Showing papers by "Dietrich Lemke published in 2002"

Far-infrared and molecular line observations of Lynds 183 - Studies of cold gas and dust

Abstract: We have mapped the dark cloud L183 in the far-infrared at 100 m and 200 m with the ISOPHOT photometer aboard the ISO satellite. The observations make it possible for the rst time to study the distribution and properties of the large dust grains in L183 without confusion from smaller grains. The observations show clear colour temperature variations which are likely to be caused by changes in the emission properties of the dust particles. In the cloud core the far-infrared colour temperature drops below 12 K. The data allow a new determination of the cloud mass and the mass distribution based on dust emission. The estimated mass within ar adius of 10 0 from the cloud centre is 25M. We have mapped the cloud in several molecular lines including DCO+(2{1) and H 13 CO+(1{0). These species are believed to be tracers of cold and dense molecular material and we detect a strong anticorrelation between the DCO+ emission and the dust colour temperatures. In particular, the DCO+(2{1) emission is not detected towards the maximum of the 100 m emission where the colour temperature rises above 15 K. The H 13 CO+ emission follows closely the DCO+ distribution but CO isotopes show strong emission even towards the 100m peak. Detailed comparison of the DCO+ and C 18 O maps shows sharp variations in the relative intensities of the species. Morphologically the 200 m dust emission traces the distribution of dense molecular material as seen e.g. in C 18 O lines. A comparison with dust column density shows, however, that C 18 O is depleted by a factor of1.5 in the cloud core. We present results of R -a ndB-band starcounts. The extinction is much better correlated with the 200 mt han with the 100m emission. Based on the 200m correlation at low extinction values we deduce a value of 17 m for the visual extinction towards the cloud centre where no background stars are observed anymore.

Far-infrared emission from intracluster dust in Abell clusters

Abstract: The ISOPHOT instrument aboard ISO has been used to observe extended FIR emission of six Abell clusters. Strip scanning measurements with crossing position angles centered on the clusters were carried out at 120 $\mathrm{\mu m}$ and 180 $\mathrm{\mu m}$. The raw profiles of the $I_{\rm 120 ~\mu m} / I_{\rm 180~\mu m}$ surface brightness ratio including zodiacal light show a bump towards Abell 1656 (Coma), dips towards Abell 262 and Abell 2670, and are without clear structure towards Abell 400, Abell 496, and Abell 4038. After subtraction of the zodiacal light, the bump towards Abell 1656 is still present, while the dips towards Abell 262 and Abell 2670 are no longer noticable. This indicates a localized excess of emitting material outside the Galaxy towards Abell 1656 with properties different from the galactic foreground cirrus, while the behavior in Abell 262 and Abell 2670 can be reconciled with galactic cirrus structures localized on the line-of-sight to these clusters. The excess of ≈ 0.2 MJy/sr seen at 120 $\mathrm{\mu m}$ towards Abell 1656 (Coma) is interpreted as being due to thermal emission from intracluster dust distributed in the hot X-ray emitting intracluster medium. The integrated excess flux within the central region of 10 ′–15 ′ diameter is ≈ 2.8 Jy. Since the dust temperature is poorly constrained, only a rough estimate of the associated dust mass of ${M_{\rm D} \approx 10^{7}\,M_{\odot}}$ can be derived. The associated visual extinction is negligible ($A_{V} \ll 0.1 ~{\rm mag}$) and much smaller than claimed from optical observations. No evidence is found for intracluster dust in the other five clusters observed. The absence of any signature for intracluster dust in five clusters and the rather low inferred dust mass in Abell 1656 indicates that intracluster dust is likely not responsible for the excess X-ray absorption seen in cooling flow clusters. These observations thereby represent a further unsuccessful attempt in detecting the presumed final stage of the cooling flow material, in accord with quite a number of previous studies in other wavelengths regions. Finally, the observed dimming of the high-redshift supernovae is unlikely be attributable to extinction caused by dust in the intracluster or even a presumed intercluster medium.

