Galaxies - Texas A&M Universitypeople.physics.tamu.edu/.../spring12/lecture_galaxies.pdfGalaxies Mid...

Galaxies

Mid 18th century, Kant and Wright suggested that the Milky Way is a finite system of stars. Turns out this is accurate.

Kant went on to suggest that the very faint “elliptical nebulae” might be similar collections of stars, well beyond the boundaries

of the Milky Way. He called these Island Universes. Immanuel Kant (1724-1804)

Virgo Cluster of Galaxies sky.google.com

“Nebulae”

Thursday, April 5, 2012

Charles Messier (1730-1817)


Charles Messier cataloged 103 “fuzzy” objects while looking for comets. This is still referred to as the Messier Catalog. Objects are given as “M#”, such as those below. It contains

planetary nebulae, spiral and elliptical nebulae.


Charles Messier (1730-1817)


M84

M86

M87

Charles Messier cataloged 103 “fuzzy” objects while looking for comets. This is still referred to as the Messier Catalog. Objects are given as “M#”, such as those below. It contains

planetary nebulae, spiral and elliptical nebulae.

M58

M88

M89

M90

M91


Cataloging “Nebulae”

Herschel expanded the cataloging of nebulae, succeeded by his son, Sir John Herschel, included objects in the Southern Hemisphere (viewed from South Africa).

William Herschel Sir John Herschel 1792-1871

Their “New General Catalog” (NGC) contains nearly 8000 objects, but the nature of these

objects was an open question.

In 1845, William Parsons, built “the

Leviathan”, telescope with 72-inch (1.8m) collecting power.

William Parsons Third Earl of Rosse

(1800-1867)



Parsons showed that some nebulae have spiral structure (henceforth known as “spiral

nebulae”). He argued they may be rotating. This was confirmed later by Vesto Slipher in 1912 who saw rotation in the Doppler-shifts of a

spectral lines in these objects.Thursday, April 5, 2012


HST Image

Parsons showed that some nebulae have spiral structure (henceforth known as “spiral

nebulae”). He argued they may be rotating. This was confirmed later by Vesto Slipher in 1912 who saw rotation in the Doppler-shifts of a

spectral lines in these objects.Thursday, April 5, 2012

The Nature of GalaxiesThe Great Shapley-Curtis Debate

At beginning of 20th century, half the astronomers thought the nebulae were objects in the Milky Way Galaxy. half thought they were “Island Universes”.

On April 26, 1920 at the National Academy of Sciences in Washington, DC, Harlow Shapley (Mt. Wilson Observatory) debated Heber D. Curtis

(1872-1932, Lick Observatory).

Shapley - Nebulae are members of our Galaxy

Major point: If Andromeda nebulae were as large as the Milky Way (100 kpc), then its angular size (3o x 1o) would imply such a great distance (2 Mpc) that the nova he observed

in Andromeda would be much, much more luminous than anything

observed in the Milky Way.

Curtis - Nebulae are Island Universes like the Milky Way

Novae showed that spiral nebulae are at least 150 kpc away to have same

luminosities as those in the Milky Way.

Doppler velocities (>500-1000 km/s) of spiral nebulae implied they would not remain bound to the Galaxy. If

transverse velocities were just as high and they are in the Galaxy, we should

be able to measure their proper motion (which we can’t).


The Nature of GalaxiesThe Great Shapley-Curtis Debate

Debate was settled by Edwin Hubble in 1923. Using the new 100-inch telescope on Mt. Wilson (Caltech),

he detected Cepheid Variable stars in M31 (Andromeda Nebula).

He measured the distance to Andromeda to be 285 kpc - well outside the Milky Way. (Current

measurement is 770 kpc.)

Spiral Nebulae (and Elliptical Nebulae) are Island Universes, other galaxies like the Milky Way.

Edwin Hubble 1889-1953

Hubble went on in his paper “Extra-Galactic Nebulae” to propose that galaxies (nebulae) be classified as ellipticals, spirals, and irregulars.

This is today known as the Hubble Sequence.


Hubble went on in his paper “Extra-Galactic Nebulae” to propose that galaxies (nebulae) be classified as ellipticals, spirals, and irregulars.

This is today known as the Hubble Sequence.

Hubble arranged his sequence on a tuning fork diagram. Originally he (incorrectly) hypothesized that galaxies evolved from the Left to the Right of this sequence.

Early types Later types


Hubble subdivided spiral sequences into Sa, Sab, Sb, Sbc, Sc (Scd, Sd), and SBa, SBab, SBb, SBbc, SBc (SBcd, SBd). Two characteristics dictate this (1) the bulge-to-disk ratio and (2) how tighltly wound the spiral arms are. Spirals with high

bulge-to-disk ratios (Lbulge/Ldisk > 0.3) and tightly wound arms are the “a” subclass. The lower sequence has nuclear “bars”.


Irregulars: galaxies lacking organized structure.

Hubble subdivided spiral sequences into Sa, Sab, Sb, Sbc, Sc (Scd, Sd), and SBa, SBab, SBb, SBbc, SBc (SBcd, SBd). Two characteristics dictate this (1) the bulge-to-disk ratio and (2) how tighltly wound the spiral arms are. Spirals with high

bulge-to-disk ratios (Lbulge/Ldisk > 0.3) and tightly wound arms are the “a” subclass. The lower sequence has nuclear “bars”.


Ellipticals


Spirals


Spirals


Barred Spirals


Irregulars


Rotation Curves of Spiral Galaxies

Wavelength

Blue

shift

ed

Reds

hifte

d

656 nm (Hα)

Spiral galaxies do rotate.

