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|SINFONI observations of starclusters in starburst galaxies|
We have used ESO’s new NIR IFS SINFONI during its ScienceVerification period to observe the central regions of local starburstgalaxies. Being Science Verification observations, the aim was 2-fold:to demonstrate SINFONI’s capabilities while obtaining informationon the nature of starclusters in starburst galaxies. The targets choseninclude a number of the brighter clusters in NGC1808 and NGC253. Here wepresent first results.
|Modelling galaxy spectra in presence of interstellar dust - I. The model of interstellar medium and the library of dusty single stellar populations|
The advent of modern infrared astronomy has brought into evidence therole played by the interstellar dust in galaxy formation and evolution.Therefore, to fully exploit modern data, realistic spectrophotometricmodels of galaxies must include this important component of theinterstellar medium (ISM).In this paper, the first of a series of two devoted to modelling thespectra of galaxies of different morphological type in the presence ofdust, we present our description of the dust both in the diffuse ISM andin the molecular clouds (MCs).Our galaxy model contains three interacting components: the diffuse ISM,made of gas and dust, the large complexes of MCs in which active starformation occurs and, finally, the populations of stars that are nolonger embedded in the dusty environment of their parental MCs.Our model for the dust takes into account three components, i.e.graphite, silicates and polycyclic aromatic hydrocarbons (PAHs). Weconsider and adapt to our aims two prescriptions for the sizedistribution of the dust grains and two models for the emission of thedusty ISM. We cross-check the emission and extinction models of the ISMby calculating the extinction curves and the emission for the typicalenvironments of the Milky Way (MW) and the Large and Small MagellanicClouds (LMC and SMC) and by comparing the results with the observationaldata. The final model we have adopted is a hybrid one which stems fromcombining the analysis of Guhathakurta & Draine for the emission ofgraphite and silicates and Puget, Leger & Boulanger for the PAHemission, and using the distribution law of Weingartner & Draine andthe ionization model for PAHs of Weingartner & Draine.We apply the model to calculate the spectral energy distribution (SED)of single stellar populations (SSPs) of different age and chemicalcomposition, which may be severely affected by dust at least in twotypes of stars: the young, massive stars while they are still embeddedin their parental MCs and the intermediate- and low-mass asymptoticgiant branch (AGB) stars when they form their own dust shell around.We use the `ray-tracing' method to solve the problem of radiativetransfer and to calculate extended libraries of SSP SEDs. Particularcare is taken to model the contribution from PAHs, introducing differentabundances of C in the population of very small carbonaceous grains(VSGs) and different ionization states in PAHs. The SEDs of young SSPsare then compared with observational data of star-forming regions offour local galaxies successfully reproducing their SEDs from theultraviolet (UV)-optical regions to the mid- and far-infrared region(MIR and FIR, respectively).
|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.
|Unveiling the nature of Ultraluminous Infrared Galaxies with 3-4μm spectroscopy*|
We present the results of L-band spectroscopical observations of localbright Ultraluminous Infrared Galaxies (ULIRGs), performed with theInfrared Spectrometer And Array Camera (ISAAC) at the Very LargeTelescope. The excellent sensitivity of the telescope and of theinstrument provided spectra of unprecedented quality for this class ofobjects, which allowed a detailed study of the active galactic nucleus(AGN)/starburst contribution to the energy output, and of thecomposition of the circumnuclear absorber. We discuss the L-bandspectral features of seven single sources, and the statisticalproperties of a complete sample of 15 sources obtained combining ourobservations with other published 3-4 μm spectra. Our main resultsare as follows. (i) When a spectral indicator suggesting the presenceof an AGN (low equivalent width of the 3.3-μm emission line, steepλ-fλ spectrum, presence of an absorptionfeature at 3.4 μm) is found, the AGN is always confirmed byindependent analysis at other wavelengths. Conversely, in all known AGNsat least one of the above indicators is present. (ii) Two newdiagnostic diagrams are proposed combining the above indicators, inwhich starbursts and AGNs are clearly and completely separated. (iii)The above diagnostic techniques are possible with spectra of relativelylow quality, which can be obtained for several tens of ULIRGs withcurrently available telescopes. This makes L-band spectroscopy thecurrent best tool to disentangle AGN and starburst contributions inULIRGs. (iv) The L-band properties of ULIRGs are heterogeneous.However, we show that all the spectral differences among ULIRGs can bereproduced starting from pure intrinsic AGN and starburst spectra andtwo varying parameters: the amount of dust extinction of the AGNcomponent, and the relative AGN/starburst contribution to the bolometricluminosity. (v) Using the above decomposition model, we show that AGNsin ULIRGs have a low dust-to-gas ratio and a dust extinction curvedifferent from Galactic. (vi) The estimate of the presence andcontribution of AGNs in a complete sample shows that AGNs are hosted byapproximately two-thirds of ULIRGs, but their energetic contribution isrelevant (>30 per cent of the bolometric luminosity) only in ~20 percent of the sample.
