Lecture Three: Observed Properties of Galaxies, contd. Longair, chapter 3 + literature. Monday 18th Feb
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1 Lecture Three: Observed Properties of Galaxies, contd. Longair, chapter 3 + literature Monday 18th Feb 1
2 The Hertzsprung-Russell Diagram magnitude colour LOW MASS STARS LIVE A VERY VERY LONG TIME! 2
3 The Hertzsprung-Russell Diagram magnitude colour LOW MASS STARS LIVE A VERY VERY LONG TIME! 3
4 The Hertzsprung-Russell Diagram magnitude colour LOW MASS STARS LIVE A VERY VERY LONG TIME! 4
5 The Hertzsprung-Russell Diagram magnitude colour LOW MASS STARS LIVE A VERY VERY LONG TIME! 5
6 The Hertzsprung-Russell Diagram magnitude colour LOW MASS STARS LIVE A VERY VERY LONG TIME! 6
7 What do colours mean? 7
8 Spectrum of an Elliptical galaxy U B V R I 8
9 What does it mean? U B V R >10Gyr ~8Gyr ~1.5Gyr ~5Myr Stellar spectra 9
10 Star Formation History Elliptical Galaxy O Connell 1986 PASP, 98,
11 Hubble Sequence Fundamental difference between Elliptical galaxies and galaxies with disks, and variations of disk type & importance of bulges Early type Late type Hubble 1936, the Realm of Nebulae 11
12 Environment 12
13 Globular Clusters in Milky Mateo 2008, Garching workshop kpc ~140 globular clusters, 65% <8kpc from centre 13
14 Globular Clusters & galaxy formation and evolution v rot = 193 +/- 29km/s σ los = 59 +/- 14km/s metal rich (disk) Armandroff 1989 AJ v rot = 43 +/- 29km/s σ los = 116 +/- 11km/s metal poor (halo) milky way disk flattened from rotation dominated by random motions Zinn 1985 ApJ, 293,
15 Globular Clusters & galaxy formation and evolution Metallicity dispersion is large; mean metallicity decreases with increasing distance from galactic centre metal rich metal poor scatter consistent with forming from large number of independent fragments Zinn 1985 ApJ, 293,
16 Outer Halo: dsph kpc Mateo 2008, Garching workshop 16
17 Formation of Halo? Bullock & Johnston 2005 ApJ
18 Milky Way & Sagittarius Evidence for merging... 18
19 The Local Group hello Near centres of mass: gas-less pressure supported dsphs Anomalies: more distant dsph Outer regions: dominated by gas rich quiescently evolving dwarf irregulars kpc Mateo 2008, Garching workshop 19
20 Nearest Cluster 20
21 Local Super-Cluster 21
22 What is a galaxy cluster? Half the galaxies in the Universe are found in clusters or groups, systems of galaxies that are a few Mpc across. Within the central Mpc, clusters typically contain luminous galaxies (L> L * ~ 2 x L ). Most famous catalogues: Abell 1958 and it s 1989 supplement, with 4073 rich clusters, having at least 30 giant members within a radius of ~1.5h -1 Mpc. Galaxies in clusters are bound together by their mutual gravitational attraction: the cluster is generally filled with hot interstellar gas, also retained by gravity. Clusters differ from groups by having higher densities. Cluster galaxies live in such proximity that they significantly affect each others development. 22
23 Galaxies in field vs. cluster 23
24 What causes diversity of galaxy types? There are a number of ways of reducing the variables in a study of galaxy properties and one is to look at a group or cluster of galaxies. You remove uncertainties due to different distances of your sample of galaxies as well as different environments. Your sample completeness is easily defined. HOWEVER it is not clear that you obtain a complete sample of all types of galaxies, and it may not even be a good average sample. 24
25 Virgo Cluster Velocity dispersion 715km/s Virial radius 730kpc ~17Mpc distance closest rich cluster of galaxies, centred on giant elliptical galaxy M87 25
26 Virgo Cluster Binggeli, Sandage & Tammann 1985 ~17Mpc distance, ~2000 member galaxies 26
27 Brighter galaxies are redder Global Properties Elliptical galaxies in Virgo (open symbols) & Coma (closed symbols) Coma galaxies are shown 3.6 mag brighter as they would be at distance of Virgo This trend could be explained if small elliptical galaxies were either younger or more metal poor than large bright ones (or both). Bower et al
28 Virgo Cluster VIVA: VLA Imaging of Virgo in Atomic gas 28
29 Inter-cluster Medium ngc4402 falling into centre of Virgo ngc4522 in Virgo 29
30 Virgo Cluster 30
31 Velocity dispersion 148km/s Virial radius 880 kpc Ursa Major Group Only late type galaxies with no particular concentration towards any centre Verheijen & Sancisi
32 Galaxies get bluer and fainter Global Properties On average S0 galaxies are luminous and red Sd, Sm systems are fainter and bluer Studying a group: all galaxies at same distance, so the brightest are the most luminous Ursa Major Group M. Verheijen
33 Fainter galaxies have proportionately more HI Global Properties M HI /L K (solar units) Open circles, low SB galaxies (I K (0) > 19.5), the least luminous and richest in HI; not efficient at turning HI into stars Disk has lower central surface brightness Ursa Major Group M. Verheijen
34 Ursa Major Group vs. Virgo HI properties of late-type galaxies HI / optical diameter HI deficiency HI content of galaxies in centre of Virgo LESS than in the outskirts or in lower density systems (Ursa Major) Gas disks are SMALLER in centre of Virgo than in the outskirts or in lower density systems (Ursa Major) Distance to cluster centre (degrees) Verheijen