Mid-infrared spectrum of the zodiacal light observed with ISOPHOT

Abstract: We present 29 mid-infrared spectra of the zodiacal light distributed over the sky. The observed 5.9-11.7m spectral shapes are well represented by blackbody radiation with colour temperatures in the range of 255 T 300 K. The spectra are smooth and featureless. The variation of the temperature can be explained by the geometrical distribution of dust in the inner solar system. This result indicates that although the interplanetary dust particles originate from discrete sources (comets, asteroids) the interplanetary cloud of today seems to be well mixed in terms of grain composition and size distribution.

An Infrared Space Observatory Atlas of Bright Spiral Galaxies

Abstract: In this first paper in a series we present an atlas of infrared images and photometry from 1.2 to 180 μm for a sample of bright spiral galaxies. The atlas galaxies are an optically selected, magnitude-limited sample of 77 spiral and S0 galaxies chosen from the Revised Shapley-Ames Catalog (RSA). The sample is a representative sample of spiral galaxies and includes Seyfert galaxies, LINERs, interacting galaxies, and peculiar galaxies. Using the Infrared Space Observatory (ISO), we have obtained 12 μm images and photometry at 60, 100, and 180 μm for the galaxies. In addition to its imaging capabilities, ISO provides substantially better angular resolution than is available in the IRAS survey, and this permits discrimination between infrared activity in the central regions and global infrared emission in the disks of these galaxies. These ISO data have been supplemented with JHK imaging using ground-based telescopes. The atlas includes 2 and 12 μm images. Following an analysis of the properties of the galaxies, we have compared the mid-infrared and far-infrared ISO photometry with IRAS photometry. The systematic differences we find between the IRAS Faint Source Catalog and ISO measurements are directly related to the spatial extent of the ISO fluxes, and we discuss the reliability of IRAS Faint Source Catalog total flux densities and flux ratios for nearby galaxies. In our analysis of the 12 μm morphological features we find that most but not all galaxies have bright nuclear emission. We find 12 μm structures such as rings, spiral arm fragments, knotted spiral arms, and bright sources in the disks that are sometimes brighter than the nuclei at mid-infrared wavelengths. These features, which are presumably associated with extranuclear star formation, are common in the disks of Sb and later galaxies but are relatively unimportant in S0–Sab galaxies.

Small-scale structure of the galactic cirrus emission

Abstract: We examined the Fourier power spectrum characteristics of cirrus structures in 13 sky fields with faint to bright cirrus emission observed with ISOPHOT in the 90--200µm wavelength range in order to study variations of the spectral index $\alpha$. We found that $\alpha$ varies from field to field with --5.3 $\le$ $\alpha$ $\le$ --2.1. It depends on the absolute surface brightness and on the hydrogen column density. We also found different spectral indices for the same sky region at different wavelengths. Longer wavelength measurements show steeper power spectra. This can be explained by the presence of dust at various temperatures, in particular of a cold extended component. For the faintest areas of the far-infrared sky we derive a wavelength independent spectral index of $\alpha$ = --2.3$\pm$0.6 for the cirrus power spectrum. The application of the correct spectral index is a precondition for the proper disentanglement of the cirrus foreground component of the Cosmic Far-Infrared Background and its fluctuations.

Isophot - photometric calibration of point sources

Abstract: All observations by the aperture photometer (PHT-P) and the far-infrared (FIR) camera section of ISOPHOT included reference measurements against stable internal fine calibration sources (FCS) to correct for temporal drifts in detector responsivities. The FCSs were absolutely calibrated in-orbit against stars, asteroids and planets, covering wavelengths from 3.2 to 240 μm. We present the calibration concept for point sources within a flux-range from 60 mJy up to 4500 Jy for staring and raster observations in standard configurations and discuss the requisite measurements and the uncertainties involved. In this process we correct for instrumental effects like nonlinearities, signal transients, time variable dark current, misalignments and diffraction effects. A set of formulae is developed that describes the calibration from signal level to flux densities. The scatter of 10 to 20% of the individual data points around the derived calibration relations is a measure of the consistency and typical accuracy of the calibration. The reproducibility over longer periods of time is better than 10%. The calibration tables and algorithms have been implemented in the final versions of the software for offline processing and interactive analysis.

