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Ýlgili Makaleler
3D NIR spectroscopy at subarcsecond resolution
We present a scientific case approached through high quality 3D NIRspectroscopy performed with CIRPASS, attached to the Gemini Southtelescope. A binary mass concentration at the nucleus of the galaxy M 83was suggested by Thatte et al. [A&A 364 (2000) L47] and Mast et al.[BAAA 45 (2002) 98. Astroph#0505264] determined the possible position ofthe hidden secondary mass concentration with 2D H-alpha kinematics. Thepreliminary results of the NIR study presented here are based in almost1500 spectra centered in the wavelength 1.3 μm, with a spectralresolving power of 3200. They allow us to unveil, with 0.36″ (6.4pc) sampling and subarcsecond resolution, the velocity field in a regionof 13″ × 9″ around the optical nucleus. We confirmthat the optical nucleus is not located at the most important center ofsymmetry of the ionized gas velocity field. The largest black hole thatcould fit to the circular motion in this kinematic center should have amass not larger than 3 × 106(sin i)‑1Mȯ solar masses.
| Oxygen abundances in the most oxygen-rich spiral galaxies
Oxygen abundances in the spiral galaxies expected to be richest inoxygen are estimated. The new abundance determinations are based on therecently discovered ff relation between auroral and nebular oxygen-linefluxes in high-metallicity HII regions. We find that the maximumgas-phase oxygen abundance in the central regions of spiral galaxies is12+log(O/H) ~ 8.75. This value is significantly lower (by a factor of>~5) than the previously accepted value. The central oxygen abundancein the Milky Way is similar to that in other large spirals.
| Soft gamma repeaters outside the Local Group
We propose that the best sites to search for soft gamma repeaters (SGRs)outside the Local Group are galaxies with active massive-star formation.Different possibilities to observe SGR activity from these sites arediscussed. In particular, we have searched for giant flares from thenearby galaxies (~2-4 Mpc away) M82, M83, NGC 253 and 4945 in the Burstand Transient Source Experiment (BATSE) data. No candidate giant SGRflares were found. The absence of such detections implies that the rateof giant flares with energy release in the initial spike above 0.5× 1044 erg is less than 1/30 yr-1 in ourGalaxy. However, hyperflares similar to that of 2004 December 27 can beobserved from larger distances. Nevertheless, we do not see anysignificant excess of short GRBs from the Virgo galaxy cluster or fromthe galaxies Arp 299 and NGC 3256 (both with extremely high starformation rates). This implies that the Galactic rate of hyperflareswith energy release ~1046 erg is less than ~10-3yr-1. With this constraint the fraction of possibleextragalactic SGR hyperflares among BATSE's short GRBs should not exceeda few per cent. We present the list of short GRBs coincident with thegalaxies mentioned above, and discuss the possibility that some of themare SGR giant flares. We propose that the best target for theobservations of extragalactic SGR flares with Swift is the Virgocluster.
