M104


Fine Shades of a Sombrero

A New Look at an Unusual Galaxy

PR Photo 07a/00 of the Sombrero Galaxy (Messier 104) was obtained with
FORS1 multi-mode instrument at VLT ANTU on January 30, 2000.  It is a
composite of three exposures in different wavebands, cf. the technical
note below. The full-resolution version of this photo retains the
orginal pixels. PR Photo 07b/00 shows the eastern area, with the
pronounced dust bands and many background galaxies. North is up and
East is left.
  


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.