Ram-Ppressure Effects on Dense Molecular Arms in the Central Regions of Spiral Galaxies by Intracluster Medium

10 Pages·0.1 MB·English

by  Makoto Hidaka

Ram-Ppressure Effects on Dense Molecular Arms in the
Central Regions of Spiral Galaxies by Intracluster Medium
2
Makoto HIDAKA, and Yoshiaki SOFUE
0
0 Institute of Astronomy, The University of Tokyo, Mitaka, Tokyo, 181-0015
2
[email protected]

n
a February 5, 2008
J

8

1 Abstract thus, been discussed mainly in relation to H i
v gas stripping and outer disk structures. How-
3 We investigated ram-pressure effects by an ever,littleattentionhasbeenpaidtoitseffect
0
intracluster wind on an inner disk of spi- on the inner molecular disk and arms. Only a
1
1 ral galaxies by a hydrodynamical simulation. few authors have discussed the ram effect on
0 Evenifthewindismildandnotstrongenough thespiralstructure(Tosa1994)andmolecular
2 tostripthegasdisk,therampressuredisturbs clouds (Kenney et al. 1990; Sofue 1994).
0 orbits of the inter-arm gas significantly. This
/ Kenneyetal. (1990)haveshownthatVirgo
h resultsinasymmetricdensemoleculararmsin
galaxiesoftenexhibitasymmetricinnermolec-
p the inner few kpc region of a galaxy. This
ular disks. A recent high-resolution CO-line
- mechanismwouldexplainthe asymmetric CO
o survey of Virgo cluster galaxies such as NGC
r gas distributions in the central regions often
4254 and NGC 4654 (Sofue et al. in prepara-
st observed in Virgo spirals. tion) has revealed that some of them show a
a Key words: galaxies: spiral — galaxies:
: significant asymmetry of the inner molecular
v kinematics and dynamics — galaxies: ISM —
disks and arms. It is not clear if such an in-
i galaxies: cluster of — intergalactic matter
X nerdeformationofdensegasdiskscanbepro-
r ducedbyrampressureeffect,whichisthought
a 1 Introduction tobethecauseoftheirdeformedHienvelops.
Sincethesetwogalaxieshavenomassivecom-
panion that can disturb such inner disks, and
Ram-pressure by intracluster medium (ICM)
theybothshowahead-tailHiouterstructure
causes the stripping of interstellar matter
(Phookun et al. 1993, 1995), the inner defor-
(ISM) from galaxies (Farouki, Shapiro 1980;
mation of molecular disks could also be due
Kritsuk 1984; Gaetz et al. 1987; Balsara et
tothe ram-pressureeffect,whilesuchaninner
al. 1994; Sofue 1994; Quilis et al. 2000). It
ram effect has not yetbeen investigated.
also produces a disturbed distribution of ISM
in galaxies, such as head-tail H i structures If the wind is very strong, such as assumed
as observed in Virgo galaxies (Cayatte et al. by Quilis et al. (2000) for the core of a rich
1990; Vollmer et al. 2000, 2001; Phookun et cluster with an ICM density of 3 10−3
al. 1993, 1995). The ram-pressure effect has, atoms cm−3 and a wind velocity∼highe×r than
1
2000 kms−1,theISMofanygalaxieswould where ρ and ρ are the gas densities
ICM ISM
∼
be completely stripped. On the other hand, if of ICM and ISM, respectively, s is the sur-
thewindismild,insuchacaseasfortheVirgo face area of the element, and α is the angle
cluster, where the ICM density is of the order between the wind direction and the galactic
of 10−3 10−4 andthe velocity is 1000 km plane (Farouki, Shapiro 1980; Kritsuk 1984;
s−1 , ou−ter H i envelopesaredeform∼edto pro- Sofue 1994; Tosa 1994). The motion of the
ducehead-tailstructures(Vollmeretal. 2000, undisturbed ISM element is governed by the
2001). Incurrentsimulations,suchasthoseby gravitational force, which is approximately
Vollmer et al., which were aimed at gas strip- equal to the centrifugal force,
ping and tailing of the outer H i disks, the
f ρ d s v2 /R, (2)
detailed behavior of the inner disk gas inside ISM ∼ ISM rot
10 kpc was not well understood because of
∼ where d is the thickness of the gas disk, and
the resolution.
R is the galactocentric radius of the element.