|Caroline Herschel's catalogue of nebulae|
|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.
|A 2 Millimeter Spectral Line Survey of the Starburst Galaxy NGC 253|
We present the first unbiased molecular line survey toward anextragalactic source, namely the nuclear region of the starburst galaxyNGC 253. The scan covers the frequency band from 129.1 to 175.2 GHz,i.e., most of the 2 mm atmospheric window. We identify 111 spectralfeatures as transitions from 25 different molecular species. Eight ofwhich (three tentatively) are detected for the first time in theextragalactic interstellar medium. Among these newly detected species,we detected the rare isotopomers 34SO andHC18O+. Tentative detections of two deuteratedspecies, DNC and N2D+, are reported for the firsttime from a target beyond the Magellanic Clouds. In addition, threehydrogen recombination lines are identified, while no organic moleculeslarger than methanol are detected. Column densities and rotationtemperatures are calculated for all the species, including an upperlimit to the ethanol abundance. A comparison of the chemical compositionof the nuclear environment of NGC 253 with those of selected nearbygalaxies demonstrates the chemical resemblance of IC 342 and NGC 4945 tothat of NGC 253. On the other hand, the chemistries characterizing NGC253 and M82 are clearly different. We also present a comparison of thechemical composition of NGC 253 with those observed in Galacticprototypical sources. The chemistry of NGC 253 shows a strikingsimilarity with the chemistry observed toward the Galactic centermolecular clouds, which are thought to be dominated by low-velocityshocks. This resemblance strongly suggests that the heating in thenuclear environment of NGC 253 is dominated by the same mechanism asthat in the central region of the Milky Way.
|Does the Milky Way Produce a Nuclear Galactic Wind?|
We detect high-velocity absorbing gas using Hubble Space Telescope andFar Ultraviolet Spectroscopic Explorer medium-resolution spectroscopyalong two high-latitude active galactic nucleus (AGN) sight lines (Mrk1383 and PKS 2005-489) above and below the Galactic center (GC). Theseabsorptions are most straightforwardly interpreted as a wind emanatingfrom the GC that does not escape from the Galaxy's gravitationalpotential. Spectra of four comparison B stars are used to identify andremove foreground velocity components from the absorption-line profilesof O VI, N V, C II, C III, C IV, Si II, Si III, and Si IV. Twohigh-velocity (HV) absorption components are detected along each AGNsight line, three redshifted and one blueshifted. Assuming that the fourHV features trace a large-scale Galactic wind emanating from the GC, theblueshifted absorber is falling toward the GC at a velocity of 250+/-20km s-1, which can be explained by ``Galactic fountain''material that originated in a bound Galactic wind. The other threeabsorbers represent outflowing material; the largest derived outflowvelocity is +250+/-20 km s-1, which is only 45% of thevelocity necessary for the absorber to escape from its current positionin the Galactic gravitational potential. All four HV absorbers are foundto reach the same maximum height above the Galactic plane(zmax=12+/-1 kpc), implying that they were all ejected fromthe GC with the same initial velocity. The derived metallicity limits of>~10%-20% solar are lower than expected for material recently ejectedfrom the GC unless these absorbers also contain significant amounts ofhotter gas in unseen ionization stages.
