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PAH Strength and the Interstellar Radiation Field around the Massive Young Cluster NGC 3603
We present spatial distribution of polycyclic aromatic hydrocarbons(PAHs) and ionized gas within the Galactic giant H II region NGC 3603.Using the IRS instrument on board the Spitzer Space Telescope, we studyin particular the PAH emission features at ~5.7, 6.2, 7.7, 8.6, and 11.3μm, and the [Ar II] 6.99 μm, [Ne II] 12.81 μm, [Ar III] 8.99μm, and [S IV] 10.51 μm forbidden emission lines. The observationsprobe both ionized regions and photodissociation regions. Silicateemission is detected close to the central cluster while silicateabsorption is seen further away. We find no significant variation of thePAH ionization fraction across the whole region. The emission of verysmall grains (VSGs) lies closer to the central stellar cluster thanemission of PAHs. The PAH/VSG ratio anticorrelates with the hardness ofthe interstellar radiation field suggesting a destruction mechanism ofthe molecules within the ionized gas, as shown for low-metallicitygalaxies by Madden et al.
| The ISO LWS high-resolution spectral survey towards Sagittarius B2
A full spectral survey was carried out towards the giant molecular cloudcomplex, Sagittarius B2 (SgrB2), using the Infrared Space Observatory(ISO) Long Wavelength Spectrometer (LWS) Fabry-Pérot mode. Thisprovided complete wavelength coverage in the range 47-196μm(6.38-1.53THz) with a spectral resolution of 30-40kms-1. Thisis a unique data set covering wavelengths inaccessible from the ground.It is an extremely important region of the spectrum as it contains boththe peak of the thermal emission from dust, and crucial spectral linesof key atomic (OI, CII, OIII, NII and NIII) and molecular species(NH3, NH2, NH, H2O, OH,H3O+, CH, CH2, C3, HF andH2D+). In total, 95 spectral lines have beenidentified and 11 features with absorption depth greater than 3σremain unassigned. Most of the molecular lines are seen in absorptionagainst the strong continuum, whereas the atomic and ionic lines appearin emission (except for absorption in the OI 63μm and CII 158μmlines). SgrB2 is located close to the Galactic Centre and so many of thefeatures also show a broad absorption profile due to material locatedalong the line of sight. A full description of the survey data set isgiven with an overview of each detected species and final line lists forboth assigned and unassigned features.Based on observations with Infrared Space Observatory (ISO), an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, the Netherlands and the United Kingdom) withthe participation of ISAS and NASA.E-mail: e.t.polehampton@rl.ac.uk
| The 15-20 μm Spitzer Spectra of Interstellar Emission Features in NGC 7023
We present 15-20 μm long-slit spectra of NGC 7023 from the InfraredSpectrograph (IRS) on Spitzer. We observe recently discoveredinterstellar emission features at 15.9, 16.4, 17.0, 17.4, 17.8, and 18.9μm, throughout the reflection nebula. The 16.4 μm emission featurepeaks near the photodissociation front northwest of the star, as thearomatic emission features (AEFs) at 3.3, 6.2, and 11.3 μm do. The16.4 μm emission feature is thus likely related to the AEFs andradiates by nonequilibrium emission. The new 18.9 μm emissionfeature, by contrast, decreases monotonically with stellar distance. Weconsider candidate species for the 18.9 μm feature, includingpolycyclic aromatic hydrocarbons, fullerenes, and diamonds. We describefuture laboratory and observational research needed to identify the 18.9μm feature carrier.
| Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
IR emission spectra are calculated for dust heated by starlight, formixtures of amorphous silicate and graphitic grains, including varyingamounts of PAH particles. The models are constrained to reproduce theaverage Milky Way extinction curve. The calculations include the effectsof single-photon heating. Updated IR absorption properties for the PAHsare presented that are consistent with observed emission spectra,including those newly obtained by Spitzer. We find a size distributionfor the PAHs giving emission band ratios consistent with the observedspectra of the Milky Way and other galaxies. Emission spectra arepresented for a wide range of starlight intensities. We calculate howthe efficiency of emission into different IR bands depends on PAH size;the strong 7.7 μm emission feature is produced mainly by PAHparticles containing <103 C atoms. We also calculate howthe emission spectrum depends on U, the starlight intensity relative tothe local interstellar radiation field. The submillimeter andfar-infrared emission is compared to the observed emission from thelocal interstellar medium. Using a simple distribution function, wecalculate the emission spectrum for dust heated by a distribution ofstarlight intensities, such as occurs within galaxies. The models areparameterized by the PAH mass fraction qPAH, the lower cutoffUmin of the starlight intensity distribution, and thefraction γ of the dust heated by starlight withU>Umin. We present graphical procedures using Spitzer IRACand MIPS photometry to estimate the parameters qPAH,Umin, and γ, the fraction fPDR of the dustluminosity coming from photodissociation regions with U>100, and thetotal dust mass Mdust.
