Accounting For Something You Dont Want To: Radiation Field Ni Nick - - PowerPoint PPT Presentation

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Accounting For Something You Dont Want To: Radiation Field Ni Nick - - PowerPoint PPT Presentation

Nov 19, 2022 377 likes •567 views

Accounting For Something You Dont Want To: Radiation Field Ni Nick Gnedin ck Gnedin Why We Need To Unless we are modeling reionization, we do not really want to add an RT solver to our code We also do not need to reionization

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SLIDE 1

Ni Nick Gnedin ck Gnedin

Accounting For Something You Don’t Want To: Radiation Field

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SLIDE 2

Why We Need To

  • Unless we are modeling reionization, we do not

really want to add an RT solver to our code…

  • We also do not need to – reionization has little

effect on star formation in galaxies.*

  • There are 3 physical effects that must account

for the radiation field:

  • Molecular hydrogen formation
  • Gas cooling and heating
  • Radiation pressure

* And yes, claims to the contrary that you might have heard are wrong!

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SLIDE 3

HI-H2 Transition

  • There exist several models for that transition, all

mutually compatible.

  • Molecular gas determines star formation rate,

atomic gas does not.

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SLIDE 4

HI-H2 Transition

  • One small correction: line overlap enhances H2

formation in very low metallicity environments. (Gnedin & Draine 2014)

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SLIDE 5

HI-H2 Transition

  • Dependence of the HI-H2 transition on radiation

field is weak, only ~ logarithmic for RF like in the Milky Way and stronger.

  • Hence, one does not need to know the radiation

field very precisely.

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SLIDE 6

Cooling Function 101

  • Most general cooling and heating functions:

One needs a code like Cloudy to compute L in its full glory.

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SLIDE 7

Cooling Function 101

  • Simplification #1: optically thin
  • Simplification #2: ionization/excitation balance
  • Often, this is what is actually called a “cooling

function”.

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SLIDE 8

Sunny Places are Warm

log10(U) = -3 log10(U) = -2.5 log10(U) = -2 log10(U) = -1.5 log10(U) = -1 log10(U) = -0.5 log10(U) = 0 log10(U) = 0.5 log10(U) = 1 log10(U) = 1.5 log10(U) = 2 log10(U) = 2.5 log10(U) = 3

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SLIDE 9

Cute Example

  • A host halo of a quasar:
  • No cooling at all – novel feedback mechanism!
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SLIDE 10

Does It Really Matter?

  • Proximity zones

around galaxies are not large.

  • Cooling/heating

functions are strongly RF dependent only in the ISM.

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SLIDE 11

99.9999% Of the Volume Is Easy

  • When the cosmic background dominates

( ):

  • …
  • Wiersma, Schaye, & Smith 2009
  • …
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SLIDE 12

00.0001% Of the Volume May Not Matter

  • We never model stellar feedback, we always

model SF+feedback together.

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SLIDE 13

In A Relationship Both Sides Matter

3/2 model Linear model Blastwave with tBW=30Myr

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SLIDE 14

00.0001% Of the Volume May Not Matter

  • We never model stellar feedback, we always

model SF+feedback together.

  • One can just add cooling to the pool.
  • In fact, that’s what most people actually do!

SF Feedback Cooling SF Feedback

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SLIDE 15

If You Are Stubborn: RT-Lite

  • Disk galaxies are RT solvers.
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SLIDE 16

If You Are Stubborn: RT-lite

  • Climb the tree to the right branch.

MFP MFP

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SLIDE 17

Conclusions

  • How to include radiation fields in galaxy

formation simulations? Don’t.

  • In the worst case when you must, use a simple

estimate and your code tree data structure: RF = SFR within the MFP.

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SLIDE 18

The End