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[sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9)

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Subject: Introduction sci.astro is a newsgroup devoted to the discussion of the science of astronomy. As such its content ranges from the Earth to the farthest reaches of the Universe. However, certain questions tend to appear fairly regularly. This document attempts to summarize answers to these questions. This document is posted on the first and third Wednesdays of each month to the newsgroup sci.astro. It is available via anonymous ftp from <URL:>, and it is on the World Wide Web at <URL:> and <URL:>. A partial list of worldwide mirrors (both ftp and Web) is maintained at <URL:>. (As a general note, many other FAQs are also available from <URL:>.) Questions/comments/flames should be directed to the FAQ maintainer, Joseph Lazio (
Subject: H.00 Galaxies, Clusters, and Quasars (QSOs) [Dates in brackets are last edit.] H.01 How many stars, galaxies, clusters, QSO's etc. in the Universe? [1997-08-06] H.02 Is there dark matter in galaxies? [1997-12-02] H.03 What is the Hubble constant? What is the best value? [1995-07-19] H.04 How are galaxy distances measured? [1995-06-29] H.05 When people speak of galaxies X billion light years, does this mean they are that far away now or were that far away when the light left them? [1997-08-06] H.06 What are QSO's ("quasars")? [1995-06-29] H.07 Are the QSO's really at their redshift distances? [2003-02-18] H.08 What about apparent faster-than-light motions? [1995-06-29] H.09 What's the Local Group? [1999-05-19] For an overall sense of scale when talking about galaxies, see the Atlas of the Universe, <URL:>.
Subject: H.01 How many stars, galaxies, clusters, QSO's etc. in the Universe? The various parts of this question will be considered separately. Also, rather consider how many stars there are in the Universe, we'll consider how many stars there are in the Milky Way. The number of stars in the Universe can be estimated by multiplying the number of stars in the Milky Way by the number of galaxies in the Universe. ------------------------------ Subject H.01.1 How many stars are there in the Milky Way? Author: William Keel <> My standard answer in introductory astronomy classes is "about as many as the number of hamburgers sold by McDonald's." Being more precise requires an extrapolation, because we can't see all the individual stars in the Milky Way for two reasons---distance and dust absorption. Both factors make stars appear dimmer. Observations at visible wavelengths are limited to a region of (more or less) 5000 light-years radius about the Sun, with a few windows in the intervening dust giving us glimpses of more distant areas (especially near the Galactic center). Our map of the Galaxy gets correspondingly more sketchy with distance. Guided somewhat by observations of other spiral galaxies, we think that the overall run of star density with radius is fairly well known. Getting a total stellar head count is more of a problem, because the stars that we can see to the greatest distances are also the rarest. Measurements of the relative numbers of stars with different absolute brightness (known in the trade as the luminosity function) shows that, for example, for every Sun-like star there are about 200 faint red M dwarfs. These are so faint that the closest, Proxima Centauri, despite being closer to the Sun than any other (known) star, takes very large binoculars or a telescope to find. So, to get the total stellar population in the Milky Way, we must take the number of luminous stars that we can see at large distances and assume that we know how many fainter stars go along with them. Recent numbers give about 400,000,000,000 (400 billion) stars, but a 50% error either way is quite plausible. Much of the interest in "brown dwarfs" stems from a similar issue---a huge number of brown dwarfs would not change how bright the Galaxy appears (at visible wavelengths), but would change its total mass quite substantially. Oddly enough, within a particular region, we probably know the total mass and luminosity rather more accurately than we do just how many stars are producing that light (since the most common stars are by far the dimmest).
Subject: H.01.2 How many galaxies in the Universe? Author: William Keel <> A widely-distributed press release about the Hubble Deep Field observations, <URL:>, reported the discovery of a vast number of new galaxies. The existence of many galaxies too faint to be hitherto detected was no surprise, and calculations of the number of galaxies in the observable Universe and searches for how they change with cosmic time must always allow for the ones we can't detect, through some combination of intrinsic faintness and great distance. What was of great interest in the Hubble Deep field (and similar) data was just how any faint galaxies were detected and what their colors and forms are. Depending on just what level of statistical error can be tolerated, catalogs of galaxies in the Hubble Deep Field list about 3000. This field covers an area of sky of only about 0.04 degrees on a side, meaning that we would need 27,000,000 such patches to cover the whole sky. Ignoring such factors as absorption by dust in our own Galaxy, which make it harder to see outside in some directions, the Hubble telescope is capable of detecting about 80 billion galaxies (although not all of these within the foreseeable future!). In fact, there must be many more than this, even within the observable Universe, since the most common kind of galaxy in our own neighborhood is the faint dwarfs which are difficult enough to see nearby, much less at large cosmological distances. For example, in our own local group, there are 3 or 4 giant galaxies which would be detectable at a billion light-years or more (Andromeda, the Milky Way, the Pinwheel in Triangulum, and maybe the Large Magellanic Cloud). However, there are at least another 20 faint members, which would be difficult to find at 100 million light-years, much less the billions of light years to which the brightest galaxies can be seen.