| Gas and stellar dynamics in NGC 1068: probing the galactic gravitational potential
We present SAURON integral field spectrography of the central 1.5 kpc ofthe nearby Seyfert 2 galaxy NGC 1068, encompassing the well-knownnear-infrared (NIR) inner bar observed in the K band. We havesuccessively disentangled the respective contributions of the ionizedgas and stars, thus deriving their two-dimensional distribution andkinematics. The [OIII] and Hβ emission lines exhibit a verydifferent spatial distribution and kinematics, the latter followinginner spiral arms with clumps associated with star formation. Stronginward streaming motions are observed in both the Hβ and [OIII]kinematics. The stellar kinematics also exhibit clear signatures of anon-axisymmetric tumbling potential, with a twist in both the velocityand Gauss-Hermite h3 fields. We re-examined the long-slitdata of Shapiro, Gerssen & van der Marel using a pPXF: a strongdecoupling of the Gauss-Hermite term h3 is revealed, and thecentral decrease of Gauss-Hermite term h4 hinted in theSAURON data is confirmed. These data also suggest that NGC 1068 is agood candidate for a so-called σ drop. We confirm the possiblepresence of two separate pattern speeds applying the Tremaine-Weinbergmethod to the Fabry-Perot Hα map. We also examine the stellarkinematics of bars formed in N-body+smoothed particle hydrodynamics(SPH) simulations built from axisymmetric initial conditionsapproximating the luminosity distribution of NGC 1068. The resultingvelocity, dispersion and higher order Gauss-Hermite moments successfullyreproduce a number of properties observed in the two-dimensionalkinematics of NGC 1068 and the long-slit data, showing that thekinematic signature of the NIR bar is imprinted in the stellarkinematics. The remaining differences between the models and theobserved properties are likely mostly due to the exclusion of starformation and the lack of the primary large-scale oval/bar in thesimulations. These models nevertheless suggest that the inner bar coulddrive a significant amount of gas down to a scale of ~ 300 pc. Thiswould be consistent with the interpretation of the σ drop in NGC1068 being the result of central gas accretion followed by an episode ofstar formation.
| Near-Infrared [Fe II] Emission in Starburst Galaxies. I. Measured Properties
We used the near-infrared [Fe II] emission line signature to detectsupernova remnants (SNRs) in the nearby starburst galaxies NGC 1569, NGC3738, and NGC 5253. The near-infrared narrowband imaging program has ledto the detection of 10 SNR candidates in NGC 1569, 7 in NGC 5253, andnone in NGC 3738. The luminosity of the SNRs candidates varies from 72to 780 Lsolar and from 69 to 331 Lsolar for NGC1569 and NGC 5253, respectively. Also, a spatially extended component tothe [Fe II] line emission is observed in NGC 1569 and NGC 5253. Thiscomponent dominates the integrated [Fe II] luminosity in both galaxies,the compact sources accounting for 14% and 7% of the total [Fe II]luminosity of NGC 1569 and NGC 5253, respectively.
| The Survey for Ionization in Neutral Gas Galaxies. I. Description and Initial Results
We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG),a census of star formation in H I-selected galaxies. The survey consistsof Hα and R-band imaging of a sample of 468 galaxies selected fromthe H I Parkes All Sky Survey (HIPASS). The sample spans three decadesin H I mass and is free of many of the biases that affect otherstar-forming galaxy samples. We present the criteria for sampleselection, list the entire sample, discuss our observational techniques,and describe the data reduction and calibration methods. This paperfocuses on 93 SINGG targets whose observations have been fully reducedand analyzed to date. The majority of these show a single emission linegalaxy (ELG). We see multiple ELGs in 13 fields, with up to four ELGs ina single field. All of the targets in this sample are detected inHα, indicating that dormant (non-star-forming) galaxies withMHI>~3×107 Msolar are veryrare. A database of the measured global properties of the ELGs ispresented. The ELG sample spans 4 orders of magnitude in luminosity(Hα and R band), and Hα surface brightness, nearly 3 ordersof magnitude in R surface brightness and nearly 2 orders of magnitude inHα equivalent width (EW). The surface brightness distribution ofour sample is broader than that of the Sloan Digital Sky Survey (SDSS)spectroscopic sample, the EW distribution is broader than prism-selectedsamples, and the morphologies found include all common types ofstar-forming galaxies (e.g., irregular, spiral, blue compact dwarf,starbursts, merging and colliding systems, and even residual starformation in S0 and Sa spirals). Thus, SINGG presents a superior censusof star formation in the local universe suitable for further studiesranging from the analysis of H II regions to determination of the localcosmic star formation rate density.