|M33 X-7: ChASeM33 Reveals the First Eclipsing Black Hole X-Ray Binary|
The first observations conducted as part of the Chandra ACIS survey ofM33 (ChASeM33) sampled the eclipsing X-ray binary M33 X-7 over a largepart of the 3.45 day orbital period and have resolved eclipse ingressand egress for the first time. The occurrence of the X-ray eclipseallows us to determine an improved ephemeris of mid-eclipse and binaryperiod as HJD (2,453,639.119+/-0.005)+/-N(3.453014+/-0.000020) andconstrain the eclipse half-angle to 26.5d+/-1.1d. There are indicationsfor a shortening of the orbital period. The X-ray spectrum is bestdescribed by a disk blackbody spectrum typical for black hole X-raybinaries in the Galaxy. We find a flat power density spectrum, and nosignificant regular pulsations were found in the frequency range of10-4 to 0.15 Hz. HST WFPC2 images resolve the opticalcounterpart, which can be identified as an O6 III star with the help ofextinction and color corrections derived from the X-ray absorption.Based on the optical light curve, the mass of the compact object in thesystem most likely exceeds 9 Msolar. This mass, the shape ofthe X-ray spectrum, and the short-term X-ray time variability identifyM33 X-7 as the first eclipsing black hole high-mass X-ray binary.
|Morphology of Spitzer 24 μm Detected Galaxies in the UDF: The Links between Star Formation and Galaxy Morphology|
We have studied the morphologies of infrared-luminous galaxies at0.3<=z<1.4 in the HST Ultra Deep Field (UDF) by calculatingconcentration and asymmetry indices and comparing the results withsimilar calculations for: (1) galaxies at similar redshift that are lessinfrared-active, and (2) local luminous infrared galaxies [LIRGs;LIR(8-1000 μm)>1011 Lsolar]. Wefind that the high-redshift samples are dominated by galaxies withconcentrations similar to local late-type disk galaxies; however, theyare significantly more asymmetric than most local galaxies but aresimilar in both regards to local LIRGs. On average, the high-redshiftinfrared-active galaxies are slightly more asymmetric than the lessactive ones, although they do include a significantly higher portion ofhighly asymmetric (merging?) systems and a lower portion of moreconcentrated, symmetric ones. The morphological similarity ofinfrared-active and typical infrared-inactive galaxies at high redshiftsuggests that they may be from the same parent population, but are indifferent stages of an episodic star formation process. The similaritybetween high redshift and local LIRGs suggests that a certain level ofasymmetry is generally associated with LIRG-level activity.
|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.
|Very Large Array H53α and H92α Line Observations of the Central Region of NGC 253|
We present new VLA observations toward NGC 253 of the recombination lineH53α (43 GHz) at an angular resolution of 1.5"×1.0". Thefree-free emission at 43 GHz is estimated to be ~140 mJy, implying astar formation rate of 2 Msolar yr-1 in thenuclear region of this starburst galaxy. A reanalysis is made forpreviously reported H92α observations carried out with angularresolution of 1.5"×1.0" and 0.36"×0.21". Based on the lineand continuum emission models used for the 1.5"×1.0" angularresolution observations, the RRLs H53α and H92α are tracersof the high-density (~105 cm-3) and low-density(~103 cm-3) thermally ionized gas components inNGC 253, respectively. The velocity fields observed in the H53αand H92α lines (1.5"×1.0") are consistent. The velocitygradient in the central ~18 pc of the NE component, as observed in boththe H53α and H92α lines, is in the opposite direction to thevelocity gradient determined from the CO observations. The enclosedvirial mass, as deduced from the H53α velocity gradient over theNE component, is ~5×106 Msolar in thecentral ~18 pc region. The H92α line observations at high angularresolution (0.36"×0.21") reveal a larger velocity gradient, alonga P.A.~-45deg on the NE component, of ~110 km s-1arcsec-1. The dynamical mass estimated using the high angularresolution H92α data (~7×106 Msolar)supports the existence of an accreted massive object in the nuclearregion of NGC 253.
|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.
