A Brief Review of Galactic Winds

0.71 MB·

by  Timothy M. Heckman

A Brief Review of Galactic Winds
TimothyM.Heckman&ToddA.Thompson
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1 Abstract Galacticwindsfromstar-forminggalaxiesplayatkeyroleintheevolu-
3 tionofgalaxiesandtheinter-galacticmedium.Theytransportmetalsoutofgalaxies,

chemically-enrichingtheinter-galacticmediumandmodifyingthechemicalevolu-
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A tionofgalaxies.Theyaffectthesurroundinginter-stellarandcircum-galacticmedia,
therebyinfluencingthegrowthofgalaxiesthroughgasaccretionandstar-formation.
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Inthiscontributionwefirstsummarizethephysicalmechanismsbywhichthemo-
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h mentum and energy output from a population of massive stars and associated su-
p pernovae can drive galactic winds. We use the proto-typical example of M82 to
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o illustrate the multiphase nature of galactic winds. We then describe how the ba-
r sicpropertiesofgalacticwindsarederivedfromthedata,andsummarizehowthe
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s propertiesofgalacticwindsvarysystematicallywiththepropertiesofthegalaxies
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thatlaunchthem.Weconcludewithabriefdiscussionofthebroadimplicationsof
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galacticwinds.

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6 1 Introduction
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0 Rapidstarformationingalaxiesisassociatedwiththeefficientejectionofgas,the
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1 fuelforstarformation.Thesegalacticwinds,poweredbythemomentumandenergy
0 injectedbymassivestarsintheformofsupernovae,stellarwinds,andradiationare
7 not only interesting in their own right. They also play a crucial role in the evolu-
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tion of galaxies and the inter-galactic medium (IGM). By transporting metals out
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X TimothyM.Heckman
r DepartmentofPhysicsandAstronomy,TheJohnsHopkinsUniversity,3400N.CharlesStreet,
a Baltimore,MD21218e-mail:[email protected]
ToddA.Thompson
Department of Astronomy and Center for Cosmology and Astro-Particle Physics, The Ohio
State University, 140 W. 18th Ave, Columbus, OH 43210 e-mail: thompson@astronomy.
ohio-state.edu
1
2 Heckman&Thompson
ofgalaxies,theyhelpestablishthetightempiricalrelationshipbetweenthegalaxy
massandthemetallicityofstarsandgasingalaxies.Thisprocessalsopollutesthe
IGMwiththeheavyelementscreatedbynuclearreactionsinstarsandsupernovae
withingalaxies.Themomentumand/orkineticenergyassociatedwiththeoutflows
candrivegas-phasebaryonsoutofgalaxiesand/orheatbaryonsinthegalaxyhalo
andpreventthemfromcoolingtoformstars.Thisprocessisakeypartofestablish-
ingthedecreasingbaryonicmass-fractionindarkmatterhalosofdecreasingmass.
By carrying low-angular momentum material out of the central regions of galax-
ies they also help establish the mass-radius relationship for disk galaxies. Finally,
galacticwindsmaybethemostextrememanifestationofstarformationfeedback,
whichdrivesturbulenceandhelpsregulatestarformationwithinthegalaxies.
Inthismonographwegiveabriefsummaryofgalacticoutflows.Insection2,we
summarizethecurrenttheoreticalmodelsforthecreationofoutflows,theirsubse-
quentdynamicalevolution,andthewayinwhichtheyinteractwiththesurrounding
gasintheinterstellarandcircumgalacticmedium(CGM).Insection3wereviewthe
observational properties of outflows, including a description of the various phases
observedintheoutflow(fromrelativisticplasmatocoldmoleculargas),asummary
ofhowthekeypropertiesofoutflowsaredeterminedfromthedata,andadiscussion
ofhowthesepropertiesscalewiththebasicpropertiesofthepopulationofmassive
stars that drive the outflow and of the surrounding galaxy. In section 4, we com-
mentontheimplicationsoftheobservedpropertiesofoutflowsfortheevolutionof
galaxiesandtheIGM.Wesummarizeourconclusionsinsection5.
