Properties of Galaxies

1. Properties of Galaxies • Many subjects here are covered superficially and I can give references to review-type articles or even the original scientific papers if you want. There is no single reference book that covers everything in these lectures, mostly because research in this field is fairly rapidly moving (for example about a quarter of what is covered was discovered in the last few years). • An outline is given below. This is not entirely a useful thing, because everything is connected to everything else. The intention is to start from the beginning and end up so that you can understand the framework in which most current research into extragalactic astronomy is placed. • www.ast.cam.ac.uk/~trentham/course.ppt

2. 1. Definition of a galaxy in terms of its components. 2. Fluxes, luminosities, magnitudes, wavebands 3. Types of Galaxies - ellipticals, spirals, and others - their gross properties. 4. Nomenclature 5. Components of Galaxies 1. Stars - formation, HR diagram, evolution, remnants 6. Components of Galaxies 2. Active Galactic Nuclei - quasars, radio jets 7. Components of Galaxies 3. Gas - hot and cold 8. Components of Galaxies 4. Dark matter - properties, what it might be? 9. Local Galaxies 10. Distribution of galaxies in the Universe - environments, large-scale structure 11. Emission mechanisms and spectral energy distributions – bolometric luminosities 12. Luminosity functions 13. Light profiles and surface-brightnesses 14. Stellar and gas dynamics 15. Galaxy formation 1. Dark matter and cosmology 16. Galaxy formation 2. The high redshift Universe 17. Galaxy formation 3. The formation of stars in galaxies - the Madau Plot

3. 1 Definition of a galaxy • A galaxy is a self-gravitating collection of about 106 to 1011 stars, plus an amount up to 1/2 of as much by mass of gas, and about 10 times as much by mass of dark matter. The stars and gas are about 70% hydrogen by mass and 25% helium, the rest being heavier elements (called "metals"). • Typical scales are: masses between 106 to 1012 solar masses (1 solar mass is 2 x 1030 kg), and sizes 10 kpc (1 pc = 3.1 x 1016 m, 1 kpc = 1000 pc). Galaxies that rotate do so in about 10-100 Myr at about 100 km/s. The average separation of galaxies is about 1 Mpc. • Between galaxies there is very diffuse gas, called the intergalactic medium. It was much denser in the past before galaxies formed and took up all the gas and made it into stars.

4. The Milky Way is known in a fair amount of detail, and both the gas and stars split cleanly into different populations or phases. Stars: Disk: 5 1010 Msun Bulge: 1 1010 Msun Halo: 109 Msun Globulars: 108 Msun Gas: H2 clouds: 1 109 Msun HI gas: 4 109 Msun HII regions: 108 Msun Dark matter: Halo: 2 1012 Msun

5. Cosmic Inventory (Fukugita & Peebles 2004, Read & Trentham 2005) 73 % - dark energy or cosmological constant 23 % - dark matter, probably CDM 4 % - normal baryonic matter, about 10% of which is in galaxies (mostly in the form of stars). The rest is in the IGM.

6. 2 Fluxes, luminosities, magnitudes, wavebands • The total light from a galaxy comes out at all wavelengths of the electromagnetic spectrum. When we observe a galaxy, we usually consider the light from some waveband (crudely thought of as all the light between two wavelengths lmin and lmax ; in fact a more complex transmission function is usually required). For example the B band is a narrow band between about 4200 and 4600 Angstroms, the K band is a band between 2.1 and 2.3 microns. • mX = -2.5 log10 (FX / F0), where F0 is the flux measured in band X from the star Vega. The magnitude we measure from a galaxy is called its apparent magnitude. A more useful quantity for relating galaxies at different distances to each other is the absolute magnitude MX = mX - 5 log10 (distance/10 pc). Absolute magnitudes for galaxies are negative, the more negative the absolute magnitude, the brighter the galaxy. Typical values range from - 7 (the faintest galaxies known) to -26 (the brightest galaxies known) in the B band. • As far as I can tell the way the magnitude scale is defined is purely historical. It is not used at long (radio or far-infrared) wavelengths or at short (UV or X-ray or gamma ray) wavelengths, where fluxes are usually quoted instead. One other system in use is the AB magnitude system in which the zero-point is a flat spectrum (instead of the spectrum of Vega).