|Chandra Observations of Gas Stripping in the Elliptical Galaxy NGC 4552 in the Virgo Cluster|
We use a 54.4 ks Chandra observation to study ram pressure stripping inNGC 4552 (M89), an elliptical galaxy in the Virgo Cluster. Chandraimages in the 0.5-2 keV band show a sharp leading edge in the surfacebrightness 3.1 kpc north of the galaxy center, a cool(kT=0.51+0.09-0.06 keV) tail with mean densityne~(5.4+/-1.7)×10-3 cm-3extending ~10 kpc to the south of the galaxy, and two 3-4 kpc horns ofemission extending southward away from the leading edge. These are allfeatures characteristic of supersonic ram pressure stripping of galaxygas, due to NGC 4552's motion through the surrounding Virgo ICM. Fittingthe surface brightness profile and spectra across the leading edge, wefind the galaxy gas inside the edge is cooler(kT=0.43+0.03-0.02 keV) and denser(ne~0.010 cm-3) than the surrounding Virgo ICM[kT=2.2+0.7-0.4 keV andne=(3.0+/-0.3)×10-4 cm-3]. Theresulting pressure ratio between the free-streaming ICM and cluster gasat the stagnation point is ~7.6+3.4-2.0 for galaxygas metallicities of 0.5+0.5-0.3Zsolar, which suggests that NGC 4552 is moving supersonicallythrough the cluster with a velocity v~1680+390-220km s-1 (Mach 2.2+0.5-0.3) at an angleξ~35deg+/-7deg toward us with respect to theplane of the sky.
|Extended Mid-Infrared Aromatic Feature Emission in M82|
We present new images (ground-based optical and mid-infrared [MIR] fromthe Spitzer Space Telescope) and spectra (from Spitzer) of thearchetypal starburst galaxy M82. The Spitzer data show that the MIRemission extends at least 6 kpc along the minor axis of the galaxy. Weuse the optical and infrared data to demonstrate that the extendedemission is dominated by emission from dust. The colors of the MIRemission and the spectra indicate that there is a strong component ofaromatic feature emission (the MIR features commonly attributed topolycyclic aromatic hydrocarbons). The dust continuum and aromaticfeature emission are both strong in the well-known superwind region ofthis galaxy; clearly, the carrier of the aromatic features can survivein close proximity to the wind, far from the plane of the galaxy. Wealso see significant emission by dust well outside the superwind region,providing the clearest picture to date of the dust distribution in thehalo of this galaxy.
|Probing Cosmic Star Formation Using Long Gamma-Ray Bursts: New Constraints from the Spitzer Space Telescope|
We report on IRAC 4.5 μm, IRAC 8.0 μm, and MIPS 24 μm deepobservations of 16 gamma-ray burst (GRB) host galaxies performed withthe Spitzer Space Telescope, and we investigate in the thermal infraredthe presence of evolved stellar populations and dust-enshroudedstar-forming activity associated with these objects. Our sample isderived from GRBs that were identified with subarcsecond localizationbetween 1997 and 2001, and only a very small fraction (~20%) of thetargeted sources are detected down to f4.5μm~3.5 μJyand f24μm~85 μJy (3 σ). This likely arguesagainst a population dominated by massive and strongly starbursting(i.e., SFR>~100 Msolar yr-1) galaxies as hasbeen recently suggested from submillimeter/radio and optical studies ofsimilarly selected GRB hosts. Furthermore, we find evidence that someGRBs do not occur in the most infrared luminous regions-hence the mostactively star-forming environments-of their host galaxies. Should theGRB hosts be representative of all star-forming galaxies at highredshift, models of infrared galaxy evolution indicate that >~50% ofGRB hosts should have f24μm>~100 μJy. Unless theidentification of GRBs prior to 2001 was prone to strong selectioneffects biasing our sample against dusty galaxies, we infer in thiscontext that the GRBs identified with the current techniques cannot bedirectly used as unbiased probes of the global and integrated starformation history of the universe.Based on observations made with the Spitzer Space Telescope, operated bythe Jet Propulsion Laboratory under NASA contract 1407.
