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I*M*P*R*S on ASTROPHYSICS at LMU Munich

Astrophysics Introductory Course
Lecture given by:

Ralf Bender and Roberto Saglia in collaboration with: Chris Botzler, Andre Crusius-WДtzel, Niv Drory, Georg Feulner, Armin Gabasch, Ulrich Hopp, Claudia Maraston, Michael Matthias, Jan Snigula, Daniel Thomas
Powerpoint version with the help of Hanna Kotarba

Fall 2007
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Chapter 11 Dwarf Galaxies

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11.1 Overview
The Hubble Sequence of Giant Galaxies:

M 1010 M

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Dwarf Galaxies:
Hubble could only detect luminous galaxies with

M

vis

> 1010 M

Most of the galaxies in the Universe are dwarf galaxies Giant galaxies still dominate the light in the Universe. Dwarf galaxies are often observed as satellites of giant galaxies building blocks of massive galaxies. Two dwarf galaxies orbit Andromeda: M32, NGC 205.

M32

N 205

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(Binggeli & Jerjen, 97)

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The Zoo of Dwarf Galaxies:

(Binggeli 93)
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Core Properties of Stellar Systems in the Universe:
All stellar systems in the Universe can be subdivided into three distinct classes. Core definition: surface density

(rc ) = 0.5 (r = 0)

1. 2. 3.

Massive ellipticals and bulges. Dwarf galaxies and disks. Globular Clusters.

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Normal ellipticals and dwarfs define separate sequences of effective radii as function of absolute magnitude.

bulges

(Carroll & Ostlie)

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Normal ellipticals and dwarf systems define separate sequences of average surface brightness versus absolute magnitude.

(Carroll & Ostlie)

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11.1. Dwarf ellipticals (dE) or dwarf spheroidals (dSph)
Morphological similar to bright ellipticals Smooth surface brightness distribution Similar ellipticity distribution as bright ellipticals No young stars

But: exponential profiles
Sersic profiles:
- b n ( r / re

I(r ) = I(0) e

)1

n

Sersic index

(Binggeli & Jerjen, 97)
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Giant ellipticals are described by the de Vaucouleurs profile:

I(r ) = I(0) e
More generalized profile:

-7.67( r re

)1

4

I(r ) = I(0) e
10

- b n ( r re

)1

n

n
1

-12

MB M

-24
B

There exists a puzzling correlation of Sersic n with galaxy luminosity.

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The dwarf spheroidals of the Local Group: 9 dSph galaxies named for the constellations in which they appear.

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Dwarf Galaxies and Globular Star Clusters:
Similar masses but very different radii The Sculptor Dwarf Galaxy Globular Cluster M55

2 kpc
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0.1 kpc
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The dwarf spheroidals of the Local Group:

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The Sagittarius Dwarf Elliptical Galaxy:
Galaxies like the Milky Way formed from smaller galaxies. Even after becoming a giant galaxy, the Milky Way still devours smaller companions that move too close to it. In 1994 a new object was discovered by Ibata et al. on the opposite side of the Galactic center SagDEG

c
(Ibata)

b

a

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One of our nearest neighbors and comprised of mostly old stars (carbon stars). About 20 kpc from the Sun on the opposite side of the Milky Way. The galaxy is torn apart by immense tidal forces of hundreds of millions of yrs In 1996, a stream of stars was found that completely encircles the Milky Way. Mass: 7 8

M 10 - 10 M

(Johnston et al.)

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11.2. Dwarf irregulars:
Brightness distribution in U,B,V shows knots, which correspond to star formation regions. The brightness distribution in the IR is smoother (old stars) and close to an exponential profile. HI column density of Leo A

(Lo et al. 94, in Panchromatic View on Galaxies)

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GR8: Wendelstein image by Claus Goessl

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Dwarf irregulars again show an ellipticity distribution similar to ellipticals and dwarf ellipticals, but not to spirals. The HI gas distribution is often much more extended than the distribution of stars. HI dominates the baryonic mass of the faintest objects. Blue compact dwarfs: extreme type of dIrr with star bursts concentrated in one very bright region. Some BCD may be genuinely young objects which form stars for the first time (e.g. I Zw 18) Star formation in irregulars and BCDs leads to bubbles of HII gas that expands and can cause significant gas loss.

