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Massimo Capaccioli » 1.Introduction to the realm of nebulae

Foreword

This is my first attempt to convert into the new format of Federica a random series of multicolored PPT slides dealing with the physics of galaxies. They were produced discontinuously over 10 years or so, either in Italian or in English, with the purpose of supporting my lectures with pretty figures and well-drawn diagrams and of sustaining my memory while on stage. The presentation is as concise as possible. The language is English, since the course is addressed to students already exposed to scientific literature. The syllabus is the standard one for the subject, but many issues are only quickly touched, and others simply overlooked, while important. Luckily, excellent books on the subject are available to fill the gaps.
Students are expected to have some familiarity with the astronomical jargon, the principles of celestial mechanics and of stellar structure/evolution, the astronomical spectroscopy, and the radiative processes in astrophysics. In order to help them to meditate further on what they are studying, some remarks and questions to them have been inserted here and there, in bold-faced italics and between square brackets.
A set of references is provided, chapter by chapter, as well as a list of books and of links to e-texts. However students should find their way to the Department library and quickly learn how to interrogate SAO-NASA ADS and to scan astro-ph.
In this effort I have been effectively and competently flanked by my colleagues Nicola R. Napolitano of the Astronomical Observatory of INAF at Capodimonte, Naples, and Giovanni Covone of the University of Naples Federico II. They have several merits and the direct responsability for some chapters, but the blame for possible errors has to be fully on me. I finally thank Maurizio Paolillo for helping with AGNs.

Naples, March 31, 2011

Massimo Capaccioli

Forewarning

Astrophysics vs. physics

Astrophysics is not just the physics of the stars, as the name seems to suggest. Since the second half of the Nineteenth century it has been definitely accepted that the laws of physics are the same everywhere in the universe under the same conditions, and that there is a unique methodology to investigate Nature, the one devised by Galilei. But the conditions under which we investigate Nature on Earth are quite different from those applying to the sky. With the due and obvious exceptions, the sky is an already built laboratory that we just observe, while terrestrial labs are places where we make experiments decided and built by us. This fact has several consequences which establish the differences between physics and astrophysics, and some pros and cons that the student will try to evaluate.

Another important warning, which applies to physical entities in general with respect to everyday life objects, is the lack of an immediate feeling of the order of magnitude of the parameters characterizing them. Let us make an example. You know by experience that the characteristic length of a car is a few meters, and that the characteristic weight of a man is, say, 70 kg. You would be surprised and unwilling to accept the result if you were told that a car is either 1 mm or 1 light year (1 lyr) long, and that a man weights either 10^-4 or 10⁷kg. But this is not the case when the quantity in play is the radius of a galaxy or the mass of a star, for instance. Before approaching astrophysics, none of you would be surprised if told that a galaxy is 10¹⁰ km wide rather than 10³¹ km, and that a stars weight 10¹⁰ or 10⁴⁴gr. This a priory ignorance makes the second step of the Galileian method, i.e. the model construction, require that first of all you establish, by the most general principles, which are the orders of magnitude of the entities in play.

What is a galaxy?

A galaxy is a massive, self-gravitating system made of:

stars and stellar remnants (RS). Stars may come in a manifold of masses, ages, and chemical compositions. Thus they happen to have an assortment of luminosities and colors [it would be wise to recap the observational properties of stars and their dependence of mass, age, and chemical composition];
an interstellar medium (ISM) of gas (in various phases: molecular, atomic, ionized) and dust;
an essential but yet mysterious ingredient named dark matter (DM) as its bulk does not interact with electromagnetic radiation (a small fraction of DM is actually baryonic). It dominates the overall mass.

The picture shows an image of the spiral galaxy M81 with its many visible ingredients: stars, dust, gas. Dark Matter is there but not visible.

Credit: NASA-JPL, Galex team, J. Huchra et al. (Harvard CfA).

Is it now clear what a galaxy is?

[Think about the meaning of the various terms used in the previous slide.
1. What does it mean "self-gravitating"?
2. What are stars?
3. How can matter be "dark"?]

