Normal Galaxies (2) ASTR 2120 Sarazin - people.virginia.edu · star, or merger of galaxies ... Form in low density regions 2. Gas pressure low in collapsing cloud 3. Star formation

Normal Galaxies (2)ASTR 2120

Sarazin

Hubble Classes

Sequence in: round (spheroid) → flat (disk)

random orbits → circular orbits in disk red → blue

old stars → young stars little cool gas and dust → lots of cool gas and dust

dense environments → sparse environments

1) Luminosity, Mass, Diameter Ellipticals:

Very large range: dwarf ellipticals, 105 L� (globular clusters?)

→  giant ellipticals → brightest cluster galaxies (BCGs or cDs), 1013 L�

BCGs are the largest galaxies in Universe

Spirals: Smaller range of sizes

Galaxy Properties

2) Light Distributions E, Sp & S0 Bulges:

I(r) = I(0) exp[ - 7.67 (r / re)1/4] De Vaucouleurs Law re = “effective radius”, 1/2 of light comes inside of re in

projection re = 0.1 - 50 kpc I(0) ~ constant for normal ellipticals (Freeman’s Law)

Galaxy Properties (Cont.)

r

2) Light Distributions (cont.) Sp & S0 Disks:

I(r) = I(0) exp( - r / ro) ro = 1 - 5 kpc I(0) ~ constant for spirals


3) Motions Sp & S0 Disks:

Nearly circular orbits in the plane of the disk, all in the same direction


Sp & S0 Disks: vrot(r) ~ constant outside of center (same as MW)


3) Motions Ellipticals:

Why are ellipticals elliptical? What is their 3D shape?

a)  Rotation? No

Shape would be oblate spheroid

vr would vary across galaxy, not true

b)  Random stellar motions


Elliptical motions

b) Random stellar motions No rotation, but many different vr along line-of sight → broaden spectral lines

σr2 ≡ <vr

2> in CM frame If spherical, σ2 = 3 σr

2


Elliptical motions: Why not spherical?

Stars move different speeds in different directions

Can be different in all 3 directions

triaxial ellipsoids


faster slower

4) Masses Sp & S0 Disks:


€

vrot (r) =GM (r)r

M (r) =vrot

2 (r) rG

vrot (r) ≈ constantMtot (r)∝ r, most mass at large radii

ρ tot (r)∝1r2 , but light and stars ∝ e−r /r0

4) Masses Sp & S0 disks:

Massive Dark Matter Halos

Extend out to ~ 100 kpc

M(Dark Matter) > 10 x M(stars and gas)


4) Masses Es, Sp & S0 bulges:

Random velocities


€

σ r = radial velocity dispersionσ = 3D velocity dispersionVirial Theorem : KE = - PE/212Mσ 2 =

12GM 2

R

M =σ 2 RG

4) Masses Elliptical galaxies:

Massive Dark Matter Halos

Extend out to ~ 100 kpc

M(Dark Matter) > 10 x M(stars and gas)


5) Dark Matter Density Profiles


ρtot (r) ≈ ρDM (r) ~ 1r2 at large radii

But, this gives M (r) ~ r, diverges at large radiiTheoretical models give a more detailed form

ρDM (r) = ρ0

rrs

1+ rrs

"

#$

%

&'

2 , rs ≡ "scale radius"

Navarro-Frank-White Profile (NFW)


ρDM (r) = ρ0

rrs

1+ rrs

!

"#

$

%&


ρDM (r) ~ r−1 at small radii (r << rs )Cusp at center, but mass goes to zero anyway

Navarro-Frank-White Profile (NFW)


ρDM (r) = ρ0

rrs

1+ rrs

!

"#

$

%&


ρDM (r) ~ r−3 at large radii (r >> rs )Mass diverges but only logarithmicallyCut off at "virial radius", outside this not in equilibriumrvir = c rs , c ≡ "concentration parameter"c decreases with mass, ranges from ~4 to ~40

*** Vitally Important Technical Point !!! ***

Do not confuse the

Navarro-Frank-White Profile


ρDM (r) = ρ0

rrs

1+ rrs

!

