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1
Overview of galaxy properties
Enrico Maria Corsini
Dipartimento di Astronomia
Università di Padova
Lectures of Astrophysics of Galaxies
Astromundus/Laurea Magistrale Astronomia
A.A. 2010-2011
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Outline
Morphology
Photometry
Kinematics
Scaling laws
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Morphology
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It is the most widely-used classification
It gives the basic terminology
Hubble recognizes four galaxy families:
- ellipticals (E)
- normal (S0) and barred (SB0) lenticulars
- normal (S) and barred (SB) spirals
- irregulars (Irr)
and puts them along the tuning-fork diagram
Hubble: morphological types
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Irr I
Irr II
Ellipticals Lenticulars Spirals Irregulars
Hubble: tuning-fork diagram
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Round or elliptical (apparent) shape
Smooth and structureless
Classes are defined according to the apparent
flattening (ellipticity)
En, n=0,1,…7 with n = 10 e = 10 (1-b/a)
Hubble: ellipticals
b
a
e = 1 – b/a
7
b/a 1 0.7 0.5 0.3
1-b/a 0 0.3 0.5 0.7
type E0 E3 E5 E7
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Lens shaped
Two components: central bulge and disk with no
evidence of spiral arms
Two types: normal (S0) and barred (SB0) lenticulars
Classes S01, S02, S03 are defined by:
- the strength of dust absorption in the disk
Classes SB01, SB02, SB03 are defined by:
- the prominence of the bar
Hubble: lenticulars
9NGC 5866 S03
NGC 3245 S01 NGC 4111 S02
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Spiral shaped
Two components: central bulge and disk with evidence
of spiral arms
Types: normal (S) e barred (SB) spirals
Classes Sa, Sb, Sc are defined according to the:
- prominence of the bulge with respect to the disk
- tightness of the spiral arms
- resolution of the spiral arms into stars, dust
knots, and nebulae
Hubble: spirals
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Sa
• Prominent bulge
• Tightly wound arms
• Not highly-resolved arms
Sc
• Small bulge
• Loosely wound arms
• Highly-resolved arms
edge-on face-on
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NGC 1302 Sa NGC 2841 Sb NGC 628 Sc
NGC 175 SBa NGC 1300 SBb NGC 7741 SBc
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No symmetric shape
Classes: Type I (Irr I) and Type II (Irr II)
- Irr I: highly-resolved in stars (e.g. LMC)
- Irr II: highly-disturbed (e.g. M82)
Hubble: irregulars
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LMC Irr I M82 (NGC 3034) Irr II
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Not-classified galaxies
2% of nearby galaxies are not E, S0, S, Irr
Most of them are interacting systems
NGC 5128 S0+S pec NGC 4038/39 Sc (tides)
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De Vaucouleurs classifies galaxies according to:
- the main morphological sequence (Hubble stage)
E-E+-S0--S0-S0+-Sa-Sb-Sc-Sd-Sm-Im
- the bar presence
SA = no bar, SAB = weak bar, SB = strong bar
- three varieties
(r) = ring shape, (s) = s shape (spiral arms),
(rs) = mixed shape
and adopts the spindle diagram
de Vaucouleurs: morphological types
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de Vaucouleurs: spindle diagram
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van den Bergh classifies galaxies according to:
- the presence of the disk
ellipticals (E) – disk galaxies (S0,A,S)
- the gas abundance
S0 = no gas, A = gas poor, S = gas rich
and puts them along the trident diagram
Classes a,b,c for S0,A,S are defined according to the
bulge-to-disk ratio
van den Bergh: morphological types
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van den Bergh: trident diagram
Ellipticals Disks
Lenticulars
Anemics
Spirals
D/B
1-3 3-10 >10
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The Milky Way is a SBbc spiral
Most of LG members are dwarf and irregular galaxies
Mophology of Local Group galaxies
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Hubble: history
Irr I
Irr II
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Photometry
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For any point of an extended luminous suorce
surface brightness =
I = F/
is the SB in linear units (e.g. L
pc-2)
= -2.5 log I + costant
is the SB in magnitude units (e.g. mag arcsec-2)
[ B =25 means SB = 25 mag arcsec-2 in B band]
flux
unit solid angle
Surface photometry
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F L / 4 D2 L
A / D2 4 A
SB does not depend
on distance (in nearby
universe):
A,L
D
I = = =
F = measured flux
L = source luminosity
A = source area
D = distance
= source solid angle as seen by the observer
F
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An isophote connects points at the same SB level
1’
N
E
B=16.78 B=21.28
10”
NGC 1291 has two bars
Isophotes
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If I(r, ) is SB in P(r, ) then the total luminosity LT is:
For circular isophotes LT:
Total magnitudine mT:
Luminosity and total magnitude
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The equivalent radius r* of an isophote with area A is:
The integrated luminosity L(r*) within r*:
The relative integrated luminosity k(r*) within r* is:
The effective radius re corresponds to:
k(re)=1/2
Equivalent and effective radius
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Surface-brightness radial profile
The SB radial
profile (as a function
of r*) is a global
description of the SB
distribution of the
galaxy
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de Vaucouleurs’ (or r1/4) law
It is a description of the SB radial profile of
ellipticals and bulges
It is a straight line in the r1/4- plane
Ie (or e) = effective SB
re = effetctive radius
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1”
r 103
14
I 106
e=22.25
effective radius: re=56.6”
sky=22.7
effective SB:
22’
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In the outskirts, SB is
brighter than the
extrapolation of the r1/4 law
( bright halo contributes
8% of the total luminosity).
