Lya as a Probe of the (High-z) Universe

Transcription

1 Lya as a Probe of the (High-z) Universe Mark Dijkstra (CfA) Main Collaborators: Adam Lidz, Avi Loeb (CfA) Stuart Wyithe (Melbourne), Zoltan Haiman (Columbia)

2 Lya as a Probe of the (High-z) Universe Outline of Talk: I: Background: The redshifted Lya line provides an excellent tracer of high-z galaxies. II:Observed Luminosity Functions (UV + Lya) of LAEs *in combination with* Equivalent Width distributions probe both the ionization state of the IGM + the (stellar) content of high-z galaxies. III: Lya emission is among the most highly polarized signals in our Universe. Lya polarization encodes additional information on the ISM (+IGM). IV: Lya radiation pressure may be important in driving outflows of HI gas in the ISM (and possibly the IGM).

3 I: Introduction Young star forming galaxies are luminous in Lya (e.g. Partridge & Peebles 1967) HII regions surrounding O and B stars convert 68% of ionizing photons into Lya. Flux Flux Frequency Partridge & Peebles 67 Wavelength Schaerer 02 L lya ~0.07 L bol (PP67) L lya ~0.24 L bol (Schaerer 02)

4 I: Introduction Prominence of Lya line depends on abundance of O and B stars-> I.e. the IMF. Also on the surface temperature of the stars-> I.e. on gas metallicity (e.g Schaerer 03). Prominence of Lya line is quantified by its Equivalent Width L Ly" = EW # f $ (1216) EW=70 A EW= A

5 I: Introduction For comparison, observed EW distributions LBGs, z=3 LAEs, z=5.7 Number EW EW Shapley+ 03 Shimasaku+ 06

6 I: Introduction Lya photons scatter in the interstellar medium blue: Lya red: Ha Ostlin et al, 2008 green: rest-frame UV >90% of Lya in diffuse component: scattered to the observer (Ostlin et al, 2008; Atek et al 2008, Hayes et al 2008)

7 I: Introduction Lya photons scatter in the intergalactic medium Residual HI in mildly overdense ( δ> 1) regions produce absorption lines to sightlines to high-z quasars (denser=more opaque). Galaxies populate the densest regions. Scattering of Lya in IGM is therefore expected. When quantifying the IGM opacity, one needs to 1. Account for local overdensity near galaxies 2. Resulting departures in Hubble flow 3. Enhanced local ionizing background (see e.g. D. Wyithe & Lidz 07, Iliev + 08) From: Faucher-Giguere+ 08

8 I: Introduction 7-25% of the total bolometric luminosity of high-redshift star forming galaxies is emitted in the Lya line. The prominence of the Lya line is quantified by the equivalent width (EW). Regular star forming galaxies emit a maximum EW of EW~200 A. Primordial (pop III) galaxies may emit EW~1500 A. Lya scatters in both ISM + IGM, and understanding Lya radiative transfer is key to using Lya emitters as a probe of the high-redshift Universe.

9 II: Brief history of the Universe. Big bang Time Now Dark Ages Epoch of Reionization No observations From: the internet. ~1 billion yrs (A.B.B)

10 II:Constraints from the LFs + EW distribution of LAEs. (Cumulative) [Lya/UV] Luminosity function: number density of galaxies brighter than x in Lya / the rest frame UV EW distribution: distribute galaxies among bins of a given EW-range data: number density of galaxies with UV luminosity > L UV theory: number density of dark matter halos with mass > M what is the relation between M and L UV? = what is star formation efficiency? Kashikawa+ 06

11 II:Constraints from the LFs + EW distribution of LAEs. Assume a fixed fraction f * of baryons is converted into stars over a time scale ε DC x t hub (e.g. Wyithe & Loeb 2006, D, Wyithe & Haiman 07). L UV =8x10 27 erg/s/hz (SFR/ 1 M sun /yr) (e.g. Kennicut 1998). Constrain parameters f * and ε DC with data. Texas D. & Symposium, Wyithe 07 Melbourne

12 II:Constraints from the LFs + EW distribution of LAEs. UV-data constrains star formation efficiencies: only free parameter that can be constrained by Lya LF: observed Lya luminosity / SFR, I.e. the observed Lya Equivalent width D & Wyithe 07 Shimasaku+ 06 EW~ 50 Angstrom

13 II:Constraints from the LFs + EW distribution of LAEs. Model that has a 1-to-1 correspondence between Lya luminosity & SFR cannot simultaneously reproduce LFs + EW dist. Expand model: use Lya luminosity that decreases in time (UV luminosity remains constant) Simple approach, repeat analysis, but assume that Lya luminosity is high (EW III ) for a fraction f III -> fit to mean observed EW D & Wyithe 07 EW III ~600 Ang, f III ~0.1 (10 Myr): EW is larger than can be generated by normal stellar population