| The Mid-Infrared Spectrum of Star-forming Galaxies: Global Properties of Polycyclic Aromatic Hydrocarbon Emission
We present a sample of low-resolution 5-38 μm Spitzer IRS spectra ofthe inner few square kiloparsecs of 59 nearby galaxies spanning a largerange of star formation properties. A robust method for decomposingmid-infrared galaxy spectra is described and used to explore thebehavior of PAH emission and the prevalence of silicate dust extinction.Evidence for silicate extinction is found in ~1/8 of the sample, atstrengths that indicate that most normal galaxies undergoAV<~3 mag averaged over their centers. The contribution ofPAH emission to the total infrared power is found to peak near 10% andextend up to ~20% and is suppressed at metallicitiesZ<~Zsolar/4, as well as in low-luminosity AGNenvironments. Strong interband PAH feature strength variations (2-5times) are observed, with the presence of a weak AGN and, to a lesserdegree, increasing metallicity shifting power to the longer wavelengthbands. A peculiar PAH emission spectrum with markedly diminished 5-8μm features arises among the sample solely in systems with relativelyhard radiation fields harboring low-luminosity AGNs. The AGNs may modifythe emitting grain distribution and provide the direct excitation sourceof the unusual PAH emission, which cautions against using absolute PAHstrength to estimate star formation rates in systems harboring activenuclei. Alternatively, the low star formation intensity often associatedwith weak AGNs may affect the spectrum. The effect of variations in themid-infrared spectrum on broadband infrared surveys is modeled andpoints to more than a factor of 2 uncertainty in results that assume afixed PAH emission spectrum, for redshifts z=0-2.5.
| Observations of Ultraluminous Infrared Galaxies with the Infrared Spectrograph on the Spitzer Space Telescope. II. The IRAS Bright Galaxy Sample
We present spectra taken with the Infrared Spectrograph on Spitzercovering the 5-38 μm region of the 10 ultraluminous infrared galaxies(ULIRGs) found in the IRAS Bright Galaxy Sample (BGS). There is a factorof 50 spread in the rest-frame 5.5-60 μm spectral slopes, and the 9.7μm silicate optical depths range from at leastτ9.7<=0.4 (AV~8) toτ9.7>=4.2 (AV>=78). There is evidencefor water ice and hydrocarbon absorption and C2H2and HCN absorption features in 4 and possibly 6 of the 10 BGS ULIRGs,indicating shielded molecular clouds and a warm, dense ISM. We havedetected [Ne V] emission in 3 of the 10 BGS ULIRGs, at flux levels of5-18×10-14 ergs cm-2 s-1 and [NeV] 14.3/[Ne II] 12.8 line flux ratios of 0.12-0.85. The remaining BGSULIRGs have limits on their [Ne V]/[Ne II]line flux ratios, which rangefrom <=0.15 to <=0.01. Among the BGS ULIRGs, the AGN fractionsimplied by either the [Ne V]/[Ne II] or [O IV]/[Ne II] line flux ratios(or their upper limits) are significantly lower than implied by the MIRslope or strength of the 6.2 μm PAH EQW feature. There is evidencefor hot (T>300 K) dust in five of the BGS ULIRGs, with the fractionof hot dust to total dust luminosity ranging from ~1% to 23%, beforecorrecting for extinction. When integrated over the IRAC-8, IRS bluepeak-up, and MIPS-24 filter bandpasses, the IRS spectra imply very bluecolors for some ULIRGs at z~1.3. The large range in diagnosticparameters among the nearest ULIRGs suggests that matching surveyresults to a small number of templates may lead to biased results aboutthe fraction of luminous dusty starbursts and AGNs at high z.Based on observations obtained with the Spitzer Space Telescope, whichis operated by the Jet Propulsion Laboratory, California Institute ofTechnology, under NASA contract 1407.
| A Cometary Bow Shock and Mid-Infrared Emission Variations Revealed in Spitzer Observations of HD 34078 and IC 405
We present new infrared observations of the emission/reflection nebulaIC 405 obtained with the Spitzer Space Telescope. Infrared images in thefour IRAC bands (3.6, 4.5, 5.8, and 8.0 μm) and two MIPS bands (24and 70 μm) are complemented by IRS spectroscopy (5-30 μm) of twonebular filaments. The IRAC (8.0 μm) and MIPS imaging shows evidenceof a bow shock associated with the runaway O9.5 V star, HD 34078,created by the interaction between the star and nebular material. Theratio of emission at 24 to 70 μm is higher in the immediate vicinityof HD 34078 than in the outer filaments, providing evidence for elevateddust temperatures (Td>~90 K) in the shock region. Thenebular imaging reveals that the morphology is band dependent, withvarying contributions from aromatic emission features, H2,and dust emission. Nebular spectroscopy is used to quantify thesecontributions, showing several aromatic emission bands between 6-14μm, the S(5), S(3), S(2), and S(1) pure rotational emission lines ofH2, and atomic fine-structure lines of Ne, S, and Ar. Thelow-dispersion spectra provide constraints on the ionization state ofthe large molecules responsible for the aromatic infrared features.H2 rotational temperatures of the two bright nebularfilaments are determined from the observed line strengths. An averageT(H2)~400 K is inferred, with evidence for additionalnonuniform excitation by UV photons in the intense radiation field of HD34078. The photoexcitation hypothesis is supported by direct measurementof the far-UV H2 fluorescence spectrum, obtained with FUSE.Based 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.