Subject: H.01.3 How many globular clusters in the Milky Way? Author: William Keel <> We are on firmer ground with this one, since globular clusters are fairly large and luminous. The only places where our census in the Milky Way is incomplete are regions close to the galactic disk and behind large amounts of absorbing dust, and for the fainter clusters that are farthest from the Milky Way just now. The electronic version of the 1981 Catalogue of Star Clusters and Associations. II. Globular Clusters by J. Ruprecht, B. Balazs, and R.E. White lists 137 globular clusters in and around the Milky Way. More recent discoveries have added a handful, especially in the heavily reddened regions in the inner Galaxy. As a rough estimate accounting for the regions that cannot yet be searched adequately, our galaxy should have perhaps 200 total globulars, compared with the approximately 250 actually found for the larger and brighter Andromeda galaxy.
Subject: H.01.4 How many open clusters? Author: William Keel <> Here we must extrapolate again, since open clusters can be difficult to find against rich star fields in the plane of the Milky Way, and since richer clusters may be identified farther away than poor ones. The electronic version of the catalogue of open cluster data compiled by Gosta Lynga, Lund Observatory, Box 43, S-221 00 Lund, Sweden, 1987 version, lists 1111 identified open clusters in our galaxy. There are certainly at least ten times this number, since we have trouble seeing even rich open clusters more than about 7000 light-years away in most directions through the obscuring dust in the plane of our Galaxy. This effect is especially acute since young star clusters are strongly concentrated to this plane (no coincidence since the gas from which new clusters are formed is associated with dust).
Subject: H.02 Is there dark matter in the Universe? Author: Will Sutherland <>, William Keel <> Dark matter is matter that is detected by its gravitational effect on other matter rather than because of its electromagnetic radiation (i.e., light). This might be because of one of two reasons: 1. The matter may emit light, but the light is so faint that we cannot detect it; an example of this kind of matter is interstellar planets. 2. The matter might not interact with light at all; an example of this kind of matter is neutrinos. The first astronomical instances of "dark matter" were probably the white dwarf Sirius B and the planet Neptune. The existence of both objects was inferred by their gravitational effects on a nearby object (Sirius A and the planet Uranus, respectively) before they were seen directly.
Subject: H.02.1 Evidence for dark matter There are many independent lines of evidence that most of the matter in the universe is dark. Essentially, many of these measurements rely on "weighing" an object such as a galaxy or a cluster of galaxies by observing the motions of objects within it, and calculating how much gravity is required to prevent it flying apart. (1) Rotation patterns in spiral galaxies. (2) Velocities of galaxies in clusters. (3) Gravitational lensing. (4) Hot gas in galaxies and clusters. (5) Large-scale motions. (1) Rotation patterns in spiral galaxies. The disks of spirals are full of stars and gas in nearly circular coplanar orbits, making them wonderful tracers for the gravitational field in which they move. In centrally-concentrated masses, such as within the solar system (where most of the mass is concentrated in the Sun), the velocity-vs.-distance relation approaches Kepler's 3rd Law, velocity^2 = constant * central mass / distance. Once we sample outside the central concentration of stars, using observations of the 21cm line emitted by neutral hydrogen clouds, spiral galaxies violate this velocity-distance relation quite flagrantly; velocity=constant is a good approximation (hence the moniker "flat rotation curves"). A sample picture and rotation curve is at <URL:>. To get this pattern, one needs a mass distribution that goes as density proportional to 1/radius^2, much fluffier than the observable stars and gas in the galaxy, and in an amount that may be 10 or more times the total mass we can account for with stars, dead stellar remnants, gas, and dust. There were hints of this issue for a while, but it was a series of observations by Vera Rubin and collaborators in the mid-1970's that really rubbed our noses in it. (2) Velocities of galaxies in clusters. Galaxies in clusters have random orbits. By measuring the dispersion for, e.g., 100 galaxies in the cluster, one finds typical dispersions of 1000 km/s. The clusters must be held together by gravity, otherwise the galaxies would escape in less than 1 billion years; cluster masses are required to be at least 10 times what the galaxies' stars can account for. This problem was first demonstrated in 1938 by Fritz Zwicky who studied