Table Of ContentModels of the Morphology, Kinematics, and Star Formation
History of the Prototypical Collisional Starburst System: NGC
7714/7715 = Arp 284
Curtis Struck
3 Department of Physics and Astronomy, Iowa State University, Ames, IA 50011
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2 Beverly J. Smith
n Department of Physics and Astronomy, East Tennessee State Univ., Box 70652, Johnson City, TN 37614
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J
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ABSTRACT
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v WepresentnewN-body,hydrodynamicalsimulationsoftheinteractionbetweenthestarburst
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galaxyNGC 7714and its post-starburstcompanionNGC 7715,focusing on the formationofthe
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collisionalfeatures, including: 1)the gas-richstar forming bridge,2) the largegaseousloop(and
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1 stellartails)tothewestofthesystem,3)theveryextendedHItailtothewestandnorthofNGC
0 7714,and 4) the partial stellar ring in NGC 7714. Our simulations confirm the results of earlier
3 workthatanoff-centerinclinedcollisionbetweentwodiskgalaxiesisalmostcertainlyresponsible
0
for the peculiar morphologies of this system. However, we have explored a wider set of initial
/
h galaxy and collisional encounter parameters than previously,and have found a relatively narrow
p range of parameters that reproduce all the major morphologies of this system.
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Thesimulationssuggestspecificmechanismsforthedevelopmentofseveralunusualstructures.
o
r We find that the complex gas bridge has up to four distinct components, with gas contributed
t
s from two sides of NGC 7715, as well as from NGC 7714. The observed gas-star offset in this
a bridge is accounted for in the simulations by the dissipative evolution of the gas. The models
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v suggestthat the most recently formed gas bridge component from NGC 7715 is interacting with
i gas from an older component. This interaction may have stimulated the band of star formation
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on the north side of the bridge. The models also indicate that the low surface brightness HI
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a tail to the far west of NGC 7714 is the end of the NGC 7715 countertail, curved behind the
two galaxies. The sensitivities of the tidal structures to collision parameters is demonstrated by
comparisons between models with slightly different parameter values.
Comparisonofmodelandobservational(HI)kinematicsprovidesanimportantcheckthatthe
morphologicalmatchesarenotmerelyfortuitous. Lineofsightvelocityanddispersionfieldsfrom
the model are found to match those of the observations reasonably well at current resolutions.
Spectral evolutionary models of the NGC 7714 core by Lanc¸on et al. suggest the possibility
of multiple starbursts in the last 300 Myr. Our hydrodynamic models suggest that bursts could
be triggered by induced ring-like waves, and a post-collision buildup of gas in the core of the
galaxy.
Subjectheadings: galaxies: individual(NGC7714/7715)—galaxies: interactions—galaxies: kinematics
and dynamics
1. Introduction
Detailed investigations of individual collisional
galaxiescanprovide importantinformationabout
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the physics of the encounter, including enhance- (1990); Bernl¨ohr (1993); Gonz´alez-Delgado et al.
mentandsuppressionofstarformationininteract- (1995);PaperII; O’Halloranet al. (2000)), oran
inggalaxiesandhydrodynamicalprocessessuchas HI counterpart(PaperII), unlike manycollisional
shock wave production, heating and cooling, and rings (Appleton & Struck-Marcell 1996). NGC
re-accretion. In practice, however, it has proven 7714 has three apparent optical tidal tails/arms.
very difficult to reconstruct the details of a spe- The outer southwestern stellar arm may be asso-
cific encounter between galaxies, and to match ciated with a large HI loop (Feature 2 in Figure
preciselythe observedpropertiesofaninteracting 1, alsosee Figure 3). The inner southwesternarm
system, particularly when one includes not just (Feature 3 in Figure 1) is brighter, with a promi-
the stellar morphology but also the gas distribu- nent HII region complex at the base of this arm
tion and kinematics and the star formation mor- (Paper II). In optical images, the northeastern
phology. stellararm(Feature4inFigure1)isclearlyphys-
Thereare,however,somespecialcircumstances icallyseparatefromthebridge,however,intheHI
in which such a detailed modeling project is maps, it is not well-resolved (Paper II; see Figure
tractable. The first of these is when the galaxies 3). In addition, low resolution HI observations
are observed at a time shortly after closest ap- reveal a low surface brightness HI tail extending
proach, and well before a merger. In such cases, 6′ (71 kpc) to the west of NGC 7714 with no
there has not been time for phase mixing, and observed optical counterpart (Paper I).