| A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds
We present a Far Ultraviolet Spectroscopic Explorer survey of highlyionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with(S/N)1030>8. We search the spectra for high-velocity (100km s-1<|vLSR|<400 km s-1) O VIabsorption and find a total of 63 absorbers, 16 with 21 cm emitting H Icounterparts and 47 ``highly ionized'' absorbers without 21 cm emission.The highly ionized HVC population is characterized by =38+/-10 km s-1 and =13.83+/-0.36, with negative-velocity clouds generally found atl<180deg and positive-velocity clouds found atl>180deg. Eleven of these highly ionized HVCs arepositive-velocity wings (broad O VI features extending asymmetrically tovelocities of up to 300 km s-1). We find that 81% (30 of 37)of highly ionized HVCs have clear accompanying C III absorption, and 76%(29 of 38) have accompanying H I absorption in the Lyman series. Wepresent the first (O VI selected) sample of C III and H I absorptionline HVCs and find =30+/-8 km s-1,logNa(C III) ranges from <12.5 to >14.4, =22+/-5 km s-1, and log Na(H I) ranges from<14.7 to >16.9. The lower average width of the high-velocity H Iabsorbers implies the H I lines arise in a separate, lower temperaturephase than the O VI. The ratio Na(C III)/Na(O VI)is generally constant with velocity in highly ionized HVCs, suggestingthat at least some C III resides in the same gas as the O VI.Collisional ionization equilibrium models with solar abundances canexplain the O VI/C III ratios for temperatures near1.7×105 K; nonequilibrium models with the O VI ``frozenin'' at lower temperatures are also possible. Photoionization models arenot viable since they underpredict O VI by several orders of magnitude.The presence of associated C III and H I strongly suggests the highlyionized HVCs are not formed in the hotter plasma that gives rise to OVII and O VIII X-ray absorption. We find that the shape of the O VIpositive-velocity wing profiles is well reproduced by a radiativelycooling, vertical outflow moving with ballistic dynamics, withT0=106 K, n0~2×10-3cm-3, and v0~250 km s-1. However, theoutflow has to be patchy and out of ionization equilibrium to explainthe sky distribution and the simultaneous presence of O VI, C III, and HI. We found that a spherical outflow can produce high-velocity O VIcomponents (as opposed to the wings), showing that the possible range ofoutflow model results is too broad to conclusively identify whether ornot an outflow has left its signature in the data. An alternative model,supported by the similar multiphase structure and similar O VIproperties of highly ionized and 21 cm HVCs, is one where the highlyionized HVCs represent the low N(H I) tail of the HVC population, withthe O VI formed at the interfaces around the embedded H I cores.Although we cannot rule out the possibility that some highly ionizedHVCs exist in the Local Group or beyond, we favor a Galactic origin.This is based on the recent evidence that both H I HVCs and themillion-degree gas detected in X-ray absorption are Galactic phenomena.Since the highly ionized HVCs appear to trace the interface betweenthese two Galactic phases, it follows that highly ionized HVCs areGalactic themselves. However, the nondetection of high-velocity O VI inhalo star spectra implies that any Galactic high-velocity O VI exists atz distances beyond a few kpc.
| 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.
| A FUSE Survey of High-Latitude Galactic Molecular Hydrogen
Measurements of molecular hydrogen (H2) column densities arepresented for the first six rotational levels (J=0-5) for 73extragalactic targets observed with the Far Ultraviolet SpectroscopicExplorer (FUSE). All of these have a final signal-to-noise ratio largerthan 10 and are located at Galactic latitude |b|>20deg.The individual observations were calibrated with the FUSE calibrationpipeline CalFUSE version 2.1 or higher and then carefully aligned invelocity. The final velocity shifts for all the FUSE segments arelisted. H2 column densities or limits are determined for thesix lowest rotational (J) levels for each H I component in the line ofsight, using a curve-of-growth approach at low column densities(<16.5) and Voigt-profile fitting at higher column densities.Detections include 65 measurements of low-velocity H2 in theGalactic disk and lower halo. Eight sight lines yield nondetections forGalactic H2. The measured column densities range fromlogN(H2)=14 to 20. Strong correlations are found betweenlogN(H2) and T01, the excitation temperature ofthe H2, as well as between logN(H2) and the levelpopulation ratios (log[N(J')/N(J)]). The average fraction ofnuclei in molecular hydrogen [f(H2)] in each sight line iscalculated; however, because there are many H I clouds in each sightline, the physics of the transition from H I to H2 cannot bestudied. Detections also include H2 in 16intermediate-velocity clouds in the Galactic halo (out of 35 IVCs).Molecular hydrogen is seen in one high-velocity cloud (the Leading Armof the Magellanic Stream), although 19 high-velocity clouds areintersected; this strongly suggests that dust is rare or absent in theseobjects. Finally, there are five detections of H2 in externalgalaxies.