In the present paper, we consider a mild
Now, the ratio of f to f is given by
ICM ISM
ICM wind, and discuss its effect on the inner
disk gas based on 2D hydrodynamicalsimula- nICMR δv 2
η cosα sinα, (3)
tions with higher resolution than those aimed ∼ nISM d(cid:0)vrot(cid:1)
at outer H i stripping, as above. If we sim-
where n and n are the number densi-
ICM ISM
ply apply the ram-stripping condition to an
ties of hydrogen atoms of the ICM and ISM,
azimuthally structure-less gas disk, the ram
respectively. If η exceeds unity, the ram force
pressure would hardly affect the inner disk.
can disturb the ISM motion, while if it is
However,ifweconsideraspiralstructurewith
smaller than unity, the ISM motion is little
an arm-to-interarm density contrast, it may
affected.
happen that the ram-pressure can affect the
Let us consider the inner part of a galaxy
low-density interarm gas. We consider here
at R 5 kpc with d 100 pc, rotating at
the possibility that the ram-pressure can af- v ∼200 km s−1 . F∼or an ICM wind with
rot
fectthedensemoleculargaswithinthecentral n ∼ 10−4 cm−3,V 103 kms−1 ,and
ICM ICM
few kpc region of spiral galaxies through dis- ∼◦ ∼
α 45 , we obtain the ratio to be
turbances of the orbits of inter-arm gas, even ∼
if the ICM wind is not strong enough to strip η 0.6 n−1 [H cm−3]. (4)
∼ ISM
the disk gas.
This relation implies that the ISM is stripped
ifn 1Hcm−3,sincetheforceperpendic-
ISM
≪
ularto the galacticplane is ofthe sameorder.
2 Ram-Pressure Force on
Westress,however,thattherelationindicates
the Arm and Interarm thattheorbitsofthegaswithinthediskplane
is significantly disturbed if n 1 H cm−3,
Gases ISM ∼
even if the wind is not strong enough to strip
the gas. This may indeed apply to the inter-
Thecomponentoftheramforceparalleltothe arm ISM in the inner disk within a few kpc
galacticplaneexertedbyanintergalacticwind radius. Ontheotherhand,high-densitygalac-
on a gas element is given by tic shocked arms, where n 1 H cm−3,
ISM
≫
would be hardly disturbed. We, thus, antici-
f ρ s δv2 cosα sinα, (1) pate that, even if the ICM wind is mild, not
ICM ICM
∼
2
strong enough to strip the disk, the interarm We assume that the galactic disk is thin
gasintheinnerdiskwouldbesignificantlydis- and faces the ICM wind everywhere, being
turbed, whichmayresultindeformedgalactic not shielded by neighboring clouds. Because
shock waves. Since δv is greater on the head- this assumption would not apply to an edge-
windsideofthe rotationaxiscomparedtothe on wind, we consider here a wind with a mild
following-windside,thedeformedshockwaves inclination. We consider the inner disk of a
could be asymmetric with respectto the rota- galaxy, where the gravitational potential is
tion axis. deep and the ISM is dense enough so that
Figure 1 illustrates the ram-deformation stripping does not occur, as discussed in the
mechanism of the dense molecular arms, and previous section, and we treat a 2D disk in a
howalopsidedspiralpatterniscreated. Since fixed rotating potential.
the density of the inter-arm gas (A and A′ in ForthevelocityofagalaxyintheICMV~
ICM
Fig. 2) is much lower than the average den- andtheICMdensityn ,weadoptthreeval-
ICM
sityofthedisk,theramforcebytheICMwind ues for each parameter. The galaxy’s veloc-
(thin lines) easily disturbs the orbits of inter- ity, V~ , is taken to be 530, 1000, and 1500
ICM
arm gas (thick curved arrows). The gas on km s−1 , where the first value is suggested by
the distorted orbits encounters density waves Phookun, Mundy (1995)for NGC 4654 in the
′
(dashed spirals) at different places (B and B) Virgocluster. TheICMdensity,n istaken
ICM
from those expected for undisturbed orbits tobe1 104cm−3,5 10−4cm−3,and1 10−3
(dashed lines), and produces deformed dense cm−3,×where the firs×t value is typical f×or the
molecular arms (thick spirals). In the next intergalactic density.
sections we discuss a numerical simulation of
the ram deformation of spiral arms in order 3.2 Numerical Method
tounderstandwhetherthismechanismcanin-
deed create deformed shocked arms, and how For simplicity, we assumed that the interstel-
the deformed arms look like in realistic model lar gas is ideal, inviscid, and compressible.
