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2: GALAXY TYPES Flashcards

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1
Q

What are the three main morphological types of galaxies + named examples for each?

A
  1. Spirals like M31 and our Galaxy
  2. Irregulars like Magellanic Clouds
  3. Ellipticals (M87)
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2
Q

What does the appearance of elliptical galaxies depend on?

A

The stars they contain. As they emit most light at longer wavelengths they must contain mostly red stars.

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3
Q

What do spiral galaxies contain and what colour do they appear and why?

A

Spirals contain gas and dust
between the stars and the spiral, patterns are delineated by both dark dust lanes and bright regions where new stars are forming from the gas. As a population of young stars will contain bright blue ones, spirals are also bluer than ellipticals.

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4
Q

What are features of the tuning fork diagram produced by Hubble?

A

The handle contains
elliptical galaxies, from circular (as projected on the sky) E0s to more and more elongated E1s to E7s.

The prongs contain spiral galaxies. They can look very elongated if seen edge-on, but often also have a central spheroidal ‘bulge’.

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5
Q

What do the numbers in the elliptical galaxies in the tuning fork diagram represent?

A

The numeral (0 to 7) represents the shape of the galaxy’s image via the quantity 10(1 − b/a), where
b/a is the ratio of the short (minor) to long (major) axis lengths, e.g. an E3 has an axis ratio of 0.7:1.

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6
Q

What do a/b/c/d etc represent in the tuning fork diagram for spirals?

A

Sa galaxies have tightly wound spiral arms, while the pattern in Sb, Sc and Sd galaxies
becomes progressively more open.

Sa have large bulges while Sc have small bulges and bulges are almost non-existent in Sd galaxies

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7
Q

What do spiral galaxies contain and what colour do they appear and why?

A

Spirals contain gas and dust
between the stars and the spiral, patterns are delineated by both dark dust lanes and bright regions where new stars are forming from the gas. As a population of young stars will contain bright blue ones, spirals are also bluer than ellipticals.

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8
Q

Why are there two prongs to the fork diagram?

A

Spirals separate into two sequences depending
on whether the arms start from the central bulge or from the ends of a further component, a central
‘bar’ – the types SBa, SBb, etc.

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9
Q

What are Irregulars in the tuning fork diagram?

A

‘Flat’ but with chaotic
patterns of bright regions.

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10
Q

What are Irregulars often referred to as?

A

Im galaxies with the m standing for Magellanic.

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11
Q

What are Intermediates in the tuning fork diagram?

A

Sm have fragmentary arm-like structures.

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12
Q

S0 (lenticular) galaxies are where the prongs join the handle. What are characteristic of these?

A

These have disc
components, but with no sign of any spiral pattern, and large bulges.

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13
Q

Which galaxies are referred to as ‘early type’ galaxies?

A

E and S0 galaxies.

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14
Q

What are referred to as early type spirals and late type spirals?

A

Sa are early type spirals.
Sc are late type spirals.

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15
Q

What was the numerical sequence introduced by de Vaucouleurs?

A

To represent the main morphological types. These run from T = −5 to 0 for ellipticals and S0s, to 1 for Sa galaxies and so on up to 5 for Sc and 9 for Sm
(and 10 for irregulars), i.e. one numerical class for each Hubble sub-class.

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16
Q

van den Bergh’s scheme attempts to indicate the luminosity of a galaxy. What do ‘grand design’ and ‘flocculent’ represent?

A

More luminous spirals have well-defined continuous arms - grand design.

Low luminosity ones have weak, patchy arms - flocculent.

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17
Q

A luminous Sc galaxy with very clearly defined arms is a ScI and a less bright one
with indistinct arms is a ScIII. What do the Roman numerals represent?

A

the ‘luminosity class’

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18
Q

What are luminosity classes IV and V used for?

A

Im galaxies with low surface brightness (SB)

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19
Q

What do spirals emit and what is it associated with?

A

Radio emission associated with star formation and interstellar matter.

