Table Of ContentBipolar hypershell models of the extended hot interstellar medium in
spiral galaxies
1. Institute of Astronomy, University of Tokyo, Mitaka, Tokyo 181-0015, Japan
([email protected])
2. CEA/Saclay, DAPNIA, Service d’Astrophys, L’Ormes des Mersiers, Bˆat. 709,
1 F-91191 Gif-sur-Yvette, France
0
0 February 1, 2008
2
n
a Abstract
Keywords galaxies: general – galaxies: individual:
J
NGC 253, NGC 1808, NGC 3079, NGC 4258, M 82
0
3 We simulated the million degree interstellar medium (NGC 3034), the Milky Way – galaxies: evolution –
andits softX-rayimagesin the disk andhalo ofspiral galaxies: ISM – galaxies: starburst– galaxies: spiral–
1 galaxies using the bipolar hypershell (BHS) model. In galaxies: structure–galaxies: X-rays–X-rays: general
v
this model dumbbell- or hourglass-shaped expanding – X-rays: galaxies
5
shells of several kpc radii are produced by a sudden
2
5 energy release in the central region. We then applied
1 our model to a mini-sample of starburst galaxies seen 1 Introduction
0 underdifferentinclinations,namelythenearlyedge-on
1
galaxies NGC 253, NGC 3079 and M 82, the highly A bipolar hypershell (BHS) model has been proposed
0
inclined galaxy NGC 4258 as well as the nearly face- to explain the double-horn features observed in the
/
h on galaxies NGC 1808 and M 83. For all galaxies, our radio-continuumhaloofthestarburstgalaxyNGC253
p
results reproducethe X-raycharacteristicsobservedin (Sofue 1984). Recent soft-X-ray observations of NGC
-
o the 0.1–2.4keV ROSAT energyband: The bipolar hy- 253 with ROSAT revealed extended outer-halo emis-
r pershell morphology, the spectral energy distribution sion with double-horn morphology, and is interpreted
t
s ofthe diffuse disk andhalo emissionas wellas absorp- as being a hypershell driven by a central starburst
a
: tion gaps in the diffuse X-ray emission caused by a (Vogler and Pietsch1999a;Pietsch et al. 2000). Many
v shadowingofsoftX-raysduetocoldinterveninggasin spiral galaxies exhibit similar galactic-scale outflows
i
X the disks ofthe galaxies. Ingeneral,the requiredtotal in the form of dumbbell-shaped hypershells, bipolar
r energyforthe starburstis estimatedto be ofthe order cylinders, or galactic-scale jets. Examples of proto-
a of 1055 ergs, corresponding to the overall kinetic en- types of such galaxies are M 82 (NGC 3034), NGC
ergy generated in ∼ 104 type-II supernova explosions. 1808 and NGC 3079 (see next section for references).
The expansion velocity of the shells is estimated to be Starburst-origin models for extended halo features in
∼ 200 km s−1, which is necessary to heat the gas to galaxies were proposed by several authors (Heckman
∼ 0.2 keV (2.3 million K), and the age to be of the et al. 1993;Tomisaka andIkeuchi1987;Suchkovet al.
order of 3×107 years. In the case of the very nearby, 1994, 1996).
nearly edge-on galaxy NGC 253 all characteristics of The Milky Way also exhibits bipolar hypershells ex-
the BHS model can be studied with high spatial reso- tending above and below the galactic plane in soft X-
lution. rays (Snowden et al. 1997) and radio continuum (e.g.
Usingthe propertythatthe shellmorphologyissen- Haslam et al. 1982), which are also interpreted as an
sitive to the ambient density distribution, we propose effect ofa giantΩ-shapedshock front originatingfrom
using soft X-ray data to probe the gas distributions in a starburst some 107 years ago in the Galactic Center
the disk, halo and intergalactic space in general. The (Sofue 1977, 1984, 1994,2000). These hypershells and
applicationofourmodeltoimagesathigherspatialand flows,includingthatintheMilkyWay,requireasimilar
spectralresolution,asprovidedbyChandraandXMM, amountoftotal energyofthe orderof∼1055 ergscor-
willhelpus tofurtherdisentanglethe ISMdensity dis- responding to the energy release from 104 to 105 type
tributions and will lead to a better understanding of II supernovae. This fact suggests that they may have
the disk halo interface. acommonpropertyandorigin,andcanbemodeledby
1
a unified scheme of energy release in the galactic cen- halo emission is attributed to a superwind emanating
ter and its propagation into the halo and intergalactic fromthe starburstnucleus. The latterassumptioncan
space. also explain the “hour-glass-” or “dumbbell-shaped”
In this paper, we simulate the extended soft X-ray morphology of the outer X-ray halo. Pietsch & Vogler
halocomponentsinvariousgalaxiesbasedonthebipo- (2000) estimate temperature of the outer halo compo-
lar hypershell (BHS) model developed for the Milky nent at 0.2 keV (2.3 million K), corresponding to an
Way halo (Sofue 1977, 1984, 1994, 2000). We further equivalent shock-heating velocity of 200 km s−1.