35 Fraction of E & Sp trend of size with type compactness First paper to quantify this effect. Oemler
36 Morphology-Density Relation First large (55 clusters, 6000 galaxies) study of morphological segregation (Dressler 1980). The frequency of different galaxy types was found to vary as a function of the number density of galaxies in which they are found. Is this related to R? Or N? Difficult to ascertain: N R -1. galaxy type appears to be dictated by LOCAL DENSITY of galaxies, although presumably galaxies move through a range of densities, thus there must be coherent sub-structure. study of poor groups (Postman & Geller 1984) - the centres of which have similar densities to outer regions of clusters follow same relations as clusters. galaxies with a nearby companion are more likely to be Es (Whitmore, Gilmore & Jones 1993), so morphology-density a local phenomenon. there is a single universal morphologydensity relation over 6 orders of magnitude in density. Fraction Sp/E goes up moving out from cluster centre Dressler 1980 fraction of Sp goes down with size of cluster. Effect of sub-structure? 36
37 What causes diversity? Galaxies in clusters more likely to be Es or S0s than those in the field Environment plays a role Not all clusters are the same - large E fraction correlates to regular symmetric clusters; low values to ratty ones Oemler (1974) Also E/Sp varies with position in a cluster -> depends on density. Fraction of spirals increases out from centre; essentially no spirals in cluster cores MORPHOLOGY-RADIUS RELATION Spirals closer to the centre have less gas than those further away Why? Spatial segregation should give rise to kinematic differences - ie., spirals follow more energetic orbits - ie., spirals at a given distance from centre of cluster should have larger random velocities than E 37
38 Simulating Interacting Systems Lack of spirals compared to ellipticals in dense environments has lead people to consider that merging spirals result in an elliptical galaxy... Josh Barnes 1998 Toomre & Toomre 1972, ApJ 38
39 Luminosity Functions 39
40 Galaxy luminosity function Just as the distribution of stellar luminosities reflects the physics of star formation and stellar structure, we might hope to learn about galactic evolutionary processes by studying the distribution of galaxy luminosities. The galaxy luminosity fn. Φ(M), Φ(M)dM is proportional to the number of galaxies that have absolute magnitudes in the range (M, M+dM): Where ν is the total number of galaxies per unit volume The field galaxy luminosity function, in its simplest form, involves measuring the apparent magnitudes of all the galaxies in some representative sample. The individual brightnesses are converted to absolute magnitudes by estimating the galaxies distances usually by applying the Hubble law to their observed redshifts. 40
41 M* break luminosity Driver 2004 PASA, 21,
42 Short comings Malmquist bias - magnitude limited surveys - luminosity function distorted if function has a finite spread in luminosity. Even if all galaxies have intrinsically identical luminosities, but a range of estimated absolute magnitudes due to errors in their adopted distances. Estimating distances using Hubble law intrinsically approximate process. Particular problem for nearby galaxies - local motions dominate over Hubble flow. Particular problem for low luminosity galaxies - which can only be observed nearby. So faint end of luminosity fns remains rather poorly defined. Spatial structure - incomplete sampling of variations in galaxy distribution (filaments vs. Voids). 42
43 Luminosity Functions of galaxies In an attempt to find a general analytic fit to galactic luminosity functions, Schechter (1976) ApJ 203, p297 proposed the functional form: Which can also be written (in terms of magnitudes): In both forms α (the slope of the power-law at low luminosities) and L * (the break luminosity) are free parameters that are used to obtain the best fit to the available data. Local: α= -1.0 and M * B = -21 Virgo: α= and M * B = -21 ± 0.7 A power law with a high luminosity exponential cut-off i.e., this is NOT a universal luminosity function. It seems to depend upon environment. 43
44 44
45 Press-Schechter Thus α sets the slope of the luminosity fn at the faint end L * or M * gives the characteristic luminosity above which the number of galaxies falls sharply and Φ * sets the overall normalisation of the galaxy density. This formula was initially motivated by a simple model of galaxy formation (Press & Schechter 1974 ApJ ), but has proved to have a wider range of application than originally envisaged. Integration over previous eqn has limitation that it effectively predicts infinite number of small faint galaxies (alpha lies close to ~-1) However we know universe is finite (dark sky) When we can detect low lum galaxies, they exist in large numbers 45
46 Galaxy Counts Schechter function Number of galaxies Φ(M) per 10Mpc cube between absolute magnitude M R and M R
47 bright faint Galaxy luminosity function in the Virgo cluster (Sandage, Binggeli & Tammann 1985, AJ 90, 1759) 47