The definition of galaxy that we have just temptatively given is however “weak” as it is not unambiguous.

For instance, the globular cluster (GC) pictured on the right-side of this slide is a self-gravitating ensemble of many stars, but it is not a galaxy. It is instead a member of the subsystem of GCs of our galaxy. Note however that there are some very distant Milky Way GCs which are not much different from some dwarf galaxies satellite of our own. This rises the question of whether the former are bona fide globular clusters or the latter bona fide galaxies.

The giant globular cluster Omega Cen. Credit: VST: ESO & INAF-OAC.

Use mass to remove ambiguity

A way out to resolve the ambiguity in defining galaxies is to constrain the range for their masses. The smallest galaxy masses are equivalent to $\sim 10^6$ stars (actually Suns, each with a mass of $1 M_\odot = 1.99\times10^{33} gr$ ), the largest to $\sim10^{13}$ stars or more.

[How would you measure the mass of the Sun? Does your measure provide the gravitational or the inertial mass? What is the difference between these two? What is mass? In what does it differ from energy?]

The mass of the Milky Way, and of the Andromeda Nebula M31, is equivalent to that of $\sim 2\times10^{11}$ Suns. [Do these galaxies really consist of such a number of stars?]

The picture at the RHS shows the Pegasus dwarf spheroidal (dSph) galaxy, a new member of the Local Group, in a 3-color image taken with the Keck telescope at Mauna Kea.

Credit: E. Grebel (Washington Univ.), P. Guhathakurta (UCO/Lick)

Is there an upper limit to the mass?

An interesting question is: if galaxy masses are inferiorly limited, are they also superiorly limited?
The answer is yes: the largest masses so far measured are of the order of $10^{13} M_\odot$ , that is 100 times the Milky Way. So, galaxy masses may differ one from the other by 7 orders of magnitude, far larger a range than that over where masses of stars vary (~10³).
[What does it limit the mass of stars, and in which range?]

Remember: a Solar mass is $1 M_\odot = 1.989\times10^{30} kg$ . [How do you measure it?]

In the false colours picture at the RHS the giant elliptical NGC 1132 ottenied combining X-ray (Chandra Obs.) and green/IR (HST) images.

Credit: NASA, ESA, M. West (ESO, Chile), and CXC/Penn State Univ./G. Garmire et al.

Plain anatomy of a galaxy: from core to halo

In a galaxy the spatial (and surface) matter and light densities [not necessarily proportional; meditate on this!] decrease typically from a center [the brightest spot; the system does not need to be symmetric] to the edges.
At, or near, the photometric center there may seat a supermassive black hole (BH).
About the center there is the nucleus, made of stars and gas, and possible location of an intense activity (which makes the object an AGN = Active Galactic Nucleus).
The nucleus is surrounded by a central bulge of old stars, which merges smoothly into an outer halo of very old stars.
The latter is a vast subsystem, usually free of dust and cold gas and containing up to thousands of globular clusters (GC).

The location of the main substructures of a typical galaxy is sketched at the RHS. Other features as jets, gas and dust clouds, star clusters but GCs, and ingredients such as plasma envelopes or magnetic field force lines, are not shown.

Plain anatomy of a galaxy: other ingredients

There may be flat structures, thin or thick disks, made of relatively young stars, some of which are rich in dust and cold gas and decorated by spiral arms.
The baryonic (luminous [clarify]) components of a galaxy are [likely: see alternative theories to gravity] embedded in a dark halo (DH, made of DM), invisible but for its gravitational effects.
Stars have various masses and ages [draw phenomenological consequences].
Gas may be cold as in the molecular clouds, neutral as in the neutral hydrogen HI disk, ionized as in the HII complexes, a hot plasma as in the X-ray haloes [draw consequences for observations].
Galaxies may be associated with intense magnetic fields.

Hot gas (blue) about the spiral galaxy NGC 5746. Credit NASA (Chandra X-ray Observatory).