proposeto use the simulationto date andmeasure the
explosive events at the nuclei, such as their starburst 2.2 NGC 1808
history. The simulation can also be used to obtain
informationaboutthegalactic-halo-intergalacticinter- NGC1808isagalaxywithlarge-scalecylindricaldusty
face in disk galaxies. The X-ray absorption morphol- jets emanatingfromthenucleusathighvelocities,and
ogy near the galactic plane can be used to probe the is supposed to be a mildly tilted M82 type galaxy
interstellar gas distribution in the galactic disk. (V´eron-Cetty and V´eron 1985; Phillips 1993). Soft X-
ray imaging observations with the ROSAT PSPC and
HRI revealed an active X-ray nucleus and extended
2 Soft X-ray Hypershells in emissioninthediskregion(Junkesetal. 1995;Dahlem
et al. 1994),as wellas a veryextended X-rayemission
Nearby Spiral Galaxies
in the halo (Iwaki et al. 1996). The total soft X-ray
luminosity is of the order of 3×1040 ergs s−1. Iwaki
2.1 NGC 253 et al. (1996) obtained wide area ASCA images in the
0.5–2 keV band, and found that the extended compo-
ROSAT images of the nearby (d = 2.6 kpc, Puche & nent is highly elongated in the direction of the dusty
Carignan 1988) starburst galaxy NGC 253 revealed jets on both sides of the galactic plane. Furthermore,
diffuse X-ray emission from the nuclear region, the theX-rayhaloappearstobebifurcatedintotworidges,
disk and the halo hemispheres (Pietsch et al. 2000). suggesting that it comprises double-horn feature or a
The emission measures of the three components are partial shell structure.
nearlyidentical,andtheintegraldiffuseemission(Lx =
4×1039 ergs−1)accountsfor∼80%ofthetotalX-ray
2.3 NGC 3079
luminosity from NGC 253. The diffuse bulge emission
is due to an extended, highly absorbed source with an Thenearlyedge-ongalaxyNGC 3079isknowntohost
extent of 250 kpc, and a hollow-cone shaped “X-ray an active nucleus classified as a LINER or Seyfert ii
plume” with an extent of ∼ 700 pc along the SE mi- nucleus, and giant radio lobes and Hα superbubbles
nor axis. The X-rays in the bulge region are thought emanate along the minor axis (Irwin & Seaquist 1991;
to trace the interaction between a galactic super-wind Cecil etal. 1995). ROSAT observations(Pietschet al.
driven by the starburst nucleus and dense interstellar 1998) detected an integral 0.1–2.4 keV luminosity of
medium in the disk. Lx =3×1040 ergs−1,whichcanberesolvedintothree
Inthe disk,the highlyabsorbeddiffuse emissionfol- components. One component consists of extended
lows the spiral arm structure. It most probably re- emissionfromthecentralregion(Lx =1×1040ergs−1)
flects a superposition of diffuse X-ray emission from originatingfromthe regionofthe centralsuperbubble.
hot interstellar and discrete sources, such as emission Whilemostofthisemissionisthoughttorepresenthot
frombinaries,SN-remnantsorHiiregions. Twomajor gas in the superbubble, one cannot exclude additional
components of the diffuse halo emission can be sepa- contributions of ≤ 30% from the active nucleus. The
rated: “coronal emission” arising from hot interstel- second main emission component arises from the disk
lar medium close to the disk (scale height ∼ 1 kpc) ofthegalaxy(Lx =7×1039ergs−1)andcanbepartly
and emission from the “outer halo”. NGC 253 is not resolved into point-like sources. Each of the three de-
completely oriented edge-on, and the NW side of the tectedpointsourceshasLx ∼6×1038ergs−1. Finally,
galaxy is closer to us. Due to this viewing geome- emission from the halo (Lx = 6×1039 erg s−1) is de-
try and absorption of the soft X-rays by the disk of tected. AsinthecaseofNGC253,thehotgasdoesnot
NGC 253, the coronal emission can only be detected fill the halo hemispheres uniformly, but an hourglass-
from the near (SE) side of the galaxy, while emission shaped structure of the halo emission is reported. A
fromtheouterhalocomponentispartlyshieldedbythe meantemperatureforthehalogaswasestimatedtobe
Hi disk of NGC 253 to the NW. Following Pietsch et 3.5×106 K. Interestingly, the lowest contours of the
al. (2000),theX-raycoronamightbefueledbyhotgas extendedemissionshowasaddleshape,witha neckat
fromgalacticfountainswithinthedisk,whiletheouter the galactic plane, suggestive of bipolar double-horns.
