NGC4945.vlt-3

PR Photo 18a/99 shows a sky field around the spiral galaxy NGC
 4945. It is assembled from five 15-minute R(ed)-narrowband (shown in
 red), four 5-minute B(lue)-band (shown in green), and five
 1000-second U(ltraviolet)-band (shown in blue) exposures, obtained in
 January 1999 during the Science Verification phase with the
 Wide-Field-Imager (WFI) at the MPG/ESO 2.2-m telescope at La Silla,
 cf. ESO Press Release 02/99. At the recession velocity of NGC 4945,
 the red filter, centred at 665 nm with an FWHM (full width at half
 maximum) of only 1.2 nm, does not include the H-alpha emission line
 of interstellar hydrogen in this galaxy. The original resolution of
 about 1 arcsec corresponds to roughly 62 light-years at the distance
 of NGC 4945 (13 million light-years).

 In addition to NGC 4945 itself, some much more distant galaxies can
 be recognized as faint, slightly red light patches in the field. The
 vast majority of the point-like sources are stars in the Milky
 Way. However, a fair number of those near NGC 4945 are globular
 clusters belonging to this galaxy.

 Each frame records 8184 x 8196 = 67,076,064 pixels, and thus the
total number of data points (pixels) of the 14 CCD frames used to make
this photo is almost 109. Their collective information content
corresponds to more than 70 x 109 photons (not counting those from the
Earth's upper atmosphere that were recorded simultaneously). The
composite photo shown here has been compressed to smaller sizes (as
indicated) in order to be transportable over the web. The field covers
32 x 32 arcmin2. East is to the left and North to the top.

 PR Photo 18b/99 is an enlargment at full resolution of a smaller
 region (about 1/7 by area) of Photo 18a/99. It covers 2997 x 2998
 pixels and measures about 12 x 12 arcmin2 in the sky. Note the many
 background galaxies in the field.

 PR Photo 18c/99 is an enlargment at full resolution of a smaller
 region (in the upper half of the lower left quadrant, vertically
 above the bright star at the bottom; about 1/67 by area) of Photo
 18a/99. It covers 1001 x 1002 pixels and measures about 4 x 4 arcmin2
 in the sky. The structure of the faint and distant spiral galaxy at
 the centre is well visible in this reproduction.

 PR Photo 18d/99 is a further enlargment of part of the same area with
 a faint spiral galaxy as in PR Photo 18c/99, now showing the
 individual pixels. It covers 214 x 210 pixels and measures about 50 x
 50 arcsec2 in the sky. The area is only 1/1500 of the full field;
 this illustrates the enormous amount of information that can be
 obtained with the WFI in a short time and hence, the great efficiency
 of this instrument. Please note that the small, "full resolution"
 image has the original resolution as recorded at the telescope, while
 the "enlarged version" is enlarged four times for easy viewing on the
 screen.



The portrait of NGC 4945 above adorns the centrefold of the most
recent issue of the ESO Messenger (no. 95; March 1999). It provides
another impressive demonstration of the observational capabilities of
the new WFI Camera at the MPG/ESO 2.2-m telescope at La Silla.

NGC 4945 is a relatively nearby spiral galaxy, not too different from
the Milky Way Galaxy in which we live. However, since it happens to be
seen almost edge-on (under an angle of about 10°), the spiral
structure is hardly discernible. This particular perspective is
essentially the same as the inside-view we have of the Milky Way,
because our Solar System is located near its central plane. The
distance from the Sun to the centre is about 30,000 light-years, or
2/3 of the radius of the Milky Way galaxy.

In both galaxies, the interstellar gas (that emits its own light and
also reflects the stellar light) and dust (that absorbs the stellar
light), imprint a high-contrast structure on the much smoother
distribution of their hundreds of billions of stars.  Most of the
light recorded in the photo of NGC 4945 is from stars, while the dark
lanes are areas where the dust blocks the light. The patchy and
filamentary appearance of the interstellar matter indicates that it is
a very dynamical medium.

In those regions, where the density is sufficiently large,
self-gravitation causes clouds of dust and gas to contract to the
point where new stars will begin to form. This process takes hundreds
of thousands to millions of years. In galaxies like these, the
equivalent of about one solar mass is transformed into new stars every
year.

In NGC 4945, observations at X-ray and infrared wavelengths have found
evidence of very energetic processes in the nucleus (which is of the
so-called Seyfert 2 type and may harbour a black hole) and a major
burst of star formation takes place in its vicinity.

NGC 4945 belongs to the Centaurus group of galaxies, seen in the
constellation of that name and located at a distance of approximately
13 million light-years. This is only six times the distance to the 6th
magnitude Andromeda Galaxy, the large galaxy in the northern sky that
is visible to the unaided eye and is the most prominent member of the
Local Group of Galaxies. NGC 4945 (9th magnitude; i.e. 15 times
fainter) is thus a rather nearby object. Its velocity with respect to
the Milky Way is comparatively small, 560 km/sec, and is not governed
by the expansion of the Universe but reflects the local motions of the
two galaxies.