ASTR 209 - Ch. 24: Galaxies
The most useful standard candle must (2)
- Have a well-defined luminosity so that uncertainty in estimating its brightness is small - Be bright enough to be seen at large distances (e.g. novae, emission nebulae, planetary nebulae, globular clusters, Type I supernovae and even entire galaxies)
Describe some of the basic properties of Seyfert galaxies.
- Properties lie between those of normal galaxies and those of active galaxies - Most energy emitted from galactic nucleus which is 10,000x brighter than ours - Has spiral arms and a galactic disk - Rapid luminosity fluctuations
As a class, all active galaxies have the same 6 properties:
- high luminosities - nonstellar energy emission - highly variable energy output (implying a small nucleus) - may exhibit jets and other explosive activity - broad emission lines indicating rapid internal motion - activity associated with interactions between galaxies
What is the most likely range of values for Hubble's constant? What are the uncertainties in its value?
70 km/s/Mpc (kilometers per megaparsec) The uncertainty in Hubble's constant translates directly into similar uncertainty in all distances determined from Hubble's law.
Galaxy Rotation
A galaxy's rotation causes some of the radiation it emits to be blue shifted and some to be red shifted. Alternative to standard candles in determining luminosities and therefore distances of spiral galaxies.
Tully-Fisher Relation
A relation used to determine the absolute luminosity of a spiral galaxy. The rotational velocity, measured from the broadening of spectral lines, is related to the total mass, hence the total luminosity.
Quasar
A starlike radio source with an observed redshift that indicates an extremely large distance from Earth. The brightest nucleus of a distant galaxy. - Not stars because of high luminosity - Closest quasar 240 mpc from Milky Way
Radio galaxies
Active galaxies that emit large amounts of energy in the radio portion of the electromagnetic spectrum. They differ from Seyfert's not only in the wavelengths at which they radiate, but also in both the appearance and the extent of their emitting regions.
Standard Candle
Any object with an easily recognizable appearance and known luminosity. Supernovae, which all have the same luminosity (depending on type) are good examples of standard candles and are used to determine distances to other galaxies.
Barred Spiral Galaxy
Barred spirals differ from ordinary spirals mainly by the presence of an elongated 'bar' of stellar and interstellar matter passing through the center and extending beyond the bulge, into the disk. Stellar density is highest in the center. SBa-----SBb-----SBc largest to smallest tight to loose spiral arms
What are some of the problems astronomers encounter in measuring the distances of far away galaxies?
Because distance-measurement techniques ultimately rely upon the existence of very bright objects whose luminosities can be inferred by other means. Such objects become increasingly hard to find and calibrate the farther we look into intergalactic space.
Why do astronomers prefer to speak in terms of redshift rather than distances to far away objects?
Because it is a directly observable property of an object, whereas distance is derived from a redshift with use of Hubble's Constant, whose value is not accurately known.
What distinguishes one spiral galaxy from another?
Both are composed of a flattened, star-forming disk with a large central galactic bulge and spiral arms. However, one type also has an elongated bar of stellar and interstellar medium passing through the centre and extending beyond the bulge, into the disk. Spiral galaxies of both types are also characterized by their size and the corresponding tight or looseness of their spiral arms.
How is the Tully-Fisher relation used to measure distances to galaxies?
By measuring the amount of broadening in emission lines, we can determine a galaxy's rotation. Once we know the rotation, we can determine luminosity. Comparing a galaxy's luminosity with its apparent brightness gives us its distance.
What is the evidence that the radio lobes of some active galaxies consist of material ejected from the galaxy's center?
Centarus A is a member of a small cluster of galaxies. Its radio lobes are roughly symmetrical, jutting out from the center of the galaxy and perpendicular to the dust lane, suggesting that they consist of material ejected in opposite directions from the galactic nucleus.
Galaxy cluster
Collection of galaxies held together by their mutual gravitational attraction.
Spiral Galaxy
Galaxy composed of a flattened, star-forming disk component which may have spiral arms and large central galactic bulge. Steller density is highest in the center. Sa ----Sb ----Sc largest to smallest tight to loose spiral arms
Elliptical galaxies
Have no spiral arms and no obvious galactic disk. Other than possessing a dense central nucleus, they often exhibit little internal structure of any kind. Stellar density highest in center. E0-E1-E2-E3-E4-E5-E6-E7 Most circular to most elongated - giant ellipticals can be 100s of kpc across and contain trillions of stars - dwarf ellipticals can be as small as 1kpc, these are the most common type - most ellipticals contain almost no gas or dust
Astronomers classify elliptical galaxies by
How flattened they appear
How do we know that the energy-emitting regions of many active galaxies must be very small?