2
2.4 NGC 4258 Suchkov et al. (1994). From their comparison, they
favor the model of Suchkov et al. (1994). This model
NGC 4258’s anomalous arms could also be due to
assumes a two component cold rotating dense disk, a
out-of-plane partial shells (van Albada and van der
non-rotating hot tenuous halo, and a starbursthistory
Hulst 1982; Vogler and Pietsch 1999b). Diffuse X-
incorporating the milder mass and energy input from
ray emission of the galaxy was detected with Lx ∼ stellarwindsintothehalobeforemoreenergeticsuper-
2×1040 erg s−1 and fills ∼ 40% of the D25 diameter nova explosions dominate the scenario. Shocked halo
of NGC 4258. At least one half of the diffuse emission
gasthenprovidesthemajoremissioncomponentinthe
is caused by the anomalous spiral arms of this galaxy,
ROSAT band.
andthe X-rayemissionmorphologyclosely follows the
pattern detected in the radio wavelengths. A reanalysis of the ROSAT data (Lehnert et al.
In the literature, those arms are discussed to be (a) 1999) and new Hα images showed the existence of a
jets confined to the disk of the galaxy and expelled by 4 kpc × 1 kpc regionof spatially coincidentX-ray and
the active nucleus (e.g., Martin et al. 1989, Hummel Hα emission about 10 kpc north of the nucleus. The
et al. 1989, Plante et al. 1991, Cecil et al. 1995), (b) luminosityinbothwavebandsaccountsforroughly1%
outflows (jets or plumes) from the active nucleus leav- of the total M 82 luminosities in both bands. The au-
ingtheplaneofthegalaxyandpossibly‘rainingdown’ thors attribute the Hα emission to ionizing radiation
onto the disk again (van der Kruit et al. 1972), (c) a from the starburst that propagates into the halo, and
hypershell of hot interstellar medium as presentin the the X-ray emission to a shock-heated massive ionized
nuclear region of NGC 253 and expanding from the cloud in the halo, which is encountered by the super-
diskintothehalo(Sofue1984,2000),or(d)barshocks wind. With this scenario, they follow theoretical pre-
heating the interstellar medium due to the high differ- dictions of Suchkov et al. (1996).
ential velocities between the bar orbit and interstellar
mediuminthebulgeandinnerdiskofNGC4258(e.g.,
Cox & Downes 1996).
Remaining diffuse emission not correlated with the
2.6 The Milky Way
anomalousarmsmightoriginatefromthe disk andthe
halo of the galaxy. For a possible halo component,
the temperature was estimated to be 2×106 K. This Using ROSAT All Sky Survey(RASS) data,presented
temperature is lowerby a factor of 2 than the temper- in several energy bands from 0.1 to 2.4 keV in Snow-
ature of the hot gas along the anomalous spiral arms den et al. (1997), we showed that the North Polar
(T ≥4×106 K). Spur(NPS)anditswesternandsoutherncounter-spurs
In addition to the diffuse emission, 14 point sources trace a giant dumbbell-shape wit a neck at the galac-
(integral Lx ∼ 2 × 1039 erg s−1) were found in the tic plane (Sofue 2000). These features are interpreted
galaxy. For possible contributions of the active nu- as effects of a shock front originating from a starburst
cleus to the X-ray emission, an upper limit of Lx ∼ 15 million years ago with a total released energy of
1×1038 erg s−1 was found in the ROSAT 0.1–2.4keV the order of ∼ 1056 ergs or 105 type II supernovae.
band. We simulated all-sky distributions of radio continuum
and soft X-ray intensities based on the bipolar hyper-
shell galactic center starburst model. The simulations
2.5 M 82
nicely reproduce the radio NPS and related spurs, as
wellasradiospursinthetangentialdirectionsofspiral
M82 is an edge-on starburst galaxy, which ejects
arms. Simulated X-raymaps in 0.25,0.75 and1.5 keV
a galactic-scale flow through bipolar cylindrical jets
bands reproduce the ROSAT X-ray NPS, its western
(Nakai et al. 1987) to which larger-scale X-ray ex-
and southern counter-spurs, and the absorption layer
tended features are associated (Lehnert et al. 1999).
