Leif Gjesing Hansen

Deep Far Infrared ISOPHOT Survey in “Selected Area 57”

We present here the results of a deep survey in a 0.4 deg2 blank field in Selected Area 57 conducted with the ISOPHOT instrument aboard ESAs Infrared Space Observatory (ISO1) at both 60 μm and 90 μm. We describe our data reduction and analysis scheme with special emphasis on source extraction and flux calibration. With a formal signal to noise ratio of 6.75 we have source detection limits of 90 mJy at 60 μm and 50 mJy at 90 μm. To these limits we find cumulated number densities of 5±3.5 deg-2 at 60 μm and 14.8±5.0 deg-2 at 90 μm. These number densities of sources are found to be lower than previously reported results from ISO but the data do not allow us to discriminate between no-evolution scenarios and various evolutionary models.

A Deep X-ray Spectroscopic Survey of the ESO Imaging Survey Fields

The deepest ROSAT surveys have shown, that, in the energy range 0.5–2.0 keV, QSOs can account for ~ 30 per cent of the Diffuse X-ray Background (DXRB), and Narrow Emission Line Galaxies (NELG) and clusters of galaxies for about 10 per cent each. But, by assuming characteristic spectral behaviour, known from bright archetypes of these classes, the faint ROSAT sources will, due to the hardness of the DXRB spectrum, only give an insignificant contribution at say 10 keV. By exploiting the excellent capability of the SODART telescopes to obtain broad band x-ray spectroscopy at energies > 2 keV, we propose to perform a spectroscopic survey with SODART and push these telescopes to the limit at the high energies. By selecting the four ESO Imaging Survey fields EIS (each 6 deg2) we will take full advantage of the large, systematic effort ESO is putting into the optical survey of these fields. This spectroscopic x-ray survey will provide a large, statistically complete, sample of sources detected at high energies, more than an order of magnitude fainter than obtained by previous missions. The study of these sources will significantly improve our understanding not only of the origin of DXRB, but also provide new insight into the evolution of galaxies, clusters of galaxies and AGNs.

IUE observations of star formation in a cooling flow

Star formation in cooling flows is usually found to have an initial mass function deficient in massive stars, but the center of the cooling flow in Hydra A has been shown to contain a significant number of early type stars. Here we use UV-spectra obtained with the IUE satellite together with ground based data to constrain the nature of this young population. The data is compatible with a burst of star formation with a Salpeter-like initial mass function, a burst age of about 4 107 yr, and a total mass ≈ 108 M⊙.

A Spiral-Like Disk of Ionized Gas in IC 1459: Signature of a Merging Collision?

The authors report the discovery of a large (15 kpc diameter) H alpha + (NII) emission-line disk in the elliptical galaxy IC 1459, showing weak spiral structure. The line flux peaks strongly at the nucleus and is more concentrated than the stellar continuum. The major axis of the disk of ionized gas coincides with that of the stellar body of the galaxy. The mass of the ionized gas is estimated to be approx. 1 times 10 (exp 5) solar mass, less than 1 percent of the total mass of gas present in IC 1459. The total gas mass of 4 times 10(exp 7) solar mass has been estimated from the dust mass derived from a broad-band color index image and the Infrared Astronomy Satellite (IRAS) data. The authors speculate that the presence of dust and gas in IC 1459 is a signature of a merger event.

Diffuse Lyα Emission around the cD Galaxy in A1795

We report briefly the discovery of diffuse Lyα emission coinciding with the optical filaments around the cD galaxy in the X-ray cluster A1795. The derived Lyα/Hβ ratio is close to the recombination value. From this ratio we find that the content of smoothly distributed HI is less than 100 solar masses. The non-detection of a stellar UV continuum implies that the formation rate of stars more massive than 3M⊙ is less than 1 per cent of the accretion rate. Combining UV, optical and radio data for the nonthermal nuclear source we find α ~.8.