Portrait of One Hundred Thousand and One Galaxies

Rich and Inspiring Experience with NGC 300 Images from the ESO Science Data
Archive

Summary

A series of wide-field images centred on the nearby spiral galaxy NGC 300,
obtained with the Wide-Field Imager (WFI) on the MPG/ESO 2.2-m telescope
at the La Silla Observatory, have been combined into a magnificent colour
photo.

These images have been used by different groups of astronomers for various
kinds of scientific investigations, ranging from individual stars and
nebulae in NGC 300, to distant galaxies and other objects in the
background.

This material provides an interesting demonstration of the multiple use of
astronomical data, now facilitated by the establishment of extensively
documented data archives, like the ESO Science Data Archive that now is
growing rapidly and already contains over 15 Terabyte.

Based on the concept of Astronomical Virtual Observatories (AVOs), the use
of archival data sets is on the rise and provides a large number of
scientists with excellent opportunities for front-line investigations
without having to wait for precious observing time. In addition to
presenting a magnificent astronomical photo, the present account also
illustrates this important new tool of the modern science of astronomy and
astrophysics.

• PR Photo 18a/02: WFI colour image of spiral galaxy NGC 300 (full field).
  
• PR Photo 18b/02: Cepheid stars in NGC 300
  
• PR Photo 18c/02: H-alpha image of NGC 300
  
• PR Photo 18d/02: Distant cluster of galaxies CL0053-37 in the NGC 300
  field
  
• PR Photo 18e/02: Dark matter distribution in CL0053-37
  
• PR Photo 18f/02: Distant, reddened cluster of galaxies in the NGC 300
  field
  
• PR Photo 18g/02: Distant galaxies, seen through the outskirts of NGC 300
  
• PR Photo 18h/02: “The View Beyond”
  

Located some 7 million light-years away, the spiral galaxy NGC 300 [1] is a
beautiful representative of its class, a Milky-Way-like member of the
prominent Sculptor group of galaxies in the southern constellation of that
name. NGC 300 is a big object in the sky – being so close, it extends over
an angle of almost 25 arcmin, only slightly less than the size of the full
moon. It is also relative bright, even a small pair of binoculars will
unveil this magnificent spiral galaxy as a hazy glowing patch on a dark sky
background.

The comparatively small distance of NGC 300 and its face-on orientation
provide astronomers with a wonderful opportunity to study in great detail
its structure as well as its various stellar populations and interstellar
medium. It was exactly for this purpose that some images of NGC 300 were
obtained with the Wide-Field Imager (WFI) on the MPG/ESO 2.2-m telescope at
the La Silla Observatory. This advanced 67-million pixel digital camera has
already produced many impressive pictures, some of which are displayed in
the WFI Photo Gallery [2].

With its large field of view, 34 x 34 arcmin^2, the WFI is optimally suited
to show the full extent of the spiral galaxy NGC 300 and its immediate
surroundings in the sky, cf. PR Photo 18a/02.

NGC 300 and “Virtual Astronomy”

In addition to being a beautiful sight in its own right, the present
WFI-image of NGC 300 is also a most instructive showcase of how astronomers
with very different research projects nowadays can make effective use of the
same observations for their programmes.

The idea to exploit one and the same data set is not new, but thanks to
rapid technological developments it has recently developed into a very
powerful tool for the astronomers in their continued quest to understand the
Universe. This kind of work has now become very efficient with the advent of
a fully searchable data archive from which observational data can then –
after the expiry of a nominal one-year proprietary period for the observers
– be made available to other astronomers.

The ESO Science Data Archive was established some years ago and now
encompasses more than 15 Terabyte [3]. Normally, the identification of
specific data sets in such a large archive would be a very difficult and
time-consuming task. However, effective projects and software “tools” like
ASTROVIRTEL and Querator now allow the users quickly to “filter” large
amounts of data and extract those of their specific interest. Indeed,
“Archival Astronomy” has already led to many important discoveries, cf. the
ASTROVIRTEL list of publications.

There is no doubt that “Virtual Astronomical Observatories” will play an
increasingly important role in the future, cf. ESO PR 26/01. The present
wide-field images of NGC 300 provide an impressive demonstration of the
enormous potential of this innovative approach. Some of the ways they were
used are explained below.

In 1999, Wolfgang Gieren (Universidad de Concepcion, Chile) and his
colleagues started a search for Cepheid-type variable stars in NGC 300.
These stars constitute a key element in the measurement of distances in the
Universe. It has been known since many years that the pulsation period of a
Cepheid-type star depends on its intrinsic brightness (its “luminosity”).
Thus, once its period has been measured, the astronomers can calculate its
luminosity. By comparing this to the star’s apparent brightness in the sky,
and applying the well-known diminution of light with the second power of the
distance, they can obtain the distance to the star. This fundamental method
has allowed some of the most reliable measurements of distances in the
Universe and has been essential for all kinds of astrophysics, from the
closest stars to the remotest galaxies.

