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Ly α emitters in galaxy formation models. Alvaro Orsi Cedric Lacey Carlton Baugh. Supercomputing techniques in Astrophysics workshop. Emission-line Galaxies. Galaxies with detectable emission lines Tracers of star formation activity UV →Ly α ,H α , H β , [OII], [OIII],. Motivation.

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ly emitters in galaxy formation models

Lyα emitters in galaxy formation models

Alvaro Orsi

Cedric Lacey

Carlton Baugh

Supercomputing techniques in Astrophysics workshop

emission line galaxies
Emission-line Galaxies
  • Galaxies with detectable emission lines
  • Tracers of star formation activity
  • UV →Lyα,Hα, Hβ, [OII], [OIII],...
motivation
Motivation
  • Spatial distribution
  • Infer mass of dark matter haloes hosting galaxies
  • Galaxy formation in the high redshift Universe
  • What ingredients do we need?
  • Dark energy surveys

- How galaxies trace DM structure on very large scales?

motivation1
Motivation

Orsi et al (2009)

  • Spatial distribution
  • Infer mass of dark matter haloes hosting galaxies
  • Galaxy formation in the high redshift Universe
  • What ingredients do we need?
  • Dark energy surveys

- How galaxies trace DM structure on very large scales?

Emission line galaxies at z=1

motivation2
Motivation

Orsi et al (2009)

  • Spatial distribution
  • Infer mass of dark matter haloes hosting galaxies
  • Galaxy formation in the high redshift Universe
  • What ingredients do we need?
  • Dark energy surveys

- How galaxies trace DM structure on very large scales?

H-band selected

galaxies at z=1

motivation3
Motivation

Orsi et al (2009)

  • Spatial distribution
  • Infer mass of dark matter haloes hosting galaxies
  • Galaxy formation in the high redshift Universe
  • What ingredients do we need?
  • Dark energy surveys

- How galaxies trace DM structure on very large scales?

Luminosity function of Hα emitters

at z ~ 1

motivation4
Motivation
  • Spatial distribution
  • Infer mass of dark matter haloes hosting galaxies
  • Galaxy formation in the high redshift Universe
  • What ingredients do we need?
  • Dark energy surveys

- How galaxies trace DM structure on very large scales?

Euclid space mission: Hα emitters slitless survey, 0.5<z<2

motivation5
Motivation
  • Spatial distribution
  • Infer mass of dark matter haloes hosting galaxies
  • Galaxy formation in the high redshift Universe
  • What ingredients do we need?
  • Dark energy surveys

- How galaxies trace DM structure on very large scales?

HETDEX: Lyα emitters IFU survey at 3<z<5

ly emitters
Lyα emitters

Narrow band Lyα search

  • Hydrogen recombination line
  • Strongest transition
  • λ0 = 1216 Å
  • Tracer of high redshift galaxies (2 < z < 7), aiming to z > 7
  • Resonant scattering + dust

Small fraction of photons

escape from the galaxy

Lyα spectrum at z=3

(Gronwall et al. 2006)

modelling ly emitters
Modelling Lyα emitters
  • We use the semi-analytic model GALFORM developed at Durham
  • Simulate galaxy populations in cosmological volumes
  • Star formation and galaxy merger history from first principles
modelling ly emitters1
Modelling Lyα emitters
  • Lyα emitters are modelled using the Baugh et al (2005) model:

- Kennicut IMF for quiescent galaxies

  • Top-heavy IMF for starbursts
  • SN + Superwind mode of feedback
  • Monte Carlo merger trees
  • Lyα emitters have a fixed escape fraction
  • fesc(Lyα) = 0.02
  • constant!
motivation for baugh model
Motivation for Baugh model
  • Baugh et al (2005) model was not designed to predict Lyα emitters properties :
  • Submillimitre number counts and redshift distributions
  • Luminosity function of Lyman break galaxies
  • Galaxy evolution in the IR (Lacey et al 2008, 2009)

Baugh et al (2005)

motivation for baugh model1
Motivation for Baugh model

Baugh et al (2005)

  • Baugh et al (2005) model was not designed to predict Lyα emitters properties :
  • Submillimitre number counts and redshift distributions
  • Luminosity function of Lyman break galaxies
  • Galaxy evolution in the IR (Lacey et al 2008, 2009)
slide14

Evolution of Lyα LF

Orsi et al (2008)

Luminosity functions well reproduced in a wide range of redshifts, even with a fixed escape fraction!

slide15

Evolution of Lyα LF

Orsi et al (2008)

Luminosity functions well reproduced in a wide range of redshifts, even with a fixed escape fraction!

slide16

Evolution of Lyα LF

Orsi et al (2008)

Luminosity functions well reproduced in a wide range of redshifts, even with a fixed escape fraction!

spatial distribution
Spatial distribution
  • We combine GALFORM with the Millennium Simulation
    • Box size: 500[Mpc/h]
    • Mhalo > 1.72 x 1010 M/h
  • Alternatively, N-body merger trees can be used
slide18

Lyα emitters at z = 0

  • Low abundance due to modest star formation activity

Dark matter

Galaxies

slide19

Lyα emitters at z = 3.3

  • Near peak of star
  • formation activity
slide20

Lyα emitters at z = 5.7

  • Star formation
  • decreases again
slide21

Lyα emitters at z = 8.5

  • Star formation
  • decreases more
slide22

Clustering of Lyα emitters

Two point correlation function fit by:

comparing to observational data
Comparing to observational data

Mock catalogues of SXDS

  • Median w() of mock catalogues
comparing to observational data1
Comparing to observational data