2 TheoryofGalacticWinds
Anytheoryofgalacticwindsmusthopetoexplain(ambitiously!)alloftheexisting
observations, including their trends, and to make predictions for new observations
totesttheunderlyingmodel.
As described in Section 3, winds are truly multi-phase. The observations span
theveryhot,hot,warm,cold,andrelativistic(cosmicrays)phases,andareprobed
through X-ray, UV/optical continuum and atomic resonance line, mid-IR, far-IR,
andmolecularemissionandabsorption,andUV/opticaldustscattering.
In making sense of the multitude of observations, it is useful to focus on a few
aspects.First,oneexpectssomesuper-heatedveryhotgas(T ∼108 K)thatisun-
boundfromthegalacticpotentialinthesenseofhavingatemperaturegreaterthan
the escape temperature T >T locally. How prevalent that gas is, and whether it
esc
participates in driving out the cold gas is a separate question. The second is that
the merely hot gas (T ∼106−7 K) and warm diffuse ionized gas (T ∼104 K) is
ubiquitous along the minor axes of wind-driving galaxies, and in M82 and other
well-studiedsystemsthereisatightspatialcorrespondencebetweenthetwophases
(section 3). The third and most constraining fact is that the ionized gas and neu-
tralatomicgasreacheshighvelocitiesinmanysystems.Thesevelocities,whichare
relatively easy to measure in absorption on lines of sight toward the galaxies, are
GalacticWinds 3
constrainingsincetheyrangefrom100−1000km/s.Thekeyphysicalproblemfor
theoristsishowtogetgas,eitherbydirectacceleration,orbytransformationfrom
someotherphasetomanyhundredsofkm/swithoutshockheatingittotemperatures
whereitwouldceasetoproducetheemissionandabsorptionseen.
2.1 HotWinds
Thepictureofsupernova-heatedhotwindsiswell-developed.Weimaginearegion
ofsizeR–eitheranindividualstarclusteroranentiregalaxy–wherekineticen-
ergyisinjectedintheformofcore-collapsesupernovaeandstellarwinds,andthat
this kinetic energy thermalizes. It then drives a super-heated outflow that escapes
the region. The energy and mass injection rates within the volume (r≤R) are E˙
andM˙,respectively.Neglectingradiativecooling,gravity,andothereffects,energy
conservationimpliesthattheasymptoticvelocityisV =(2E˙/M˙)1/2,andthatthe
∞
characteristictemperatureisT ∝V2.Assumingtheflowissteady-state,massconti-
∞
nuitygivesthedensityofthehotgaswithintheenergyinjectionregion.Forr>R,
oneexpectsadiabaticexpansionwithT ∝r−4/3(forγ=5/3),n∝r−2,andV ∼V .
∞
These expressions are the essence of the Chevalier & Clegg (1985) (CC85)
model,whichgivesaself-similarsolutionforahotflowvalidforr≤Rthatsmoothly
connectstotheadiabaticexpansionr>Rregionthroughasonicpoint.Ingeneral,
the sonic point for such a flow (without gravity) is located at the “edge” (r=R)
oftheenergy/massinjectionregion(Wang1995),andforparameterstypicalofsu-
pernovaheatingfromstarformation(seebelow)gravitycanbeneglectedatR.The
criticalpointtopologychangesiftheflowisslowenough–eitherbecauseofineffi-
cientheatingorheavymass-loading–thatgravityshouldbeincluded,asshownby
Johnson & Axford (1971). Radiative cooling in the context of early-type galaxies
andheatingwasexploredbyMathews&Baker(1971).