M87 shows departures
from the r1/4 law at large
radii from the center
Departures from r1/4 at large radii
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The seeing blurs the
inner SB (=“core”)
Typical angular
resolution of ground-
based observations 1”.
V
1”
Departures from r1/4 at small radii
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HST images are diffraction-limited but not blurred by
seeing. Its typical angular resolution is 0.1”.
This SB
flattening is
real and not
due to seeing
or PSF effects 0.05”1”
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Nuker’s law
• rb = break radius (slope change)
• Ib = SB at rb
• for r rb slope -
• for r rb slope -
• = maximal curvature
It is adopted for the SB profile in the inner regions
of elliptical galaxies
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core profiles
power-law profiles
rb = break radius
Ib
r-r-
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K = scale SB
rc = core radius
rt = tidal radius
It is used for the SB of ellipticals (dwarfs and nuclei) and
globular clusters
Parametric law with a teorethical basis (spherical
systems with isotropic velocities)
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King’s law
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C = log (rt/rc) = concentration parameter
c
42 Comparison between King’s and r1/4 laws
King
de Vaucouleurs
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The SB profile of E1 NGC 3379 fitted by the King’s law
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It is used for the SB profile of disks
It is a straight line in the r- plane
I0 (or 0) = central SB
h = scale length
Freeman’s (or exponential) law
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central SB:
0=21.9
scale length: h =43.0”
sky
(h)= 0+1.086
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exponential disk
r1/4 bulge
bulge+disk
data
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Sometimes r1/4 bulge+exponential disk gives a “good”
model of the data
B/D=0.28 B/T=0.22 B/D=1.51 B/T=0.60
B = bulge, D = disk, B+D = T = total
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NGC 7013
bulge+disk+ring+lens
exponential disk
r1/n bulge
data
ring
lens
Sometimes r1/4
bulge+exponential disk is
not “sufficient” to account
for the data
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Usually (in
ellipticals) isophotes
have an elliptical
shape
isophote
fitted ellipse
Shape of the isophotes
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19/03/2003 R.P. Saglia 15
NGC 4278
N
E
PA
b
a
(x0,y0)E
19/03/2003 R.P. Saglia 15
NGC 4278
N
E
PA
b
a
Each isophote is defined by:
SB level:
center coordinates: x0,y0
length of the semiaxes: a,b
PA of the major semiaxis: PA
PA
N
NGC 4278
PA twist
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x0 y0
e=1-b/a
PA
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R
P(x,y) P(R, )
a
b
x
y
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isophote Riso( )
fitted ellipse Rell( )
Sometimes isophotes
are not perfect ellipses
An and Bn describe the
deviations of the shape of
the isophote from perfect
ellipse
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X0Y0
PAe
symmetric dev.
X axis
boxy/disky
symmetric dev.
Y axis
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disky a4>0
boxy a4<0 NGC 5322
NGC 4660
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NGC 4660
disky a4>0
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NGC 4365
boxy a4<0
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Kinematics
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The observed spectrum of a galaxy is the sum of the
spectra of the stars along l.o.s shifted according to the their
radial velocities. If S(λ) is the stellar (i.e. template)
spectrum, then the measured galaxy spectrum G(λ) is the
weighted integral of S(λ) with the (Gaussian) distribution
function of the radial velocities along the l.o.s. B(V,σ,…).