48 Galaxy cluster LFs Different environment. Easier to obtain LF members lie in small region of sky. So photometry can be obtained efficiently, and all members at same distance. Reducing distance errors. Only problem is rich clusters are rare. So typically at large distances. Making it hard to detect fainter members. faint bright Jerjen & Tammann 1997 Faint end slope in cluster significantly steeper than in the field: encounters don t end in mergers as often as in field because relative velocities are higher in clusters 48
49 see: Thomas: ESO Astrophysics Symposia (1999) 49
50 Relative numbers of different types LF of cluster & local field broken down into different types Largest fraction in either environment of all galaxies are dwarfs (de and Irr). Even though Sp and E the most prominent in terms of mass and luminosity. More E in Virgo... Binggeli, Sandage, Tammann ARAA (1988) ARAA,26, 509 The total luminosity function in either environment is the sum of the individual luminosity functions of each Hubble type. All dis and des 50
51 How to determine the mass of a galaxy? 51
52 Masses of galaxies Spiral Galaxies: application of Gauss theorem to Newton s law of gravity: Rotation curves allow mass determination. The constant rotational velocities in the outer regions - suggests that mass increases linearly with distance from the centre. In stark contrast to the light distribution, which decreases exponentially over the same distance. Meaning a rapidly increasing mass-to-light ration (M/L) and a hidden dark matter halo in spiral galaxies (Bosma 1981). Elliptical Galaxies: application of virial theorem, assuming isotropic stellar distribution This kind of analysis has led to the prediction of large dark matter haloes around elliptical galaxies (e.g., Côte et al. 2001, 2003), using globular clusters as tracers. The line-of-sight velocity dispersion remains remarkably constant out to the limits of observation. This has the same explanation as flat rotation curves in HI. To bind globular clusters with large velocity dispersions at large radii means that the mass within R must increase proportional to R. 52
53 Rotation Curves of Galaxies Surface Brightness profiles sample the distribution of luminous matter in a galaxy. This does not necessarily tell us about the mass of the galaxy - about the presence and amount of DARK MATTER. The most direct way to do this is via the rotation curve of the HI. When rotation curves are compared with either luminosity or Hubble type a number of correlations are found: for increasing L B rotation curves tend to rise more rapidly with distance from centre and peak at higher maximum velocity (V max ). Tully-Fisher for equal L B spirals of earlier type have larger V max. within a given Hubble type more luminous galaxies have larger V max. for a given value of V max the rotation curves tend to rise slightly more rapidly with radius for earlier type galaxies. The fact that galaxies of different Hubble types, and therefore different bulge-to-disk luminosity ratios, exhibit rotation curves that are very similar in form if not in amplitude suggests that the shapes of the gravitational potential do not necessarily follow the distribution of luminous matter. V max is significantly lower in Irrs (50-70km/s). This suggests that this is the minimum rotation speed required for the development of a well ordered spiral pattern Bosma
54 Internal dynamics of Ellipticals Source of galaxies shape? It might be thought that the internal dynamics of elliptical galaxies would be relatively simple - the surface brightness distributions appear to be ellipsoidal, with a range of flattenings, which it might be thought could be attributed to rotation. This can be tested by measuring the mean velocities and velocity dispersions of the stars through out the body of a galaxy. These measurements can be compared with the rotation and internal velocity dispersions expected if the flattening can be attributed to rotation. Solid line: amount of rotation necessary to account for observed ellipticity of galaxy relative to σ of stars. Ellipticals rotate too slowly for centrifugal forces to be the causes of their observed flattening. This means that the assumptions of asymmetric spatial distribution and/or an isotropic velocity distribution of stars at all points within galaxy must be wrong. TRIAXIAL SYSTEMS this means a system with 3 unequal axes and consequently anisotropic stellar velocity distributions from Davies et al
55 Converting luminosity to mass IMF (initial mass function) Ψ(m, t), number of stars formed per unit volume at t=0 often approximated as a power law: Ψ(m) dm = Ψ 0 m -α LF (luminosity function) currently observed number of stars observed per unit luminosity per unit volume PDMF (present day mass function) number of stars observed today per unit mass per unit volume. This needs to be corrected for the time evolution of the IMF up to the present day, low mass, long lived stars IMF PDMF Kroupa, Tout & Gilmore 1993 MNRAS, 262, 545 high mass, short lived stars Star Formation History 55
56 fin 56
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