Evolution of our understanding of galaxies: pre-history

In the pre-telescopic era only one galaxy was known: our own. Perceived as a diffuse band of light crossing the sky (we live at a rim of the disk), it was usually recognized by primitive cultures as a “celestial road”. The Greeks named it “Galaxias kuklos” = Milky Band.
While the understanding of the feature was mythological [cf. the myth of Alcmena, the mortal mother of Heracles], philosophers indeed reflected on its true (physical) nature.

This is apparent in Dante Alighieri’s Paradise (14: 97-99):
Come distinta da minori e maggi
lumi biancheggia tra’ poli del mondo
Galassia sì, che fa dubbiar ben saggi.

In the philosophical essay Convivio (II-15) the Florentine poet attributed (wrongly) to the Greek Aristotle the idea that the Milky Way is a cluster of stars not visible individually as they are too small.

Milky Way panorama produced by ESO for IYA2009. Find bulge, disk, dust lanes, and Magellanic Clouds. Credit: ESO.

Pre-historical pioneers

In the X Century the Persian astronomer ‘Abd al-Rahman al-Sufi (903-986) was the first to mention the existence of celestial nebulae. He described the nebula in Andromeda (M31) and the Large Magellanic Cloud (LMC).

The two Clouds were cited as “nebelle” by Antonio Pigafetta (1492-1531) from Vicenza, one of the very few survivors of Magellan’s expedition around the world, in his report of the journey, La mia longa et pericolosa navigatione: la prima circumnavigazione del globo: 1519-1522.

In 1610, looking through his small refractor, Galilei discovered that the Milky Way «is nothing else than a congeries of innumerable stars distributed in clusters» («Est enim GALAXIA nihil aliud, quam innumerarum Stellarum coacervatim consitarum congeries: in quamcumque enim regionem illius Perspicillum dirigas, statim Stellarum ingens frequentia sese in conspectum profert, quarum complures satis magnæ ac valde conspicuæ videntur; sed exiguarum multitudo prorsus inexplorabilis est.»)

The Andromeda Nebula was re-discovered in 1650 ca. by the Sicilian priest Giovanni Battista Hodierna (1597-1660) using a simple Galilean refractor. He likely detected also the Triangulum Nebula (M33) and NGC 891.

M31, the giant spiral companion of the Milky Way, with its spheroidal satellite NGC 205. Credit: KPNO.

Understanding the Milky Way

Moved by theological considerations, in 1750 the English architect and astronomer Thomas Wright (1711-1786) speculated on the nature and shape of the Milky Way, that he pictured it as a spherical shell made of clumps of stars. Seen from the Sun located midway between the inner and outer boundaries, the shell would act as a thin layer and thus perceived as a band onto the sky. Wright argued also that the nebulae [see next slides] were indeed similar to the Milky Way, i.e. distant galaxies.

These provocative thoughts were soon re-elaborated by Immanuel Kant (1724-1804) in his Theory of the Heavens (1755) upon the reading of a loose report of Wright’s model. The German philosopher conjectured that the Milky Way was a rotating disk of stars held together by gravity (Newton’s theory was only 70 years old), with no special place for the Sun [notice how advanced was this Copernican attitude] and that the nebulae were distinct stellar systems of the same nature of our own.

This latter notion was later rendered by the German scientist Alexander von Humboldt (1769-1859) in the expression of “island universes” (Cosmos, 1845).

Sketch of Wright's last model of the Galaxy.

Understanding the Milky Way: Herschel

In the last quarter of the Eighteen century, the German-born English astronomer William Herschel (1738-1822) started his famous star gauging with the assistance of the sister Caroline (1758-1848); at first she annotated the real-time observations of the brother, then became an independent and talented observer.

Herschel himself was polishing the larger and larger mirrors of his telescopes, the most famous being a 40-inch, the largest a 48-inch.

His survey work brought to many discoveries: a new planet, Uranus, and a few new satellites, several double stars (found as the most distant tests of Newton’s gravity, but actually hunted while applying an intuition by Galilei about a way of measuring parallaxes), and a rich catalogue of nebular objects.

Picture of William Herschel's 40-foot telescope published in Leisure Hour in 1867.