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20
Q

Giant elliptical galaxies are radio galaxies. What are its physical features?

A

Powered by central engines containing massive black holes and exhibit jets and twin lobes of emission on either side.

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21
Q

When do we see thermal re-emission from dust at ~ 10 - 100K?

A

At millimetre, sub-millimetre, and far infra-red (FIR) wavelengths.

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22
Q

What did observations made by IRAS lead to?

A

The discovery of ultra-luminous infra-red galaxies (ULIRGS) powered by starbursts, the rapid formation of large numbers of stars over a short time period.

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23
Q

What are star burst regions enshrouded in?

A

Thick dust layers and ULIRGs have L_FIR&raquo_space; L_opt.

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24
Q

How are x-rays produced?

A

Thermally in galaxies by material at temperatures T > 10^6 K. They can arise from x-ray binary stars or hot gas in or between galaxies, but the most impressive source are Active Galactic Nuclei (AGN) harbouring black holes, where the emission is due to accretion of gas onto the central object.

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25
How many stars does our Galaxy contain and what is its luminosity?
10^11 stars and L ~ 2 x 10^10 solar luminosity.
26
What is the Local Group?
Consists of our Galaxy and M31 and have a number of small companions of 30-40 galaxies.
27
What kind of galaxies are in the Local Group?
3 giant galaxies (our Galaxy, M31, M33), the rest are irregulars (mostly dwarf irregulars, dIs), dwarf ellipticals (dEs) and dwarf spheroidals (dSphs).
28
What are M31's companions?
M32 and NGC 205 are prototype dEs.
29
What do dEs look like? What is a nucleated dE?
Small versions of a normal (giant) E galaxy, ellipsoidal in shape with little or no internal structure. Nucleated dEs contain bright central star clusters.
30
How are dIs similar and different to dEs?
They have similar luminosity but have very clumpy structures.
31
Are dwarf types included in the tuning fork?
No, they have low SB.
32
What are the brightest giant Es in galaxy clusters called and how bright are they?
cD galaxies. Range up to M_B ~ 24, 25 times brighter than our Galaxy.
33
What is the selection effect?
1. Very luminous galaxies really are rare, but we will be biased against including low L systems in our samples, as they must be nearby to look bright. 2. High L galaxies will be visible throughout a large volume of space. Exactly this problem is encountered with stars; apparently, bright stars like Vega and Rigel are mostly distant stars intrinsically much brighter than the Sun, but in a representative volume of space most stars are less luminous than the Sun.
34
What are the brightest giant Es in galaxy clusters called and how bright are they?
cD galaxies. Range up to M_B ~ 24, 25 times brighter than our Galaxy.
35
What does the luminosity function,φ, represent?
The number of galaxies per unit volume (in practice per Mpc3) per unit luminosity interval.
36
What is the power law slope value for Es, S0s, and spirals?
Es and S0s have α ~ −0.5 while spirals have α ~ −1.2.
37
E0s and S0s provide just over half the bright galaxies. What does the less negative faint end slope parameter suggest?
The numbers per magnitude interval do not increase towards faint MB like those of galaxies in general. Thus the overall fraction of galaxies which are giant Es and S0s is fairly small.
38
How do late-type vs early-type spirals differ?
1. Late types have fainter M∗ but steeper α than earlier type spirals. 2. A typical Sd is significantly less luminous than an average Sa. 3. The fractions of galaxies in each spiral type are relatively poorly defined, but amongst brighter spirals there are similar numbers of Sa and Sb galaxies combined as there are of Sc s. At the fainter end Sd s and Sm s dominate.
39
What does α = -1.2 imply?
Implies no. of galaxies per magnitude interval rises slowly towards fainter objects.
40
What does α = - 1 imply?