Detection of solid ammonia, methanol, and methane with ISOPHOT

Abstract: We report on a search for the NH3 inversion band at 9.0m, the CH4 band at 7.7m, and the CH3OH band at 9.7m in the spectra of 10 heavily obscured stellar objects taken with the spectrophotometer ISOPHOT-S aboard the Infrared Space Observatory (ISO). Four of them show indications of the NH3 inversion band at 9.0m superimposed on the always strong silicate band. The results show that ammonia ice may be abundant in some cool and dense molecular circumstellar regions. The 9.7m band of solid CH3OH was detected in 5 of the sources, and also the 7.7m band of solid CH4 was found in 5 sources. The PHT-S detections were cross-checked against ISO-SWS spectra from archive data. Considering the dierent beam sizes (24 24u 00 versus 14 20u 00 ), there is good agreement.

Star Formation in the Infrared Space Observatory Atlas of Bright Spiral Galaxies

Abstract: We investigate star formation along the Hubble sequence using the Infrared Space Observatory Atlas of Bright Spiral Galaxies. Using mid-infrared and far-infrared flux densities normalized by K-band flux densities as indicators of recent star formation, we find several trends. First, star formation activity is stronger in late-type (Sc–Scd) spirals than in early-type (Sa–Sab) spirals. This trend is seen both in nuclear and disk activity. These results confirm several previous optical studies of star formation along the Hubble sequence but conflict with the conclusions of most of the previous studies using IRAS data, and we discuss why this might be so. Second, star formation is significantly more extended in later type spirals than in early-type spirals. We suggest that these trends in star formation are a result of differences in the gas content and its distribution along the Hubble sequence, and it is these differences that promote star formation in late-type spiral galaxies. We also search for trends in nuclear star formation related to the presence of a bar or nuclear activity. The nuclear star formation activity is not significantly different between barred and unbarred galaxies. We do find that star formation activity appears to be inhibited in low ionization nuclear emission regions and transition objects compared with H II galaxies. The mean star formation rate in the sample is 1.4 M⊙ yr-1, based on global far-infrared fluxes. Combining these data with CO data gives a mean gas consumption time of 6.4 × 108 yr, which is ~5 times lower than the values found in other studies. Finally, we find excellent support for the Schmidt law in the correlation between molecular gas masses and recent star formation in this sample of spiral galaxies.

Star Formation in the ISO Atlas of Spiral Galaxies

Abstract: We investigate star formation along the Hubble sequence using the ISO Atlas of Spiral Galaxies. Using mid-infrared and far-infrared flux densities normalized by K-band flux densities as indicators of recent star formation, we find several trends. First, star formation activity is stronger in late-type (Sc - Scd) spirals than in early-type (Sa - Sab) spirals. This trend is seen both in nuclear and disk activity. These results confirm several previous optical studies of star formation along the Hubble sequence but conflict with the conclusions of most of the previous studies using IRAS data, and we discuss why this might be so. Second, star formation is significantly more extended in later-type spirals than in early-type spirals. We suggest that these trends in star formation are a result of differences in the gas content and its distribution along the Hubble sequence, and it is these differences that promote star formation in late-type spiral galaxies. We also search for trends in nuclear star formation related to the presence of a bar or nuclear activity. The nuclear star formation activity is not significantly different between barred and unbarred galaxies. We do find that star formation activity appears to be inhibited in LINERs and transition objects compared to HII galaxies. The mean star formation rate in the sample is 1.4 Msun/yr based on global far-infrared fluxes. Combining these data with CO data gives a mean gas consumption time of 6.4 x 10^8 yr, which is ~5 times lower than the values found in other studies. Finally, we find excellent support for the Schmidt Law in the correlation between molecular gas masses and recent star formation in this sample of spiral galaxies.

Far infrared observations of pre-protostellar sources in Lynds 183

Abstract: Using ISOPHOT maps at 100 and 200um and raster scans at 100, 120, 150 and 200um we have detected four unresolved far-infrared sources in the high latitude molecular cloud L183. Two of the sources are identified with 1.3mm continuum sources found by Ward-Thompson et al. and are located near the temperature minimum and the coincident column density maximum of dust distribution. For these two sources, the ISO observations have enabled us to derive temperatures (about 8.3 K) and masses (about 1.4 and 2.4 solar masses). They are found to have masses greater than or comparable to their virial masses and are thus expected to undergo gravitational collapse. We classify them as pre-protostellar sources. The two new sources are good candidates for pre-protostellar sources or protostars within L183.