Scaling the energy injection rate to E˙ =αE˙ , where α is the thermalization
SN
efficiency and E˙ is the energy injection rate expected from core-collapse super-
SN
novae (1051ergs per 100M of star formation), and the mass injection rate to the
(cid:12)
starformationrateM˙ =βSFR,whereSFRisthestarformationrate,onefindsthat
V (cid:39)(2E˙/M˙)1/2(cid:39)103km/s(α/β)1/2. (1)
hot,∞
EquatingtheBernoulliintegral,
B=constant=V2/2+(5/2)P/ρ,
atthesonicpoint(r=R)withitsvalueatinfinity,wehavethat
T (cid:39)(m /k )(3/20)V2 (cid:39)2×107K(α/β). (2)
hot p B hot,∞
Usingmasscontinuity,thedensityatthesonicpointis
4 Heckman&Thompson
β3/2 SFR3/2
n (cid:39)1×10−2cm−3 1 , (3)
hot α1/2 R2
kpc
whereR =R/kpcandSFR =SFR/M yr−1.Theasymptotickineticpowerof
kpc 1 (cid:12)
thewindisE˙ =(1/2)M˙V2,anditsmomentuminjectionrateis
∞
P˙ =M˙V (cid:39)(2E˙M˙)1/2(cid:39) 5(αβ)1/2L/c (4)
hot ∞
where for the last approximate equality we have used the fact that the bolometric
luminosityofsteadystatestarformationisrelatedbothtothesupernovarateandto
E˙,suchthatL(cid:39)1010L SFR .Themomentuminjectionrateofsupernovaheated
(cid:12) 1
winds is thus comparable to the expectation from radiation pressure in the single
scattering limit (∼L/c) discussed below. Limits on the parameters α and β can
be derived for individual systems, or for collections of star-forming galaxies from
X-rayobservations(e.g.,Strickland&Heckman2009;Zhangetal.2014).
Hot winds in the spirit of CC85 and their interaction with the ISM have been
investigated numerically by a number of groups, both in idealized setups of blow-
outfromasmoothgalacticdisk(e.g.,Strickland&Stevens2000)andinthemore
realistic fully 3d case (e.g., Cooper et al. 2009), and in planar geometry, where
the turbulence of the galaxy and the outflow are directly coupled (e.g., Creasey et
al.2013).
TheCC85theoryhasalsobeenappliedonmuchsmallerscaleinthecontextof
individualsuperstarclusters(e.g.,Silichetal.2003,2004).Thepictureofindividual
starformingregionspunchingoutofthelocaldiskandinjectingoutflowintoalarge-
scale galactic wind may be more realistic than the picture of an entire starburst
functioningasenvisionedbytheCC85model.Numericalgalaxyformationmodels
attempttocapturethisdynamics(e.g.,Hopkinsetal.2012).
2.2 Warm,Cool,andColdWinds
An important puzzle in the theory of galactic winds is how to accelerate the cool
atomic and warm ionized gas we see in emission and absorption to hundreds or
evenathousandkm/s.Severalmechanismshavebeenproposed.Oneideaisthatthe
outflowinghotsupernova-heatedphasecoolsradiatively.Otherideasincludethedi-
rectaccelerationofISMmaterialfromthehostgalaxy,eitherwiththerampressure
ofahotCC85-likeoutflow,theradiationpressureofstarlightondustygas,and/or
thepressuregradientfromcosmicrays.
HighVelocityCoolGasfromRadiativeCoolingoftheHotWind
One way to produce fast outflowing cool/warm gas is to precipitate it directly
fromthehotphase.Ifthehotwindissufficientlymass-loaded–highβ ineqs.(1)-
(3)–theradiativecoolingtimefortheoutflowcanbecomeshorterthanthedynam-
icaltimescaleandweexpectanyhotwindlaunchedfromagalaxyorstar-forming
GalacticWinds 5
regiontocoolonlargerscales(r>R)(Wang1995).ThewindmaystartatRwith
T given by equation (2), but as it cools adiabatically, the temperature drops to
hot
∼107K,belowwhichmetallinecoolingdominatesoverbremsstrahlung.Here,the
coolingrateincreasesasthetemperaturedecreasesandthemediumcanbecomera-
diativelyunstable.Forsolarmetallicitygas,thecoolingradiuscanbeapproximated
as(seeThompsonetal.2016fordetails)
α2.13 (cid:18) Ω (cid:19)0.789
r (cid:39)4kpcR1.79 4π , (5)
cool 0.3kpcβ2.92 SFR
10
wheretheopeningangleofthewindisΩ =Ω/4π andwehavescaledthestar-
4π
burst for parameters typical of M82 or a high-redshift star-forming clump. The
strong parameter dependencies for the cooling radius follow from the strong den-
sityandtemperaturedependenceofthecoolingfunctionintheregionwheremetal
coolingdominatesatT <107K.
