| Galaxies Introduction - Island UniversesGalaxies are the lighthouses that plumb the Universe - constituents ofthe largest-scale texture we know. They span a vast range of properties,from dwarf galaxies with a few million stars barely outshining the brightest individual star clusters in our own galaxy, to vast assemblagesof a trillion stars in the centers of great clusters. Our own galaxy,a reasonably bright spiral system, can be traced at least fifty thousandlight-years from its nucleus, and we know of many galaxies much larger yet. Some elliptical galaxies show no evidence of having formed starssince a brilliant epoch early in cosmic history, while spiral and irregular galaxies have been making stars briskly over their entire lifetimes. Some galaxies produce most of their energy deep in the infrared, and some are so diffuse and faint as to be barely detectable against the faint glowof the Earth's night sky. Some HistoryOur appreciation of the universe beyond the Milky Way is entirelyan achievement of the twentieth century. The objects which wenow know to be galaxies had, to be sure, occasionally drawnthe curiosity of visual observers from the days of Charles Messierforward, particularly William Parsons (the Earl of Rosse), whose72-inch (1.8-meter) telescope with its speculum-metal mirrorhad revealed the intriguing spiral forms of certain dim, cloudyobjects (nebulae) seen, by and large, far from the encircling bandof the Milky Way. However, further tools were to be needed to unravel the true nature of some of these objects.By the 1920s, photography had revealed that there must be tens of thousandsof these objects, by then known as white nebulae to distinguish them fromthe clearly different gaseous nebulae such as the famous Orion Nebula,accessible to the telescopes of the time. They showed a variety ofspiral, elongated, or oval forms. The most plausible theories to account forthese nebulae made them either nearby objects - perhaps planetary systems in formation - or extremely distant, truly ``island universes"of which our Milky Way, hitherto the entire known Universe, would bemerely one among myriads.The key observation in resolving this dispute came from Edwin Hubble,using the recently completed 100-inch (2.5-meter) telescope on MountWilson, California. Targeting the largest and brightest of the ``whitenebulae", as the ones most likely to be nearby in space, he repeatedlyphotographed selected portions of them as deeply as the availablephotographic plates would allow. Faint starlike points had been recognizedin these nebulae, but could one show that these were in fact stars suchas we know in the solar neighborhood, and thus at the enormous distancesrequired to make them appear so faint? Hubble's breathrough came in identifying stars with a particular kindof cyclic change in brightness, which them ``standard candles" whoseabsolute brightness could be determined - Cepheid variable stars.Henrietta Leavitt at Harvard had shown that this class of pulsating starshas the useful property of a tight correlation between the periodrequired to complete one pulsation in surface temperature and size(and thus brightness) and the amount of energy the star gives off(usually expressed as absolute magnitude, the stellar brightnesswhich we would measure if a star were located at a reference distanceof ten parsecs). Cepheid variables gave Hubble the first necessaryyardstick in the ladder of extragalactic distances. (One of the majorprograms for the Hubble Space Telescope is the measurement of galaxydistances beyond the reach of ground-based instruments, by identifyingCepheid variables in more and more distant galaxies. One HST team hasrecently reported success in measuring Cepheids in galaxies of the VirgoCluster, about twenty times as distant as the galaxies Hubble theastronomer was observing).This discovery, in one stroke, opened a whole new vista of the Universe.Within a decade, many of the major strands of galaxy research had begun.Clusters and groups were recognized, classification schemes were proposed,and spectroscopic measurements were begun. Spectra of galaxies provedespecially rewarding. Early measures by V.M. Slipher at Lowell Observatory,using very delicate multi-night exposures, had shown that some ``spiralnebulae" exhibited unusually large Doppler shifts. It eventualy developedthat galaxies exhibit, on average, a relation between the redshift offeatures in their spectra and their estimated distances. This gavea way to estimate the distances of ever fainter and more remote galaxies,and provided the first glimpse of an