Rupert Croft (Carnegie Mellon)

Studing Radiation-Induced LSS: Motivation

We know a lot about the growth of large-scale structure due to gravitational instability from small seed fluctuations.

-> seen in galaxy surveys, Lya forest etc…

-> widely studied: linear and higher order PT, halo model, cosmic web morphology etc…

What about large-scale structure caused by other mechanisms?

-> one example is structure in the neutral hydrogen density field caused by sources of radiation: “Radiation Induced LSS”

-> how is it different in visual appearance and statistically from gravitationally induced LSS?

-> what can statistics tell us about sources of radiation (first stars, quasars, decaying DM etc…) and about cosmology?

Talk plan:

(1)Structure formation during reionization: radiative transfer vs gravity.

(2) Quasar light echos and how to find them. (with Eli Visbal, CMU)

(3) The quasar proximity effect from the SDSS and joint constraints on the ionizing BG and baryon density. (with Taotao Fang, UCB)

(All using cosmological hydro simulations)

(All work in progress)

Many codes exist for doing RT around the first stars/QSOs

e.g., Abel et al 1999, Razoumov & Scott 1999, Gnedin 2000, Ciardi et al 2000, Sokasian et al 2003, Cen 2002, Bolton 2004, Iliev et al 2005, Mellema et al 2005

Only recently have simulations started to resolve DM halosof mass 109 Msun which dominated the ionizing radiation output, as well as having box sizes large enough for bubbles of diameter 10 Mpc/h or more:

e.g., Iliev et al 2005, Kohler et al 2005

Code: Monte Carlo Radiative transfer

-> raytraces photon paths through SPH kernels -> source photons and recombination photons

No gridding needed, so keeps the high resolution of thesimulation: 10 Kpc/h vs 0.4 Mpc/h for typical gridded sim.

source

better than this

(a) Gadget hydro simulation: 2 x 2563 particles 40 Mpc/h box 10 Kpc/h resolution

(b) RT run as post-processing

(c) Sources of radiation associated with DM halos

-> simplest idea: we assign a mass/light ratio in a fashion similar to Zahn et al 2006 (astr0-ph/0604177) actually 1.2 x 1042 ionizing photons/sec/M

this is ~ like Pop II stars forming with efficiency f*=0.1 from a Salpeter IMF, with stellar lifetime 5x107 yrs and escape fraction fesc~0.05

this is our fiducial case

..

Ingredients

the idea is to vary unknown parameters, and see what effect this has on the LSS.

-> 9 different RT runs so far:

(others have softer nu-4 spectrum)

models 5-9 are normalized to have the same total radiationoutput as model 1 (fiducial) by z=6

1 Mpc/h thick slice

40 Mpc/h

90% neutral by mass at z=10.0

50% neutral by mass at z=8.2

10% neutral by mass at z=7.8

neutral remnants mostly in voids - can they be detected/ tell us anything?

• effect of recombinations quite small for these late reionizing models(density is relatively low)• reionization process is fastest for sources hosted by large halos only

mass-weighted neutral fraction vs redshift

• early on, mfp is strongly affected by recombinations as photons try to escape from dense regions around sources

• mfp of hard spectrum model is always > 0.3 Mpc/h. For a more realistic AGN spectrum it would be even more. Thisaffects recombinations.

we will compare models when there is 50% neutral fraction by mass

fiducial model

plots of neutral density(rho x neutral fraction):

fiducial (all halos > 109 Msun contribute)

only halos > 1010 Msun contribute

only halos < 1010 Msun contribute

neutral fractions

10% ionized 30% ionized

70% ionized 90% ionized

(for fiducial case)

for all models 50% ionized

Plot of ionizeddensity insteadof neutraldensity

-easier to see sources

ionized density fiducial

big galaxies only

small galaxies only

clusteringgoes downand then upagain

slope andamplitudecan be verydifferent frommass xi

“bias” > 10for 1% neutralfield

10% ionized

90% ionized

correlation function of mass and HI mass

fiducial case

97% ionized

99% ionized

“growth factor” for HIfluctuationsmuch largerduring thisshort epochthan for rho(this plot spansz from 10-7.5).

quarter fiducial luminosity

fiducial case

ratio of HI to matter correlation function

We will instead compare to a Very simplistic bubble model:

(Babul & White 1991 give analytic form for bubbles of filling factor f and radius r in a uniform medium)

€

1+ ξΗΙ = (1+ ξ ρ )(1+ ξ bubble )

Assumesno correlationbetween densityand bubbles

(actually we mean intrabubble)

Many detailed semi-analytic models for reionization exist: e.g.Miralda-Escude 2000, Furlanetto 2003, Zahn et al 2006

For mean neutral fraction < 0.6, this bubble model doesn’t work (we haveantibias)

(for all models 50% ionized)

neutral fraction:

10% ionized 40% ionized 70% ionized

fiducial

nu-2

spectrum

Non-Gaussianity? e.g., how is S3 different from under gravitational evolution?

