The physics ofintergalactic gas

Serena Bertoneserena@scipp.ucsc.edu

March 9th 2009

Overview• What is the IGM?

• Why do we care?

• IGM properties from simulations

• Low vs. high redshift IGM

• Observational techniques to detect the IGM:

– absorption lines

– emission lines

• Future prospects

• References

Neutrinos:0.3%

Metals: 0.03%

Dark energy: 70%

Free H & He: 4%

Stars: 0.5%

Dark matter: 25%

> 90% of the baryonic mass is in diffuse gas(Persic & Salucci 1992, Fukugita et al 1998, Cen & Ostriker 1999)

Baryons in the Universe

Prochaska & Tumlinson 2008

Why do we care about IGM?

Fundamental ingredient in galaxy formation:

• unbiased information on power spectrum of density fluctuations on largest scales

• reservoir of fuel for star and galaxy formation

• IGM metallicity constrains the cosmic star formation history

• interplay IGM-feedback constrains galaxy formation models

How can we study the IGM?

• with theory

• with cosmological, hydrodynamical simulations of structure formation

• with observations:

– in the local universe

– in the low redshift universe

– in the high redshift universe

IGM simulations

Basic recipe to simulate the IGMIngredients:

• initial conditions CMB

• hydrodynamic physical model

• numerical code Gadget, Enzo, Flash, Ramses…

• big computer

Shake well

Evolution of gas density, temperature, metallicity, ionisation state etc…

Mock observations

100 Mpc

z=6 z=2 z=0

Springel 2003

cosmic time

IGM density evolution

Simulations have just about everything…

Temperature Density Metallicity

BUT: it doesn’t have to be correct!What you get is simply the result of the physics you put in

IGM temperature distribution at z=0

Cen & Ostriker 1999

T<105 K 105 K <T<107 K T>107 K

Warm gas:• diffused• photo-ionised by UV background• traced by Lyα forest

Warm-hot gas: WHIM• mildly overdense• collisionally ionised• shock heated by gravitational shocks

Hot gas:• dense• metal enriched • haloes of galaxies and clusters

z=0

Where are the baryons?

• T>107 K hot cluster gas: small fraction @ all redshifts

• 105 K<T<107 K WHIM: strongly increases with z

• T<105 K diffuse Lyα forest gas: strong decrease

• condensed: cold ISM + stars

Average IGM temperature increases with timeDave’ et al. 2001

What is the fraction of gas mass in given temperature/density ranges? How do these fractions evolve?

Where are the metals?

Simulations predict that:

• most metals locked in stars at z=0

• most metals in star forming gas at z>2

• ~30% of metals in WHIM at all times

• small fraction of metals in cool diffuse gas

Wiersma et al 2009

WHIMICM

diffuse IGM

halo gas

How does this compare with observational

results?

Observing the IGM

How can we detect the IGM?Temperature:

wavelength of transition increases with temperature

Redshift:wavelength of transitions decreases with redshift

wavelengthIR optical UV X-rays

T=105 K @ z=0O VI λ=1032ÅExample: oxygen

T=106 K @ z=0O VII λ=21.60Å

T=107 K @ z=0O VIII λ=18.97Å

T=105 K @ z=3O VI λ=1032Å λ=4100Å

Emission vs. absorption

Emission Absorption

PRO• 3-D data on spatial distribution, velocity field, metallicity…

PRO• Easier to detect• Underdense regions investigated with current optical/UV instruments

CONS• Hard to detect – low fluxes• Requires new instruments with high sensitivity and large FoV

CONS• 1-D info along a LOS• Requires bright background sources

T<106 KRest-frame UV linesLyα, OVI, CIV…

T>106 KSoft X-rays metal lines

OVII, OVIII, FeXVII

Absorption

z<1Nicastro et al 2005

Rasmussen et al 2007Buote et al 2009

z<1: UVTripp et al 2007Lehner et al 2007Danforth & Shull 2005

z>1.5: opticalKim et al 2001Simcoe et al 2006

z<1Fang et al 2005Bertone et al 2009a

z<1: UVFurlanetto et al 2004 Bertone et al 2009a

z>1.5: opticalWeidinger et al 2004Bertone et al 2009b

Emission

✔✔

✗✔✗

✗

?

