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To understand and interpret the observed spectral energy distributions (SEDs) of starbursts, theoretical or semi-empirical SED models are necessary. Yet, while they are well founded in theory, independent verification and calibration of these models, including the exploration of possible degeneracies between their parameters, are rarely made. As a consequence, a robust fitting method that leads to unique and reproducible results has been lacking. Here we introduce a novel approach based on Bayesian analysis to fit the Spitzer-Infrared Spectrometer spectra of starbursts using the SED models proposed by Groves et al.. We demonstrate its capabilities and verify the agreement between the derived best-fit parameters and actual physical conditions by modeling the nearby, well-studied, giant H II region 30 Doradus in the LMC. The derived physical parameters, such as cluster mass, cluster age, interstellar medium pressure, and covering fraction of photodissociation regions, are representative of the 30 Doradus region. The inclusion of the emission lines in the modeling is crucial to break degeneracies. We investigate the limitations and uncertainties by modeling subregions, which are dominated by single components, within 30 Doradus. A remarkable result for 30 Doradus in particular is a considerable contribution to its mid-infrared spectrum from hot (300K) dust. The demonstrated success of our approach will allow us to derive the physical conditions in more distant, spatially unresolved starbursts.

Abridged: Using free-free emission measured in the Ka-band (26-40GHz) for 10 star-forming regions in the nearby galaxy NGC6946, including its starbursting nucleus, we compare a number of SFR diagnostics that are typically considered to be unaffected by interstellar extinction: i.e., non-thermal radio (i.e., 1.4GHz), total infrared (IR; 8-1000um), and warm dust (i.e., 24um) emission, along with the hybrid (obscured + unobscured) indicators of H\alpha+24um and UV+IR. The 33GHz free-free emission is assumed to provide the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24um, H\alpha+24um and UV+IR SFR calibrations are in good agreement with the 33GHz free-free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimate the nuclear SFR by a factor of ~2. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the free-free SFRs for the nucleus, and underestimate the SFRs from the extranuclear star-forming regions by a factor of ~2. This result likely arises from the CR electrons decaying within the starburst region with negligible escape compared to the young extranuclear star-forming regions. Finally, we find that the SFRs estimated using the total 33GHz emission agree well with the free-free SFRs due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission., Comment: 18 pages, 7 Figures, Accepted for publication in ApJ

We present the detection of extended (~30 kpc^2) dust emission in the tidal \hi\ arm near NGC 3077 (member of the M\,81 triplet) using SPIRE on board Herschel. Dust emission in the tidal arm is typically detected where the \hi\ column densities are >10^21 cm^-2. The SPIRE band ratios show that the dust in the tidal arm is significantly colder (~13 K) than in NGC 3077 itself (~31 K), consistent with the lower radiation field in the tidal arm. The total dust mass in the tidal arm is ~1.8 x 10^6 M_sun (assuming beta=2), i.e. substantially larger than the dust mass associated with NGC 3077 (~2 x 10^5 M_sun). Where dust is detected, the dust-to-gas ratio is 6+/-3 x 10^-3, consistent within the uncertainties with what is found in NGC 3077 and nearby spiral galaxies with Galactic metallicities. The faint HII regions in the tidal arm can not be responsible for the detected enriched material and are not the main source of the dust heating in the tidal arm. We conclude that the interstellar medium (atomic HI, molecules and dust) in this tidal feature was pre-enriched and stripped off NGC 3077 during its recent interaction (~3 x 10^8 yr ago) with M 82 and M 81. This implies that interaction can efficiently remove heavy elements and enriched material (dust, molecular gas) from galaxies. As interactions were more frequent at large lookback times, it is conceivable that they could substantially contribute (along with galactic outflows) to the enrichment of the intergalactic medium., Comment: ApJ Letters, in print

We investigate the emission properties of polycyclic aromatic hydrocarbons (PAHs) in various metallicity environments with the Infrared Spectrograph on board Spitzer. Local giant HII regions are used as references as they enable access to the distinct interstellar medium components that contribute to the mid-infrared spectrum of star-forming galaxies: photodissociation regions (PDRs), photoionized gas, stellar clusters, and embedded regions. Three objects are considered, NGC3603 in the Milky Way, 30Doradus in the Large Magellanic Cloud, and N66 in the Small Magellanic Cloud. From the variations of the PAH/14um ratio, we find that PAHs are destroyed in the ionized gas for a radiation field such that [NeIII]/[NeII]>3. From the variations of the PAH/Hu-alpha ratio, we find that the PAH emission sources in the giant HII regions follow the same photodestruction law regardless of metallicity. We then compare these results with observations of starburst galaxies, HII galaxies, and blue compact dwarf galaxies (BCDs). While the integrated mid-infrared spectra of BCDs are reminiscent of a warm dusty ionized gas, we observe a significant contribution to the PAH emission in starburst galaxies that is not arising from PDRs., Comment: Accepted for publication in the Astrophysical Journal

We use WIRC, IR images of the Antennae (NGC 4038/4039) together with the extensive catalogue of 120 X-ray point sources (Zezas et al. 2006) to search for counterpart candidates. Using our proven frame-tie technique, we find 38 X-ray sources with IR counterparts, almost doubling the number of IR counterparts to X-ray sources first identified in Clark et al. (2007). In our photometric analysis, we consider the 35 IR counterparts that are confirmed star clusters. We show that the clusters with X-ray sources tend to be brighter, K_s ~16 mag, with (J-K_s) = 1.1 mag. We then use archival HST images of the Antennae to search for optical counterparts to the X-ray point sources. We employ our previous IR-to-X-ray frame-tie as an intermediary to establish a precise optical-to-X-ray frame-tie with <0.6 arcsec rms positional uncertainty. Due to the high optical source density near the X-ray sources, we determine that we cannot reliably identify counterparts. Comparing the HST positions to the 35 identified IR star cluster counterparts, we find optical matches for 27 of these sources. Using Bruzual-Charlot spectral evolutionary models, we find that most clusters associated with an X-ray source are massive, ~10^6 M_sun, young, ~10^6 yr, with moderate metallicities, Z=0.05., Comment: 9 page, 8 figures, accepted for publication in MNRAS

