Galaxies

Chapter 20

Galaxy Classification

In 1924, Edwin Hubble divided galaxies into different

“classes” based on their appearance.

Why begin here?

•Hubble classification serves as the basic language of the field.

•The morphological sequence reflects a fundamental physical and, in some ways, evolutionary sequence, which offers important clues to galactic structure, formation and evolution.

Hubble Tuning Fork diagram (Hubble 1936)

EllipticalsLenticular (S0)Spiral and Barred SpiralIrregular

Spiral Galaxies

•Disk + spiral arms + bulge (usually)•Subtype a b c defined by 3 criteria:

•Bulge/disk luminosity ratio•Sa: B/D>1 Sc: B/D<0.2

•Spiral pitch angle•Sa: tightly wound arms Sc: loosely wound arms

•Degree of resolution into knots, HII regions, etc.

Barred Spiral Galaxies

•Contain a linear feature of nearly uniform brightness centered on nucleus

•Subclasses follow those of spirals with subtypes a b and c

MW may be SBb, depending on prominence of the bar.

Spiral Galaxies

Comprise about 2/3rds of bright galaxiesGrand Design Spiral - well defined spiral structureFlocculent - less organized spiral design

Spirals clearly contain much gas and dust Most starlight is from young, blue stars - ongoing star formation

Sizes - radius = 10 to 30 kpcMasses - M = 107 to 1011 Msun

Milky Way and Andromeda are both bright, spiralsMV ~ -21 or LV ~ 2 x 1010 LV,sun

Elliptical Galaxies

•Smooth structure and symmetric, elliptical contours•Subtype E0 - E7 defined by flattening

•En where n = 10(a-b)/aa and b are the projected major and minor axes (not necessarily a good indicator of the true 3-D shape)

Types of Elliptical Galaxies

Giant Ellipticals – few 100kpc across, 1 trillion starsMV ~ -23 or LV ~ 1011 LV,sun

Dwarf Ellipticals (dE) - as small as 1 kpc across, 1 million starsMV ~ -18 or LV ~ 109 LV,sunEllipticals:• contain little or no gas (only ~1% of mass)• no current star formation• mostly older, redder stars - like those in the halo of our galaxy• Unlike halo stars, metal abundances of these stars are high (about twice solar metallicity)

NGC 205

Leo I dSph

Dwarf Spheroidal - Classification dSphVery low stellar density and surface brightnessMV ~ -14 or LV ~ 3 x 107 LV,sun

S0 Galaxies (Lenticulars)

•Smooth, central brightness concentration (bulge similar to E) surrounded by a large region of less steeply declining brightness (similar to a disk)

•No spiral arm structure but some contain dust and gas

•Originally thought to be transition objects between Sa and E but typical S0 is 1-2 mags fainter than typical Sa, E (van den Bergh 1998)

Irregular Galaxies

•Little morphological symmetry

•Lots of young, blue stars and interstellar material

•Smaller than most spirals and elliptical galaxies

•Two major subtypes:•Irr I: spiral-like but without defined arms, show bright knots with O,B stars•Irr II: contain many dust lanes and gas filaments (e.g. M82) - explosive

M82-Irr IINGC 4485-Irr II LMC - Irr I

Limitations of the Hubble Classification Scheme

1. Only includes massive galaxies (doesn’t include dwarf spheroidals, dwarf irregulars, blue compact dwarfs)

2. Three different parameters for classifying spirals is unsatisfactory because the parameters are not perfectly correlated.

3. Bars are not all-or-nothing. There is a continuum of bar strengths.

General trends within Hubble sequence E Sc:

Decreasing Bulge/DiskDecreasing stellar ageIncreasing fractional gas contentIncreasing ongoing star formation

de Vaucouleurs’ Revised Hubble Classification System(de Vaucouleurs 1958, Handbuch der Phys. 53, 275)(de Vaucouleurs2 1964, Reference Catalog of Bright Galaxies)

Basic idea: retain Hubble system, but add lots of additional options:Rings (inner and outer), range of bar-like structures….

No Bar

Bar

Spiralshaped

Ringshaped

Cross section of diagram

E E+ S0- S0 S0+Sa Sb Sc Sd Sm Im

Limitations:Rings and bars are not independentDoes not take into consideration mass or other important parameters.

Morphological Distributions

The range and frequency of different morphological types is sensitive to the sample of galaxies studied. Some key results:

•The Local Group is the only sample that includes a significant number of very faint galaxies. Of the ~35 galaxies in the Local Group, only the 3 brightest (M31, MW and M33) are spirals, the remainder are equally divided between irregular and dwarf elliptical /spheroidal galaxies.

•Within rich clusters of galaxies, the population of bright galaxies is dominated by Ellipticals.

•Samples of galaxies outside of clusters (in the “field”) are mainly Spirals.