because most encounters are quite impulsive in The presence of the ring suggests a direct im-
the early stages, the immediate response is pri- pactonthedisk,whilethespiralssuggestthatthe
marily kinematic (e.g., Gerber (1993), Dubinski collision was somewhat off-center, and generated
et al. (1999)). A second helpful factor is the significant tidal torques (Paper I). The ring itself
existence of a special symmetry in the collision. constrains the collision parameters,but the spiral
The symmetric collisional ring galaxies provide morphologies are so distinctive that they should
a notable example (Appleton & Struck-Marcell yield even stronger constraints. The bridge con-
1996). Another example is provided by the oc- sists of HI gas, old stars, and young stars, with
ular galaxies, in which the characteristic eye-like a significant offset between the old stars and the
morphologyresultsfromastrongprogradedistur- other components (Paper II; see Figure 3). This
bance, implying that the companion orbitalplane offsetis clearlyevidentinFigure2,whereastring
has a small inclination relative to the disk plane of luminous H II regions lies to the north of an
of the ocular (Elmegreen et al. (1991), also see opticalcontinuumbridge. Sincemodelsshowthat
Kaufman et al. (1997), Kaufman et al. (1999), connecting bridges are relatively short-lived fea-
and Elmegreen et al. (2000)). Long tidal tails tures (Struck 1997), its presence provides strong
also result from nearly planar prograde interac- evidence for a recent encounter. The edge-on
tions (Toomre & Toomre (1972), and review of shape of NGC 7715 and the stellar countertail of
Struck (1999)), and can provide important clues NGC7715alsoconstrainthecollisionparameters.
to the nature of an ongoing interaction. Still more contraints are provided by the map of
The interacting system NGC 7714/15 (Arp line-of-sightHIvelocities(PaperII),whichdefines
284), with a relatively nearby distance of 37 Mpc the sense of rotationof the two galaxies andkine-
(assuming H = 75 km s−1 Mpc−1), has many of maticline ofnodes. NGC 7714hasanuclearstar-
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these exceptionalproperties. The mostimportant burst (French (1980); Weedman et al. (1981);
features of NGC 7714/15 are summarized in Ta- Keel (1984); Gonz´alez-Delgado et al. (1995);
bles l and 2, and are visible in the optical Arp and Kotilainen (2001)), while the center of NGC
(1966)Atlasphotograph(Figure1,withaHubble 7715 shows a post-starburst spectrum (Bernl¨ohr
Space Telescope archive image in Figure 2), and 1993). The stellar evolutionary timescales associ-
in the published 21 cm HI and Hα maps (Smith ated with these phenomena give additional input
& Wallin (1992), hereafter Paper I, and Smith et on the time since the collision.
al. (1997), hereafter Paper II). NGC 7714 has a In Paper I, we presented a restricted 3-body
prominentstellarring (Arp 1966)whichdoes not modeloftheNGC7714/5encounterthatmatched
haveon-goingstarformation(Bushouse&Werner the stellar morphology of the system and the line
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of nodes and sense of rotationof NGC 7714. This tion 5.
scenarioconsistedofanoff-centerinclinedcollision
between two unequal mass galaxies, with the en- 2. Numerical Models
counterbeingretrogradewithrespecttothe main
As noted above, the general nature of the col-
galaxy NGC 7714 and prograde with respect to
lision can be immediately deduced from the pres-
the companion. Thissimulation,however,didnot
ence of a few distinctive morphologies. The NGC
include hydrodynamical effects. The later acqui-
7714 ring and the relatively straight bridge con-
sition of the high resolution HI data (Paper II)
necting the two galaxies provide prima facie evi-
allowed a stronger test of the collision scenario.
dence for a nearly head-oncollision. On the other
In Paper II, we presented a preliminary hydro-
hand, the offset of the NGC 7714 nucleus from
dynamical model of the NGC 7714/5 encounter
the ring center, and the presence of loops and
with two gas disks and rigid halos, using param-
tidaltailssuggestsomeasymmetryinthecollision.