| A Study of the Isolated Dark Globule DC 314.8-5.1: Extinction, Distance, and a Hint of Star Formation
The southern dark globule DC 314.8-5.1 contains a reflection nebulailluminated by a normal B9 V star, HD 130079. This serendipitousassociation provides the opportunity to evaluate the distance to theglobule with greater accuracy than would otherwise be possible, subjectto accurate accounting for the effects of interstellar extinction andreddening. It is shown that the dust in the line of sight has opticalproperties characterized by elevated values of the ratio of extinctionto reddening (RV=AV/EB-V) and thewavelength of maximum polarization (λmax), signifyinggrowth that most probably results from grain-grain coagulation withinthe globule. Taking this into account yields a distance of 342+/-50 pc,significantly lower than previous estimates that assume the standarddiffuse interstellar medium average extinction law. Comparison of thisresult with the loci of other major sources of extinction along the lineof sight suggests that the globule is isolated; it appears to not bephysically associated with the adjacent Circinus molecular cloud andstar formation complex G317.0-4.0. Estimates are made of the mean numberdensity (nH>~9×103 cm-3) andmass (30-100 Msolar) of DC 314.8-5.1. A stellar census of theregion using the Two Micron All Sky Survey suggests that it is not asite of vigorous star formation, although two (out of 387) sources areidentified that appear to be good candidates for young stellar objectstatus on the basis of their near-infrared colors. A deep mid-infraredsurvey will be needed to determine whether DC 314.8-5.1 is, indeed,starless (with respect to indigenous birth) or a site of sedentarylow-mass star formation. The globule may also prove to be a valuablelaboratory for future study of the interaction of dense molecular gasand dust in a quiescent core with the relatively soft UV radiation fieldemanating from the embedded B9 V star.
| Optical Emission Band Morphologies of the Red Rectangle
We present narrowband images of the Red Rectangle (RR) Nebula thatreveal the distinct morphologies of this intriguing nebula in differentoptical emission bands. The morphology of the RR nebula in blueluminescence (BL) and extended red emission (ERE) are almost mutuallyexclusive. We also present the optical detection of the circumbinarydisk of the RR in the light of the BL. The total intensities from thetwo optical band emissions (BL and ERE) when summed over the nebula areof comparable magnitude. Their spatial distributions with respect to theembedded illumination sources lead us to suggest that they may beattributed to different ionization stages of the same family ofcarriers.
| Shock Processing of Interstellar Dust and Polycyclic Aromatic Hydrocarbons in the Supernova Remnant N132D
We observed the oxygen-rich Large Magellanic Cloud (LMC) supernovaremnant N132D (SNR 0525-69.6), using all instruments on board the Spitzer Space Telescope, IRS, IRAC, and MIPS (Infrared Spectrograph,Infrared Array Camera, and Multiband Imaging Photometer for Spitzer).The 5-40 μm IRS spectra toward the southeastern shell of the remnantshow a steeply rising continuum with [Ne III] and [O IV], as well as PAHemission. We also present the spectrum of a fast moving ejecta knot,previously detected at optical wavelengths, which is dominated by strong[Ne III] and [O IV] emission lines. We interpret the continuum asthermal emission from swept-up, shock-heated dust grains in theexpanding shell of N132D, which is clearly visible in the MIPS 24 μmimage. A 15-20 μm emission hump appears superposed on the dustcontinuum, and we attribute this to PAH CCC bending modes. We alsodetect the well-known 11.3 μm PAH CH bending feature, and find theintegrated strength of the 15-20 μm hump about a factor of 7 strongerthan the 11.3 μm band in the shell of the remnant. IRAC 3-9 μmimages do not show clear evidence of large-scale, shell-like emissionfrom the remnant, partly due to confusion with the ambient ISM material.However, we identified several knots of shocked interstellar gas basedon their distinct infrared colors. We discuss the bright infraredcontinuum and the polycyclic aromatic hydrocarbon features with respectto dust processing in young supernova remnants.
