Ulrich Klaas

The Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory

The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on ESAs far infrared and submil- limetre observatory. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with 16 × 25 pixels, each, and two filled silicon bolometer arrays with 16 × 32 and 32 × 64 pixels, respectively, to perform integral-field spectroscopy and imaging photom- etry in the 60−210 μm wavelength regime. In photometry mode, it simultaneously images two bands, 60−85 μ mo r 85−125 μ ma nd 125−210 μm, over a field of view of ∼1.75 � × 3.5 � , with close to Nyquist beam sampling in each band. In spectroscopy mode, it images afi eld of 47 �� × 47 �� , resolved into 5 × 5 pixels, with an instantaneous spectral coverage of ∼ 1500 km s −1 and a spectral resolution of ∼175 km s −1 . We summarise the design of the instrument, describe observing modes, calibration, and data analysis methods, and present our current assessment of the in-orbit performance of the instrument based on the performance verification tests. PACS is fully operational, and the achieved performance is close to or better than the pre-launch predictions.

Infrared to millimetre photometry of ultra-luminous IR galaxies ?;?? : New evidence favouring a 3{stage dust model

Infrared to millimetre spectral energy distributions (SEDs) have been obtained for 41 bright ultra- luminous infrared galaxies (ULIRGs). The observations were carried out with ISOPHOT between 10 and 200 m and supplemented for 16 sources with JCMT/SCUBA at 450 and 850 m and with SEST at 1.3 mm. In addition, seven sources were observed at 1.2 and 2.2 m with the 2.2 m telescope on Calar Alto. These new SEDs represent the most complete set of infrared photometric templates obtained so far on ULIRGs in the local universe. The SEDs peak at 60{100 m and show often a quite shallow Rayleigh-Jeans tail. Fits with one single modied blackbody yield a high FIR opacity and small dust emissivity exponent T >30 K) to cold (30 K >T >10 K) temperatures. This provides evidence for two dust stages, the cool starburst dominated one and the cold cirrus-like one. The third stage with several hundred Kelvin warm dust is identied in the AGN dominated ULIRGs, showing up as a NIR-MIR power- law flux increase. While AGNs and SBs appear indistinguishable at FIR and submm wavelengths, they dier in the NIR-MIR. This suggests that the cool FIR emitting dust is not related to the AGN, and that the AGN only powers the warm and hot dust. In comparison with optical and MIR spectroscopy, a criterion based on the SED shapes and the NIR colours is established to reveal AGNs among ULIRGs. Also the possibility of recognising evolutionary trends among the ULIRGs via the relative amounts of cold, cool and warm dust components is investigated.

The Herschel PACS photometer calibration

This paper provides an overview of the PACS photometer flux calibration concept, in particular for the principal observation mode, the scan map. The absolute flux calibration is tied to the photospheric models of five fiducial stellar standards (α Boo, α Cet, α Tau, β And, γ Dra). The data processing steps to arrive at a consistent and homogeneous calibration are outlined. In the current state the relative photometric accuracy is ∼2 % in all bands. Starting from the present calibration status, the characterization and correction for instrumental effects affecting the relative calibration accuracy is described and an outlook for the final achievable calibration numbers is given. After including all the correction for the instrumental effects, the relative photometric calibration accuracy (repeatability) will be as good as 0.5 % in the blue and green band and 2 % in the red band. This excellent calibration starts to reveal possible inconsistencies between the models of the K-type and the M-type stellar calibrators. The absolute calibration accuracy is therefore mainly limited by the 5 % uncertainty of the celestial standard models in all three bands. The PACS bolometer response was extremely stable over the entire Herschel mission and a single, time-independent response calibration file is sufficient for the processing and calibration of the science observations. The dedicated measurements of the internal calibration sources were needed only to characterize secondary effects. No aging effects of the bolometer or the filters have been found. Also, we found no signs of filter leaks. The PACS photometric system is very well characterized with a constant energy spectrum νFν = λFλ = const as a reference. Colour corrections for a wide range of sources SEDs are determined and tabulated.