We present the wavelength and spectral resolution characterisation of the Integral Field Unit (IFU) Medium Resolution Spectrometer for the Mid-InfraRed Instrument (MIRI), to fly onboard the James Webb Space Telescope in 2014. We use data collected using the Verification Model of the instrument and develop an empirical method to calibrate properties such as wavelength range and resolving power in a portion of the spectrometer's full spectral range (5-28 microns). We test our results against optical models to verify the system requirements and combine them with a study of the fringing pattern in the instrument's detector to provide a more accurate calibration. We show that MIRI's IFU spectrometer will be able to produce spectra with a resolving power above R=2800 in the wavelength range 6.46-7.70 microns, and that the unresolved spectral lines are well fitted by a Gaussian profile., Comment: 12 pages, submitted to SPIE Proceedings vol. 7731, Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave

Using photometry of NGC 1097 from the Herschel PACS (Photodetector Array Camera and Spectrometer) instrument, we study the resolved properties of thermal dust continuum emission from a circumnuclear starburst ring with a radius ~ 900 pc. These observations are the first to resolve the structure of a circumnuclear ring at wavelengths that probe the peak (i.e. lambda ~ 100 micron) of the dust spectral energy distribution. The ring dominates the far-infrared (far-IR) emission from the galaxy - the high angular resolution of PACS allows us to isolate the ring's contribution and we find it is responsible for 75, 60 and 55% of the total flux of NGC 1097 at 70, 100 and 160 micron, respectively. We compare the far-IR structure of the ring to what is seen at other wavelengths and identify a sequence of far-IR bright knots that correspond to those seen in radio and mid-IR images. The mid- and far-IR band ratios in the ring vary by less than +/- 20% azimuthally, indicating modest variation in the radiation field heating the dust on ~ 600 pc scales. We explore various explanations for the azimuthal uniformity in the far-IR colors of the ring including a lack of well-defined age gradients in the young stellar cluster population, a dominant contribution to the far-IR emission from dust heated by older (> 10 Myr) stars and/or a quick smoothing of local enhancements in dust temperature due to the short orbital period of the ring. Finally, we improve previous limits on the far-IR flux from the inner ~ 600 pc of NGC 1097 by an order of magnitude, providing a better estimate of the total bolometric emission arising from the active galactic nucleus and its associated central starburst., Comment: Accepted for publication in the A&A Herschel Special Edition

NGC 1097 is a nearby SBb galaxy with a Seyfert nucleus and a bright starburst ring. We study the physical properties of the interstellar medium (ISM) in the ring using spatially resolved far-infrared spectral maps of the circumnuclear starburst ring of NGC 1097, obtained with the PACS spectrometer on board the Herschel Space Telescope. In particular, we map the important ISM cooling and diagnostic emission lines of [OI] 63 $\mu$m, [OIII] 88 $\mu$m, [NII] 122 $\mu$m, [CII] 158 $\mu$m and [NII] 205 $\mu$m. We observe that in the [OI] 63 $\mu$m, [OIII] 88 $\mu$m, and [NII] 122 $\mu$m line maps, the emission is enhanced in clumps along the NE part of the ring. We observe evidence of rapid rotation in the circumnuclear ring, with a rotation velocity of ~220$ km s$^{-1}$ (inclination uncorrected) measured in all lines. The [OI] 63 $\mu$m/[CII] 158 $\mu$m ratio varies smoothly throughout the central region, and is enhanced on the northeastern part of the ring, which may indicate a stronger radiation field. This enhancement coincides with peaks in the [OI] 63 $\mu$m and [OIII] 88 $\mu$m maps. Variations of the [NII] 122 $\mu$m/[NII] 205 $\mu$m ratio correspond to a range in the ionized gas density between 150 and 400 cm$^{-3}$., Comment: Accepted for publication on the A&A Herschel Special Issue

Stellar density and bar strength should affect the temperatures of the cool (T ~ 20-30 K) dust component in the inner regions of galaxies, which implies that the ratio of temperatures in the circumnuclear regions to the disk should depend on Hubble type. We investigate the differences between cool dust temperatures in the central 3 kpc and disk of 13 nearby galaxies by fitting models to measurements between 70 and 500 microns. We attempt to quantify temperature trends in nearby disk galaxies, with archival data from Spitzer/MIPS and new observations with Herschel/SPIRE, which were acquired during the first phases of the Herschel observations for the KINGFISH (key insights in nearby galaxies: a far-infrared survey with Herschel) sample. We fit single-temperature modified blackbodies to far-infrared and submillimeter measurements of the central and disk regions of galaxies to determine the temperature of the component(s) emitting at those wavelengths. We present the ratio of central-region-to-disk-temperatures of the cool dust component of 13 nearby galaxies as a function of morphological type. We find a significant temperature gradient in the cool dust component in all galaxies, with a mean center-to-disk temperature ratio of 1.15 +/- 0.03. The cool dust temperatures in the central ~3 kpc of nearby galaxies are 23(+/-3)% hotter for morphological types earlier than Sc, and only 9(+/-3)% hotter for later types. The temperature ratio is also correlated with bar strength, with only strongly barred galaxies having a ratio over 1.2. The strong radiation field in the high stellar density of a galactic bulge tends to heat the cool dust component to higher temperatures, at least in early-type spirals with relatively large bulges, especially when paired with a strong bar., Comment: Accepted for publication on the A&A Herschel Special Issue

We present deep Hubble Space Telescope (HST) NICMOS 2 F160W band observations of the central 56*57" (14pc*14.25pc) region around R136 in the starburst cluster 30 Dor (NGC 2070) located in the Large Magellanic Cloud. Our aim is to derive the stellar Initial Mass Function (IMF) down to ~1 Msun in order to test whether the IMF in a massive metal-poor cluster is similar to that observed in nearby young clusters and the field in our Galaxy. We estimate the mean age of the cluster to be 3 Myr by combining our F160W photometry with previously obtained HST WFPC2 optical F555W and F814W band photometry and comparing the stellar locus in the color-magnitude diagram with main sequence and pre-main sequence isochrones. The color-magnitude diagrams show the presence of differential extinction and possibly an age spread of a few megayears. We convert the magnitudes into masses adopting both a single mean age of 3 Myr isochrone and a constant star formation history from 2 to 4 Myr. We derive the IMF after correcting for incompleteness due to crowding. The faintest stars detected have a mass of 0.5 Msun and the data are more than 50% complete outside a radius of 5 pc down to a mass limit of 1.1 Msun for 3 Myr old objects. We find an IMF of dN/dlog(M) M^(-1.20+-0.2) over the mass range 1.1--20 Msun only slightly shallower than a Salpeter IMF. In particular, we find no strong evidence for a flattening of the IMF down to 1.1 Msun at a distance of 5 pc from the center, in contrast to a flattening at 2 Msun at a radius of 2 pc, reported in a previous optical HST study. We examine several possible reasons for the different results. If the IMF determined here applies to the whole cluster, the cluster would be massive enough to remain bound and evolve into a relatively low-mass globular cluster., Comment: Accepted in ApJ. Abstract abridged

