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The VLT Moves into the Infrared

A Selection of ISAAC Spectra

27 februari 1999

Various observations were made with ISAAC at the Nasmyth focus of VLT UT1 during the recent commissioning periods for this infrared multi-mode instrument. Some of the first results from the VLT Infrared Spectrometer And Array Camera (ISAAC) were published earlier. The following photos illustrate the type of front-line work that is now possible with ISAAC. The possibility to obtain high-quality infrared spectra of even quite faint (and remote) objects is particularly interesting.

The Birth of a Supernova Remnant

The prominent ring consists of material blown off the progenitor star during its evolution. It was illuminated by the UV radiation emerging from the supernova explosion and has been recombining for the past 12 years. Travelling at 30,000 km/s (which corresponds to the fastest material seen in the explosion itself), the shock front from the supernova has only now reached the ring, located 1 light-year away from the exploding star, and has re-ionized it. The brighter region corresponds to the material closest to us which has a shorter light travel time to earth (and the effect is therefore seen first in this region).

Of most interest in this image is the prominent jet like structure (to the left of the ring) in the dispersion direction which reveals the presence of a broad, blueshifted, HeI component that presumably originates in the shock ionized ejecta.

These data were taken with the medium resolution grating centred close to the HeI spectral line at 1.083 µm. The total exposure time was 20 minutes and the pixels are 0.046 nm wide in the dispersion direction (horizontal) and 0.147 arcsec along the slit (vertical).

Star Forming Galaxies at z ~ 0.62

The faint, horizontal white lines show their continuum emission (from stars) while the brighter regions close to the centre are due to H-alpha line emission from gas that has been photoionized by recently formed hot stars. The measured intensity of these features can be used to estimate the rate of star formation in the galaxies.

The line emission in the lower, brighter, galaxy can also be seen to be tilted, relative to the continuum. This is due to and allows an estimate of the rotation speed of the galaxy and hence its mass . A second emission line from [NII] (ionized nitrogen) is also visible to the right.

The dark 'reflections' above and below the centre result from the method used to subtract the sky emission while the vertical bands correspond to increased noise at the position of bright sky lines from the OH radical in the earth's atmosphere. Between these lines, the sky is almost as dark from the ground as in space.

The lower panel shows the plot of intensity versus wavelength, extracted for the brighter of the two galaxies. The redshifted H-alpha emission line is seen as the broad feature at the centre.

A z = 2.4 Radio Galaxy with Companions ?

The fact that these, and other very red galaxies, are found so close to the radio galaxy on the sky has led to speculation that they may be members of a cluster at the same redshift.

The radio galaxy spectrum is characterized by a number of bright "blobs" which are due to the emission lines [O II] (from ionized oxygen), H-beta (hydrogen), [O III] (double ionized oxygen), [O I] (atomic oxygen), Halpha and [S II] (ionized sulphur) whose wavelengths fall in the visible spectrum of objects at rest, but are shifted into the infrared in this case due to the high recession speed of the radio galaxy.

These lines can be seen more clearly in the extracted spectrum shown in the lower panel. They confirm the high redshift of the radio galaxy and provide more detailed information about its nature. The fact that neither of the two very red objects shows any of these emission lines appears to exclude the possibility that they are active or star-forming galaxies at the same redshift, but still leaves their actual nature and distance unclear.

These spectra were obtained using the low resolution spectroscopic mode and with total integration times of 2 hours in the J band (right) and 1 hour in each of the H (middle) and K (left) bands.

Mer information

The first image displays a "spectral image" of SN1987A, the supernova that exploded 12 years ago in the Large Magellanic Cloud. It was obtained using the medium resolution spectroscopic mode of ISAAC with a 2 arcsec wide slit and the grating centred close to the HeI emission line at 1.083 µm (1083 nm). The orange bands above and below the SN are due to stars.

The upper panel of the second image shows medium resolution (R ~ 3000) spectral images around 1.06 µm of two galaxies from the Canada France Redshift Survey catalogue. Their redshifts are z ~ 0.62 and they are only a few arcsecs apart on the sky.

The third image is a montage of low resolution (R ~ 500) spectra that cover most of the 1 - 2.5 µm range of the radio galaxy MRC0406-244, as well as two extremely red (colour index R-K > 5) galaxies below and an anonymous galaxy above.

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