7. Here as some of the quantities and units that are currently in use. The way in which people quote results is often subjective. Note in that in the penultimate equation, the first transmission coefficient is a function of the physics of the Earth’s atmosphere, the second transmission coefficient is a function of the filter used, the luminosity distance is a function of the cosmology, and the attenuation coefficient is a function of the line of sight to the object in question.

9. Apparent magnitudes Values in the V-band (appropriate for the eye) are -27 for the Sun, -13 for the Moon, -4 for Venus, -1 for Sirius, +6 for the faintest stars visible with the naked eye, +4 for Andromeda (this is difficult to see, because it is spread out over a large area of the sky), +18 for the star-galaxy transition in the sky, and +30 for the faintest galaxies observed in the Hubble Deep Field. • Absolute magnitudes Values in the V-band are -20 for the Milky Way, +5 for normal solar-type stars, -9 for globular clusters, -28 for bright quasars. The brightest object ever observed at optical wavelengths, the prompt afterglow of GRB 990123, had an absolute magnitude that reached -36. • Colours These can be computed from either apparent or absolute magnitudes. Typical B-V values are 0.6 for the Sun, 0 for an A0 star, 1.0 for an elliptical galaxy and 0.5 for a spiral galaxy.

10. Fukugita et al. (1995) PASP 107, 945

11. 3 Types of galaxies • In most galaxy samples there are roughly equal numbers of elliptical, spiral, and peculiar (irregular) galaxies. • Elliptical galaxies come in two types - giant ellipticals, which have high brightnesses at their centres and absolute B magnitudes between about -25 and -15, and dwarf ellipticals, which have low brightnesses at their centres and absolute B magnitudes fainter than about -18. The faintest galaxies known are dwarf ellipticals. Elliptical galaxies are featureless, with brightness profiles that are high in the centre and lower far away from the centre. • Spiral galaxies like the Milky Way and Andromeda have absolute B magnitudes between about -24 and -18. They often look smooth near their centres (where the brightnesses are highest), and have spiral arms at large radius from the centre. The spiral arms are often irregular in form and one can see many condensations or knots, like HII regions which are making stars. • Irregular galaxies are irregular. They can be big systems of interacting galaxies, or (more commonly) small blue galaxies with absolute blue magnitudes > -18 and no regular morphology, usually just diffuse fuzz with a few condensations. The most famous examples are the Magellanic clouds which can be seen from the Southern hemisphere. • If a galaxy has a bright quasar at the centre, the quasar is usually so bright that you can't see the rest of the galaxy. Such objects therefore look pointlike (that is, like stars). • Finally, there do exist other, rarer, less classifiable galaxies, like giant low-surface-brightness galaxies that can barely be seen above the sky brightness.

12. Spiral Elliptical Dwarf irregular Dwarf elliptical

13. The Hubble sequence is normally used to classify giant galaxies and is illustrated in the tuning fork diagram. For ellipticals, the classification seems to depend an orientation. Conventionally, many astronomers use the classifications Sd and Sm to describe galaxies intermediate between Sc and irregular galaxies.