Same number in each magnitude bin at faint end
41
What does α = - 1.5 imply?
Dwarfs
42
What does α = - 2 imply?
Not possible or total amount of light from the Galaxy population would diverge.
43
What do we need to measure the luminosity function?
Distances. From Hubble's law, we can use redshift as a substitute for distance.
44
What is the k-correction factor for?
Accounts for the fact that we observe a different range of rest frame wavelengths through a given filter if the galaxy spectrum is redshifted.
45
If Fλ is constant, why is there still a k-correction of 2.5 log(1 + z)?
Because a narrower range of rest frame wavelengths contributes to the observed flux from the redshifted galaxy.
46
How do we obtain the LF from 1/Vmax?
We then obtain the LF by summing (1/Vmax) for all galaxies in each magnitude bin.
47
Why is there a greater statistical uncertainty at the faint end of the LF than at the bright end?
We still see far fewer dwarfs than giants in any sample. This uncertainty is increased by the fact that many low L galaxies are also low SB and not all surveys are equally sensitive to such objects.
48
What does the LF give us?
The average number density of galaxies, but galaxies are not uniformly distributed.
49
In the Local Group, how do dEs, dSphs, and dIs occupy the space?
dEs and dSphs occur as satellites of larger galaxies, while dIs can be free-flying and fill the overall volume, which is a region of radius 1.5 Mpc around the mid-point between M31 and the Galaxy.
50
How do galaxy groups tend to arrange themselves?
They tend to line up, making larger filamentary structures. Few galaxies are completely isolated.
51
Are clusters rare? What do they contain?
They are rarer than groups. They contain hundreds to thousands of galaxies.
52
How does the environment influence general galaxy properties?
The denser the region, the greater the fraction of early type galaxies.
53
How does the morphology-density relation affect galaxy distribution?
In small groups and other low density regions (the ‘field’), the fraction of bright galaxies which are spirals is around 80%, with about 20% S0s and very few Es.
54
As we move in from the outskirts of clusters, how does the fraction of spirals, S0s, and E fraction change?
The fraction of spirals decreases steadily (to near zero by the time we reach the densest regions), while the S0 fraction rises steadily (up to ∼ 50%) but is rapidly caught up by the E fraction at high densities.
55
Where do Dwarf galaxies, dEs, dSphs and dIs dominate?
Dwarf galaxies also show a dependance on density, with greater numbers (relative to giants) in clusters; dEs and dSph dominate towards the centres, dIs in the outskirts.
56
What is an example of the Galaxy LF being different in different environments?
The increased function of dwarfs in clusters is equivalent to a steeper faint end of the LF in clusters than the field.
57
What do we need to describe the physical size of a galaxy?
We need the stars’ (or other constituents’) distribution as a function of distance R from the galaxy’s centre.
58
How is the radial distribution seen for stars?
In intensity I, the light emitted per unit area (surface brightness, SB).
59
What is a general measure of the size of the galaxy?
It's half-light or effective radius, Re.
60
What is effective radius, Re?
It's the radius of the circle projected on the sky which encloses half of the total light.
61
What is the approximate radius of large spirals, ellipticals, cD galaxies, and dwarf galaxies?
10kpc, ~ 100 kpc, 200 pc or less
62
What does the part of a galaxy we see depend on?
It's contrast against the night sky and the noise in observations.
63
Measurements of an area of galaxy which emits ng photons can never be more accurate than what?
The Poisson noise, sqrt(ng)
64
When can a galaxy not be distinguished from the background? What is this called?
1. When S/N is small (e.g. 3) 2. Isophotal detection threshold, a minimum intensity level I_min, typically a few percent of the sky background level.
65
The size of the galaxy out to this limit (where I = Imin is its ‘isophotal diameter’. We are not sensitive to the remaining light outside this isophote, so what is a more directly measurable quantity?