No cold dust within the supernova remnant Cassiopeia A

A large amount (about three solar masses) of cold (18 K) dust in the prototypical type II supernova remnant Cassiopeia A was recently reported. It was concluded that dust production in type II supernovae can explain how the large quantities (∼ 108 solar masses) of dust observed in the most distant quasars could have been produced within only 700 million years after the Big Bang. Foreground clouds of interstellar material, however, complicate the interpretation of the earlier submillimetre observations of Cas A. Here we report far-infrared and molecular line observations that demonstrate that most of the detected submillimetre emission originates from interstellar dust in a molecular cloud complex located in the line of sight between the Earth and Cas A, and is therefore not associated with the remnant. The argument that type II supernovae produce copious amounts of dust is not supported by the case of Cas A, which previously appeared to provide the best evidence for this possibility.

The spiral galaxy M33 mapped in the FIR by ISOPHOT A spatially resolved study of the warm and cold dust

The Sc galaxy M 33 has been mapped with ISOPHOT in the far-infrared, at 60, 100, and 170 µm. The spatial resolution of these FIR maps allows the separation of spiral arms and interarm regions and the isolation of a large number of star-forming regions. The spectral energy distribution in the FIR indicates a superposition of two components, a warm one originating from dust at ∼45 K, and a cold one, at ∼16 K. The warm component is concentrated towards the spiral arms and the star-forming regions, and is likely heated by the UV radiation from OB stars. The cold component is more smoothly distributed over the disk, and heated by the diffuse interstellar radiation. For the about 60 star-forming regions detected the Hα/FIR flux ratio increases significantly with the distance from the galaxy center, probably due to decreasing extinction. An anti-correlation of FHa/F60 with F170 suggests the intrinsic extinction to be related to the cold dust surface brightness according to AV /S 170 ∼ 0.03 mag MJy −1 sr. For the total galaxy the star formation rate (SFR) derived from the FIR is in agreement with that derived from the de-extincted Hα emission. For individual star-forming regions, a consistency between SFRs derived from the optical and from the FIR requires only a fraction of the UV radiation to be absorbed locally. The individual star-forming regions also show a local radio-FIR correlation. This local correlation is, however, due to quite different components than to those that lead to the well-known global radio-FIR correlation for entire galaxies.

Dust emission from 3C radio galaxies and quasars: New ISO observations favour the unified scheme

In order to test the unied scheme for luminous radio galaxies and quasars we observed 10 galaxy/quasar pairs from the 3CR catalogue with ISOPHOT at infrared wavelengths between 5 and 180 m. Each pair was selected such that both the 178 MHz luminosity and the redshift match as close as possible between the radio galaxy and the quasar in order to minimize eects of cosmic evolution. 13 of the 20 sources were detected in at least one waveband. 12 sources show clear evidence of a thermal bump at FIR wavelength, while in the remaining 7 sources the upper limits are still compatible with the presence of luminous dust emission. In agreement with the predictions of the unied scheme, the quasars and galaxies in our sample cannot be distinguished by their observed mid- and far-infrared properties. This is in contrast to the ndings on the basis of the IRAS scans which indicated that radio galaxies radiate signicantly less mid- to far-infrared emission than quasars. However, the IRAS samples are dominated by low-redshift sources ( z 0:8). At lower redshifts ( z< 0:5), however, some of the lobe-dominated FRII radio galaxies contain active nuclei which emit less UV-optical continuum than the quasars of similar radio power. As this division is mainly a function of redshift and less one of absolute radio power, we suggest that it is caused by the evolution of the nuclear fueling rate with cosmic epoch. In order to quantify the deviation from the purely aspect-dependent unied scheme at low redshifts a larger fraction of 3C radio galaxies has to be observed at mid- to far-infrared wavelengths with sensitivities which suce to yield secure detections rather than upper limits.

The relation of PAH strength with cold dust in galaxies

Mid-infrared spectra and submillimetre maps are investigated for ve galaxies covering a range of star forming activity. We nd a good spatial coincidence between the 850m continuum emission and the strength of the PAH 7.7m line. The PAH 7.7m peak to 850m flux ratio lies in the range around 2 with a moderate dispersion across the galaxies. Both PAH and cold dust emission correlate also with the emission from very small grains at 14.3m, but regions with starbursts show an excess in the very small grain emission. This suggests that the PAH carriers are preferentially related to the regions dominated by cold dust and molecular clouds, where they are excited mainly by the interstellar radiation eld. The lack of increased PAH/submm ratio in starburst knots suggests that starbursts play a minor role for powering the PAH emission in galaxies.

