ABSTRACT The Infrared Spectrograph (IRS) on board the Spitzer Space Telescope observed about objects during the cryogenic mission lifetime. Observations provided low-resolution ( ) spectra over m and ...high-resolution ( ) spectra over 10-37 m. The Cornell Atlas of Spitzer/IRS Sources (CASSIS) was created to provide publishable quality spectra to the community. Low-resolution spectra have been available in CASSIS since 2011, and here we present the addition of the high-resolution spectra. The high-resolution observations represent approximately one-third of all staring observations performed with the IRS instrument. While low-resolution observations are adapted to faint objects and/or broad spectral features (e.g., dust continuum, molecular bands), high-resolution observations allow more accurate measurements of narrow features (e.g., ionic emission lines) as well as a better sampling of the spectral profile of various features. Given the narrow aperture of the two high-resolution modules, cosmic ray hits and spurious features usually plague the spectra. Our pipeline is designed to minimize these effects through various improvements. A super-sampled point-spread function was created in order to enable the optimal extraction in addition to the full aperture extraction. The pipeline selects the best extraction method based on the spatial extent of the object. For unresolved sources, the optimal extraction provides a significant improvement in signal-to-noise ratio over a full aperture extraction. We have developed several techniques for optimal extraction, including a differential method that eliminates low-level rogue pixels (even when no dedicated background observation was performed). The updated CASSIS repository now includes all the spectra ever taken by the IRS, with the exception of mapping observations.
We present the spectral atlas of sources observed in low resolution with the Infrared Spectrograph on board the Spitzer Space Telescope. More than 11,000 distinct sources were extracted using a ...dedicated algorithm based on the SMART software with an optimal extraction (AdOpt package). These correspond to all 13,000 low-resolution observations of fixed objects (both single source and cluster observations). The pipeline includes image cleaning, individual exposure combination, and background subtraction. Particular attention is given to bad pixel and outlier rejection at the image and spectra levels. Most sources are spatially unresolved so that optimal extraction reaches the highest possible signal-to-noise ratio. For all sources, an alternative extraction is also provided that accounts for all of the source flux within the aperture. CASSIS provides publishable quality spectra through an online database together with several important diagnostics, such as the source spatial extent and a quantitative measure of detection level. Ancillary data such as available spectroscopic redshifts are also provided. The database interface will eventually provide various ways to interact with the spectra, such as on-the-fly measurements of spectral features or comparisons among spectra.
We present 5-38 km 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 of starburst SEDs. The silicate absorption ranges from essentially no absorption to heavily obscured systems with an optical depth of t sub(9.8) k sub(m) 6 5. The spectral slopes can be used to discriminate between starburst and AGN powered sources. The monochromatic continuum fluxes at 15 and 30 km 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 sub(IR) as both continuum and PAH feature scale proportionally. However, the luminosity of the 6.2 km feature scales with L sub(IR) and can be used to approximate the total infrared luminosity of the starburst. Although our starburst sample covers about a factor of 10 difference in the Ne III/Ne II ratio, we found no systematic correlation between the radiation field hardness and the PAH equivalent width or the 7.7 km/11.3 km 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.
We report the spectroscopic detection of silicate emission at 10 and 18 mu m in five PG quasars, the first detection of these two features in galaxies outside the Local Group. This finding is ...consistent with the unification model for active galactic nuclei (AGNs), which predicts that an AGN torus seen pole-on should show a silicate emission feature in the mid-infrared. The strengths of the detected silicate emission features range from 0.12 to 1.25 times the continuum at 10 mu m and from 0.20 to 0.79 times the continuum at 18 mu m. The silicate grain temperatures inferred from the ratio of 18 mu m to 10 mu m silicate features under the assumption of optically thin emission range from 140 to 220 K.
We present mid-infrared spectra of 32 high-redshift ultraluminous infrared galaxies, selected via the stellar photospheric feature at rest- frame 1.6 mum, and an observed-frame 24 mum flux of >500 ...muJy. Nearly all the sample reside in a redshift range of image and have rest-frame 1- 1000 mum luminosities of 10 super(12.9)-10 super(13.8) L sub(image). Most of the spectra exhibit prominent polycyclic aromatic hydrocarbon emission features and weak silicate absorption, consistent with a starburst origin for the IR emission. Our selection method appears to be a straightforward and efficient way of finding distant, IR-luminous, star-forming galaxies in narrow redshift ranges. There is, however, evidence that the mid-IR spectra of our sample differ systematically from those of local ULIRGs; our sample have comparable PAH equivalent widths but weaker apparent silicate absorption, and (possibly) enhanced PAH 6.2 mum/7.7 mum and 6.2 mum/11.2 mum flux ratios. Furthermore, the composite mid-IR spectrum of our sample is almost identical to that of local starbursts with IR luminosities of 10 super(10)-10 super(11) L sub(image), rather than that of local ULIRGs. These differences are consistent with a reduced dust column, which can plausibly be obtained via some combination of (1) star formation that is extended over spatial scales of 1-4 kpc and (2) star formation in unusually gas-rich regions.
Full low-resolution (65 < R < 130) and high-resolution (R 6 600) spectra between 5 and 37 km 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.