Adaptive Optics at mid-IR wavelengths has long been seen as either not necessary or easy. The impact of atmospheric turbulence on the performance of 8-10 meter class telescopes in the mid-IR is relatively small compared to other performance issues like sky background and telescope emission. Using a relatively low order AO system, Strehl Ratios of larger than 95% have been reported on 6-8 meter class telescopes. Going to 30-42 meter class telescopes changes this picture dramatically. High Strehl Ratios require what is currently considered a high-order AO system. Furthermore, even with a moderate AO system, first order simulations show that the performance of such a system drops significantly when not taking into account refractivity effects and atmospheric composition variations. Reaching Strehl Ratios of over 90% at L, M and N band will require special considerations and will impact the system design and control scheme of AO systems for mid-IR on ELTs. In this paper we present an overview of the effects that impact the performance of an AO system at mid-IR wavelengths on an ELT and simulations on the performance and we will present a first order system concept of such an AO system for METIS, the mid-IR instrument for the E-ELT., Comment: 5 pages, 7 figures; Proceedings of the AO4ELT Conference, Paris, France (22-26 June 2009)

We present an atlas of Spitzer/IRS high resolution (R~600) 10-37um spectra for 24 well known starburst galaxies. The spectra are dominated by fine-structure lines, molecular hydrogen lines, and emission bands of polycyclic aromatic hydrocarbons. Six out of the eight objects with a known AGN component show emission of the high excitation [NeV] line. This line is also seen in one other object (NGC4194) with, a priori, no known AGN component. In addition to strong polycyclic aromatic hydrocarbon emission features in this wavelength range (11.3, 12.7, 16.4um), the spectra reveal other weak hydrocarbon features at 10.6, 13.5, 14.2um, and a previously unreported emission feature at 10.75um. An unidentified absorption feature at 13.7um is detected in many of the starbursts. We use the fine-structure lines to derive the abundance of neon and sulfur for 14 objects where the HI 7-6 line is detected. We further use the molecular hydrogen lines to sample the properties of the warm molecular gas. Several basic diagrams characterizing the properties of the sample are also shown. We have combined the spectra of all the pure starburst objects to create a high S/N template, which is available to the community., Comment: 25 pages (emulate apj), 6 tables, 14 figures, Accepted for publication in ApJS

(Aims): We study the LMC interstellar medium in the field of the nebula N157B, which contains a supernova remnant, an OB association, ionized gas, and high-density dusty filaments in close proximity. We investigate the relative importance of shock excitation by the SNR and photo-ionization by the OB stars, as well as possible interactions between the supernova remnant and its environment. (Methods): We apply multiwavelength mapping and photometry, along with spatially resolved infrared spectroscopy, to identifying the nature of the ISM using new infrared data from the Spitzer space observatory and X-ray, optical, and radio data from the literature. (Results): The N157B SNR has no infrared counterpart. Infrared emission from the region is dominated by the compact blister-type HII region associated with 2MASS J05375027-6911071 and excited by an O8-O9 star. This object is part of an extended infrared emission region that is associated with a molecular cloud. We find only weak emission from the shock-indicator [FeII], and both the excitation and the heating of the extended cloud are dominated by photo-ionization by the early O stars of LH99. (Conclusions): Any possible impact by the expanding SNR does not now affect the extended cloud of molecules and dust, despite the apparent overlap of SNR X-ray emission with infrared and Ha emission from the cloud. This implies that the supernova progenitor cannot have been more massive than about 25 solar masses., Comment: 10 pages, 8 figures, published in A&A

Using the Infrared Spectrograph on the Spitzer Space Telescope, we observed the Antennae galaxies obtaining spectral maps of the entire central region and high signal-to-noise 5-38 um spectra of the two galactic nuclei and six infrared-luminous regions. The total infrared luminosity of our six IR peaks plus the two nuclei is L_IR = 3.8x10^10 L_o, with their derived star formation rates ranging between 0.2 and 2 M_o/yr, with a total of 6.6 M_o/yr. The hardest and most luminous radiation originates from two compact clusters in the southern part of the overlap region, which also have the highest dust temperatures. PAH emission and other tracers of softer radiation are spatially extended throughout and beyond the overlap region, but regions with harder and intenser radiation field show a reduced PAH strength. The strong H_2 emission is rather confined around the nucleus of NGC 4039, where shocks appear to be the dominant excitation mechanism, and the southern part of the overlap region, where it traces the most recent starburst activity. The luminosity ratio between the warm molecular gas (traced by the H_2 lines) and the total far-IR emission is ~1.6x10^-4, similar to that found in many starburst and ULIRGs. The total mass of warm H_2 in the Antennae is 2.5x10^7 M_o, with a fraction of warm to total H_2 gas mass of about 0.35%. The average warm H_2 temperature is 302+/-26 K and appears anti-correlated with the radiation field hardness, possibly due to an evolution of the PDR morphology. The previously reported tight correlation between the H_2 and PAH emission was not found but higher total PAH emission to continuum ratios were found in PDRs with warmer gas., Comment: 24 pages, 13 figures, accepted by ApJ, full resolution version at http://www.strw.leidenuniv.nl/~brandl/ngc4038_brandl.pdf