|The Disk and Extraplanar Environment of NGC 247|
The stellar content of the spiral galaxy NGC 247 is investigated usingdeep visible and near-infrared images. The main-sequence turnoff (MSTO)in the inner 12 kpc of the disk corresponds to an age of ~6 Myr. A meanstar formation rate (SFR) of 0.1 Msolar yr-1during the past 16 Myr is computed from star counts. The color of thered supergiant plume does not change with radius, suggesting that themean metallicity of young stars does not vary by more than ~0.1 dex. Thenumber of bright main-sequence stars per local stellar mass densityclimbs toward larger radii out to a distance of 12 kpc; the scalelengths that characterize the radial distributions of young and oldstars in the disk thus differ. The density of bright main-sequence starswith respect to projected H I mass gradually drops with increasingradius. The population of very young stars disappears in the outer disk;the MSTO at galactocentric radii between 12 and 15 kpc corresponds to~16 Myr, while between 15 and 18 kpc the age is >=40 Myr. Red giantbranch (RGB) stars are resolved at a projected minor-axis galactocentricdistance of ~12 kpc. There is a broad spread in metallicity among theRGB stars, with a mean [M/H]~-1.2. The RGB tip occurs ati'=24.5+/-0.1, indicating that the distance modulus is27.9+/-0.1. Luminous AGB stars with an age ~3 Gyr are also seen in thisfield.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council of Canada (Canada), CONICYT (Chile), theAustralian Research Council (Australia), CNPq (Brazil), and CONICET(Argentina).This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
|The Polycyclic Aromatic Hydrocarbon Emission Deficit in Low-Metallicity Galaxies-A Spitzer View|
Archival observations of 18 starburst galaxies that span a wide range inmetallicity reveal for the first time a correlation between the ratio ofemission-line fluxes of [Fe II] at 26 μm and [Ne II] at 12.8 μmand the 7.7 μm PAH strength, with the [Fe II]/[Ne II] flux ratiodecreasing with increasing PAH strength. We also find a strongcorrelation between the [Fe II]/[Ne II] flux ratio and the host galaxymetallicity, with the flux ratio decreasing with increasing metallicity.Since [Fe II] emission has been linked primarily to supernova shocks, weattribute the high [Fe II]/[Ne II] ratios in low-metallicity galaxies toenhanced supernova activity. We consider this to be a dominant mechanismfor PAH destruction, rather than grain destruction in photoionizedregions surrounding young massive stars. We also consider whether theextreme youth of the low-metallicity galaxies is responsible for thelack of PAH emission.
|Large Silicon Abundance in Photodissociation Regions|
We have made one-dimensional raster scan observations of the ρ Ophand σ Sco star-forming regions with two spectrometers (SWS andLWS) on board the ISO. In the ρ Oph region, [Si II] 35 μm, [O I]63 μm, 146 μm, [C II] 158 μm, and the H2 purerotational transition lines S(0) to S(3) are detected, and thephotodissociation region (PDR) properties are derived as the radiationfield scaled by the solar neighborhood value G0~30-500, thegas density n~250-2500 cm-3, and the surface temperatureT~100-400 K. The ratio of [Si II] 35 μm to [O I] 146 μm indicatesthat silicon of 10%-20% of the solar abundance must be in the gaseousform in the PDR, suggesting that efficient dust destruction is ongoingeven in the PDR and that a fraction of the silicon atoms may becontained in volatile forms in dust grains. The [O I] 63 μm and [CII] 158 μm emissions are too weak relative to [O I] 146 μm to beaccounted for by standard PDR models. We propose a simple model, inwhich overlapping PDR clouds along the line of sight absorb the [O I] 63μm and [C II] 158 μm emissions, and show that the proposed modelreproduces the observed line intensities fairly well. In the σ Scoregion, we have detected three fine-structure lines, [O I] 63 μm, [NII] 122 μm, and [C II] 158 μm, and derived that 30%-80% of the [CII] emission comes from the ionized gas. The upper limit of the [Si II]35 μm is compatible with the solar abundance relative to nitrogen,and no useful constraint on the gaseous Si is obtained for the σSco region.Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries: France, Germany, theNetherlands, and the UK) and with the participation of ISAS and NASA.
