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Galaxies > Semester 2 - Definitions > Flashcards

Semester 2 - Definitions Flashcards

1
Q

What is the key method for studying large-scale motions within galaxies?

A

Spectroscopy is the key method for probing large-scale motions within galaxies.

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2
Q

How is the Doppler effect used in spectroscopy to study galaxy dynamics?

A

Radiation emitted from gas moving radially, such as stars and nebulae, is Doppler shifted, allowing the measurement of line-of-sight velocities.

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3
Q

What does the Line of Sight Velocity Distribution (LOSVD) represent?

A

The LOSVD represents the distribution of velocities along the line of sight in a galaxy.

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4
Q

How is the observed spectrum of a galaxy affected by the LOSVD?

A

The observed spectrum of a galaxy is a smoothed version of the stellar spectrum, smeared out by the LOSVD.

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5
Q

What factors influence the observed spectrum of a galaxy?

A

The observed spectrum depends on factors such as the age, metallicity, and galaxy environment.

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6
Q

What is the Cross-Correlation Function Method used for in galaxy spectroscopy?

A

The Cross-Correlation Function Method is used to estimate line-of-sight velocities and velocity dispersions.

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7
Q

What information can be obtained from the analysis of the Line of Sight Velocity Distribution (LOSVD)?

A

The LOSVD analysis can reveal the spiral structure of galaxies and trace out features such as HII regions and molecular clouds.

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8
Q

What does the flat rotation curve of the Milky Way suggest about its mass distribution?

A

The flat rotation curve suggests the presence of a halo of dark matter surrounding the galaxy.

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9
Q

How is dark matter distribution inferred in galaxies?

A

Dark matter distribution in galaxies is inferred through observations such as gravitational lensing, rotation curves, and galaxy clustering.

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10
Q

What evidence supports the existence of non-baryonic dark matter?

A

Observations of galaxy clusters, gravitational lensing, and the cosmic microwave background radiation suggest that most dark matter is non-baryonic.

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11
Q

What are some proposed candidates for non-baryonic dark matter?

A

Proposed candidates include Weakly Interacting Massive Particles (WIMPs) such as axions and neutralinos.

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12
Q

How is the Tully-Fisher Relation used in cosmology?

A

The Tully-Fisher Relation allows for the estimation of galaxy distances based on the relationship between galaxy luminosity and rotational velocity.

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13
Q

What is the Fundamental Plane relation for elliptical galaxies?

A

The Fundamental Plane relation describes the correlation between various physical parameters of elliptical galaxies, such as luminosity, size, and velocity dispersion.

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14
Q

Describe the Faber-Jackson relation.

A

The Faber-Jackson relation is a special case of the Fundamental Plane relation, focusing on the correlation between luminosity and velocity dispersion for elliptical galaxies.

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15
Q

What improvements can be made to the Faber-Jackson relation?

A

Improvements include defining galaxy radius to a fixed isophotal value and incorporating the effective radius as a parameter.

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16
Q

How can we classify disk systems when resolving them from photometry is challenging?

A

Spectra can be used instead of photometry for classification.

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17
Q

What types of stars contribute most of the light in disk systems?

A

Most light emerges in the near-infrared from cool K and G type stars.

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18
Q

What spectral characteristics differentiate Sb and Sc galaxies?

A

Sb galaxies have emission lines at around 2000 Å and Hα (6563 Å), while Sc galaxies emit most light in UV and blue with prominent emission lines.

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19
Q

What are starburst galaxies, and what dominates their spectra?

A

Starburst galaxies are systems with recent intense star formation, dominated by UV and blue light with strong emission lines.

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20
Q

When do we get Ultra-luminous Infra-red Galaxy (ULIRG)?

A

If the starburst region of hot young stars is shrouded in dust, we don’t see the UV and blue light. It is then absorbed by the dust and re-radiated mainly in the infrared.

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21
Q

How can we observe Ultra-luminous Infrared Galaxies (ULIRGs) effectively?

