texas symposium, melbournedecember 14th 2006 theoretical properties of ly cooling radiation’ mark...

Texas Symposium, MelbourneDecember 14th 2006

Theoretical Properties of Ly Cooling Radiation’Mark Dijkstra (CfA)

Collaborators: Z. Haiman, M.Spaans & A. Lidz


‘Theoretical Properties of Ly Cooling Radiation’Mark Dijkstra (CfA)

Collaborators: Z. Haiman & M.Spaans

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motivation


‘Theoretical Properties of Ly Cooling Radiation’Mark Dijkstra (CfA)

Collaborators: Z. Haiman, M.Spaans & Lidz

Outline of talk

• Gas cooling & Ly emission• Observable properties of Ly cooling emission (DHS 06a,b)

• Observable properties of continuum cooling radiation (D, submitted to MNRAS)


Gas cooling

Primordial Cooling curve

T < 6.e4 K: H cooling dominates

Thoul & Weinberg ‘95


The Impact of Cooling on Gas Collapse

• Adiabatic collapse:Gas shell virializes at r=rmax/2

And heated to virial T of halo

• Turn on cooling mechanismtcool<<tdyn

Gas cools to 1e4 K rapidly


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Tvir =106 M

1011M⊕

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2 / 31+ zcol

5

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The Impact of Cooling on Gas Collapse.

• Gas collapses at T=1e4 K (no virial shock, for M<Mcrit)

• Cooling @ T=1.e4 K is dominated by collisional excitation of H– Collisions to 2p and 2s

states of H– 2p 1s: Ly cooling– 2s 1s: 2- emission



Gas cooling.

• Gas cooling is dominated by Ly emission

• Spatially extends up to ~100 kpc.• Luminosities in the range L=1e42-

1e44 erg/s

(Haiman et al ‘00, Fardal et al ‘01, Yang et al ‘06)

Yang et al ‘06


Lyman Alpha ‘Blobs’

Steidel et al. (2000)

• Observed Spatially extended Ly emission up to ~ 100 kpc.

• Several tens have been discovered at z=3-5. (e.g. Matsuda et al, 2004; Saito et al, 2006/2007)

• Luminosities ~ 1e42-1e44 erg/s

• Powered by cooling radiation?

• Alternatives:– Obscured starburst/AGN– Shock heating by superwinds.– Fluoresence (next talk)


Ly Cooling Radiation: Properties

• Observational signatures of cooling radiation?

• Cooling clouds are optically thick to Ly-> radiative transfer (RT). Well studied problem (> 60 years)

HARD


Ly Transfer

• A simple problem: a Ly source inside a uniform static neutral H cloud.

• Calculate emerging spectrum Harrington ‘73, Neufeld ‘90, DHS06a

• Generally no analytic solution can be found:

– Monte-Carlo.


Ly Transfer

• Calculate Ly transfer through series of models representing cooling clouds

• Goal: To extract basic properties of Ly cooling radiation

• Use Monte-Carlo: follow individual photons through the collapsing cloud.

• The code is reliable.


Ly Transfer

• Cute: deuterium• N_H=2e19

(static)



Frequency->

Data: Smith & Jarvis, 2007

Radius

Su

rfa

ce b

righ

tne

ss

• Use Monte-Carlo method to calculate emerging Ly spectrum+surface brightness profiles.

• Result 1: Radiative Transfer of Ly through collapsing (optically thick) gas results in a blueshift of the line. The opposite is true for outflows.

• Frequency off-set of Ly-line constrains gas motion.



• What if one can’t tell whether there is an off-set?• Ly cooling radiation has frequency dependent

surface brightness profile:

Red: reddest 15% of Ly

Blue: bluest 15% of Lya



• Why a frequency dependent surface brightness profile?



• Caution:Spectra shown are affected by IGM. The impact of the IGM is non-trivial

(e.g. Santos ‘04; D, Wyithe & Lidz ‘07)



• Has cooling radiation from cold accretion been seen?

– Perhaps (e.g. talk by M.Rauch)– Saito et al’07 find asymmetric Ly profiles

with enhanced blue emission.



• Cooling radiation seen?

Wilman et al ‘05


Cooling Radiation: Properties

Part II(=short)


2- Cooling Radiation: Properties

• 1 collisional excitation 1s2p is accompanied by 0.6 excitations 1s2s • 2s1s++, 2 photons have combined energy of 10.2 eV.• Results in continuum emission redward of Ly-a. The spectrum of this

emission has been calculated by Spitzer & Greenstein (‘51)



• How weak is continuum? Prominence of Ly line relative to continuum is quantified by the equivalent width (EW)

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FLyα = EW × f λ (1216)

EW=1000-1500 Å

Emitted restframe EW

However IGM opaque to Ly;

Observed restframe EW

EW< 200 Å



• Shape of continuum can also ‘betray’ cooling

Cooling powered or resonant scattering or recombination emission?


‘Theoretical Properties of Ly-a Cooling Radiation’Mark Dijkstra (CfA)

Collaborators: Z. Haiman & M.Spaans

Summary• Gas cooling is accompanied by copious Ly emission (especially the

‘cold’ mode).

• Observational signposts of this emission are:– Intrinsic blueshift of line– Steepening of surface brightness towards bluer Ly wavelength– Faint continuum redward of Ly with weird spectrum.

– Caution: Absence does not immediately rule out cooling radiation.

• Currently, no convincing observational evidence exists, but d/dt(almost convincing) > 0.

texas symposium, melbournedecember 14th 2006 theoretical properties of ly cooling radiation’ mark...

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