along the galactic plane. We further proposed to use
Stricklandetal. (1997)reportthedetectionofagalac-
tic wind. The X-ray emission was visible out ≥ 6 kpc theROSATall-skymapstoprobethephysicsofgasin
the halo-intergalactic interface of the Milky Way, and
aboveandbelowtheplaneofthegalaxy,andthelumi-
to directly date and measure the energy of a recent
nosityofthediffuseX-rayemissionoutsidethenuclear
region is ∼ 2×1040 erg s−1. From X-ray spectra, the Galactic Center starburst. The present paper is a se-
ries of the BHS modeling of extended halo X-rays in
temperature of the gas is found to vary from 0.6 keV
galaxies using the same numerical code.
near the nucleus to 0.4 keV for distances further away
from the plane. The authors compared the X-ray re-
sultswithseveraltheoreticalmodels,namelyTomisaka
& Ikeuchi (1988), Tomisaka & Bregman (1993) and
3
3 Bipolar Hypershell Starburst at ρ3 = 10−5H cm−3, while the other parameters are
Model set in various possible combinations as listed in Table
1. We also calculate a case in which the halo density
decreases with a power law. We further calculate an-
We adopt the same numerical methods to trace the
other case with uniform halo density which decreases
shockenvelope andX-rayintensity distributions as we
suddenly at a certain height facing the intergalactic
used to model the radio spurs and BHS in the Milky
gas, which represents a case for halo gas confined by
Way, which is described in detail in Sofue (2000). We
the intergalactic gas pressure.
briefly describe the methods below.
SolutionsoftheaboveequationsarepresentedinFig.
1 for various parameter sets given in Table 1. The
3.1 Adiabatic-Shock Envelope Method
total energy input is taken to be E0 = 0.75 × 1055
The propagation of a shock wave through the galac- ergs for S1 to S5, and 3×1055 ergs for S6, and the
tic halo induced by a point energy injection at the shock envelopesaredrawnevery10 millionyears. The
center can be calculated by applying the shock enve- shock velocity can be estimated by dividing the radial
7
lopetracingmethodofSakashita(1971)andMo¨llenhoff distance between succeeding envelopes by 10 years.
(1976), who extended the Laumbach and Probstein Models S1 and S2 : Exponential disk with scale
(1969)methodfortracingtheevolutionofashockfront height 0.1 and 0.2 kpc, respectively, exponential halo
toacaseofaxi-symmetricdistributionsofambientgas. with scale height 1 kpc, and intergalactic gas of uni-
The flow field is assumed to be locally radial, and the form density.
gas is adiabatic, and, therefore, the heat transfer by ModelsS3andS4: Gaussiandiskwithhalfthickness
radiationandcounter-pressureareneglected. Theden- 0.1 and 0.2 kpc, respectively, exponential disk of scale
sity contrast between the shock front and ambient gas height 1 kpc, and intergalactic gas of uniform density.
is given by (γ+1)/(γ−1)=4 for γ =5/3, where γ is Model S5: Power-law density disk, power-law den-
the adiabatic exponent of the gas. sity halo, and intergalactic gas of uniform density.
The equation of motion of the shock wave is given Model S6: Disk of 2 kpc thickness with sudden de-
as follows (M¨ollenhoff 1976): crease of density, facing the intergalactic low-density
uniform gas. This model represents a case where the
R P 2 R 1 ∂r 2 2 halo-to-disk gas is not gravitationally stratified (e.g.,
E0 =Z0 γ−14πr dr+Z0 2(cid:18)∂t(cid:19) ρ04πr0dr0 (1). as exponentially), but is confined by intergalactic gas
pressure, so that the internal pressure and density in
Here, E0 is the total energy of the explosive event, P the halo are more uniform.
is the internal pressure, γ is the adiabatic exponent,
— Fig. 1 —
ρ0 istheunperturbedambientgasdensity,andr isthe
radiusfromtheexplosioncenter,withsuffix0denoting
the quantities of the unperturbed ambient gas, and R 3.2 X-ray Emission and Absorption
is the radius of the shock front. This equation then
The X-ray intensity distribution for a model galaxy
leads to an equation of the shock radius, as described
havingahypershelliscalculatedasbelow. Thecenters
in Sofue (2000).
of the shells are assumed to be at z = ±6 kpc on the
The unperturbed density distribution of gas in a
rotation axis, and the radii 6 and 9 kpc in the radial
galaxy is assumed to comprise a stratified disk, a halo
andverticaldirections,respectively. Thevolumeemis-
with an exponentially decreasing density, and inter-
sivity is calculated from the density contrast of the
galactic gas with a uniform density. The differential
shocked gas, which is assumed to be proportional to
distribution is approximated by the following expres-
sion. ρ/ρ0 =(γ+1)/(γ−1),whereρandρ0 aregasdensities
in the shockedshell andunperturbed halo gas,respec-
ρ0 =ρ1exp(−(z/z1)η))+ρ2exp(−z/z2)+ρ3 (2). tively. Inthepresentsimulation,theprofileofemissiv-
ity perpendicular to the shell surface is simply repre-
Here, suffices 1, 2 and 3 denote quantities for the disk, sented by an exponentially decreasing function behind
halo and intergalactic gas, respectively, ρ is the den- the shock front toward the center with a scale thick-
sity, z is the height from the galactic plane, z is the ness of 500 pc. The emissivity also decreases with the
i
scalethicknessofthediskandhalo. Thepowerindexη height from the galactic plane with a scale height of 3
represents the tightness of the disk component toward kpc,correspondingtotheexponentiallydecreasinggas
the galactic plane, and is taken either 1 (exponential) density in the halo.
