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Register a new userArtifacts at 4.5 and 8.0 um in Short Wavelength Spectra from the Infrared Space Observatory
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Spectra from the Short Wavelength Spectrometer (SWS) on ISO exhibit artifacts at 4.5 and 8 um. These artifacts appear in spectra from a recent data release, OLP 10.0, as spurious broad emission features in the spectra of stars earlier than ~F0, such as alpha CMa. Comparison of absolutely calibrated spectra of standard stars to corresponding spectra from the SWS reveals that these artifacts result from an underestimation of the strength of the CO and SiO molecular bands in the spectra of sources used as calibrators by the SWS. Although OLP 10.0 was intended to be the final data release, these findings have led to an additional release addressing this issue, OLP 10.1, which corrects the artifacts.
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More than a decade ago the IRAS satellite opened the realm of external galaxies for studies in the 10 to 100 micron band and discovered emission from tens of thousands of normal and active galaxies. With the 1995-1998 mission of the Infrared Space Observatory the next major steps in extragalactic infrared astronomy became possible: detailed imaging, spectroscopy and spectro-photometry of many galaxies detected by IRAS, as well as deep surveys in the mid- and far- IR. The spectroscopic data reveal a wealth of detail about the nature of the energy source(s) and about the physical conditions in galaxies. ISOs surveys for the first time explore the infrared emission of distant, high-redshift galaxies. ISOs main theme in extragalactic astronomy is the role of star formation in the activity and evolution of galaxies.
We present results from observation, correlation and analysis of interferometric measurements between the three geodetic very long baseline interferometry (VLBI) stations at the Onsala Space Observatory. In total 25 sessions were observed in 2019 and 2020, most of them 24 hours long, all using X-band only. These involved the legacy VLBI station ONSALA60 and the Onsala twin telescopes, ONSA13NE and ONSA13SW, two broadband stations for the next generation geodetic VLBI global observing system (VGOS). We used two analysis packages: nuSolve to pre-process the data and solve ambiguities, and ASCOT to solve for station positions, including modelling gravitational deformation of the radio telescopes and other significant effects. We obtained weighted root mean square postfit residuals for each session on the order of 10-15 ps using group delays and 2-5 ps using phase delays. The best performance was achieved on the (rather short) baseline between the VGOS stations. As the main result of this work we determined the coordinates of the Onsala twin telescopes in VTRF2020b with sub-millimeter precision. This new set of coordinates should be used from now on for scheduling, correlation, as a~priori for data analyses, and for comparison with classical local-tie techniques. Finally, we find that positions estimated from phase-delays are offset $sim+3$ mm in the Up-component with respect to group-delays. Additional modelling of (elevation-dependent) effects may contribute to future understanding of this offset.
The Infrared Space Observatory observed over 900 objects with the Short Wavelength Spectrometer in full-grating-scan mode (2.4-45.2 micron). We have developed a comprehensive system of spectral classification using these data. Sources are assigned to groups based on the overall shape of the spectral energy distribution (SED). The groups include naked stars, dusty stars, warm dust shells, cool dust shells, very red sources, and sources with emission lines but no detected continuum. These groups are further divided into subgroups based on spectral features that shape the SED such as silicate or carbon-rich dust emission, silicate absorption, ice absorption, and fine-structure or recombination lines. Caveats regarding the data and data reduction, and biases intrinsic to the database, are discussed. We also examine how the subgroups relate to the evolution of sources to and from the main sequence and how this classification scheme relates to previous systems.
We determine and tabulate A(lambda)/A(K), the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25um<lambda<8.0um along two lines of sight: l~42 and l~284. The first is a relatively quiescent and unremarkable region; the second contains the giant HII region RCW 49 as well as a ``field region unrelated to the cluster and nebulosity. Areas near these Galactic longitudes were imaged at J, H, and K bands by 2MASS and at 3--8um by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A(lambda)-A(K))/(A(J)-A(K)) directly from the color distributions of observed stars. The extinction ratio between two of the filters, e.g. A(J)/A(K), is required to calculate A(lambda)/A(K) from those measured ratios. We use the apparent JHK magnitudes of giant stars along our two sightlines, and fit the reddening as a function of magnitude (distance) to determine A(J)/kpc, A(K)/kpc, and A(J)/A(K). Our values of A(lambda)/A(K) show a flattening across the 3--8um wavelength range, roughly consistent with the Lutz 1996 extinction measurements derived for the sightline toward the Galactic center.
High-redshift quasars are important to study galaxy and active galactic nuclei (AGN) evolution, test cosmological models, and study supermassive black hole growth. Optical searches for high-redshift sources have been very successful, but radio searches are not hampered by dust obscuration and should be more effective at finding sources at even higher redshifts. Identifying high-redshift sources based on radio data is, however, not trivial. Here we report on new multi-frequency Giant Metrewave Radio Telescope (GMRT) observations of eight z>4.5 sources previously studied at high angular resolution with very long baseline interferometry (VLBI). Combining these observations with those from the literature, we construct broad-band radio spectra of all 30 z>4.5 sources that have been observed with VLBI. In the sample we found flat, steep and peaked spectra in approximately equal proportions. Despite several selection effects, we conclude that the z>4.5 VLBI (and likely also non-VLBI) sources have diverse spectra and that only about a quarter of the sources in the sample have flat spectra. Previously, the majority of high-redshift radio sources were identified based on their ultra-steep spectra (USS). Recently a new method has been proposed to identify these objects based on their megahertz-peaked spectra (MPS). Neither method would have identified more than 18% of the high-redshift sources in this sample. More effective methods are necessary to reliably identify complete samples of high-redshift sources based on radio data.
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