Very Large Telescope Takes Snapshots of Two Grand-Design Spiral Galaxies

Images of beautiful galaxies, and in particular of spiral brethren of
our own Milky Way, leave no-one unmoved. It is difficult indeed to
resist the charm of these impressive grand structures. Astronomers at
Paranal Observatory used the versatile VIMOS instrument on the Very
Large Telescope to photograph two magnificent examples of such “island
universes”, both of which are seen in a southern constellation with an
animal name. But more significantly, both galaxies harboured a
particular type of supernova, the explosion of a massive star during a
late and fatal evolutionary stage.

The first image (PR Photo 33a/04) is of the impressive spiral galaxy
NGC 6118 [1], located near the celestial equator, in the constellation
Serpens (The Snake). It is a comparatively faint object of 13th
magnitude with a rather low surface brightness, making it pretty hard
to see in small telescopes. This shyness has prompted amateur
astronomers to nickname NGC 6118 the “Blinking Galaxy” as it would
appear to flick into existence when viewed through their telescopes in
a certain orientation, and then suddenly disappear again as the eye
position shifted.

There is of course no such problem for the VLT’s enormous
light-collecting power and ability to produce sharp images, and this
magnificent galaxy is here seen in unequalled detail. The colour photo
is based on a series of exposures behind different optical filters,
obtained with the VIMOS multi-mode instrument on the 8.2-m VLT Melipal
telescope during several nights around August 21, 2004.

About 80 million light-years away, NGC 6118 is a grand-design spiral
seen at an angle, with a very small central bar and several rather
tightly wound spiral arms (it is classified as of type “SA(s)cd” [2])
in which large numbers of bright bluish knots are visible. Most of
them are active star-forming regions and in some, very luminous and
young stars can be perceived.

Of particular interest is the comparatively bright stellar-like object
situated directly North of the galaxy’s centre, near the periphery
(see PR Photo 33b/04): it is Supernova 2004dk that was first reported
on August 1, 2004. Observations a few days later showed this to be a
supernova of Type Ib or Ic [3] , caught a few days before
maximum light. This particular kind of supernova is believed to result
from the demise of a massive star that has somehow lost its entire
hydrogen envelope, probably as a result of mass transfer in a binary
system, before exploding.

Also visible on the image is the trail left by a satellite, which
passed by during one of the exposures taken in the B filter, hence its
blue colour. This is an illustration that even in such a remote place
as the Paranal Observatory in the Atacama desert, astronomers are not
completely sheltered from light pollution.

The second galaxy imaged by the VLT (ESO PR Photo 33c/04) is another
spiral, the beautiful multi-armed NGC 7424 that is seen almost
directly face-on. Located at a distance of roughly 40 million
light-years in the constellation Grus (the Crane), this galaxy was
discovered by Sir John Herschel while observing at the Cape of Good
Hope.

This other example of a “grand design” galaxy is classified as
“SAB(rs)cd” [2], meaning that it is intermediate between normal
spirals (SA) and strongly barred galaxies (SB) and that it has rather
open arms with a small central region. It also shows many ionised
regions as well as clusters of young and massive stars. Ten young
massive star clusters can be identified whose sizes span the range from
1 to 200 light-years. The galaxy itself is roughly 100,000
light-years across, that is, quite similar in size to our own Milky
Way galaxy.

Because of its low surface brightness, this galaxy also demands dark
skies and a clear night to be observed in this impressive detail. When
viewed in a small telescope, it appears as a large elliptical haze
with no trace of the many beautiful filamentary arms with a multitude
of branches revealed in this striking VLT image. Note also the very
bright and prominent bar in the middle.

On the evening of 10 December 2001, Australian amateur astronomer
Reverend Robert Evans, observing from his backyard in the Blue
Mountains west of Sydney, discovered with his 30cm telescope his 39th
supernova, Supernova 2001ig in the outskirts of NGC 7424. Of magnitude
14.5 (that is, 3000 times fainter than the faintest star that can be
seen with the unaided eye), this supernova brightened quickly by a
factor 8 to magnitude 12.3. A few months later, it had faded to an
insignificant object below 17th magnitude. By comparison, the entire
galaxy is of magnitude 11: at the time of its maximum, the supernova
was thus only three times fainter than the whole galaxy. It must have
been a splendid firework indeed!

By digging into the vast Science Archive of the ESO Very Large
Telescope, it was possible to find an image of NGC 7424 taken on June
16, 2002 by Massimo Turatto (Observatorio di Padova-INAF, Italy) with
the FORS 2 instrument on Yepun (UT4). Although, the supernova was
already much fainter than at its maximum 6 months earlier, it is still
very well visible on this image (see PR Photo 33d/04).

Spectra taken with ESO’s 3.6-m telescope at La Silla over the months
following the explosion showed the object to evolve to a Type Ib/c
supernova. By October 2002, the transition to a Type Ib/c supernova
was complete. It is now believed that this supernova arose from the
explosion of a very massive star, a so-called Wolf-Rayet star, which
together with a massive hot companion belonged to a very close binary
system in which the two stars orbited each other once every 100 days
or so. Future detailed observations may reveal the presence of the
companion star that survived this explosion but which is now doomed to
explode as another supernova in due time.

The full text of this ESO PR Photo Release, with four images and all
weblinks, is available at:
http://www.eso.org/outreach/press-rel/pr-2004/phot-33-04.html

Notes

[1] NGC stands for “New General Catalogue”. Published in 1888 by
J.L.E. Dreyer, this New General Catalogue of Nebulae and Clusters of
Stars, being the Catalogue of the late Sir John F.W. Herschel contains
7840 objects of which 3200 are galaxies.

[2] Spiral galaxies take their name from the spectacular spiral arms
that wind around in a very thin disc. Following the celebrated
classification by American astronomer Edwin Hubble, spiral galaxies
are classified into two families, so-called normal spirals (SA) and
barred spirals (SB), and are further divided into types Sa, Sb and Sc
depending on the opening of the spiral arms and the relative
brightness of the central area. In barred spiral galaxies, the nucleus
is crossed by a bar of stars at the ends of which the spiral arms
begin. The (rs) in the classification testifies to the presence of an
internal ring (r) surrounding the nucleus of the galaxy as well as to
the fact that the spiral arms begin directly at the nucleus (s).

[3] Supernovae are classified into different types, depending on the
appearance of their spectrum. Type II supernovae show the presence of
hydrogen lines in their spectra while Type I lack this signature. Type
I have been subdivided into Type Ia, Ib and Ic. Type I supernovae are
all believed to arise in binary stellar systems.