S3 is skewness/ variance2

higher order perturbation theory prediction for graviational S3 is ~4 for these (0.1~10 Mpc/h) scales

PT for rho

minimum at 30% ionized

S3 becomesconstant forhigh ionizedfraction

all models 50% ionized

OK- this was structure in the HI field around reionization - what about radiation-induced LSS at later times?

We will look at z=3.

Differences from z~10:

. Ionizing photon mean free path is much longer: ~200 Mpc/h

. Radiation field is much more uniform: only very bright rare sources(quasars) will have any noticeable effects

. Observational probe is the Lya forest.

250 Mpc/h box, z=3

matter ionizing radiation

What are these weird “light echos”, and how can we detect them?

density field around a quasar (50 Mpc/h wide box)

quasar light curve

neutral hydrogen density field around quasar

Lya forest probes of density field along line of sight:

F=e-

€

∝ [ρ (r)]Γ(r)

1.6

(observable quantity)

Photoionization rate(proportional to Ionizing radiation intensity)

Density of matter For materialin photoionizationequilibrium(see e.g. Huiand Gnedin 1997)

(Tau is prop to HI density - Gunn &Peterson 1965)

lya spectra with uniform BG and with BG+quasar

5 different sightlines

Simulation test:

put 50 quasars behind a ~2 deg x 2 deg area at z=3

make simulated lya spectra(2 Angstrom res)

try to detect a light echo thatwe put in

the “noise” is structure in thedensity field

Method: make a template and slide it through thedataset. -> need 5 parameters (x,y,z, quasar luminosity, time since quasar switched off)

-> very time consuming as we need a 5 parameter grid

simulation test in 1D:

What is the chance of finding something that looks like a light echo, but is just a chance set of density fluctuations?

-> we look for light echos in 1000 simulations with only density fluctuations contributing to the Lya forest

For quasars with bolometric luminosity 5x1045 erg/s,statistical significance of detection is 1 in 1000

data like COSMOS survey (e.g. Impey et al 2006) can be used toconstruct a grid of sightlines through a volume

If we find light echos, what then?

(1) They will tell us about the lifetimes and luminosities of long dead quasars.

(2) They are interesting objects and may be useful for cosmology - for example, their angular and redshift extent can be used to construct a geometric test.

(c) Quqsar proximity effect from SDSS

Proximity effect (Bajtlik etal 1987)is deficit of lya absorption closeto observer quasar.

Unlike light echo case, we knowluminosity of quasar, so we can predict what the deficit should be

€

∝Ωb2 [ρ (r)]

Γ(r)

1.6

Here Gamma=gamma for BG +gamma for this quasar

Can use this to measure gamma for BGlargest current measurement is Scott et al 2000, from ~100 quasar spectra

€

∝Ωb2 [ρ (r)]

ΓQuasar(r) + ΓBG

1.6

F=exp(-tau)

r

small r dependson quasargamma

large rdepends on BG gammabecause of inverse square law

If we know quasar luminosity, we can in principleconstrain Gamma_BG and Omega_b

We use ~3000 quasars from SDSS DR3 -use only quasars above z=2.4 so that mean z of lya forest is 3.0

We fit continua using PCA components derived from red sideonly (method of Suzuki et al 2005) -> the important region is that close to the Lya emission line.

As a test, we can reject spectra which the PCA doesn’tfit well around the red side of the lya emission line

Scott et al 2000 proximity effectwith 1 sigma errors

Total seen from quasars onlyHaardt and Madau 1999

Results

Conclusions

We have looked at 3 examples of RIS:

(1)RIS during reionization encodes information about the radiation sources. Structure is qualitatively different from under gravity. There is a rich phenomenology to explore.

(2) Light echos from bright quasars should be detectable. We can use them to tell us about the quasar lifetime, and about long-dead quasars. They are structures generated by the radiation field itself and may have some uses for cosmology.

(3) The proximity effect (the RIS in the Lya forest very close to quasars) can be used to constrain the baryon density (using SDSS Lya spectra gives results consistent with BBN/CMB) and ionizing radiation BG intensity (need ~2 x that from quasars)