Mock catalogues of SXDS

  • Median w() of mock catalogues
  • Idealized survey over much larger solid angle
comparing to observational data2
Comparing to observational data

Mock catalogues of SXDS

  • Median w() of mock catalogues
  • Idealized survey over much larger solid angle
  • Model agrees with observational measurements at 95% confidence
comparing to observational data3
Comparing to observational data

Mock catalogues for MUSYC, z=3

Abundances and clustering properties are well reproduced in a constant escape fraction scenario!

empirical attempts to model f esc
Empirical attempts to model fesc

Kobayashi et al (2008, 2009) semianalytic model

empirical attempts to model f esc1
Empirical attempts to model fesc

Nagamine et al. (2008) SPH simulation

Escape fraction scenario:

Duty cycle scenario

Samples diluted by a fixed fraction

empirical attempts to model f esc2
Empirical attempts to model fesc

Nagamine et al. (2008) SPH simulation

Escape fraction scenario:

Duty cycle scenario

Samples diluted by a fixed fraction

detailed modelling of ly photons
Detailed modelling of Lyα photons
  • Monte Carlo Radiative Transfer
  • Follow scattering of Lyα photons by HI atoms in the ISM
  • How many of them escape (effect of dust)
  • Lyα spectrum
  • Some applications:
  • Understand observed line profiles
  • Verhamme et al. (2006,2008),
  • Surface brightness of Lyα emission in
  • SPH galaxies
  • Laursen et al (2008,2009)
  • Modelling observed DLAs
  • Dijkstra et al (2006), Barnes et al (2009)
  • Neutral gas fracion in the IGM
  • - Cantalupo et al (2008)
the monte carlo code
The Monte Carlo code
  • Define the properties of the HI region

(geometry, density, temperature, kinematics, etc)

The frequency of the photon is characterised by

the monte carlo code1
The Monte Carlo code

2. Choose the location and direction and frequency of the photon

3. The photon will travel an optical depth given by

the monte carlo code2
The Monte Carlo code

4. At the location of interaction, we calculate the probability of interacting with an H atom

5.1 If the photon interacts with dust, then the dust albedo A tells us whether the photon was

absorbed or scattered. If absorbed, then the photon is lost and we start over again.

the monte carlo code3
The Monte Carlo code

5.2 If interacts with hydrogen then we compute the cross section:

The velocity of the atom parallel to the direction of the photon depends on its frequency:

the monte carlo code4
The Monte Carlo code

6. We perform a Lorentz transformation to the atom’s frame to compute the frequency and direction after the scattering

7. We repeat the process until the photon escapes or is absorbed. The same is applied to a large number of photons

slide39

Lyα spectrum

Homogeneous, static slab

Harrington (1973) analytical prediction

τ0=104

slide40

Lyα spectrum

Homogeneous, static slab

Harrington (1973) analytical prediction

τ0=104

τ0=105

slide41

Lyα spectrum

Homogeneous, static slab

Harrington (1973) analytical prediction

τ0=104

τ0=105

τ0=106

slide42

Lyα spectrum

Homogeneous, static sphere

Dijkstra (2006) analytical prediction

escape fraction for homogeneous slab
Escape fraction for homogeneous slab

Analytical solution for this case

(Neufeld, 1990)

No analytical solution for more

general cases

effect of outflow velocity
Effect of outflow velocity

Homogeneous expanding sphere

Static case

effect of outflow velocity1
Effect of outflow velocity

Homogeneous expanding sphere

Vmax=20 km/s:

Photons are slightly redshifted

effect of outflow velocity2
Effect of outflow velocity

Homogeneous expanding sphere

Vmax=200 km/s:

Photons are completely redshifted

effect of outflow velocity3
Effect of outflow velocity

Homogeneous expanding sphere

Vmax=2000 km/s:

The optical depth becomes so thin

that photons escape very easily after

being redshifted

next step
Next step
  • Combine with GALFORM
  • Choose a suitable geometry for the ISM
  • Study dependence of escape fraction on mass, redshift, luminosities, metallicities, etc...
tracing large scale structure with h emitters
Tracing large scale structure with Hα emitters
  • Forthcoming dark energy space missions will measure BAOs using Hα emitters
  • To what accuracy?

Understand Hα emitters from a galaxy formation perspective

tracing large scale structure with h emitters1
Tracing large scale structure with Hα emitters

Baugh model

Bower model

  • Forthcoming dark energy space missions will measure BAOs using Hα emitters
  • To what accuracy?

Understand Hα emitters from a galaxy formation perspective

tracing large scale structure with h emitters2
Tracing large scale structure with Hα emitters
  • The goal is to get an accurate P(k)
  • Different survey configurations determine P(k,z) and n(z):
  • We assess different configurations calculating the effective volume:
tracing large scale structure with h emitters3
Tracing large scale structure with Hα emitters
  • Hα emitters with f>1x10-16 [erg s-1cm-2] can measure wDE with an accuracy of ~ 0.6% in a survey like EUCLID
  • Other alternatives (H-band selected sample) can reach similar results if H(AB)<22

Error in the dark energy equation of state parameter:

redshift

summary
Summary
  • Galaxy formation models are able to predict the abundances and clustering of Lyα emitters using simple prescriptions for fesc
  • Physical properties of Lyα emitters can be studied using RT models
  • RT is needed for a more physical estimate of fesc in galaxy formation models
  • Still work in progress...