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(Mac Low & Ferrara 99)

RT

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The Magellanic Clouds
The LMC in H The LMC

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HI around the SMC and LMC

Mathewson, Ford 84, IAU Symp 108

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11.3 Stellar populations and chemistry of dwarf galaxies
Dwarf ellipticals are generally old, i.e. they started to form stars > 10 Gyrs ago. Some objects may also have had more recent episodes of star formation, until a few Gyrs ago. These conclusions are mostly based on dwarf companions of the Milky Way which can be resolved in individual stars (see Hodge: 1989, ARAA 27, 139). Age determinations are more difficult for more distant non-resolved dE. Irregulars usually undergo several bursts of star formation, sometimes separated by long quiet periods. The oldest stars in the LMC are > 10 Gyrs ago. 3 Gyrs ago a burst of star formation started again. Fainter objects have fewer bursts (BCDs)

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For dwarfs in the Local Group color luminosity diagrams can be derived and metallicities follow from the color of the giant branch.

GB of globular clusters with different metallicities

(Freedman 1992, IAU Symp. 149)
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The metallicity of dwarf irregulars can be derived from the emission lines of their interstellar medium. There exists a very strong correlation between metallicity and luminosity. Note that dE follow the same relation!

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Both, dwarf ellipticals and dwarf irregulars follow a single relation between metallicity and luminosity:

Z L0.4 B
Interestingly, the gas-to-star ratio does not seem to be very important. Apparently, the metallicity of the stars depends only on the total number of stars produced (luminosity) and not on the gas mass left at present time. The enrichment history must be different from the closed box model (Skillman & Bender 95, Rev. Mex.A.A. 3,25)

galactic winds

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The Population Box:

(Hodge 1989, ARAA 27, 139)
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11.4 Kinematics of Dwarf Galaxies
Dwarf elliptical galaxies:
follow a similar relation between luminosity and central velocity dispersion as bright elliptical galaxies

LB

2.5 ... 3 0

show too little rotation for their flattening and are therefore supported by anisotropic velocity dispersion. The reason for the anisotropy is not understood yet. of very low luminosities 106 - 107 L
B,

can have extremely high M/L.

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Dark matter in dwarf ellipticals:
The gravitational potential: grav. force

F( x ) = - ( x )

The potential of a spherical galaxy:
GM (< r ) (r ) = - + 4G (r ')r ' dr ' r r

(Binney & Tremaine 1987; Galactic Dynamics) Potential of a uniform sphere of density outer radius of sphere

2 r2 (r ) = -2G a - 3
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The potential energy:

E

pot

= 0.5 (r ) (r ) 4r 2 dr
E
pot

For a homogeneous sphere:

3GM =- 5a

2

We measure a line-of-sight velocity dispersion (z-direction): For

0

x = y = z the total kinetic energy is estimated as:

E
Virial theorem:

kin

1 = M ( 2 + 2 + x y 2

2 z

)

3 = M 2

2 0

effective radius

2E

kin

= -E

pot



2 50 a M= G

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The M/L for Local Group dwarf spheroidals:
constant dark halo mass of

2.5 107 M

(Mateo)

Below M V = -12 : M/L L-1

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Structure of dark matter halos

galactic disk

(Moore, 2001)

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The density structure of dark matter halos:
CDM simulations (Navarro et al. 97, Moore et al. 98, Klypin et al. 2000) predict that dark halos have universal density profiles:

(r )
with

1 r (rs + r )

3-

1 - 1.5

log

Central density cusp results from a temperature inversion.

log

2

log r
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Dark matter in DDO 154
HI surface contour

M * 5 107 M M M
HI

3 108 M 4 109 M
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DM

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Predicted

1 (r ) 1.5 r (rs + r )1
Observed

.5

(r )

1 (rs + r )(rs2 + r 2 )

In contrast to models of hierarchical clustering, dark matter cores are isothermal with a flat density distribution.
IMPRS Astrophysics Introductory Course Fall 2007