and metallicities lower than the comparable field galaxies.Our results provide strong observational evidence for a merger scenarioin which galaxy interactions cause gas flows toward the central regions,carrying less enriched gas from the outskirts of the galaxy into thecentral regions. The less enriched gas dilutes the preexisting nucleargas to produce a lower metallicity than would be obtained prior to theinteraction. These gas flows trigger central bursts of star formation,causing strong central burst strengths and possibly aiding the formationof blue bulges. We show that the timescale and central gas dilutionrequired by this scenario are consistent with predictions fromhydrodynamic merger models.
| A Radio and X-Ray Study of Historical Supernovae in M83
We report the results of 15 years of radio observations of the sixhistorical supernovae (SNe) in M83 using the Very Large Array. We notethe near-linear decline in radio emission from SN 1957D, a Type II SN,which remains a nonthermal radio emitter. The measured flux densitiesfrom SNe 1923A and 1950B have flattened as they begin to fade belowdetectable limits; they are also Type II SNe. The luminosities for thesethree SNe are comparable with the radio luminosities of otherdecades-old SNe at similar epochs. SNe 1945B, 1968L, and 1983N were notdetected in the most recent observations, and these nondetections areconsistent with previous studies. We report the X-ray nondetections ofall six historical SNe using the Chandra X-Ray Observatory, consistentwith previous X-ray searches of other decades-old SNe and low inferredmass-loss rates of the progenitors [M˙~(10-8Msolar yr-1)(vw/10 km s-1)].
| Massive Stellar Content of Giant H II Regions in M33 and M101
Far-ultraviolet (900-1200 Å) spectral synthesis of nine giantextragalactic H II regions in M33 and M101 is performed to study theirmassive stellar content. Several parameters are quantified, predicted,and compared to the literature: age, stellar mass, initial mass function(IMF) slope, number of O-type and Wolf-Rayet stars, and Hα and5500 Å continuum fluxes. The results of this particular techniqueare consistent with other methods and observations. This work shows thata total stellar mass of a few 103 Msolar is neededto populate the IMF bins well enough at high masses to obtain accurateresults from the spectral synthesis technique in the far-ultraviolet. Aflat IMF slope seems to characterize better the stellar line profiles ofthese objects, which is likely the first sign of a small numberstatistics effect on the IMF. Finally, the H II region NGC 5461 isidentified as a good candidate for hosting a second generation of stars,not yet seen at far-ultraviolet wavelengths.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| A Comparison of Hα and Stellar Scale Lengths in Virgo and Field Spirals
The scale lengths of the old stars and ionized gas distributions arecompared for similar samples of Virgo Cluster members and field spiralgalaxies via Hα and broad R-band surface photometry. While theR-band and Hα scale lengths are, on average, comparable for thecombined sample, we find significant differences between the field andcluster samples. While the Hα scale lengths of the field galaxiesare a factor of 1.14+/-0.07 longer, on average, than their R-band scalelengths, the Hα scale lengths of Virgo Cluster members are, onaverage, 20% smaller than their R-band scale lengths. Furthermore, inVirgo, the scale length ratios are correlated with the size of thestar-forming disk: galaxies with smaller overall Hα extents alsoshow steeper radial falloff of star formation activity. At the sametime, we find no strong trends in scale length ratio as a function ofother galaxy properties, including galaxy luminosity, inclination,morphological type, central R-band light concentration, or bar type. Ourresults for Hα emission are similar to other results for dustemission, suggesting that Hα and dust have similar distributions.The environmental dependence of the Hα scale length placesadditional constraints on the evolutionary process(es) that cause gasdepletion and a suppression of the star formation rate in clusters ofgalaxies.
| Thermal and non-thermal components of the interstellar medium at sub-kiloparsec scales in galaxies
Aims. We present new radio continuum observations of ten BIMA SONGgalaxies, taken at 1.4 GHz with the Very Large Array. These observationsallow us to extend the study of the relationships between the radiocontinuum (RC) and CO emission to 22 CO luminous galaxies for whichsingle dish CO images have been added to interferometric data. NewSpitzer infrared (IR) images of six of these galaxies have beenreleased. The analysis of these high resolution images allowed us toprobe the RC-IR-CO correlations down to linear scales of a few hundredpc. Methods: .We compare the point-by-point RC, CO and mid-IRintensities across entire galaxy disks, producing radial profiles andspatially resolved images of the RC/CO and RC/mid-IR ratios.Results: .For the 22 galaxies analysed, the RC-CO correlation on scalesfrom ~10 kpc down to ~100 pc is nearly linear and has a scatter of afactor of two, i.e. comparable to that of the global correlations. Thereis no evidence for any severe degradation of the scatter below the kpcscale. This also applies to the six galaxies for which high-resolutionmid-IR data are available. In the case of NGC 5194,we find that the non-thermal radio spectral index is correlated with theRC/FIR ratio. Conclusions: .The scatter of the point-by-pointcorrelations does not increase significantly with spatial resolution. Wethus conclude that we have not yet probed the physical scales at whichthe correlations break down. However, we observe local deviations fromthe correlations in regions with a high star formation rate, such as thespiral arms, where we observe a flat radio spectrum and a low RC/FIRratio. In the intra-arm regions and in the peripheral regions of thedisk, the RC/FIR is generally higher and it is characterized by asteepening of the radio spectrum.
| Toward a clean sample of ultra-luminous X-ray sources
Context: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided.
| A catalogue of ultra-luminous X-ray source coincidences with FIRST radio sources
Aims.We search for ultra luminous X-ray source (ULXs) radio counterpartslocated in nearby galaxies in order to constrain their physicalnature. Methods: .Our work is based on a systematiccross-identification of the most recent and extensive available ULXcatalogues and archival radio data. Results: .A catalogue of 70positional coincidences is reported. Most of them are located within thegalaxy nucleus. Among them, we find 11 new cases of non-nuclear ULXsources with possibly associated radio emission.
| 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.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
Katalógusok és elnevezések:
|
NEWS@SKY (Science&Space News)
