Fine Shades of a Sombrero – A New Look at an Unusual Galaxy

In addition to their scientific value, many of the exposures now
being obtained by visiting astronomers to ESO’s Very Large Telescope
(VLT) are also very beautiful. This is certainly true for this new image
of the famous early-type spiral galaxy Messier 104, widely
known as the “Sombrero” (the Mexican hat) because of its
particular shape.

The colour image was made by a combination of three CCD images from
the FORS1
multi-mode instrument on VLT ANTU,
recently obtained by Peter Barthel from the Kapteyn Institute
(Groningen, The Netherlands) during an observing run at the Paranal Observatory. He
and Mark Neeser, also from the Kapteyn Institute, produced the
composite images.

The galaxy fits perfectly into the 6.8 x 6.8 arcmin²
field-of-view of the FORS1 camera. A great amount of fine detail is
revealed, from the structures in the pronounced dust band in the
equatorial plane, to many faint background galaxies that shine through
the outer regions.

The “Sombrero” is located in the constellation Virgo (The
Virgin), at a distance of about 50 million light-years. The overall
“sharpness” of this colour image corresponds to about 0.7 arcsec which
translates into a resolution of about 170 light-years at that
distance.

About Messier 104

Messier 104 is the 104th object in the famous catalogue of
nebulae by French astronomer Charles Messier (1730 – 1817). It
was not included in the first two editions (with 45 objects in 1774;
103 in 1781), but Messier soon thereafter added it by hand in his
personal copy as a “very faint nebula”. The recession velocity, about
1000 km/sec, was first measured by American astronomer Vesto
M. Slipher at the Lowell Observatory in 1912; he was also the
first to detect the galaxy’s rotation.

This galaxy is notable for its dominant nuclear bulge,
composed primarily of mature stars, and its nearly edge-on disk
composed of stars, gas, and intricately structured dust. The
complexity of this dust, and the high resolution of this image, is
most apparent directly in front of the bright nucleus, but is also
very evident as dark absorbing lanes throughout the disk. A
significant fraction of the galaxy disk is even visible on the far
side of the source, despite its massive bulge, cf. PR Photo
07c/00.

A large number of small and slightly diffuse sources can be seen as
a swarm in the halo of Messier 104. Most of these are
globular clusters, similar to those found in our own
Galaxy.

Measurements reveal a steep increase in the mass-to-light ratio and
increasing stellar speeds near the nucleus of Messier 104. This
is indicative of the presence of a massive black hole at the centre,
estimated at about 10⁹ solar masses.

The radio properties of Messier 104 are unusual for a
spiral galaxy – it has a variable core. The optical spectrum of the
central region displays emission lines from hot gas (of the “LINER”
type – Low Ionisation Nuclear Emission line Region). This points to
Messier 104 harbouring a weak Active Galactic Nucleus
(AGN). Although more commonly known from the much more luminous
and distant quasars and powerful radio galaxies, the weak AGN in this
galaxy lies at the opposite extreme: the most likely explanation being
a central black hole accreting circumnuclear matter at a slow
pace.

Technical information: PR Photos 07a/00 and
07b/00 are composites based on three exposures from the FORS1
instrument at VLT ANTU. They were obtained at about 6:20 hrs UT on
January 30, 2000, through V-band (central wavelength 554 nm; 112 nm
Full Width Half Maximum (FWHM); exposure time 120 sec; here rendered
as blue), R-band (657 nm; 150 nm FWHM; 120 sec; green) and I-band (768
nm; 138 nm FWHM, 240 sec; red). The seeing was 0.6 – 0.7 arcsec. PR
Photo 07c/00 was made by dividing the V-band image by itself
smoothed with a 2D gaussian profile (sigma: 10 pix); this removes the
uniform areas and enhances the high spatial frequency features,
e.g. the dust bands. Image processing by Mark Neeser (Kapteyn
Institute, Groningen) and Richard Hook (ST/ECF, Garching,
Germany).

This is the caption to ESO PR Photos 07a-c/00. They may be
reproduced, if credit is given to the European Southern
Observatory.