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New Photos from FORS at the VLT UT1

26 Novembre 1998

Following the installation and first commissioning period of FORS1 at the first 8.2-m VLT Unit Telescope (UT1), the FORS Team is now busy reducing the wealth of data obtained during this first series of test observations. Apart from allowing the assessment of the technical performance of the instrument, a number of images can also be used for extracting information of scientific interest.

Some of the new deep images from FORS1 are presented here together with a spectrum of a distant quasar.

Abell 370 Cluster of Galaxies with Gravitational Arcs

eso9857c shows a composite colour image of the Abell 370 cluster of galaxies . This cluster hosts one of the most prominent gravitational arcs known to date.

The light of a very distant object is deformed by the foreground cluster. Abell 370, at the center of the photo, contains a large number of galaxies - most of them are ellipticals. The two brightest red galaxies slightly above the gravitational arc are the most massive in the cluster and give an important contribution to the lensing event.

This gravitational arc - in combination with the cluster of galaxies - provides a natural laboratory to study the physics of gravity. The deformation of "space-time" by the large mass of Abell 370 can be derived and modeled from the slope of the gravitational arc. The exact shape of the arc allows to estimate the mass of the cluster through the equations of general relativity. This estimate agrees with independent determinations from the X-ray emission and the motion of the galaxies in the cluster.

FORS Deep Field near Q0103-260

eso9857d During the commissioning of FORS1, "deep fields" were observed in order to estimate the limiting magnitudes of the new instrument. Fields were selected which do not contain bright stars that might reduce the image quality due to scattered light.

This FORS "deep field" hosts the quasar Q0103-260 which was also investigated spectroscopically (see ESO Press Photo eso9857e below). In this image, the quasar is visible as a bright white source near the bottom, to the lower left (south-east) of the prominent spiral galaxy.

This field has a very low intergalactic extinction (absorption of light by intervening matter) and therefore allows a very deep look into the distant Universe. A preliminary reduction shows that the galaxy sample is complete down to about magnitude 26.5 in R (red) and 27.5 in the B (blue) band.

The shown field is crowded with several thousands of galaxies of different types and shapes. Their colours will allow an estimate of their distances, with blue objects being closest. Among the reddest objects, it is possible to find candidates of very distant, possibly primeval, galaxies, which will be further investigated spectroscopically with FORS.

Spectrum of Quasar Q0103-260

eso9857e displays a spectrum of the distant quasar Q0103-260 (visual magnitude V = 18.8 , redshift z = 3.36 ) with a prominent emission line at 5300 Å (530 nm), identified as the Lyman-alpha resonance line of atomic hydrogen, redshifted from 1216 Å to 5300 Å.

To the left of the Lyman-alpha emission line, the "Lyman-alpha forest" is clearly seen; it is formed by numerous Lyman-alpha absorption lines of hydrogen clouds of lower redshift and located between the quasar and us. Q0103-260 is a particularly interesting object for studying the Lyman forest, since this quasar is in a region close to the south galactic pole (i.e. the direction perpendicular to the Milky Way plane) with its exceptionally low density of foreground stars and foreground galaxies. It will accordingly be possible to obtain with FORS very deep images and spectra of faint very distant objects in the field around this quasar and thus to search for objects which may cause the hydrogen absorption features in the quasar spectrum.

Ulteriori Informazioni

Technical Information:

• The photo is a combination of three exposures through R- (red; centered at 657 nm and with FWHM 150 nm), B- (blue; 429 nm and FWHM 88 nm) and U-band (ultraviolet; 366 nm and FWHM 36 nm) filters and with exposure times of 240, 300 and 500 sec, respectively. Only one quarter (1050 x 1050 pix) of the full image is displayed here. The seeing was 0.9 arcsec in R and B, and 1.1 arcsec in the U-band filter. The field measures 3.5 x 3.5 arcmin with North at the top and East to the left.

• Technical Information: This image consists of three exposures in filters I (near-infrared; centered at 768 nm and with FWHM 140 nm), R and B with exposure times 2700, 1200 and 2200 seconds, respectively. The field shown corresponds to one quarter (1050 x1050 pix) of the full image. The seeing was 0.8 arcsec in I and R, and 0.9 arc in B. Contrary to the previous image, the intensity scale here represents the square root of the flux. The field measures 3.5 x 3.5 arcmin with North at the top and East to the left.

• Technical Information: This spectrum is based on a 30 min exposure, taken with a 1.0 arcsec slit and a grism with 600 lines/mm blazed in the blue spectral range

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