14. Many observational parameters correlate with Hubble type. For example: • Bulge-to-disk ratio decreases towards later types • Spiral arm pitch angle increases towards later types • Star formation rate per unit mass increases towards later types • HI mass fraction increases towards later types • Colour gets bluer towards later types • Total mass is approximately constant from S0 to Sc, then decreases towards later types • Dark matter mass fraction increases towards later types • Optical stellar mass-to-light ratio decreases towards later types • Ratio of molecular to atomic gas mass decreases towards later types

15. X= Mrk 1460

16. 4 Nomenclature This is mostly historical, with apparently similar galaxies having very different-sounding names. Names are usually taken from the following catalogs (if a galaxy appears in more than one catalog, the name from the first-listed is usually adopted). • Messier or M (bright local objects, many are star clusters) • NGC (New Galaxy Catalog, about 8000 objects, many are star clusters) • Zwicky catalogs (odd objects) • Arp catalog (peculiar interacting systems) • Markarian catalog (UV-bright systems) • IC (Index catalog) • UGC (Uppsala General Catalog) • IRAS (infrared-loud, discovered by the IRAS satellite) Most contemporary surveys like 2MASS and SDSS give names with the coordinates implicit. Failing all above, a galaxy can be named by its coordinates, which tell its position on the sky. For example 01305+3305 would be an object at a right ascension of 01:30.5 and a declination of +33:05.

17. 5. Components of galaxies 1. Stars • Stars like the Sun are massive spherical accumulations of gas that are undergoing nuclear fusion and releasing energy in the form of (mostly visible) electromagnetic waves. They have masses typically 0.1 to 100 times the mass of the sun, and have a blackbody spectrum that peaks at longer wavelengths for lower mass stars (it peaks at about 500 nm for the Sun). • The evolution of stars is depicted in the Hertzprung-Russell (HR) diagram (alternatively called the color-magnitude diagram), which is historically one of the most profound matches between theory and observation in astrophysics. Different mass stars follow different tracks on the diagram. • Most gross photometric properties of galaxies can be understood in terms of this diagram, remembering the fact that more massive stars evolve (that is, move along their tracks in the diagram) much faster than less massive ones. For example, a 10 solar mass star completes its track in about 0.01 billion years, a 1 solar mass star in about 10 billion years.

18. BRIGHT FAINT RED BLUE Gas cloud – gravitational contraction - pre-main sequence – nuclear burning initiated - main sequence – shell burning - subgiant – photon streaming limit - red giant branch ascent – core helium ignition – tip of the red giant branch – vigorous helium core burning + weakened hydrogen shell burning – horizontal branch – CO core and double shell burning – asymptotic giant branch – helium shell exhaustion – supergiant – mass loss to become a planetary nebula and then CO white dwarf OR core collapse supernova to become neutron star or black hole

19. Young stars form in clouds of gas and tend to be enshrouded in these clouds which contain lots of dust. Therefore we don't see these (very luminous) OB stars directly, rather we infer their presence from the thermal (infrared or submillimeter) radiation of dust which has absorbed the short-wavelength radiation from the young stars and reradiates it as a cold blackbody. • The nuclear fusion in stars is responsible for making all the elements heavier than helium that are seen in the Universe. • Remnants are white dwarfs, neutron stars, or black holes. These are condensed matter, the last two are optically invisible and are remnants of the most massive stars. Neutron stars are however visible as radio pulsars (and maybe some gamma-ray bursts). The long-duration gamma ray bursts may be linked to black holes in formation.

20. The color-magnitude for star cluster is that for a single age population. Here is the color-magnitude diagram of an old globular cluster. Note: • There are no blue stars on the main sequence. • The turn-off is well defined, and a function of age (redder is older) • White dwarfs will be below the diagram to the left • The two or three stars on the blue side of the turn-off are blue stragglers. • That the morphology of the horizontal branch. In general, older and more metal-poor stars are bluer here.

21. Stellar population synthesis modeling of galaxies is used to predict photometric properties of galaxies by combining the stellar evolution models for different age populations with a stellar IMF. • Elliptical galaxies contain old populations of stars. All the massive stars in these galaxies have completed their evolution and are remnants. The most luminous stars in elliptical galaxies are red giants -- this is why elliptical galaxies look red through a telescope. This is also true for the central parts of spiral galaxies (the bulges). • Spiral galaxies (particularly the outer parts, also irregular galaxies) contain young populations of stars. They have massive stars that haven't finished their evolution. These are the most luminous stars in the galaxies. Recall from the HR diagram that these are blue. This is why spiral galaxies look blue through a telescope.

