Gas in the Local Group

James Binney & Filippo Fraternali

Oxford University

Outline

• Missing baryons

• Infall and HVCs

• Extraplanar gas in external galaxies

• The hot halo

• Conclusions

Missing baryons

• Negative vlos of M31 ) MLG=4.8£1012M¯ (Kahn & Woltjer 59 ff)

• b/m=0.17 (Spergel et al 03)

• If MM31'1.5MMW (cf Wilkinson & Evans 99)

• But LV(MW) ' 1.5£1010L¯, so M* ' 3-5£1010M¯

• Implies most baryons missing• Klypin, Zhao & Somerville (02) have MMW=1012M¯

and half baryons missing

Still infalling?

• Muller Oort & Raimond (63) found HI at highly anomalous velocities

• HVCs mapped at ever higher sensitivity ! Leiden-Dwingeloo (Hartman & Burton 1997) & HIPASS (Barnes et al 01) surveys

• Are HVCs distant & massive? (Oort 70; Blitz et al 99)

• Efforts to detect massive extragalactic clouds in other groups repeatedly failed (Pisano & Wilcots 03)

• Clouds usually have detectable H emission (Tufte et al 02; Putman et al 03)

Extraplanar gas

• Some HVCs associated with LG galaxies (Magellanic Stream; Andromeda clouds)

• Most are within MW and of low mass (Westmeier 03)

• Extend to N<1019 cm-2 at which HI hard to detect (Hoffman et al 04; Richter et al 05)

• Significant covering factor

• Have complex shapes (Richter et al 05)

• Local clouds show net infall v ' 50 km/s (de Heij et al 02; Wakker 04)

Outside view

• Counterparts of HVCs now studied in external galaxies

• (M101: van der Hulst & Sancisi; NGC 5668: Schulman et al 94-6; NGC 891, NGC 2403: Swaters et al 97 ! Fraternali, Oosterloo & Sancisi 04)

Extra-planar gas in NGC 891

• Sancisi & Allen 1979 NH ≈ 5 1020 cm-2

• Swaters et al. 1997 NH ≈ 7 1019 cm-2

• Oosterloo et al. 2005 NH ≈ 1.7 1019 cm-2

• Sancisi & Allen 1979 NH ≈ 5 1020 cm-2

• Swaters et al. 1997 NH ≈ 7 1019 cm-2

• Oosterloo et al. 2005 NH ≈ 1.7 1019 cm-2

• Sancisi & Allen 1979 NH ≈ 5 1020 cm-2

• Swaters et al. 1997 NH ≈ 7 1019 cm-2

• Oosterloo et al. 2005 NH ≈ 1.7 1019 cm-2

NGC891: Low rotation of extra-planar gas

Fraternali 2005

vrot~15 km s-1 kpc-1

NGC 2403

.Distance: 3 Mpc

.Type: Sc

.Inclination ~ 62

.Non-interacting

.Very similar to M33

NGC2403: Extra-planar gas

Extra-planar gas

130 km/s

Forbidden gas

Fraternali, Oosterloo, Sancisi, van Moorsel 2001

Thin disc model

NGC2403: Non circular motions

Thin disc Extra-planar gas

V

Lagging haloThin disc

Non-circular motions

purerotation

pure radialinflow

rotation + outflowrotation + inflow

NGC 6946: Extra-planar gas and SF

Boomsma PhD 2005

WRST

Summary (observations) Extra-planar detected up to 15 kpc from plane

Rotation lower than the disc

High velocities (100-200 km s-1)

Global inflow motion

Link with star formation?

Evidence for accretion?

Fountain model(Shapiro & Field, ApJ 1976; Bregman, ApJ 1980)

• Clouds ejected from circular orbits with distributions in v,

• Axisymmetry exploited to build pseudo-data cube

New work (Fraternali & B 05):

• Clouds move ballistically as in Collins, Benjamin & Rand, A&A 02, but may not be visible until zmax or rmax

• Clouds return to disk on first or second passage through z=0• <4% of SN energy needed

Model constraint: vertical distribution

Vkick ~ 75 km s-1

Mhalo ~2 109 M

NGC 891: Lack of low angular momentum

Fast rotating gas

NEED FOR LOW ANGULAR MOMENTUM MATERIAL

NGC2403: lagging gas

Thin disc

Thick disc

60o

Vkick ~ 70 km s-1

Mhalo ~ 5 108 M

NGC2403: inflow/outflowThin disc gas

Extra-planar gas

Radial outflow

NEED FOR INFALLING MATERIAL

V

VR

Vz

Second-passage models

V

VR

Vz

V

VR

Vz

Phase-change models

NGC 2403

NGC 891 Fast rotating gas

Inside view

Summary (models) Models reproduce the vertical extent with reasonable energy

input (<4 % SN energy)

Failure in NGC2403: lack of inflowNeed for accretion

Failure in NGC891: lack of low angular momentumNeed for drag

Seen from inside, a successful cloud model would look like HVC population

But must reverse outflow and diminish rotation

The WHIM

CDM simulations without feedback suffer from “overcooling”

• Natural solution: fast mass loss during GF

• Direct evidence from Moutflow' MSF (Pettini et al 01; Steidel et al 04)

• Also manifest connection of outflow to HVCs (NGC 6946 and …)

• So expect accumulation of gas @

NGC 253 Boomsma et al 05

The hot halo

• Munch (52) detected Ca II and Na I interstellar lines at |v-vLSR|>20 km/s even at high b

• Spitzer (56) argued that cold absorbing clouds must be confined by pressure p/kB'104 K cm-3 of gas with T' Tvir

• At Tvir, Mgas= 0.52£109 (Rmax/R0) M¯

• So CDM requires M>1011M¯ halo to extend to Rmax'1Mpc

• Copernicus, HST and FUSE detect absorption in C IV, O VI, etc

• O VI important because ionize E(O V)=114eV; O VI emission peaks @ T = 3£105 K

HI emission & O VI absorption

• Consistent with O VI at interface of HI and WHIM• Possible evidence that O VI expanding relative to HI

Sembach et al 02

Interaction of HVCs with WHIM

• Density contrast Tvir/THI' 100-104

• Analogous to a transonic sprinkler• Ram-pressure drag (Benjamin & Danly 97)

• = 21 N19/(n-3v200) Myr• Tflight ' 100 Myr• Drag important

Evidence for drag

• Structure of leading arm of Magellanic stream

• Head-tail structure of HVCs (Bruns et al 01)

• Z < Z¯ for complex C

HVCCHVCsPutman et al 03

Problems

• Fountain circulates large mass through extraplanar gas: MHI ' 5£108 M¯ every 100 Myr

• If ejected gas loses 10% of its angular momentum, halo will become corotating if not extensive (Mgas= 5£108 (Rmax/R0) M¯)

• Naively expect moving clouds to be ablated

• Net inflow and low Z (10% Zsun) imply condensation prevails

Conclusions• CDM predicts that most baryons are hidden• Observations of external groups & galaxies

show that HVCs lie at 10 – 100 kpc distances• HVCs are generated by star formation• The basic fountain model does not reproduce:

lag in rotation & net infall• Much evidence for interaction of HI with WHIM• Likely that lag & infall result from interaction with

WHIM• LCDM predicts that WHIM contains bulk of LG

baryons & extends to > 1Mpc