| The Spitzer View of Low-Metallicity Star Formation. I. Haro 3
We present Spitzer observations of the blue compact dwarf galaxy Haro 3,with an oxygen abundance of 12+log(O/H)=8.32. These data are part of alarger study of star formation and dust in low-metallicity environments.The IRS spectrum of Haro 3 shows strong narrow PAH emission, with highequivalent widths. Gaseous nebular fine-structure lines are also seen.Despite the absence of optical high-excitation lines, a fainthigh-ionization O IV line at 25.89 μm indicates the presence ofradiation as hard as 54.9 eV. A CLOUDY model suggests that the MIR linesoriginate in two regions: a low-extinction optically emitting region,and an optically invisible one with much higher extinction. Themorphology of Haro 3 changes with wavelength. IRAC 4.5 μm tracesextended stellar photospheric emission from the body of the galaxy andhot dust continuum coming mainly from star-forming regions, 8 μmprobes extended PAH emission coming mainly from the general ISM, MIPS 24and 70 μm images map compact small-grain warm dust emissionassociated with active star formation, and 160 μm reflects coolerextended dust associated with older stellar populations. We have derivedthe optical-to-radio SED of the brightest star-forming region A in Haro3. The best-fit DUSTY model of the SED gives a total luminosity of2.8×109 Lsolar and a mass of2.8×106 Msolar for the ionizing clusters. Weinfer an extinction AV<~3, intermediate between theoptical AV~0.5 and the radio AV~8, consistent withthe picture that longer wavelength observations probe more deeply intostar-forming regions.
| Formation and destruction of polycyclic aromatic hydrocarbon clusters in the interstellar medium
Aims.The competition between the formation and destruction of coroneneclusters under interstellar conditions is investigated theoretically. Methods: .The unimolecular nucleation of neutral clusters issimulated with an atomic model combining an explicit classical forcefield and a quantum tight-binding approach. Evaporation rates arecalculated in the framework of the phase space theory and are insertedin an infrared emission model and compared with the growth rateconstants. Results: .It is found that, in interstellar conditions,most collisions lead to cluster growth. The time evolution of smallclusters (containing up to 312 carbon atoms) was specificallyinvestigated under the physical conditions of the northernphotodissociation region of NGC 7023. These clusters are found to bethermally photoevaporated much faster than they are reformed, thusproviding an interpretation for the lowest limit of the interstellarcluster size distribution inferred from observations. The effects ofionizing the clusters and density heterogeneities are also considered.Based on our results, the possibility that PAH clusters could be formedin PDRs is critically discussed.
| Study of photon dominated regions in Cepheus B
Aims. The aim of the paper is to understand the emission from the photondominated regions in Cepheus B, estimate the column densities of neutralcarbon in bulk of the gas in Cepheus B and to derive constraints on thefactors which determine the abundance of neutral carbon relative toCO. Methods: .This paper presents 15 arcmin ×15 arcmin fullysampled maps of [C I] at 492 GHz and 12CO 4-3 observed withKOSMA at 1´ resolution. The new observations have been combinedwith the FCRAO 12CO 1-0, IRAM-30 m 13CO 2-1 andC18O 1-0 data, and far-infrared continuum data fromHIRES/IRAS. The KOSMA-τ spherical PDR model has been used tounderstand the [C I] and CO emission from the PDRs in Cepheus B and toexplain the observed variation of the relative abundances of bothC0 and CO. Results: .The emission from the PDRassociated with Cepheus B is primarily at V_LSR between -14 and -11 kms-1. We estimate about 23% of the observed [C II] emissionfrom the molecular hotspot is due to the ionized gas in the H II region.Over bulk of the material the C0 column density does notchange significantly, (2.0±1.4)×1017cm-2, although the CO column density changes by an order ofmagnitude. The observed C/CO abundance ratio varies between 0.06 and 4in Cepheus B. We find an anti-correlation of the observed C/CO abundanceratio with the observed hydrogen column density, which holds even whenall previous observations providing C/CO ratios are included. Here weshow that this observed variation of C/CO abundance with total columndensity can be explained only by clumpy PDRs consisting of an ensembleof clumps. At high H2 column densities high mass clumps, which exhibitlow C/CO abundance, dominate, while at low column densities, low massclumps with high C/CO abundance dominate.
| High spatial resolution mid-infrared spectroscopy of the starburst galaxies NGC 3256, II Zw 40 and Henize 2-10
Aims.In order to show the importance of high spatial resolutionobservations of extra-galactic sources when compared to observationsobtained with larger apertures such as ISO, we present N-band spectra(8-13 μm) of some locations in three starburst galaxies. Inparticular, we show the two galactic nuclei of the spiral galaxy NGC3256, the compact IR supernebula in the dwarf galaxy II Zw 40 and thetwo brightest IR knots in the central starburst of the WR galaxy He2-10. Methods: .The spectra were obtained with TIMMI2 on the ESO3.6 m telescope. An inventory of the spectra in terms of atomicfine-structure lines and molecular bands is presented. Results:.We show the value of these high spatial resolution data in constrainingproperties such as the extinction in the mid-IR, metallicity or stellarcontent (age, IMF, etc.). We have constrained the stellar content of theIR compact knot in II Zw 40 by using the mid-IR fine-structure lines andsetting restrictions on the nebular geometry. We have constructed a newmid-/far-IR diagnostic diagram based on the 11.2 μm PAH andcontinuum, accessible to ground-based observations. We find thatextra-galactic nuclei and star clusters observed at high spatialresolution (as is the case of the TIMMI2 observations) are closer inPAH/far-IR to compact H II regions, while galaxies observed by largeapertures such as ISO are closer to exposed PDRs such as Orion. This islikely due to the aperture difference. We find a dependence between thepresence of PAHs and the hardness of the radiation field as measured bythe [S IV]/[Ne II] ratio that may be explained by the PAH-dustcompetition for FUV photons or the relative contribution of thedifferent phases of the interstellar medium.