If an object's luminosity fluctuates rapidly, then it must be compact; an object cannot 'flicker' in less time than radiation takes to cross it.
S0 and SB0 Galaxies
Intermediate between elliptical and spiral galaxies. - show evidence of a thin disk and flattened bulge, but contain no gas and no spiral arms. - S0 has no bar of stellar/interstellar matter - SB0 has a bar of stellar/interstellar matter
How does the use of Hubble's law differ from the other extragalactic distance-measurement techniques we have seen?
It doesn't use the inverse-square law. The other methods all provide a way of determining luminosity of a distant object, which is then converted to a distance using the inverse-square law. Hubble's law gives a direct connection between redshift and distance.
The energy emission from an active galactic nucleus does not resemble a blackbody curve. Why is this important?
It means that the energy source cannot simply be the summed energy of a huge number of stars - some other mechanism must be at work.
What is Hubble's Law?
Law that relates the observed velocity of recession of a galaxy to its distance from us. The velocity of recession of a galaxy is directly proportional to its distance away.
Starburst galaxies
Luminous galaxies with nonstellar emission; previously normal systems currently characterized by widespread episodes of star formation, most likely a result of interactions with a neighbour.
The Local Group
Made up of nearly 50 galaxies within approx. 1 Mpc of our Milky Way Galaxy. Only a few are spirals, most are dwarf elliptical or irregular galaxies. - Milky Way and Andromeda are largest members - Most other galaxies are gravitationally bound to them
In what way are large spirals like the Milky Way and Andromeda not representative of galaxies as a whole?
Most galaxies are not large spirals - the most common galaxy types are dwarf ellipticals and dwarf irregulars.
Young stars in the galactic disk are
Mostly found in the spiral arms
Blazar
Particularly intense active galactic nucleus in which the observer's line of sight happen to lie directly along the axis of a high-speed jet of particles emitted from the active region.
Hubble Diagram
Plots the recessional velocity and distance of a galaxy
Megallanic Clouds
Prominent features of the night sky that are dwarf irregular (Irr I) galaxies gravitationally bound to the Milky Way.
How do we know that quasars are extremely luminous?
Quasaras are very far away, therefore they appear small and unimpressive to us. However, they are extremely luminous. At their distances from our galaxy (240 Mpc) it's amazing that we can see them at all.
Radio lobe
Roundish extended region of radio-emitting gas, lying well beyond the center of a radio galaxy. - Undetectable by visible light
What are standard candles and why are they important in astronomy?
Standard candles are easily recognizable astronomical objects whose luminosities are confidently known. They are important because they extend the distance-measurement ladder by allowing us to use its estimated luminosity in comparison to apparent brightness to figure out an object's distance.
Dusty Donut
The accretion disk surrounding the massive black hole in active galaxies consists of hot gas at many different temperatures. The dusty infalling gas that ultimately powers the system is thought to form a rather fat, donut shaped disk which effectively absorbs much of the high energy radiation reaching it, re-emitting it in the form of cooler, infrared radiation. - Reprocessing mechanism for radiation
What was it about the spectra of quasars that was so unexpected and surprising?
The amount of redshifting is very large, they have very high recession velocities, meaning they are receding away from us remarkably fast.
Hubble's Constant
The constant of proportionality between recessional velocity and distance in Hubble's law. Recessional velocity = Hubble's constant x distance - Measured most commonly in megaparsecs: H0=70km/s/mpc
How does accretion onto a supermassive black hole power the energy emission from the extended radio lobes of a radio galaxy?
The energy is generated in an accretion disk in the central nucleus of the visible galaxy, then transported by jets out of the galaxy and into the lobes, where it is eventually emitted by the synchrotron process in the form of radio waves.
Active Galactic Nucleus
The leading theory for the energy source in active galactic nuclei holds that these objects are powered by material accreting onto a super massive black hole. As matter spirals toward the hole, it heats up, producing large amounts of energy. At the same time, high-speed jets of gas can be ejected perpendicular to the accretion disk, forming jets and lobes in many active objects. The jets transport magnetic fields generated in the disk out to the radio lobes, where they play a crucial role in producing the observed radiation.
Briefly describe the leading model for the central engine of an active galaxy
The leading theory for the energy source in active galactic nuclei holds that these objects are powered by material accreting onto a supermassive black hole. As matter spirals toward the hole, it heats up, producing large amounts of energy. At the same time, high-speed jets of gas can be ejected perpendicular to the accretion disk, forming the jets and lobes observed in many active objects. The jets transport magnetic fields generated in the disk out to the radio lobe, where they play a crucial role in producing the observed radiation.