Understanding the Milky Way: wrong picture

Herschel produced also a new model of the Milky Way, this time based on observations. To this end he counted stars in as many as ca. 700 distinct directions. He built his 3D view assuming that the stars were:

all of the same intrinsic luminosity, so that their apparent luminosity could provide the relative distance [say why], and
all visible to the edge of the system.

The result was a flattened system, shaped as a “grindstone”, with the Sun (again!) near the center.

Herschel's reconstruction of the Milky Way. Compare it with the modern picture by ESO (search on web): not bad indeed!

Gauging the Milky Way: Kapteyn and Shapley

Herschel’s monumental work was repeated over a century later by the Dutch astronomer Jacobus Kapteyn (1851-1922) who counted stars on photographic plates, collecting an impressive database of stellar magnitudes, colors, spectral types, radial velocities. A methodological progress was a determination of stellar distances by a statistical use of the correlation between distance and luminosity (which had been found erroneous for single stars by Herschel himself at the end of his life). The result, appeared at the end of his life (1922), was a 17 kpc wide and 3 kpc thick disk, with the Sun slightly off its center: far smaller than real since Kapteyn ignored the existence of interstellar extinction, discovered a decade later by the Swiss-born American astronomer Robert Trumpler (1886-1956) [what is the effect of extinction on distances?].

In this same season (1921) Harlow Shapley(1885-1972) built at Harvard his own model of the Milky Way based on the distribution of the globular clusters. Assuming that they were symmetrically distributed about the Galaxy center, he derived a disk 100 kpc across (6 times Kapteyn’s), with the Sun displaced by as much as 16 kpc from the center. A good guess, biased again by the ignorance of extinction which affected Shapley’s calibration of the GC distances through the cluster variable stars RR Lyrae.

Shapley's 2D map of the distribution of the Galactic globular clusters.

Gauging the Milky Way: the misterious nebulae

Since the beginning of 1700 very few were the objects with nebular (i.e. not star-like) appearance observed by astronomers. Some of them were comets, eagerly searched by specialized hunters, others unresolved star clusters, as then shown by larger/better telescopes. But some, in spite of observational improvements, remained mysterious blurred images, true “nebulae”.
Christiaan Huygens (1629-1695), likewise did later Edmond Halley (1656-1742), understood the Andromeda Nebula as a diffuse window open on a back-illuminated surface of the sky.
Later on, Pierre-Simon Laplace (1749-1827) speculated that they might be extra-solar systems under formation (this is why small mass dying stars still keep the name of Planetary Nebulae).

A first extensive list of nebulae was prepared from 1774 to 1781 by the French comet-hunter Charles Messier (1730-1817) as a by-product of his activity: «Nothing is so similar to a nebular star - he wrote - as a comet which begins to be detectable by the instruments». The catalogue contains now 110 objects of various types (open and globular clusters, emission, reflection and dark nebulae, planetary nebulae, SN remnants, one asterism, one MW field, and 39 galaxies), identifies by the entry number preceded by the letter “M” for Messier.

A sample of Messier objects with NGC (New General Catalogue) entry numbers, and with their popular names.

The New General Catalogue

A longer list with ca. 5000 entries was assembled almost a century later by John Herschel (1792-1871), the son of William. In 1864 he printed a General Catalogue of Nebulae and Clusters of Stars (GC) which included objects found by his father and many from the then poorly explored Southern sky.
The GC was upgraded and corrected by the Danish-born Irish astronomer John Dreyer (1852-1926). In 1884 he published the famous New General Catalogue (NGC) with almost 8000 entries, and then the supplement Index Catalogue (IC). Each object is indicated by an integer number (increasing, apart from errors, with the Right Ascension), preceded by the letters “NGC” or “IC” [visit these and the Messier catalogues on the Web].

The figure plots all NGC and IC objects in an equatorial coordinate system [revisit astronomical coordinate systems, and related transformations]. You may immediately appreciate the texture, with clear condensations called cluster and superclusters.

Credit: W. Steinicke, Revised New General Catalogue and Index Catalogue.