Coma cluster – red=Ellipticals; blue=Spirals; green contours= X-rays

Automated Classification

Visual classification time consuming and different observers may not always agree. This motivates the development of algorithms to automatically and impartially classify galaxy images - very important for large surveys like the Sloan Digital Sky Survey and deep extragalactic fields.

Examples: Abraham et al. (1994, 1996):Concentration parameter C - fraction of light within ellipsoidal radius 0.3 x outer isophotal radius (1.5 above sky level).Asymmetry parameter A - fraction of light in features not symmetric wrt a 180 degree rotation

faintest

brightest

Photometric Properties of Galaxies

Astronomers measure brightness in units of magnitude.

During the 2nd century BC, Hipparchus ranked all visible stars – brightest were magnitude 1, faintest were magnitude 6.

To our eyes, a change of one magnitude = a factor of 2.512 in flux (i.e. logarithmic scale).

To convert from flux units (in W/m2 or erg/s/cm2) to magnitudes:

mag2 - mag1 = 2.5 log (F1/F2)

To measure the brightness distribution of galaxies, we must determine the surface brightness of the galaxy.

Surface brightness is flux (magnitude) within 1 unit of angular area on the sky (square arc seconds).

(mag/arcsec2)

I is Intensity (W/m2/str) where 1 str = 4.25 x 1010 arcsec2

Photometric Properties of Galaxies

15

20

25

30Radius R

BSurface Brightness is independent of distance since flux decreases d2, but the area subtended by 1 square arc second increases as d2.

R

Surface Brightness profiles forElliptical Galaxies (and bulges of Spirals)

B

R1/4

log I ~ - R 1/4

I(R) = Ie exp{-7.67[(R/Re)1/4-1]} “deVaucouleurs law” (1948)or “r1/4 law”

Re = effective radius containing 50% of luminosity Re = (aebe)1/2

(factor of 7.67 chosen to make this so) -for major,minor axis

Ie = flux at Re Io = Ie103.33 = 2138Ie (central flux)

Spiral Galaxies

Bulges •Luminosity profiles fit r1/4 law•Structure appears similar to Elliptical galaxies, except bulges are more flattened (oblate spheroids).

Disk scale lengthCentral surface brightness

Disks

log I ~ -R

•Most are well-represented by an exponential profileI(R) = Ioe-R/Rd (Freeman 1970)

NGC 7331 Sb galaxyR-band isophotes

•Bulge dominates in center and again at very large radii (if bulge obeyed r1/4 to large R)

•Disk dominates at intermediate radii•Rd ~ 1 - 10 kpc•Disk in many spirals appear to end at some Rmax around 10 to 30 kpc or (3-5Rd)

(Rd)

NGC 7331

I

Multi-wavelength view of GalaxiesPhotons of different energies are created by different physical phenomena.

Spiral galaxy M31 (Andromeda) 2 million ly away (700 kpc). 1” on the sky = 3pc

HI (neutral H gas) – 21 cm

Star formation - Infrared CO (molecular gas) – 3.6 micron

Stars & hot gas (optical)

• Radio observations allow us to study HI (neutral Hydrogen) and molecular gas

• CO displays sharp drop with radius• Traces spiral arms• CO more associated with arms than HI which

permeates galaxy (except in center)• Spiral galaxies vary in the amount of

molecular to neutral Hydrogen (50% to 10%)• CO velocity map shows rotation

X-ray Visible

Infrared Radio

M82 Irregular galaxy

Infrared light produced by recent star formation inside dusty clouds – dust absorbs light and reradiates at longer wavelengths.

X-ray emitting gas heated to high temps by supernovae and is expanding in galactic wind.

Radio – 21-cm neutral Hydrogen gas

Spiral StructureThe winding dilemma

• If spiral arms are coherent physical structures displaying differential rotation like the stars and gas, they would have wound up so tightly by now that they would not be visible.

The stars and gas have made dozens of revolutions around the center of their galaxy over the age of the Universe so spiral structure should be “smeared out” if caused due to winding…

A leading theory for galactic spiral arms is spiral density waves.

Density Waves in Traffic

Spiral Structure

Spiral arms are caused by the compression of gas as it orbits the Galactic center and encounters density waves (moving more slowly). The compression of gas causes stars to form which we see as spiral arms.

Density Waves in Spiral Galaxies

•Cloud approaches arm at a relative speed of ~100km/s.•Arm acts as gravitational well, slowing down the cloud.•Arm will alter orbits of gas/stars, causing them to move along arm briefly.•Compresses HI gas and gathers small MCs to form GMCs.•GMCs produce O&B stars.•Stellar radiation disrupts the clouds.

If stars in the disk of a spiral galaxy are on slightly eccentric orbits, and the position angle of these ellipses vary with radius, a spiral-shaped density wave can be formed from a set of nested ovals.

Density wave theory is really based on the premise that mutual gravitational attraction of stars and gas clouds at different radii can offset the spiral’s tendency to wind-up.

