Galactic Evolution?
The adjacent figure shows the Hubble Deep Field. To obtain this image,
the Hubble Space Telescope was pointed at the same region
of the sky (in the constellation Ursa Major) for ten consecutive days
and images were combined to give the
most distant optical view of the Universe yet obtained. Almost every smudge is a
galaxy
and objects down to visual magnitude +30 can be seen in this remarkable image.
Galaxies and Time
Looking out to such large
distances implies looking back in time because of the finite speed of light. So
this image is actually giving
us a view of the Universe early in the history of galaxy formation (see the
discussion of look-back times in Module 3 of this
chapter). It
may represent what the
Universe looked like only a billion years or so after the big bang in which it was created.
Interactions and Evolution
Seeing such a bewildering array of galaxies from the early history of our Universe
raises an important question.
Do the galaxies evolve from one form to another, or is "once a spiral galaxy, always a spiral galaxy" the
norm. Initially, there were some attempts to interpret the Hubble galaxy diagram as an evolutionary sequence,
with one galaxy type evolving into another over time. However, such attempts met with
little success.
Today, we do not believe that isolated galaxies
evolve from, say, spiral to elliptical. However, we have increasingly become aware that many galaxies are not
isolated and in fact may interact strongly with each other. We shall discuss that more
extensively in Module 6 of this chapter,
but we note here that there now is strong evidence that
galaxies evolve because of interactions
with other galaxies.
Interacting Galaxies
The adjacent image shows two of the common galaxy types: a spiral galaxy and an giant elliptical galaxy.
This pair is in the constellation Centaurus,
at a distance of about 155 million light years.
The two galaxies appear to be interacting, with
some tidal disruption of the spiral galaxy (NGC 5091) by the elliptical galaxy (NGC 5090). The nucleus
of the spiral galaxy has a strong double radio source, indicating that something
violent is taking place there, perhaps in response to the tidal distortion by the other
galaxy.
If the interaction remains sufficiently weak in this system,
the spiral will probably remain
a spiral and the elliptical an elliptical. However, if the interaction becomes
stronger, it could change substantially the character of the two galaxies. We shall
see examples shortly where exactly such modifications have taken place because of
interactions between galaxies.