are young galaxies visible? - partridge & peebles 1967, apj 147, 686
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Are Young Galaxies Visible?R.B. Partridge & P. J. E. Peebles 1967
(Astrophysical Journal, vol. 147, p.686)
Christian Herenz
Extragalactic Science Club 2010
December 14th, 2010
Citations from ADS
The Authors
R. B. PartridgeP. J. E. Peebles
Outline
I Aim, Method & ResultsI Galaxy Model
I Time of FormationI Luminosity
I Element ProductionI Stellar Luminosity Function
I SpectrumI Predicted ObservabilityI Unsuccessful Hunt for Primeval GalaxiesI Properties of Lyα-Emitters
Are Young Galaxies Visible?
I Aim: To asses possibility of detection of individual galaxiesin an early evolutionary stage.
I Method:I Theoretical model of galaxy formation and evolutionI Model based purely on assumptions, (almost) no
connection to observations. (exc. CMB)I Derivation of observational properties of these “Young
Galaxies”.I Results:
1. “Young Galaxies” go through a stage of high luminosity(after t ∼ 108 years for ∆t ∼ 107 years)
2. For a Milky-Way like Galaxy: L ∼ 1046 erg s−1
3. 6 - 7 % of total L in Lyα-Line
The Model for the Formation of Galaxies
I Model to Adress the 2 Questions:1. Time of formation2. Luminosity of the Galaxy
I Focus on: Milky Way like galaxies(because observational values needed for computationsare available here)
I Monolithical collapse:Galaxies evolve in isolation. No dark matter.
I 2 important consequences in their picture:1. Halo stars preserve memory of the maximum size of
proto-galaxy.2. Onset of star formation = well defined moment at earliest
stages of inital collapse
Time of Formation (1)- Spherically symmetric modell- Newtonian Approximation
R(t) = − GM
(R(t))2 (1)
- Solution to Eq. (1) in parametric form1:
R(η) =
(3M
32πρp
)1/3
(1−cos η) ; t(η) =
(3
32πGρP
)1/2
(η−sin η)
(2)- Material stops expanding (i.e. η = π) at
tP =
√3π
32Gρp(3)
(tP - Protogalaxy formation time)1Multiplication with R leads to DEq. of Cycloid
Time of Formation (2)Input in (3):
I Mass of Protogalaxy:
MP = 1.2× 1011M (4)
I Radius of Protogalaxy:
RP = 20 kpc (5)
Then Eq. (3) gives:
tP = 1.4× 108 years (6)
From Eq. (2) and (3) we see→ free fall time to highly contracted state:
2× tP & 3× 108 years
Minimum time to form a disk.
Luminosity
Two ways to estimate luminosity of the young galaxies:1. Element Production2. Luminosity of the First Generation of Stars
In general:Young galaxies must have been much brighter than theyare now.
Luminosity - Element Production- At the time the disk has formed→ most metals have been produced by 1st gen. stars→ Lower Limit of Luminosity:
Ly ' 0.007Mc2 ∆X∆t' 5× 1047 ∆X
∆t8erg s−1 (7)
(∆t8 in 108 years)
- For MW: M/L = 10×M/L→ Young Galaxy brighter than now by
Ly
L' 1× 104 ∆X
∆t8(8)
→ Put in: ∆X = Z = 0.02 & ∆t8 = 0.3 (c.f.)
Ly & 3× 1046 erg s−1 or Ly & 700L (9)
Luminosity - Stars
Normalize:∑
MVΨ(Mv )M(MV ) = 1.2× 1011M [Eq. (4)] !
Luminosity for 1 MBol. = 0 star: LMBol.=0? = 3× 1035 erg s−1
⇒ Ly = 7.5× 1046 erg s−1 (10)
Assumption here: All Stars created at once...
...but consider timescales (Cols. 7 & 8)
- High M stars contribute most L (τM . 107 years)- Formation: τform. τM (τM ∼ 106)→ Adopt: ∆t = 3× 107 years [as in Eq. (9)]! ∆t & τM for high M, only fraction ε of them radiates in ∆t
with Col. (8) ⇒ Ly = 2.5× 1046 (11)Compare with Eq. (9)
Spectrum- Avg. Teff. of O & B type stars: Teff. = 30000 K- Stellar opacity→ Lyman decrement (stellar atmosphere models):
Fλ(912+)
Fλ(912−)∼ 18 (12)
⇒ 10 % of total flux is shortward of 912 A
Spectrum would look like this if ISM HI is distributed uniformly, but...
...assuming HI distributed in clouds with nHI = 10− 100 cm−3
→ optically thick case, γion.’s get converted to Lyman-γ’s→ resonant Lyα line!⇒ as much as 2/3 of γion. could be converted to Lyα
(6-7 % of total flux!)
s p d f
n=1
n=2
n=3
n=4
n=5
γLyα
(γγ)
...in the most extreme case, the spectrum would look like this:
ObservabilityDepends on Cosmology, here ΩM = 1, ΩΛ = 0 andH0 = 70 km s−1 Mpc−3 are used.
Observational Parameters:
I Redshift: z ∼ 10− 30I → Radiation: λ ∼ 1− 3µI → Angular Diameter: & 5”I → Probability of intersecting
random line of sight: & 5 %
Prediction:
With a D = 2m telescope, a narrow-band (NB) filter (∆λ = 20A) & 5 min integration time a primeval galaxy at z = 7 could be
seperated from sky-background.( mλmax.
AB ∼ 23 ⇒ mNBAB ∼ 19 )
Unsuccessful Hunt for Primeval GalaxiesI Basic problem faced by observers: PGs should be
I relatively bright &I large &I numerous.
I So where are these objects?2
I Monolithical collapse model is not appropiateI Dark Matter based models emphasize gradual assembly of
massive galaxies...I ...moreover, radiation transfer in HI regions is more
complicated (attenuation by dust etc.).
⇒ PGs are less massive!⇒ Flux limits were optimistically bright!
2Pritchet 1994, PASP 106, 1052
Observational Properties of Lyα-Emitters
- Mostly used: Narrowband-Imaging (here: z = 4.55 LAEs3 in afield around a QSO- Typical observed parameters of LAEs:
LLyα ∼ 1042 − 1043 erg s−1, rLyα ∼ 1 kpc,MLAE ∼ 109 − 1010MAvg. age of stars ∼ 106 − 108 years, z ∼ 3− 8
3Hu & McMahon 1996, Nat 382, 231
The End
“Cosmologists are always wrong, but never in doubt.”
Robert P. Kirshner4
I Questions & DiscussionI Compare with contemporary views of galaxy formation...I ...so why is this paper still important today?
I Thanks for your attention!
4PNAS 101, 8-13 (2004)
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