the galaxy-rich Coma cluster. Zwicky was very bright, very arrogant, and highly insulting to anyone he felt was beneath him, so this took a long while to sink in. Today we know that virtually all clusters of galaxies show the same thing. (3) Gravitational lensing. General relativity shows that we can treat gravity (more precisely than in Newtonian dynamics) by considering it as a matter-induced warping of otherwise flat spacetime. One of the consequences of this is that, viewed from a distance, a large enough mass will bend the paths of light rays. Thus, background objects seen past a large mass (galaxy or cluster of galaxies) are either multiply imaged or distorted into "arcs" and "arclets." Some beautiful examples can be seen at <URL:>, <URL:>, and <URL:>. When we know the distances of foreground and background objects, the mass inside the lensing region can be derived (and for some of these multi-lens clusters, its radial distribution). Same old story - we need a lot more mass in invisible than visible form. (4) Hot gas in galaxies and clusters. A real shocker once X-ray astronomy became technologically possible was the finding that clusters of galaxies are intense X-ray sources. The X-rays don't come from the galaxies themselves, but from hot, rarefied gas at typically 10,000,000 K between the galaxies. To hold this stuff together against its own thermal motions requires - you guessed it, huge amounts of unseen material. It is worth noting that these last three methods all give about the same estimate for the amount of dark matter in clusters of galaxies. (5) Less direct evidence also exists: On larger scales, there is evidence for large-scale "bulk motions" of galaxies towards superclusters of galaxies, e.g., the Great Attractor. There is also the question of reconciling the very small (1 part in 100,000) observed fluctuations in the cosmic microwave background with the "lumpy" galaxy distribution seen at the present day; dark matter helps nicely to match these two facts because the density fluctuations grow more rapidly with time in a higher-density Universe. Finally, the theory of inflation (which is an "optional extra" to the standard big bang model) usually predicts that the universe should have exactly the critical density, which could require as much as 95% of the mass in the Universe to be dark. It is worth mentioning the possibility of non-standard gravity theories, which attempt to explain the above list of observations without dark matter. It turns out that modifying the inverse-square law of gravity does not work well, essentially because the dark matter problem extends over so many different lengthscales. Modifying the F = ma law has been tried, e.g., by Milgrom, but relativistic versions of this theory have not been found, and most cosmologists are reluctant to abandon Einstein's GR which is elegant and well tested (at least on solar system scales).
Subject: H.02.2 How much dark matter is there? A convenient way of quoting mass estimates is via Omega, the ratio of the density contributed by some objects to the "critical density" = 3 H^2 / 8 pi G, where H is the Hubble constant and G is the universal constant of gravitation. The critical density is the amount of matter that would be just sufficient to stop the expansion of the Universe and is 10^{-29} g/cm^3. (Of course, portions of the Universe have a higher density than this, e.g., you, but this is an average density.) The visible stars in galaxies contribute about 1 percent of critical density, i.e., Omega_stars ~ 0.01; dark halos around galaxies contribute Omega_halos ~ 0.05; mass estimates from clusters tend to give Omega_clus ~ 0.2 (assuming the ratio of dark matter to stars is the same in clusters as everywhere else); and theoretical considerations (i.e., inflation) favor Omega_total = 1. The gap between 0.05 and 0.2 can be explained if galaxy halos extend further out than we can measure the rotation curves, but if Omega_total = 1 we may require extra dark matter in intergalactic space. It's also interesting to consider the dark matter density "locally." Within a few hundred parsecs of the Sun, this is about 0.01 Solar masses per cubic parsec, or about 0.3 proton masses per cm^3; that's only about 1/10 of the density of visible matter (mostly stars); though it's much larger than critical density because we live in a galaxy. However, because the stars are in a thin disk while the dark matter is more spherical, if you take an 8 kpc radius sphere centred on the Galaxy and passing through the Sun, roughly half the mass in this sphere is dark matter If you consider a larger sphere, e.g., out to the Large Magellanic Cloud at 50 kpc radius, over 80% of the mass in it is dark matter. This estimate was first made by Jan Oort, and the estimate of the *total* mass density near the Sun is today termed the Oort limit in his honor.
