Lecture Three:

Observed Properties of Galaxies, contd.

Longair, chapter 3

+ literature

Monday 18th Feb

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The Hertzsprung-Russell Diagram

LOW MASS STARS LIVE A VERY VERY LONG TIME!colour

magnitude

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The Hertzsprung-Russell Diagram

LOW MASS STARS LIVE A VERY VERY LONG TIME!colour

magnitude

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The Hertzsprung-Russell Diagram

LOW MASS STARS LIVE A VERY VERY LONG TIME!colour

magnitude

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The Hertzsprung-Russell Diagram

LOW MASS STARS LIVE A VERY VERY LONG TIME!colour

magnitude

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The Hertzsprung-Russell Diagram

LOW MASS STARS LIVE A VERY VERY LONG TIME!colour

magnitude

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What do colours mean?

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Spectrum of an Elliptical galaxy

U B V R I

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What does it mean?

Stellar spectra

>10Gyr

~8Gyr

~1.5Gyr

~5Myr

U B V R

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O’Connell 1986 PASP, 98, 163

Star Formation History

Elliptical Galaxy

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Hubble Sequence

Early type Late type

Fundamental difference between Elliptical galaxies and galaxies with disks, and variations of disk type & importance of bulges…

Hubble 1936, the Realm of Nebulae

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Environment

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Globular Clusters in Milky

~140 globular clusters, 65%

Globular Clusters & galaxy formation and evolution

metal rich(disk)

metal poor (halo)

milky way disk

Zinn 1985 ApJ, 293, 424

vrot = 43 +/- 29km/sσlos = 116 +/- 11km/s

vrot = 193 +/- 29km/sσlos = 59 +/- 14km/s

Armandroff 1989 AJ 97 375

flattened from rotation dominated by random motions

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Metallicity dispersion is large; mean metallicity decreases with increasing distance from galactic centre

metal rich

metal poor

Zinn 1985 ApJ, 293, 424

Globular Clusters & galaxy formation and evolution

scatter consistent with forming from large number of independent fragments

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Outer Halo: dSph

Mateo 2008, Garching workshop

kpc

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Formation of Halo?

Bullock & Johnston 2005 ApJ 635 931

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& Sagittarius Milky Way

Evidence for merging...

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The Local Grouphello

Mateo 2008, Garching workshop

kpcOuter regions: dominated by gas rich quiescently evolving dwarf irregulars

Near centres of mass: gas-less pressure supported dSphs

Anomalies: more distant dSph

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Nearest Cluster

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Local Super-Cluster

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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 50-100 luminous galaxies (L> L* ~ 2 x 1010 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-1Mpc.

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.

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Galaxies in “field” vs. “cluster”

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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.

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Virgo Cluster

closest rich cluster of galaxies, centred on giant elliptical galaxy M87

Velocity dispersion 715km/sVirial radius 730kpc~17Mpc distance

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Virgo Cluster

~17Mpc distance, ~2000 member galaxies

Binggeli, Sandage & Tammann 1985

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Global Properties

Bower et al. 1992

Brighter galaxies are redder

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).

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Virgo Cluster

VIVA: VLA Imaging of Virgo in Atomic gas28

ngc4522 in Virgo

Inter-cluster Medium

ngc4402 falling into centre of Virgo

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Virgo Cluster

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Ursa Major Group

Verheijen & Sancisi 2001

Velocity dispersion 148km/sVirial radius 880 kpc

Only late type galaxies with no particular concentration towards any centre

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Global Properties

M. Verheijen 1997

Galaxies get bluer and fainter

Ursa Major Group

On average S0 galaxies are luminous and redSd, Sm systems are fainter and bluer

Studying a group: all galaxies at same distance, so the brightest are the most luminous

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Global Properties

M. Verheijen 1997

Fainter galaxies have proportionately more HI

Ursa Major Group

Disk has lower central surface brightness

MH

I/LK

’ (s

olar

uni

ts)

Open circles, low SB galaxies (IK’(0) > 19.5), the least luminous and richest in HI; not efficient at turning HI into stars

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Ursa Major Group vs. Virgo

Verheijen 2004

HI properties of late-type galaxies

HI /

opt

ical

di

amet

erH

I def

icie

ncy

Distance to cluster centre (degrees)

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)

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Fraction of E & Sp

Oemler 1974

First paper to quantify this effect.

com

pact

ness

trend of size with type

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Morphology-Density RelationFirst 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.

Dressler 1980 Effect of sub-structure?

Fraction Sp/E goes up moving out from cluster centre

fraction of Sp goes down with size of cluster.

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 morphology-density relation over 6 orders of magnitude in density.

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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

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Simulating Interacting Systems

Toomre & Toomre 1972, ApJ

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

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Luminosity Functions

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Galaxy luminosity functionJust 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.

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Driver 2004 PASA, 21, 344

M* break luminosity

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Short comingsMalmquist 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).

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In an attempt to find a general analytic fit to galactic luminosity functions, Schechter (1976) ApJ 203, p297 proposed the functional form:

Luminosity Functions of galaxies

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 = -21Virgo: α= -1.24 and M*B = -21 ± 0.7

i.e., this is NOT a universal luminosity function. It seems to depend upon environment.

A power law with a high luminosity exponential cut-off

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Press-SchechterThus 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 187 425), 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

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Galaxy Counts

Number of galaxies Φ(M) per 10Mpc cube between absolute magnitude MR and MR + 1

Schechter function

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Galaxy luminosity function in the Virgo cluster (Sandage, Binggeli & Tammann 1985, AJ 90, 1759)

bright faint

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Galaxy cluster LFsDifferent 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 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

Jerjen & Tammann 1997

brightfaint

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see: Thomas: ESO Astrophysics Symposia (1999)

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Relative numbers of different typesB

inggeli, Sandage, Tam

mann A

RA

A (1988) AR

AA ,26, 509

The total luminosity function in either environment is the sum of the individual luminosity functions of each Hubble type.

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...

All dIs and dEs

LF of cluster & local field broken down into different types

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How to determine the mass of a galaxy?

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Masses of galaxies

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).

Spiral Galaxies: application of Gauss’ theorem to Newton’s law of gravity:

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.

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Rotation Curves of GalaxiesSurface 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.

Bosma 1981

When rotation curves are compared with either luminosity or Hubble type a number of correlations are found:• for increasing LB rotation curves tend to rise more rapidly with distance from centre and peak at higher maximum velocity (Vmax).• for equal LB spirals of earlier type have larger Vmax.• within a given Hubble type more luminous galaxies have larger Vmax. • for a given value of Vmax 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.

Vmax 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

Tully-Fisher

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Internal dynamics of EllipticalsSource 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.

from Davies et al. 1983

Ellipticals rotate too slowly for centrifugal forces to be the causes of their observed flattening.

Solid line: amount of rotation necessary to account for observed ellipticity of galaxy relative to σ of stars. 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

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Kroupa, Tout & Gilmore 1993 MNRAS, 262, 545

Converting luminosity to mass

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,

LF (luminosity function) currently observed number of stars observed per unit luminosity per unit volume

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 = Ψ0 m-α

IMF

PDMF

high mass, short lived stars

low mass, long lived stars

StarFormationHistory

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fin

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