eters similar to those of the Paper I model. This
Tailsandringscanbesimultaneouslyproducedin
model was not intended to reproduce all the fea-
collisions of intermediate inclination with closest
turesofthesystem,butrathertodemonstratetwo
approachdistances somewhatless thanthe radius
points. First,itshowedthatthegasloopcouldbe
of the gas disk of the ring galaxy (e.g., Appleton
obtainedinacollisionlikethatinthemodelofPa-
& Struck-Marcell (1996)). Tails are more easily
perIwiththeadditionofagasdiskoflargerradius
produced if the encounter is prograde relative to
thanthestellardisk. Second,itdemonstratedthat
the tailed galaxy.
theobservedoffsetintheoldstarbridgeversusthe
youngstar andgas bridgecould be explainedasa
2.1. Simulation Codes
result of the dissipative impact of the gas disks of
the twogalaxies. Thismodeldidnot,however,do Webeganourmodelingworkwithalargenum-
a good job in matching the observed location of ber of exploratory runs with a restricted three-
the gaseouslooporthe orientationofthe gas/star body code (Wallin 1990), in order to refine our
offset in the bridge. estimates of the collision parameters. We will not
In this paper we present the results of a more describe that work any further. We then used the
detailed modeling program undertaken to better hydrodynamiccodeofPaperII(seeStruck (1997)
interpretthefeatureslistedinTables1and2. The for details on this code) to study the gas dynam-
new modeling includes fully self-consistent simu- ics of the refined collision, and to further adjust
lations made with the ‘Hydra’ (version 3.0) N- it to reproduce the observed HI structure. These
body, adaptive-mesh SPH code of Couchman et runswerealsousedtostudythethermalandstar-
al. (1995), which includes radiative cooling, and forming properties of the colliding galaxies in a
also feedback heating terms in some models (see preliminary way. However, that hydrodynamic
thefollowingsectionfordetails). Thesenewmod- code does not include fully self-consistent calcu-
elsaresimilarto theearlieronesinrequiringadi- lations of the gravitational forces, and in partic-
rectimpactbetweendiskswithamoderateimpact ular dynamical friction and related effects. Thus,
parameteranda relativelylargeinclinationangle. oneofthemostprominent‘errors’oftypicalmod-
However, significant changes have been made in els(e.g.,modelsinwhichNGC7715beginsnearly
the new models in order to better fit the detailed at rest relative to NGC 7714 at a distance of at
morphologyofthesystem(seesection3.1and3.2). least several diameters away), is that NGC 7715
Comparisons to observed kinematics (section 3.3) plunges through and well away from NGC 7714,
andstarformation(SF)characteristics(section4) before the HI loop and other tidal structures can
have also been made for the first time in this sys- develop to the observed degree. Given the limi-
tem. Thegenerallygoodagreementbetweenmod- tations of these models, we will not describe their
els and observational kinematics provides strong results in any detail.
confirmation of the basic collisional model. The Fully self-gravitating simulations were then
model results on SF are tentative, but they do produced with the serial code Hydra 3.0 (hence-
provide some useful suggestions to be checked in forth simply Hydra), which has been made pub-
future work. The results are summarized in sec- liclyavailable. HydrausesanSPHalgorithm,and
3
gravity is calculated with an adaptive particle- tical luminosities). We note, however, that it is
particle (PP), particle-mesh (AP3M) algorithm possible that the companion has lost a good deal
(for details see Couchman et al. (1995), Pearce of mass, and thus, its precursor may have been
& Couchman (1997)). For a typical timestep in more massive than current observations suggest.
ourmodels,adaptiverefinementswerecarriedout A model with equal mass initial galaxies showed
on about half the gas particles, with about 6-10 that even with a massive companion it is possible
submeshes, and with the most refinement around to produce a fairly good model, with only a few
the primary center. disagreements with observation.
Thesimulationswereallrunusinganadiabatic IntheHydrasimulations,thetwomodelgalax-
equationof state. Optically thin radiative cooling ies each contain a collisionless halo, a stellar disk,
was calculatedvia the tables ofSutherland & Do- and a gas disk component, which were added and
pita (1993), which were supplied with the Hydra relaxed sequentially. The halo is approximately
code. The Sutherland and Dopita cooling curves isothermal, while the disk components have a
include atomic and ionic line and continuum pro- nearly constant vertical velocity dispersion with
cessesforT ≥104K. Coolingtimeswerenotused radius, and an approximately 1/r surface density
to limit the size of the computational timestep, distribution. The gas fraction of the disks is very
sincethedynamicaltimeisusuallylongerthanthe high, with equal masses of stars and gas. This
cooling time. Particles evolve adiabatically and may be unrealistic, but was done to provide ade-
are cooled at the end of a given timestep, at con- quateparticleresolutionofthegasdynamics. The
stant density. No feedback heating was included gas disk cools somewhat in its relaxation, and so,
in most of the Hydra simulations. is generally thinner than the stellar disk. Many
details of the model galaxies are given in Table 3.