Subject: H.02.3 What is the dark matter? Since it's detected in a negative sense---not visible in gamma rays, X-rays, ultraviolet, visible light, infrared, millimeter, or radio regimes, and it doesn't block light either---it's a theoretical happy hunting ground. First, let's list some things that can't make the dark matter. Most forms of gas are excluded, because atomic hydrogen would be seen in 21cm radiation, and hot gas would be seen in X-rays and/or distort the spectrum of the CMB. Cold molecular gas is a possibility, but it would tend to collapse into visible stars. "Snowballs" made of solid hydrogen would evaporate due to the CMB, and larger snowballs would leave too many craters on the Moon or be seen as high-speed comets. "Rocks" are unlikely because there haven't been enough stars to make the heavy elements. Faint red stars are excluded because they're not seen in deep images e.g., the Hubble Deep Field. This leaves two main classes of dark-matter candidate: large objects called MACHOs and subatomic particles, some of which are called WIMPs. MACHOs stands for Massive Compact Halo Objects; examples are "interstellar Jupiters" or "brown dwarfs," which are lumps of mostly hydrogen less than 0.08 Solar masses; objects this small don't get hot enough to fuse hydrogen into helium, and so would be extremely faint and hard to find. Other varieties of MACHOs are dead stars, such as old white dwarfs or neutron stars, and black holes. The second class is some form of sub-atomic particle; if so, there'd be millions of these passing through us every second, but they'd hardly ever interact with normal matter, hence the term "weakly interacting massive particles" or WIMPs. Many varieties of these have been suggested; the only one of these that certainly exists is the neutrino, but neutrinos may not have any mass. The number of neutrinos made in the Big Bang is similar to the number of CMB photons (few hundred per cm^3), so if they have a small mass (around 30 eV = 6 x 10^-5 electron masses) they could contribute most of the dark matter. However, computer models indicate that galaxies form much too late in a neutrino-dominated universe. Another possibility is the "axion" which is a hypothetical particle invented to solve a strange "coincidence" in particle physics (called the strong CP problem). The most popular WIMP at the moment is the "neutralino" or "lightest supersymmetric particle"; supersymmetry is a popular way to unify the strong and electroweak forces (also known as a Grand Unified Theory), which has some (tentative) experimental support. Supersymmetry predicts an unobserved new particle or "superpartner" for every known particle; the lightest of these should be stable, and lots of them would be left over from the Big Bang. These probably weigh about 30-500 proton masses. An important piece of evidence here is "primordial nucleosynthesis," which explains the abundances of He-4, Deuterium, He-3 and Li-7 produced a few minutes after the Big Bang; in order to obtain the observed abundances of these elements, the density of baryons (i.e., "ordinary" matter) must be Omega_baryon ~ 0.02--0.1. Since Omega_stars ~ 0.01, there are probably some dark baryons, but if Omega_total = 1 (as inflation predicts) most of the dark matter is probably WIMPs.
Subject: H.02.4 Searches for Dark Matter There are many searches now underway for the dark matter. For MACHOs, the most promising method is "gravitational microlensing," where we wait for a MACHO to pass between us and a distant star, and the gravity of the MACHO bends the starlight into two images. These images are too close together to resolve, but add up to more light, so the star appears to brighten and then fade back to normal as the MACHO passes by. The shape is quite distinctive, and the brightening happens only once so does not look like a variable star. The probability of such a close-enough approach is very low, so millions of stars must be monitored to have a chance of finding these events. The Large Magellanic Cloud is the most popular target. A number of groups---MACHO, EROS, OGLE, among others---have been doing this for several years, and have found a number of good candidate microlensing events. At the moment, it is too early to say that MACHOs have definitely been discovered, but it looks as though the "brown dwarf" objects are just about excluded, while perhaps as much as 50% of the dark matter could be in larger objects roughly 0.5 solar masses, e.g., white dwarfs. There is an axion search recently started at Lawrence Livermore Labs, which uses a huge superconducting magnet to convert axions (if they exist) into microwave photons. For the big bang neutrinos, there is currently no hope of detecting them because they have far less energy than the well-known solar neutrinos (see FAQ entry E.01). However, if a neutrino mass could be measured by lab experiments, we could calculate their contribution to the dark matter. For the supersymmetric particles, there are broadly three ways at detecting them: i) Direct detection by watching a crystal down a deep mine, and waiting for a WIMP to bounce off a nucleus in it with observable results such as scintillation or heating of the crystal. Very roughly 1 WIMP per day should hit each kg of detector, but the tricky part is discriminating these from natural radioactivity. The WIMPS should have a preferred direction (due to the orbit of the Sun around the galaxy), but we'll have to wait for next-generation experiments to measure this. ii) Indirect detection, whereby WIMPs get captured in the Sun, and then a WIMP + anti-WIMP annihilate into super-high energy (GeV) neutrinos which could be detected in huge volume detectors, e.g., Antarctic ice or ocean water. iii) Create WIMPs directly at next-generation accelerators like LHC, measure their properties and then calculate how many should have been produced in the Big Bang. With all these searches, there is a good chance that in the next 10 years or so we may find out what constitutes dark matter. Further reading: Astronomy magazine, Oct. 1996 issue contains many dark matter articles. The Center for Particle Astrophysics home page at <URL:> has several links including the Question of Dark Matter page. The MACHO home page at <URL:> has info on the MACHO project and links to many other dark matter searches. For cosmology background, see Ned Wright's Cosmology Tutorial at <URL:>. A more technical conference summary is at <URL:>. Krauss, L., _The Fifth Essence_, Basic Books, NY 1989. Silk, J., _The Big Bang_, Freeman, San Francisco, 1988. Peebles, P.J.E., _Principles of Physical Cosmology_, Princeton, 1992 (advanced)
