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Register a new userStacked Average Far-Infrared Spectrum of Dusty Star-Forming Galaxies from the Herschel/SPIRE Fourier Transform Spectrometer
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We present stacked average far-infrared spectra of a sample of 197 dusty, star-forming galaxies (DSFGs) at $0.005 < z < 4$ using close to 90% of the SPIRE Fourier Transform Spectrometer (FTS) extragalactic data archive from the Herschel Space Observatory based on 3.5 years of science operations. These spectra explore an observed-frame $rm 447,GHz-1568,GHz$ ($rm 191,mu m-671,mu m$) frequency (wavelength) range allowing us to observe the main atomic and molecular lines emitted by gas in the interstellar medium. The sample is sub-divided into five redshift bins at $0.005 < z < 0.05$, $0.05 < z < 0.2$, $0.2 < z < 0.5$, $0.8 < z <2$, and $2 < z < 4$. To study the dependence of observed spectral lines on total infrared luminosity, the sources in a subset of the redshift bins are stacked in luminosity bins. These stacked spectra are used to determine the average properties of the interstellar medium and dense molecular gas properties of DSFGs, in particular, the fine-structure line ([CII] 158 $mu$m and [OI] 63 $mu$m) luminosity ratios, and the line to far-IR luminosity ratios are used to model the gas density and radiation field strength in the photodissociation regions (PDRs). For the low-redshift sample, we additionally present the average spectral line energy distributions (SLED) of CO and $rm{H_2O}$ rotational transitions and also consider PDR conditions based on a combination of [CI] 370 $mu$m and 609 $mu$m and $rm CO (7-6)$ lines. For the high-z ($0.8 < z < 4$) sample PDR models suggest a molecular gas distribution in the presence of a radiation field that is at least a factor of 10$^3$ larger than the Milky-Way and with a neutral gas density of roughly 10$^3$ to 10$^5$ cm$^{-3}$. The corresponding PDR models for the low-z sample suggest a UV radiation field and gas density comparable to those at high-z.
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The first complete submillimetre spectrum (190-670um) of the Seyfert 2 galaxy NGC1068 has been observed with the SPIRE Fourier Transform Spectrometer onboard the {it Herschel} Space Observatory. The sequence of CO lines (Jup=4-13), lines from water, the fundamental rotational transition of HF, two o-H_2O+ lines and one line each from CH+ and OH+ have been detected, together with the two [CI] lines and the [NII]205um line. The observations in both single pointing mode with sparse image sampling and in mapping mode with full image sampling allow us to disentangle two molecular emission components, one due to the compact circum-nuclear disk (CND) and one from the extended region encompassing the star forming ring (SF-ring). Radiative transfer models show that the two CO components are characterized by density of n(H_2)=10^4.5 and 10^2.9 cm^-3 and temperature of T=100K and 127K, respectively. The comparison of the CO line intensities with photodissociation region (PDR) and X-ray dominated region (XDR) models, together with other observational constraints, such as the observed CO surface brightness and the radiation field, indicate that the best explanation for the CO excitation of the CND is an XDR with density of n(H_2) 10^4 cm^-3 and X-ray flux of 9 erg s^-1 cm^-2, consistent with illumination by the active galactic nucleus, while the CO lines in the SF-ring are better modeled by a PDR. The detected water transitions, together with those observed with the her sim PACS Spectrometer, can be modeled by an LVG model with low temperature (T_kin sim 40K) and high density (n(H_2) in the range 10^6.7-10^7.9 cm^-3).