∼
Thetemperatureofthecoolinggasshoulddropprecipitouslyatthecoolingra-
dius to ∼103.5−104K. The velocity of the cooling material is expected to be of
order∼700−1200km/s.Theminimumβ abovewhichtheflowmustcoolonlarge
scalesisactuallynotverylarge—β (cid:39)0.6α0.636(R Ω /SFR )0.364 —sug-
min 0.3 4π 10
gestingthatradiativecoolingofhotwindsmaybeaubiquitoussourceofthehigh-
velocitycool/warmgasseeninstarburstingsystems(Thompsonetal.2016).
AcceleratingCoolGaswiththeRamPressureoftheHotWind
Given its large momentum flux (eq. 4), it is natural to consider the ram pres-
sureaccelerationofcoolcloudsbythehotoutflow.Manypaperstreatthenumerical
problem of the interaction between a hot high velocity flow as it interacts with a
singlecoolcloud,compressing,accelerating,andshreddingit.Animportantprob-
lem with this mechanism is the short timescale for cloud destruction via hydro-
dynamical instabilities, which set in on a multiple of the cloud crushing timescale
t (cid:39)(r /V )(ρ /ρ )1/2,wherer istheinitialradiusofthe(assumed)spherical
cc c hot c hot c
cloudandρ isitsdensity(e.g.,Cooperetal.2009;Scannapieco&Bru¨ggen2015;
c
Bru¨ggen&Scannapieco2016;Banda-Barraga´netal.2016).Fortypicalparameters,
t isshortenoughthatthecloudisnotacceleratedtohighvelocitiesbeforecomplete
cc
destruction(Scannapieco&Bru¨ggen2015;Zhangetal.2015).However,Cooperet
al. (2009) find that some of their cloud contrails reach velocities of hundreds of
km/s. In addition, magnetic fields may significantly prolong the life of clouds so
that they can be accelerated (McCourt et al. 2015). Banda-Barraga´n et al. (2016)
findthatcloudsmaybeacceleratedto∼10%ofthehotwindspeed.Additionally,
conductionhasrecentlybeenshowntoincreasecloudlifetime,butwithoutincreas-
ing the acceleration to the point where ram pressure can work to explain the high
velocitycool/warmoutflowsseen(Bru¨ggen&Scannapieco2016).
Animportantadditionalconstraintonthismechanism,andallmechanismsthat
relyonmomentuminjection,isanEddington-likelimit(e.g.,Murray,Quataert,&
Thompson 2005; Zhang et al. 2015). If V is large compared to the velocity of
hot
thecoolcloud,balancingtherampressureofasphericalwindandthegravitational
6 Heckman&Thompson
forces for a cloud of surface density Σ =M /πr2 (which changes in time as the
c c c
cloudiscompressedandshredded),oneobtainsthecriticalcondition
P˙ =4πGM Σ , (6)
Edd tot c
which one can view as either a critical momentum injection rate required to ac-
celerate a cloud of some Σ , or, for a given value of P˙ (e.g., eq. 4), the critical
c
value of Σ below which the cloud is super-Eddington and should be accelerated.
c
For example, taking M =2σ2r/G, as appropriate for an isothermal sphere with
tot
velocity dispersion σ and using equation (4), one finds a critical cloud surface
density of Σ ∼P˙/(8πσ2r), which corresponds to a cloud column density of
c,crit
N∼4×1021cm−2(αβ)1/2SFR /σ2 R ,whereσ isscaledto200km/s.