The nature of the white nebulae

The nature of the unresolved nebulae remained a matter for speculations until 1860 ca., when William Huggins (1824-1910) found that some of them, named then “white nebulae”, showed a spectrum similar to that of stars rather than of a hot gas.

Many questions arose. Were they stellar systems? If so, how big? Just star clusters belonging to the Milky Way or systems comparable to our stellar system, the Galaxy? In the latter case they were to be considered “island universes” à la Kant.

Meanwhile William Parsons, Third Lord of Rosse (1800-1867), had provided the first detailed drawings [photography did not exist yet] of some nearby galaxies showing their complex morphology.

The "Leviathan of Parsonstown", a 72 inch (183 cm) telescope at Birr, Ireland, the world largest.

The nature of the white nebulae: errors and trials

Kant’s view seemed to triumph over that of a universe filled by just our Galaxy, when in 1885 a new star appeared in M31. The variable star, named S And [also SN 1885A; see the IUA rules for naming variables], seemed to be alike the Milky Way Novae. By this (wrong) assumption the Andromeda Nebula acquired a very short distance (3 kpc), which made it part of the Galaxy. Actually S And was a Supernova, an object inconceivably too bright at that time. The extragalactic scenario met another problem in the observed distribution of galaxies, which seemed to avoid systematically the Galactic equator.
[Why this anti-correlation may suggest that nebulae are of galactic origin? Are we nowadays using this same argument to establish the galactic/extragalactic nature of new classes of phenomena? Give examples and discuss.]
We know now (but they did not then) that this is due to the extinction by the thick dust layer.

Galaxy counts showing the Zone of avoidance of white nebulae all along the Galactic Equator.

Drawings of Lord Rosse

Credit for the M33 color picture: P.Massey (Lowell Obs.), N.King (STScI), S.Holmes (Charleston), G.Jacoby (WIYN/AURA/NSF).

The nature of the white nebula: more errors

A third difficulty arose by a claim of the Dutch astronomer Adriaan van Maanen (1884-1946) who reported the detection of proper motions in spiral nebulae. If so, they should have been too close [why?] to justify the name of galaxies for them. Only much later it was discovered the technical reason why van Maanen goofed.

One last problem came from a crucial observation by Vesto Slipher (1875-1969) at the Lowell Observatory in Arizona. In 1912 he noticed that the spectral lines in nearby white nebulae were prevalently shifted to the red. Interpreted as Doppler effect [why reasonable?], this shift seemed to indicate a systematic tendency of the nebulae to escape from the Galaxy. What we now call Hubble flow was then considered the sign of a close connection between spiral nebulae and the Galaxy. However, the high velocities measured (up to 1000 km/s) were inconsistent with stars.

A sample of galaxy images from the classical paper of Francis G. Pease (1981-1938) is shown at the RHS here and in the next page.

F.G. Pease, Photographs of nebulae with the 60-inch reflector 1911-1916, Ap.J., 46, 24, 1917.

The nature of the white nebulae: the party of the extragalactic view

In the first two decades of 1900, while the majority of astronomers were against the Kantian hypothesis, and in particular Harlow Shapley, still a few were supporting it.
Heber Curtis (1872-1942) at Lick Observatory, for instance, was searching for Novae in bright spirals. He found a few, which proven to be 100 times more distant than the Galactic Novae used for calibration.

F.G. Pease, Photographs of nebulae with the 60-inch reflector 1911-1916, Ap.J., 46, 24, 1917.

The Great Debate

At the end of the Great War the situation was so confused that on 26 April 1920 the National Academy of Sciences in Washington organized the so-called Great Debate between Shapley and Curtis.

Hubble discovers galaxies

Using the 100 inch Hooker reflector at Mt. Wilson, at the time the largest in the world [visit the sites of the major Observatories in the world and become acquainted with their instruments] Edwin Hubble (1889-1953) discovered several Cepheid variables in the Triangulum Nebula M33.
He used the period-luminosity relation [see lectures 12 on Distance scale - Part II] just found by Henrietta Leavitt (1868-1921) and derived a distance of 0.26 Mpc, which made the object comparable in size with the Milky Way and placed it well outside our stellar system.