or 2 (gaussian). The third component, representing The X-ray emission from a hypershell is assumed to
the intergalactic gas density, is taken to be constant bethermalfree-freeradiation,whoseemissivityisgiven
4
Table 1: Parametersfor Shock Envelope Solutions
Model Energy ρ1 h1 Disk z type ρ2 h2 ρ3
ergs H cm−3 kpc H cm−3 kpc H cm−3
S1 0.4×1055 1 0.1 exponential 0.01 1 10−5
S2 ” 1 0.2 exponential 0.01 1 ”
S3 ” 1 0.1 gaussian 0.01 1 ”
S4 ” 1 0.2 gaussian 0.01 1 ”
S5 ” 1 0.1 power law 0.01 1 ”
S6 1.6×1055 0.1 2 exp[−(z/h1)4] 0.0 – ”
The shock envelope (radius and velocity) obeys a scaling law: For equal ratios E/ρ , one obtains the same
i
envelope, independent of the individual E, ρ values
i
by H cm−2 (Snowden et al. 1997). Since the ROSAT en-
ǫ=n2Λ∝ρ2T1/2 (3). ergybandsareratherbroad,coveringhigherandlower
e
energies around the representative energies, we adopt
Here, Λ is the cooling function of the gas. Since the
two representative cross sections at 0.75 and 1.5 keV
temperatureisashighasT ∼107 Kinoursimulation,
bands,whichare1and0.1timesthevalueat0.75keV,
theradiationisalmosttotallyfree-free,andthecontri-
respectively.
bution by recombination lines from hydrogen, helium,
The absorbing disk gas is assumed to be condensed
andmetals,whicharesignificantatT ∼105 K,canbe ◦
in logarithmic spiral arms of pitch angle 6 and the
neglected. A typical emission measure along the hy-
width is taken to be 1/5 of the arm separation, as in
pershell ridge is n2L ∼ 10−2 cm−6pc for a tangential
e the same way assumed for the Milky Way. The peak
pass in the shell of about 2 kpc. For simplicity, we
hydrogendensityinanarmatadistanceof8kpcfrom
assume that the temperature of X-ray emitting gas is the center is taken to be 5 H cm−3. The hydrogen
constant in the shocked shell. In addition to the hy-
density distribution is expressed by
pershells, we assume a galactic disk X-ray component
of scale height of 500 pc, and a bulge component of nH =αn0exp(−r/rH−z/zH)cosk(θ−ηlog r/r0) (6).
scale radius 1 kpc. X-ray emissions from the ambi-
ent (unperturbed) halo gas is neglected, because the Here, θ and η are the azimuthal angle and the pitch
density would not be sufficiently high to contribute a angle of the arms.
significant 0.1-2.4 keV X-ray emission measure.
The X-ray intensity, I, is calculated by
4 Results of Simulations
dI =ǫXds−κIds (4),
4.1 NGC 253
where the first term in the right-handside is the emis-
sion measure with ǫX being the X-ray emissivity, and The expansion velocity of the outer halo is estimated
the second term represents the absorption rate with κ to be 200 km s−1, which is required for shock heating
beingabsorptioncoefficient,andsisthedistancealong of the gas up to 0.2 keV (2.3×106 K) (Pietsch et al.
the line-of-sight. 2000). TakingthisexpansionvelocityfortheBHSat5
The interstellar extinction of soft X-rays occurs due kpc height from the galactic plane in the ROSAT 0.75
tothephotoelectricabsorptionbymetals,andhasbeen keV contour map, we find that the observation is well
calculated for the solar metal abundance by Morrison fittedbyashockfrontat∼3×107 yrsforanexplosion
and McCammon (1983). The absorption coefficient κ energy of ∼4×1054 ergs as in Table 1.