A

ULIRGs can be observed comparing B, V, R, I band observations or with NICMOS near-infrared observations from HST.

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22
Q

What triggers starburst events in galaxies?

A

Starburst events are triggered when the gas density in the galactic nucleus is high enough.

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23
Q

What are nuclear star clusters, and what distinguishes them from globular clusters?

A

Nuclear star clusters are formed due to a series of starburst episodes where nuclear star clusters are similar in size and mass to globular clusters but contain both old and young stars due to ongoing star formation.

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24
Q

What are active galactic nuclei (AGNs), and what distinguishes them from normal galaxies?

A

AGNs are compact regions in galaxies emitting substantial non-stellar radiation across the electromagnetic spectrum.

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25
Q

What are the different classifications of AGNs?

A

AGNs can be classified into Seyfert galaxies, radio galaxies, quasars, and blazars based on their observed properties.

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26
Q

What are Seyfert galaxies?

A

Seyfert galaxies are a type of AGN that are generally spirals with highly luminous, unusually blue nuclei. They exhibit strong emission lines in their spectra due to Doppler motions.

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27
Q

What are the two subclasses of Seyfert galaxies originally identified?

A

Seyfert 1 galaxies which exhibit broad emission lines and Seyfert 2 galaxies which exhibit narrow emission lines.

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28
Q

What are radio galaxies?

A

Radio galaxies are a type of AGN which are elliptical or giant elliptical galaxies with bright, point-like nuclei that emit radio waves. They often exhibit highly collimated radio jets and lobes extending well beyond the optical limit of the galaxy.

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29
Q

What are the two main types of radio galaxies based on their spectra?

A

Broad Line Radio Galaxies (BLRGs) and Narrow Line Radio Galaxies (NLRGs).

30
Q

What are quasars?

A

Quasars are a type of extremely luminous and distant AGN that outshine their host galaxies. They emit radiation across the electromagnetic spectrum and exhibit strong, broad emission lines in their spectra.

31
Q

What is a distinguishing feature of quasar spectra?

A

Quasar spectra typically show a “blue bump” in the continuum and a forest of absorption lines.

32
Q

What are blazars?

A

Blazars are a type of active galactic nucleus (AGN) characterized by very rapid variability and high polarization. They exhibit significant changes in luminosity over short timescales and often have spectra devoid of emission lines.

33
Q

What is the unified scheme?

A

The unified scheme proposes that the observed properties of different types of AGN can be understood in terms of different accretion rates onto the central supermassive black hole and different orientations of the AGN and its host galaxy to the line of sight.

34
Q

What is the size of the broad line region in galaxies?

A

The broad line region is very small, with an upper limit of a few light weeks.

35
Q

What is the typical luminosity of a quasar?

A

Typical quasar luminosity is around L≈5×10^39W, close to the Eddington luminosity.

36
Q

What explains the blue bump in the quasar continuum?

A

The blue bump in the quasar continuum is explained by the UV excess from a hot accretion disk.

37
Q

Describe AGN engines.

A

AGN engines are characterized by rapidly rotating, hot, ionized accretion disks with intense magnetic fields. They accelerate particles along collimated, bipolar jets through synchrotron radiation, which open out into huge radio lobes.

38
Q

Describe superluminal motion in AGN jets.

A

Superluminal motion in AGN jets is a result of highly relativistic jets where knots move with velocities close to the speed of light. The basic physical mechanism is the same for all AGN.

39
Q

What are the power sources of AGN?

A

Supermassive Black Holes

40
Q

What are the two types of Quasars?

A

Radio Quite Quasars (90%) and Radio Loud Quasars (10%)

41
Q

Describe polarisation in Quasars?

A

Quasars typically show low polarisation but if they show high polarisation it comes the radio emission comes from the core.

42
Q

The five pieces of evidence that support the unified scheme.