Name 2 basic differences between normal galaxies and active ones.
The luminosities of active galaxies are extremely large and their spectra differ slightly. Also, their energy emission is mostly nonstellar.
Active Galaxies
The most energetic galaxies which can emit 100's or 1000's times more energy per second than the Milky Way, mostly in the form of long-wavelength non-thermal radiation. - At optical wavelengths they appear normal - "Non stellar" radiation - A system whose abnormal activity is related to violent events occuring in or near the galactic nucleus (Active galactic nuclei)
What is the Virgo Cluster?
The nearest large galaxy cluster to the Local Group. It houses 2500 galaxies, much more than our own.
Using the method of standard candles, we can, in principle,find the distance of a campfire if we know
The number of logs used
Hubble's Law
The rate at which a galaxy recedes is directly proportional to its distance from us. Using this law, we can derive the distance to a remote object by measuring the objects recessional velocity and dividing by Hubble's Constant (70 km/s/Mpc)
Describe the 4 rungs in the distance-measurement ladder used to determine the distance to a galaxy lying 5 mpc away.
The techniques show in the the bottom 4 rungs of the Extragalactic Distance Ladder are: - radar ranging - stellar parallax - spectroscopic parallax - variable stars And they take us as far as the nearest galaxies. To go further, new techniques, such as the Tully-Fisher relation, must be used.
Irregular Galaxies
Their visual appearance does not allow us to place them in other categories. Tend to be rich in interstellar matter and young blue stars, but they lack any structure. Irr I --- Irr II Irr I look like misshapen spirals - These galaxies tend to be smaller than spirals, but larger than dwarf ellipticals. - Smallest are 'dwarf irregulars', which are also most common
Describe some similarities and differences between elliptical galaxies and the halo of our own galaxy.
They are similar in that the orbits of stars in both are disordered as they exhibit little or no overall rotation. They differ in that ellipticals have amounts of very hot interstellar gas distributed throughout their interiors, often extending well beyond the visible portions of the galaxies.
How is the process of synchrotron emission related to observations of active galaxies?
This radiation is produced by electrons that whirl around, emitting electromagnetic radiation, which is nonthermal, meaning there is no link between the emission and temperature of the radiating object. Hence, the radiation is not described by a blackbody curve. Instead, its intensity decreases with increasing frequency. This is just what is needed to explain the overall nonthermal spectrums of radio galaxies and radio-loud quasars. Observations of the radiation received from the jets and radio lobes of active galaxies are completely consistent with synchrotron radiation.
Normal galaxies
Those that fall under Hubble's classification (spiral and barred spiral, elliptical and irregular) - The majority of galaxies fall in this category
Synchrotron Radiation
Type of nonthermal radiation produced by high speed charged particles, such as electrons, as they are accelerated in a strong magnetic field - Reprocessing mechanism for radiation in active galaxies - Not associated with temperature but frequency; intensity decreases with increasing frequency
Seyfert Galaxies
Types of active galaxies whose emission comes from a very small region within the nucleus of an otherwise normal-looking spiral system.
Hubble Flow
Universal recession described by the Hubble Diagram and quantified by Hubble's Law
Cosmological Redshift
Used to distinguish recessional redshift from redshifts caused by motion within an object, redshift resulting from Hubble Flow. Objects that lie so far away that they exhibit a large cosmological redshift are said to be at cosmological distances - distances comparable to the scale of the universe itself.
How is Hubble's law used by astronomer's to measure distances to galaxies?
Using Hubble's Law, we can derive the distance to a remote object simply by measuring the object's recessional velocity and dividing by Hubble's Constant (70km/s/mpc)
Irr II Galaxies
Very rare, often exhibit a distinctly explosive appearance, probably caused in some cases by a close encounter or collision between two previously 'normal' systems.
How did the determination of quasar distances change astronmers understanding of these objects?
When quasars were discovered, they were thought to be faint, relative nearby stars or starlike objects, although their unusual spectra posed problems for astronomers. Once astronomers realized that their odd spectra meant that they had very large redshifts, it become clear that quasars were actually among the most distance - and hence the most luminous - objects in the universe.
Are there any 'nearby' active galaxies, within 50 mpc of ours?
Yes! NGC7742 is roughly 24 Mpc from the Local Group. Circinus galaxy, a Seyfert with a bright compact core, is one of the closest active galaxies to ours, lying only 4 Mpc away.
Within 40 Mpc of the Sun, there are about
a few thousand galaxies
Quasar spectra
are strongly red shifted
Active galaxies are very luminous because
they contain black holes in their cores
If the light from a galaxy fluctuates in brightness very rapidly, the region producing the radiation must be
very small