Subject: H.03 What is the Hubble constant? What is the best value? Author: Steve Willner <>, Joseph Lazio <> By 1925, V. M. Slipher had compiled radial velocities for 41 galaxies. He noticed that their velocities were quite a bit larger than typical for objects within our Galaxy and that most of the velocities indicated recession rather than approach. In 1929, Edwin Hubble (and others) recognized the simple relationship that recession velocity is on average proportional to the galaxy's distance. (His distance measure was the apparent magnitude of the brightest individually recognizable stars.) This proportionality is now called "Hubble's Law," and the constant of proportionality is known as the "Hubble constant," H (often written "Ho," i.e., H subscript zero). The Hubble constant also has the property of being related to the age of the Universe, which undoubtedly explains some of the interest in its value. It is a constant of proportionality between a speed (measured in km/s) and a distance (measured in Mpc), so its units are (km/s)/Mpc. Since kilometers and megaparsecs are both units of distance, with the correct factor, we can convert megaparsecs to kilometers, and we're left with a number whose units are (km/s)/km. If we take 1/H, we see that it has units of seconds, that is 1/H is a time. We might consider 1/H to be the time it takes for a galaxy moving at a certain velocity (in km/s) to have moved a certain distance (in Mpc). If the galaxies have always been moving exactly as they now are, 1/H seconds ago all of them were on top of us! Of course the proportionality isn't exact for individual galaxies. Part of the problem is uncertainties in measuring the distances of galaxies, and part is that galaxies don't move entirely in conformity with the "Hubble Flow" but have finite "peculiar velocities" of their own. These are presumably due to gravitational interactions with other, nearby galaxies. Some nearby galaxies indeed have blue shifts; M 31 (the Andromeda galaxy) is a familiar example. In order to measure the Hubble constant, all one needs a distance and a redshift to a galaxy that is distant enough that its peculiar velocity does not matter. Measuring redshifts for galaxies is easy, but measuring distances is hard. (See the next question.) The Hubble constant is therefore not easy to measure, and it is not surprising that there is controversy about its value. In fact, there are generally two schools of thought: one group likes a Hubble constant around 55 (km/s)/Mpc, and another prefers values around 90 (km/s)/Mpc. When converted to an age of the Universe, H = 55 (km/s)/Mpc corresponds to an age of about 19 billion years and H = 90 (km/s)/Mpc is an age of 11 billion years (again if the velocities are constant). A measure of how difficult it is to determine the Hubble constant accurately can be seen by examining the different values reported. A search by Tim Thompson <tim@lithos.Jpl.Nasa.Gov> for the period 1992--1994 found 39 reported values for H in the range 40--90 (km/s)/Mpc. The linear relation between distance and recession velocity breaks down for redshifts around 1 and larger (velocities around 2E5 km/s). The true relation depends on the curvature of space, which is a whole other topic in itself (and has no clear answer). The sense, though, is that infinite redshift, corresponding to a recession velocity equal to the speed of light, occurs at a finite distance. This distance is the "radius of the observable Universe." Nothing more distant than this can be observed, even in principle.
Subject: H.04 How are galaxy distances measured? Author: Martin Hardcastle <> Galaxy distances must be measured by a complicated series of inferences known as the distance ladder. We can measure the distances to the nearest stars by parallax, that is by the apparent motion of the star in the sky as a result of the Earth's motion round the Sun. This technique is limited by the angular resolution that can be obtained. The satellite Hipparcos will provide the best measurements, giving the parallax for around 100,000 stars. At present parallax can be used accurately to determine the distances of stars within a few tens of parsecs from the Sun. [ 1 parsec = 3.26 lt yrs.] Statistical methods applied to clusters of stars can be used to extend the technique further, as can `dynamical parallax' in which the distances of binary stars can be estimated from their orbital parameters and luminosities. In this way, or by other methods, the distance to the nearest `open clusters' of stars can be estimated; these can be used to determine a main sequence (unevolved Hertzsprung-Russell diagram) which can be fitted to other more distant open clusters, taking the distance ladder out to around 7 kpc. Distances to `globular clusters', which are much more compact clusters of older stars, can also have their distances determined in this way if account is taken of their different chemical composition; fitting to the H-R diagram of these associations can allow distance estimates out to 100 kpc. All of these techniques can be checked against one another and their consistency verified. The importance of this determination of distance within our own galaxy is that it allows us to calibrate the distance indicators that are used to estimate distances outside it. The most commonly used primary distance indicators are two types of periodic variable stars (Cepheids and RR Lyrae stars) and two types of exploding stars (novae and supernovae). Cepheids show a correlation between their period of variability and their mean luminosity (the colour of the star also plays a part) so that if the period and magnitude are known the distance can in principle be calculated. Cepheids can be observed with ground-based telescopes out to about 5 Mpc and with the Hubble space telescope to at least 15 Mpc. RR Lyrae stars are variables with a well-determined magnitude; they are too faint to be useful at large distances, but they allow an independent measurement of the distance to galaxies within 100 kpc, such as the Magellanic Clouds, for comparison with