H. W. W. Spoon

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 Spitzer covering the 5-38 μm region of the 10 ultraluminous infrared galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample (BGS). There is a factor of 50 spread in the rest-frame 5.5-60 μm spectral slopes, and the 9.7 μm silicate optical depths range from at least τ_(9.7) ≤ 0.4 (A_V ~ 8) to τ_(9.7) ≥ 4.2 (A_V ≥ 78). There is evidence for water ice and hydrocarbon absorption and C_2H_2 and HCN absorption features in 4 and possibly 6 of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [Ne V] emission in 3 of the 10 BGS ULIRGs, at flux levels of 5-18 × 10^(-14) ergs cm^(-2) s^(-1) and [Ne V] 14.3/[Ne II] 12.8 line flux ratios of 0.12-0.85. The remaining BGS ULIRGs have limits on their [Ne V]/[Ne II]line flux ratios, which range from ≤0.15 to ≤0.01. Among the BGS ULIRGs, the AGN fractions implied 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 MIR slope or strength of the 6.2 μm PAH EQW feature. There is evidence for hot (T > 300 K) dust in five of the BGS ULIRGs, with the fraction of hot dust to total dust luminosity ranging from ~1% to 23%, before correcting for extinction. When integrated over the IRAC-8, IRS blue peak-up, and MIPS-24 filter bandpasses, the IRS spectra imply very blue colors for some ULIRGs at z ~ 1.3. The large range in diagnostic parameters among the nearest ULIRGs suggests that matching survey results to a small number of templates may lead to biased results about the fraction of luminous dusty starbursts and AGNs at high z.

Polycyclic Aromatic Hydrocarbons as a Tracer of Star Formation

Infrared (IR) emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 ?m are generally attributed to IR fluorescence from (mainly) far-ultraviolet (FUV) pumped large polycyclic aromatic hydrocarbon (PAH) molecules. As such, these features trace the FUV stellar flux and are thus a measure of star formation. We examined the IR spectral characteristics of Galactic massive star-forming regions and of normal and starburst galaxies, as well as active galactic nuclei (AGNs) and ultraluminous infrared galaxies (ULIRGs). The goal of this study is to analyze whether PAH features are a good qualitative and/or quantitative tracer of star formation, and hence to evaluate the application of PAH emission as a diagnostic tool in order to identify the dominant processes contributing to the infrared emission from Seyfert galaxies and ULIRGs. We develop a new mid-infrared (MIR)/far-infrared (FIR) diagnostic diagram based on our Galactic sample and compare it to the diagnostic tools of Genzel and coworkers and Laurent and coworkers, with these diagnostic tools also applied to our Galactic sample. This MIR/FIR diagnostic is derived from the FIR normalized 6.2 ?m PAH flux and the FIR normalized 6.2 ?m continuum flux. Within this diagram, the Galactic sources form a sequence spanning a range of 3 orders of magnitude in these ratios, ranging from embedded compact H II regions to exposed photodissociation regions (PDRs) and the (diffuse) interstellar medium (ISM). However, the variation in the 6.2 ?m PAH feature-to-continuum ratio is relative small. Comparison of our extragalactic sample with our Galactic sources revealed an excellent resemblance of normal and starburst galaxies to exposed PDRs. While Seyfert 2 galaxies coincide with the starburst trend, Seyfert 1 galaxies are displaced by at least a factor of 10 in 6.2 ?m continuum flux, in accordance with general orientation-dependent unification schemes for AGNs. ULIRGs show a diverse spectral appearance. Some show a typical AGN hot dust continuum. More, however, either are starburst-like or show signs of strong dust obscuration in the nucleus. One characteristic of the ULIRGs also seems to be the presence of more prominent FIR emission than either starburst galaxies or AGNs. We discuss the observed variation in the Galactic sample in view of the evolutionary state and the PAH/dust abundance and discuss the use of PAHs as quantitative tracers of star formation activity. Based on these investigations, we find that PAHs may be better suited as a tracer of B stars, which dominate the Galactic stellar energy budget, than as a tracer of massive star formation (O stars).IR emission bands at 3.3, 6.2, 7.7, 8.6 and 11.3 um are generally attributed to IR fluorescence from (mainly) FUV pumped PAHs. As such, they trace the FUV stellar flux and are a measure of star formation. We examined the IR spectral characteristics of Galactic star forming regions, normal and starburst galaxies, AGNs and ULIRGs. The goal is to analyze if PAH bands are a good qualitative and/or quantitative tracer of star formation and hence the application of PAH bands as a diagnostic in order to identify the dominant processes contributing to the IR emission from Seyferts and ULIRGs. We develop a MIR/FIR diagnostic and compare it to known diagnostics, with these also applied to the Galactic sample. This diagnostic is based on the FIR normalized 6.2 um PAH flux and the FIR normalized 6.2 um continuum flux. The Galactic sources form a sequence spanning a range of 3 orders of magnitude, from embedded compact HII regions to exposed PDRs and the (D)ISM. The variation in the 6.2 um PAH/continuum ratio is relative small. Normal and starburst galaxies ressemble exposed PDRs. While Seyfert-2s coincide with the starburst trend, Seyfert-1s are displaced by at least a factor 10 in 6.2 um continuum flux. ULIRGs show a diverse spectral appearance (AGN hot dust continuum, starburst-like or strong dust obscuration in the nucleus). ULIRGs also seems to have more prominent FIR emission than either starburst galaxies or AGNs. We discuss the observed variation in the Galactic sample in view of the evolutionary state and the PAH/dust abundance and the use of PAHs as quantitative tracers of star formation activity. We find that PAHs may be better suited as a tracer of B stars, which dominate the Galactic stellar energy budget, than as a tracer of massive star formation (O stars).

