The Pressure and Polarization of Scattered Ly Mark Dijkstra (CfA)

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

2. 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.

3. 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

4. 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

5. 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)

6. 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:

7. 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.)

8. 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

9. 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

10. 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

11. 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

12. 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.

13. III: Polarization of Scattered Lya • Chicken chicken chicken chicken chicken (Chicken & Chicken 08) • Chicken chicken chicken chicken chicken chicken. 45% Polarization 18% Chicken Chicken

14. 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

15. 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)

16. 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

17. 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

18. 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.