The History of the Sombrero Galaxy

 

The Sombrero Galaxy is also known as Messier 104 because it was the 104 th object to be registered in the famous catalogue of Nebula. It was not included in Messier’s original published catalogue.  But Messier (1730-1817), a French astronomer, added it to his personal copy by hand on the 11 th May 1781. He described it as a “very faint nebula”. We know now that it is not a nebula (which is actually a cloud of dust and gas) but in fact a very large galaxy. M104 was probably not discovered by Messier but by his colleague Pierre Mechain.

In 1912, Vesto M.Slipher, an American astronomer at the Lowell Observatory discovered that the Sombrero Galaxy had a large “red shift” (see later heading ‘Observations’).  A large “red shift” meant that the M104 was quite distant and outside of our own galaxy.  Slipher also detected the rotation of the galaxy.

Characteristics of the sombrero galaxy

We chose the Sombrero galaxy as our ‘object’ because of its appearance.  It has a very definite ‘bulge’ and this is why it was called ‘sombrero’, because it looked like the famous Mexican hat!  We loved the photos of the galaxy; it looked strange but beautiful.

Why does the galaxy have that unusual shape?  It is because there are billions of old, faint stars around the core of the galaxy.  The rim of the ‘hat’ is in fact the disc of the galaxy, which we see as an edge-on disc because of our line-of-sight; the dust in the disc appears like a shadow against the bright bulge of all the stars.  The bulge is called a ‘nuclear bulge’.

Our imagination sees a ‘sombrero’, but what about the facts behind this beautiful galaxy?

It is catalogued as M104 (NGC 4594) and classified as a type Sa (spiral galaxy).

It is over 50 million light years away from us.

It is located in the constellation Virgo, which you can observe in the southern skies.

It can be found at right ascension (RA) 12:40.0 (h:m) and declination –11:37 (deg:m).

It has an apparent dimension of 9x4 (arc min).

It has a visual magnitude of 9.5.

Astronomers have measured increasing stellar speeds near the nucleus of the Sombrero galaxy, and think that this means there is a massive black hole at its centre, estimated to be 10 9 solar masses.

Other measurements made by astronomers (see next heading ‘Observations’) have led them to conclude that the Sombrero galaxy has a weak Active Galactic Nucleus (AGN).  This is another clue that there is a central black hole in the galaxy, slowly stripping matter away from around the nucleus.

 

Observations

How did astronomers first find out about the Sombrero galaxy?  It is thought that William Herschel first discovered the dark ‘dust lane’ in his famous reflector telescope.  This is because Messier’s largest telescope was only 20cm and used speculum metal mirrors; this would compare to the light gathering power of an 8-10cm telescope today, so Messier probably wouldn’t have been able to see the detail that Herschel could.

In 1912, Slipher measured the red shift of the galaxy as being 1000 km per second.  In those early days this measurement was very significant, because astronomers at that time thought that ‘spiral nebulas’ (as they thought M104 was) were part of our own Milky Way galaxy.  The large red shift meant that M104 was probably outside the Milky Way.

A lot of X-ray radiation has been found in the Sombrero galaxy’s centre.  This observation, combined with the measurement of high stellar velocities near the centre, have made astronomers think that there is a black hole at the nucleus of this galaxy.  Other measurements of the Sombrero galaxy that may indicate a black hole are:

It shows unusual radio properties for a spiral galaxy;

The optical spectrum of the central region shows emission lines fromhot gas (of the ‘LINER’ type – Low Ionisation Nuclear Emission line Region);

A steep increase in the mass-to-light ratio near the nucleus.

Comparing the Sombrero galaxy and the Milky Way galaxy

Both the Sombrero galaxy and the Milky Way are similar in that they are spiral galaxies.  However, the classification of the two seems to be different, because:

The Sombrero galaxy is classified as Sa/Sb,

The Milky Way galaxy is classified as Sb or Sc; but astronomers have not yet decided whether the Milky Way has a ‘bar’ structure, which would then make it a type SB galaxy (see ‘tuning fork’ diagram below).

Both the Sombrero galaxy and the Milky Way have a ‘bulge’, which is made up from globular clusters of stars.  The Milky Way is thought to have about 200 globular clusters.   Large telescopes on Earth have found several hundred globular clusters in the Sombrero galaxy.

The Milky Way galaxy has other ‘companion’ galaxies.

One of the main differences between the Sombrero galaxy and the Milky Way is that the nucleus of the Sombrero galaxy is several billion times that of the Milky Way nucleus.

Sombrero galaxy – Practical work

We have designed this activity for a Junior Science class.  It will mean learning about something new, the classification of galaxies (we do not usually study this in class), using the ‘tuning fork’ diagram.

After learning about the different types of galaxies, we will ask the class to try to classify a few galaxies using images from telescopes.

1. Give out information sheet with ‘tuning fork’ classification diagram.  Explain the different names for the types of galaxies, e.g. spiral, barred spiral.

Ask pupils to study the different shapes and note the classification types.

2. Give out photos of galaxies and ask groups of pupils to identify each type, e.g.

An E0 type galaxy An Sb type galaxy

The answers could be printed on the reverse of the photos so that pupils could check their classification.

You could make this a competition and see which group was able to correctly identify most galaxies.

References

www.seds.org/messier/more/mw

www.seds.org/messier/m/m104

www.absolutebeginnersastronomy.com/finder104

www.seds.org/messier/more/m104_dfm_image

www.kopernik.org/images/archive/m104

www.hermes.physics.ox.ac.uk/users/Galactic/layman/gal_types

www.smv.org/hastings/teacher

www.herts.ac.uk/astro_ub/a21_ub

www.astro.soton.ac.uk/PH308/galaxies/classification