Watching the Birth of a Galaxy Cluster?

First Visiting Astronomers to VLT ANTU Observe the Early Universe

When the first 8.2-m VLT Unit Telescope (ANTU) was "handed over" to the scientists on April 1, 1999, the first "visiting astronomers" at Paranal were George Miley and Huub Rottgering from the Leiden Observatory (The Netherlands) [1].

They obtained unique pictures of a distant exploding galaxy known as 1138 - 262 . These images provide new information about how massive galaxies and clusters of galaxies may have formed in the early Universe.

Formation of clusters of galaxies

An intriguing question in modern astronomy is how the first galaxies and groupings or clusters of galaxies emerged from the primeval gas produced in the Big Bang. Some theories predict that giant galaxies, often found at the centres of rich galaxy clusters, are built up through a step-wise process. Clumps develop in this gas and stars condense out of those clumps to form small galaxies. Finally these small galaxies merge together to form larger units.

An enigmatic class of objects important for investigating such scenarios are galaxies which emit intense radio emission from explosions that occur deep in their nuclei. The explosions are believed to be triggered when material from the merging swarm of smaller galaxies is fed into a rotating black hole located in the central regions. There is strong evidence that these distant radio galaxies are amongst the oldest and most massive galaxies in the early Universe and are often located at the heart of rich clusters of galaxies.

They can therefore help pinpoint regions of the Universe in which large galaxies and clusters of galaxies are being formed.

The radio galaxy 1138-262

The first visiting astronomers pointed ANTU towards a particularly important radio galaxy named 1138-262 . It is located in the southern constellation Hydra (The Water Snake). This galaxy was discovered some years ago using ESO's 3.5-m New Technology Telescope (NTT) at La Silla.

Because 1138-262 is at a distance of about 10,000 million light-years from the Earth (the redshift is 2.2), the VLT sees it as it was when the Universe was only about 20% of its present age.

Previous observations of this galaxy by the same team of astronomers showed that its radio, X-ray and optical emission had many extreme characteristics that would be expected from a giant galaxy, forming at the centre of a rich cluster. However, because the galaxy is so distant, the cluster could not be seen directly.

Radio data obtained by the Very Large Array (VLA) in the USA and X-ray data with the ROSAT satellite both indicated that the galaxy is surrounded by a hot gas similar to that observed at the centres of nearby rich clusters of galaxies.

Most telling was a picture taken by the Hubble Space Telescope that revealed that the galaxy comprises a large number of clumps, and which bore a remarkable resemblance to computer models of the birth of giant galaxies in clusters. From these observations, it was concluded that 1138-262 is likely to be a massive galaxy in the final stage of assemblage through merging with many smaller galaxies in an infant rich cluster and the most distant known X-ray cluster.

VLT obtains Lyman-alpha images

ESO PR Photo 33a/99

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Caption to ESO PR Photo 33a/99 : False-colour picture of the ionized hydrogen gas surrounding 1138-262 (Lyman-alpha). The size of this cloud is about 5 times larger than the optical extent of the Milky Way Galaxy. A contour plot, as observed with VLT ANTU + FORS1 in a narrow-band filter around the wavelength of the redshifted Lyman-alpha line, is superposed on a false-colour representation of the same image. The contour levels are a geometric progression in steps of 2^1/2. The image has not been flux calibrated, so the first contour level is arbitrary. The field measures 35 x 25 arcsec², corresponding to about 910,000 x 650,000 light-years (280 x 200 kpc). The linear scale is indicated at the lower left. North is up and East is left.

The Leiden astronomers used the FORS1 instrument on ANTU to take long-exposure pictures of 1138-262 and a surrounding field of 36 square arcmin. Images were obtained through two optical filters, one which tunes in to light produced by hydrogen gas (the redshifted Lyman-alpha line) and the other which is dominated by light from stars (the B-band).

The "difference" between the images shows that the hydrogen gas surrounding the galaxy and from which the galaxy is presumably forming is huge (Photo 33a/99 ). The measured size is about 20 arcsec or, at the distance of the cluster, somewhat more than 500,000 light-years (160 kpc), making it the largest such structure ever seen. It corresponds to about 5 times the size of the optical extent of the Milky Way Galaxy!

ESO PR Photo 33b/99

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Caption to ESO PR Photo 33b/99 : Three small fields near radio galaxy 1138-262 as observed with VLT ANTU + FORS1 in a narrow-band filter at the redshifted wavelength of Lyman-alpha emission in that galaxy (left) and a broader filter in the surrounding spectral region (right), respectively. Three excellent candidates of Lyman-alpha emitters are seen at the centres of the fields. They are clearly visible in the narrow-band image (that mostly shows the gas), but are not detected in the broad-band image (that mostly shows the stars). Each field measures 24 x 24 arcsec², corresponding to about 620,000 x 620,000 light-years (190 x 190 kpc); North is up and East is left.

Even more intriguing is the presence of a number of objects in the gas picture (to the left in PR Photo 33b/99 ), but absent from the stars' picture (right).

These are galaxies whose hydrogen gas is emitting the bright Lyman-alpha spectral line within a distance of the order of about 3 million light-years (1 Mpc) from the radio galaxy, and probably in the surrounding cluster. The team has pinpointed a total of 26 objects in the surrounding field that may be companion galaxies with fainter hydrogen emission.

The detection by the VLT of the huge gas halo and of the companion galaxies is further evidence that 1138-262 is a massive galaxy, forming in a group or cluster of galaxies.

The next step

The next step in the project will be to confirm the distances of the candidate companion galaxies and establish that they are indeed members of a cluster of galaxies surrounding 1138-262 . This can be done using one of the spectrographs on the VLT.

Note

[1] The project on 1138-262 is being carried out by a large international consortium of scientists led by astronomers from the Leiden Observatory. Besides George Miley and Huub Rottgering, the team includes Jaron Kurk, Laura Pentericci, and Bram Venemans (Leiden), Alan Moorwood (ESO), Chris Carilli (US National Radio Astronomy Observatory - NRAO), Wil van Breugel (University of California, USA) Holland Ford and Tim Heckman (Johns Hopkins University, Baltimore, USA) and Pat McCarthy (Carnegie Institute, Pasadena, USA).

Technical information about the VLT images of 1138-262

Narrow and broad-band imaging was carried out on April 12 and 13, 1999, with the ESO VLT ANTU (UT1), using the FORS1 multi-mode instrument in imaging mode. A narrow-band filter was used which has a central wavelength of 381.4 nm and a bandpass of 6.5 nm. For 1138-262 (redshift z = 2.2), the emission of Lyman-alpha at 121.6 nm is redshifted to 383.8 nm, which falls in this narrow band. The broad-band filter was a Bessel-B with central wavelength of 429.0 nm. The detector was a Tektronix CCD with 2048 x 2046 pixels and an image scale of 0.20 arcsec/pixel. Eight separate 30-min exposures were taken in the narrow band and six 5-min in the broad band, shifted by about 20 arcsec with respect to each other to minimize problems due to flat-fielding and to facilitate cosmic ray removal. The average seeing was 1.0 arcsec. Image reduction was carried out by means of the IRAF reduction package. The individual images were bias subtracted and flat-fielded using twilight exposures (narrow band) or an average of the unregistered science exposures (broad-band). The images were then registered by shifting them in position by an amount determined from the location of several stars on the CCD. The registered images were co-added and dark pixels from cosmic rays were cleaned. To improve the signal-to-noise ratio, the resulting images were smoothed with a Gaussian function having full-width-at half-maximum (FWHM) = 1 arcsec (5 pixels).

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