Finding chart of M33 Cepheids used by Hubble (Ap.J., 63, 236, 1926) to derive the distance of the object.

Hubble discovers galaxies. discovery paper

Hubble did not dare to present himself this fundamental result, and asked Joel Stebbins (1868-1966), pioneer of the photoelectric photometry in astronomy, to read his note at the 1926 meeting of the American Astronomical Society. Since then, everybody knew that M33 was another island universe as the Galaxy was. [Read the discovery paper, "A spiral nebula as a stellar system. M33", Ap.J., 63, 236, 1926).] It is fair to say that four years before, Ernst Julius Öpik (1893-1985) from Tartu had already found a rather high value for the distance of M31 (450 kpc; Ap.J. 55, 406, 1922), enough to make it “a stellar universe comparable with our Galaxy”.

Light curves of two Chepeids in M33, from Hubble's original paper (Ap.J., 63, 236, 1926).

“The paper”

Abstract of Hubble's historical paper on galaxies, published in Ap.J., 64, 321, 1926.

1900: the extragalactic century

In 1610, using the first astronomical telescope, Galilei almost closed the ancient cosmological problem about the center of the planetary motions (he falsified a key Aristotelian postulate [which one?]), and at the same time opened the way to the sidereal astronomy. Three centuries later, using the world largest reflector, Hubble closed the “Great Debat”e in favor of Curtis’ view that white nebulae are stellar systems akin to our own and opened the way to the extragalactic astronomy.

In a few years Hubble himself provided a wealth of information about galaxies, from their morphological classification to a synthetic description of how their surface brightness varies with the distance from the center (the so called surface photometry). It is during these studies that he made the key discovery about the expansion of the universe: the so called Hubble law.
[Meditate on the role of the new instruments in astronomical discoveries.]

Modern observations

In the last few decades there has been an extraordinary progress in the number, quality, and performances of astronomical instrumentation both for observations and for data reduction, analysis, and archiving. Telescopes with apertures of 10 m and more, wide field (WF) digital cameras, multi-object spectrographs (MOS), large radio telescopes, space-based observatories, coupled with supercomputers for numerical simulations, sophisticated software (SW) ambient for data management, massive data bases on line, are some of the facilities which have deeply modified the strategy of today extragalactic astrophysics, now vaguely distinct from cosmology (galaxies are key probes for the cosmologist).

At the RHS a picture of the VLT Survey Telescope: a 2.6 m wide field (WF) imager (1 square degree field) devoted to optical surveys at the ESO Paranal Observatory in Chile: a joint venture of the Capodimonte Astronomical Observatory (OAC) of the Istituto Nazionale di Astrofisica (INAF) with the European Southern Observatory (ESO).

Credit: INAF-OAC and ESO.

Modern observations

Hubble observed a few tens of galaxies. Modern surveys, carried out by multipurpose or dedicated instruments (e.g. the VLT Survey Telescope, VST) deal with up to many millions of objects to great cosmic distances (and time) with a manifold of information (e.g., classification, multiband photometry and digital images, spectroscopy etc.). These data are normally released for public use – cf. the Hubble Space Telescope and the Sloan Digital Sky Survey (SDSS) archives — and available directly via Internet. The statistics is far richer than before, and we may now ask how galaxies evolve looking at their evolution with redshift. Giant ground-based telescopes and a wealth of new space observatories are presently under construction (e.g. the ESO Estremely Large Telescope (E-ELT) and the NASA James Webb Space Telescope, JWST), as well as a new generation of focal plane instruments.

Wide surveys: GALEX and SDSS

The student must familiarize with the contemporary prime surveys listed below with the corresponding Web links. From there he will find the inputs for older surveys as the Palomar Sky Survey (PSS) and the POS II, the Byurakan Schmidt Survey, APM, COSMOS, XMM-Newton and Chandra X-ray surveys etc.