10 200 0.3
Fortheelongatedcometarycloudmorphologyfoundinsimulations(e.g.,Cooper
etal.2009),thecloudcolumndensityshouldnaivelyincreaseandforagivenwind
P˙,andthecloudmaynotbeacceleratedintheextendedgravitationalfieldofgalax-
ies. However, the cool cloud gas mass rapidly decreases on the cloud crushing
timescale, and thus the increase in Σ may be mitigated. Overall, the simulations
c
donesofarimplythatcloudscannotbeacceleratedbyahotCC85-likeflowtothe
asymptoticvelocitiesseeningalacticwinds,unlessmagneticfieldsdramaticallyin-
creasethecloudlifetime(McCourtetal.2015;Banda-Barraga´netal.2016).
Accelerating Cool Gas with Radiation Pressure on Dusty Gas and Supernova
Explosions
Galactic winds are dusty. As a result, the same massive stars that produce su-
pernovaemayalsodrivethecoolgasoutofgalaxiesviatheabsorptionandscatter-
ing of starlight by dust grains (Murray, Quataert, & Thompson 2005, 2010; Mur-
ray, Menard, & Thompson; Zhang & Thompson; Hopkins et al. 2012; Krumholz
&Thompson;Davisetal.;Thompsonetal.2015).Thismechanismisparticularly
attractive for young massive star clusters that disrupt their natal gas clouds before
the first supernovae have gone off, but radiation pressure may also act on galaxy
scales in systems that are both IR and UV bright. Observations suggest the direct
single-scattering radiation pressure force may have dominated the dynamics of 30
Dor(Lopezetal.2011;Pellegrinietal.2011).
Assumingthatthedustandgasaredynamicallycoupledsothatdustgrainsshare
theirmomentumwiththesurroundinggas,andassumingthatthestarlightisdomi-
natedbyUV,asappropriateforaveryyoungstellarpopulation,therearethreelimits
forthewindmedium:(1)optically-thintotheUV(τ <1),(2)optically-thickto
UV∼
the UV, but optically-thin to the re-radiated FIR (the so-called ”single-scattering”
limit; τ >1 and τ <1), and (3) optically-thick to both the incident UV and
UV IR
there-radiatedIRphotons(Andrews&Thompson2011).Foratypicalgas-to-dust
massratiofortheISM,aUVopacityofκ ∼103cm2/gofgas,andanIRopacity
UV
oforderκ ∼1cm2/gofgas,thetwobreakpointsbetweenthesethreelimitscor-
IR
respondtogassurfacedensitiesoforder5M /pc2 and5×103M /pc2 foraMilky
(cid:12) (cid:12)
Way-likegas-to-dustratio.
GalacticWinds 7
CombiningeachoftheseregimesintoasingleEddingtonlimitforthedustygas
M inthegalaxy,onefindsthat(e.g.,Thompsonetal.2015)
g
GM(<r)M c
L (cid:39) g [1+τ −exp(−τ )]−1 (7)
Edd r2 IR UV
whereM isthetotalmass,τ (cid:39)κ M /4πr2,κ istheRosseland-meanopacity,
IR IR g IR
whichisafunctionoftemperature,τ (cid:39)κ M /4πr2,andκ istheflux-mean
UV UV g UV
opacity over the radiation field from the stellar population. Note that equation (7)
assumes a spherical distribution of gas M at R around a point source of radia-
g
tion, and that it is simply generalized to a thin plane parallel disk geometry. Tak-
ingthelimitτ (cid:28)1orτ (cid:29)1,equation(7)reducestothemorefamiliarlimits
UV IR
L (cid:39)4πGMc/κ and(cid:39)4πGMc/κ ,respectively.Inthesingle-scatteringlimit,
Edd UV IR
applicableoverawiderangeofcolumndensitiesfrom∼5−5000M /pc2,
(cid:12)
L (cid:39) 4πGMΣ c (cid:39) 2×1011L M N (8)
Edd g (cid:12) 10 21
whereΣ =M /4πr2,N =N/1021cm−2istheparticlecolumndensity,andM =
g g 21 10
M/1010M .ThisexpressionfortheEddingtonluminosityshouldbecomparedwith
(cid:12)
equation(4).