| The Opaque Nascent Starburst in NGC 1377: Spitzer SINGS Observations
We analyze extensive data on NGC 1377 from the Spitzer Infrared NearbyGalaxies Survey (SINGS). Within the category of nascent starbursts thatwe previously selected as having infrared-to-radio continuum ratios inlarge excess of the average and containing hot dust, NGC 1377 has thelargest infrared excess yet measured. Optical imaging reveals amorphological distortion suggestive of a recent accretion event.Infrared spectroscopy reveals a compact and opaque source dominated by ahot, self-absorbed continuum (τ~20 in the 10 μm silicate band).We provide physical evidence against nonstellar activity being theheating source. H II regions are detected through the single [Ne II]line, probing <1% of the ionizing radiation. Not only is the opticaldepth in different gas and dust phases very high, but >85% ofionizing photons are suppressed by dust. The only other detectedemission features are molecular hydrogen lines, arguably excited mainlyby shocks, besides photodissociation regions, and weak aromatic bands.The new observations support our interpretation in terms of an extremelyyoung starburst (<1 Myr). More generally, galaxies deficient in radiosynchrotron emission are likely observed within a few Myr of the onsetof a starburst and after a long quiescence, prior to the replenishmentof the interstellar medium with cosmic rays. The similar infrared-radioproperties of NGC 1377 and some infrared-luminous galaxies suggest thatNGC 1377 constitutes an archetype that will be useful to betterunderstand starburst evolution. Although rare locally because observedin a brief evolutionary stage, nascent starbursts may represent anonnegligible fraction of merger-induced starbursts that dominate deepinfrared counts. Since they differ dramatically from usual starbursttemplates, they have important consequences for the interpretation ofdeep surveys.
| Infrared Spectroscopy of Gas-Phase Complexes of Fe+ and Polycyclic Aromatic Hydrocarbon Molecules
The observed depletion of iron in the interstellar medium has beensuggested to result from its efficient complexation with, among others,polycyclic aromatic hydrocarbon molecules (PAHs). We present here thefirst experimental vibrational spectra of cationic iron-PAH complexeswith benzene, naphthalene, and fluorene. The spectra were obtained byinfrared multiple-photon dissociation (IRMPD) spectroscopy of thecomplex ions trapped in a Fourier transform ion cyclotron resonance(FT-ICR) mass spectrometer. A continuously tunable free electron laserprovided intense radiation in the astrophysically interesting wavelengthrange of 6-16 μm. Supporting calculations of the geometries, relativestabilities, and harmonic vibrational frequencies of the complexes werecarried out using density functional theory with theMPW1PW91/6-31+G(d,p) functional/basis set. In all cases, theexperimental spectra indicate that Fe+ is bound to thesix-membered carbon ring of the aromatic ligand in a high-spin (quartet)electronic ground state for the mono-complexes, Fe(benzene)+and Fe(naphthalene)+, and in a low-spin (doublet) electronicground state for the bis-complexes, Fe(benzene)+2,Fe(naphthalene)+2, andFe(fluorene)+2 complexes. Comparison of thecomplex spectra to the bare (neutral and cationic) PAH spectra revealstheir spectroscopic fingerprint, e.g., in the splitting of theout-of-plane CH-bending modes, which could aid in their interstellardetection.