expanding universe. Kinds of GalaxiesScientists and naturalists alike have the urge to sort, classify and organize new phenomena, in the hope of seeing underlying patterns thathave physical meaning. Several classifications for galaxies were proposedearly in their study; Hubble's classification system has proven remarkablyrobust, correlating well with physically interesting measurements suchas stellar content, gas content, and environment despite being designedonly to describe the appearance of the galaxy as seen on photographswith blue-sensitive emulsions. With later extensions by Gerard deVaucouleurs and Sidney van den Bergh, this remains the most commonlyused description of galaxy forms.Elliptical galaxies were denoted by the letter E and a number describingthe galaxy's apparent shape - 0 for a completely round form, 5 forone twice as long as wide, and 7 for the apparently flattest genuineellipticals. We do not know, solely from an image, the true shape ofsuch a galaxy; the same galaxy might have quite different degrees offlattening if viewed from different directions. Elliptical galaxiesare, in general, characterized by old stellar populations and very little of the gas and dust needed to form new stars.Spirals are divided into ordinary and barred spirals; in barred systems thespiral arms arise from a straight ``bar" passing through the center, whileordinary spirals have a more S-shaped inner configuration. Ordinaryspiral are denoted S and barred systems SB. Both usually contain acentral bulge, often sharing many properties with elliptical galaxies,surrounded by a thin rotating disk containing whatever spiral structurethere may be. Spirals are subdivided into a sequence jointly definedby the winding and prominence of the spiral arms, and the relativeimportance of the central bulge. Sa galaxies have a bright bulge andtightly wound arms, while Sc galaxies have loosely wound arms anda relatively less important bulge. This sequence Sa-Sb-Sc-Sd hascounterparts SBa-SBb-SBc-SBd in the barred spirals. As more detail wasobserved in some galaxies, intermediate substeps (Sab,Sbc,Scd,S0/a)could be added when necessary.Some galaxies show no particular organization, either because some recentevent has left them in a disturbed state or because they simply lackthe organizing rotation and wave motions of a spiral. These are simplycalled irregulars; the ones that do not result from external disturbanceform, in many respects, an extension beyond Sd of the spiral sequence.Hubble recognized that the connections among various types left anapparent hole which he called S0 - objects with a bulge and disk,but little or no star formation, dust, or gas. They would form abridge between ellipticals and spirals. Later, many genuine S0galaxies were in act recognized, and understanding their originpromises to tell us much about the history and development ofgalaxies in general.Several refinements of the Hubble classification have proven widelyuseful. Gerard de Vaucouleurs introduced distinctions dependingon whether the spiral structure proceeds from the nucleus in anS-shape (s) or from an inner ring (r), or some combination (rs) or(sr). He also recognized intermediate cases SAB between barred andnonbarred galaxies. These new dimensions allowed a finer discriminationof galaxy structure and opened the way for more detailed study ofthe physical properties of spiral galaxies. Sidney van den Berghnoted that the most luminous spirals have long, well-developedspiral arms, and introduced a luminosity classification driven by theorganization and distinctness of the arms; Sc I galaxies are in themean the brightest Sc galaxies, and Sc V the faintest. Note that theclassification is based solely on a galaxy's appearance, with itsabsolute magnitude a correlating quantity. Some galaxies are not well described by the Hubble system or its variants,even excluding ``train wrecks" resulting from galaxy collisions.There exist, usually in rich clusters, enormous elliptical-like systemsthat may span millions of light-years with more extended outer regionsthan a similarly huge elliptical would show. These are given thedesignation cD, from a scheme developed at Yerkes Observatory byW.W. Morgan. Dwarf galaxies may be irregular, elliptical, orspheroidal, depending on their degree of symmetry and centralconcentration. Recent work has turned up galaxies of very lowsurface brightness, which must have had a rather different historyfrom familiar spirals. While many of these look like the ghostsof ordinary spirals, it is not at all clear how they connect tothe familiar Hubble