the pressure and polarization of scattered ly mark dijkstra cfa n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
The Pressure and Polarization of Scattered Ly Mark Dijkstra (CfA) PowerPoint Presentation
Download Presentation
The Pressure and Polarization of Scattered Ly Mark Dijkstra (CfA)

Loading in 2 Seconds...

play fullscreen
1 / 18

The Pressure and Polarization of Scattered Ly Mark Dijkstra (CfA) - PowerPoint PPT Presentation


  • 91 Views
  • Uploaded on

The Pressure and Polarization of Scattered Ly Mark Dijkstra (CfA). Main Collaborators: Adam Lidz, Avi Loeb (CfA) Stuart Wyithe (Melbourne), Zoltan Haiman (Columbia). The Pressure and Polarization of Scattered Ly. Summary of Talk:

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'The Pressure and Polarization of Scattered Ly Mark Dijkstra (CfA)' - thiery


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
the pressure and polarization of scattered ly mark dijkstra cfa
The Pressure and Polarization of Scattered Ly Mark Dijkstra (CfA)

Main Collaborators:

Adam Lidz, Avi Loeb (CfA)

Stuart Wyithe (Melbourne), Zoltan Haiman (Columbia)

the pressure and polarization of scattered ly
The Pressure and Polarization of Scattered Ly

Summary of Talk:

  • Theoretically, the IGM is expected to be (very) opaque to Lya radiation, even at frequencies redward of Lya (restframe).
  • The relevance of this statement depends on the prominence of outflowing HI gas in the ISM + the fraction of Lya that (back)scatters off this gas.
  • Backscattered Lya radiation is among the most highly polarized signals in our Universe. Ly polarimetry provides additional (independent) constraints.
  • The pressure exerted by Ly radiation itself may be important in driving outflows of HI gas in the ISM.
the pressure and polarization of scattered ly1
The Pressure and Polarization of Scattered Ly
  • The following discussion is driven by two observations (but applies in general):
  • To infer ionization state of IGM, we need to understand Lya RT through the IGM.
  • The same applies when interpreting Equivalent Widths of Lya emitting galaxies (IGM affects Lya, but not the continuum).

Kashikawa+ 06

Shimasaku+ 06

Cum. Number density

EW

i igm opacity to lya
I: IGM Opacity to Lya
  • ‘First-order’ treatment the IGM:

Lya line before IGM processing assumed to be a Gaussian with FWHM set by bulk motions of HII regions within galaxy (~vcirc of host DM halo)

  • Photons emitted blueward of Lya resonance eventually redshift into Lya resonance where IGM is opaque: transmission is ~TIGM (1) blueward (redward) of Lya resonance, I.e T>0.5.

Faucher-Giguere+ 08

BlueRed

-ln TIGM

i igm opacity to lya1
I: IGM Opacity to Lya
  • This prescription for the IGM only works when galaxies are randomly distributed throughout the Universe.
  • However, in CDM galaxies preferentially form in overdense regions of the Universe and are highly clustered.
  • When quantifying the opacity of the IGM around Lya emitting galaxies one must account for (e.g. D. Lidz & Wyithe 07, Iliev+08):
    • local overdensity of IGM gas around galaxies
    • Infall of IGM gas near galaxies (gas is *not* comoving with Hubble flow )
    • Enhancement of local ionizing background (due to source clustering)
i igm opacity to lya2
I: IGM Opacity to Lya
  • Impact IGM in more realistic model.
  • Predicted schematic lines are given by the red lines.
  • Extra absorption by overdense IGM close to the galaxy.
  • Infalling gas can scatter photons emitted redward of Lya:
i igm opacity to lya3
I: IGM Opacity to Lya
  • IGM transmits only 5-30% of Lya -> intrinsic EWs higher than previously assumed?
  • Infalling IGM gas can give rise to P-cygni type profiles (e.g D, Haiman & Spaans 06).
  • Transmission @ z=5.7 ~ 1.0-1.3 times Transmission @ z=6.5 (enough to explain obs.)
ii radiative transfer in the ism
II: Radiative Transfer in the ISM
  • However, the impact of the IGM is depends on prominence ‘back-scattering’ mechanism.
  • Depending on velocity + HI column density, majority of Lya photons can escape from galaxy with large enough redshift for the IGM to become irrelevant.