is, then, represented by Among the models, however,a better fit is obtained
by Models S2 and S6 than by the other models, which
κ=(nH/NH0)(EX/E0X)−2.5 (5), indicates that the decrease of halo gas density in the
direction perpendicular to the disk is not as steep as
wherenHisthenumberdensityofhydrogenatoms,and in the other models. In Fig. 2 we show the shock
N0 is the e-folding column density of interstellar neu- front for Models S2 and S6 superposed on the 0.75
H
tral hydrogen at the photon energy EX (Ryter 1996; keV ROSAT map of NGC 253 in contours overlaid on
0
Sofue 2000). At EX = 0.75 keV, the e-folding column a B-band optical photograph from DSS. The ROSAT
0
density for EX = 0.75 keV is given as N0 = 3×1021 result shows that the hypershells tend to break and
0 H
5
become round at high altitudes, when the shock front BHS model, which was originally used to model the
reaches a few kpc from the galactic plane, mimicking North Polar Spur NPS and associated spurs in the
a dumbbell shape. Hence, Model S6 appears to be Milky Way halo (Sofue 2000). It is, therefore, inter-
preferable to S2, for which the BHS is more like an esting to see, whether the BHS model also applies to
“hourglass”. Model S6 represents a case in which the extended X-ray features observedin other galaxies ex-
gas density is uniform within the disk and halo up to hibitingstarburstsandoutflows. Here,wecomparethe
a certain height (e.g. 2 kpc), where the halo faces a observed morphology with the outcomes of our theo-
uniformintergalacticgasoflowerdensity. Suchdensity retical description. We calculated 0.75 keV intensity
◦
distribution will be possible if the halo gas is confined distributions for various inclination angles at 0 (face-
◦ ◦ ◦ ◦ ◦
by the intergalactic gas pressure, but not stratified by on),30 ,45 ,60 ,80 ,and90 (edge-on). Fig. 5shows
a hydrostatic equilibrium under the disk gravity. thecalculatedresults,andtheyarecomparedinFig. 6
◦
—- Fig. 2 —- with observed examples for M83 (i=24 ), NGC 1808
◦ ◦ ◦
AlthoughModelS6appearsbetter,overallreproduc- (i = 58 ), NGC 4258 (i = 67 ), NGC 253 (i = 78 ),
tion of the observations is obtained by both models. NGC 3079 (edge-on), M82 (edge-on) and the Milky
For simplicity, we hereafter use the Model S2 in which Way (edge-on).
the shape of the shell can be well approximated by a — Fig. 5 —
part of an ellipsoidal sphere. This allows us a precise — Fig. 6 —
prediction for the X-ray emissivity and density distri- M83: When a galaxy with BHS is observed face-
butions. Fig. 3showsthecalculatedintensitydistribu- on, the X-ray images exhibit a bright ring feature as
tions for NGC 253for two differentopacity (κ) values, in Fig. 5. If the galaxy is slightly inclined such as
which correspond to (a) a case of 0.75 keV band, and i = 30◦, the BHS ridges in both sides of the galactic
(b)1.5keVband. WealsoshowtheROSATX-rayim- plane are superposed to exhibit a box-shaped bright
ages in the right panels: right-topfor 0.75 keV in gray region. Such a box-shaped X-ray distribution appears
scale, and the right-middle and right-bottom are for to be observed indeed in the ROSAT PSPC image of
0.75 and 1.5 keV, respectively, overlaid on an optical M83,asshowninFig. 6(Stricklandetal. 1997),where
photograph. As seen in the figures, the BHS simula- thearrowpointstowardthedirectionoftheminoraxis
tion reproduces the following observed characteristics: as the near side. Although it is difficult to distinguish
(1)double-hornmorphologyofthediffuseemissionfea- the halo emission from disk contributions, spiral arms
turesinthe 0.75keVbandemergingfromthe disk,(2) and/or the bar, some emission along the boxy edges
strongabsorptionalongthe galacticplane, and(3)en- could be due to overlapped ridges of BHS.
ergy dependence of the intensity distribution, namely,
NGC 1808: The extended 0.5–2 keV features bifur-
harder emission near the disk.
catedinthehaloofNGC1808asobservedwithASCA
— Fig. 3 —
(Iwaki et al. 1996) are also reproduced by our simula-
The Combination of three NGC 253 images in in-
tions, althoughthe lopsidedness cannotbe reproduced
dependent energy bands centered at 0.25, 0.75 and
well. ThearrowinFig. 6indicatesthe directionofthe
1.5 keV enabled the construction of a true color im- dusty jet along the minor axis in the nearer side.
age (Pietsch et al. 2000). For this purpose, the dif-
NGC 4258: This is a galaxy known for its anoma-
ferent subbands were associated with the colors red,
lous radio arms, along which X-ray extended emission
green and blue, for the 0.25, 0.75 and 1.5 keV band,
isobserved,asisshowninthe0.1–0.4keVROSATmap
respectively. Fig. 4 shows the thus obtained colored
(Vogler and Pietsch 1999b). Although the resolution
intensity distribution compared to the observed spec-
is still too crude, some spur features are mimicked by
tral intensity distribution taken from Pietsch et al.