| 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.
| Imaging Molecular Gas in the Luminous Merger NGC 3256: Detection of High-Velocity Gas and Twin Gas Peaks in the Double Nucleus
Molecular gas in the merging starburst galaxy NGC 3256 has been imagedwith the Submillimeter Array at a resolution of1''×2'' (170×340 pc at 35 Mpc). Thisis the first interferometric imaging of molecular gas in the mostluminous galaxy within z=0.01. There is a large disk of molecular gas(r>3 kpc) in the center of the merger with a strong gas concentrationtoward the double nucleus. The gas disk having a mass of~3×109 Msolar in the central 3 kpc rotatesaround a point between the two nuclei that are 850 pc apart on the sky.The molecular gas is warm and turbulent and shows spatial variation ofthe intensity ratio between CO isotopomers. High-velocity molecular gasis discovered at the galactic center. Its velocity in our line of sightis up to 420 km s-1 offset from the systemic velocity of thegalaxy; the terminal velocity is twice as large as that due to therotation of the main gas disk. The high-velocity gas is most likely dueto a molecular outflow from the gas disk, entrained by thestarburst-driven superwind in the galaxy. The molecular outflow isestimated to have a rate of ~10 Msolar yr-1 and toplay a significant role in the dispersal or depletion of molecular gasfrom the galactic center. A compact gas concentration and steep velocitygradient are also found around each of the twin nuclei. They aresuggestive of a small gas disk rotating around each nucleus. If theseare indeed minidisks, their dynamical masses are ~109Msolar within a radius of 170 pc.
| Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from Spitzer MIPS Observations
We use very deep Spitzer MIPS 24 μm observations to examine thebolometric luminosities (Lbol) and UV extinction propertiesof more than 200 spectroscopically identified, optically selected(UnGR) z~2 galaxies, supplemented with near-IR-selected(``BzK'' and ``DRG'') and submillimeter galaxies at similar redshifts,in the GOODS-N field. Focusing on redshifts 1.51012 Lsolar, with a mean~=2×1011 Lsolar. Using24 μm observations as an independent probe of dust extinction, wefind that, as in the local universe, the obscurationLIR/L1600 is strongly dependent on Lboland ranges in value from <1 to ~1000 within the sample considered.However, the obscuration is generally ~10 times smaller at a givenLbol at z~2 than at z~0. We show that the values ofLIR and obscuration inferred from the UV spectral slopeβ generally agree well with the values inferred fromL5-8.5μm for Lbol<1012Lsolar. Using the specific SFRs of galaxies as a proxy forcold gas fraction, we find a wide range in the evolutionary state ofgalaxies at z~2, from galaxies that have just begun to form stars tothose that have already accumulated most of their stellar mass and areabout to become, or already are, passively evolving.Based, in part, on data obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation. Alsobased in part on observations made with the Spitzer Space Telescope,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under a contract with NASA.
