Where are all the baryons?
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Where are all the baryons? Baryon census (2013) of the local universe, contd. PowerPoint PPT Presentation

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Where are all the baryons? Baryon census (2013) of the local universe, contd. Now that we've introduced DM, chemistry and gas evolution, we'll put these all together in a story starting with the first stars, the first galaxies, moving towards modern day galaxies.

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Where are all the baryons? Baryon census (2013) of the local universe, contd.

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Where are all the baryons?

Baryon census (2013) of the local universe, contd.

Now that we've introduced DM, chemistry and gas evolution, we'll put these all together in a story starting with the first stars, the first galaxies, moving towards modern day galaxies.

Joss Bland-Hawthorn University of Sydney

Baryon census (z~0)


fixed cosmic abundances

This is a real problem since we don’t know how metals vary between ISM and IGM

Most recent summary

from Shull+ 2012

  • Collapsed baryons (18%)

  • Uncollapsed baryons (82%)

  • 2013 baryon census

    • Galaxies

    • Cold gas

    • ICM

    • CGM

    • LyaF

    • WHIM

    • ? missing ?

Baryon census - overview




Schechter luminosity function (1976)

n★ is a normalization factor which defines the overall density of galaxies (number per cubic Mpc)

L★ is a characteristic galaxy luminosity. An L★ galaxy is a bright galaxy, roughly comparable in luminosity to the Milky Way. A galaxy with

L < 0.1 L★ is a dwarf.

α defines the `faint-end slope’ of the luminosity function. α is typically negative, implying large numbers of galaxies with low luminosities.


Large galaxy surveys of the local universe – 2dFGRS (Colless+ 2001), SDSS (Blanton+ 2001), GAMA (Driver+ 2012)

GF favours disks over spheroids

2:1 (Driver+07; cf. FHP96)

Schechter luminosity function (1976)

we see this function vary across galaxy types and in cosmic time

Cold gas (2%)

There is a good reason for why we only include cold gas in galaxies (and not in the IGM) in the inventory.


corrected for cold He I, H2 (FP04)

Confirmation: ALFALFA survey (Darling+11)





Sancisi dictat: All cold gas is associated with galaxies (i.e. dark matter)

Extragalactic HI always associated with galaxies, e.g. tidal tails like the Magellanic Stream.

If you can find a truly isolated cloud, you will be famous!

Leo Ring



What about hot gas in galaxies?

We see it in massive ellipticals which are often in big clusters that have their own intracluster gas (ICM)

blue = hot x-ray gas (107 K)

bremmstrahlung = emission ("braking rays" when e- decelerated by p)

What about hot gas in spirals ?

The Galaxy – classic paper (followed by Kahn & Woltjer 1959 for the LG)

Pedersen+ 06; Rasmussen+ 06

not confirmed: smoothed out nuclear source!


Vrot ~ 400 km/s

This appears to be first reliable detection in emission after the Galaxy

see also Dai+12 (UGC 12591; Vrot ~ 470 km/s)

INTRACLUSTER MEDIA (4%): can often dominate over stars


Where does this hot gas come from?

Large-scale shocks associated with accretion onto A3376

This provides us with a clue about why so much gas is rendered (almost) invisible to modern instruments.

Cooling times are longer than the age of the Universe (Hubble time) at low densities (n α r-2) beyond the virial radius...

Sutherland & Dopita (1993):

Solar, 0.3, 0.1, 0.01, 0.001 Solar

This is an important clue that lots of gas can escape detection by being hot and at low densities...

Mass vs Temp calibration

harder to detect

(hard UV photons)

easier to detect

(soft and hard x-rays)


Rvir=1.6 Mpc

The first systematic survey has found warm gas at the outskirts of the Virgo cluster

(100%; NHI > 1013 cm-2) – circumcluster medium ? Not just as simple as hot halos.

Baryon fractions in groups & clusters

Joss' rule of thumb:

below 0.1Z from outside, above 0.2Z

from inside

1013 Mo

1015 Mo


Way more scatter than implied by McGaugh+ 2010 figure.

Baryon fractions down to galaxy masses

(note where the Milky Way is)

Dai+ 2012

So even with corrections for missing hot gas (e.g. Anderson, NGC 1961), galaxies appear to fall below the cosmic baryon fraction. So maybe the gas never got in? or maybe the gas was once there but then removed?

Circumgalactic media (~5%)

− the new frontier −

gas distributed on the scale of the dark matter halo, i.e. out to the so-called virial radius

Circumgalactic media (5%)

Lyα halos at z~3 – 92 LBGs stacked

So huge haloes of warm/cool gas do exist. We need to look to high z to see these because Lyα map only detectable when redshifted out of extreme UV. Where do these come from?

CGM − Why the new frontier?

It may soon be possible

to resolve how gas gets

into galaxies:

Coherent flows?

Cosmic rain?

Something else?

Keres+ 2005, 2009

Do cold/cool/warm flows exist? Are they figments of our imaginative simulations? Not seen to date at any redshift.

Circumgalactic media (5%)

See also OVI absorbers in the Galaxy (Richter+09)

OVI absorbers in


L★ galaxies


Important clue to

metals in the IGM?

The intergalactic medium:sheets, filaments & voids

How does gas move out of voids?

Sheth & van de Weygaert 2004

Sheth & vdW 2004

Mo et al 2010:

a lot of detail handled

in on-line codes that borrow from many sources, e.g. Fyris,

Gadget II, VH-1, etc.

First to simulate the IGM and noted relevance to missing baryons. Very simple shock physics was used.

WHIM: large-scale shocks

You've been introduced to shocks in BMG's lectures.

Cen: you have a lump of dark matter of size L at some redshift z. Gas has been infalling for the age of the Universe, i.e. To = 1/Ho(z). The infall velocity VI ~ HoL.

The potential energy of the gas is converted into internal (heat) energy through the shock, which will heat the gas to the virial temperature TVIR of the halo. The sound speed Cs in the shocked gas (TSH~TVIR) is comparable to the infall speed ~ rotation speed Vc.

The post-shock conditions depend on geometry,

e.g. no = pre-shock, n1 = post-shock density

3D: n1/no = 64 (collapse onto DM sphere)

2D: n1/no = 16 (collapse onto DM filament)

1D: n1/no = 4 (collapse onto DM sheet)

(adiabatic case, i.e. no heat lost or gained from system)

δ > 10

δ > 100

δ > 1000 (galaxy halos)

T ~ 105-7 K

Formation of the WHIM near sheets and filaments (O'Shea)

vs < 150 km s-1

Ts < 105 K

WHIM: large-scale shocks

vs > 700 km s-1

Ts > 107 K

Kang+ 2005

Chandra/XMM spectroscopy

HST COS UV spectroscopy

It's not all hot in the IGM. The mysterious Lyman α forest (LyαF) appears to be huge diffuse clouds of photoionized, low density gas. Near to galaxies, groups and filaments?

We detect metals in the LyαF.

Evidence for shocks:

much of this appears

collisionally ionized


Danforth & Shull 05,08

Thom & Chen 08

really need e.g. Ne VIII

missing phase e.g. hotter WHIM?



broad Lyα absorbers


integrated to log NH > 12.5

rises to 31% if down to 12.0

much of this is photoionized

15% baryons collapsed (GF is an inefficient & complex process);

85% baryons uncollapsed, half still missing.

We need to understand the physical state of each gas phase before talking about a complete inventory (e.g. CGM inflow? outflow?).

We know little about the dynamics of gas flows onto mass structures on any scale, although there may be some evidence that these processes are now being observed. Most of the action was at high redshift but the same processes are still ongoing.


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