This and interacting galaxies. 1 Introduction 1.1

This
experiment requires that we use results from the Herschel and Hubble space telescopes
to show the fraction of stars that form in isolating galaxies and interacting
galaxies. First we will understand the general questions surrounding galaxy
evolution, including the idea of galaxy interactions and the effect
interactions have on the gas and the stars. Further in the assignment we will
classify galaxy types from Hubble Space Telescope images and Herschel of
GOODS-South into isolated and interacting. By measuring the output of energy we
shall conclude this experiment by determining the fractions of stars formed in
isolating and interacting galaxies.

 

1 Introduction

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1.1 Background
Information

It was first thought that
galaxies were formed and evolved in isolation. However, as research continues
and extra evidence is attained this is found to be false. Strong evidence
suggests that most galaxies have not only interacted with another but a high
proportion of stars are formed due to galaxies interacting. Insert Reference. Galaxies
are often found in groups and being immensely heavy objects they can interact
quite strongly with one another, with a substantial fraction of galaxies having
interacted in the early stages of the universe. Theories suggest that most
galaxies are subject to some form of interaction over their lifespan. (Mihos et
al. 2006).

1.2
Galaxy Types, Classification, and formation

Galaxies
are separated into three main categories: Spiral, Elliptical, and Irregular. Irregular
galaxies are smaller galaxies that do not have a distinctive profile (Butz et
al. 2002) they are frequently liable to collisions with larger galaxies
(Elmegreen et al. 2002). However, the main focus of this experiment will be
elliptical and spiral galaxies. Elliptical galaxies, known as early type
galaxies (ETGs), are categorised based on their
ellipticity
from “E0” seeming to be perfectly circular and “E7” appearing to be flattened.
Spiral galaxies, known as late type galaxies (LTGs), are classified by letters starting
at “a” going to “c”. The letters correspond to how condensed the spiral arms
are, “Sc” represents spirals that are less compact and “Sa” signifying
constricted arms. LTGs can be further categorised into barred spirals and
normal spirals. A barred spiral has a bar running through the central bulge
made of stars. This is shown as “SBa” for a tightly bound barred spiral galaxy.
“S0” galaxies, known as lenticular, are alike to E7 elliptical galaxies, with
similarities in colour and shape to some extent.  S0 galaxies show signs of a disk which is not
present in elliptical galaxies. This can be seen in the Hubble sequence in figure
1.1 below.

ETGs
can also be split up into fast and slow rotators where fast rotators share
similar properties to that of LTGs and slow rotators being less abundant than
fast rotators (Emsellem et al. 2011).

ETGs
are often formed of low-mass, older, red stars and scarce of dust this results
in ETGs having a low SFR and seeming to be red in appearance. ETGs are not void
of dust however as 50% of ETGs contain dust, implying that there is a steady variation
in the properties of dust across the Hubble sequence (Eales et al. 2015). LTGs
on the other hand are filled with gas, leading to high SFR, from this LTGs
contain young blue stars giving a blue colour appearance. There is not only a
large overlap between the red passive, quiescent galaxies and ETGs but also
between the star-forming blue galaxies and LTGs (Eales et al. 2015). This has
been confirmed true for both low redshift and high redshift (Kennicutt 1998;
Bell et al. 2004).

The
process to how galaxies are formed is not fully understood but there are strong
theories as to how they form. One theory is that the rotational velocity of the
cloud of gas that eventually formed the galaxy has a part to play in the shape
of the galaxy produced. If the cloud was spinning as it collapsed its velocity
would increase, flattening into a spiral galaxy. This is opposed to a cloud of
gas that was not rotating which in turn would form an elliptical galaxy (Erin
McNally 2000; Pearson Eductation 2004). The second theory is that spiral
galaxies interact and merge with each other to form elliptical galaxies, this
will be covered in chapter 1.5.

1.3
Galaxy Structure

The
different Galaxies have varying structures; these structures can determine
several characteristics about the galaxy which can contribute to the star
formation rate (SFR).