The figure at the RHS reproduces a 2.5 degrees thick slice through the 3-D map of the distribution of galaxies made by the Sloan Digital Sky Survey (SDSS). Earth is at center. The radius of the outer circle is $2\times10^9 \ lyr$ . Each point is a galaxy, colored according to ages of its stars: the redder, more strongly clustered galaxies are made of older stars.

Credit: M. Blanton and SDSS.

Wide surveys: GALEX and SDSS

UV (135 to 280 nm) imaging: Galaxy Evolution Explorer = GALEX 0.5 m orbiting telescope, spanning 80% of cosmic evolution (history of star formation).
UV optical imaging and spectroscopy: Sloan Digital Sky Survey = SDSS. Resources from Alfred P. Sloan Foundation. The most ambitious and fertile astronomical survey ever made (SDSS-I, 2000-2005; SDSS-II, 2005-2008, SDSS-III in progress). Dedicated 2.5m telescope on Apache Point, New Mexico, with a 120-Mpx camera (1.5 deg² field) and a pair of 600 optical fiber spectrographs. Deep, multicolor (u, g, r, i, z) photometric and spectroscopic observations of 8,000 deg² within a volume containing ~10⁶ galaxies (median redshift z = 0.1) and 10⁵ quasars. SDSS site with a splendid collection of galaxy pictures in color.

Surveys: other windows

IR photometry: Two Microns All Sky Survey = 2MASS. Two twin 1.3 m telescopes in the two hemispheres (Arizona/Chile). Three-channel J (1.25 μm), H (1.65 μm), K (2.17 μm) IR camera. View of Milky Way nearly free of interstellar dust. The survey was intended to log over 10⁶ galaxies (position and total magnitude) including in the Zone of Avoidance.
Spectroscopic survey: Two degree Field = 2dF survey. 4 m Anglo-Australian Telescope (AAT) with a 2 deg² WF camera and a 400 optical fibers spectrograph. Covered area about 1500 deg² (North and South Galactic Poles); same as APM Galaxy Survey. Photometry and spectra for about 20,000 galaxies. Photometric catalog limit = 19.5 mag (redshift z < 0.3). See also: 6dFGRS.
Radio surveys: HI Parkes All Sky Survey (HIPASS) , and the Arecibo Legacy Fast ALFA Survey.

The distribution of 10⁶ brightest IR extended sources, mostly galaxies, detected by 2MASS (RHS) proves that galaxies are not distributed at random. Many galaxys are gravitationally bound in clusters, which in turn are loosely bound into superclusters. The vertical warped strip is the Milk Way.

Credit: 2MASS, T.H. Jarrett, J. Carpenter, R. Hurt.

Smaller surveys

Some narrower and/or more detailed survey programs are:

Galaxy kinematical mapping with the Spectroscopic Areal Unit for Research on Optical Nebulae = SAURON;
The Spitzer Infrared Nearby Galaxy Survey = SINGS;
The HI Nearby Galaxy Survey = THINGS;
Hubble Space Telescope (HST) large programs (e.g., Virgo Cluster survey).

At the RHS a combination of Spitzer IR (red), HST (yellow), and Chandra X-ray (blue + violet) images of the dusty Center of the Milky Way (size of a full moon) shows the complex morphology of our Galaxy.

Credit: NASA.

A few numbers and some global properties

A typical galaxy has an (effective [see later]) radius $R_e = 5 kpc$ and hosts $n_\star = 10^{11}$ stars in a spheroidal volume $V$ with axis ratio $b/a = 0.1\div1.0$ . Thus the separation between two stars is: $\Delta_\star=\left(V/n_\star \right)^{1/3}\simeq 1 pc.$ .

A typical stellar radius is $r_\star = R_\odot = 7\times 10^5 km \simeq 1/300 UA = 2\times10^{-8} pc \simeq 2\times10^{-8} \Delta_\star.$

If stars were sand grains ( 0.1 mm), a galaxy would be a shore where the sand grains are 5 km apart on average (at the galaxy center it is 10² times smaller).

The mean free path $1/n_\star\sigma = \Delta_\star^3/\pi r_\star^2 \simeq 10^{15} pc = 3\times10^{10} {\rm orbits} (1 {\rm ~orbit}=2\pi R_e),$ where σ is the geometrical cross section of a star.

