“What types from the Hubble Space Telescope (HST) images

“What fraction of stars
were formed in Isolated Galaxies and Interacting Galaxies?”

Mary Lavis

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The aims of
this project were to understand the general questions of galaxy evolution and
to understand the idea of galaxy interactions and the effects the interactions
have on the gas and stars. This knowledge was then used to classify galaxy
types from the Hubble Space Telescope (HST) images of Great Observatories
Origins Deep Survey (GOODS-South)
into isolating and interacting so that the total energy output from each class
could be calculated.


1. Introduction.

1.1 Background

To find the
fraction of stars that were made in interacting and isolated galaxies, we need
to know which features of a galaxy instigate the formation of stars. Once the
properties of a galaxy that cause this are known, we will be able to identify
which classes of galaxy are producing the most stars. Using this knowledge, and
data from the HST and the Herschel Survey, we will be able to calculate the
energy emitted from these galaxy classes and confirm the origin of the stellar
population. The current theory is that energy emission is intrinsically linked
to the amount of star formation in a galaxy as, simply put, more radiation
detected suggests more star formation activity has occurred.

1.2 Galaxy Types and

1.2.1 Interacting and
Isolated Galaxies.

Galaxies can
be separated into different classes. For the purpose of this project, the
galaxy types we were most interested in are ‘isolated galaxies’ and ‘interacting
galaxies’. The obvious difference between these two classes is that isolated
galaxies do not get affected by other galaxies whereas the interacting ones do.
However, this basic difference causes many changes in the way that the galaxy

interactions are a common part of galaxy evolution. An interacting galaxy is
defined as a galaxy whose gravitational field disturbs the gravitational field
of another. A common, minor form of interaction that has been observed is
‘satellite interaction’. This is where the spiral arms of the primary galaxy
are attracted towards a satellite galaxy, drawing the satellite towards the
primary galaxy which in itself is a cause of increased star formation. 1
Gravitational collisions are another type of common galactic interaction,
however these are not true ‘collisions’, they are just interactions which are
caused by the galaxy’s mass distribution. These collisions can turn into galaxy
mergers if the momentum of one or both galaxies is too high. During the merger
process, the galaxies cannot continue travelling after the collision, leaving
them to ‘fall’ into each other repeatedly. These galaxies will eventually
combine to become one after each passes through the other multiple times, with
the larger galaxy finally consuming the smaller.2

cannibalism is the name given to a type of collision where high mass galaxy is
seen to interact and merge with a smaller galaxy. It is thought that these
merger types result in the formation of mainly irregular and rarely elliptical
Another interesting form of galactic interaction is Galaxy Harassment. This
refers to an interaction between two high and low luminosity galaxies. When
these two galaxies have high relative speeds, they interact recurrently with
each other, caused by the luminous galaxy’s high galactic density. The
interactions are thought to morph the two galaxies into disturbed barred spiral
galaxies which are rich in starbursts leading to a sudden massive increase in
star formation rates.
Whether it is an interaction, collision or merger, star formation is almost
always triggered due to the gravitational effects on the matter within the
galaxy; disturbance to the gas and dust causes an increase of star formation

Galaxies are
not typically found in isolation as a galaxy will commonly have multiple
satellite galaxies. However, rarely a galaxy will have very few -if any- galactic
neighbours; this is defined as an isolated galaxy. These, unlike the
interacting galaxies, are not subject to external forces caused by the
interaction of gravitational fields. In the case of elliptical isolated
galaxies, many are classed as ‘fossil groups’. These are thought to be remains
left behind after a colossal merging of a group of large galaxies.

Figure 1: Hubble Tuning Fork Diagram.a

1.2.2 Early-type and Late-type Galaxies

Galaxies can
also be sorted into late type galaxies (LTGs) and early type galaxies (ETGs).
LTGs includes all types of spiral and irregular galaxies whereas ETGS covers
elliptical and lenticular types. These galaxies are so called due to the period
in which they are believed to have formed which is based on the ‘Hubble tuning
fork diagram’. This is a simple morphological classification scheme for
galaxies invented by Hubble and is shown in figure 1.

Figure 2: Graph to show variation of sersic index with
galactic radius.b


1.2.3 Sersic Index


Sersic index
is a useful way by which galaxies can be sorted into disks and elliptical, and
from there they can loosely be sorted into LTG and ETG respectively. Sersic
index is a function that describes the variation of a galaxy’s intensity with
distance from its centre. The index itself Is calculated using equation 1 and
the variation of sersic index with radius can be seen in the graph in figure 2.
The majority of galaxies have an index between 0.5 and 10, however it is the
difference between the galaxies with a sersic index of high or lower than 2.5
that we are using in this project. If n> 2.5 then we say the galaxy is
elliptical, however if n