We present a mid-infrared spectroscopic data cube of the central part of 30 Doradus, observed with Spitzer's IRS and MIPS/SED mode. Aromatic dust emission features and emission lines from molecular and atomic hydrogen are detected but not particularly strong. The dominant spectral features are emission lines from moderately ionized species of argon, neon, and sulphur, which are used to determine the physical conditions in the ionized gas. The ionized gas excitation shows strong variations on parsec scales, some of which can plausibly be associated with individual hot stars. We fit the ionic line strengths with photoionization and shock models, and find that photoionization dominates in the region. The ionization parameter U traces the rim of the central bubble, as well as highlighting isolated sources of ionization, and at least one quiescent clump. The hardness of the ionizing radiation field T_rad reveals several "hot spots" that are either the result of individual very hot stars or trace the propagation of the diffuse ionizing field through the surrounding neutral cloud. Consistent with other measurements of giant molecular hydrogen regions, log(U) ranges between -3 and -0.75, and T_rad between 30000 and 85000K., Comment: 32 pages, 26 figures, ApJ accepted. A version with high-resolution images can be found at http://www.astro.virginia.edu/~ged3j/indebetouw20090125.pdf

We present new mid-infrared ($5 - 35\mu$m) and ultraviolet (1539 -- 2316 \AA) observations of the interacting galaxy system Arp 143 (NGC 2444/2445) from the Spitzer Space Telescope and GALEX. In this system, the central nucleus of NGC 2445 is surrounded by knots of massive star-formation in a ring-like structure. We find unusually strong emission from warm H$_2$ associated with an expanding shock wave between the nucleus and the western knots. At this ridge, the flux ratio between H$_2$ and PAH emission is nearly ten times higher than in the nucleus. Arp 143 is one of the most extreme cases known in that regard. From our multi-wavelength data we derive a narrow age range of the star-forming knots between 2 Myr and 7.5 Myr, suggesting that the ring of knots was formed almost simultaneously in response to the shock wave traced by the H$_2$ emission. However, the knots can be further subdivided in two age groups: those with an age of 2--4 Myr (knots A, C, E, and F), which are associated with $8\mu$m emission from PAHs, and those with an age of 7-8 Myr (knots D and G), which show little or no $8\mu$m emission shells surrounding them. We attribute this finding to an ageing effect of the massive clusters which, after about 6 Myr, no longer excite the PAHs surrounding the knots., Comment: 19 pages, 11 figures, including tables at the end; accepted by ApJ

We discuss the effect of atmospheric dispersion on the performance of a mid-infrared adaptive optics assisted instrument on an extremely large telescope (ELT). Dispersion and atmospheric chromaticity is generally considered to be negligible in this wavelength regime. It is shown here, however, that with the much-reduced diffraction limit size on an ELT and the need for diffraction-limited performance, refractivity phenomena should be carefully considered in the design and operation of such an instrument. We include an overview of the theory of refractivity, and the influence of infrared resonances caused by the presence of water vapour and other constituents in the atmosphere. `Traditional' atmospheric dispersion is likely to cause a loss of Strehl only at the shortest wavelengths (L-band). A more likely source of error is the difference in wavelengths at which the wavefront is sensed and corrected, leading to pointing offsets between wavefront sensor and science instrument that evolve with time over a long exposure. Infrared radiation is also subject to additional turbulence caused by the presence of water vapour in the atmosphere not seen by visible wavefront sensors, whose effect is poorly understood. We make use of information obtained at radio wavelengths to make a first-order estimate of its effect on the performance of a mid-IR ground-based instrument. The calculations in this paper are performed using parameters from two different sites, one `standard good site' and one `high and dry site' to illustrate the importance of the choice of site for an ELT., Comment: 11 pages, to be published in SPIE Proceedings vol. 7015, Adaptive Optics Systems, eds. N. Hubin, C.E. Max and P.L. Wizinowich, 2008

We use the previously-identified 15 infrared star-cluster counterparts to X-ray point sources in the interacting galaxies NGC 4038/4039 (the Antennae) to study the relationship between total cluster mass and X-ray binary number. This significant population of X-Ray/IR associations allows us to perform, for the first time, a statistical study of X-ray point sources and their environments. We define a quantity, \eta, relating the fraction of X-ray sources per unit mass as a function of cluster mass in the Antennae. We compute cluster mass by fitting spectral evolutionary models to K_s luminosity. Considering that this method depends on cluster age, we use four different age distributions to explore the effects of cluster age on the value of \eta and find it varies by less than a factor of four. We find a mean value of \eta for these different distributions of \eta = 1.7 x 10^-8 M_\sun^-1 with \sigma_\eta = 1.2 x 10^-8 M_\sun^-1. Performing a \chi^2 test, we demonstrate \eta could exhibit a positive slope, but that it depends on the assumed distribution in cluster ages. While the estimated uncertainties in \eta are factors of a few, we believe this is the first estimate made of this quantity to ``order of magnitude'' accuracy. We also compare our findings to theoretical models of open and globular cluster evolution, incorporating the X-ray binary fraction per cluster., Comment: 20 pages, 6 figures, accepted by ApJ

The Magellanic Clouds (MCs) offer an outstanding variety of young stellar associations, in which large samples of low-mass stars (with masses less than 1 solar mass) currently in the act of formation can be resolved and explored sufficiently with the Hubble Space Telescope. These pre-main sequence (PMS) stars provide a unique snapshot of the star formation process, as it is being recorded for the last 20 Myr, and they give important information on the low-mass Initial Mass Function (IMF) of their host environments. We present the latest results from observations with the Advanced Camera for Surveys (ACS) of such star-forming regions in the MCs, and discuss the importance of Hubble}for a comprehensive collection of substantial information on the most recent low-mass star formation and the low-mass IMF in the MCs., Comment: To appear in the proceedings of the 41st ESLAB Symposium: The Impact of HST on European Astronomy, 4 pages, LaTeX ESA Publications style, 5 Figures