Previous to Gieren’s new project, only about a dozen Cepheids were known in
NGC 300. However, by regularly obtaining wide-field WFI exposures of NGC 300
from July 1999 through January 2000 and carefully monitoring the apparent
brightness of its brighter stars during that period, the astronomers
detected more than 100 additional Cepheids. The brightness variations (in
astronomical terminology: “light curves”) could be determined with excellent
precision from the WFI data. They showed that the pulsation periods of these
Cepheids range from about 5 to 115 days. Some of these Cepheids are
identified on PR Photo 18b/02, in the middle of a very crowded field in NGC
300.

When fully studied, these unique observational data will yield a new and
very accurate distance to NGC 300, making this galaxy a future cornerstone
in the calibration of the cosmic distance scale. Moreover, they will also
allow to understand in more detail how the brightness of a Cepheid-type star
depends on its chemical composition, currently a major uncertainty in the
application of the Cepheid method to the calibration of the extragalactic
distance scale. Indeed, the effect of the abundance of different elements on
the luminosity of a Cepheid can be especially well measured in NGC 300 due
to the existence of large variations of these abundances in the stars
located in the disk of this galaxy.

Gieren and his group, in collaboration with astronomers Fabio Bresolin and
Rolf Kudritzki (Institute of Astronomy, Hawaii, USA) are currently measuring
the variations of these chemical abundances in stars in the disk of NGC 300,
by means of spectra of about 60 blue supergiant stars, obtained with the
FORS multi-mode instruments at the ESO Very Large Telescope (VLT) on
Paranal. These stars, that are among the optically brightest in NGC 300,
were first identified in the WFI images of this galaxy obtained in different
colours – the same that were used to produce PR Photo 18a/02. The nature of
those stars was later spectroscopically confirmed at the VLT.

As an important byproduct of these measurements, the luminosities of the
blue supergiant stars in NGC 300 will themselves be calibrated (as a new
cosmic “standard candle”), taking advantage of their stellar wind properties
that can be measured from the VLT spectra. The WFI Cepheid observations in
NGC 300, as well as the VLT blue supergiant star observations, form part of
a large research project recently initiated by Gieren and his group that is
concerned with the improvement of various stellar distance indicators in
nearby galaxies (the “ARAUCARIA” project).

But there is much more to discover on these WFI images of NGC 300!

The WFI images obtained in several broad and narrow band filters from the
ultraviolet to the near-infrared spectral region (U, B, V, R, I and H-alpha)
allow a detailed study of groups of heavy, hot stars (known as “OB
associations”) and a large number of huge clouds of ionized hydrogen (“HII
shells”) in this galaxy. Corresponding studies have been carried out by
Gieren’s group, resulting in the discovery of an amazing number of OB
associations, including a number of giant associations.

These investigations, taken together with the observed distribution of the
pulsation periods of the Cepheids, allow to better understand the history of
star formation in NGC 300. For example, three distinct peaks in the number
distribution of the pulsation periods of the Cepheids seem to indicate that
there have been at least three different bursts of star formation within the
past 100 million years. The large number of OB associations and HII shells
(PR Photo 18c/02) furthermore indicate the presence of a numerous, very
young stellar population in NGC 300, aged only a few million years.

Dark matter and the observed shapes of distant galaxies

In early 2002, Thomas Erben and Mischa Schirmer from the “Institut fuer
Astrophysik and extraterrestrische Forschung” (IAEF, Universitaet Bonn,
Germany), in the course of their ASTROVIRTEL programme, identified and
retrieved all available broad-band and H-alpha images of NGC 300 available
in the ESO Science Data Archive. Most of these have been observed for the
project by Gieren and his colleagues, described above.

However, the scientific interest of the German astronomers was very
different from that of their colleagues and they were not at all concerned
about the main object in the field, NGC 300. In a very different approach,
they instead wanted to study those images to measure the amount of dark
matter in the Universe, by means of the weak gravitational lensing effect
produced by distant galaxy clusters.

Various observations, ranging from the measurement of internal motions
(“rotation curves”) in spiral galaxies to the presence of hot X-ray gas in
clusters of galaxies and the motion of galaxies in those clusters, indicate
that there is about ten times more matter in the Universe than what is
observed in the form of stars, gas and galaxies (“luminous matter”). As this
additional matter does not emit light at any wavelengths, it is commonly
referred to as “dark” matter – its true nature is yet entirely unclear.

Insight into the distribution of dark matter in the Universe can be gained
by looking at the shapes of images of very remote galaxies, billions of
light-years away, cf. ESO PR 24/00. Light from such distant objects travels
vast distances through space before arriving here on Earth, and whenever it
passes heavy clusters of galaxies, it is bent a little due to the associated
gravitational field. Thus, in long-exposure, high-quality images, this “weak
lensing” effect can be perceived as a coherent pattern of distortion of the
images of background galaxies.

The WFI NGC 300 images appeared promising for gravitational lensing research
because of the exceptionally long total exposure time. Although the large
foreground galaxy NGC 300 would block the light of tens of thousands of
galaxies in the background, a huge number of others would still be visible
in the outskirts of this sky field, making a search for clusters of galaxies
and associated lensing effects quite feasible. To ensure the best possible
image sharpness in the combined image, and thus to obtain the most reliable
measurements of the shapes of the background objects, only red (R-band)
images obtained under the best seeing conditions were combined. In order to
provide additional information about the colours of these faint objects, a
similar approach was adopted for images in the other bands as well.