A summary is presented for 130 galaxies observed with the Herschel Photodetector Array Camera and Spectrometer instrument to measure fluxes for the CII 158 mu m emission line. Sources cover a wide ...range of active galactic nucleus to starburst classifications, as derived from polycyclic aromatic hydrocarbon strength measured with the Spitzer Infrared Spectrograph. Redshifts from CII and line to continuum strengths (equivalent width (EW) of CII) are given for the full sample, which includes 18 new CII flux measures. Calibration of L(C II)) as a star formation rate (SFR) indicator is determined by comparing CII luminosities with mid-infrared NeII and NeIII emission line luminosities; this gives the same result as determining SFR using bolometric luminosities of reradiating dust from starbursts: log SFR = log L(C II)) - 7.0, for SFR in M sub(middot in circle) yr super(-1) and L(CII) in L sub(middot in circle). We conclude that L(CII) can be used to measure SFR in any source to a precision of ~50%, even if total source luminosities are dominated by an active galactic nucleus (AGN) component. The line to continuum ratio at 158 mu m, EW(CII), is not significantly greater for starbursts (median EW(CII) = 1.0 mu m) compared to composites and AGNs (median EW(CII) = 0.7 mu m), showing that the far-infrared continuum at 158 mu m scales with CII regardless of classification. This indicates that the continuum at 158 mu m also arises primarily from the starburst component within any source, giving log SFR = log vL sub(v)(158 mu m) - 42.8 for SFR in M sub(middot in circle) yr super(-1) and vL sub(v)(158 mu m) in erg s super(-1).
Results are presented for CII 158 mu m line fluxes observed with the Herschel PACS instrument in 112 sources with both starburst and active galactic nucleus (AGN) classifications, of which 102 ...sources have confident detections. Results are compared with mid-infrared spectra from the Spitzer Infrared Spectrometer and with L sub(ir) from IRAS fluxes; AGN/starburst classifications are determined from equivalent width of the 6.2 mu m polycyclic aromatic hydrocarbon (PAH) feature. It is found that the CII line flux correlates closely with the flux of the 11.3 mu m PAH feature independent of AGN/starburst classification, log functionof(CII 158 mu m)/functionof(11.3 mu m PAH) = -0.22 + or - 0.25. It is concluded that the CII line flux measures the photodissociation region associated with starbursts in the same fashion as the PAH feature. A calibration of star formation rate (SFR) for the starburst component in any source having CII is derived comparing CII luminosity L(CII) to L sub(ir) with the result that log SFR = log L(C II)) - 7.08 + or - 0.3, for SFR in M sub(middot in circle) yr super(-1) and L(CII) in L sub(middot in circle). The decreasing ratio of L(CII) to L sub(ir) in more luminous sources (the "CII deficit") is shown to be a consequence of the dominant contribution to L sub(ir) arising from a luminous AGN component because the sources with the largest L sub(ir) and smallest L(CII)/L sub(ir) are AGNs.
A sample of 196 AGNs and ULIRGs observed by the Infrared Spectrograph (IRS) on Spitzer is analyzed to study the distribution of the strength of the 9.7 km silicate feature. Average spectra are ...derived for quasars, Seyfert 1 and Seyfert 2 AGNs, and ULIRGs. We find that quasars are characterized by silicate features in emission and Seyfert 1s equally by emission or weak absorption. Seyfert 2s are dominated by weak silicate absorption, and ULIRGs are characterized by strong silicate absorption (mean apparent optical depth about 1.5). Luminosity distributions show that luminosities at rest frame 5.5 km are similar for the most luminous quasars and ULIRGs and are almost 10 super(5) times more luminous than the least luminous AGN in the sample. The distributions of spectral characteristics and luminosities are compared to those of optically faint infrared sources at z62 being discovered by the IRS, which are also characterized by strong silicate absorption. It is found that local ULIRGs are a similar population, although they have lower luminosities and somewhat stronger absorption compared to the high-redshift sources.
Obscuration in Extremely Luminous Quasars Polletta, M; Weedman, D; Hönig, S ...
Astrophysical journal/The Astrophysical journal,
03/2008, Letnik:
675, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The SEDs and IR spectra of a remarkable sample of obscured AGNs selected in the MIR are modeled with recent clumpy torus models. The sample contains 21 AGNs at image discovered in the largest Spitzer ...surveys (SWIRE, NDWFS, and FLS) by means of their extremely red IR to optical colors. All sources show the 9.7 mum silicate feature in absorption and have extreme MIR luminosities image . The IR SEDs and spectra of 12 sources are well reproduced with a simple torus model, while the remaining nine sources require foreground extinction from a cold dust component to reproduce both the depth of the silicate feature and the NIR emission from hot dust. The best-fit torus models show a broad range of inclinations. Based on the unobscured QSO MIR luminosity function (Brown and coworkers) and on a color- selected sample of AGNs, we estimate the surface densities of obscured and unobscured QSOs with image and image to be about 17-22 and 11.7 deg super(- 2), respectively. Overall we find that image35%-41% of luminous QSOs are unobscured, 37%-40% are obscured by the torus, and 23%-25% are obscured by a cold absorber detached from the torus. These fractions are consistent with a decrease of the torus covering fraction at large luminosities as predicted by receding torus models. An FIR component is observed in eight objects with luminosity greater than image, implying SFRs of image. In the whole sample, the average contribution from a starburst component to the bolometric luminosity, as estimated from the PAH 7.7 mum luminosity in the composite IR spectra, is <=20% of the total bolometric luminosity.
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