Absorption lines

The Lyα Forest

1-D information on:

• spatial distribution of H I

• spatial distribution of metal absorbers

• column density of species gives information on:– temperature– density– metallicity– ionisation state

John Webb UNSW

Absorption by warm photo-ionised gas along the LoS to a bright QSORest-frame UV lines: Lyα, C IV, O VI, N V, Si IV…

Recovering this information is non-trivial!

QSO 1422+23, zem=3.62Womble et al, 1996

Lyα forest evolution

• Gas becomes hotter and progressively more ionised

• Lyα forest becomes “thinner”: lines have lower column densities

• overall spectrum moves to shorter wavelengths from optical to UV band

Becker et al. 2001 “Neutral” IGM

Tripp et al 2004STIS, zem=0.3

wavelength (Angstroms)

wavelength (Angstroms)

The high-redshift Lyα forest

• Rest-frame UV lines:

– Lyman series lines: Lyα, Lyβ, Lyγ…

– metal lines: C IV, O VI, N V, Si IV…

• high resolution spectra from Keck and VLT needed for line properties

• low resolution spectra (SDSS) good for power spectrum Kim et al 2001

UV absorption lines at z<1• Current observations:

FUSE, HST/STIS

• Lyα, O VI and C IV narrow absorption lines prove prevalently cool and photoionised gas

• Abundance of O VI at z<0.5 corresponds to Ωb(O VI) ~ 5-7% of baryons

• Other UV lines: C IV, N V, Ne VIII, Si IV, Si III, C III… detectable with HST/COS

Tripp et al 2007

Line density and opacity evolution

• number of absorption lines decreases with redshift• mean optical depth of lines decreases with redshift thinning of forest

Kim et al 2002

Column density distributions• way to characterize the forest: number of lines vs. line column

density

• H I column density ≈ H I density, not hydrogen in general!

• same analysis can be done for each species: C IV, O VI etc.

Column density distribution of ionised (Lyα forest) and neutral (DLAs, neutral H I and CO) gas.Prochaska & Tumlinson 2008

z=0z=3Lyα forest

LLS

DLAs

The Lyα forest power spectrum at z=2

• The flux power spectrum can be translated into the matter power spectrum of density fluctuations: τ ≈ ρα

• the conversion is non-trivial: contamination from peculiar velocities, metal lines and non-linear structures (eg DLAs)

• flux power spectrum used with CMB and other data to estimate cosmological parameters

Kim et al 2004

Baryonic mass density• fraction of baryons in the Lyα forest decreases with time• fraction of mass in stars increases with time• results comparable to predictions of simulations (see previous slides)• WHIM abundance not known observationally still missing!

z=3 z=0

Estimates of baryonic mass densities, relative to Ωb, at z=3 and z=0. Prochaska & Tumlinson 2008

Ryan-Weber et al 2009

C IV mass density evolution

• C IV abundance is NOT a prior for carbon

• C IV abundance roughly constant with redshift up to z≈5

• C IV downturn at z≈6 indicates:

– declining carbon abundance with z?

– change in UV background?

IGM metallicity

• Estimating the metallicity from ion abundances (eg C IV) requires non-trivial ionisation corrections

• strong dependence on ionising UV background

• metallicity roughly depends on density of systems: lowest metallicity in lowest density regions and viceversa

Schaye et al 2003Carbon abundance vs. overdensity Carbon abundance vs. redshift

z=3 δ=0.5

The Lyα forest traces warm gas with T<105 K.

What about the WHIM?

Broad Lyα absorptionlines (BLA) at z<1

Evidence of WHIM gas?

• Lyα absorption from collisionally ionised gas with 105<T<106 K

• Thermally broadened lines: bth> 40 km/s

• upper limit on temperature

STIS spectra (Richter et al 2006)

X-ray absorption linesLines at z=0:

what are they?

• galactic halo?

• local group gas?

• nearby gas outside the local group?

(Williams et al. 2005;Bregman 2007)

Williams et al 2005Chandra LETG spectrum of Mrk 421

z=0 absorbers

The X-ray forest?Nicastro et al 2005

WHIM “detections” in X-rays are controversial

2-σ detection of twoO VII absorption lines at z=0.011 and z=0.027 in the spectrum of Mkn 421 WHIM evidence or statistical fluke?