disks. We used a freely downloadable and usable
— Fig. 1 — hydrodynamical code, VH-1 (Blondin, Lufkin
1993). This is a multidimensional hydrody-
namics code for an ideal compressible fluid
3 Numerical Simulation
written in FORTRAN, developed by the nu-
merical astrophysics group at the University
3.1 Basic assumptions of Virginia based on the Piecewise Parabolic
MethodLagrangianRemap(PPMLR)scheme
The ram-pressure acceleration per unit mass
of Colella, Woodward (1984). The PPMLR
is given by
has the advantage of maintaining contact dis-
continuitieswithouttheaidofacontactsteep-
~a =Cn V~ ~v V~ ~v ,
ram ICM(cid:12)(cid:12) ICM− rot(cid:12)(cid:12)(cid:16) ICM− rot(cid:17) ener, and is sufficiently good to be applied
(cid:12) (cid:12) (5) toagalactic-scalehydrodynamicalsimulation.
where n is the ICM density, V~ is the The code does not take into account the
ICM ICM
ICM velocity with respect to the galaxy, ~v gas’s self-gravity, artificial viscosity, variable
rot
is the circular velocity of the clouds, and C is gamma equation of state, and radiative heat-
evaluatedtobeontheorderofΣ−1 (e.g.,Tosa ingand/orcooling. Theself-gravityofthegas
1994). isnottakenintoaccount,becauseweconsider
3
acasewherethegasmassisnotsomuchasto where ε is the strength of the bar of the order
contributetothedensitywavepotentialbythe of ε = 0.15. Spiral shocked arms of gas are
stellardisk,andalsobecausewedonotintend produced by this potential.
to discuss such processes as clumping of gas
andcloud formationin the armandinter-arm
3.4 Initial Conditions
regions. The interstellar gas can be assumed
to be isothermal, as is assumed here, while if Initially,weset256 256two-dimensionalcells
×
cooling is taken into account, the shocked gas corresponding to 12.8 kpc 12.8 kpc field,
×
arms would become much denser than those whilesettingthefieldcenteratthecoordinates
calculated below. However, all such neglec- origin. The initial number density was taken
tion would not affect the physical essence of tobe5cm−1 theinnerdiskatR 8kpcdisk,
the present study, which was aimed at simu- and 1 cm−1 at R > 8 kpc. The≥initial rota-
lating ram-deformation of the orbits of inter- tion velocity of each gas cell was set so that
armgasandresultingdisturbedshockedarms, the centrifugal force would balance the gravi-
as illustrated in figure 1. tation. The bar pattern speed, Ω , was taken
p
to be 23 km s−1 , andthe strengthof the bar
3.3 Gravitational Potential ε was taken to be 0.10.
We implicitly give a gravitational potential,
whichcomprisesthefollowingtwoterms: (i)a 4 Ram-Pressure Deforma-
static axisymmetric potential, and (ii) a non-
tion of Dense Molecular
axisymmetric,rotatingbarpotential. The po-
tential is expressed by Arms
Φ(R,φ)=Φ (R)+Φ (R,φ). (6)
0 1 4.1 Deformation of Inner Spiral
Structure
We adopt a “Toomre disk” (Toomre 1981)
potential for the axisymmetric component as
Figure 1 shows the result of a simulation in-
given by
cludingram-pressureeffectsforthevariouspa-
c2 1 rametercombinations,asdescribedinthepre-
Φ0(R)=−a (R2+a2)1/2, (7) vioussection. BoththeISMandspiralpattern
rotate counterclockwise, and the ICM wind
where a is the core radius and c = blowsfromlefttoright. Thesimulationshows
v (27/4)1/4a. Through our numerical sim- thatthe orbitsof the diffuse inter-armgasare
max
ulation, we fixd the core radius and maxi- easily disturbed by the ram force, which re-
mum circular velocity to be a = √2 kpc and sults in a significant displacement of galactic
v =200 km s−1 . shockwavesfromtheirundisturbedsymmetric
max
The nonaxisymmetric potential was taken positions, as illustrated in figure 1.