22. 6 Components of galaxies 2. Active galactic nuclei • Nucleosynthesis in stars is not the only way that one can convert gas into other material and release electromagnetic radiation along the way. Accretion onto a black hole in the centres of galaxies (the Active Galactic Nuclei, or AGN phenomenon) is another way and is about 400 times more efficient. This phenomenon is however quite rare since one requires lots of gas in a small region, and this usually only happens in the centres of galaxies. • In its most extreme phenomenon, AGNs can be QSOs or quasars, which are more luminous than the most luminous galaxies (up to absolute blue magnitudes of - 28). These are probably quite shortlived, as they require enormous gas supplies. More common weaker AGNs like Seyfert galaxies, etc. are also found. Many of the most luminous AGNs are dust-enshrouded, particularly when they are very young and have just formed. • AGNs have non-thermal spectra and so emit quite a lot of energy outside visible wavebands. They are much brighter than stars at X-ray and radio wavelengths, for a given optical luminosity. • AGNs vary considerably in their radio properties. Optically luminous QSOs can be either radio-loud quasars, or radio quiet. Quasars may be radio galaxies that happened to be observed down the jet.

23. The entire theoretical framework on which AGNs are based is somewhat less secure than for stars. This is in part due to the extreme conditions near these very massive black holes of 106 to 109 solar masses, which is quite unlike anything we can test in laboratories on Earth (we are in the strong field limit of general relativity). On the other hand, atomic spectroscopy and nuclear physics (the physics on which stellar astrophysics is based) is much more thoroughly tested. • The black-hole models for AGNs predict that when the AGN has run out of gaseous fuel, one should still find very massive (now dark) objects in the centres of nearby galaxies. These can be found by virtue of their dynamical effects on stars in the centres of galaxies. Recent observations strongly suggest that these massive dark objects do in fact exist. Interestingly, there is a very tight relationship between the mass of the black hole and the mass (or velocity dispersion) of the stars in the spheroid population of the host galaxy: MSMBH ~ 0.0015 M* (spheroid) This ties together black-hole formation and star formation together in some (presumably complex) way.

25. 7. Components of galaxies 3. Gas • The space between the stars is occupied by gas that is not very dense (about 1 atom per cc, a vacuum more perfect than we can get on Earth in laboratories). However, the galaxies are so big that the total mass in this gas is an appreciable (though usually less than a percent) fraction of the mass of the galaxy. • The gas is about 70% H and 25% He by mass. Other elements are present in trace amounts (mostly CNO), and have been made in stars. These are of astronomical interest as they offer useful probes of conditions, such as the integrated chemical evolution history of the galaxies. • Gas may be ionized (hot), atomic (cold), or molecular (very cold, less than 50 K). In elliptical galaxies most gas is hot. In spiral in irregular galaxies, most gas is cold, 50% or less of which is molecular. The molecular gas is very clumpy, the others are more evenly spread. • It is in the molecular gas clumps that stars form. Often associated with these cold molecular gas clumps are substantial amounts of dust (about 1% of the cloud mass). It is this dust that obscures the light from young stars. • The gas in galaxies, called the interstellar medium, is much denser than the gas between galaxies, called the intergalactic medium (1 atom/cc in the interstellar medium compared to 10-8 atom/cc in the intergalactic medium).

26. 8. Components of galaxies 4. Dark matter • This occupies about 90% (at least) of the mass of galaxies. It is distributed in a smooth halo that envelopes the stars and gas. • Spatially the dark matter halo occupies the same region as a number of small (< 106 Msun) dense old star clusters, the globular clusters. There about 150 globular clusters in the Milky Way, more in bigger galaxies. There are also about 109 Msun of individual stars in the Milky Way field halo.