| Spitzer/IRAC and ISOCAM/CVF insights on the origin of the near to mid-IR Galactic diffuse emission
Spitzer/IRAC images of extended emission provide a new insight on thenature of small dust particles in the Galactic diffuse interstellarmedium. We measure IRAC colors of extended emission in several fieldscovering a range of Galactic latitudes and longitudes outside of starforming regions. We determine the nature of the Galactic diffuseemission in Spitzer/IRAC images by combining them with spectroscopicdata. We show that PAH features make the emission in the IRAC 5.8 and8.0 μm channels, whereas the 3.3 μm feature represents only 20 to50% of the IRAC 3.6 μm channel. A NIR continuum is necessary toaccount for IRAC 4.5 μm emission and the remaining fraction of theIRAC 3.6 μm emission. This continuum cannot be accounted by scatteredlight. It represents 9% of the total power absorbed by PAHs and 120% ofthe interstellar UV photon flux. The 3.3 μm feature is observed tovary from field-to-field with respect to the IRAC 8.0 μm channel. Thecontinuum and 3.3 μm feature intensities are not correlated. Wepresent model calculations which relate our measurements of the PAHsspectral energy distribution to the particles size and ionization state.Cation and neutral PAHs emission properties are inferred empiricallyfrom NGC 7023 observations. PAHs caracteristics arebest constrained in a line of sight towards the inner Galaxy, dominatedby the Cold Neutral Medium phase: we find that the PAH cation fractionis about 50% and that their mean size is about 60 carbon atoms. Asignificant field-to-field dispersion in the PAH mean size, from 40 to80 carbon atoms, is necessary to account for the observed variations inthe 3.3 μm feature intensity relative to the IRAC 8.0 μm flux.However, one cannot be secure about the feature interpretation as longas the continuum origin remains unclear. The continuum and 3.3 μmfeature emission process could be the same even if they do not sharecarriers.
| Keys to Spitzer Observations of Luminous Star Forming Regions
Spitzer Space Telescope observations are opening a new perspective onmassive star formation in the Galaxy, nearby galaxies and luminousinfrared galaxies. We summarize results from an analysis of 30 DoradusSpitzer observations together with near infrared H[2] 1-0 S(1) and Brγ images and spectro-imaging observations obtained with ISO. Werelate the spatial and spectral distribution of the 30 Doradusmid-infrared emission to the radiative and dynamical impact of the R136super star cluster on the ISM structure and dust composition. Thisanalysis provides keys to the interpretation of mid-IR observations ofmassive star forming regions and insight on massive clusterspre-supernova feedback on their environment.
| Far-ultraviolet scattering by dust in Orion
We have modelled diffuse far-ultraviolet (FUV) spectrum observed by theFar Ultraviolet Spectroscopic Explorer (FUSE) near M42 as the scatteringof the starlight from the Trapezium stars by dust in front of thenebula. The dust grains are known to be anomalous in Orion withRV= 5.5 and these are the first measurements of the FUVoptical properties of the grains outside of `normal' Milky Way dust. Wefind an albedo varying from 0.3 +/- 0.1 at 912 Å to 0.5 +/- 0.2 at1020 Åwhich is consistent with theoretical predictions.
| Detection of CO+ in the Nucleus of M82
We present the detection of the reactive ion CO+ toward theprototypical starburst galaxy M82. This is the first secure detection ofthis short-lived ion in an external galaxy. Values of[CO+]/[HCO+]>0.04 are measured across the inner650 pc of the nuclear disk of M82. Such high values of[CO+]/[HCO+] have previously only been measuredtoward the atomic peak in the reflection nebula NGC 7023. This detectioncorroborates the scenario in which the molecular gas reservoir in theM82 disk is heavily affected by the UV radiation from recently formedstars. Comparing the column densities measured in M82 with those foundin prototypical Galactic photon-dominated regions (PDRs), we need ~20clouds along the line of sight to explain our observations. We havecompleted our model of the molecular gas chemistry in the M82 nucleus.Our PDR chemical model successfully explains the[CO+]/[HCO+] ratios measured in the M82 nucleusbut fails by an order of magnitude to explain the large measuredCO+ column densities [~(1-4)×1013cm-2]. We explore possible routes to reconcile the chemicalmodel and the observations.
| IRS Spectra of Two Ultraluminous Infrared Galaxies at z = 1.3
We present low-resolution (6420) galaxies in the NDWFS. Its mid-infraredspectrum lacks emission features, but the broad 9.7 μm silicateabsorption band places this source at z~1.3. Optical spectroscopyconfirms a redshift of z=1.293+/-0.001. Given this redshift, SST24J142827.19+354127.71 has among the largest rest-frame 5 μmluminosities known. The similarity of its SED to those of knownAGN-dominated ULIRGs and its lack of either PAH features or largeamounts of cool dust indicate that the mid-infrared emission isdominated by an AGN rather than a starburst.
| First Detection of PAHs and Warm Molecular Hydrogen in Tidal Dwarf Galaxies
We observed two faint tidal dwarf galaxies (TDGs), NGC 5291 N and NGC5291 S, with the Infrared Spectrograph on the Spitzer Space Telescope.We detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2,7.7, 8.6, 11.3, 12.6, and 16.5 μm, which match models of groups of~100 carbon atoms with an equal mixture of neutral and ionized PAHs. TheTDGs have a dominant warm (~140 K) dust component in marked contrast tothe cooler (40-60 K) dust found in starburst galaxies. For the firsttime we detect the low-J rotational lines from molecular hydrogen.Adopting LTE, there is ~105 Msolar of ~400 K gas,which is <0.1% of the cold gas mass inferred from 12CO(1-0) measurements. The combination of one-third solar metallicity witha recent (<5 million year) episode of star formation is reflected inthe S and Ne ratios. The excitation is higher than typical values forstarburst galaxies and similar to that found in BCDs. Using the InfraredArray Camera, we identify an additional 13 PAH-rich candidate TDGs.These sources occupy a distinct region of IRAC color space with[3.6]-[4.5]<0.4 and [4.5]-[8.0]>3.2. Their disturbed morphologiessuggest past merger events between companions; for example, NGC 5291 Shas a projected 11 kpc tail. NGC 5291 N and S have stellar masses of(1.5 and 3.0)×108 Msolar, which iscomparable to BCDs, although still roughly 10% of the LMC's stellarmass. The candidate TDGs are an order of magnitude less massive. Thissystem appears to be a remarkable TDG nursery.
