Galaxy Spectra
•  Galaxy spectra
– 
– 
– 
– 
Continuum
Absorption Lines
Emission Lines
Typical Spectra
•  Elliptical
•  Spiral
•  Irregular
•  Galaxy motion
– 
– 
– 
– 
Radial velocity
Redshift
Redshift à distance
Peculiar velocities
•  Galaxy inclination
•  Gas, dust and stars
Continuum
•  The combina,on of many Black-­‐Body spectra spanning a range in temperatures •  This produces a fairly flat overall spectrum I
Blue
λ
•  The main feature is the 4000A-­‐break Red
The 4000A-break
•  Caused by: –  blanket absorp,on of high energy radia,on from metals in the stellar atmospheres –  the lack of hot blue stars •  Hence: –  Ellip,cals => A strong 4000A-­‐Break –  Spirals => A weak 4000A-­‐Break –  Irregulars => No 4000A-­‐Break Absorption Lines
•  Mainly caused by Atoms/Molecules in a star’s atmosphere that absorb specific wavelengths ATOMS
RADIATION
ATMOSPHERE
CORE
•  Can also be due to COLD gas in the interstellar medium which can EXTRACT energy from the passing radia,on Emission Lines
•  Caused by gas being ionized and heated and then re-­‐radia,ng at specific allowed wavelengths Stellar spectrum
Emission lines
GAS
•  Stars form from gas so are oSen embedded •  Young stars ionise gas which releases radia,on at a specific wavelength as it recombines M101
Emission nebulae (HII regions) in spiral arms Absorption / Emission Lines
•  Absorp,on Lines –  Need metals in stellar atmospheres or cold gas in the interstellar medium •  Implies –  Old stellar popula,on = old galaxy •  From –  Ellip,cals –  Spiral Bulges •  Emission Lines –  Need very hot gas and O and B type stars •  Implies –  Newly formed stars = star-­‐
forming/young galaxy •  From –  Spiral Disks –  Irregulars Typical Spectral features
•  Absorp,on – 
– 
– 
– 
– 
Ca(H) = 3933.7A Ca(K) = 3968.5A G-­‐band = 4304.4A Mg = 5175.3A Na = 5894.0 A •  Emission – 
– 
– 
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– 
– 
[OII] = 3727.3A Hδ = 4102.8A Hγ = 4340.0A Hβ = 4861.3A [OIII] = 4959.0A [OIII] = 5006.8A Hα = 6562.8A S2 = 6716.0A Example Spectrum: Elliptical
Strong
4000A-Break
Mg
G-Band
CaH&K
Na
S2
Strong absorp6on lines due to metals in the stellar atmospheres of a mostly low luminosity stellar popula6on. No evidence of any emission lines and hence no young stars and no gas Example Spectrum: Spiral
Some emission and some absorp6on indica6ng both a young and old stellar popula6on. Hα
[OIII]
Hβ
S2
[OII]
Mg
4000A-Break
CaH&K
Example Spectrum: Irregular
A strong emission-­‐line spectrum indica6ng many hot young stars hea6ng the gas which is re-­‐radia6ng at specific wavelengths which depend on the chemical composi6on of the gas Hα
[OIII]
[OII]
Hβ
No 4000A-Break
S2
Radial Velocities
•  Most galaxy spectra are REDSHIFTED, which means their spectral features are offset compared to those measured for gasses in the lab •  i.e., characteris,c combina,ons of lines are systema,cally offset to longer wavelengths •  This is interpreted as a DOPPLER shiS and implies that galaxies are moving away •  Posi,ve veloci,es: RECEEDING •  Nega,ve veloci,es: APPROACHING !" !v
=
"
c
  or !OBSERVED
v+c
=
!CALIBRATION
c
Example Radial Velocity
Hα
OII
OII is at 4000A
v = c(
Hα is at 7030A
"OBS ! "CAL
4000 ! 3727
467
) = c(
) = 21,974km/s v = c(
) = 21,500km/s
"CAL
3727
6563
GALAXY IS MOVING AWAY AT ABOUT 21,750 km/s v
d=
Ho
Reminder: Cepheid P-­‐L rela,on • 
• 
• 
• 
• 
• 
Well studied stellar objects
Very bright (Mv ~ -2)
Pulsate regularly (~ few days)
Pulsation period depends on luminosity
P-L relation calibrated to 220 stars via Hipparcos parallax distances (1997)
Measuring the pulsation and apparent magnitude for a distant Cepheid
provides a direct distance measurement given a known P-L relation. RedshiS •  We now know the Universe is expanding (see later lectures) •  An expansion implies a stretching of space-­‐,me. •  The more space-­‐,me there is between you and an object the faster it will appear to be moving away. •  It is the expansion which causes a galaxy’s spectrum to be REDSHIFTED: STATIONARY: DOPPLER SHIFT: REDSHIFT: REDSHIFT IS NOT THE SAME AS DOPPLER SHIFT RedshiS •  A useful parameter for cosmology is the redshiS: ! # !o "!
