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“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<2.5, the galaxy is a disk. This knowledge can be used to identify galaxy type when calculating energy emission later. 1.3 Galaxy Structure. A lot can be told about star formation within a galaxy by its external structure. Spiral galaxies, or ETGs, can be assumed to have a high rate of star formation. This is due to their structure; spiral galaxies have arms and two thirds of them contain a bar3.  Spiral galaxies, due to their nature of their outreaching arms, are a common cause of galactic interaction and barred spiral galaxies especially so. Bars in a galaxy, caused by 'density waves' which move from the centre of the galaxy and disturb the inner stars' orbits, can be described as 'stellar nurseries'. The orbital resonance of the bar tends to move gas from the outer arms of the spiral towards the centre, which is a highly effective method of triggering star formation. The assembled star mass in a galaxy is another property which gives us an insight into the SFR. For galaxies with a low stellar mass, the SFR is much higher than those with a high stellar mass, implying that assembled star mass is indirectly proportional to SFR of a galaxy. Using this and the knowledge that ETGs contain 89% of the universe's total stellar mass4 it can be assumed that ETGs have a lower SFR than LTGs. When taking into account that LTGs are made up of disk galaxies and ETGs are made up of bulge galaxies, this information seems intuitive due to the properties of each of these galaxy types. Dark matter halos are another structural component of galaxies. It is thought that they not only make up the majority of the galaxy's mass, but also completely encases it and extends out into space- hence the term 'halo'. Initially, dark matter halo theory was conceived due to the motions of spiral galaxies; it was observed that matter at the edge of spiral galaxies moves just as fast as matter at the centre when this should not be the case. This phenomenon could be explained by assuming that galaxies have a dark matter halo. Not only does this explain the motions of the spiral galaxy, but also it explains why galaxy interactions occur so frequently; without dark matter halos, interactions would be rare. When galaxies move past each other, the dark matter halos interact causing dynamical friction which leads to rapid orbital decay. When the halos pass through each other, the mass distributions are disturbed, creating a gravitational drag between the galaxies. Angular momentum and energy that result from this are absorbed by the halo which allows mergers to commence. This, naturally, triggers star formation in the merged galaxies.5 1.4 Galactic Redshift. Redshift is caused by the movement of an object away from or towards the observer.  Defined as a displacement in wavelengths, redshift gives rise to an apparent 'redshift' or 'blueshift' depending on which direction the object is moving. When the object is moving away, the wavelengths appear to have stretched towards the longer (red) end of the spectrum, whereas when the object is moving closer the opposite happens with an apparent stretch towards the shorter (bluer) end of the spectrum. As discovered by Hubble in the 1930s, all galaxies are redshifted. This is due to the fact that all galaxies are moving away from the Milky Way, which is innate in an expanding universe. However, some galaxies – those that are moving away at a higher speed- have a higher redshift than others. These are the galaxies which tend to have a higher rate of star formation. This is thought to be due to the higher density of gas in these galaxies, which in turn will result in a higher SFR. Figure 3: GOODS-South sky.c   2. Data The data used in this experiment came from the Hubble Space Telescope and the Herschel observatory. The Hubble Space Telescope was launched in 1990 and uses optical wavelengths, among others, to produce detailed images of the sky. Herschel was the largest infrared space telescope ever launched and was active from 2009 to 2013. Herschel had a larger mirror than the HST does, but the detectable wavelength was lower, meaning that the image's resolution was not as high.  The area of sky used in this project was GOODS-South and can be seen in figure 2. This section of sky is used regularly by numerous observatories trying to determine galaxy population, formation and evolution in the universe. Each telescope had produced an image of GOODS-South, for which the data had then been tabulated to include information about the galaxies in the images. 3.Methodology 3.1 Past Method These tables had a record of the required data for each galaxy in the image. This included RA, Dec, Sersic index and fluxes at 5 different wavelengths. Using a program called TopCat, these two tables were matched together using the RA column in each to ensure that the data was corresponding correctly. Figure 3: Graph produced to show background radiation against wavelength. This new 'matched' table was then used to calculate the sum of the total flux received at each wavelength. Using this information, the brightness per steradian could be calculated and put in a graph of flux against wavelength to display the background radiation. Next, the sersic index was used. The sersic index column allowed the galaxies to be split into 'disk' or 'elliptical' based on whether the sersic index was above or below 2.5. This then allowed the total flux from each galaxy type to be calculated which gave the answer to the question of which galaxy type emits the most radiation. 3.2 Future Method A similar method will be followed to the past method described. Instead of the match table being used as before, 1200 images of galaxies will be taken from an HST image- again of GOODS-South. These galaxies will then be sorted into the two categories 'interacting' and 'isolated'. After this has been done, the fluxes of each galaxy for both categories will be summed to give the total radiation emitted. This summation, as star formation rate and galatic energy emission are so closely linked, will be used to determine what fraction of the stellar population were formed in each galaxy type. References.   1 2 3 P. B. Eskridge; J. A. Frogel (1999). "What is the True Fraction of Barred Spiral Galaxies?". Astrophysics and Space Science. 269/270: 427–430.) 4  Eales, S et al (2002). "H:ATLAS/GAMA: Quantifying the Morphological Evolution of the Galaxy Population Using Cosmic Calorimetry". 5 C. Mihos (2006), "Galaxies: Interactions and Mergers" from P. Murdin, "Encyclopedia of Astronomy & Astrophysic". A Las Cumbres Observatory B C    

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