| 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.
| The Pre-Main-Sequence Population of L988
L988 is a large (~0.5d×0.7d) dark cloud complex at about 600 pcthat contains several bright pre-main-sequence objects (such as V1331Cyg and LkHα 321), but this paper deals in detail only with asmall region on its eastern edge, near the HAeBe star LkHα 324.That star and its distant companion LkHα 324SE lie at the apex ofa V-shaped area apparently excavated from the edge of L988, and are thebrightest members of a small cluster containing about 50Hα-emission stars. A median age of about 0.6 Myr (with largedispersion) is inferred from its color-magnitude diagram, constructedfrom VRI photometry to V=22. Keck HIRES spectra show that LkHα324SE is probably also an HAeBe. Its image is nonstellar, and within 3"to the northwest are three condensations having complex [S II] and [O I]profiles and radial velocities up to -200 km s-1. Theyprobably originate in an outflow from LkHα 324SE. A bright Ap starwith strong Si II lines is embedded in the heavy obscuration 8' to thewest. It illuminates a small reflection nebulosity, has several faintHα emitters nearby, and shares the radial velocity of L988, soclearly it was formed in that cloud. It demonstrates again that suchchemical peculiarities can be established very early in young stars ofmoderate mass.
| A Spitzer Space Telescope Infrared Survey of Supernova Remnants in the Inner Galaxy
Using Infrared Array Camera (IRAC) images at 3.6, 4.5, 5.8, and 8 μmfrom the GLIMPSE Legacy science program on the Spitzer Space Telescope,we searched for infrared counterparts to the 95 known supernova remnantsthat are located within Galactic longitudes65deg>|l|>10deg and latitudes|b|<1deg. Eighteen infrared counterparts were detected.Many other supernova remnants could have significant infrared emissionbut are in portions of the Milky Way too confused to allow theseparation of bright H II regions and pervasive mid-infrared emissionfrom atomic and molecular clouds along the line of sight. Infraredemission from supernova remnants originates from synchrotron emission,shock-heated dust, atomic fine-structure lines, and molecular lines. Thedetected remnants are G11.2-0.3, Kes 69, G22.7-0.2, 3C 391, W44, 3C 396,3C 397, W49B, G54.4-0.3, Kes 17, Kes 20A, RCW 103, G344.7-0.1,G346.6-0.2, CTB 37A, G348.5-0.0, and G349.7+0.2. The infrared colorssuggest emission from molecular lines (nine remnants), fine-structurelines (three remnants), polycyclic aromatic hydrocarbons (fourremnants), or a combination; some remnants feature multiple colors indifferent regions. None of the remnants are dominated by synchrotronradiation at mid-infrared wavelengths. The IRAC-detected sampleemphasizes remnants interacting with relatively dense gas, for whichmost of the shock cooling occurs through molecular or ionic lines in themid-infrared.
| Cloudshine: New Light on Dark Clouds
We present new deep near-infrared images of dark clouds in the Perseusmolecular complex. These images show beautiful extended emission that wemodel as scattered ambient starlight and name ``cloudshine.'' Thebrightness and color variation of cloudshine complicates the productionof extinction maps, the best tracer of column density in clouds.However, since the profile of reflected light is essentially a functionof mass distribution, cloudshine provides a new way to study thestructure of dark clouds. Previous work has used optical scattered lightto study the density profile of tenuous clouds; extending this techniqueinto the infrared provides a high-resolution view into the interiors ofvery dense clouds, bypassing the complexities of using thermal dustemission, which is biased by grain temperature, or molecular tracers,which have complicated depletion patterns. As new wide-field infraredcameras are used to study star-forming regions at greater depth,cloudshine will be widely observed and should be seen as a newhigh-resolution tool, rather than an inconvenience.