Cepheids. Novae show a relationship between luminosity at maximum light and rate of magnitude decline, though not a very tight one; however, they are brighter than Cepheids, so this method may allow distance estimates for more distant objects. Finally, supernovae allow distance determination on large scales (since they are so bright), but the method requires some input from theory on how they should behave as they expand. The advantage of using supernovae is that the derived distances are independent of calibration from galactic measurements; the disadvantage is that the dependence of the supernova's behaviour on the type of star that formed it is not completely understood. The best primary distance indicators (generally Cepheids) can be used to calibrate mainly empirical secondary distance indicators; these include the properties of H II regions, planetary nebulae, and globular clusters in external galaxies and the Tully-Fisher relation between the width of the 21-cm line of neutral hydrogen and the absolute magnitude of a spiral galaxy. These can all be used in conjunction with type Ia supernovae to push the distance ladder out to the nearest large cluster of galaxies (Virgo, at around 15--20 Mpc) and beyond (the next major goal is the Coma cluster at around 5 times farther away). Other empirical estimators such as a galaxy size-luminosity relation or a constant luminosity for brightest cluster galaxies are of uncertain value. The goal in all of this is to get out beyond the motions of our local group of galaxies and determine distances for much more distant objects which can reasonably be assumed to be moving along with the expansion of the universe in the Big Bang cosmology. Since we know their velocities from their redshifts, this would allow us to determine Hubble's constant, currently the `holy grail' of observational cosmology; if this were known we would know the distances to _all_ distant galaxies directly from their recession velocity. Sadly different methods of this determination, using different steps along the distance ladder, give different results; this leads to a commonly adopted range for H of between 50 and 100 km/s/Mpc, with rival camps supporting different values. There are a number of ongoing attempts to reduce the complexity of the distance ladder and thus the uncertainty in H. One has been the recent (and continuing) use of the Hubble Space Telescope to measure Cepheid variables directly in the Virgo cluster, thereby eliminating several steps; this leads to a high (80--100) value of H, although with large uncertainty (which should hopefully be reduced as more results arrive). Other groups are working on eliminating the distance ladder, with its large uncertainty and empirical assumptions, altogether, and determining the distances to distant galaxies or clusters directly, for example using the Sunyaev-Zeldovich effect together with X-ray data on distant clusters or using the time delays in gravitational lenses. The early results tend to support lower values of H, around 50.
Subject: H.05 When people speak of galaxies X billion light years away, does this mean they are that far away now or were that far away when the light left them? Author: William Keel <> Distance is indeed a slippery thing in an expanding universe such as ours. There are at least three kinds of distances: * angular-diameter distance---the one you need to make the usual relation sine(angular size) = linear size/distance work; * luminosity distance---makes the typical relationship observed flux = luminosity / 4 pi (distance**2) work; and * proper distance---the piece-by-piece distance the light actually travelled. Of the three, the proper distance is perhaps the most sensible of the three. In this case, distance doesn't mean either when the light was emitted or received, but how far the light travelled. Since the Universe expands, we have been moving away from the emitting object so the light is catching up to us (at a rate set by the rate of expansion and our separation from the quasar or whatever at some fiducial time). You can of course turn this distance into an extrapolated distance (where the quasar or it descendant object is "today") but that gets very slippery. Both special and general relativity must be taken into account, so simultaneity, i.e., "today," has only a limited meaning. Nearby galaxies are pretty much where we see them; for example, the light from the Andromeda galaxy M31 has been travelling only about 0.01% of the usually estimated age of the Universe, so its distance from us would have changed by about that fraction, if nothing but the Hubble expansion affected its measured distance (which is not the case, because gravitational interactions between the Andromeda galaxy and our Galaxy affect the relative velocity of the two galaxies). To muddy the waters further, observers usually express distances (or times) not in light-years (or years) but by the observable quantity the redshift. The redshift is, by definition, the amount by which light from an object has been shifted divided by the emitted or laboratory wavelength of the light and is usually denoted by z. For an object with a redshift z, one can show that (1+z) is the ratio of the scale size of distances in the Universe between now and the epoch when the light was given off. Turning this into an absolute distance (i.e., some number of light-years) requires us to plug in a rate for the expansion (the Hubble constant) and its change with time (the deceleration parameter), neither of which is as precisely known as we might like. As a result ages and distances are usually quoted in fairly round numbers. If the expansion rate has remained constant (the unrealistic case of an empty Universe), the age of the Universe is the reciprocal of the Hubble constant. This is from 10--20 billion (US, 10^9) years for the plausible range of Hubble constants. If we account for the matter in the Universe, the Universe's age drops to 7--15 billion years. A quick estimate of the look-back time (i.e., how long the light from an object has been travelling to us) for something at redshift z is t = (z/[1+z])*1/H0 for Hubble constant H0. For example, the author has published a paper discussing a cluster of galaxies at z=2.4. For the press release we quoted a distance of 2.4/3.4 x 15 billion light-years (rounded to 11 since that 15 is fuzzy).