The German astronomers indeed measured a significant lensing effect for one
of the galaxy clusters in the field (CL0053-37, see PR Photo 18d/02); the
images of background galaxies around this cluster were noticeably distorted
in the direction tangential to the cluster center. Based on the measured
degree of distortion, a map of the distribution of (dark) matter in this
direction was constructed (PR Photo 18e/02). The separation of unlensed
foreground (bluer) and lensed background galaxies (redder) greatly profited
from the photometric measurements done by Gieren’s group in the course of
their work on the Cepheids in NGC 300.

Assuming that the lensed background galaxies lie at a mean redshift of 1.0,
i.e. a distance of 8 billion light-years, a mass of about 2 x 10^14 solar
masses was obtained for the CL0053-37 cluster.

This lensing analysis in the NGC 300 field is part of the Garching-Bonn Deep
Survey (GaBoDS), a weak gravitational lensing survey led by Peter Schneider
(IAEF). GaBoDS is based on exposures made with the WFI and until now a sky
area of more than 12 square degrees has been imaged during very good seeing
conditions. Once complete, this investigation will allow more insight into
the distribution and cosmological evolution of galaxy cluster masses, which
in turn provide very useful information about the structure and history of
the Universe.

In addition to allowing a detailed investigation of dark matter and lensing
effects in this field, the present, very “deep” colour image of NGC 300
invites to perform a closer inspection of the background galaxy population
itself.

No less than about 100,000 galaxies of all types are visible in this amazing
image. Three known quasars ([ICS96] 005342.1-375947, [ICS96]
005236.1-374352, [ICS96] 005336.9-380354) with redshifts 2.25, 2.35 and
2.75, respectively, happen to lie inside this sky field, together with many
interacting galaxies, some of which feature tidal tails. There are also
several groups of highly reddened galaxies – probably distant clusters in
formation, cf. PR Photo 18f/02. Others are seen right through the outer
regions of NGC 300, cf. PR Photo 18g/02.

More detailed investigations of the numerous galaxies in this field are now
underway. From the nearby spiral galaxy NGC 300 to objects in the young
Universe, it is all there, truly an astronomical treasure trove, cf. PR
Photo 18h/02!

Notes

[1]: “NGC” means “New General Catalogue” (of nebulae and clusters) that was
published in 1888 by J.L.E. Dreyer in the “Memoirs of the Royal Astronomical
Society”.

[2]: Other colour composite images from the Wide-Field Imager at the MPG/ESO
2.2-m telescope at the La Silla Observatory are available at the ESO
Outreach website at http://www.eso.org/outreach/gallery/astro/wfi/. The most
recent addition to this collection is an image of the Tarantula Nebula in
the LMC, cf. ESO PR Photos 14a-g/02.

[3]: 1 Terabyte = 10^12 byte = 1000 Gigabyte = 1 million million byte.

Technical information about the photos

PR Photo 18a/02 and all cutouts were made from 110 WFI images obtained in
the B-band (total exposure time 11.0 hours, rendered as blue), 105 images in
the V-band (10.4 hours, green), 42 images in the R-band (4.2 hours, red) and
21 images through a H-alpha filter (5.1 hours, red). In total, 278 images of
NGC 300 have been assembled to produce this colour image, together with
about as many calibration images (biases, darks and flats). 150 GB of hard
disk space were needed to store all uncompressed raw data, and about 1 TB of
temporary files was produced during the extensive data reduction. Parallel
processing of all data sets took about two weeks on a four-processor Sun
Enterprise 450 workstation. The final colour image was assembled in Adobe
Photoshop. To better show all details, the overall brightness of NGC 300 was
reduced as compared to the outskirts of the field.

The (red) “rings” near some of the bright stars originate from the H-alpha
frames – they are caused by internal reflections in the telescope.

The images were prepared by Mischa Schirmer at the Institut fuer Astrophysik
und Extraterrestrische Forschung der Universitaet Bonn (IAEF) by means of a
software pipeline specialised for reduction of multiple CCD wide-field
imaging camera data. The raw data were extracted from the public sector of
the ESO Science Data Archive.

The extensive observations were performed at the ESO La Silla Observatory by
Wolfgang Gieren, Pascal Fouque, Frederic Pont, Hermann Boehnhardt and La
Silla staff, during 34 nights between July 1999 and January 2000. Some
additional observations taken during the second half of 2000 were retrieved
by Mischa Schirmer and Thomas Erben from the ESO archive.

CD-ROM with full-scale NGC 300 image soon available

PR Photo 18a/02 has been compressed by a factor 4 (2 x 2 rebinning). For PR
Photos 18b-h/02, the largest-size versions of the images are shown at the
original scale (1 pixel = 0.238 arcsec). A full-resolution TIFF-version
(approx. 8000 x 8000 pix; 200 Mb) of PR Photo 18a/02 will shortly be made
available by ESO on a special CD-ROM, together with some other WFI images of
the same size. An announcement will follow in due time.