|Detection of the Buried Active Galactic Nucleus in NGC 6240 with the Infrared Spectrograph on the Spitzer Space Telescope|
We present mid-infrared spectra of the nearby ultraluminous infraredgalaxy NGC 6240 taken with the Infrared Spectrograph (IRS) on theSpitzer Space Telescope. The spectrum of NGC 6240 is dominated by strongfine-structure lines, rotational H2 lines, and polycyclicaromatic hydrocarbon (PAH) emission features. The H2 linefluxes suggest molecular gas at a variety of temperatures. A simpletwo-temperature fit to the S(0) through S(7) lines implies a mass of~6.7×106 Msolar at T~957 K and~1.6×109 Msolar at T~164 K, or about 15% ofthe total molecular gas mass in this system. Notably, we have detectedthe [Ne V] 14.3 μm emission line, with a flux of5×10-14 ergs cm-2 s-1, providingthe first direct detection of the buried active galactic nucleus (AGN)in the mid-infrared. Modeling of the total spectral energy distribution(SED) from near- to far-infrared wavelengths requires the presence of ahot dust (T~700 K) component, which we also associate with the buriedAGN. The small [Ne V]/[Ne II] and [Ne V]/IR flux ratios, the relativefraction of hot dust emission, and the large 6.2 μm PAH equivalentwidth (EQW), are all consistent with an apparent AGN contribution ofonly 3%-5% to the bolometric luminosity. However, correcting themeasured [Ne V] flux by the extinction implied by the silicate opticaldepth and our SED fitting suggests an intrinsic fractional AGNcontribution to the bolometric luminosity of ~20%-24% in NGC 6240, whichlies within the range implied by fits to the hard X-ray spectrum.Based on observations obtained with the Spitzer Space Telescope, whichis operated by the Jet Propulsion Laboratory, California Institute ofTechnology, under NASA contract 1407.
|Massive Coronae of Galaxies|
There is reason to suspect that about half of the baryons in theuniverse are in pressure-supported plasma in the halos of normalgalaxies, drawn in by gravity along with about half of the dark matter.We present a model for this substantial baryonic component, the galacticcoronae, that fits the available observational constraints. Thisphenomenological approach requires departures from state-of-the-artnumerical models of galaxy formation, but the adjustments are not solarge as to seem unreasonable. In particular, massive coronae would haveto be hotter than the kinetic temperature of the halo dark matter so asto produce acceptable central electron densities. This highertemperature might result from the difference of the fluid dynamics ofthe baryons and the collisionless dynamics of the dark matter during theassembly of the protogalaxy, in an analogy to what seems to happen incluster formation. The cooling time of a massive corona would be longerthan the gravitational collapse time but, in the inner parts, shorterthan the Hubble time, making the corona thermally unstable to theformation of a cloudy structure that is settling and adding to the massin interstellar matter and stars. Since in this picture the mass in thecorona of a spiral galaxy is much larger than the mass in condensedbaryons, the corona would be a substantial reservoir that could supplymatter for star formation in isolated spirals continuing well past thepresent epoch.
|The First INTEGRAL AGN Catalog|
We present the first INTEGRAL AGN catalog, based on observationsperformed from launch of the mission in 2002 October until 2004 January.The catalog includes 42 AGNs, of which 10 are Seyfert 1, 17 are Seyfert2, and 9 are intermediate Seyfert 1.5. The fraction of blazars is rathersmall, with five detected objects, and only one galaxy cluster and nostarburst galaxies have been detected so far. A complete subset consistsof 32 AGNs with a significance limit of 7 σ in the INTEGRAL ISGRI20-40 keV data. Although the sample is not flux limited, thedistribution of sources shows a ratio of obscured to unobscured AGNs of1.5-2.0, consistent with luminosity-dependent unified models for AGNs.Only four Compton-thick AGNs are found in the sample. Based on theINTEGRAL data presented here, the Seyfert 2 spectra are slightly harder(Γ=1.95+/-0.01) than Seyfert 1.5 (Γ=2.10+/-0.02) and Seyfert1 (Γ=2.11+/-0.05).