A very young star forming region detected by the ISOPHOT Serendipity Survey

Abstract: We present a multi-wavelength study of the star forming region ISOSS J 20298+3559, which was identified by a cross-correlation of cold compact sources from the 170 micron ISOPHOT Serendipity Survey (ISOSS) database coinciding with objects detected by the MSX, 2MASS and IRAS infrared surveys. ISOSS J 20298+3559 is associated with a massive dark cloud complex (M ~ 760 M$_{\odot}$) and located in the Cygnus X giant molecular cloud. We derive a distance of 1800 pc on the basis of optical extinction data. The low average dust temperature (T ~ 16 K) and large mass (M ~ 120 M$_{\odot}$) of the dense inner part of the cloud, which has not been dispersed, indicates a recent begin of star formation. The youth of the region is supported by the early evolutionary stage of several pre- and protostellar objects discovered across the regio n: I) Two candidate Class 0 objects with masses of 8 and 3.5 M$_{\odot}$, II) a gravitationally bound, cold (T ~ 12 K) and dense (n(H$_{2}$) \~ 2 x 10$^{5}$ cm$^{-3}$) cloud core with a mass of 50 M$_{\odot}$ and III) a Herbig B2 star with a mass of 6.5 M$_{\odot}$ and a bolometric luminosity of 2200 L$_{\odot}$, showing evidence for ongoing accretion and a stellar age of less than 40000 years. The dereddened SED of the Herbig star is well reproduced by an accretion disc + star model. The externally heated cold cloud core is a good candidate for a massive pre-protostellar object. The star formation efficiency in the central cloud region is about 14 %.

Multi-wavelength data mining of the ISOPHOT Serendipity Sky Survey

Abstract: The ISOPHOT C200 stressed Ge:Ga array aboard the Infrared Space Observatory was used to carry out scientific observations while the telescope was moved from one target to the next. These strip scanning measurements of the sky in the far-infrared (FIR) at 170 μm comprise the ISOPHOT Serendipity Survey, the first slew survey designed as an integral part of a space observatory mission. The ISOPHOT Serendipity Survey is the only large scale sky survey in the unexplored wavelength region beyond the IRAS 100 μm limit to date. Within nearly 550 hours more than 12000 slew measurements with a total slew length of more than 150000 degrees were collected, corresponding to a sky coverage of about 15%. The slew data analysis has been focused on the detection of compact sources, which required the development of special algorithms. A severe problem at 170 μm is the confusion of genuine compact sources with foreground galactic cirrus knots and ridges. The selection and identification of objects therefore necessarily requires a multi-wavelengths approach, which makes use of a broad variety of additional data from databases and other surveys. Known galaxies were identified by cross-correlating the Serendipity Survey source positions with galaxy entries in the NED and Simbad databases and a subsequent cross-check of optical images from the Digital Sky Survey. A large catalogue with 170 μm fluxes for ≈2000 galaxies is being complied. The particularly interesting rare galaxies with very cold dust and very large dust masses further require additional FIR data from the IRAS survey as well as measured redshifts. A large fraction of the compact galactic structures are prestellar cores inside cold star forming regions. Early stages of medium and high mass star forming regions are identified by combining compact bright and cold Serendipity Survey sources with the near-infrared 2MASS and MSX surveys, the combination of which indicates large dust masses in conjunction with embedded young stars of early spectral types. In all the studied samples of different object classes the 170 μm flux provides the crucial data point for a complete characterization of the FIR spectral energy distributions and the derivation of total dust masses. Follow-up observations are underway to study selected objects in more detail.

Zodiacal light observations with the infrared space observatory

Abstract: ISO performed an extensive observing programme on the infrared zodiacal light, including multi-filter photometry of the global brightness distribution, observations of the asteroidal bands and cometary dust trails, and investigation of the small scale brightness fluctuation. The detailed determination of the spectral energy distribution of the zodiacal light, including mid-infrared spectrophotometry, opens the possibility for the separation of the main components of the infrared sky. The mid-infrared spectrum may provide information on the nature of the constituents, and on the size distribution of the interplanetary grains. Two years after the ISO mission we review the observations and summarize the results of ISOPHOT and ISOCAM.