(Abridged) We present R~600, 10-37um spectra of 53 ULIRGs at z<0.32, taken using the IRS on board Spitzer. All of the spectra show fine structure emission lines of Ne, O, S, Si and Ar, as well as molecular Hydrogen lines. Some ULIRGs also show emission lines of Cl, Fe, P, and atomic Hydrogen, and/or absorption features from C_2H_2, HCN, and OH. We employ diagnostics based on the fine-structure lines, as well as the EWs and luminosities of PAH features and the strength of the 9.7um silicate absorption feature (S_sil), to explore the power source behind the infrared emission in ULIRGs. We show that the IR emission from the majority of ULIRGs is powered mostly by star formation, with only ~20% of ULIRGs hosting an AGN with a comparable or greater IR luminosity than the starburst. The detection of the 14.32um [NeV] line in just under half the sample however implies that an AGN contributes significantly to the mid-IR flux in ~42% of ULIRGs. The emission line ratios, luminosities and PAH EWs are consistent with the starbursts and AGN in ULIRGs being more extincted, and for the starbursts more compact, versions of those in lower luminosity systems. The excitations and electron densities in the NLRs of ULIRGs appear comparable to those of lower luminosity starbursts, though there is evidence that the NLR gas in ULIRGs is more dense. We show that the combined luminosity of the 12.81um [NeII] and 15.56um [NeIII] lines correlates with both IR luminosity and the luminosity of the 6.2 micron and 11.2 micron PAH features in ULIRGs, and use this to derive a calibration between PAH luminosity and star formation rate. Finally, we show that ULIRGs with 0.8 < S_sil < 2.4 are likely to be powered mainly by star formation, but that ULIRGs with S_sil < 0.8, and possibly those with S_sil > 2.4, contain an IR-luminous AGN., Comment: 62 pages in preprint format, 4 tables, 23 figures. ApJ accepted

We compare the CO 2-1 observations against previously taken CO 4-3 observations and analyze the spatial distribution of young stellar objects (YSO's) within the cloud using the Spitzer IRAC observations of the 30 Doradus complex. Both peaks of CO 2-1 and 4-3 emitting clouds coincide with the densest region of the filaments where multiple shells are colliding. We find that the YSO's are clustered in the southern ridge of the warm and dense molecular gas clouds traced by CO J=4-3, indicating a filamentary structure of star formation throughout the 30 Doradus. We also find that some of Class I YSO's candidates which are likely to be associated with a high-velocity component of CO 4-3 emitting clouds are present. This is a bona fide place where the triggered star formation had happened and newly formed stars may have produced such a high-velocity outflow interacting with the surrounding molecular cloud material., Comment: 17 pages, 7 figures, Submitted to ApJ

We present spatial distribution of polycyclic aromatic hydrocarbons and ionized gas within the Galactic giant HII region NGC3603. Using the IRS instrument on board the Spitzer Space Telescope, we study in particular the PAH emission features at ~5.7, 6.2, 7.7, 8.6, and 11.3um, and the [ArII] 6.99um, [NeII] 12.81um, [ArIII] 8.99um, and [SIV] 10.51um forbidden emission lines. The observations probe both ionized regions and photodissociation regions. Silicate emission is detected close to the central cluster while silicate absorption is seen further away. We find no significant variation of the PAH ionization fraction across the whole region. The emission of very small grains lies closer to the central stellar cluster than emission of PAHs. The PAH/VSG ratio anticorrelates with the hardness of the interstellar radiation field suggesting a destruction mechanism of the molecules within the ionized gas, as shown for low-metallicity galaxies by Madden et al. (2006)., Comment: Accepted for publication in ApJ. Corrected typos

IZw18, a blue compact dwarf (BCD) galaxy with the 2nd lowest metallicity measured in a star-forming object, has been observed with all three instruments on board the Spitzer Space Telescope. We present the deepest 5-36 micron mid-infrared (mid-IR) spectrum of this galaxy as yet obtained, as well as 3.6 to 70 micron imaging results. As with SBS0335-052E, another BCD with similar metallicity, IZw18 shows no detectable emission from polycyclic aromatic hydrocarbons (PAHs). However, the continuum emission, from 15 to 70 micron, of IZw18 has a much steeper slope, more characteristic of a typical starburst galaxy of solar abundance. The neon abundance as measured from the infrared fine-structure lines is ~1/23 Z_sun, and the sulfur abundance is ~ 1/35 Z_sun, generally consistent with the nebular oxygen abundance of 1/30 Z_sun derived from optical lines. This suggests that the extinction to the infrared emitting regions of this galaxy is low, also in agreement with the optical Balmer line ratios., Comment: Accepted by ApJ, 7 pages, 5 figures

We combine the the stellar spectral synthesis code Starburst99, the nebular modelling code MAPPINGSIII, and a 1-D dynamical evolution model of HII regions around massive clusters of young stars to generate improved models of the spectral energy distribution (SED) of starburst galaxies. We introduce a compactness parameter, C, which characterizes the specific intensity of the radiation field at ionization fronts in HII regions, and which controls the shape of the far-IR dust re-emission, often referred to loosely as the dust ``temperature''. We also investigate the effect of metallicity on the overall SED and in particular, on the strength of the PAH features. We provide templates for the mean emission produced by the young compact HII regions, the older (10 - 100 Myr) stars and for the wavelength-dependent attenuation produced by a foreground screen of the dust used in our model. We demonstrate that these components may be combined to produce a excellent fit to the observed SEDs of star formation dominated galaxies which are often used as templates (Arp 220 and NGC 6240). This fit extends from the Lyman Limit to wavelengths of about one mm. The methods presented in both this paper and in the previous papers of this series allow the extraction of the physical parameters of the starburst region (star formation rates, star formation rate history, mean cluster mass, metallicity, dust attenuation and pressure) from the analysis of the pan-spectral SED., Comment: 35 pages, 21 figures, accepted for publication in ApJS full-res available at http://www.strw.leidenuniv.nl/~brent/publications/SEDIV.pdf

We study the correlation between the radio, mid-infrared and far-infrared properties for a sample of 28 blue compact dwarf (BCD) and low metallicity star-forming galaxies observed by Spitzer. We find that these sources extend the same far-infrared to radio correlation typical of star forming late type alaxies to lower luminosities. In BCDs, the 24um (or 22um) mid-infrared to radio correlation is similar to starburst galaxies, though there is somewhat larger dispersion in their q_24 parameter compared to their q_FIR. No strong correlations between the q parameter and galaxy metallicity or effective dust temperature have been detected, though there is a hint of decreasing q_24 at low metallicities. The two lowest metallicity dwarfs in our sample, IZw18 and SBS0335-052E, despite their similar chemical abundance, deviate from the average q$_{24}$ ratio in opposite manners, displaying an apparent radio excess and dust excess respectively., Comment: 8 pages, 4 figures, accepted by ApJ