On the Far-Infrared Emission of Quasars*

Infrared spectral energy distributions between 4.8 and 200 μm are presented for a subsample of 10 quasars and radio galaxies from the European Central Quasar Programme observed with the Infrared Space Observatory. For three of the sources, this represents the first far-infrared detection. The spectral energy distributions (SEDs) reveal signatures of thermal dust as well as synchrotron emission. In most cases, one of the two components is so dominant that the other remains hidden. The SEDs of the radio-quiet and steep-spectrum quasars show a bump around 60 μm and a decline longward of 100 μm—strong evidence for thermal emission. It can be described as a superposition of several modified blackbodies, showing the broad variety of temperatures from hot (≈ 600 K) to cool (≈ 30 K) dust present in these objects. The infrared emission of the flat-spectrum radio quasars is in smooth continuation of the radio spectrum, supporting the interpretation as synchrotron emission. For one of these sources (3C 279), a bump is prying above the synchrotron spectrum, and we interpret it as thermal emission. For comparison within the framework of unified schemes, the radio galaxies Cyg A (3C 405) and 3C 20 are observed. While 3C 20 has not been detected, Cyg A reveals an infrared spectrum surprisingly similar to that of 3C 48 and the radio-quiet quasars, which strongly supports the interpretation of the radio galaxy Cyg A as a hidden quasar.

SPECTRAL ENERGY DISTRIBUTIONS OF QSOs AT z > 5: COMMON ACTIVE GALACTIC NUCLEUS-HEATED DUST AND OCCASIONALLY STRONG STAR-FORMATION

We present spectral energy distributions (SEDs) of 69 QSOs at z > 5, covering a rest frame wavelength range of 0.1 μm to ~80 μm, and centered on new Spitzer and Herschel observations. The detection rate of the QSOs with Spitzer is very high (97% at λrest 4 μm), but drops toward the Herschel bands with 30% detected in PACS (rest frame mid-infrared) and 15% additionally in the SPIRE (rest frame far-infrared; FIR). We perform multi-component SED fits for Herschel-detected objects and confirm that to match the observed SEDs, a clumpy torus model needs to be complemented by a hot (~1300 K) component and, in cases with prominent FIR emission, also by a cold (~50 K) component. In the FIR-detected cases the luminosity of the cold component is of the order of 1013 L ☉ which is likely heated by star formation. From the SED fits we also determine that the active galactic nucleus (AGN) dust-to-accretion disk luminosity ratio declines with UV/optical luminosity. Emission from hot (~1300 K) dust is common in our sample, showing that nuclear dust is ubiquitous in luminous QSOs out to redshift 6. However, about 15% of the objects appear under-luminous in the near infrared compared to their optical emission and seem to be deficient in (but not devoid of) hot dust. Within our full sample, the QSOs detected with Herschel are found at the high luminosity end in L UV/opt and L NIR and show low equivalent widths (EWs) in Hα and in Lyα. In the distribution of Hα EWs, as determined from the Spitzer photometry, the high-redshift QSOs show little difference to low-redshift AGN.

The Small Magellanic Cloud in the far infrared. II. Global properties

We present global IR properties of the Small Magellanic Cloud using an ISOPHOT 170 µm map and re-visited IRAS data. The integrated 170 µm flux is 14 950 ± 2300 Jy, and the integrated IR flux is F1−1000 = 7.48 × 10 −10 Wm −2 . Discrete sources contribute 28%, 29%, and 36% to the integrated flux at 60 µm, 100 µm, and 170 µm, respectively. Most of the total emission arises from diffuse emission regions surrounding the HII regions. The SED can be modelled by the composition of a 45 K, a 20.5 K and a 10 K blackbody component with emissivity index n = 2. A color temperature map is constructed from the ISOPHOT 170 µm and the IRAS 100 µm HiRes maps. The average dust temperature is TD = 20.3 K. The total dust mass is found to be MD = 7.8 × 10 5 M� , yielding a gas-to-dust mass ratio Mgas/Mdust ≈ 540, a value 25 times lower than found by former studies. The global star formation rate is estimated to SFRtotal ≈ 0.05 Myr −1 . The comparison with other irregular