SETI Watching the Birth of a Galaxy Cluster?


Larry Klaes (lklaes@bbn.com)
Fri, 30 Jul 1999 14:38:21 -0400


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>Date: Friday, July 30, 1999
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>Text with all links is available on the ESO Website at URL:
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>http://www.eso.org/outreach/press-rel/pr-1999/pr-13-99.html
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>Dear subscribers,
>
>the following ESO Press Release describes observations of faint and
>distant galaxies, carried out by the first visiting astronomers to the
>VLT ANTU telescope. The original URL is:
>
>http://www.eso.org/outreach/press-rel/pr-1999/pr-13-99.html
>
>Please note the message about the ESO-NEWS mailing list, attached
>below.
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> Information from the European Southern Observatory
>
> ESO Press Release 13/99
>
> 30 July 1999 [ESO Logo]
>
> For immediate release
> ------------------------------------------------------------------------
>
>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
>
> [Preview - JPEG: 483 x 400 pix - 86k]
>
> [Normal - JPEG: 966 x 800 pix - 230k]
>
> [High-Res - JPEG: 2894 x 2396 pix - 1.1M]
>
>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 2e1/2. 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
>
> [Preview - JPEG: 400 x 593 pix - 149k]
>
> [Normal - JPEG: 800 x 1185 pix - 335k]
>
> [High-Res - JPEG: 1982 x 2935 pix - 1.1M]
>
>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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