Spectrum taken during QSO burst (Nicastro et al 2005).

Detection not confirmed by XMM in deeper exposure (Rasmussen et al 2007).

Statistical analysis not favorable (Kaastra et al 2006).

A real detection?• New strategy: look for

absorption from known structures such as galaxy overdensities and filaments

• Buote et al 2009: 3-σ detection of O VII absorption from Sculptor Wall at correct redshift

• 3-σ still not extremely high significance…

• More sensitive instruments needed! Buote et al 2009

Emission lines

or3D maps of the gas in the universe

X-ray emission @ z<1

• O VIII strongest line

• lines from lower ionisation states and whose emissivity peaks at lower temperatures trace moderately dense IGM: C V, C VI, N VII, O VII, O VIII and Ne IX

• lines from higher ionisation states trace denser, hotter gas: C VI, O VIII, Ne X, Mg XII, Si XIII, S XV and Fe XVII

• Fe emission has different spatial distribution than other elements: later enrichment by SN Ia

Bertone et al 2009a

UV emission@ z<1

• C IV strongest line: traces gas in proximity of galaxies

• different spatial distribution: O VI and Ne VIII trace more diffuse gas than C IV

• no emission from the hottest gas in groups

Bertone et al 2009a

Rest-frame UV emission @ z>1.5

• At z>1.5 rest-frame UV lines are redshifted in to the optical band

• A number of upcoming optical instrument might detect IGM emission lines at 1.5<z<5:

- Cosmic Web Imager on Palomar (CWI, Rahman et al 2006) this year!

- Keck Cosmic Web Imager (KCWI)

- Antarctic Cosmic Web Imager (ACWI, Moore et al 2008)

- MUSE on VLT (Bacon et al 2009)

• IFUs with large fields of view & high spatial resolution

• Great chance to observe the 3-D structure of the IGM for the first time!

Future prospectsCurrently, the IGM is only observed through rest-frame UV

absorption lines at z<7

In the future it will be desirable to:

• theory and simulations need to help understand observations several features still unexplained/not reproduced

• understand shape,origin etc of UV background

• detect the IGM in emission

• detect the IGM in absorption in X-rays

• detect the IGM in multiple bands multiphase gas?

• extend surveys to higher redshifts (eg 21 cm line at z>7)

• compare multi-wavelength data to obtain a deeper understanding of the physics of galaxy formation, as is currently done for galaxies

Wish-list of future instrumentsDetection of IGM in emission requires:• large field of view• high spatial resolution• high spectral resolution

Telescopes to do the job:• X-ray missions:

– Constellation-X & XEUS for absorption– EDGE, Xenia or better for emission– >10 years in the future

• UV missions:– ISTOS or similar– > 5 years in the future

• Optical instruments to study z>2:– CWI– KCWI or similar– 1 to 5 years in the future

The missing baryons in the low redshift universe are the hardest to detect

because of instrument

requirements

Feedback on the IGM

• C IV optical depth: indicator of C IV column density• Previous simulations (e.g. Springel & Hernquist 2003) were unable to predict the

observed high column density of C IV at z>2• Reason: metals are too hot in the simulations• Missing ingredients:

– Metal cooling? Aguirre et al 2005; Wiersma et al 2009– Wrong SN feedback model? Oppenheimer & Dave’ 2005+

Comparisons with IGM data help improve feedback models

Aguirre et al 2005

BEFORE: no cooling LATER: metal cooling better agreement!

HI optical depth

CIV

optic

al d

epth

References

• J.X. Prochaska & J. Tumlinson: Baryons: What, when and where? 2008, arXiv:0805.4635

• S. Bertone, J. Schaye, K. Dolag: Numerical simulations of the warm-hot intergalactic medium, 2008, SSR, 134, 259

• J. Bregman: The search for the missing baryons at low redshift, 2007, ARAA, 45, 221

• G. Stasinska: What can emission lines tell us?, 2007, Lectures XVIII Canary Islands Winter School

• M. Rauch: The Lyα forest in the spectra of quasi-stellar objects, 1998, ARAA, 36, 267