the BHS model. The origin of the anomalous arms is
(2000),whereredstandsforsoftandblueforhard. The
controversial, as described in section 2, but we com-
present BHS model can nicely reproduce the observed
ment that the anomalous radio arms (van Albada and
spectrum distribution, such as the harder spectra in
vander Hulst 1982)arenicely fitted by twooppositely
the disk and spiral arms due to stronger absorption of
extendingridgesoftheBHS.Inparticular,theirround
softer X-rays.
shapesandcurvatureofthe armsreturningtothe disk
— Fig. 4. —
canbeexplainediftheyarepartofBHS.However,the
lobsidednessoftheanomalousarmswithrespecttothe
4.2 BHS at Various Inclination: Com- rotationaxisremainsaquestioninthe BHSmodel,al-
though some lopsidedness is seen in the BHS of the
parison with Other Galaxies
Milky Way (Fig. 6 bottom, constructed from the en-
We have thus shown that the extended X-ray features ergy bands presented in Snowden et al. 1997).
in the halo of NGC 253 are well reproduced using the M82: The ROSAT HRI image (Strickland et al.
6
1997) shows a bipolar extended halo elongated in the able to reproduce the dumbbell shape of the observed
direction perpendicular to the disk, and the emission X-ray lobes. This suggests that the disk-to-halo gas
appears to be centrally filled, indicating that the out- densityisratheruniformuntilitfacestheintergalactic
flowiswellcollimated,aswellsimulatedbythe“jet-like low-density region. Such uniform density distribution
wind” modelby Suchkovet al. (1994,1996). We men- may be possible if the gas is confined by the gas pres-
tion, however, that the northern end of the flow at a sure, not hydrostatic equilibrium in the disk gravity.
′
height of about 2.5 kpc (3) from the disk appears to Inordertoclarifythis andfittheobservationsinmore
bemoreopenedintotheintergalacticspace,suggesting detail, higher resolution imaging in the soft X-rays is
double-horn ridges or a partial shell. necessary.
NGC 3079: The radio-lobe edge-on galaxy NGC
3079 shows a box-shaped X-ray features, suggesting
a double-horn morphology, as observed in the ROSAT 5.2 Origin of the Hypershells
PSPC image (Pietsch et al. 1998), and is well repro-
duced by an edge-on BHS model. In our model, the energy injection at the galactic cen-
Milky Way: As mentioned in Sect. 2.6, the RASS teris assumedtobe impulsive,andthe shockis strong
enoughtocreateawell-definedshellstructure. There-
reveals bipolar hypershells, which make up the North
quiredtotalenergygiventotheinterstellargasisofthe
PolarSpuranditscounter-spurs. Theobservationsare
order of ∼ 1055 ergs. The time scale of the explosive
wellfitted by the BHS model. A detailed modeling for
event responsible for the energy release is assumed to
thecaseoftheMilkyWayisdescribedinSofue(2000).
besignificantlyshorterthantheexpansiontimescaleof
the hypershell,t∼r/v ∼107 yearsfor r ∼ severalkpc
5 Discussion andv ∼300kms−1. So,theexplosion(starburst)time
scale must be shorter than a few million years. Such
5.1 Probing Starburst and Halo Gas impulsiveness androbustnessofthe energyreleasecan
Distribution be explained if the nucleus of the galaxy has experi-
enced a starburst ∼ 107 years ago, lasting for a few
In the presentshock approximation,the expansionve- million years or shorter, during which ∼104...5 type II
locityv atradiusr isrelatedtothemeanambientden- supernovaeexploded. Itis worthmentioningthatsim-
sity ρ0 and the explosion energy E0 as E0 ∼ ρ0r30v2. ilar timescales are predicted from starburst modeling
Here, r is measured from the observed shell radius, based on infrared observations (e.g., F¨orster-Schreiber
and the expansion velocity is also measured from the et al. 2000).
gaseous temperature inferred from X-ray spectrum. Alternative models have also been discussed by sev-
Forgivenradiiandvelocities(temperature),whichare eral authors: Suchkov et al. (1994, 1996) developed a
both known parameters from observations, a scaling mass-loading outflow model, and simulated the X-ray
law can be established: In the case of constant E/ρ jets in M82 using their hydrodynamical code. In their
values, we obtain identical shock envelopes. model, the blowout itself is as hot or as highly ener-
For a given temperature (velocity), the emission getic as one would detect in the ROSAT band. This
measureis estimatedfromthe intensity, andtherefore, model may provide a better fit in the case of M82,
the accumulated total amount of gas in the shell is where we observe a centrally-filled high energy flow
known, which is related to the mean gas density ρ0. well collimated in the direction perpendicular to the
Hence, we can measure ρ0, r and v from observations, disk. A stellar-wind driven Galactic wind (Heckman
and thus, we can finally estimate the total energy of etal. 1990)couldalsoproduceashellstructure,asob-
the explosion. servedwithROSATforNGC253(Pietschetal. 2000).