A Large Mid-Infrared Spectroscopic and Near-Infrared Imaging Survey of Ultraluminous Infrared Galaxies: Their Nature and Evolution

We present a low-resolution mid-infrared spectroscopic survey of an unbiased sample of 62 ultraluminous infrared galaxies (ULIRGs) (LIR > 1012L⊙, z ≤ 0.3) using ISOPHOT-S on board the Infrared Space Observatory (ISO). For comparison, we also present ISOPHOT-S spectra for 23 active galactic nuclei (AGNs) and 15 starburst and normal galaxies. The line-to-continuum ratio of the 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission feature is used as a discriminator between starburst and AGN activity in ULIRGs. We find that the majority of ULIRGs are predominantly powered by starbursts. The ratio of PAH over infrared luminosities, LPAH/LIR, for starburst-dominated ULIRGs is very similar to the ratio found for template starbursts. The shapes of the PAH features are sometimes unusual. Extinction has a noticeable effect on the PAH spectra of ULIRG starbursts. We have obtained high-resolution near-infrared imaging for the southern ISOPHOT-S ULIRGs in order to investigate their evolution stage. The majority (68%) of the ULIRGs imaged are double systems, and all show distorted morphologies. Of the 23 double-nuclei systems, 17 appear at linear separations between 4 and 14 kpc, with a mean separation of 6.5 kpc. Using the separations measured from our new near-infrared imaging as well as from the literature, we have examined whether ULIRGs that are advanced mergers are more AGN-like. We have found no such evidence, contrary to what is postulated by the classical evolutionary scenario. No correlation is found between the stage of merger in ULIRGs and their infrared luminosity. In fact, we find that systems in the early stages of merging may well put out maximum luminosity. We also find that the total mass of interstellar gas, as estimated from the CO (1 → 0) luminosity, does not decrease with decreasing merger separation. When both an AGN and a starburst occur concurrently in ULIRGs, we find that the starburst dominates the luminosity output. We propose that the available gas reservoir and the individual structure of the interacting galaxies plays a major role in the evolution of the system.

Mid-Infrared Galaxy Classification Based on Silicate Obscuration and PAH Equivalent Width

We present a new diagnostic diagram for mid-infrared spectra of infrared galaxies based on the equivalent width of the 6.2 μm PAH emission feature and the strength of the 9.7 μm silicate feature. Based on the positions in this diagram, we classify galaxies into nine 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 nonclumpy obscuration. Spectra of ULIRGs are found along the full length of both branches, reflecting the diverse nature of the ULIRG family.