Cold
filamentary streams of gas are thought to be how galaxies get their energy for
star formation (Keres et al. 2005). A spiral galaxy with a strong bar will have
more gas flow into the central parsec of the galaxy. If the galaxy does not
have a bar, gas tends to be left in the inner few kilo parsecs (Mihos et al.
2006). With more gas fuelling the galaxy SFR will be higher than a normal spiral
galaxy. Bar galaxies are disk dominated, so are abundant of stars. The disk is
the region that surrounds the bulge of the galaxy in a flat disk shape, found
only in spiral galaxies. It is filled with stars, gas, and dust, found in the
spiral arms. The spiral arms extend out of the centre of the galaxy and can be
‘loose’ or ‘tight’ as seen in figure 1.1.

Elliptical
and spiral galaxies both contain a bulge structure, a sphere shaped area found
in the surrounding the centre of the galaxy made up of closely gathered stars.
Though for elliptical galaxies they contain old stars and have minimal star
production, these are called classical bulges. In spiral galaxies studies from
the Hubble Space Telescope have “pseudobulges”. The stars in these pseudo
bulges orbit in an organised fashion and the SFR can be as high as the SFR in
disk galaxies (Kormendy et al. 2004). It was considered that pseudobulges were
a result of galaxies merging, though there is doubt as it is unlikely that the
disks endure the merging. It is proposed that it is the galaxy repositioning
the stars and gas to counter instabilities. However, Classical bulges are
likely caused in merges (Kormendy et al. 2004). Pseudobulges are also found in
some ETGs (Kormendy et al. 2013).

The bulge is surrounded by a
halo that comprises of old stars and dark matter in a spherical volume that in
cases the galaxy. This can be seen if figure 1.2.

Outside of the visible halo
is a hypothetical component of galaxies, though it cannot be observed or
detected there is strong evidence for their existence. They have a
gravitational effect on the stars and gas inside the galaxy. Rotational
velocity is expected to decrease at the distant edges of the galaxy. However,
this is not the case, the rotational velocity plateaus and remains very
consistent. This is believed to be due to the large quantities of mass from the
dark matter halo. These halos can extend out up to hundreds of kilo parsecs
from the galaxy (Kormendy et al. 2005).

1.4
Redshift and Dust 

As
galaxies are billions of years old it makes it problematic to comprehend how
they evolve. Redshift aids this field of study as we are able to observe
galaxies millions of years in the past, as it has taken the time equivalent to
distance to reach Earth. Galaxies at higher redshift have been observed to have
a higher SFR than similar galaxies today (Wuyts et al. 2011).

·        
Talk more about redshift, refer to paper 1

·        
Peak sfr was at z-2

When
observing galaxies, they can often appear passive in the optical waveband this
is due to cosmic dust in galaxies. Cosmic dust are miniscule dust particles
found in space and are likely from the remanence of supernovae. Cosmic dust
absorbs about 50% of all energy emitted from stars, this is higher at z > 1
(Eales et al. 2015). This absorbed energy makes it difficult to observe the
galaxies at high redshift in the optical wavelength. The Herschel space
telescope can overcome this issue as it can measure in the far-infrared
wavelength. This will be discussed later.

1.5
Interactions and Mergers

Galaxies
can interact and merge with most galaxies being believed to have already
encountered another galaxy in their lifetime. There is evidence of past mergers
that can be seen in the structure of galaxies, these galaxies are likely to
have bars, bridges, and tails. The tails are caused by the strong gravitational
waves acting as a tidal force that stretches the stars and gas when the
galaxies interact (Mihos et al. 2006). This can be seen in figure 1.3.