Subject: H.06 What are QSO's ("quasars")? Author: Martin Hardcastle <> "Quasi-stellar objects" (or QSO's) are defined observationally as objects that appear star-like on photographic plates but have high redshifts (and thus appear extragalactic; see above). The luminosity (if we accept that the redshift correctly indicates the distance) of a QSO is much larger than that of a normal galaxy, and many QSO's vary on time scales as short as days, suggesting that they may be no more than a few light days in size. QSO spectra typically contain strong emission lines, both broad and narrow, so that the redshift can be very well determined. In a few cases, a nebulosity reminiscent of stars in a normal galaxy has been detected around a QSO. Quasars (a shortened version of "quasi-stellar radio source") were originally discovered as the optical counterparts to radio sources, but the vast majority of QSO's now known are radio-quiet. Some authors reserve the term "quasar" for the radio-loud class and use the term "QSO" generically; others (especially in the popular literature) use "quasar" generically. In the standard model, QSO's are assumed to lie at the centre of galaxies, and to form the most extreme example of the class of active galactic nuclei (AGN); these are compact regions in the centre of galaxies which emit substantially more radiation in most parts of the spectrum than would be expected from starlight. From the energy output in QSO's, together with some guess at their lifetime (about 10^8 years) the mass of the central engine can be estimated as of order 10^7 solar masses or more (this is consistent with estimates of the masses of other, related types of AGN). A compact, massive object of this kind is most likely (on our current understanding of physics) to be a black hole, and most astronomers would accept this as the standard assumption. The luminosity ultimately derives from matter falling into the black hole and gravitational potential energy being converted to other forms, but the details are unexplained and very much an active research topic.
Subject: H.07 Are the QSO's really at their redshift distances? Author: Martin Hardcastle <> It's often suggested that QSOs are not at the distances that would be inferred from their redshifts and from Hubble's law; this would avoid the enormous powers and necessity for general-relativistic physics in the standard model. Many arguments of this type are flawed by a lack of consideration of the other types of galaxies and active galactic nuclei (AGN): unless it's believed that _no_ galaxy is at its redshift distance, i.e., that the whole concept of redshift is wrong, then we know that there are objects very similar to QSOs which _are_ at their redshift distances. (Cosmological theories that overthrow the whole idea of redshift and the big bang are beyond the scope of this discussion, although several have been proposed based on the apparent spatial association of objects with very different redshifts.) Another argument favoring QSOs being at their redshift distance comes from gravitational lensing. Gravitational lenses occur when two objects are nearly aligned, and the mass of the foreground object lenses (magnifies and/or distorts) the background object. In every gravitational lens for which redshifts are known, the galaxy (or galaxies) acting as the lens has a lower redshift than the galaxy being lensed. A recent analysis of data available from the 2-degree field (2dF survey) also showed no evidence for a connection between galaxies and QSOs. This analysis is particularly significant because the people who carried out the analysis spoke to proponents on both sides of the argument *before* conducting their analysis (Hawkins, Maddox, & Merrifield 2002, Mon. Not. R. Astron. Soc., vol. 336, p. L13). More generally, though, like many arguments in science, this one also has an element of aesthetics. The proponents of the standard model argue that the physics we know (general relativity, special relativity, electromagnetism) is sufficient to explain QSOs, and that, by Occam's razor, no model introducing new physics is necessary. Its opponents argue either that there are features of QSOs which cannot be explained by the standard model or that the predictions of the standard model (and, in particular, its reliance on supermassive black holes) are so absurd as clearly to require some new physics. A good deal of bad science has been put forward (on both sides) on sci.astro. Readers should be aware that the scientific community isn't as insanely conservative as some posters would have them believe, and that a number of other possibilities for QSO physics were considered and rejected when they were first discovered. For example, the frequent suggestion that the redshifts of QSOs are gravitational does not work in any simple model. Species having different ionization potentials ought to exist at different distances from the central source and thus should have different redshifts, but in fact emission lines from all species are observed to have the same redshift. For examples of claims of galaxy-QSO associations, see papers by Stockton, either of the Burbidges, or Arp. For additional, technical discussions of why these conclusions are not valid, see papers by Newman & Terzian; Newman, Terzian, & Haynes; and Hawkins, Maddox, & Merrifield (2002).