z=
=
!o
!o
•  This is analogous to the defini,on of Doppler shiS such that: from which follows: v
z!
c
zc
d=
Ho
Hubble constant •  Although this is the wrong interpreta,on of redshiS it is a good approxima,on for low-­‐z (z < 0.1) •  Hubble constant = rate of expansion in units of (km/s)/Mpc –  Current value ~75 km/s/Mpc Calcula,ng distances •  Using Hubble’s Law (which we’ll discuss more later) we can easily es,mate distances from a galaxy s measured redshiS.   e.g., If H0 = 75 km/s/Mpc and the redshiS is measured to be 0.1 what is its distance ? zc 0.1! 3 !105
d=
=
= 400 Mpc
Ho
75
  This implies that for example its [OII] line, normally at 3727A, occurred at 4100A Peculiar Veloci,es •  Gravita,onal aprac,on between galaxies and larger objects (clusters, groups, superclusters, filaments) •  Velocity we measure is not just the expansion of the universe VRADIAL = VRECESSIONAL ± VPECULIAR
•  For example the MW is falling into Virgo which in turn is falling into The Great Apractor. •  If we know a galaxy’s peculiar velocity we can correct for this addi<onal velocity component. Peculiar Veloci,es •  What we measure from spectra:   VRADIAL or VLINE OF SIGHT VRECESSION VINFALL VIRGO •  Objects velocity w.r.t. our surroundings:   VPECULIAR or VINFALL •  Velocity due to expansion: VBULK VBULK   VRECESSION or VEXPANSION VRADIAL = VRECESSIONAL ± VPECULIAR
THE GREAT ATTRACTOR Example •  The MW is falling towards Coma at Vinfall =1000km/s. •  Distance is 50 Mpc (from Cepheids) •  If the redshiS, z = 0.01, what is H0 ? VRADIAL VRECESSION ! VINFALL
z=
=
c
c
VRECESSION cz + VINFALL 3000 + 1000
Ho =
=
=
d
d
50
H o = 80km / s / Mpc
Note: Be very careful with the sign of VINFALL & VPECULIAR, it is intui,ve Typically VINFALL is subtracted but for VPECULIAR it will depend on the direc,on of the peculiar velocity (towards us= -­‐ve, away= +ve) Galaxy Inclination
FACE-ON
EDGE-ON
o
Inclination=0
o
Inclination=90
b=Minor Axis
Majority of galaxies are somewhere in between a=Major Axis
Calculating the Inclination
•  Assuming galaxies are circular: •  Inclina,on, i, is given by: b
cos(i ) =
a
b
i
o
•  a=b, i=0 o
•  b=0, i=90   NB: a is always measurable a
Line of sight velocity
•  When we measure the rota,onal velocity from a spectral line we need to correct for inclina,on. i
Line of sight Velocity=Vobs   Hence if,   i=90 , Vobs = Vrot   i=0 , Vobs = 0 b
i
a
VROT
VOBS
=
sin(i )
Vobs
Example
  A long slit spectrum aligned along a galaxy s major axis indicates a varia,on in the [OII] line of 5A, The midpoint of the [OII] line is observed to be at 3900A and the major-­‐to-­‐minor axis ra,o is 3. What is the rota,onal velocity of the outermost stars ? i = cos !1 (1 / 3) = 70.50
Note: 5/2 A
"#
2.5
c=
! 3 !105 = 192km / s
#
3900
V
= OBS = 204km / s
sin(i )
VOBS =
VROT
Note: λ=3900 and not 3727 (as reference is to the galaxy centre) Stars, Dust and Gas in Galaxies
•  Dust mass is negligible but can block up to 90% of the light and provides a good indica,on of where the fresh stars are •  Stars form from gas in galaxy •  In the high-­‐density regions the gas is converted into Stars –  Elliptical: very little gas content
–  ~ all gas converted into stars =>
M HI
= 0.01" 0.1
MSTARS
–  Spiral: some gas content
–  most gas converted
=>
–  Irregular: lots of gas
–  little gas converted
=>
M HI
= 0.1"1.0
MSTARS
M HI
# 1.0
MSTARS
Warm dust and starlight in Antenae galaxy NGC1512 Op,cal and radio image of NGC891 Distribution of Gas and Stars
M82
M81
NGC 3077
STARLIGHT
GAS (HI)