27. The existence of dark halos around galaxies comes from dynamical measurements, X-ray profiles of elliptical galaxies and measurements of gravitational lensing by individual galaxies. • The existence of dark matter in galaxy clusters, comes from X-ray, velocity dispersion and gravitational lensing (both weak and strong) measurements. • The existence of dark matter in a cosmological context comes from the consideration of a large number of datasets in conjunction with each other.for example, information comes from measurements of cosmic shear and the Lyman forest. • Cosmological simulations, like the Millennium simulation, show that the three are the same material. We require a universe with about 30% of the critical density in normal matter, of which about 15% (about 4% of the total) is in baryons. By “matter”, we mean material that obeys a pressureless equation of state. This is what distinguishes it from dark energy, which makes up the other 70% or so. This dark matter must be cold in order to form galaxies, since hot dark matter (like neutrinos) will free-stream out of perturbations in the cosmological fluid at early times. By “cold”, we mean that the material has low thermal velocity. That all observations point towards one consistent picture is another example of a successful match between theory and observation in astrophysics.

28. HI and Ha rotation curves are both flat for many spiral galaxies. These galaxies are all at about 5000 km/s, where 1 arcmin corresponds to about 30 kpc.

30. Science (2003) 301, 1696

32. ACS image of a Abell 1689 (z=0.18) from Broadhurst et al. 2005 ApJ, 621, 53 The ACS is the optical camera on board the HST. It has a field of view 202 X 202 arcsec. At z=0.18, 1 kpc = 0.3 arcseconds, so the cluster core radius 300 kpc = 1.5 arcminutes

33. This is one of large number of clusters for which measurements like this have been made. Clusters like Abell 1689 and Abell 2218 are particularly good, because they had gravitational arcs near the center. So the results can be calibrated by strong gravitational lensing (the green points in the figure). The dark matter often has structure, sometimes with lumps of dark matter that are quite massive but have no optical galaxies (for example Abell 1942; Erben et al. 2000, A&A, 355, 23)

34. Dark matter masses via X-ray halos This equation can be applied to either galaxies or clusters but T(r) is more straightforward to measure in clusters. The main complication is lack of spherical symmetry. For a review of the applications of these techniques to clusters, see the earlier paper by Buote.

35. Title: The ESO Nearby Abell Cluster Survey. XII. The mass and mass-to-light-ratio profiles of rich clustersAuthors: Peter Katgert (Sterrewacht Leiden), Andrea Biviano (INAF/OAT, Trieste), Alain Mazure (OAMP/LAM, Marseille)Journal-ref: Astrophys.J. 600 (2004) 657-669

36. If perturbations in the microwave background arise due to baryons falling within dark matter fluctuations, then galaxies (and clusters) will grow out of the perturbations with the observed clustering properties. They will then have dark matter halos around them. This requires the dark matter to be cold, meaning that it does not free-stream of fluctuations. This is a fairly robust prediction of simulations. Since dark matter particles interact only by gravitational forces, we can have some confidence in the result. This phenomenon is implicit in the results of most simulation papers. For example, Springel et al., Nature, 2005, 435, 629. An early reference for clusters is Croft & Efstathiou, 1994, NN RAS, 267, 390

37. Read & Trentham (2005) CDM theory also predicts a galaxy dark matter (halo) mass function for galaxies that is steeper than the baryonic mass function at both the bright and faint ends. The former is probably due to AGN feedback preventing arbitrarily large numbers of atoms from falling into galaxies at late times. The latter is probably due to feedback from supernovae in small galaxies and maybe reionization.