Forcontinuousoptically-thinradiationpressuredrivenflowinapoint-massgrav-
itational potential, from the momentum equation one finds that the asymptotic ve-
locityofthegasis
V =V (R )(Γ−1)1/2 (9)
∞ esc 0
where V2 (R )=2GM(R )/R , R is the launch radius, and Γ =L/L is the
esc 0 0 0 0 Edd
Eddingtonratio.ForanEddingtonratiooforderΓ ∼few,theexpectationisthatthe
bulkofthematerialshouldbeacceleratedtooforderthelocalescapevelocity.For
anisothermalsphereofvelocitydispersionσ,V ∼2σ.
∞
Forageometrically-thininitiallyoptically-thickdustyshell,theflowcanachieve
highervelocitybecausewhileitisinthesingle-scatteringlimit,itgathersallofthe
momentum.Inthiscase(Thompsonetal.2015)
(cid:18)2R L(cid:19)1/2
V (cid:39) UV (cid:39)600km/s L1/2κ1/4 M−1/4, (10)
∞ M c 12 UV,3 g,9
g
whereR =(κ M /4π)1/2istheradialscalewheretheshellbecomesoptically-
UV UV g
thin to the incident UV radiation, L =L/1012L (SFR(cid:39)100M /yr), κ =
12 (cid:12) (cid:12) UV,3
κ1/4/103cm2/g,andM =M /109M .
UV g,9 g (cid:12)
Many questions about the importance of radiation pressure feedback in galax-
iesremain.Thefirstisthatalthoughradiationpressuremaydominatethedispersal
ofgasinGMCsbeforethefirstsupernovaeexplode,onaverage,supernovaexplo-
sions inject more total momentum into the ISM than photons under standard as-
sumptions, and may thus dominate the driving of turbulence within galaxies, and
potentially wind driving. The total momentum of a given supernova remnant is
enhanced relative to the initial value of the explosion as it sweeps up ISM mate-
8 Heckman&Thompson
rial during its energy-conserving phase. This boost to the asymptotic momentum
means that the momentum injection from supernova remnants can be as large as
P˙ ∼10L/cforsteady-statestarformation.Howthemomentuminjection
SN,remnants
and turbulence driven by supernovae couples to a galaxy-scale wind is a topic of
activeresearch.
Asecondissueforradiationpressurefeedbackisthatindensestarclustersand
starburst galaxies, the IR optical depth may significantly exceed unity, leading to
the question of whether or not there is a significant boost to the total momentum
deposited, or whether the trapped photons escape via low-column density sight-
lines.Inprinciple,themomentuminputcouldbeaslargeasτ L/c.Finally,thefull
IR
dynamical radiation transport problem has not yet been solved self-consistently in
multi-dimensionalsimulationsofgalaxies,starformation,andwinds.
2.3 CosmicRayDrivenWinds
Although massive stars deposit energy and momentum directly into the ISM via
their supernova explosions, there is another way they may drive the emergence
oflarge-scalegalacticwindsinstar-forminggalaxies:cosmicrays.Approximately
10% of the 1051 ergs in kinetic energy of supernova explosions is thought to go
into primary cosmic ray protons (and other nuclei), with a power-law spectrum of
particleenergiesfromGeVtoPeVproducedbyFermishockacceleration.Thetotal
energyinjectionrateincosmicraysisthenoforder
L (cid:39)3×1040ergss−1 SFR (cid:39)8×10−4L. (11)
CR 1
Once injected by supernovae, cosmic rays scatter off of magnetic inhomo-
geneities in the ISM with pc-scale mean free path λ as they diffuse out of the
hostgalaxy.Thescatteringprocesstransferscosmicraymomentumtothegas,and
the large implied scattering optical depth (τ ∼R/λ ∼kpc/pc∼103) implies a
CR
largesteady-statecosmicraypressureandenergydensity(Ipavich1975;Breitschw-
erdt,McKenzie,&Voelk1991;Everettetal.2008;Jubelgasetal.2008;Socrates,
Davis, Ramirez-Ruiz 2008). In the Milky Way, the local cosmic ray energy den-
sity is roughly comparable to the energy density in magnetic fields, photons, and
turbulence.Eachhasanassociatedpressureroughlycomparabletothatrequiredto
supportthegasoftheGalaxyinverticalhydrostaticequilibrium(Boulares&Cox
1990).