types. Content of GalaxiesWe observe stars, gas, and dust in galaxies. Stars come in a wide rangeof age and mass, and are intricately linked to interstellar matter byprocesses of stellar birth and death. This means that galaxies have ahistory, which we can probe either by investigating the makeup of agalaxy in detail, or in a kind of fossil probe unique to astronomy,lok at galaxies so distant that the light we observe left them when theywere much younger than they are ``today".In tracing the makeup of galaxies, there are numerous clues as to the populations of stars present. Different kinds of stars (giant/dwarf,hot/cool, higher/lower abundances of heavy elements) have differentpatterns and intensities of features in their spectra. In mostgalaxies, we can observe only their overall (integrated) spectrum,so that a mathematical solution can give constraints on the overallpopulation, but the solution is not completely well-determined withoutadditional assumptions (such a a smoothly varying star-formation rate,or fixed ratios of stars at various masses). To resolve these ambiguities,observations of very nearby galaxies are crucial, where individual(luminous) stars can be observed and counted.Some components of a galaxy stand out in specific kinds of observations,so that interstellar matter and certain kinds of stars can be studiedin isolation. The 21-cm radio emission of cold atomic hydrogen tracesthis component of a galaxy cleanly, giving one index of its gas contentand tracing internal motions beautifully. The gas most immediatelyassociated with the birth of new stars is colder and denser than neutral hydrogen, being mostly molecular hydrogen and best observedvia the trace molecule CO, which emits spectral lines in the 1-3 mmrange. The most massive young stars emit copious ultraviolet radiation,which may be absorbed by surrounding gas and re-emitted as spectrallines including H-alpha in the visible region, so that using specialfilters and image processing allows a view of these star-formingregions alone (so long as they are not hidden from view by interveningdust). The dust itself emits longer-wavelength infrared radiation,so we can trace the location of interstellar dust and locate theregions where it is warmed by starlight. Going to the ultraviolet,only the hottest stars give off enough radiation to see, so this regionalso allows us to trace regions of active star formation. Finally,loking at a galaxy in X-rays, we see only the highest-energy components -binary stars in which material falling onto a neutron star or black holegives rise to extreme temperatures, emission from gas at millionsof degrees, and sometimes emission from central quasar-like activenuclei which may not give an accurate indicator of temperature, sinceso-called nonthermal processes may be involved.There is growing evidence that we may be completely ignorant of one ofthe most important constituents of galaxies - the dark matter.If gravity behaves over ranges of thousands of light years in theway that it does over smaller scales, the motions of stars and gasin galaxies, and of gas and galaxies in clusters, require that mostof the mass in these systems is on some completely invisible forms.The main lines of evidence include: -- flat rotation curves in spiral galaxies. The orbital speed measured for material in the outer parts of spirals is nearly constant with distance, without the dropoff which would show that we are observing orbits outside the main mass concentration. -- velocities of galaxies in clusters. Similarly, the measured motions of galaxies in clusters are too fast for them to be held together by the gravity of the the visble stars comprising the galaxies. Hot gas between the galaxies, revealed by its X-ray emission, adds about an equal amount of mass to the galaxies' stars, but a discrepancy often reaching a factor 10 remains between visible and gravitating masses. -- the extent of the hot gas in clusters of galaxies. At its observed temperatures, the amount of mass needed to hold it in place by gravity is comparable to that deduced to from galaxy motions. In fact, in many cases, the gas is regarded as a more reliable tracer, since a cluster contains only so many galaxies which can act as tracer particles, while the hot gas is a continuous medium which can be observed in as much detail as instrumentation permits.The nature of this unseen matter remains elusive, and has provided a happyhunting ground for observer and theoretician alike. Proposals have includedbrown dwarf stars, Jupiter-like objects, quantum black holes producedin the