2.2. Scalings and Boundary Conditions
In the models we adopt the x-y plane as the
Thecodesusedinthisprojectrunindimension- plane of the sky, and then the model galaxy ini-
tial conditions and the orbital orbital trajectories
less variables, and many of the graphs below are
plotted in those units. The Hydra runs are made wereoptimizedtomatchthe observationsatsome
on a cubic volume, where x,y and z coordinates later time. Specifically, with the initial orienta-
tions given in Table 3 the disk of the primary ap-
all run from 0.0 to 1.0 in code units. Most of the
computationalcubeisemptymostofthetime(see pears relatively face-on in the x-y plane, and is
edge-on in the x-z plane. The companion disk is
Figures 1-3 below). Very few particles reach the
more nearly face-on in the x-z plane. Inner disk
boundaries and when they do they are taken out
of the calculation. The adopted scalings for the orientationsareroughlypreservedthroughthecol-
lision.
Hydra model are: one computational length unit
equals100kpc,one time unit equals109yr, (which Given the limited number of particles that can
implies that the velocity unit is 97.7 km/s), and be used, we cannot represent an extended halo
the mass unit equals 1010M⊙. with great accuracy with the Hydra code. How-
With these scalings, the model results sug- ever, we find that the most successful models
gest that closest approach occurred about 100− of this system require relatively compact halos,
250Myr ago; we will refine this estimate below. which can be modeled quite well.
In the following sections, discussions of model
2.3. Initial Conditions and Model Differ- resultsandobservationalcharacteristicsofthesys-
ences temwillbe highly interwoven. Toavoidconfusion
aboutwhichisbeingreferredtowewillrefertothe
We ran about a dozen simulations using the
model galaxies as galaxies A and B, correspond-
Hydra code; in this paper we will focus on the re-
ing toNGC 7714andNGC 7715respectively,and
sultsofthebestHydrasimulation(henceforth’the
reserve the latter names for the real galaxies.
model’ or ’best model’), with only brief mention
of the results of other model runs. In the best
models the mass ratio of the galaxies was about
1/3, in accord with the observations (e.g., the op-
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3. Model Results: Reconstructing the 3.2. Specific Collisional Structures
Collision
3.2.1. The NGC 7714 Ring (Feature 1)
3.1. Overview of the Collision and Forma-
The high inclination orbit of galaxy B relative
tion of the Large Scale Morphology
tothediskplaneofA,andtheimpactofthecenter
In this section, we will focus on the morpho- ofBwithinorneartheAdisk,shouldgiverisetoa
logical and kinematic results, and briefly discuss substantialm=0,ring-likecollisionalperturbation
thermal effects and star formation in the follow- ofA.Inopticalimages(Fig. 1)aprominentstellar
ingsection. WebeginwithFigures4and5,which ring is visible. Rings are clear in the A gas disk
show the evolution ofthe gas disk of the model in in Figure 5, but fainter in the stellar disk. The
three orthogonal projections. The orbital trajec- primary reason for this is the fact that the dissi-
tories of the galaxy centers are also shown in the pationless starparticles havemuchlargerthermal
firstrowofFigure4. Theappearanceofthestellar velocities in the model than the gas particles, so
disk during the last three time steps is shown in stellarwavesareheavilysmoothedbyparticledif-
Figure 6. fusion (also see Figure 9).