We present high spatial resolution (~ 35 parsec) 5-38 um spectra of the central region of M82, taken with the Spitzer Infrared Spectrograph. From these spectra we determined the fluxes and equivalent widths of key diagnostic features, such as the [NeII]12.8um, [NeIII]15.5um, and H_2 S(1)17.03um lines, and the broad mid-IR polycyclic aromatic hydrocarbon (PAH) emission features in six representative regions and analysed the spatial distribution of these lines and their ratios across the central region. We find a good correlation of the dust extinction with the CO 1-0 emission. The PAH emission follows closely the ionization structure along the galactic disk. The observed variations of the diagnostic PAH ratios across M82 can be explained by extinction effects, within systematic uncertainties. The 16-18um PAH complex is very prominent, and its equivalent width is enhanced outwards from the galactic plane. We interpret this as a consequence of the variation of the UV radiation field. The EWs of the 11.3um PAH feature and the H_2 S(1) line correlate closely, and we conclude that shocks in the outflow regions have no measurable influence on the H_2 emission. The [NeIII]/[NeII] ratio is on average low at ~0.18, and shows little variations across the plane, indicating that the dominant stellar population is evolved (5 - 6 Myr) and well distributed. There is a slight increase of the ratio with distance from the galactic plane of M82 which we attribute to a decrease in gas density. Our observations indicate that the star formation rate has decreased significantly in the last 5 Myr. The quantities of dust and molecular gas in the central area of the galaxy argue against starvation and for negative feedback processes, observable through the strong extra-planar outflows., Comment: 15 pages, 12 figures, 3 tables, ApJ, emulateapj

We investigate the chemical abundances of NGC3603 in the Milky Way, of 30Doradus in the Large Magellanic Cloud, and of N66 in the Small Magellanic Cloud. Mid-infrared observations with the Infrared Spectrograph onboard the Spitzer Space Telescope allow us to probe the properties of distinct physical regions within each object: the central ionizing cluster, the surrounding ionized gas, photodissociation regions, and buried stellar clusters. We detect [SIII], [SIV], [ArIII], [NeII], [NeIII], [FeII], and [FeIII] lines and derive the ionic abundances. Based on the ionic abundance ratio (NeIII/H)/(SIII/H), we find that the gas observed in the MIR is characterized by a higher degree of ionization than the gas observed in the optical spectra. We compute the elemental abundances of Ne, S, Ar, and Fe. We find that the alpha-elements Ne, S, and Ar scale with each other. Our determinations agree well with the abundances derived from the optical. The Ne/S ratio is higher than the solar value in the three giant HII regions and points toward a moderate depletion of sulfur on dust grains. We find that the neon and sulfur abundances display a remarkably small dispersion (0.11dex in 15 positions in 30Doradus), suggesting a relatively homogeneous ISM, even though small-scale mixing cannot be ruled out., Comment: Accepted for submission to ApJ. The present version replaces the submitted one. Changes: new title, new figure, the text was modified in the discussion

We present a study of elemental abundances in a sample of thirteen Blue Compact Dwarf (BCD) galaxies, using the $\sim$10--37$\mu$m high resolution spectra obtained with Spitzer/IRS. We derive the abundances of neon and sulfur for our sample using the infrared fine-structure lines probing regions which may be obscured by dust in the optical and compare our results with similar infrared studies of starburst galaxies from ISO. We find a good correlation between the neon and sulfur abundances, though sulfur is under-abundant relative to neon with respect to the solar value. A comparison of the elemental abundances (neon, sulfur) measured from the infrared data with those derived from the optical (neon, sulfur, oxygen) studies reveals a good overall agreement for sulfur, while the infrared derived neon abundances are slightly higher than the optical values. This indicates that either the metallicities of dust enshrouded regions in BCDs are similar to the optically accessible regions, or that if they are different they do not contribute substantially to the total infrared emission of the host galaxy., Comment: 11 pages, 6 figures, accepted by ApJ

MIDIR is the proposed thermal/mid-IR imager and spectrograph for the European Extremely Large Telescope (E-ELT). It will cover the wavelength range of 3 to at least 20 microns. Designed for diffraction-limited performance over the entire wavelength range, MIDIR will require an adaptive optics system; a cryogenically cooled system could offer optimal performance in the IR, and this is a critical aspect of the instrument design. We present here an overview of the project, including a discussion of MIDIR's science goals and a comparison with other infrared (IR) facilities planned in the next decade; top level requirements derived from these goals are outlined. We describe the optical and mechanical design work carried out in the context of a conceptual design study, and discuss some important issues to emerge from this work, related to the design, operation and calibration of the instrument. The impact of telescope optical design choices on the requirements for the MIDIR instrument is demonstrated., Comment: for publication in SPIE Proceedings vol. 6692, Cryogenic Optical Systems and Instrumentation XII, eds. J.B. Heaney and L.G. Burriesci, San Diego, Aug 2007

We explore the relationships between the Polycyclic Aromatic Hydrocarbon (PAH) feature strengths, mid-infrared continuum luminosities, far-infrared spectral slopes, optical spectroscopic classifications, and silicate optical depths within a sample of 107 ULIRGs observed with the Infrared Spectrograph on the Spitzer Space Telescope. The detected 6.2 micron PAH equivalent widths (EQWs) in the sample span more than two orders of magnitude (0.006-0.8 micron), and ULIRGs with HII-like optical spectra or steep far-infrared spectral slopes (S_{25} / S_{60} < 0.2) typically have 6.2 micron PAH EQWs that are half that of lower-luminosity starbursts. A significant fraction (~40-60%) of HII-like, LINER-like, and cold ULIRGs have very weak PAH EQWs. Many of these ULIRGs also have large (tau_{9.7} > 2.3) silicate optical depths. The far-infrared spectral slope is strongly correlated with PAH EQW, but not with silicate optical depth. In addition, the PAH EQW decreases with increasing rest-frame 24 micron luminosity. We argue that this trend results primarily from dilution of the PAH EQW by continuum emission from dust heated by a compact central source, probably an AGN. High luminosity, high-redshift sources studied with Spitzer appear to have a much larger range in PAH EQW than seen in local ULIRGs, which is consistent with extremely luminous starburst systems being absent at low redshift, but present at early epochs., Comment: 15 pages, 9 Figures; Accepted for publication in ApJ