REDBLUE

Lya source

Verhamme+06

ii radiative transfer in the ism1
II: Radiative Transfer in the ISM
  • Backscattering mechanism can explain some observed Lya line shapes quite beautifully (Verhamme+08,Schaerer & Verhamme08)
  • Note that spectral fitting yields not always yields unique constraints on NHI and shell speed. Additionally, unclear whether fits exists in which an IGM is a-priori included. Possible way to distinguish different models: polarization?

BlueRed

D, Haiman & Spaans 06

iii polarization of scattered lya
III: Polarization of Scattered Lya
  • Scattered photons can appear polarized to an observer (electric vectors of photons have some preferred directions).
  • Consider photon whose path is indicated with
iii polarization of scattered lya1
III: Polarization of Scattered Lya
  • Scattered photons can appear polarized to an observer (electric vectors of photons have some preferred directions).
  • Lya scattering can in practise be described accurately by Rayleigh scattering, for which scattering by  deg, results in 100[sin2 /(1+cos2 )] % polarization.

Electric vector of photon

Propagation direction of photon

iii polarization of scattered lya2
III: Polarization of Scattered Lya
  • Compute polarization of backscattered Lya radiation using a Monte-Carlo radiative transfer code (D & Loeb ‘08, also see Lee & Ahn ‘98). In this code:
    • the trajectories of individual photons are simulated as they scatter off H atoms (microphysics of scattering is accurate)
    • can attach a polarization vector to each photon, and
    • compute observed quantities such as the Lya spectrum, surface brightness profile, and the polarization
  • Polarization quantified as P=|Il-Ir|/(Il+Ir). Single photon contributes cos2 to Il and sin2 to Ir (Rybicki & Loeb 99).
  • Apply Monte-Carlo code to a central Lya emitting source, completely surrounded by a thin, single, expanding shell of HI gas (as in Verhamme+06,08). Free parameters are NHI and vexp.
iii polarization of scattered lya3
III: Polarization of Scattered Lya
  • Chicken chicken chicken chicken chicken (Chicken & Chicken 08)
  • Chicken chicken chicken chicken chicken chicken.

45%

Polarization

18%

Chicken

Chicken

iii polarization of scattered lya4
III: Polarization of Scattered Lya
  • Lya can reach high levels of polarization (~40%, D & Loeb ‘08)
  • Polarization depends on NHI and vsh, and therefore provides additional constraints on scattering medium (frequency dependence of polarization also constrains sign of vsh , see D & Loeb ‘08).

45%

Polarization

18%

Impact parameter

Impact parameter

vii pressure of scattered lya
VII: Pressure of Scattered Lya
  • What (when they exist) drives these HI outflows?
  • Supernovae? Continuum radiation pressure? Lya radiation pressure?
  • Assume each supernova ejects 10 Msun of material at 3000 km/s. Further assume 1 SN per unit SFR per 100 yr. Then, the momentum injection rate due to SN ejecta (Murray+05)
  • The total momentum flux in the radiation field is
  • Total pressure exerted by continuum radiation may be equal (or greater) than the kinetic pressure exerted by supernova exjected (Murray+05)
vii pressure of scattered lya1
VII: Pressure of Scattered Lya
  • Total momentum flux in Lya is fLya Lbol/c, and fLya~0.07-0.24
  • However, Lya radiation can be efficiently ‘trapped’ by a neutral medium, which boosts the total Lya momentum flux by a factor MF.
  • MFrelates to the total number of times a Lya photons -on average- bounces back and forth between neutral walls of the HI shell (D & Loeb 08).

HI

vii pressure of scattered lya2
VII: Pressure of Scattered Lya
  • Compute MF for a suite of models (NHI,vexp) using the Monte-Carlo RT code.
  • Results: MF can exceed unity by > 1 order of magnitude. In these cases, Lya radiation pressure dominates over continuum radiation + supernova pressure.

D & Loeb 08

MF>10

the pressure and polarization of scattered ly2
The Pressure and Polarization of Scattered Ly

Summary of Talk:

  • Theoretically, the IGM is expected to be (very) opaque to Lya radiation, even at frequencies redward of Lya (restframe).
  • The relevance of this statement depends on the prominence of outflowing HI gas in the ISM + the fraction of Lya that (back)scatters off this gas.
  • Backscattered Lya radiation is among the most highly polarized signals in our Universe. Ly polarimetry provides additional (independent) constraints.
  • The pressure exerted by Ly radiation itself may be important in driving outflows of HI gas in the ISM.