| Spatially Resolved Spitzer Spectroscopy of the Starburst Nucleus in NGC 5253
We present new Spitzer Space Telescope data on the nearby,low-metallicity starburst galaxy NGC 5253, from the Infrared ArrayCamera (IRAC) and the Infrared Spectrograph (IRS). The mid-IR luminosityprofile of NGC 5253 is clearly dominated by an unresolved cluster nearthe center, which outshines the rest of the galaxy at longerwavelengths. We find that the [Ne III]/[Ne II] ratio decreases from ~8.5at the center to ~2.5 at a distance of ~250 pc. The [S IV]/[S III]follows the [Ne III]/[Ne II] ratio remarkably well, being about 4-5times lower at all distances. Our spectra reveal for the first time aPAH emission feature at 11.3 μm, and its equivalent width increasessignificantly with distance from the center. The good anticorrelationbetween the PAH strength and the product between hardness and luminosityof the UV radiation field suggests photodestruction of the PAH moleculesin the central region. The high-excitation [O IV] 25.91 μm line wasdetected at 0.42×10-20 W cm-2. Our resultsdemonstrate the importance of spatially resolved mid-IR spectroscopy.
| 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
| Late-Time Radio Observations of 68 Type Ibc Supernovae: Strong Constraints on Off-Axis Gamma-Ray Bursts
We present late-time radio observations of 68 local Type Ibc supernovae,including six events with broad optical absorption lines(``hypernovae''). None of these objects exhibit radio emissionattributable to off-axis gamma-ray burst jets spreading into our line ofsight. Comparison with our afterglow models reveals the followingconclusions. (1) Less than ~10% of Type Ibc supernovae are associatedwith typical gamma-ray bursts initially directed away from our line ofsight; this places an empirical constraint on the GRB beaming factor of<~104, corresponding toan average jet opening angle, θj>~0.8d. (2) Thisholds in particular for the broad-lined supernovae (SNe 1997dq, 1997ef,1998ey, 2002ap, 2002bl, and 2003jd), which have been argued to host GRBjets. Our observations reveal no evidence for typical (or evensubenergetic) GRBs and rule out the scenario in which every broad-linedSN harbors a GRB at the 84% confidence level. Their large photosphericvelocities and asymmetric ejecta (inferred from spectropolarimetry andnebular spectroscopy) appear to be characteristic of the nonrelativisticSN explosion and do not necessarily imply the existence of associatedGRB jets.
| Impact of Dark Matter Subhalos on Extended H I Disks of Galaxies: Possible Formation of H I Fine Structures and Stars
Recent observations have discovered star formation activities in theextreme outer regions of disk galaxies. However, it remains unclear whatphysical mechanisms are responsible for triggering star formation insuch low-density gaseous environments of galaxies. In order tounderstand the origin of these outer star-forming regions, wenumerically investigate how the impact of dark matter subhalos orbitinga gas-rich disk galaxy embedded in a massive dark matter halo influencesthe dynamical evolution of the outer H I gas disk of the galaxy. We findthat if the masses of the subhalos (Msb) in a galaxy with anextended H I gas disk are as large as 10-3Mh,where Mh is the total mass of the galaxy's dark halo, localfine structures can be formed in the extended H I disk. We also findthat the gas densities of some apparently filamentary structures canexceed a threshold gas density for star formation and thus be likely tobe converted into new stars in the outer part of the H I disk in somemodels with larger Msb. These results thus imply that theimpact of dark matter subhalos (``dark impact'') can be important forbetter understanding the origin of recent star formation discovered inthe extreme outer regions of disk galaxies. We also suggest thatcharacteristic morphologies of local gaseous structures formed by thedark impact can indirectly prove the existence of dark matter subhalosin galaxies. We discuss the origin of giant H I holes observed in somegas-rich galaxies (e.g., NGC 6822) in the context of the dark impact.
