D. Devost

The SMART Data Analysis Package for the Infrared Spectrograph* on the Spitzer Space Telescope **

SMART is a software package written in IDL to reduce and analyze Spitzer data from all four modules of the Infrared Spectrograph, including the peak-up arrays. The software is designed to make full use of the ancillary files generated in the Spitzer Science Center pipeline so that it can either remove or flag artifacts and corrupted data and maximize the signal-to-noise in the extraction routines. It may be run in both interactive and batch mode. The software and Users Guide will be available for public release in December 2004. We briefly describe some of the main features of SMART including: visualization tools for assessing the data quality, basic arithmetic operations for either 2-d images or 1-d spectra, extraction of both point and extended sources and a suite of spectral analysis tools. Subject headings: methods: data analysis — techniques: spectroscopic — telescopes: Spitzer Space Telescope

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.

High-Resolution Mid-Infrared Spectroscopy of Ultraluminous Infrared Galaxies

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

Mid-Infrared Spectra of Classical AGNs Observed with the Spitzer Space Telescope

Full low-resolution (65 < R < 130) and high-resolution (R ~ 600) spectra between 5 and 37 μm obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope are presented for eight classical active galactic nuclei (AGNs) that have been extensively studied previously. Spectra of these AGNs are presented as comparison standards for the many objects, including sources at high redshift, that are being observed spectroscopically in the mid-infrared for the first time using the IRS. The AGNs are NGC 4151, Mrk 3, I Zw 1, NGC 1275, Centaurus A, NGC 7469, Mrk 231, and NGC 3079. These sources are used to demonstrate the range of infrared spectra encountered in objects that have widely different classification criteria at other wavelengths but that unquestionably contain AGNs. Overall spectral characteristics, including continuum shape, nebular emission lines, silicate absorption and emission features, and PAH emission features, are considered to understand how spectral classifications based on mid-infrared spectra relate to those previously derived from optical spectra. The AGNs are also compared to the same parameters for starburst galaxies such as NGC 7714 and the compact, low-metallicity starburst SBS 0335-052 previously observed with the IRS. Results confirm the much lower strengths of PAH emission features in AGNs, but there are no spectral parameters in this sample that unambiguously distinguish AGNs and starbursts based only on the slopes of the continuous spectra.

PAH Emission from Ultraluminous Infrared Galaxies

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 in a sample of 107 ULIRGs observed with the Infrared Spectrograph on the Spitzer Space Telescope. The detected 6.2 μm PAH equivalent widths (EWs) in the sample span more than 2 orders of magnitude (~0.006-0.8 μm), and ULIRGs with H II-like optical spectra or steep far-infrared spectral slopes (S_(25)/S_(60) 2.3) silicate optical depths. The far-infrared spectral slope is strongly correlated with PAH EW, but not with silicate optical depth. In addition, the PAH EW decreases with increasing rest-frame 24 μm luminosity. We argue that this trend results primarily from dilution of the PAH EW 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 EW than seen in local ULIRGs, which is consistent with extremely luminous starburst systems being absent at low redshift, but present at early epochs.

Detection of the Buried Active Galactic Nucleus in NGC 6240 with the Infrared Spectrograph on the Spitzer Space Telescope

We present mid-infrared spectra of the nearby ultraluminous infrared galaxy NGC 6240 taken with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The spectrum of NGC 6240 is dominated by strong fine-structure lines, rotational H2 lines, and polycyclic aromatic hydrocarbon (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 ~6.7 × 106 M☉ at T ~ 957 K and ~1.6 × 109 M☉ at T ~ 164 K, or about 15% of the total molecular gas mass in this system. Notably, we have detected the [Ne V] 14.3 μm emission line, with a flux of 5 × 10-14 ergs cm-2 s-1, providing the first direct detection of the buried active galactic nucleus (AGN) in the mid-infrared. Modeling of the total spectral energy distribution (SED) from near- to far-infrared wavelengths requires the presence of a hot dust (T ~ 700 K) component, which we also associate with the buried AGN. The small [Ne V]/[Ne II] and [Ne V]/IR flux ratios, the relative fraction of hot dust emission, and the large 6.2 μm PAH equivalent width (EQW), are all consistent with an apparent AGN contribution of only 3%-5% to the bolometric luminosity. However, correcting the measured [Ne V] 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 ~20%-24% in NGC 6240, which lies within the range implied by fits to the hard X-ray spectrum.