At a velocity of 200 km/s, this implies a collision every $\sim 5\times 10^{18}\ yr$ , far more than a Hubble time.

At variance with a perfect gas, galaxies are likely collisionless systems. In any case, when galaxies collide, stars do not run one into the other. The rarefaction suggests that every star of a galaxy moves according to an overall smooth profile; the fine grain of the stellar distribution is generally ineffective. The very rare collisions allow the galaxy to keep memory of the past: e.g. cannibalized systems are metabolized very slowly, so we can see the persistence of plumes, shells, ripples, counter-rotating cores etc.

Galaxy interaction: “The Mice”

The galaxy NGC 4676 named "The Mice" shows the result of the interaction of two galaxies. Credit: NASA (HST).

A few numbers and some global properties

In a stellar system the number of mass points (stars) are conserved (few stars appear/disappear), as well as their total mass, angular momentum and kinetic energy (continuity and conservation equations).

The distribution is governed (if at steady state) by the function: $F\left({\bf r},{\bf V} \right) d\bf r{}d{\bf V},$ analogous to the Maxwell-Boltzmann distribution function, where $\bf V$ is the combination of a bulk flow (mean rotational velocity) of the elementary cell at $\bf r$ with a velocity dispersion $\sigma_V$ (analogous to the thermal motion). If $d\sigma_V/d{\bf r}\equiv 0$ anywhere, the system is said isothermal. Systems at equilibrium with negligible $\sigma_V$ (cold) are rotationally supported; their bulk velocity balances exactly the gravity and causes the system to flatten along the rotational axis.

A few numbers and some global properties

If rotation is negligible, $\sigma_V$ is maximum (no coordination of the individual orbital motions) and the galaxy is said to be pressure supported. However, unlike ordinary gas, a gas of stars may have an anisotropic and non-Gaussian velocity dispersion tensor. This has several consequences on the shape of the system (it may flatten objects with no net rotation or keep rotating systems spherical as it sets different scale-heights in the three coordinates).

Note that, being gravitationally bound, stellar systems have a negative specific heat: $dC/dT< 0,$ as they contact and heat up (stars move more quickly) as you remove energy from the system. Gas is dissipative: gas atoms collide frequently enough, so they quickly develop, on top of a bulk flow (mean shear), an isotropic Maxwell-Boltzmann velocity distribution obeying the overall potential (if equilibrium is reached).

Same basic questions about galaxies

Is morphology physically meaningful? [Is it always useful?]
What does it produce the variety of galaxy types?
What does is set the mass and luminosity functions?
What does it determine the values/ranges for the masses, dimensions, and luminosities of galaxies?
What does it rule the star formation?
What is the ratio of baryons over DM in the various morphological types and luminosity classes?
What is the set of the initial conditions (e.g. specific angular momentum) leading to the present structure of galaxies and
how much does the latter depend on:
- basic physical properties as the dynamical/chemical evolution,
- feedback processes (e.g. re-heating of the gas by an AGN phase),
- secondary evolution (interactions and merging)?
- environment, i.e. are galaxies different in different environments?

Quests and strategy

How can you investigate the structure and properties of galaxies? By looking at nearby objects which are better resolved.

How do investigate evolution? By comparing the properties of galaxies at various redshifts (ages?).

This requires large telescopes. A complementary approach is to trace the evolution by theoretical cosmological models, mostly numerical.

Reading material

Here we list the best books on the subject. Numerous and excellent reviews of specific topics are also available in the literature (often accessible via Web). In the Web there are some excellent courses on the physics of galaxies that you may conveniently consult.

“Galactic Dynamics” by James Binney & Scott Tremaine
“Galaxy Formation and Evolution” by Houjun Mo, Frank Van Den Bosch & Simon White
“Extragalactic Astronomy and Cosmology” by Peter Schneider
“Pdf Lectures” by Mark Whittle
“Dynamics of Collisionless System lectures” by Frank Van Den Bosch