In these figures, we see that galaxy B begins Despitethisnumericalcomplication,itremains
at some distance below A in the z-direction. The true that this type of collisional perturbation has
galaxies swing around to almost return to their strong m=0, 1 and 2 Fourier components. The
starting positions. Because of the disk tilt in A, m=0componentgivesrisetosuccessiverings. The
theangleofattackofBatclosestapproachislarge m=2tidalcomponentislargelyresponsibleforthe
(>80◦). Bothgalaxiesfeelaprogradedisturbance. tidal tails and two-armed spirals in the interior,
as well as the general bar-like appearance of the
Thetidaltailsarethemostdramaticstructures
disk. The m=1 and higher odd moments gener-
atthelatertimesshowninFigure5. Thecomplex
ate asymmetry in these structures. All of these
bridge is almost as prominent. The same struc-
features are apparentin Figure 9 which showsen-
tures are alsoevident, though more diffuse, in the
larged views of star (left panel) and gas (right
disk stars in Figure 6.
panel) particles in the galaxy A disk at a time
Figure7showsthreelatetimesnapshotsofjust
near the present. The left panel of the figure pro-
the gas particles originating in A, while Figure 8
videsoneofthebestviewsoftheasymmetricstel-
provides the corresponding views of gas particles
lar ring. Rings are more evident in the gas disk
originating in B. These figures show that there is
shown in the right panel, but are rather complex
substantial mass transfer from each galaxy to the
structures. This is because the rings are also spi-
other. However, galaxy B loses much more mass.
rals, which are more or less tightly wrapped at
Nextwewillexamineindividualcollisionstruc-
different positions. These spirals also smoothly
tures in more detail. We note at the outset
connectsuccessiverings. Thisisaresultofhaving
that the figures illustrating these structures have
comparable m=0 and m=2 perturbations.
been chosen at a variety of times in the range
Infact,thetidalcountertail(whichcorresponds
100−220Myr,thoughinmostcasest=120Myr.
totheHIloopandFeature2ofNGC7714)andthe
The latter figure is essentially a default, but if
companion-side tidal tail are connected at early
the specific structure is better illustrated by the
times. (Theyarealsoconnectedatlatetimes,but
model output at a different time, we have shown
the connection is represented by so few particles,
it at that time. In some cases multiple times are
itisvirtuallyinvisible.) Sincethese structuresde-
shown to illustrate structure development. The
velopedevenbefore closestapproachwecallthem
range above approximates our uncertainty in the
the 0 order ring. This ’ring’ is very asymmetric,
time since closestapproach,andsince the lifetime
with the HI loop representing the strongly posi-
of most structures is longer than this range, all
tively torqued side. Each successive ring also has
the illustrated features should be visible at the
both positively and negatively torqued sides. The
“present.”
former(denotedbyaplussign)movesradiallyout-
ward earlier and farther than the latter (denoted
by a minus sign). At the time shown in Figure
5
9 ring 0 persists because it is largely a material to unity than in the double-ringed Cartwheel
rather than phase wave. Much of ring 1 (a phase galaxy, or most models of double rings. The-
wave)haspropagatedthroughthediskanddisap- ory suggests that collisional rings are closer to-
peared. Ring 2 is maturing. gether in galaxies with declining rotation curves
Part of ring 1+ remains visible on optical im- (e.g., Struck-Marcell & Lotan (1990); Appleton
ages as a very faint loop outside the bright east & Struck-Marcell (1996)), and so, this provides
side ring/loop. It contributes much to the ring- some evidence that the halos of these galaxies are
like appearance of the stellar disk in Figure 9. It less extensive than most. (Paper I shows that the
loops inward more tightly than the corresponding NGC 7714 rotationcurve is flat or slightly declin-
gas feature, which has a wispy appearance. We ing, but this curve is affected by the collisional
propose that the strong NGC 7714 stellar ring is disturbances, and doesn’t extend much beyond
equivalent to ring 2(+ and -) in the model. The the optical disk.) On the other hand, the asym-
high resolution HST (WFPC2) partial image in metry of the ring, and the fact that they have
Figure 2 of Lanc¸on et al. (2001) looks very simi- been torqued in the interaction, complicates this
lar to the right panel of Figure 9, except that the interpretation.
2+ ring arc is not so prominent on the west side.
3.2.2. OuterSouthwestTailandHILoopofNGC
Inthe HST imagethere appearto be bubbles and
7714 (Feature 2)
shellsonthisarc,soSFfeedbackmayhaveeffected
its strength and appearance.