| Chemical Enrichment of the Complex Hot ISM of the Antennae Galaxies. II. Physical Properties of the Hot Gas and Supernova Feedback
We investigate the physical properties of the interstellar medium (ISM)in the merging pair of galaxies known as the Antennae (NGC 4038/4039),using the deep co-added ~411 ks Chandra ACIS-S data set. The method ofanalysis and some of the main results from the spectral analysis, suchas metal abundances and their variations from ~0.2 to ~20-30 timessolar, are described in Paper I (Baldi et al.). In the present paper weinvestigate in detail the physics of the hot emitting gas, derivingmeasures for the hot gas mass (~107 Msolar),cooling times (107-108 yr), and pressure(3.5×10-11-2.8×10-10 dynecm-2). In at least one of the two nuclei (NGC 4038), the hotgas pressure is significantly higher than the CO pressure, implying thatshock waves may be driven into the CO clouds. Comparison of the metalabundances with the average stellar yields predicted by theoreticalmodels of SN explosions points to SNe of Type II as the maincontributors of metals to the hot ISM. There is no evidence of anycorrelation between radio-optical star formation indicators and themeasured metal abundances. Although due to uncertainties in the averagegas density we cannot exclude that mixing may have played an importantrole, the short time required to produce the observed metal masses(<~2 Myr) suggests that the correlations are unlikely to have beendestroyed by efficient mixing. More likely, a significant fraction ofType II SN ejecta may be in a cool phase, in grains, or escaping in hotwinds. In each case, any such fraction of the ejecta would remainundetectable with soft X-ray observations.
| 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.
| 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.
| A Study of Compact Radio Sources in Nearby Face-on Spiral Galaxies. I. Long-Term Evolution of M83
We present analyses of deep radio observations of M83 taken with theVery Large Array spanning 15 yr, including never-before-publishedobservations from 1990 and 1998. We report on the evolution of 55individual point sources, which include four of the six known historicalsupernovae in this galaxy. A total of 10 sources have X-ray counterpartsfrom a Chandra survey. Each of these sources show nonthermal spectralindices, and most appear to be X-ray supernova remnants. Comparing theradio source list to surveys in the optical and X-ray, we identify threeoptical/X-ray supernova remnants. Nearly half of the detected radiosources in these observations are coincident with known H II regionslying in the spiral arm structures of the galaxy. We also report onchanges in emission from the complex nuclear region, which has shownvariability at 20 cm wavelengths. We confirm that the peak radioemission from the nucleus is not coincident with the known opticalcenter. One lesser nuclear peak is consistent with the optical/IRnucleus. Previous dynamical studies of a ``dark'' nuclear mass indicatea possible match to other radio nuclear emission regions in M83.
| 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.
| Double Nucleus in M83
M83 is one of the nearest galaxies with enhanced nuclear star formation,and it presents one of the best opportunities to study the kinematicsand physical properties of a circumnuclear starburst. Ourthree-dimensional spectroscopy data in the R band confirm the presenceof a secondary nucleus or mass concentration (previously suggested byThatte and coworkers). We determine the position of this hidden nucleus,which would be more massive than the visible one and was not detected inthe optical Hubble Space Telescope images due, probably, to the strongdust extinction. Using a Keplerian approximation, we estimated for theoptical nucleus a mass of (5.0+/-0.8)×106Msolar/sini (r<1.5"), and for the hidden nucleus, located4''+/-1'' to the northwest (position angle of271deg+/-15deg) of the optical nucleus, a mass of(1.00+/-0.08)×107 Msolar/sini (r<1.5").The emission-line ratio map also unveils the presence of a secondcircumnuclear ring structure, previously discovered by IR imaging(Elmegreen and coworkers). The data allow us to resolve the behavior ofthe interstellar medium inside the circumnuclear ring and around thebinary mass concentration.
| 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)].