A

1.The broad line region is very small
2. Supermassive black hole in core - AGN luminosities and Schwarzschild radius
3. Accretion of matter onto black hole only realistic power source
4.Hot accretion disk explains the “blue bump” in the quasar continuum
5. AGN “engine” explains jets and radio lobes

43
Q

What is the significance of the viewing angle in a cross sectional view of an AGN?

A

The type of AGN observed depends mainly on the viewing angle. Directly viewing down a jet results in high polarization due to beaming, rapid variability due to time dilation, and significant brightness caused by special relativity effects.

44
Q

What evidence suggests a connection between supermassive black holes and galaxies?

A

There exists a tight relation between the mass of the supermassive black hole at the center of a galaxy and the velocity dispersion of the galaxy bulge, suggesting a strong connection between the formation of the black hole and the galaxy itself.

45
Q

What are the main ingredients in the recipe for galaxy formation?

A

The main ingredients in the recipe for galaxy formation include gravity (dark matter), gas physics (stars), and the background cosmological model.

46
Q

Describe the process known as Dynamical Friction.

A

When galaxies merge, their stars don’t collide, but some of their kinetic energy is transferred to the random motion of the stars. This process, known as Dynamical Friction which depends on factors such as the mass of the galaxy, its velocity, and the mass density of neighboring galaxies.

47
Q

Why do we get a Drag Force?

A

Gravitational pull of the star cluster attracts background matter
towards its location from all directions. Creating an overdensity of matter where the star cluster
was. This gravitational wake acts like a drag force, pulling the star
cluster back and slowing it down.

48
Q

How do galaxies change after mergers or interactions and what happens if there are multiple close encounters?

A

After mergers or interactions, galaxies can experience changes such as disheveled disks, where stars acquire random motions, leading to the formation of spiral arms or bars. Multiple close encounters may destroy disks altogether, explaining the lack of disk galaxies in the cores of rich clusters.

49
Q

Describe a head-on collision between galaxies.

A

A head-on collision between galaxies can produce a polar ring galaxy. During such collisions, the potential energy of the galaxies changes, with one galaxy gaining kinetic energy while the other loses it. After some time, virial equilibrium is restored, often through processes such as the conversion of excess kinetic energy into potential energy.

50
Q

Describe the interaction between gas-rich spirals.

A

Close passages of two gas-rich spirals can produce starburst galaxies, where disk gas is pulled away from near-circular orbits. This leads to high-speed collisions between gas clouds, triggering shocks and compressing gas to very high densities, thereby causing large amounts of star formation.

51
Q

Describe a slower collision or fly by.

A

In a slower collision there is a much greater disturbance. Where the relative velocities of stars in the two galaxies is significantly smaller. Where interactions can draw out tidal tails.

52
Q

What do star formation models aim to understand?

A

Star formation models aim to understand how and where stars form in galaxies, what determines stellar masses and chemical compositions, and how these factors depend on galaxy type, age, and redshift. They also aim to understand what determines stellar luminosities.

53
Q

Describe the formation of the dust lane.

A

instead of a disturber drawing out a tidal tail from the first galaxy. The first galaxy can tidally strip gas and dust leaving behind a dust lane which is a fresh supply of gas that can kick start new star formation.

54
Q

How can star formation models be compared with observations?

A

Star formation models can be compared with observations using spectral synthesis. This involves computing a synthetic spectrum for a model galaxy, considering factors such as its age, chemical composition, initial mass of stars and gas, rate of new star formation, and redshift of observation, and then comparing it with the observed spectrum to best fit galaxy properties.

55
Q

Where does the Lyman break occur.

A

A sharp drop in flux where λ < 400nm is known as
the Lyman Break.

56
Q

Why does the Lyman break occur.

A

The Lyman break occurs because there aren’t many stars hot enough to produce UV photons in great numbers. And any UV photons that are produced can ionise HI clouds, so a large fraction of them are absorbed before they reach us.