Subject: H.08 What about apparent faster-than-light motions? Author: Martin Hardcastle <> The apparently faster-than-light motions observed in the jets of some radio-loud quasars have misled a number of people into believing that the speed of light is not really a limit on velocity and that astrophysics has provided a disproof of the theory of relativity. In fact, these motions can be easily understood without any new physics; you just need trigonometry and the idea of the constancy of the speed of light. Consider the situation shown in the diagram below. A blob B of radio-emitting plasma starts at O and moves with velocity v at some angle a to our line of sight. At a time t, B has moved across the sky a distance vt sin a. The light from when it was at O has travelled a distance ct towards us (c is the speed of light). But the light from its position at time t only has to travel an additional distance (ct - vt cos a) to reach us. Thus we measure the time between the two events as (distance / speed of light) = t(1 - (v/c) cos a). If we derive an apparent velocity by dividing the (measurable) transverse motion of the source by the measured time difference, we get vt sin a v sin a v(apparent) = ------------------ = --------------- t(1 - (v/c) cos a) 1 - (v/c) cos a ^ O ^ | |\ | | | \ | | | \ vt cos a | | a \ | ct | \ | | | \ | | | B v | | ^ | | ct - vt cos a v | v \_____I_____/ (Earth, radio telescope) This apparent velocity can clearly be greater than c if a is small and v is close to c. There are other independent reasons for believing that the jets in radio-loud quasars have velocities close to c and are aligned close to the line of sight, so that this explanation is a plausible one.
Subject: H.09 What's the Local Group? Author: Hartmut Frommert <>, Christine Kronberg <> This is "our" group of galaxies. It was first recognized by Hubble, in the time of the first distance determinations and redshift measurements. The Local Group contains the Andromeda Galaxy (M31) and its satellites M32 and M110, as well as the Triangulum galaxy (M33). Other members (over 30 in all) include our Milky Way Galaxy, the Large and the Small Magellanic Cloud (LMC and SMC), which have been known before the invention of the telescope (as was the Andromeda Galaxy), as well as several smaller galaxies which were discovered more recently. These galaxies are spread in a volume of nearly 10 million light years diameter, centered somewhere between the Milky Way and M31. Membership is not certain for all these galaxies, and there are possible other candidate members. Of the Local Group member galaxies, the Milky Way and M31 are by for the most massive, and therefore dominant members. Each of these two giant spirals has accumulated a system of satellite galaxies, where * the system of the Milky Way contains many (nearby) dwarf galaxies, spread all over the sky, namely Sag DEG, LMC, SMC, and the dwarf galaxies in Ursa Minor, Draco, Carina, Sextans (dwarf), Sculptor, Fornax, Leo I and Leo II; and * the system of the Andromeda galaxy is seen from outside, and thus grouped around its main galaxy M31 in Andromeda, containing bright nearby M32 and M110 as well as fainter and more far-out NGC 147 and 185, the very faint systems And I, And II, And III, and, possibly, And IV. The third-largest galaxy, the Triangulum spiral M33, may or may not be an outlying gravitationally bound companion of M31, but has itself probably the dwarf LGS 3 as a satellite. The other members cannot be assigned to one of the main subgroups, and float quite alone in the gravitational field of the giant group members. The substructures of the group are probably not stable. Observations and calculations suggest that the group is highly dynamic and has changed significantly in the past: The galaxies around the large elliptical Maffei 1 have probably been once part of our galaxy group. As this shows, the Local Group is not isolated, but in gravitational interaction, and member exchange, with the nearest surrounding groups, notably: * the Maffei 1 group, which besides the giant elliptical galaxy Maffei 1 also contains smaller Maffei 2, and is associated with nearby IC 342. This group is highly obscured by dark dust near the Milky Way's equatorial plane. * the Sculptor Group or South Polar Group (with members situated around the South Galactic pole), dominated by NGC 253; * the M81 group; and * the M83 group. In the future, interaction between the member galaxies and with the cosmic neighborhood will continue to change the Local Group. Some astronomers speculate that the two large spirals, our Milky Way and the Andromeda Galaxy, may perhaps collide and merge in some distant future, to form a giant elliptical. In addition, there is evidence that our nearest big cluster of galaxies, the Virgo Cluster, will probably stop our cosmological recession away from it, accelerate the Local Group toward itself so that it will finally fall and merge into this huge cluster of galaxies. A table of the currently known Local Group member galaxies is at <URL:>. A (somewhat technical) review of the Local Group is at <URL:>.
Subject: Copyright This document, as a collection, is Copyright 1995--2003 by T. Joseph W. Lazio ( The individual articles are copyright by the individual authors listed. All rights are reserved. Permission to use, copy and distribute this unmodified document by any means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted, provided that both the above Copyright notice and this permission notice appear in all copies of the FAQ itself. Reproducing this FAQ by any means, included, but not limited to, printing, copying existing prints, publishing by electronic or other means, implies full agreement to the above non-profit-use clause, unless upon prior written permission of the authors. This FAQ is provided by the authors "as is," with all its faults. Any express or implied warranties, including, but not limited to, any implied warranties of merchantability, accuracy, or fitness for any particular purpose, are disclaimed. If you use the information in this document, in any way, you do so at your own risk.

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