Instead of the total luminosity L_T, it's is the isophotal luminosity L_iso, which only integrates the light out to the isophotal radius.
66
Do high luminosity galaxies also have high SB?
Yes
67
Why are Dwarf galaxies hard to see?
They are usually low SB, hence diffuse and hard to see against the foreground glow from our atmosphere
68
What is low SB caused by?
Low surface density of stars.
69
What are the units of SB?
Solar luminosity/pc^2 or magnitudes per square arcsec (µ).
70
What is 1 solar L/pc^2 in Wm^-2?
4.05E-7
71
How do you calculate the mean SB inside the effective radius?
(L/2*pi*R_e^2)
72
What is the mean SB inside the effective radius for giant ellipticals and spirals?
100 solar L/pc^2
73
Ellipticals have I(r) rising more steeply towards the centre than spiral disks so the central values differ widely. What are the approximate values?
A few 10^2 solar luminosities/pc^2 for disks and ~10^4 solar luminosities/pc^2 for Es and bulges
74
How much lower is the surface brightness in Dwarf galaxies than giants?
A factor of 10 lower.
75
Do all Dwarf galaxies have low SB? (2)
No, Blue Compact Dwarves contain small, blue high SB central regions. Ultra Compact Dwarfs have properties between normal dwarfs and globular clusters.
76
What is the equation for azimuthally averaged intensity distribution for spiral galaxy disks seen face-on? Give in magnitudes too.
I(R) = I_0 exp (-R/a) µ(R) = µ_0 + 1.086(R/a)
77
What do R and a represent?
R. and a can be either physical radii (in kpc) or observed angular sizes on the sky (in arcsec).
78
What is the de Vaucouleurs/R^(1/4) law?
I(R) = I_0 exp (-(R/a)^1/4) or µ(R) = µ_0 + 1.086(R/a)^1/4
79
What law do bulge components or spiral galaxies follow?
The R^1/4 law
80
Which galaxies have profiles close to exponentials?
dE, dSph, dI
81
Typical E has R_e is approximately a few kpc, what does the length scale to?
~ 1 pc
82
What is the core in very luminous elliptical galaxies?
A region where I is nearly constant
83
How does the SB behave in less luminous ellipticals?
They rise steeply to a cusp in the centre.
84
For a cuspy profile, what must the 3D density do and what is the luminosity density at the centre of cuspy ellipticals?
It must be rising faster than r^-1. The luminosity density can be 10^6 solar luminosity/pc^3.
85
What is the relationship between luminosity and surface brightness? (one in terms of a and one in terms of Re)
L ∝ I_0*a^2 ∝ I_e*Re^2
86
What is the Kormendy relation?
Brighter Es are larger but the increase in a more than accounts for the increase in L, so the SB goes down.
87
Dwarf ellipticals with MB > -18 have L-SB relationships. How do these relationships behave?
They go in the opposite direction.
88
What is the equation that represents a good fit for the Kormendy's original relation for giant Es? (2) What does this imply?
µe = 20.2 + 3.0 logRe MB = −19.3 − 2.0 logRe. L ∝ Re^0.8 and Ie ∝ Re^-1.2 ∝ L^−1.5 .
89
Assuming Es have regular isodensity surfaces, what are there 3 possible 3D shapes?
1. Oblate spheroids 2. Prolate spheroids 3. Tri-axial ellipsoid
90
What do oblate spheroids look like?
They are squashed down at the poles. Look flattened in 3D projection. For most viewing angles, oblate spheroids should appear nearly circular, so the relatively number of E0 galaxies implies that Es aren't usually oblate.
91
What do prolate spheroids look like?
Has one long and two short axes
92
Of which 3D shapes is a luminous E most likely to take?
Tri-axial .
93
What gives rise to twisting isophotes?
Projection at general tri-axial isodensity surfaces
94
What are the two kinds of isophotes and how do they differ?
Disky isophotes - more elongated along the major axis (pointer) Boxy isophotes - Compressed along the axes (more rectangular)
95
What are boxy Es more likely to be?
Radio & x-ray transmitters and have central cores.
96
What are medium L, cuspy Es more likely to be?
Disky and also faster rotating.
97
What are disky Es intermediate between?
Other Es and S0s
98
How do S0s resemble Es?