|Extended X-Ray Emission from QSOs|
We report Chandra ACIS observations of the fields of four QSOs showingstrong extended optical emission-line regions. Two of these show noevidence for significant extended X-ray emission. The remaining twofields, those of 3C 249.1 and 4C 37.43, show discrete (but resolved)X-ray sources at distances ranging from ~10 to ~40 kpc from the nucleus.In addition, 4C 37.43 also may show a region of diffuse X-ray emissionextending out to ~65 kpc and centered on the QSO. It has been suggestedthat extended emission-line regions such as these may originate in thecooling of a hot intragroup medium. We do not detect a general extendedmedium in any of our fields, and the upper limits we can place on itspresence indicate cooling times of at least a few 109 yr. Thediscrete X-ray emission sources we detect cannot be explained as theX-ray jets frequently seen associated with radio-loud quasars, nor canthey be due to electron scattering of nuclear emission. The mostplausible explanation is that they result from high-speed shocks fromgalactic superwinds resulting either from a starburst in the QSO hostgalaxy or from the activation of the QSO itself. Evidence from thedensities and velocities found in studies of the extended opticalemission around QSOs also supports this interpretation.Based in part on data obtained with the Chandra X-Ray Observatory, whichis operated for the National Aeronautics and Space Administration by theSmithsonian Astrophysical Observatory. Also based in part onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the Data Archive at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
|Evidence for Chimney Breakout in the Galactic Supershell GSH 242-03+37|
We present new high-resolution neutral hydrogen (H I) images of theGalactic supershell GSH 242-03+37. These data were obtained with theParkes Radiotelescope as part of the Galactic All-Sky Survey (GASS). GSH242-03+37 is one of the largest and most energetic H I supershells inthe Galaxy, with a radius of 565+/-65 pc and an expansion energy of3×1053 ergs. Our images reveal a complicated shell withmultiple chimney structures on both sides of the Galactic plane. Thesechimneys appear capped by narrow filaments about 1.6 kpc above and belowthe Galactic midplane, confirming structures predicted in simulations ofexpanding supershells. The structure of GSH 242-03+37 is extremelysimilar to the only other Galactic supershell known to have blown out ofboth sides of the plane, GSH 277+00+36. We compare the GASS H I datawith X-ray and Hα images, finding no strong correlations.
|Molecular Superbubbles in the Starburst Galaxy NGC 253|
The central 2×1 kpc of the starburst galaxy NGC 253 have beenimaged using the Submillimeter Array at a 60 pc resolution in the J=2-1transitions of 12CO, 13CO, and C18O, aswell as in the 1.3 mm continuum. Molecular gas and dust are distributedmainly in a circumnuclear disk of ~500 pc radius, with warm (~40 K) andhigh area filling factor gas in its central part. Two gas shells orcavities have been discovered in the circumnuclear disk. They have ~100pc diameters and have large velocity widths of 80-100 km s-1,suggestive of expansion at ~50 km s-1. Modeled as anexpanding bubble, each shell has an age of ~0.5 Myr and needed kineticenergy of ~1×1046 J, as well as mean mechanicalluminosity of ~1×1033 W, for its formation. The largeenergy allows each to be called a superbubble. A ~106Msolar super star cluster can provide the luminosity andcould be a building block of the nuclear starburst in NGC 253.Alternatively, a hypernova can also be the main source of energy foreach superbubble, not only because it can provide the mechanical energyand luminosity but also because the estimated rate of superbubbleformation and that of hypernova explosions are comparable. Ourobservations indicate that the circumnuclear molecular disk harboringthe starburst is highly disturbed on 100 pc or smaller scales,presumably by individual young clusters and stellar explosions, inaddition to being globally disturbed in the form of the well-knownsuperwind.
|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.
|Simple Model of the Outflow from Starburst Galaxies: Application to Radio Observations|
We present an analytical model for the hydrodynamic outflow from thedisk of a starburst galaxy. The model is used to calculate thecosmic-ray propagation and the radio intensity distribution in thenuclear starburst region of NGC 253. We find that the cosmic-ray energyproduction rate of the central 600 pc of NGC 253 is about3×1041 ergs s-1, which is about 15% of thetotal mechanical supernova power. For this inner region, we estimate aterminal outflow velocity of 900 km s-1 and a mass-loss rateof M˙=2-4 Msolar yr-1.
|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.
|Magnetic fields in halos of spiral galaxies|
Observations of magnetic fields in halos of edge-on disk galaxies arediscussed in relation to the interstellar disk-halo interface in diskgalaxies. The distribution of extra-planar diffuse ionized gascorrelates on local and global scales with cosmic rays and magneticfields as inferred from observations of the non-thermal radio continuumradiation and its polarisation. From the polarisation a large-scale andwell-ordered magnetic field in these gaseous halos can be deduced. Forseveral objects a significant poloidal component of the halo field islikely. These observations indicate the presence of physical processeswhich generate and maintain magnetic fields on galactic scales. Theimportance of differential rotation of the gaseous halos for suchprocesses is briefly discussed.
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