38. Most simulations show that the dark matter obeys an NFW profile down to the resolution limit………….. Trentham et al. 2001, MNRAS, 322, 658

39. …………….but there is increasing evidence for deviations on small scales, particularly near galaxy centres. A core has a=0 and a cusp has a<0. Astrophys.J. 583 (2003) 732-751

40. The dwarf spheroidal galaxy Ursa Minor, which is composed almost entirely of dark matter, also shows evidence for substructure. Astrophys.J. 588 (2003) L21-L24

41. Lower luminosity galaxies are increasingly dark-matter dominated. astro-ph/0407321

42. Much about the global properties of dark matter is known, specifically that it is cold and that of the four fundamental physics forces it only responds to gravity (not electroweak for example, as it emits no radiation). • Nothing about the detailed nature of dark matter (like what particle it is) is known. Figuring it out is one major current area of study in astrophysics. It is probably NOT baryonic (big-bang nucleosynthesis + dynamical constraints), neutrinos (phase-space constraints) or black-holes that result from stellar evolution (chemical constraints). • Possibilities include primordial black-holes, axions (QCD predicts a <10^-5eV cold non-interacting particle), or some supersymmetric entity (beyond the standard theory of particle physics or even GUTs), or something else. If it is either the first or second, there are hopes of progress as far as figuring it out. If not, it will be difficult, at least for the foreseeable future, despite the fact that there are about 70 orders of magnitude difference in mass between the candidates. • Dark matter is important in understanding galaxy formation since it is dark matter perturbations in the early universe that grow gravitationally and drag the baryons with them (meaning the gas that later forms stars). So it's worth repeating: its global properties are well-studied without knowing what it is!

43. 9. Local galaxies • The Milky Way is one of two big galaxies in the Local Group, which is a region of the local Universe about 1 Mpc across. Andromeda (M31) is the other and is about twice as big. On the blue absolute magnitude scale, these are at about -20. • The two big galaxies are separated by 0.7 Mpc. Both have a number of smaller satellites. The Milky Way has two well-known ones, the Magellanic Clouds. The largest satellite of M31 is M33, a small spiral galaxy. There are a few other small galaxies between and around the two large galaxies. • M31, and the Magellanic Clouds can be seen with the naked eye. The LMC and SMC are obvious, but we are too far north to see them here. You can see M31 at the moment at the beginning of the night, but it is quite faint (it is about as big as the moon).

47. Beyond the Local Group: Over the past few years, Karachentsev and collaborators have been compiling a list of all galaxies within 10 Mpc, the Local Volume sample. This includes all galaxies in the nearby Sculptor and M81 Groups. The program is essentially a two-stage project. Firstly candidate galaxies need to be identified, usually from photographic all-sky surveys. Secondly, distances to the galaxies need to be established; any one of a variety of methods may be used, the best being the TRGB method. The first results from this program were published in 2004.

48. 10. Distribution of galaxies Galaxies tend to cluster. This means that the probability of finding a galaxy at some point P is highest when P is known to be close to another galaxy. We saw a hint of this in the Local Group because there are 2 big galaxies, not one. On much larger scales, the clustering is more obvious. Most big galaxies exist in small groups of 2, 3, or 4 like the Local Group (these are called groups). Garcia 1993, A&AS, 100, 47 This kind of study can be performed in much more detail with the new redshift surveys. An example this kind of study is Eke et al., 2004, MNRAS, 348, 866, which uses the 2dfGRS.

49. Some big galaxies exist in much bigger aggregations called galaxy clusters. In a galaxy cluster there are hundreds or even thousands of galaxies within a few megaparsecs radius. Group tend to cluster too. So do galaxy clusters. The result is large-scale structure. AJ 114, 2205 Large-scale structure can be mathematically quantified by the power spectrum of fluctuations. Redshifts surveys provide three-dimensional positions. Two recent papers are: Tegmark et al. 2004, ApJ, 606, 702 (SDSS) and Percival et al. 2004, MNRAS, 353, 1201 (2dF)

50. Large-scale structure is commonly seen in simulations of galaxy formation like the Millennium Simulation. This is the natural result of the dark matter fluctuations that we see imprinted in the microwave background. The dark matter grows by gravity and each dark matter condensation is accompanied by baryons, which may turn into a galaxy according to some prescription. The natural result is a galaxy distribution with considerable large-scale structure. The picture about is about 100 Mpc on the side and looks rather like the picture in the previous viewgraph.