These
tidal torque forces caused by the merger channels gas and dust towards the
centre of the galaxies, into the active galactic nuclei (AGN), causing a rapid
increase in SFR. This is known as nuclear starburst (Barnes & Herquist
1991). Galaxies undergoing a nuclear starburst can produce over 100 stars a
year, where an average galaxy will produce 1-4 a year. Due to the extremely
high SFR, supernova dispersing gas, gas-exhaustion, and AGN feedback the galaxy
will completely use its gas reserves. This leads to SFR in the galaxy to
drastically plummet, leaving a red and dead quiescent galaxy. This is called a post starburst
galaxy (Wuyts et al. 2010; Snyder et al. 2011). After a few million years these
galaxies would have run out of gas. The timescale in which a galaxy undergoes
nuclear starburst is far shorter than the lifetime of the galaxy but it changes
the galaxy dramatically.

Galaxies
can also be interacting even when they appear not to pass by one another. This
is due to the dark matter halos discussed earlier that extend far beyond the
visible borders of the galaxy. The dark matter halos from the galaxies can
interact with one another, this causes a quick orbital decay triggered by
dynamical friction. The halos are sped up as the galaxies collide and slow down
as the energy and angular momentum are transferred. The halos absorb massive
amounts of energy, and it is believed that galaxy interactions occur because of
dark matter halos (Mihos et al. 2006).  This is due to the sheer mass of the dark
matter halos that cause strong gravitational forces on each other.

There
is strong evidence to support the theory that elliptical galaxies are formed
from the merging of other galaxies. Elliptical galaxies are scarce of gas and
dust, implying that they have used up their gas reservoirs, likely due to going
through a nuclear star burst phase as discussed previously. The likelihood of
elliptical galaxies previously interacting is expected as in the early stages
of the universe the galaxies were in a closer proximity so interactions were
more probable. Elliptical galaxies are also frequently found in galaxy clusters
where mergers were also expected (Erin McNally 2000; Pearson Education 2005). It
is also believed that when galaxies collide it sends the stars and dust in arbitrary
directions, this leads to the stars in the elliptical galaxy travelling in moderately
circular pattern.

As
we found in our results and from previous studies, at higher redshifts, LTGs
contained the majority of the stellar mass density (SMD) and also had higher
SFR. However, the majority of SMD is in ETGThis
experiment requires that we use results from the Herschel and Hubble space telescopes
to show the fraction of stars that form in isolating galaxies and interacting
galaxies. First we will understand the general questions surrounding galaxy
evolution, including the idea of galaxy interactions and the effect
interactions have on the gas and the stars. Further in the assignment we will
classify galaxy types from Hubble Space Telescope images and Herschel of
GOODS-South into isolated and interacting. By measuring the output of energy we
shall conclude this experiment by determining the fractions of stars formed in
isolating and interacting galaxies.

 

1 Introduction

1.1 Background
Information

It was first thought that
galaxies were formed and evolved in isolation. However, as research continues
and extra evidence is attained this is found to be false. Strong evidence
suggests that most galaxies have not only interacted with another but a high
proportion of stars are formed due to galaxies interacting. Insert Reference. Galaxies
are often found in groups and being immensely heavy objects they can interact
quite strongly with one another, with a substantial fraction of galaxies having
interacted in the early stages of the universe. Theories suggest that most
galaxies are subject to some form of interaction over their lifespan. (Mihos et
al. 2006).

1.2
Galaxy Types, Classification, and formation

Galaxies
are separated into three main categories: Spiral, Elliptical, and Irregular. Irregular
galaxies are smaller galaxies that do not have a distinctive profile (Butz et
al. 2002) they are frequently liable to collisions with larger galaxies
(Elmegreen et al. 2002). However, the main focus of this experiment will be
elliptical and spiral galaxies. Elliptical galaxies, known as early type
galaxies (ETGs), are categorised based on their
ellipticity
from “E0” seeming to be perfectly circular and “E7” appearing to be flattened.
Spiral galaxies, known as late type galaxies (LTGs), are classified by letters starting
at “a” going to “c”. The letters correspond to how condensed the spiral arms
are, “Sc” represents spirals that are less compact and “Sa” signifying
constricted arms. LTGs can be further categorised into barred spirals and
normal spirals. A barred spiral has a bar running through the central bulge
made of stars. This is shown as “SBa” for a tightly bound barred spiral galaxy.
“S0” galaxies, known as lenticular, are alike to E7 elliptical galaxies, with
similarities in colour and shape to some extent.  S0 galaxies show signs of a disk which is not
present in elliptical galaxies. This can be seen in the Hubble sequence in figure
1.1 below.