early Universe, and a whole zoo of exotic particles which wouldalso be remnants of the early Universe. Assorted astronomical and laboratorysearches have yet to tell us what makes up most of the matter in theUniverse. We are left with the sobering realization that all of our vauntedtechnology and apparatus has been telling us about only 10% of the cosmos. Clusters of galaxiesEarly surveys of galaxies on the sky showed that certain regions havemore than their share of galaxies; such concentrations as the Virgocluster were known long before the nature of galaxies was understood.More complete statistics have shown that the distribution of galaxiesin space is far from the uniform "sea" first envisaged, with many(perhaps most) galaxies arrayed in groups, clusters with thousands ofmembers, superclusters, and even larger sheets and fingers stretchingas far across the Universe as we can reliably map.Clusters come in a variety of kinds, just as galaxies do. The richestand densest clusters are round assemblages, while sparser clustershave flattened or irregular shapes. The cluster environment is reflectedin it sgalaxy content - dense environments like cluster cores arepopulated almost solely by elliptical and S0 galaxies, nearly devoid ofgas and star formation. Less extreme environments can host, as well,spiral and irregular galaxies. This so-called morphology-density relationhas engendered a classic heredity-environment question - were spiralgalaxies never formed in those regions which would one day be richclusters, or are they somehow destroyed or transformed in such clusters?The jury is still out, though there is strong evidence that in some clustersspirals were once numerous and have been tranformed by external factorsinto elliptical or S0 systems. One such transforming mechanism is via galaxymergers, which, while not common at the high speeds typical of clusterencounters today, might have been more common early on.A second transforming mechanism could be provided if clusters contain somekind of external medium - intergalactic gas. Such a medium was indeeddiscovered by early X-ray astronomy satellites, and is known to beubiquitous in clusters and even galaxy groups. Random motions in thecluster heat this gas to temperatures of 10,000,000 Kelvin,making it visible only by its own X-ray emission. This gas typicallyhas as much mass as do stars in the visible galaxies, and as galaxiesmove through it, will provide an external wind. This would in principlebe strong enough to sweep gas out of a spiral galaxy, and a gas-free spiralwill cease star formation and quickly look like an S0. Detailed observationsin local clusters such as Virgo in fact show that spirals nearest thecenter seem to have lost the outermost parts of their gas distributions.Detailed studies of the distance and redshifts of nearby galaxies have addedanother dynamic aspect to our understanding of clusters - they are stillgrowing. At greater and greater distances, the gravity of a cluster takeslonger to affect the motions of its surrounding galaxies, so that galaxiesat larger distances will eventually turn around against the expansion ofthe Universe and fall into the cluster. Our own local group has a detectablemotion toward the Virgo Cluster (more precisely, the core of the LocalSupercluster), and such large-scale motions can be found near manynearby clusters. In this sense, the cosmic epoch of cluster formationis now. Galaxies and cosmologyThe very recognition of galaxies as objects at vast distances led tothe first attempts to use thems as tracers of the structure of theUniverse as a whole - observational cosmology. Hubble's promulgationof the evidence for a relation between a galaxy's distance and itsredshift led to a picture of an expanding Universe. As observationalcapabilities have increased, so has the volume of space whereastronomers can search for signatures of the geometry of space-time.The Hubble law for galaxy redshifts implied a uniform expansion -one in which every galaxy sees the same linear relation betweendistance and redshift when looking at other galaxies. The rateof this expansion is characterized by the Hubble constant - the ratiobetween redshift and distance for a fictitious average galaxy withno peculiar motion of its own with respect to the expansion. Notonly does this value give us the size scale of the Universer, butit gives a measure of its age as well. If one runs the clock backwardson a uniform Hubble expansion, at a constant rate, the age of theexpansion is the numerical inverse of the Hubble constant. Even if there has been deceleration