ThesamemaybetrueofstarburstgalaxieslikeM82,andifso,cosmicraysmay
beimportantinwinddriving.Indeed,analyticargumentsakintotheEddingtonlimit
for photons discussed above have been made by Socrates et al. (2008) that show
cosmicraysmaybeimportantinregulatingstarformationanddrivingoutflows.The
large scattering optical depth implies that the total effective momentum injection
ratewouldbeP˙ ∼τ L /c∼(L/c)(τ /103),oforderthemomentuminputin
CR CR CR CR
lightfrommassivestars,butwithverydifferenttransportproperties.
GalacticWinds 9
Multi-dimensionalsimulationsofgalaxiesarebeginningtosimulateCR-driven
winds in detail (e.g., Girichidis et al. 2016). Several questions still need to be ad-
dressed. The first is the importance of pion production via inelastic scattering of
cosmicraysoffofISMgas.Thesecollisionsproducechargedandneutralpionsthat
decay to secondary electron/positron pairs, neutrinos, and gamma-rays. Because
nearby starbursts like M82 are observed to be gamma-ray bright, the implication
isthatmanycosmicraysinteracttoproducepionemissionbeforeescapingthehost
galaxy. This may limit the effective total scattering optical depth, the steady-state
pressure,andthetotalcosmicraymomentumavailabletodriveoutflows.Arelated
issueishowthecosmicrayscoupletothegas,andwhetherornottheysamplethe
average density gas, or a lower-density medium. Since the timescale for pion pro-
ductionisinverselyproportionaltothegasdensitysampled,thisisakeyissuefor
determining how much momentum is transferred from the cosmic rays to the gas
before pion production. Nevertheless, extended radio emission is observed along
the minor axis of M82 (Seaquist & Odegaard 1991), which indicates the presence
ofrelativisticelectrons/positronsandmagneticfields,andcosmicray-drivenmodels
remainatopicofactiveresearch.
3 ObservationalPropertiesofOutflows
3.1 AGuidedTouroftheMulti-phaseOutflowinM82
Theconditionsneededtodrivegalacticoutflowsarerareinthelocaluniverse,but
commonatredshiftsaboveaboutone(wewillquantifythesestatementsinsection
3.4 below). In fact, in the local universe, galactic outflows are only observed in
galaxiesundergoingunusuallyintenseepisodesofstar-formation(“starburstgalax-
ies”). We therefore begin our discussion of observations of galactic outflows with
asummaryoftheproto-typicalexampleassociatedwiththestarburstgalaxyM82.
Located at a distance of only 3.6 Mpc, this is the brightest and best-studied ex-
ampleofastarburst-drivenoutflow.Whilethedataarethereforethebestandmost
complete,observationsofotherstarburst-drivenoutflowsarequalitativelyconsistent
withthoseofM82.
TheM82starbursthasastar-formationrateofabout7to10M peryear(assum-
(cid:12)
ingastandardChabrier/Kroupa)initialmassfunction.Thestarbursthasaradiusof
400pc,yieldingastar-formationrateperunitarea(SFR/A)ofabout15to20M
(cid:12)
year−1kpc−2.Forcontext,thisisovertwoorders-of-magnitudelargerthanthechar-
acteristicvalueinthediskoftheMilkyWay,butistypicalofpresent-daystarbursts
andstar-forminggalaxiesathigh-redshift.