| The Star Formation Threshold in NGC 6822
We investigate the star formation threshold in NGC 6822, a nearby LocalGroup dwarf galaxy, on subkiloparsec scales using high-resolution,wide-field, deep H I, Hα, and optical data. In a study of the H Ivelocity profiles we identify cool and warm neutral components in theinterstellar medium of NGC 6822. We show that the velocity dispersion ofthe cool component (~4 km s-1) when used with a ToomreQ-criterion gives an optimal description of ongoing star formation inNGC 6822 superior to that using the more conventional dispersion valueof 6 km s-1. However, a simple constant surface densitycriterion for star formation gives an equally superior description. Wealso investigate the two-dimensional distribution of Q and the starformation threshold and find that these results also hold locally. Therange in gas density in NGC 6822 is much larger than the range incritical density, and we argue that the conditions for star formation inNGC 6822 are fully driven by this density criterion. Star formation islocal, and in NGC 6822 global rotational or shear parameters areapparently not important.
| 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.
| [CII] emission and star formation in the spiral arms of M 31
Context: .The [Cii] 158 μm line is the most important coolant of theinterstellar medium in galaxies but substantial variations are seen fromobject to object. The main source of the emission at a galactic scale isstill poorly understood and candidates range from photodissociationregions (PDRs) to the cold neutral or diffuse warm ionized medium.Previous studies of the [Cii] emission in galaxies have a resolution ofseveral kpc or more so the observed emission is an average of differentISM components. Aims: .The aim of this work is to study, for thefirst time, the [Cii] emission at the scale of a spiral arm. We want toinvestigate the origin of this line and its use as a tracer of starformation. Methods: . We present [Cii] and [Oi] observations of asegment of a spiral arm of M 31 using the Infrared Space Observatory.The [Cii] emission is compared with tracers of neutral gas (CO, Hi) andstar formation (Hα, Spitzer 24 μm). Results: . Thesimilarity of the [Cii] emission with the Hα and 24 μm imagesis striking when smoothed to the same resolution, whereas thecorrelation with the neutral gas is much weaker. The [Cii] cooling rateper H atom increases dramatically from 2.7 ×10-26 erg s-1 atom-1 in the border ofthe map to 1.4 × 10-25 erg s-1atom-1 in the regions of star formation. The[Cii]/FIR{42-122} ratio is almost constant at 2%, a factor 3 higher thantypically quoted. However, we do not believe that M 31 is unusual.Rather, the whole-galaxy fluxes used for the comparisons include thecentral regions where the [Cii]/FIR ratio is known to be lower and theresolved observations neither isolate a spiral arm nor include data asfar out in the galactic disk as the observations presented here. A fitto published PDR models yields a plausible average solution ofG0 100 and n 3000 for the PDR emission in theregions of star formation in the arm of M 31.
| [CII] 158 μm emission and metallicity in photon dominated regions
We study the effects of a metallicity variation on the thermal balanceand [CII] fine-structure line strengths in interstellar photon dominatedregions (PDRs). We find that a reduction in the dust-to-gas ratio andthe abundance of heavy elements in the gas phase changes the heatbalance of the gas in PDRs. The surface temperature of PDRs decreases asthe metallicity decreases except for high density (n>106cm-3) clouds exposed to weak (χ< 100) FUV fields wherevibrational H2-deexcitation heating dominates over photoelectric heatingof the gas. We incorporate the metallicity dependence in our KOSMA-τPDR model to study the metallicity dependence of [CII]/CO line ratios inlow metallicity galaxies. We find that the main trend in the variationof the observed CII/CO ratio with metallicity is well reproduced by asingle spherical clump, and does not necessarily require an ensemble ofclumps as in the semi-analytical model presented by Bolatto et al.(1999).