of the expansion due to gravity, thisage - the Hubble time - gives a scaling value for the age of the Universe.The exact value of the Hubble constant has been contentious, with strongarguments presented for values from 50 to 100 km/sec per megaparsec.These correspond to Hubble times of, respectively, 20 and 10 billionyears. Some of the disagreement between workers on the cosmic distancescale comes from different treatments of local galaxy motions superimposedon the smooth expansion, and some from regarding various measures ofdistance as primary or secondary. One of the major projects for theHubble Space Telescope deals with direct measures of galaxies distantenough to expect a clean measurement of the Hubble constant.The distribution of galaxies into groups, clusters, and superclusterscarries information on masses and motions in the early Universe. In brief, if the initial distribution of pre-galactic material was as uniform as the COBE satellite data suggests, then in orderto form clusters today surrounded by relatively empty areas, thecluster-galaxies-to-be must have been able to move fast enough to cross (at least) the size of these empty areas in a Hubble time. The necessary gravitational clumping to propel such motion early enough proves to be an important constraint on the early Universe.Classical cosmology was once described as a search for two numbers -the Hubble constant H_0, and a second value, the deceleration parameterq_0. The deceleration parameter described how fast the Hubble constantchanges with cosmic time as the overall gravity of all matter in theUniverse slows the expansion. A value of 0 would indicate an emptyUniverse - mathematically simple and appealing, but not veryinteresting to us! There are there different cases - an open Universe,in which the expansion will never stop; a closed Universe, in whichthe expansion will someday stop and reverse; and the critical pointbetween them, where the expansion will constantly slow and approach(but never quite reach) zero. These are separated by the critical valueq_0=1/2. Many classical tests for the value of q_0 relied on usinggalaxies as standard candles, but have been defeated by the fact thatgalaxies evolve on the same timescales that must be probed to measureq_0. Current efforts in this direction use different probes, such asgravitational lensing or the mean mass density in the local Universe,in efforts to circumvent the unknowns of galaxy evolution. In any case,it is remarkable that q_0 is close to its critical value; otherwisethe Universe might not have the right properties for us to exist anddiscover cosmology. Galaxy nomenclature and catalogs: Galaxies are generally denoted only by catalog numbers; only a handfulare well-known or unusual enough to rate distinctive names (such asthe Whirlpool, Antennae, Pinwheel, and Cartwheel). A given galaxy may sport numbers from several catalogs. The most citedsources are:Messier number - from a list compiled visually bu Charles Messier andseveral colleagues during the eighteenth century. Many of the brightestand most conspicuous galaxies (as well as gaseous nebulae and star clusters)appear in the Messier lists.NGC/IC (New General Catalog and Index Catalog) - compilations by J.L.E.Dreyer from the 1860s-1880s. These included results of the complete skysweeps performed by William and Jihn Herschel and discoveries byothers, plus the first harvests of celestial photography. These catalogsinclude (besides the usual round of clusters and nebulae) about 10,000 of the most conspicuous galaxies. Until recently, almost all galaxieswhich could be studiesd in detail had NGC or IC numbers.Arp - Halton Arp produced an atlas of peculiar and interacting galaxies,which first drew tha attention of many astronomers to the strange andspectacular forms that galaxies outside the normal Hubble classificationcan take.UGC (Uppsala General Catalog) - galaxies only, covering the sky north of-2 degrees 30'. Peter Nilsson produced this catalog of positions,sizes, orientations, and magnitudes from Palomar Sky Survey photographs.Several other kinds of name comprise coordinate designations (first digits of the object right ascension and declination, either for epoch 1950 or 2000)and a survey name. Examples are the PKS (radio sources from the Parkesradio telescope in Australia) and IRAS (Infarred Astronomical Satellite)surveys. Thus we may have PKS 1413+003 or IRAS 09104+4109.Many of the originally published versions of these catalogs are eitherrather obscure (university observatory transaction series, for example), or out of print (the