They resemble as smooth ensembles of stars but with large disk contribution.
99
In S0s, how much does the bulge contribute to light? How does the disk to bulge ratio vary?
50-60%. Varies from about 0.1 to 2.
100
How are spirals characterised?
Thin, flat disks and a central bulge.
101
Which galaxies have the largest B/D ratio of ~1?
Earliest type spirals and S0s
102
What is the B/D ratio for Sb galaxies?
~ 0.3
103
How much is the B/D ratio for later type spirals
< 0.1
104
What do Sd and Sm galaxies contain and what are the absent of?
They have no real bulge but can have a central star cluster
105
What 3D shape does a bulge take?
Fairly round spheroids, quite flattened, or even prolate tri-axial ellipsoids.
106
How are bulges in our Galaxy best viewed?
In the IR where the dust extinction in the intervening disk is minimised
107
How much of the stellar mass does the bulge contain?
20%
108
How many spirals and S0s have a bar?
Roughly half of them
109
Where does the bar co-exist with the bulge?
In early type barred spirals
110
Name the maximum axis ratio of bars, how much of total galaxy light they can contain, and their shape.
Up to 5. 30%. Flat
111
Shape of the pattern of a spiral arm can be represented by a log scale in polar coordinates. What is the equation?
ln(R/R0) = k*theta
112
What does the pitch angle measure and what is the equation?
It measures the angle between the direction of the arm and the tangent to a circle at the same R. tan i = (1/R)(dr/d*theta)
113
What is the pitch angle for Sa galaxies, mid-type spirals, and late types?
i ~ 5 degrees i ~ 10-12 degrees i ~ 20 degrees
114
In two armed spirals, where are the brightness peaks?
At radius R there are two brightness peaks 180 degrees apart.
115
Where are the spiral arms superimposed?
At the exponential disk of starlight.
116
State Freeman's law.
Spiral galaxies have the same surface brightness at the center i.e. within a small range, if any bulge is excluded
117
What will the scale size and isophotal radius be for a galaxy of very high SB?
Small scale size and its isophotal radius will be small.
118
Even if the central disk regions are hidden by a bulge, how can we estimate the disk brightness at the centre?
We can extrapolate the radial profile from further out to estimate the disk brightness at the centre, the extrapolated central SB, µ0.
119
How does having the same L_T but different µ0 affect the SB and isophotal radius of a galaxy?
A galaxy of very high SB will have a very small scale size and its isophotal radius will be small. If the SB is very low, only the central region is above the limiting isophote and against the isophotal radius is small.
120
How does the bias affect the inclusion of galaxies in samples? (4)
1. If we only include objects with images above some minimum angular diameter,. Galaxies with large physical isophotal sizes will be included out to much larger distances and hence in relatively large numbers. 2. Galaxies with high or low SB exceed the angular diameter limit only if quite local. 3. Very low SB galaxies may not rise above a survey’s SB threshold at all. Similarly for isophotal magnitudes, low SB galaxies have a smaller fraction (possibly none) of their total light inside the limiting isophote, so are less likely to be included. 4. Galaxies with low SB and low L (i.e. dwarfs) are doubly selected against.
121
What is the radial surface density profile of face-on disks?
Exponential.
122
In our Galaxy, how is vertical structure found?
Via star counts
123
What stars are likely to lie in the thin disk?
Young luminous stars
124
What is the signature of the thick disk?
K dwarves, a "tail" to higher Z distances
125
What is the halo and what astrophysical body is associated with it?
A diffuse spheroidal extension of the bulge with stellar density fall off r^-3. GCs associated with it.
126
What are the number of GCs related to?
The luminosity of the galaxy.
127
Define specific frequency SN
The number per unit galaxy light (usually 10E8 solar masses)
128
What is the SN value for spiral galaxies and Es?
Spirals ~ 0.4 Es ~ 2

Galaxies (6 decks)

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