arXiv subjects

P. Gallais

Publications and source records attributed to P. Gallais.

23 records · Page 2Linked to original sources

arXiv1999

The ISOCAM/LW Detector Dark Current Behaviour

We describe the calibration, measurements and data reduction, of the dark current of the ISOCAM/LW detector. We point-out the existence of two significant drifts of the LW dark-current, one throughout the ISO mission, on a timescale of days, another within each single revolution, on a timescale of hours. We also show the existence of a dependence of the dark current on the temperature of the ISOCAM detector. By characterizing all these effects through polynomial fittings, we build a model for the LW calibration dark, that depends on the epoch of observation (parametrized with the revolution number and the time elapsed in that given revolution since the activation) and on the temperature of the ISOCAM detector. The model parameters are tuned for each of ISOCAM/LW pixel. We show that the modelling is very effective in taking into account the dark-current variations and allows a much cleaner dark subtraction than using a brute average of several calibration dark images. The residuals of the LW model-dark subtraction are, on average, similar to the pre-launch expectation.

astro-ph↗

arXiv1999

Mid-IR imaging of Toomre's Merger Sequence

We present results on mid-IR (5--16micron) spectral imaging of a sequence of interacting galaxies, observed by ISOCAM. The galaxies are part of the well known Toomre's ``merger sequence'' which was defined as a sample of galaxies depicting progressive snapshots in the time evolution of a merging event. To trace the intensity of the radiation field in a starburst, we use the ratio of the 15 to 7 micron flux. Our analysis indicates that this ratio increases as galaxies move from the pre-starburst to the starburst phase and goes again down to ~1 in the post-starburst phase, a value typical of normal star forming regions in galactic disks. Moreover, we find that the variation of this ratio is well correlated with the one of the IRAS 25micron/12micron and 60micron/100micron flux ratios.

astro-ph↗

arXiv1998

A barred spiral at the centre of the giant elliptical radio galaxy Centaurus A

We report observations at mid-infrared and sub-millimeter wavelengths of Centaurus A (CenA, NGC 5128), the giant elliptical galaxy that harbors the closest radio loud Active Galactic Nucleus (AGN) to Earth. The dust emission from the deep interior of CenA reveals a bisymmetric structure with a diameter of 5' (5 kpc), centred at the AGN. This structure is remarkably similar to that of a barred spiral galaxy, with the bar lying in a plane that is tilted ~18 degrees from the line of sight. The true nature of the distribution of dust in the inner regions of CenA is noticeably displaced from the more chaotic and widespread optical obscuration. The barred spiral is a quasi-stable structure formed at the center of the giant elliptical from the tidal debris of a gas-rich object(s) accreted in the past 10^9 years. The total size and mass of interstellar gas in the barred spiral at the center of CenA is comparable to the small Local Group spiral galaxy Messier 33. The observation of this remarkable structure opens the more general question on whether the dusty hosts of giant radio galaxies like CenA, are "symbiotic" galaxies composed of a barred spiral inside an elliptical, where the bar serves to funnel gas toward the AGN.

astro-ph↗

arXiv1998

Dust in the Wheel: The Cartwheel Galaxy in the Mid-IR

We present mid-infrared images at $6.7 μm$ and $15 μm$ of ``The Cartwheel'' (AM~0035-33), the prototypical collisional ring galaxy. The observations, taken with ISOCAM, reveal the distribution of hot dust in the galaxy and its two companions in the north-east. The intensity of the mid-IR emission from the outer star forming ring of the Cartwheel shows considerable azimuthal variation and peaks at the most active H$α$ region of the ring. The 15 $μm$ to 6.7 $μm$ flux ratio of 5.2 is the highest among all the galaxies of our sample. A surprising result of our observations is the discovery of significant emission from the inner regions of the galaxy, including the inner ring, spokes and nucleus, where previously only low level H$α$ emission had been reported. At 6.7 $μm$, this emission is stronger than the one from the outer star forming ring, and at 15 $μm$, it represents 40% of the emission from the outer ring. The H$α$ to mid-IR flux ratios from the inner regions are consistent with the heating of grains from weak star formation activity.

astro-ph↗

arXiv1998

The dark side of star formation in the Antennae galaxies

We compare mid-infrared images of the Antennae galaxies (NGC 4038/39) from the Infrared Space Observatory, with optical images from the Hubble Space Telescope. The mid-infrared observations show that the most intense starburst in this colliding system of galaxies takes place in an off-nucleus reg ion that is inconspicuous at optical wavelengths. The analyses of the mid-infrar ed spectra indicate that the most massive stars are being formed in an optically obscured knot of 50 pc radius, which produces about 15% of the total luminosity from the Antennae galaxies between 12.5 microns and 18 microns. The mid-infrared observations reported here demonstrate that the interpretation of star formation properties in colliding/merging systems based on visible wavelengths alone can be profoundly biased due to dust obscuration. The multiwavelength view of this nearby prototype merging system suggests caution in deriving scenarios of early evolution of high redshift galaxies using only observations in the narrow rest-frame ultraviolet wavelength range.

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