We use deep J and Ks images of the Antennae (NGC 4038/9) obtained with WIRC on the Palomar 200-inch telescope, together with the Chandra X-ray source list of Zezas et al. (2002a), to search for IR counterparts to X-ray point sources. We establish an X-ray/IR astrometric frame tie with 0.5" rms residuals over a \~4.3' field. We find 13 ``strong'' IR counterparts brighter than Ks = 17.8 mag and < 1.0" from X-ray sources, and an additional 6 ``possible'' IR counterparts between 1.0" and 1.5" from X-ray sources. The surface density of IR sources near the X-ray sources suggests only ~2 of the ``strong'' counterparts and ~3 of the ``possible'' counterparts are chance superpositions of unrelated objects. Comparing both strong and possible IR counterparts to our photometric study of ~220 Antennae, IR clusters, we find the IR counterparts to X-ray sources are \~1.2 mag more luminous in Ks than average non-X-ray clusters. We also note that the X-ray/IR matches are concentrated in the spiral arms and ``overlap'' regions of the Antennae. This implies that these X-ray sources lie in the most ``super'' of the Antennae's Super Star Clusters, and thus trace the recent massive star formation history here. Based on the N_H inferred from the X-ray sources without IR counterparts, we determine that the absence of most of the ``missing'' IR counterparts is because they are intrinsically less luminous in the IR, implying that they trace a different (possibly older) stellar population., Comment: 27 pages, 10 Poscript figures, accepted by ApJ

We present a new diagnostic diagram for mid-infrared spectra of infrared galaxies based on the equivalent width of the 6.2 micron PAH emission feature and the strength of the 9.7 micron silicate feature. Based on the position in this diagram we classify galaxies into 9 classes ranging from continuum-dominated AGN hot dust spectra and PAH-dominated starburst spectra to absorption-dominated spectra of deeply obscured galactic nuclei. We find that galaxies are systematically distributed along two distinct branches: one of AGN and starburst-dominated spectra and one of deeply obscured nuclei and starburst-dominated spectra. The separation into two branches likely reflects a fundamental difference in the dust geometry in the two sets of sources: clumpy versus non-clumpy obscuration. Spectra of ULIRGs are found along the full length of both branches, reflecting the diverse nature of the ULIRG family., Comment: 5 pages, 3 figures. Accepted for publication in ApJ Letters

We present spectra taken with the Infrared Spectrograph on Spitzer covering the 5-38 micron region of the ten Ultraluminous Infrared Galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample. Among the BGS ULIRGs, we find a factor of 50 spread in the rest-frame mid to far-infrared spectral slope. The 9.7 micron silicate optical depths range from less than 0.4 more than 4.2, implying line of sight extinctions of A(V) ~ 8 - 78 mag. There is evidence for water ice and hydrocarbon absorption and C2H2 and HCN absorption features in four and possibly six of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [NeV] emission in three of the ten BGS ULIRGs, at flux levels of 5-18E-14 erg/cm^2/sec and [NeV] 14.3/[NeII] 12.8 line flux ratios of 0.12-0.85. The remaining BGS ULIRGs have limits on their [NeV]/[NeII] line flux ratios which range from less than 0.15 to less than 0.01. Among the BGS ULIRGs, the AGN fractions implied by either the [NeV]/[NeII] or [OIV]/[NeII] line flux ratios (or their upper limits) are significantly lower than implied by the MIR slope or EQW of the 6.2 micron PAH feature. Fitting the SEDs, we see evidence for hot (T > 300K) dust in five of the BGS ULIRGs, with the fraction of hot dust to total dust luminosity ranging from ~1-23%, before correcting for extinction. When integrated over the IRAC-8, IRS blue peakup, and MIPS-24 filter bandpasses, the IRS spectra imply very blue colors for some ULIRGs at z ~ 1.3. This is most extreme for sources with significant amounts of warm dust and deep silicate absorption., Comment: accepted for publication in the Astrophysical Journal

We present 5-38um mid-infrared spectra at a spectral resolution of R~65-130 of a large sample of 22 starburst nuclei taken with the Infrared Spectrograph IRS on board the Spitzer Space Telescope. The spectra show a vast range in starburst SEDs. The silicate absorption ranges from essentially no absorption to heavily obscured systems with an optical depth of tau(9.8um)~5. The spectral slopes can be used to discriminate between starburst and AGN powered sources. The monochromatic continuum fluxes at 15um and 30um enable a remarkably accurate estimate of the total infrared luminosity of the starburst. We find that the PAH equivalent width is independent of the total starburst luminosity L_IR as both continuum and PAH feature scale proportionally. However, the luminosity of the 6.2um feature scales with L_IR and can be used to approximate the total infrared luminosity of the starburst. Although our starburst sample covers about a factor of ten difference in the [NeIII]/[NeII] ratio, we found no systematic correlation between the radiation field hardness and the PAH equivalent width or the 7.7um/11.3um PAH ratio. These results are based on spatially integrated diagnostics over an entire starburst region, and local variations may be ``averaged out''. It is presumably due to this effect that unresolved starburst nuclei with significantly different global properties appear spectrally as rather similar members of one class of objects., Comment: 22 pages, accepted for publication in ApJ, a high-resolution version is available from http://www.strw.leidenuniv.nl/~brandl/IRS_starbursts.pdf

We present the first Spitzer Infrared Spectrograph (IRS; The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and the Ames Research Center.) observations of the disks around classical T Tauri stars: spectra in the 5.2-30 micron range of six stars. The spectra are dominated by emission features from amorphous silicate dust, and a continuous component from 5 to 8 microns that in most cases comprises an excess above the photosphere throughout our spectral range. There is considerable variation in the silicate feature/continuum ratio, which implies variations of inclination, disk flaring, and stellar mass accretion rate. In most of our stars, structure in the silicate feature suggests the presence of a crystalline component. In one, CoKu Tau/4, no excess above the photosphere appears at wavelengths shortward of the silicate features, similar to 10 Myr old TW Hya, Hen 3-600, and HR 4796A. This indicates the optically thick inner disk is largely absent. The silicate emission features with peaks at 9.7 and 18 microns indicate small dust grains are present. The extremely low 10-20 micron color temperature of the dust excess, 135 K, indicates these grains are located more than 10 AU from the star. These features are suggestive of gravitational influence by planets or close stellar companions and grain growth in the region within 10 AU of the star, somewhat surprising for a star this young (1 Myr)., Comment: AASTEX, 10 pages text, 2 figures, 1 table, published September 2004 in The Astrophysical Journal Supplement Series