"#

$

%&

2

*** Vitally Important Technical Point !!! ***

With the

Navarro-Frank-White Profiles


6) Luminosity - Velocity Laws (Fundamental Plane) Elliptical galaxies:

L ∝ σ4

Spiral galaxies:

L ∝ vrot4 (Tully-Fisher relation)

(derive in problem set)

More generally, luminosity L, velocity (vrot or σ), and radius R lie nearly on a plane in their 3-d space = Fundamental Plane


Review from last semester:

Galaxy Formation

Collapsing gas clouds make rotating disks

Collapse of Gas:Formation of Disks

L

L

1.  Accretion disks

1.  Binary stars

2.  Supermassive BHs in galaxies

Disks are Ubiquitous


2.  Disks in MW and other spiral galaxies



2.  Disks in MW and other spiral galaxies

3.  Disks around protostars, protoplanetary disks, Solar System plane


Collapse of Ball of Stars:Formation of Spheriods

Ri

Randomly oriented halo orbits

Ellipticals: Start with collapse of ball of gas and

star, or merger of galaxies Assume that either no gas at start, or

gas is quickly consumed by star formation →

Collapse of ball of stars → elliptical galaxy

Formation of Galaxies

Formation of Elliptical Galaxy

Ball of stars on randomly oriented halo orbits

Formation of Spirals Start with collapse of ball of gas and

stars, or merger of gas rich galaxies Assume that gas survives initial

collapse, and is not completely consumed by star formation →

Collapse of ball of stars and gas → spiral galaxy

Formation of Spirals

L

Gas and star disk with circular orbits

Density Dependence

Sequence in: round (spheroid) → flat (disk)

random orbits → circular orbits in disk red → blue

old stars → young stars little cool gas and dust → lots of cool gas and dust

dense environments → sparse environments

Density Dependence

E S0

Sp

Fraction

log projected galaxy density

for galaxies in clusters

Field

Why are elliptical & S0’s mainly in dense regions (clusters of galaxies), while spirals and irregulars are in more isolated environments

Heredity vs. Environment?

Nature vs. Nurture?

1.  Galaxy morphology is set at formation, reflects density then

2.  Galaxy morphology is affected by environmental effects during the life of the galaxy

Density Dependence

Ellipticals: 1.  Form in dense regions

2.  Gas pressure high in collapsing cloud

3.  Star formation rapid

4.  All gas converted to stars

5.  Collapse of ball of star = pure bulge

Galaxy Morphology Set at Formation

Formation of Elliptical Galaxy

Ball of stars on randomly oriented halo orbits

Spirals: 1.  Form in low density regions

2.  Gas pressure low in collapsing cloud

3.  Star formation slow

4.  Much of gas survives to form rotating disk

5.  Stars form in rotating disk

6.  Gas, dust, most of stars in disk

Galaxy Morphology Set at Formation

Formation of Spirals

L

Gas and star disk with circular orbits

Environmental Effects:

1.  Galaxy mergers

2.  Ram pressure stripping of gas from galaxies

Environmental Effects on Galaxies

1.  Elliptical-elliptical mergers → elliptical Gas-poor mergers (“dry” mergers)

2.  Violent, similar mass E-Sp and Sp-Sp mergers can → E

Gas-rich mergers (“wet” mergers)

Violent merger → increase pressure → rapid star formation → collapsing ball of stars → E

3.  Minor, less-violent E-Sp and Sp-Sp → larger Sp

Galaxy Mergers

Galaxy Mergers

Cartwheel Galaxy: one of galaxies at right passed through

Galaxy Mergers

Early stage spiral-spiral merger

Galaxy Mergers

Antennae: mid stage spiral-spiral merger

Galaxy Mergers

Antennae: HST

Galaxy Mergers

NGC7252

“Atoms for Peace”

Late stage spiral-spiral merger

Galaxy Mergers

NGC7252 “Atoms for Peace”

Galaxy Mergers - Star Burst

Galaxy Mergers - Star Burst

M82 Starburst

Galaxy Cluster

Abell 1689 Cluster of Galaxies

Ram Pressure Stripping of Gas from Spiral Galaxies

Clusters of galaxies are full of hot gas at T ~ 108 K

Galaxies move through gas at ~1000 km/s

Ram-pressure from hot gas strips gas from spiral galaxies.

Spirals → disk galaxies without much gas = S0’s

Ram Pressure Stripping

Contours = HI gas In Virgo cluster


NGC4438

In Virgo cluster


Ram Pressure Stripping Virgo Cluster

Orange = X-rays from hot gas

Colored disks = atomic hydrogen gas from spirals, all blown up by ~100x

Gas disks in center are very small

Spiral Arms in Galaxies:What Are They?

ASTR 2120Sarazin

Disk Galaxies vs. Spiral Galaxies

Explained disk galaxies, but what are spiral arms?

Are Spiral Arms “Things” or “Waves”?

•  Things? –  Differential rotation → wrap up with time

•  vrot ~ constant → Ω = vrot/r ∝ 1/r , Prot ∝ r •  Outer stars take longer to orbit galaxy

If spirals were “things”, too wound up unless recreated constantly

Documents

Normal Galaxies (2) ASTR 2120 Sarazin - people.virginia.edu · star, or merger of galaxies ... Form in low density regions 2. Gas pressure low in collapsing cloud 3. Star formation