NGC and IC being notable exceptions).Users of personal computers can get modern versions of these andmany more on CD-ROMs (from the Astronomical Data Center at the NASAGoddard Space Flight Center, or the Almageste package from the ASP). Such a level of access, without need of a professional astronomical library, makes many kinds of advanced study and observing programs possible.Note that wavelength/flux/surface brightness selection enters into whatgalaxies get selected for a particular catalog or study. Malin 1, despitebeing large and luminous, was long missed for being _too_ large. Severalresearchers have pointed out that galaxy catalogs are dominated by thosekinds of galaxies which are easiest to see against the natural glowof the night sky, so we may still be ignorant of important parts ofthe extragalactic census. Observing galaxies Even though most are difficult objects for visual inspection,the emotional impact of seeng "fossil" light makes galaxies populartargets for amateur astronomers. My own "best" list, including a fewshown in the UA collection of WWW images, athttp://www.astr.ua.edu/choosepic.html, includes:Name Constellation RA (2000) Dec Mag Notes----------- ------------ -------- ------ -- --------------------------M31=NGC 224 Andromeda 00 42.7 +41 16 4 Great spiralM32=NGC 221 Andromeda 00 42.7 +40 52 9 Elliptical companion of M31M110=NGC 205 Andromeda 00 40.4 +41 41 9 Elliptical companion of M31M81=NGC 3031 Ursa Major 09 55.5 +69 04 8 M82=NGC 3034 Ursa Major 09 55.9 +69 41 9 edge-on dusty starburst galaxyCentaurus A Centaurus 13 25.5 -43 01 8 peculiar radio galaxyLMC Doradus 05 23 -69 45 2 Large Magellanic CloudSMC Tucana 00 52 -72 50 3 Small Magellanic CloudM77=NGC 1068 Cetus 02 42.6 -00 01 10 Seyfert nucleusM87=NGC 4486 Virgo 12 30.8 +12 23 9 Center of Virgo clusterNGC 4565 Coma 12 36.3 +25 59 10 edge-on spiral, dust laneNGC 3556 Ursa Major 11 11.5 +55 40 11 edge-on spiralNGC 891 Andromeda 02 22.5 +42 21 10 edge-on spiralM104=NGC 4594 Virgo 12 40.0 -11 37 9 Sombrero galaxyM65=NGC 3623 Leo 11 18.9 +13 05 10M66=NGC 3627 Leo 11 20.2 +12 59 10NGC 2903 Leo 09 32.1 +21 30 10 barred spiralM83=NGC 5236 Hydra 13 37.0 -29 52 8 barred spiralNGC 7331 Pegasus 22 37.1 +34 25 10 inclined, elongatedM94=NGC 4736 Canes Venatici 12 50.9 +41 07 9 very bright nucleusFor more of a challenge:M33=NGC 598 Triangulum 01 33.8 +30 39 6 large but dimComa cluster Coma Berenices 12 59.6 +27 57 13+NGC 1300 Eridanus 03 19.7 -19 25 11 strongly barred spiralNGC 147 Cassiopaeia 00 33.2 +48 30 11 distant companion of M31NGC 185 Cassiopeia 00 39.0 +48 20 10 distant companion of M31NGC 4038/9 Corvus 12 01.9 -18 52 10 "Antennae" interacting pairNGC 6822 Sagittarius 19 44.9 -14 48 9 Local Group irregularThe whole region around M87, in the Virgo Cluster, contain numerousbright galaxies of various types. Take a good map.As always, any particular size and magnitude listed for a galaxy may notgauge the visual impression for a given telescope, magnitude, andset of sky conditions. Look for yourself, your mileage may vary. For Further ReadingThe Hubble Atlas, by A. Sandage (Carnegie Institution of Washington 1961). This virtually defines the Hubble classification, and offers a stunning collection of galaxy photographs from the Mount Wilson and Palomar telescopes. It will soon be superceded by the Carnegie Atlas of Galaxies.The Color Atlas of Galaxies, by J.D. Wray (Cambridge 1988). The fruits of a long-term project to photograph galaxies through selected filters and produce composite color images. A wide range of galaxy types is illustrated, with care devoted to the color reproduction and its interpretation.Man Discovers the Galaxies, by R. Berendzen, R. Hart, and D. Seeley (Science History Publications 1976). From the recognition of galaxies through early distance measurements and the beginnings of modern observational cosmology. Galaxies, by Timothy Ferris (Sierra Club Books 1980). A classic coffee-table book full of beautiful galaxy images. Useful even for career astronomers who need to remind themselves of what drew them into galaxy study.Lonely Hearts of the Cosmos, by Denis Overbye. An engaging account of the quest for the cosmic distance scale.The Universe of Galaxies, edited by Paul Hodge (Freeman 1984). Articles excerpted from Scientific American, covering dark matter, galactic tides, clusters, and active galactic nuclei.Galaxies, by Paul Hodge(text originally produced for the Astronomical Society of the Pacific "Galaxies"slide set, by Bill Keel - see http://www.astr.ua.edu/keel) |
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