ThesehighvaluesforSFR/Amayallowfortheefficientconversionofthekinetic
energy supplied by core-collapse supernovae and winds from massive stars into
the thermal energy of a very hot fluid since most supernovae will explode in the
hot rarified gas created by prior supernovae. Subsequently, there can be efficient
10 Heckman&Thompson
conversion of this thermal energy into the bulk kinetic energy of a volume-filling
“windfluid”(asdiscussedaboveinsection2).Thedirectobservationalevidencefor
theexistenceforthishotfluidofthermalizedstellarejectaintheM82starburstis
providedbyhardX-rayobservationswhichrevealthatdominantionicstageofFein
thediffusehotgasinsidethestarburstisHelium-like.Theimpliedtemperatureofthe
gasisbetween30and80×106K.Analysisofthepropertiesofthisgasshowsthat
it is consistent with the simple Chevalier & Clegg (1985) model described above,
with a thermalization efficiency of α ∼0.3−1 and a mass-loading factor of β ∼
0.2−0.6. The implied terminal velocity for an outflow fed by this very hot gas is
V (cid:39)1400to2200kms−1(Strickland&Heckman2009;eq.1).
hot,∞
DirectobservationalevidenceforanoutflowinM82datesbackdecades(Lynds
& Sandage 1963) to the discovery of an extensive system of filamentary optical-
emission-line gas extending to radii of several kpc from the central starburst out
along the minor axis of the edge-on galaxy (Figure 1). This gas can also be ob-
servedthroughnebularlineandcontinuumemissioninthevacuumultraviolet,and
throughmid-andfar-IRfine-structurelineemission(Hoopesetal.2005;Contursi
etal.2013;Beira˜oetal.2015).DetailedspectroscopyhasshownthatthisT ∼104
Kgashasemission-lineratiosconsistentwithamixtureofgasthatisphoto-ionized
by radiation leaking out of the starburst and shock-heated by the outflowing wind
fluid generated within the starburst (Heckman, Armus, & Miley 1990). We will
henceforth refer to this as the warm ionized phase. The kinematics of this gas im-
plies that we are seeing material located largely along the surfaces of a bi-conical
orbi-cylindricalstructurethatoriginatesatthestarburst.Theinteriorofthisstruc-
tureispresumablyfilledbytheoutflowingwindfluid(Shopbell&Bland-Hawthorn
1998).Correctingthemeasuredoutflowspeedofthewarmionizedgasforline-of-
sight effects yields intrinsic outflow speeds of about 600 km sec−1. Note that this
significantly slower than the inferred outflow velocity for the hot wind fluid itself
(∼1400−2200km sec−1). The velocity field shows rapid acceleration of the gas
fromthestarburstitselfouttoaradiusofabout600pc,beyondwhichtheflowspeed
isroughlyconstant.
The morphological structure of this warm ionized phase is strongly correlated
withthestructureoftheco-spatialsoft(<2keV)X-rayemission(Lehnert,Heck-
man, & Weaver 1999; Figure 2). This X-ray emission primarily traces gas with a
characteristic temperature of ∼5−10×106K (hereafter, the “hot phase”). A de-
tailed comparison shows that while there is a global correspondence between the
emissionfromthewarmionizedandhotphases,onalocallevel,theemissionfrom
the hot phase appears to be systematically located upstream of, or interior to, that
fromthewarmionizedphase.OnenaturalinterpretationisthattheX-rayemission
maybesomesortofinterfacebetweenthetenuouswindfluidandthewarmerand
denser gas that the wind fluid is interacting with (e.g., shocks or turbulent mixing
layers). The clearest example of the relationship between the two phases is in the
“cap” of M 82, a filamentary structure located about 12kpc in projection above
the starburst, with an orientation roughly perpendicular to the outflow (Lehnert,
Heckman,&Weaver1999;seeFigure1).OneinterpretationisthatthesoftX-ray
emission — which is located about 0.5kpc upstream of the region of optical line