ETGs
can also be split up into fast and slow rotators where fast rotators share
similar properties to that of LTGs and slow rotators being less abundant than
fast rotators (Emsellem et al. 2011).

ETGs
are often formed of low-mass, older, red stars and scarce of dust this results
in ETGs having a low SFR and seeming to be red in appearance. ETGs are not void
of dust however as 50% of ETGs contain dust, implying that there is a steady variation
in the properties of dust across the Hubble sequence (Eales et al. 2015). LTGs
on the other hand are filled with gas, leading to high SFR, from this LTGs
contain young blue stars giving a blue colour appearance. There is not only a
large overlap between the red passive, quiescent galaxies and ETGs but also
between the star-forming blue galaxies and LTGs (Eales et al. 2015). This has
been confirmed true for both low redshift and high redshift (Kennicutt 1998;
Bell et al. 2004).

The
process to how galaxies are formed is not fully understood but there are strong
theories as to how they form. One theory is that the rotational velocity of the
cloud of gas that eventually formed the galaxy has a part to play in the shape
of the galaxy produced. If the cloud was spinning as it collapsed its velocity
would increase, flattening into a spiral galaxy. This is opposed to a cloud of
gas that was not rotating which in turn would form an elliptical galaxy (Erin
McNally 2000; Pearson Eductation 2004). The second theory is that spiral
galaxies interact and merge with each other to form elliptical galaxies, this
will be covered in chapter 1.5.

1.3
Galaxy Structure

The
different Galaxies have varying structures; these structures can determine
several characteristics about the galaxy which can contribute to the star
formation rate (SFR).

Cold
filamentary streams of gas are thought to be how galaxies get their energy for
star formation (Keres et al. 2005). A spiral galaxy with a strong bar will have
more gas flow into the central parsec of the galaxy. If the galaxy does not
have a bar, gas tends to be left in the inner few kilo parsecs (Mihos et al.
2006). With more gas fuelling the galaxy SFR will be higher than a normal spiral
galaxy. Bar galaxies are disk dominated, so are abundant of stars. The disk is
the region that surrounds the bulge of the galaxy in a flat disk shape, found
only in spiral galaxies. It is filled with stars, gas, and dust, found in the
spiral arms. The spiral arms extend out of the centre of the galaxy and can be
‘loose’ or ‘tight’ as seen in figure 1.1.

Elliptical
and spiral galaxies both contain a bulge structure, a sphere shaped area found
in the surrounding the centre of the galaxy made up of closely gathered stars.
Though for elliptical galaxies they contain old stars and have minimal star
production, these are called classical bulges. In spiral galaxies studies from
the Hubble Space Telescope have “pseudobulges”. The stars in these pseudo
bulges orbit in an organised fashion and the SFR can be as high as the SFR in
disk galaxies (Kormendy et al. 2004). It was considered that pseudobulges were
a result of galaxies merging, though there is doubt as it is unlikely that the
disks endure the merging. It is proposed that it is the galaxy repositioning
the stars and gas to counter instabilities. However, Classical bulges are
likely caused in merges (Kormendy et al. 2004). Pseudobulges are also found in
some ETGs (Kormendy et al. 2013).

The bulge is surrounded by a
halo that comprises of old stars and dark matter in a spherical volume that in
cases the galaxy. This can be seen if figure 1.2.

Outside of the visible halo
is a hypothetical component of galaxies, though it cannot be observed or
detected there is strong evidence for their existence. They have a
gravitational effect on the stars and gas inside the galaxy. Rotational
velocity is expected to decrease at the distant edges of the galaxy. However,
this is not the case, the rotational velocity plateaus and remains very
consistent. This is believed to be due to the large quantities of mass from the
dark matter halo. These halos can extend out up to hundreds of kilo parsecs
from the galaxy (Kormendy et al. 2005).