Stellar kinematics
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Elliptical galaxies
KIII star
(nm)
(nm)
Flu
x
Flu
xG( )= S[ (1+v/c)]B(v|V, ,h3,h4)dv
-
+
G = S B (Direct Fitting Method)
G = S • B (Fourier Quotient Method)~ ~ ~
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Line of sight velocity distribution (LOSVD)
B(v) = I0exp(-y2/2)[1+h3H3(y)+h4H4(y)]
where
y = (v-vfit)/ fit
and
H3(y) = (2 2y3-3 2y)/ 6
H4(y) = (4y4-12y2+3)/ 24
are the Gauss-Hermite function.
Gerhard (2003)
van der Marel & Franx (2003)
_ _ _
__
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Stellar kinematics: LOSVD
ln ln
F/F
continuum
-1
HR6018 (K1III) NGC4807 (S0) r=0”
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F/F
continuum
-1
ln ln
star & galaxy star (v=6993 km/s) & galaxy
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F/F
continuum
-1
LOSVD & fit
ln
v = 6993 km/s
= 228 km/s
h3 = -0.001
h4 = 0.002
v (km/s)
star & galaxy
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NGC 4889 cD
major axis minor axis
NGC 4931 S0
major axis minor axis
Kinematics profiles
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70Bender et al. (1990)
V>0 (receding) h3<0
V<0 (approaching) h3>0
1
2
2
LOSVD: h3
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tangential anisotropy h4<0
radial anisotropy h4>0
(R. Saglia)
LOSVD: h4
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Scaling laws
73spheroid disk
no bar
bar
Extension of Hubble including disky/boxy galaxies
boxy disky
disk
Kormendy & Bender: morphological types
74boxy disky boxy disky
rotation
pressure
strong grad.
weak grad.
bright
faint
ellipticity
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CORES
SLOW ROT
log rb
(pc)
POWER-LAW remaining
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CORES
BOXY
log rb
(pc)
POWER-LAW remaining
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E with power-law profiles:
smaller
fainter
disky
rotation supported
E with core profiles:
larger
brighter
boxy
pressure supported
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Brighter ellipticals have larger velocity dispersions. This
relation by Faber & Jackson (1976) is given by
log LT = a log + b
LT4
FJ links distance-dipendent LT with distance-independent .
By measuring apparent magnitude and calculating absolute
magnitude by with FJ, we measure galaxy distance.
Faber-Jackson relation
79R = 0”
0
Stellar kinematics of M87
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Larger ellipticals have fainter effective SB. This relation by
Kormendy (1977) is given by
e = a log Re + b
with a = 3.02, b = 19.74 (con H0 = 50 km s-1 Mpc-1 in V band) or
alternatively
e = a’ log Re + b’
Re I e-0.90
Since Le = I e Re2 it is
I e Le–3/2
which menas that brighter ellipticals have fainter effective SB
Kormendy relation
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Ellipticals are not randomly distributed in the (log Re, e, log
) space but are lying on the fundamental plane (FP, Djorgovski
& Davis 1987, Dressler et al. 1987) defined as
log Re = a log + b e + c
with a = 1.39, b = 0.36, c = -6.71 (with H0 = 50 km s-1 Mpc-1 in rG
band) and a = 1.25, b = 0.32, c=cost (with H0 = 50 km s-1 Mpc-1
in r band). For log I e it is b=-0.82.
FP links distance-dipendent Re with distance-indipendent
e and . By measuring Re in arcsec and calculating its value
in kpc with FP, we measure galaxy distanza (with 20% error)
Fundamental plane
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Fundamental plane
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a) Face-one view
b) Long-edge view
c) Short-edge view
Jorgensen et al. (1996)
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Brighter spirals spin faster. This relation found
by Tully & Fisher (1977) is given by
LT V4
log LT = 4 log V + cost
Brighter galaxies are more massive
Tully-Fisher relation
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gas (= Vc)
V = v/sini
kinematics:
v
imaging:
mT ,i
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NGC 3198
Optical
isophotes
Radio
HI map
Rotation
curve
on the
major axis
HI line
profile
W20
20%
v
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LT V4
90
Found in radio (HI) it works in optical too (HII)
Different definition of V: W20, WR, 2Vmax, 2Vflat
TF calibrated with galaxies of known distance
with B=0.25 e V=0.06 empirical arbitrary corrections to
correct for color differences between field and cluster galaxies.