OBSERVATIONS OF ULTRALUMINOUS INFRARED GALAXIES WITH THE INFRARED SPECTROGRAPH (IRS) ON THE SPITZER SPACE TELESCOPE: EARLY RESULTS ON MARKARIAN 1014, MARKARIAN 463, AND UGC 5101

We present spectra taken with the Infrared Spectrograph on the Spitzer Space Telescope covering the 5-38 μm region of three ultraluminous infrared galaxies (ULIRGs): Mrk 1014 (z = 0.163), Mrk 463 (z = 0.051), and UGC 5101 (z = 0.039). The continua of UGC 5101 and Mrk 463 show strong silicate absorption suggesting significant optical depths to the nuclei at 10 μm. UGC 5101 also shows the clear presence of water ice in absorption. Polycyclic aromatic hydrocarbon (PAH) emission features are seen in both Mrk 1014 and UGC 5101, including the 16.4 μm line in UGC 5101. The fine-structure lines are consistent with dominant active galactic nucleus (AGN) power sources in both Mrk 1014 and Mrk 463. In UGC 5101 we detect the [Ne V] 14.3 μm emission line, providing the first direct evidence for a buried AGN in the mid-infrared. The detection of the 9.66 μm and 17.03 μm H2 emission lines in both UGC 5101 and Mrk 463 suggest that the warm molecular gas accounts for 22% and 48% of the total molecular gas masses in these galaxies.

Probing Cosmic Star Formation Using Long Gamma-Ray Bursts: New Constraints from the Spitzer Space Telescope*

We report on IRAC 4.5 μm, IRAC 8.0 μm, and MIPS 24 μm deep observations of 16 gamma-ray burst (GRB) host galaxies performed with the Spitzer Space Telescope, and we investigate in the thermal infrared the presence of evolved stellar populations and dust-enshrouded star-forming activity associated with these objects. Our sample is derived from GRBs that were identified with subarcsecond localization between 1997 and 2001, and only a very small fraction (~20%) of the targeted sources are detected down to f4.5 μm ~ 3.5 μJy and f24 μm ~ 85 μJy (3 σ). This likely argues against a population dominated by massive and strongly starbursting (i.e., SFR 100 M☉ yr-1) galaxies as has been recently suggested from submillimeter/radio and optical studies of similarly selected GRB hosts. Furthermore, we find evidence that some GRBs do not occur in the most infrared luminous regions—hence the most actively star-forming environments—of their host galaxies. Should the GRB hosts be representative of all star-forming galaxies at high redshift, models of infrared galaxy evolution indicate that 50% of GRB hosts should have f24 μm 100 μJy. Unless the identification of GRBs prior to 2001 was prone to strong selection effects biasing our sample against dusty galaxies, we infer in this context that the GRBs identified with the current techniques cannot be directly used as unbiased probes of the global and integrated star formation history of the universe.

The Magellanic Zoo: Mid-Infrared Spitzer Spectroscopy of Evolved Stars and Circumstellar Dust in the Magellanic Clouds

We observed a sample of evolved stars in the Large and Small Magellanic Clouds (LMC and SMC) with the Infrared Spectrograph on the Spitzer Space Telescope. Comparing samples from the SMC, LMC, and the Galaxy reveals that the dust production rate depends on metallicity for oxygen-rich stars, but carbon stars with similar pulsation properties produce similar quantities of dust, regardless of their initial metallicity. Other properties of the oxygen-rich stars also depend on metallicity. As the metallicity decreases, the fraction of naked (i.e., dust-free) stars increases, and among the naked stars, the strength of the 8 μm absorption band from SiO decreases. Our sample includes several massive stars in the LMC with long pulsation periods that produce significant amounts of dust, probably because they are young and relatively metal-rich. Little alumina dust is seen in circumstellar shells in the SMC and LMC, unlike in Galactic samples. Three oxygen-rich sources also show emission from magnesium-rich crystalline silicates. Many also show an emission feature at 14 μm. The one S star in our sample shows a newly detected emission feature centered at 13.5 μm. At lower metallicity, carbon stars with similar amounts of amorphous carbon in their shells have stronger absorption from molecular acetylene (C2H2) and weaker emission from SiC and MgS dust, as discovered in previous studies.

THE EXTRAORDINARY MID-INFRARED SPECTRUM OF THE BLUE COMPACT DWARF GALAXY SBS 0335-052

SBS 0335-052 is a blue compact dwarf galaxy (BCD) with one of the lowest known metallicities, Z ~ Z⊙/41, making it a local example of how primordial starburst galaxies and their precursors might appear. A spectrum obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope clearly shows silicate absorption features and emission lines of [S IV] and [Ne III], and puts strong upper limits on the polycyclic aromatic hydrocarbon (PAH) emission features. The observed low-resolution spectrum (R ~ 90) extends from 5.3 to 35 μm and peaks at ~28 μm. The spectrum is compared to IRS observations of the prototypical starburst nucleus NGC 7714. SBS 0335-052 is quite unlike normal starburst galaxies, which show strong PAH bands, low-ionization emission lines, and a continuum peak near 80 μm. The continuum difference for λ > 30 μm implies a substantial reduction in the mass of cold dust. If the spectrum of this very low-metallicity galaxy is representative of star-forming galaxies at higher redshifts, it may be difficult to distinguish them from active galactic nuclei, which also show relatively featureless flat spectra in the mid-IR.