57
Q

What are the assumptions of the closed-box chemical evolution model or one zone instantaneous recycling closed box model?

A

The closed-box model assumes that 1) the galaxy’s gas is well-mixed with the same composition everywhere
2) massive stars rapidly return their nuclear products to the interstellar medium (ISM)
3) no gas escapes from the galaxy or is added to it
4) all elements heavier than Helium maintain the same proportion relative to each other.

58
Q

What are the three models for star formation rate?

A

The three main variations are:
a) Instantaneous burst (delta function)
b) Constant star formation rate (SFR)
c) Steep rise followed by exponential decay

59
Q

What does the closed-box model predict about the distribution of metallicities?

A

The closed-box model predicts that the distribution of metallicities should fall off exponentially over time as stars are formed and the gas in the ISM is steadily used up.

60
Q

Explain the G-dwarf problem.

A

The G-dwarf problem arises when testing the closed-box model with observations in the solar neighborhood. It is expected that more than half of the stars in the solar neighborhood should have metallicities less than a quarter of the Sun’s, however, this is not observed.

61
Q

How is the G-dwarf problem resolved?

A

The G-dwarf problem can be resolved by supposing that the initial metallicity of the solar neighborhood was not zero, meaning that the gas was pre-enriched when it arrived at the solar neighborhood.

62
Q

What additional complexities arise in the closed-box model?

A

In the closed-box model, additional complexities arise from variations in the initial metallicity of the gas such that there is a subsequent inflow of fresh, metal-poor gas that leads to the uneven mixing of this gas with the existing ISM. Gas dynamics and sophisticated star formation models are required.

63
Q

What does the Cosmological Principle state about the large-scale structure of the Universe?

A

The Cosmological Principle states that on very large scales the Universe is homogeneous. This is supported by evidence from galaxy surveys and the Cosmic Microwave Background Radiation (CMBR).

64
Q

How is the galaxy distribution on smaller scales characterized?

A

On smaller scales, the galaxy distribution is strongly clustered, forming structures such as galaxies, groups and clusters, filaments, sheets, and voids. This clustered structure was assembled by gravity, causing density fluctuations in the early Universe to grow as the smooth background Universe expands.

65
Q

What is the Concordance model in cosmology?

A

The Concordance model incorporates cold dark matter and a non-zero cosmological constant (Λ). The cosmological constant, often referred to as “dark energy,” dominates the Universe’s energy density and causes its expansion to accelerate.

66
Q

How do galaxy clusters serve as cosmological probes?

A

Galaxy clusters are the largest gravitationally bound structures in the Universe and are not expanding with the background cosmological model. They can be used as probes to study the background cosmology by examining properties such as their abundance as a function of redshift, masses, mass densities, velocity dispersions, and alignments with large-scale structure.

67
Q

What information can be obtained from quasar spectra?

A

Quasar spectra can provide information about the early Universe, including highly redshifted broad emission features and narrow absorption lines, such as the Lyman α forest. Analysis of absorption lines can reveal details about the redshift distribution, density, and filling factor of protogalactic gas clouds and newly formed galaxies.

68
Q

Describe the Gunn-Peterson test.

A

The Gunn-Peterson test compares the flux level on either side of an emission line (Lyman Alpha) to determine the presence of neutral hydrogen. The absence of significant differences suggests that the neutral hydrogen has been ionized, indicating a re-ionized Universe.

69
Q

What can we obtain from observations of the CMBR?

A

Observations of the CMBR, particularly its temperature anisotropies, provide constraints on the re-ionization of the Universe. The scattering of CMBR photons by free electrons during re-ionization smears out temperature anisotropies on small angular scales, allowing us to infer the characteristic redshift at which re-ionization occurred.

70
Q

Where does the Lyman alpha forest originate?

A

From proto-galactic clouds along the line of sight to the quasar where the neutral hydrogen in these clouds partially absorbs the Lyman alpha photons.

Galaxies (4 decks)

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