from Sanders (1977), assuming rigid rotation Highly asymmetric dense spiral arms in the
at a pattern speed, Ω , which has the form centralregionareproducedbythismechanism
p
if the wind speed is higher than 1000 km
aR2 s−1 and the ICM density is great∼er than sev-
Φ1(R,φ)=ε(R2+a2)3/2Φ0(R)cos2(φ−Ωpt),eral 10−4 H cm−3. A head-tail structure of
(8) densegasesslantedtotheICMwind,likeNGC
4
4654nucleus, can be producedby this mecha- 4.3 Comparison with Observa-
nism,ifn V2 isgreaterthan 3 1012 tions and Wind Velocities
cm−1 s−2I.COMn×e sIpCiMralarmon the do∼wns×tream
side is prominent, reproducing the lopsided WenowcomparetheresultswithHiandCO-
arms, as observed in NGC 4254 and NGC line observations of the Virgo galaxies, NGC
4654. 4254 and NGC 4654. The heliocentric radial
— Fig. 2 — velocityofNGC 4254,2407 kms−1 , isabout
1100 kms−1 differentfromthatofNGC4486
(M 87), the center of the Virgo cluster, 1282
4.2 Deformed Molecular Arms kms−1 (deVaucouleursetal. 1991). Also,by
comparing with the lopsided H i distribution
We then calculated the distribution of molec-
of NGC 4254 (Phookun et al. 1993) and the
ular fraction (Elmegreen 1993) corresponding
rampressuresimulationongalaxies(Abadi et
to figure 1. The molecular fraction is defined
al. 1999),wecanexcludethepossibilityofany
by
face-on motion of NGC 4254. Assuming that
ρ 2n
fmol = H2 = H2 , (9) NGC 4254’s orbit is inclined by 45◦ from the
ρ +ρ n +n
HI H2 HI H2 line of sight, we may estimate the velocity of
where ρ , ρ , n and n are the mass motion of NGC 4254 in the Virgo cluster as
H2 HI H2 HI
and number densities of molecular and H i being 1500 km s−1 . Then, the rotation
gases, respectively. We used a method de- velocit∼yofNGC4254( 150 kms−1 )isneg-
∼
scribedbySofueetal. (1995)andHonmaetal. ligible compared to the wind velocity, so that
(1995) to calculate the galaxy-scale molecular the H i tail grows toward downstream. The
fraction; they investigated molecular fronts in location of a prominent spiral arm (Iye et al.
spiral galaxies using a phase-transition model 1982)and the direction of rotationare consis-
proposedby Elmegreen(1993). Inthis model, tent with our numerical simulation. However,
the molecular fraction is determined by three if we assume a slower wind velocity, e.g., on
parameters; the interstellar pressure, P, the the order of, or smaller than, 750 km s−1 ,
∼
UV radiation field, U, and the metallicity, Z. the simulation cannot reproduce the observed
ForU andZ,weadoptedanexponentialfunc- features.
tion of galactocentric radius, and calculated Our simulation for moderate ICM veloc-
fmol for corresponding gas pressure, P, which ity, which predicts an off-center bar of dense
is determined by the gas density in each cell. gas tilted toward the ICM wind, is consis-
Figure 3 shows the result of a numerical tent with the observations of NGC 4654 in
simulation of the molecular fraction. The in- H i (Phookun, Mundy 1995) and CO (Sofue
ner few kpc region is dominated by molecular et al. in preparation). Our simulations show
gas, where the molecular fraction is as large that spiral arm in the upstream side becomes
as 70 90%. Also, the molecular frac- stronger than that in the downstream side.
∼ −
tion increasessuddenly atthe galactic shocks, Considering the location of the prominent op-
which are already deformed from symmetric tical arm (Frei et al. 1996) and an elongation
arms. Thus, the simulation has revealed that ofthe observedmolecular bar and H i tail, we
highly deformed inner molecular arms can be findthatthe directionofmotionofNGC 4654
producedbyram-pressuredisturbancesonthe is toward the northwest, with a significantly
inter-arm low-density regions. high velocity comparedto the velocity disper-
— Fig. 3 — sion of the Virgo cluster. Taking it into ac-
5
count that the rotation velocity of NGC 4654 present2Dsimulation. Therefore,detailed3D
is relativelyslow, the velocity of NGC 4654in simulations will be crucial to thoroughly un-
the Virgo cluster would be greater than 1000 derstand the inner ram effect in more detail,
km s−1 , although the heliocentric radial ve- while the present 2D results can tell us about
locity,1054 kms−1,isclosetoVirgo’scentral some essential mechanism to cause the non-
velocity. axisymmetric molecular structures in the in-
ner disk.
However, in the cases of much stronger
5 Discussion
winds, the 2D assumption cannot be applied
in any way, and 3D treatment of stripping is
Wehaveconsideredtheram-pressureeffectsof crucial. In fact, ram effect by a wind with a
a mild ICM wind on gaseous disks in cluster pressure much higher than that considered in
galaxies. Galaxies in the central region of the this paper has been simulated by Quilis et al.
Virgo galaxies show H i deficiency, where the (2000) with a 3D hydrodynamical simulation;
molecular gas fraction is higher than that of they have shown that the ISM in a galaxy is
the galaxies in the outer region of the cluster completely stripped within one galactic rota-
(Kenney et al. 1990). This fact has been nat- tion.
urallyexplainedbytheram-pressurestripping
of the H i gas: the ram effects are negligible
on the inner molecular disks, while it is cru-
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Figure Captions
Fig. 1. Schematic illustration of the ram-
deformation mechanism to cause asymmetric
inner moleculararms. Orbits(thin dashedar-
′
rows)of low-density inter-arm gas (A and A)
are disturbed by the ram pressure of the ICM
wind (thin arrows). The gas on the distorted
7
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