This produces a pattern which rotates rigidly within the galaxy disk.

Spiral StructureHow do density waves form?

Kennicutt (1992)

Galaxy Spectra

Useful information about galaxies can be learned by looking at the spectrumSpectra are dominated by the brightest stars in the galaxy

Elliptical galaxy spectra dominated by old, K-type stars. They have strong absorption lines seen in stellar spectra

Spiral galaxies have absorption but also bright emission lines from star forming gas regions heated by newly formed stars

Spiral galaxies – Rotation Curves and Mass

Spiral disks seen at inclination i

q = cos i where q=b/a (b=semiminor and a=semimajor axis)b

a

Where vr is radial velocity and vr,0 is radial velocity at galaxy center

For M31, vr along the major axis is -30 km/s and central velocity is -270 km/s. Velocity curve is fairly constant out to 36 kpc from center and yields vc ~ 230 km/s.

Evidence of Dark Matter in Spiral GalaxiesRotation curves determined from Doppler shifts in spectral emission lines

Vr stays roughly constant with R as far as luminous matter can be detected

Masses from 1011 to 2x1012 Msun

50% to 80% of mass is Dark matter.

DM does not have to be confined to the disk – distributed spherically

Elliptical galaxies – Velocity Dispersion and Mass

K Giant star

NGC 2549 S0 galaxy

Added width of the absorption lines in elliptical galaxies reveals large random motions of stars – not much rotational motion as observed in spirals.

The velocity dispersion σ can be used to estimate the galaxy’s mass.Use Virial Theorem - relation between kinetic and gravitational potential energy for a system in equilibrium 2K = -U

K = ½ M <v2> and U ~ -GM2/r or U = -0.4 GM2/rh where rh is half mass radius

Let <v2> = 3σ2 then M = 7.5 σ2rh/G

For the dSph Leo I, rh = 290 pc and σ = 8.8 km/s giving

M/L ratio of 8 indicates significant DM

Determining Distances to Other Galaxies

With their high luminosities (~10,000 Lsun), Cepheid variables extend the distance scale to nearby galaxies, out ~25 Mpc (80 million light years).

Cepheid variable stars, when found in other galaxies, tell us the distance to those galaxies since their luminosities can be determined using the period-luminosity relation

Cepheids

• Galaxy rotation measured via the Doppler shifting of the 21cm atomic hydrogen line.

• Rotation speed (width of H line) is proportional to the galaxy’s mass.

• Galaxy luminosity is also proportional to galaxy mass (number of stars).

• The correlation between luminosity and rotation speed is referred to as the Tully-Fisher relation.

(a broadened line)

Tully-Fisher Relation

Determining Distances to Other Galaxies

Type-I supernova result from the detonation of white dwarf stars when their mass (slightly) exceeds 1.4 Msun.

The brightness of the explosion should be (roughly) the same for every Type-1 supernova.Type-I Supernovae are Type-I Supernovae are standard candlesstandard candles. Knowing their luminosity, . Knowing their luminosity, and comparing to their measured flux, yields the distance via the and comparing to their measured flux, yields the distance via the inverse-square law.inverse-square law.

Useful for determining distances out to (3 billion light years - 1 Gpc).

Type 1a SN

Determining Distances to Other Galaxies

Hubble’s LawHubble’s Law

In 1912, Vesto Slipher discovered that with few exceptions, every galaxy is receding from us,

i.e. has redshifted spectral lines.

Redshift is defined by:

z =

Determining Distances to Other Galaxies

•In the 1920’s, Edwin Hubble discovered that more distant galaxies (using distances determined from Cepheids) are receding faster (have larger redshifts).

•The relationship, well fit by a straight line, is called Hubble’s Law.

Hubble’s Law is written:

Recessional Velocity (in km/sec) = Ho distance (in Mpc)

V = Ho D

where Ho is Hubble’s constant (slope of the line)

Universal Expansion

· You can think of space as the surface of a balloon. As the balloon expands, the space between galaxies stretches.

· This means that the wavelength of light emitted by galaxies is also stretched as space expands. The wavelength of light is redshifted – we call this a Cosmological Redshift.

If the rate of expansions stays constant over time, and all objects are together at t=0, current distance between two objects is

d = v to where to is current age of Universe

Then, v = (1/to)d

Same as Hubble’s law where Ho = 1/to

The value 1/Ho is called the Hubble Time age of the Universe if expansion is constant.

Ho has units of km/s/Mpc to express velocity and distance in convenient units.

Each distance technique has uncertainties which then add to the error in determining the Hubble Constant

Current values hover around Ho = 70 km/s/Mpc with an error of +/- 8 km/s/Mpc

The Cosmic Distance Ladder

Hubble’s law allows us to measure distances to the “ends of the visible universe,” (~13 billion light years).

It is less accurate for distances < 100 Mpc because of the “peculiar” velocities of galaxies (i.e. motions affected by local gravitational fields).