Post on 28-Feb-2019
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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
• The main feature is the 4000A-‐break
I
Blue λ Red
• 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
The 4000A-break
Absorption Lines • Mainly caused by Atoms/Molecules in a star’s atmosphere that
absorb specific wavelengths
• Can also be due to COLD gas in the interstellar medium which can EXTRACT energy from the passing radia,on
ATOMS ATMOSPHERE
CORE RADIATION
• Caused by gas being ionized and heated and then re-‐radia,ng at specific allowed wavelengths
• Stars form from gas so are oSen embedded • Young stars ionise gas which releases radia,on at a specific
wavelength as it recombines
Emission Lines
GAS
Stellar spectrum
Emission lines
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 – [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
CaH&K
G-Band
Mg Na
Strong 4000A-Break
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
Some emission and some absorp6on indica6ng
both a young and old stellar popula6on.
Example Spectrum: Spiral
[OII]
CaH&K
Hβ
[OIII]
Hα
S2
Mg
4000A-Break
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
Example Spectrum: Irregular
[OII]
Hβ
[OIII]
Hα
S2
No 4000A-Break
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
or
cv!
=!
""
ccv
NCALIBRATIO
OBSERVED +=
!!
Example Radial Velocity
OII is at 4000A Hα is at 7030A
OII
Hα
km/s974,21)372737274000()c(v
CAL
CALOBS =!
=!
= c"
""km/s500,21)
6563467c(v ==
GALAXY IS MOVING AWAY AT ABOUT 21,750 km/s
d = vHo
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:
• This is analogous to the defini,on of Doppler shiS such that: from which follows:
• 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
oo
oz!!
!!! "
=#
=
cz v!
oHzcd =
Hubble constant
• 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 ?
This implies that for example its [OII] line, normally at 3727A, occurred at 4100A
Calcula,ng distances
MpcHzcdo
400751031.0 5
=!!
==
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
• 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.
PECULIARLRECESSIONARADIAL VVV ±=
Peculiar Veloci,es • What we measure from spectra:
VRADIAL or VLINE OF SIGHT
• Objects velocity w.r.t. our surroundings: VPECULIAR or VINFALL
• Velocity due to expansion: VRECESSION or VEXPANSION
VINFALL
VRECESSION
VBULK VBULK
VIRGO
THE GREAT ATTRACTOR PECULIARLRECESSIONARADIAL VVV ±=
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 ?
MpcskmHdVcz
dVH
cVV
cVz
o
INFALLRECESSIONo
INFALLRECESSIONRADIAL
//805010003000
=
+=
+==
!==
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 Inclination=0 Inclination=90 Majority of galaxies are somewhere in between
o o
b=Minor Axis
a=Major Axis
Calculating the Inclination • Assuming galaxies are circular: • Inclina,on, i, is given by:
• a=b, i=0 • b=0, i=90 NB: a is always measurable
abi =)cos(
b
a i
o
o
Line of sight velocity • When we measure the rota,onal velocity from a spectral
line we need to correct for inclina,on.
Hence if, i=90 , Vobs = Vrot
i=0 , Vobs = 0 )sin(iVV OBS
ROT =
b
a i Vobs
i
Line of sight Velocity=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 ?
01 5.70)3/1(cos == !i
skmi
VV
skmcV
OBSROT
OBS
/204)sin(
/19210339005.2 5
==
=!!="
=##
Note: 5/2 A
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 =>
– Spiral: some gas content – most gas converted =>
– Irregular: lots of gas – little gas converted =>
!
MHI
MSTARS
= 0.01" 0.1
MHI
MSTARS
= 0.1"1.0
MHI
MSTARS
#1.0
Warm dust and starlight in Antenae galaxy
NGC1512
Op,cal and radio image of NGC891
Distribution of Gas and Stars
STARLIGHT GAS (HI)
M82
NGC 3077
M81