| The Excitation of Extended Red Emission: New Constraints on Its Carrier from Hubble Space Telescope Observations of NGC 7023
The carrier of the dust-associated photoluminescence process causing EREin many dusty interstellar environments remains unidentified. Severalcompeting models are more or less able to match the observed broad,unstructured ERE band. We now constrain the character of the ERE carrierfurther by determining the wavelengths of the radiation that initiatesthe ERE. Using the imaging capabilities of the HST, we have resolved thewidth of narrow ERE filaments appearing on the surfaces of externallyilluminated molecular clouds in the bright reflection nebula NGC 7023and compared them with the depth of penetration of radiation of knownwavelengths into the same cloud surfaces. We identify photons withwavelengths shortward of 118 nm as the source of ERE initiation, not tobe confused with ERE excitation, however. There are strong indicationsfrom the well-studied ERE in the Red Rectangle Nebula and in thehigh-|b| Galactic cirrus that the photon flux with wavelengths shortwardof 118 nm is too small to actually excite the observed ERE, even with100% quantum efficiency. We conclude, therefore, that ERE excitationresults from a two-step process. The first, involving far-UV photonswith E>10.5 eV, leads to the creation of the ERE carrier, most likelythrough photoionization or photodissociation of an existing precursor.The second, involving more abundant near-UV/optical photons, consists ofthe optical pumping of the previously created carrier, followed bysubsequent deexcitation via photoluminescence. The latter process canoccur many times for a single particle, depending upon the lifetime ofthe ERE carrier in its active state. While none of the previouslyproposed ERE models can match these new constraints, we note that underinterstellar conditions most PAH molecules are ionized to the dicationstage by photons with E>10.5 eV and that the electronic energy levelstructure of PAH dications is consistent with fluorescence in thewavelength band of the ERE. Therefore, PAH dications deserve furtherstudy as potential carriers of the ERE.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555. These observations are associated with program9471.
| Insights into the Carbon Chemistry of Monoceros R2
Aiming to learn about the chemistry of the dense PDR around theultracompact (UC) H II region in Mon R2, we have observed a series ofmillimeter-wavelength transitions of C3H2 andC2H. In addition, we have traced the distribution of othermolecules, such as H13CO+, SiO, HCO, andHC3N. These data, together with the reactive ions recentlydetected, have been considered to determine the physical conditions andto model the PDR chemistry. We then identified two kinds of molecules.The first group, formed by the reactive ions (CO+ andHOC+) and small hydrocarbons (C2H andC3H2), traces the surface layers of the PDR and ispresumably exposed to a high UV field (hence we call it high UV or HUV).HUV species are expected to dominate for visual absorptions 2mag5 mag). Whilethe abundances of the HUV molecules can be explained by gas-phasemodels, this is not the case for the studied LUV ones. Although someefficient gas-phase reactions might be lacking, grain chemistry soundslike a probable mechanism able to explain the observed enhancement ofHCO and SiO. Within this scenario, the interaction of UV photons withgrains produces an important effect on the molecular gas chemistry andconstitutes the first evidence of an ionization front created by the UCH II region carving its host molecular cloud. The physical conditionsand kinematics of the gas layer that surrounds the UC H II region werederived from the HUV molecules. Molecular hydrogen densities>4×106 cm-3 are required to reproduce theobservations. Such high densities suggest that the H II region could bepressure-confined by the surrounding high-density molecular gas.
| Blue Luminescence and the Presence of Small Polycyclic Aromatic Hydrocarbons in the Interstellar Medium
Blue luminescence (BL) was first discovered in a proto-planetary nebula,the Red Rectangle (RR), surrounding the post-AGB star HD 44179. BL hasbeen attributed to fluorescence by small, 3-4 ringed neutral polycyclicaromatic hydrocarbon (PAH) molecules and was thought to be unique to theRR environment, where such small molecules are actively being producedand shielded from the harsh interstellar radiation by a densecircumstellar disk. In this paper we present the BL spectrum detected inseveral ordinary reflection nebulae illuminated by stars havingtemperatures between 10,000 and 23,000 K. All these nebulae are known toalso exhibit the infrared emission features called aromatic emissionfeatures (AEFs) attributed to large PAHs. We present the spatialdistribution of the BL in these nebulae. In the case of Ced 112, the BLis spatially correlated with mid-IR emission structures attributed toAEFs. These observations provide evidence for grain processing andpossibly for in situ formation of small grains and large molecules fromlarger aggregates. Most importantly, the detection of BL in theseordinary reflection nebulae suggests that the BL carrier is anubiquitous component of the interstellar medium and is not restricted tothe particular environment of the RR.
| Dust Properties in the Far-Ultraviolet in Ophiuchus
We have derived the albedo (a) and phase function asymmetry factor (g)of interstellar dust grains at 1100 Å using archival Voyagerobservations of diffuse radiation in Ophiuchus. We have found that thegrains are highly forward-scattering, with g=0.55+/-0.25 anda=0.40+/-0.10. Even though most of the gas in this direction is in theOphiuchus molecular cloud, the diffuse FUV radiation is almost entirelydue to scattering in a relatively thin foreground cloud. This suggeststhat one cannot assume that the UV background is directly correlatedwith the total amount of gas in any direction.
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