| Outer structure of the Galactic warp and flare: explaining the Canis Major over-density
Aims.In this paper we derive the structure of the Galactic stellar warpand flare. Methods: .We use 2MASS red clump and red giant stars,selected at mean and fixed heliocentric distances ofRȯ≃3, 7 and 17 kpc. Results: .Our resultscan be summarized as follows: (i) a clear stellar warp signature isderived for the 3 selected rings, proving that the warp starts alreadywithin the solar circle; (ii) the derived stellar warp is consistent(both in amplitude and phase-angle) with that for the Galacticinterstellar dust and neutral atomic hydrogen; (iii) the consistency andregularity of the stellar-gaseous warp is traced out to aboutR_GC20 kpc; (iv) the Sun seems not to fall on the line of nodes.The stellar warp phase-angle orientation (φ15°) is closeto the orientation angle of the Galactic bar and this, most importantly,produces an asymmetric warp for the inner Rȯ≃3 and7 kpc rings; (v) a Northern/Southern warp symmetry is observed only forthe ring at Rȯ≃17 kpc, at which the dependency onφ is weakened; (vi) treating a mixture of thin and thick diskstellar populations, we trace the variation with R_GC of the diskthickness (flaring) and derive an almost constant scale-height (~0.65kpc) within R_GC15 kpc. Further out, the disk flaring increasegradually reaching a mean scale-height of ~1.5 kpc at R_GC23 kpc;(vii) the derived outer disk warping and flaring provide further robustevidence that there is no disk radial truncation at R_GC14 kpc. Conclusions: .In the particular case of the Canis Major (CMa)over-density we confirm its coincidence with the Southern stellarmaximum warp occurring near l240° (forRȯ≃7 kpc) which brings down the Milky Waymid-plane by ~3° in this direction. The regularity and consistencyof the stellar, gaseous and dust warp argues strongly against a recentmerger scenario for Canis Major. We present evidence to conclude thatall observed parameters (e.g. number density, radial velocities, propermotion etc) of CMa are consistent with it being a normal Milky Wayouter-disk population, thereby leaving no justification for more complexinterpretations of its origin. The present analysis or outer diskstructure does not provide a conclusive test of the structure or originof the Monoceros Ring. Nevertheless, we show that a warped flared MilkyWay contributes significantly at the locations of the Monoceros Ring.Comparison of outer Milky Way H I and CO properties with those of othergalaxies favors the suggestion that complex structures close to planarin outer disks are common, and are a natural aspect of warped andflaring disks.
| Mid infrared properties of distant infrared luminous galaxies
We present evidence that the mid infrared (MIR, rest frame 5-30 μm)is a good tracer of the total infrared luminosity, L(IR)(=L[8{-}1000μm]), and star formation rate (SFR), of galaxies up to z 1.3. Weuse deep MIR images from the Infrared Space Observatory (ISO) and theSpitzer Space Telescope in the Northern field of the Great ObservatoriesOrigins Deep Survey (GOODS-N) together with VLA radio data to computethree independant estimates of L(IR). The L(IR, MIR) derived from theobserved 15 and/or 24 μm flux densities using a library of templateSEDs, and L(IR, radio), derived from the radio (1.4 and/or 8.5 GHz)using the radio-far infrared correlation, agree with a 1-σdispersion of 40%. We use the k-correction as a tool to probe differentparts of the MIR spectral energy distribution (SED) of galaxies as afunction of their redshift and find that on average distant galaxiespresent MIR SEDs very similar to local ones. However, in the redshiftrange z= 0.4-1.2, L(IR, 24 μm) is in better agreement with L(IR,radio) than L(IR, 15 μm) by 20%, suggesting that the warm dustcontinuum is a better tracer of the SFR than the broad emission featuresdue to polycyclic aromatic hydrocarbons (PAHs). We find marginalevidence for an evolution with redshift of the MIR SEDs: two thirds ofthe distant galaxies exhibit rest-frame MIR colors (L(12 μm)/L(7μm) and L(10 μm)/L(15 μm) luminosity ratios) below the medianvalue measured for local galaxies. Possible explanations are examinedbut these results are not sufficient to constrain the physics of theemitting regions. If confirmed through direct spectroscopy and if itgets amplified at higher redshifts, such an effect should be consideredwhen deriving cosmic star formation histories of dust-obscured galaxies.We compare three commonly used SED libraries which reproduce thecolor-luminosity correlations of local galaxies with our data anddiscuss possible refinements to the relative intensities of PAHs, warmdust continuum and silicate absorption.
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