We present new Spitzer Space Telescope data on the nearby, low-metallicity starburst galaxy NGC 5253, from the Infrared Array Camera IRAC and the Infrared Spectrograph IRS. The mid-IR luminosity profile of NGC 5253 is clearly dominated by an unresolved cluster near the center, which outshines the rest of the galaxy at longer wavelengths. We find that the [NeIII]/[NeII] ratio decreases from $\sim8.5$ at the center to $\sim2.5$ at a distance of $\sim250$ pc. The [SIV]/[SIII] follows the [NeIII]/[NeII] ratio remarkably well, being about 4-5 times lower at all distances. Our spectra reveal for the first time PAH emission feature at 11.3$\mu$m and its equivalent width increases significantly with distance from the center. The good anti-correlation between the PAH strength and the product between hardness and luminosity of the UV radiation field suggests photo-destruction of the PAH molecules in the central region. The high-excitation [OIV]25.91$\mu$m line was detected at $0.42\times10^{-20}$W cm$^{-2}$. Our results demonstrate the importance of spatially resolved mid-IR spectroscopy., Comment: 5 pages, 4 figures, accepted for publication in ApJL

We present an X-ray tour of diffuse emission in the 30 Doradus star-forming complex in the Large Magellanic Cloud using high-spatial-resolution X-ray images and spatially-resolved spectra obtained with the Advanced CCD Imaging Spectrometer aboard the Chandra X-ray Observatory. The dominant X-ray feature of the 30 Doradus nebula is the intricate network of diffuse emission generated by interacting stellar winds and supernovae working together to create vast superbubbles filled with hot plasma. We construct maps of the region showing variations in plasma temperature (T = 3--9 million degrees), absorption (N_H = 1--6 x 10^{21} cm^{-2}), and absorption-corrected X-ray surface brightness (S_X = 3--126 x 10^{31} ergs s^{-1} pc^{-2}). Enhanced images reveal the pulsar wind nebula in the composite supernova remnant N157B and the Chandra data show spectral evolution from non-thermal synchrotron emission in the N157B core to a thermal plasma in its outer regions. In a companion paper we show that R136, the central massive star cluster, is resolved at the arcsecond level into almost 100 X-ray sources. Through X-ray studies of 30 Doradus the complete life cycle of such a massive stellar cluster can be revealed., Comment: 42 pages, 15 bitmapped figures, 4 tables; accepted to AJ

We discuss the signature of a peculiar constellation of very massive stars at a projected distance of 2-3 pc around R136a. We discuss various scenarios for its possible origin, such as independent clusters, triggered star formation, supernovae, and ejections via dynamical interactions. If the latter scenario were the correct one this would have significant implications on the way to probe the conditions in dense stellar cores, and on the evolution of massive clusters in general., Comment: To appear in the proceedings of "Massive Stars in Interacting Binaries", eds. A.J.F. Moffat & N. St-Louis

30 Doradus is the closest massive star forming region and the best studied template of a starburst. In this conference paper we first summarize the properties of 30 Doradus and its stellar core, R136. We discuss the effects of insufficient spatial resolution and cluster density profiles on dynamical mass estimates of super star clusters, and show that their masses can be easily overestimated by a factor of ten or more. From a very simple model, with R136-like clusters as representative building blocks, we estimate typical luminosities of the order 10^11 L_o for starburst galaxies., Comment: To be published in "Starbursts: From 30 Doradus to Lyman Break Galaxies", eds. R. de Grijs & R.M. Gonzalez Delgado

We have surveyed a field covering 9.0 degrees^2 within the NOAO Deep Wide-Field Survey region in Bootes with the Multiband Imaging Photometer on the Spitzer Space Telescope (SST) to a limiting 24 um flux density of 0.3 mJy. Thirty one sources from this survey with F(24um) > 0.75 mJy which are optically very faint (R > 24.5 mag) have been observed with the low-resolution modules of the Infrared Spectrograph on SST. Redshifts derived primarily from strong silicate absorption features are reported here for 17 of these sources; 10 of these are optically invisible (R > 26 mag), with no counterpart in B_W, R, or I. The observed redshifts for 16 sources are 1.7 < z < 2.8. These represent a newly discovered population of highly obscured sources at high redshift with extreme infrared to optical ratios. Using IRS spectra of local galaxies as templates, we find that a majority of the sources have mid-infrared spectral shapes most similar to ultraluminous infrared galaxies powered primarily by AGN. Assuming the same templates also apply at longer wavelengths, bolometric luminosities exceed 10^13 L(solar)., Comment: Accepted for publication on 7 Feb 2005 in ApJL. 7 pages 2 figures

We present mid-infrared spectra of the nearby, Ultraluminous Infrared Galaxy NGC 6240 taken with the IRS on Spitzer. The spectrum of NGC 6240 is dominated by strong fine-structure lines, rotational H2 lines, and PAH emission features. The H2 line fluxes suggest molecular gas at a variety of temperatures. A simple two-temperature fit to the S(0) through S(7) lines implies a mass of 6.7E6 solar masses at T=957K and 1.6E9 solar masses at T=164K, or about 15% of the total molecular gas mass in this system. Noteably, we have detected the [NeV] 14.3 micron emission line, with a flux of 5E-14 erg/cm^2/s, providing the first direct detection of the buried AGN in the mid-infrared. Modelling of the total SED from near to far-infrared wavelengths requires the presence of a hot dust (T=700K) component, which we also associate with the buried AGN. The small [NeV]/[NeII] and [NeV]/IR flux ratios, the relative fraction of hot dust emission, as well as the large 6.2 micron PAH EQW, are all consistent with an apparent AGN contribution of only 3-5% to the bolometric luminosity. However, correcting the measured [NeV] flux by the extinction implied by the silicate optical depth and our SED fitting, suggests an intrinsic fractional AGN contribution to the bolometric luminosity of about 20-24% in NGC 6240, which lies within the range implied by fits to the hard X-ray spectrum., Comment: 18 pages, 4 figures, accepted for publication in The Astrophysical Journal