A systematic programme of calibration observations was carried out to monitor the performance of the SPIRE FTS instrument on board the Herschel Space Observatory. Observations of planets (including the prime point-source calibrator, Uranus), asteroids, line sources, dark sky, and cross-calibration sources were made in order to monitor repeatability and sensitivity, and to improve FTS calibration. We present a complete analysis of the full set of calibration observations and use them to assess the performance of the FTS. Particular care is taken to understand and separate out the effect of pointing uncertainties, including the position of the internal beam steering mirror for sparse observations in the early part of the mission. The repeatability of spectral line centre positions is <5km/s, for lines with signal-to-noise ratios >40, corresponding to <0.5-2.0% of a resolution element. For spectral line flux, the repeatability is better than 6%, which improves to 1-2% for spectra corrected for pointing offsets. The continuum repeatability is 4.4% for the SLW band and 13.6% for the SSW band, which reduces to ~1% once the data have been corrected for pointing offsets. Observations of dark sky were used to assess the sensitivity and the systematic offset in the continuum, both of which were found to be consistent across the FTS detector arrays. The average point-source calibrated sensitivity for the centre detectors is 0.20 and 0.21 Jy [1 sigma; 1 hour], for SLW and SSW. The average continuum offset is 0.40 Jy for the SLW band and 0.28 Jy for the SSW band.
We present the data processing pipeline to generate calibrated data products from the Spectral and Photometric Imaging Receiver (SPIRE) imaging Fourier Transform Spectrometer on the Herschel Space Observatory. The pipeline processes telemetry from SPIRE observations and produces calibrated spectra for all resolution modes. The spectrometer pipeline shares some elements with the SPIRE photometer pipeline, including the conversion of telemetry packets into data timelines and calculation of bolometer voltages. We present the following fundamental processing steps unique to the spectrometer: temporal and spatial interpolation of the scan mechanism and detector data to create interferograms; Fourier transformation; apodization; and creation of a data cube. We also describe the corrections for various instrumental effects including first- and second-level glitch identification and removal, correction of the effects due to emission from the Herschel telescope and from within the spectrometer instrument, interferogram baseline correction, temporal and spatial phase correction, non-linear response of the bolometers, and variation of instrument performance across the focal plane arrays. Astronomical calibration is based on combinations of observations of standard astronomical sources and regions of space known to contain minimal emission.
We describe an update to the Herschel-SPIRE Fourier-Transform Spectrometer (FTS) calibration for extended sources, which incorporates a correction for the frequency-dependent far-field feedhorn efficiency, $eta_mathrm{FF}$. This significant correction affects all FTS extended-source calibrated spectra in sparse or mapping mode, regardless of the spectral resolution. Line fluxes and continuum levels are underestimated by factors of 1.3-2 in the Spectrometer Long-Wavelength band (SLW, 447-1018 GHz; 671-294 $mu$m) and 1.4-1.5 in the Spectrometer Short-Wavelength band (SSW, 944-1568 GHz; 318-191 $mu$m). The correction was implemented in the FTS pipeline version 14.1 and has also been described in the SPIRE Handbook since Feb 2017. Studies based on extended-source calibrated spectra produced prior to this pipeline version should be critically reconsidered using the current products available in the Herschel Science Archive. Once the extended-source calibrated spectra are corrected for $eta_mathrm{FF}$, the synthetic photometry and the broadband intensities from SPIRE photometer maps agree within 2-4% -- similar levels to the comparison of point-source calibrated spectra and photometry from point-source calibrated maps. The two calibration schemes for the FTS are now self-consistent: the conversion between the corrected extended-source and point-source calibrated spectra can be achieved with the beam solid angle and a gain correction that accounts for the diffraction loss.
The ESA Herschel Spectral and Photometric Imaging Receiver (SPIRE) Fourier Transform Spectrometer (FTS) Spectral Feature Finder (FF) project is an automated spectral feature fitting routine developed within the SPIRE instrument team to extract all prominent spectral features from all publicly available SPIRE FTS observations. In this work, we demonstrate the use of the FF information extracted from three observations of the edge-on spiral galaxy NGC 891 to measure the rotation of NII and CI gas at Far-infrared frequencies in complement to radio observations of the HI 21cm line and the CO(1-0) transition as well as optical measurements of Halpha. We find that measurements of both NII and CI gas follow a similar velocity profile to that of HI and Halpha showing a correlation between neutral and ionized regions of the interstellar medium (ISM) in the disk of NGC 891.
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