14 II:Constraints from the LFs + EW distribution of LAEs. Simplified model in which galaxies are bright in Lya for ~ 10 Myr, and then fainter for ~90 Myr 1) reproduces observed LFs+ EWs (it was constructed this way) 2) implies that large EW emitters are ~10 Myr, while lower EW emitters are older (consistent with observations, bimodal distribution of Lya ages? See Finkelstein+ 08, talk by Wang) 3) alleviates friction between EW-distributions from Lya and UV selected galaxies D & Wyithe 07 (data from Shimasaku+06, Stanway+06)

15 II:Constraints from the LFs + EW distribution of LAEs. Model furthermore constrains to what degree Lya attenuation (dust or IGM) evolves between z=5.7 and z=6.5 D & Wyithe 07 T 57 T 65 =1.2 ± 0.1 (68%) We observe ~20% more Lya that was emitted by galaxies from z=5.7 than z=6.5. Theoretically, an evolving cosmic density of ionized IGM could result in 30% (D, Wyithe & Haiman 07)-> Even if entirely due to IGM, not necessarily evidence for reionization.

16 II: Constraints from the EWs + LFs In recent years, the quality of published luminosity functions at high-z Lya emitters + their equivalent width distribution has increased significantly (see e.g. Ouchi et al 2008) Models that assign 1-to-1 relations between SFR and Lya cannot simultaneously reproduce both LFs + EW distributions. However, this is possible with a model in which star forming galaxies go through a short phase during which they are very bright (EW~600 A) in Lya. This model shows that =1.2 ± 0.1, I.e. at z=5.7 20% more Lya is transmitted to the observer. T 65 T 57 T 57 In principle =1.3, solely because of resonant scattering in an ionized IGM. T 65 However, this conclusion may be modfied by RT effects in the ISM of galaxies.

17 III:The Polarization of Scattered Lya How important is RT in the ISM? Local star burst galaxies, and also z=3 LBGs, show P-Cygni type Lya lines. Schaerer & Verhamme 08 (data Pettini+02)

18 III:The Polarization of Scattered Lya The presence of single outflowing (super)shells of HI gas with N HI ~ cm -2 can naturally explain the shape of many observed emitters (see Verhamme+ 08). Note that sometimes, spectral fitting yields no unique constraints on N HI and shell speed. (some additional caution is needed: an infalling overdense IGM can also result in P-Cygni type profiles, e.g. D. Lidz & Wyithe 07).

19 III:The Polarization of Scattered Lya Determining shell speed + HI column is important because it sets the amount by which backscattering redshifts Lya photons: the more these photons redshift, the less likely they are to resonantly scatter in the IGM. Scattered photons can appear polarized to an observer. Photon scatters. Perform Monte-Carlo Lya RT transfer calculations that include polarization.

20 III:The Polarization of Scattered Lya Lya can reach high levels of polarization (~40%, D & Loeb 08) Polarization depends on N HI and v sh (additional constraints on scattering medium). How is this possible given that these thin shells are extremely opaque to Lya (which results in numerous scattering events, and hence little polarization)?

21 III:The Polarization of Scattered Lya Because of the Doppler effect, the Lya photons hit that the moving HI shell appear redder (I.e. off-resonance) to atoms in the shell, and can pass through more easily. E.g., consider the shell with N HI =10 19 cm -2 Solid line: PDF of N scat Histrogram: average polarization D & Loeb 08 The shell is not opticall thick, and the majority of photons scatter only once.

22 III:The Polarization of Scattered Lya Furthermore, the more photons scatter, the less polarized. For this reason, polarization increases toward longer (shorter) wavelengths when scattering off an expanding (contracting) HI shell. D & Loeb 08

23 III:The Polarization of Scattered Lya Evidence for outflows of HI in the ISM of galaxies Lya line shape can constrain both outflow speed + HI column density However, this is not true in all galaxies: multiple solutions possible (+ possible confusion with IGM infall signature). Lya that scatters off outflows is polarized: level of polarization yields extra constraints on N HI and outflow speed. Determining HI and speed of outflows is crucial when assessing the importance of e.g. resonant scattering of Lya in the IGM.

24 IV: Lya Radiation Pressure What drives these outflows? Ans: energy input by supernovae. What about Lya radiation pressure itself? Order of magnitude estimate: spherical HI shell with N HI =10 20 cm -2, r=1 kpc, surrounding a Lya source of luminosity L lya =10 43 erg/s for 50 Myr. If all photons scatter once then the shell is accelerated to 250 km/s. However, photons scatter more than once, and the total momentum transfer rate from Lya to shell (I.e. the total force) is L F Lya = M Lya F c Here M F is some force multiplier that depends on both shell speed + HI column. We can compute M F for a suite of models using a Monte-Carlo RT code.

25 IV: Lya Radiation Pressure M F can greatly exceed unity (especially for slow moving slabs): I.e. previous estimate was conservative. D & Loeb, submitted

26 Lya as a Probe of the (High-z) Universe Summary: Observed Luminosity Functions (UV + Lya) of LAEs *in combination with* Equivalent Width distributions provides the best probe ionization state of the IGM + the (stellar) content of high-z galaxies. Lya emission is among the most highly polarized signals in our Universe. Lya polarization encodes additional information on the ISM (+IGM) Lya radiation pressure may be important in driving outflows of HI gas in the ISM (and possibly the IGM).