The Nature and Evolution of Ultraluminous Infrared Galaxies:A Mid-Infrared Spectroscopic Survey

We report the first results of a low-resolution mid-infrared spectroscopic survey of an unbiased, far-infrared-selected sample of 60 ultraluminous infrared galaxies (ULIRGs) (LIR

High-Resolution Mid-Infrared Spectroscopy of Ultraluminous Infrared Galaxies

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Mid-infrared properties of low-metallicity blue compact dwarf galaxies from the Spitzer infrared spectrograph

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Massive Star Formation and Evolution in Starburst Galaxies: Mid-infrared Spectroscopy with the ISO Short Wavelength Spectrometer*

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Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2. II. Diagnostics

--> > 10 -->12 L?) using ISOPHOT-S on board the Infrared Space Observatory (ISO). We use the ratio of the 7.7 ?m polycyclic aromatic hydrocarbon (PAH) emission feature to the local continuum as a discriminator between starburst and active galactic nucleus (AGN) activity. About 80% of all ULIRGs are found to be predominantly powered by star formation, but the fraction of AGN-powered objects increases with luminosity. Whereas only about 15% of ULIRGs at luminosities below 2 ? 10 -->12 L? are AGN powered, this fraction increases to about half at higher luminosity. Observed ratios of the PAH features in ULIRGs differ slightly from those in lower luminosity starbursts. This can be plausibly explained by the higher extinction and/or different physical conditions in the interstellar medium of ULIRGs. The PAH feature-to-continuum ratio is anticorrelated with the ratio of feature-free 5.9 ?m continuum to the IRAS 60 ?m continuum, confirming suggestions that strong mid-infrared continuum is a prime AGN signature. The location of starburst-dominated ULIRGs in such a diagram is consistent with previous ISO-Short Wavelength Spectrograph spectroscopy, which implies significant extinction even in the mid-infrared. We have searched for indications that ULIRGs that are advanced mergers might be more AGN-like, as postulated by the classical evolutionary scenario. No such trend has been found among those objects for which near-infrared images are available to assess their likely merger status.

CASSIS: THE CORNELL ATLAS OF SPITZER/INFRARED SPECTROGRAPH SOURCES

We present R � 600, 10Y37 � m spectra of 53 ultraluminous infrared galaxies (ULIRGs), taken using the Infrared SpectrographonboardSpitzer.Thespectra showfine-structureemissionlines of neon,oxygen,sulfur,silicon, argon, chlorine, iron, and phosphorous; molecular hydrogen lines, and C2H2, HCN, and OHabsorption features. We em- ploy diagnostics based on the fine-structure lines, the polycyclic aromatic hydrocarbon (PAH) features and the 9.7 � m silicate absorption feature, to show that the infrared emission from most ULIRGs is powered mostly by star formation, with only � 20% of ULIRGs hosting an AGN with a greater IR luminosity than the starburst. The detection of (Ne v) k14.32injustunderhalfthesample,however,impliesthatanAGNcontributessignificantlytothemid-IRfluxin � 42% of ULIRGs.ThestarburstsandAGNsinULIRGsappearmoreextincted,andforthestarburstsmorecompactthanthose in lower luminosity systems. The excitations and electron densities in the narrow-line regions of ULIRGs appear comparable to those of starbursts with LP10 11.5 L� , although the NLR gas in ULIRGs may be more dense. We show thatthe(Neii)k12.81+(Neiii)k15.56 luminositycorrelateswithbothinfraredluminosityandtheluminosityof the 6.2 and 11.2 � m PAH features, and derive a calibration between PAH luminosity and star formation rate. Finally, we show thatULIRGswithsilicateabsorptionstrengthsSsilof 0:8PSsilP2:4 arelikelytobepoweredmainlybystarformation, but that ULIRGs with Ssil P0:8, and possibly those with Ssil k2:4, contain an IR-luminous AGN. Subject headingg galaxies: active — galaxies: evolution — galaxies: starburst — infrared: galaxies Online material: color figures