The morphology of the BHS is sensitive to the den- If the duration of the starburst is short enough com-
sity distribution in the disk and halo. The shell shape pared to the expansion time scale of the shell, namely,
in the uppermost part manifests the halo-intergalactic if the starburst is “impulsive” compared to the shell’s
density structure. If the intergalactic gas density is life time, the wind model and the our shocked BHS
very low, the shell will have a largeron-averageradius model give essentially the same result. On the other
and a more conical form. In the case of higher den- hand, if the wind is steady and longer-lived than the
sity, the shell becomes compact and round. Among shell’s expansiontime, e.g. a few tens of million years,
the various ambient density distributions in Table 1, theflowwouldbecomeanopen-coneshape,whichdoes
ModelsS2andS6appeartobetter fitthe observations not apply to roundshells observedin the case of NGC
than the other models. Although both models allow 253.
a sufficient global reproduction of the observed mor-
phology of NGC 253, Model S6 appears to be prefer-
7
6 Summary and conclusions
thanROSAThad(0.2–10keVversus0.1–2.4keV)and
offer superior spectral resolution. They allow us to
Wepresentedatheoreticalmodelofthemilliondegrees studyspiralgalaxiesatsub-arcsecresolution(Chandra)
phaseoftheinterstellarmediuminthe bulge,diskand and to probe the ISM with an extremely high photon
haloofspiralgalaxies. Inthismodel,weassumeasud- collecting power at a resolutionof ∼10′′ (XMM). The
den energy release in the nuclear region of the galaxy, combinationofChandraandXMMresultswillprovide
as provided, e.g., by a nuclear starburst. The energy spatially highly resolvedimages of the bright ISM fea-
releaseissupposedtoformabipolarhyper-shell(BHS) tures, as well as detailed X-ray spectra of the ISM in
ofionizedISM,whichisheatedtoamilliondegreesand general. The BHS model, in which the shell morphol-
is thus visible in the soft X-ray band. Our simulations ogyissensitivetotheambientdensitydistribution,will
are based on the assumption of an adiabatic shock en- then allow us to probe the distributions of the million
velope, and our projections of the three-dimensional degree interstellar medium in the disk, halo and inter-
data cubes for different galaxy inclinations take into galacticspace aswellas the disk halointerface athigh
account the absorption of soft X-rays by intervening spatial and spectral resolution.
cold gas in the disk of the galaxy. The model predicts
We are grateful to W. Pietsch for helpful comments
differentspectralenergydistributionsofthesoftX-ray
and providing the NGC 3079 and NGC 253 data. We
emissionfordifferentdistancesfromthegalacticplane.
would like to thank Sue Madden for carefully proof-
Weappliedourmodeltoamini-sampleofnearbyspi-
reading the manuscript.
ralgalaxies,which are all classified as starburstgalax-
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Figure Captions
Fig. 1: Calculated shock front in the galactic halo
at every 2.5×107 yrs after an explosion and/or a
starburstatthe nucleus withatotalenergyof0.4×
55
10 erg for various parameters as models S1 to S6
in Table 1
Fig. 2: Calculated shock fronts for Model S2 and S6
overlaid on the ROSAT 0.75 keV contours and op-
tical photograph of NGC 253 from Pietsch et al.
(2000)
Fig. 3: Simulated intensity distributions for NGC
253 at 0.75 and 1.5 keV (left panel) compared with
ROSAT images at 0.75 and 1.5 keV taken from
Pietsch et al. (2000) (right panel)
Fig. 4: Additionof0.25(R), 0.75(G) and1.5(B)keV
intensities,comparedwiththatofNGC253(Pietsch
et al. 2000)
Fig. 5: Simulated0.75keVviewsofagalaxyatvarious
inclination angles
Fig. 6: Comparison of the model with observed X-ray
images of several galaxies with various inclinations:
From top to bottom, M83, NGC 1808, NGC 253,
NGC 4258, M82, NGC 3079, and the Milky Way.
The arrows in M83 and NGC 1808 point the direc-
tionsoftheminoraxisinthenearside. Seethetext
9
This figure "fig1.gif" is available in "gif"(cid:10) format from:
http://arXiv.org/ps/astro-ph/0101525v1