Measure “rotation curve” by measuring doppler shifted light from spectral lines as a function of galacticentric

distance.



Rotation curve for our Galaxy. Strange thing is.... rotation curve is flat beyond the Solar circle, R0 = 8.5 kpc.

Clemens 1985, ApJ, 295, 422



Observations !v ~ constant (r0)



Rubin, Thonnard, & Ford, 1978, ApJ, 225, L107

Vera Rubin (b1928)

Responsible for most of the work

on the “galaxy rotation rate”

problem.


This is the Dark Matter distribution in galaxies.

Navarro-Frenk-White (NFW) profile:

For Spiral Galaxies, we can extend the relation between Mass and Velocity over the whole galaxy using the “maximum” rotational velocity, V, at R=size of galaxy:

2



There is a relationship between the Maximum rotation velocity and the Galaxy’s Absolute Magnitude (Luminosity).



There is a relationship between the Maximum rotation velocity and the Galaxy’s Absolute

Magnitude (Luminosity).

This relation is now referred to as the Tully-Fisher Relation, after Brent Tully and

Richard Fisher who first determined it in 1977.

They derived:

MB = -9.95 log10 ( vmax ) + 3.15 (Sa)

MB = -10.2 log10 ( vmax ) + 2.71 (Sb)

MB = -11.0 log10 ( vmax ) + 3.31 (Sc)

As with other quantities, this relation is tightened when using Infrared magnitudes:

MH = -9.50( log10 VR - 2.50) - 21.67

Rubin et al. 1985, ApJ, 289, 81Thursday, April 5, 2012


Rubin et al. 1985, ApJ, 289, 81Thursday, April 5, 2012


Rubin et al. 1985, ApJ, 289, 81

Milky Way



Rubin et al. 1985, ApJ, 289, 81

Milky Way

Vera Rubin, measuring spectra


Luminosity-Metallicity RelationZaritsky, Kennicutt, Huchra 1994, ApJ, 420, 87

More luminous galaxies have high metallicities. The more luminous galaxies have had more metal enrichment.

Milky Way

Metallicity (metal fraction) of gas

Metallicity of stars

Sun

Sun


Mass-Metallicity Relation

Tremonti et al. 2004, ApJ, 613, 898

More luminous means more mass to first order. Higher-mass galaxies have had more metal enrichment.


Ellipticals


Elliptical Galaxies

M 87Giant Elliptical

Mass ~ 3 x 1012 M⊙

Size is ~10 x diameter of Milky Way


Elliptical Galaxies

Recall that the gravitational potential of a collection of points with total mass M and radius R is:

And the Kinetic energy is :

Where σ2 is the velocity dispersion (average velocity of all particles in all dimensions).

Using the Virial Theorem 2K + U = 0, and solving for the velocity dispersion:

Solving for the Mass gives:

What the heck is σ ?

This is the velocity dispersion, which is the average of radial velocities in a galaxy.


Elliptical GalaxiesWhat the heck is σ ?

This is the velocity dispersion, which is the average of radial velocities in a galaxy.

telescope

elliptical galaxy (made of lots of stars on radial, keplerian orbits)

continuum

wavelength

flux

Δλ

λ0

Normally, measure velocity dispersion in only 1-degree of freedom, so the mass will be: Absorption line:

many stars at different velocities


Elliptical Galaxies

Recall that the gravitational potential of a collection of points with total mass M and radius R is:

And the Kinetic energy is :

Where σ2 is the velocity dispersion (average velocity of all particles in all dimensions).

Using the Virial Theorem 2K + U = 0, and solving for the velocity dispersion:

Assume that the Mass-to-light ratio is constant over the galaxy:

Introduce the surface brightness (luminosity per area), and assume its constant (not necessarily true, but OK for now) :

Combining the above two relations gives:

Solving for L gives:


Elliptical GalaxiesAll elliptical galaxies have a relationship between their central radial-velocity dispersion,

σ, and their absolute magnitude. This is the Faber-Jackson relation, L ~ σ4.

Sandra Faberb. 1944

This is a consequence of the Virial Theorem.

cD EllipticalsMass ~ 1013-1014 M⊙

Diameter ~ 300-1000 kpc

“Normal” EllipticalsMass ~ 108-1013 M⊙

Diameter ~ 1-200 kpc

“Dwarf” EllipticalsMass ~ 107-108 M⊙

Diameter ~ 1-10 kpc


Tighter fit to the data uses the radius (size) of the galaxy. This is the “fundamental plane” of elliptical galaxies. L ~ σ2.65 re0.65.

The effective radius is related to the surface brightness, which is not constant over all types of Ellipticals. You can rewrite the “fundamental plane” as

re ~ σ1.24 Ie-0.82

cD Ellipticals

giant & normal Ellipticals

dwarf Ellipticals


Supermassive Blackholes in Galaxies


Mass ~ 3 x 1012 M⊙

Size is ~10 x diameter of Milky WayThursday, April 5, 2012

Supermassive Blackholes in Galaxies


Mass ~ 3 x 1012 M⊙

Size is ~10 x diameter of Milky Way

Jet of material emitted by nucleus


Supermassive Blackhole in M87

rotational velocity

velocity dispersion, σ ≈ FWHM / 2.35

Modeling gives Virial Mass of (3.2+/-0.9) x 109 M⊙ within r < 0.05” = 3.5 pc.

Macchetto et al. 1997

(This density is 1.8 x 107 M⊙ pc-3. Recall, the solar neighborhood has 0.05 M⊙ pc-3.)


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