1.4
Redshift and Dust 

As
galaxies are billions of years old it makes it problematic to comprehend how
they evolve. Redshift aids this field of study as we are able to observe
galaxies millions of years in the past, as it has taken the time equivalent to
distance to reach Earth. Galaxies at higher redshift have been observed to have
a higher SFR than similar galaxies today (Wuyts et al. 2011).

·        
Talk more about redshift, refer to paper 1

·        
Peak sfr was at z-2

When
observing galaxies, they can often appear passive in the optical waveband this
is due to cosmic dust in galaxies. Cosmic dust are miniscule dust particles
found in space and are likely from the remanence of supernovae. Cosmic dust
absorbs about 50% of all energy emitted from stars, this is higher at z > 1
(Eales et al. 2015). This absorbed energy makes it difficult to observe the
galaxies at high redshift in the optical wavelength. The Herschel space
telescope can overcome this issue as it can measure in the far-infrared
wavelength. This will be discussed later.

1.5
Interactions and Mergers

Galaxies
can interact and merge with most galaxies being believed to have already
encountered another galaxy in their lifetime. There is evidence of past mergers
that can be seen in the structure of galaxies, these galaxies are likely to
have bars, bridges, and tails. The tails are caused by the strong gravitational
waves acting as a tidal force that stretches the stars and gas when the
galaxies interact (Mihos et al. 2006). This can be seen in figure 1.3.

These
tidal torque forces caused by the merger channels gas and dust towards the
centre of the galaxies, into the active galactic nuclei (AGN), causing a rapid
increase in SFR. This is known as nuclear starburst (Barnes & Herquist
1991). Galaxies undergoing a nuclear starburst can produce over 100 stars a
year, where an average galaxy will produce 1-4 a year. Due to the extremely
high SFR, supernova dispersing gas, gas-exhaustion, and AGN feedback the galaxy
will completely use its gas reserves. This leads to SFR in the galaxy to
drastically plummet, leaving a red and dead quiescent galaxy. This is called a post starburst
galaxy (Wuyts et al. 2010; Snyder et al. 2011). After a few million years these
galaxies would have run out of gas. The timescale in which a galaxy undergoes
nuclear starburst is far shorter than the lifetime of the galaxy but it changes
the galaxy dramatically.

Galaxies
can also be interacting even when they appear not to pass by one another. This
is due to the dark matter halos discussed earlier that extend far beyond the
visible borders of the galaxy. The dark matter halos from the galaxies can
interact with one another, this causes a quick orbital decay triggered by
dynamical friction. The halos are sped up as the galaxies collide and slow down
as the energy and angular momentum are transferred. The halos absorb massive
amounts of energy, and it is believed that galaxy interactions occur because of
dark matter halos (Mihos et al. 2006).  This is due to the sheer mass of the dark
matter halos that cause strong gravitational forces on each other.

There
is strong evidence to support the theory that elliptical galaxies are formed
from the merging of other galaxies. Elliptical galaxies are scarce of gas and
dust, implying that they have used up their gas reservoirs, likely due to going
through a nuclear star burst phase as discussed previously. The likelihood of
elliptical galaxies previously interacting is expected as in the early stages
of the universe the galaxies were in a closer proximity so interactions were
more probable. Elliptical galaxies are also frequently found in galaxy clusters
where mergers were also expected (Erin McNally 2000; Pearson Education 2005). It
is also believed that when galaxies collide it sends the stars and dust in arbitrary
directions, this leads to the stars in the elliptical galaxy travelling in moderately
circular pattern.

As
we found in our results and from previous studies, at higher redshifts, LTGs
contained the majority of the stellar mass density (SMD) and also had higher
SFR. However, the majority of SMD is in ETGs, around 71% (Eales et al. 2015).
This shows evidence that there must be an evolution process that causes ETGs to
have such high SMD.s, around 71% (Eales et al. 2015).
This shows evidence that there must be an evolution process that causes ETGs to
have such high SMD.