The‘outersouthwesttail’seeninopticalimages
To date, the observationsdo not show a strong (Feature2ofFigure1)appearstobejustthebase
gas ring, like that visible in Figures 5 and 7. In of the great HI loop described in Paper II (see
part, this may be the result of limited observa- Figure 3). This stellar feature becomes broader
tionalresolutionandsensitivity. However,the ab- and fainter at a position angle of about 270◦, due
sence of star formation in the ring also suggests west ofthe NGC 7714nucleus, and at low surface
that there is not much highly compressed gas. In brightness levels traces the HI loop to at least a
fact,the gasringinFigure9isweakeronthe east position angle of 330◦.
(left) side. This is the sector of disk-disk overlap
Our Hydra models (Figures 5 and 6, lower left
in the collision, and also the bridge side. Large
panels) also show a low surface brightness stel-
scale shock dissipation is likely to have separated
lar feature coincident with a gaseous loop. As in
the gas and stars in this region, with the gas ei-
the observations,the stellar feature is shorterand
therbeingpushedinwardorpulledintothebridge.
less prominentthan the gaseousfeature. This is a
This accounts for the fact that the east side ring
common property of tidal tails (see e.g., Hibbard
consists of old stars, with little evidence of com-
& van Gorkom (1996), Elmegreen et al. (2000),
pressed gas or young stars.
Mihos (2001)); it is the result of the fact that
On the other hand, the west side gas ring (2+) tails are pulled out of the disk like taffy out of a
is strong in Figure 9. We have already noted that pot, with the outer boundary of the tail derived
HST observations suggest SF feedback may have from the outer edge of the disk, which is typically
heated and scattered the gas on this side. Pre- gas-rich, while the inner base of the tail includes
liminary models with feedback also support this materialfromdeeper within the disk, whichhas a
idea, and further show that the 2+ ring could be larger proportion of stars.
completelydisruptedbyreasonableamountsfeed-
To help understand how such a large mass of
back. Observations of the stellar populations in gas(≃1.1×109M⊙) waspulled out into the huge
this region would be very desireable.
loop, we investigated the history of the particles
As we will discuss in Section 3.3, there is evi- in the loop in the Hydra models (Figure 10). In
dence that the northeastern tail (Item 3 of Table the left panel are shown all gas particles above
1)andthesouthernfilamentofFig. 3arepartsof the plane y = 0.31+0.48x, i.e., most of the loop
an older ring. These may well be remnants of the material that is not too close to the A disk.
gas component to ring 1+.
The right-hand panel of Figure 10 shows the
The ratio of the successive ring radii are closer locationofthesamegasparticlesatatimeshortly
6
before the impact the two disks. It suggests that to a range of similar models, like ring waves and
most of the outer disk on the south side is pulled the tidal tails, versus those which depend sensi-
out into the loop as the companion swings by. tively on structural and orbital parameters. The
However, Figures 4 and 5 show that while this bridge contains components of both types.
loop has characteristics of a classical tidal tail, it Thefirstbridgecomponent(LabelediinFigure
alsohasastrongring-likeaspect. E.g.,itisalways 11) is the B tidal (counter) tail, which loops back
aloop,whichconnectsbacktotheAdisk,andnot down into projection onto the bridge in this view
a tail with a detached end point. Thus, this loop and especially in the x-y view. It is not physi-
mayalsoberegardedaspartofthefirstring-wave. cally associatedwith the other (true) bridge com-
The fact that most of the material in the loop ponents. The second component (ii) is a strong
originatesintheouterpartofthepre-collisiondisk tidal bridge from B to A. This component, the
alsohelps explainthelackofdetectablemolecular late or post-collision B bridge, begins as a dark,
material in the loop, despite the large gas mass nearly vertical line on the left side of the gas be-
it contains (see Smith & Struck (2001)). We tween the two galaxies in the upper left panel. It
notethatthenucleusofNGC7714hasmoderately also dominates the left side of the bridge in the
low chemical abundances (12 + log(O/H) ∼ 8.5; alternate model at this time (upper right panel).
(French 1980; Gonz´alez-Delgado et al. 1995)), Athirdcomponentofthebridge(iii)isthetidal
and the material in this loop may be even more stream from A to B. Bridge particles and galaxy
metal-poor than that in the nucleus. contoursareshowninFigure12. Thisstreamgen-
Note that the contour levels, while arbitrary, erally misses its target initially in three dimen-
arethesameinbothpanelsofFigure10. However, sions. Inthe bestmodel,thiscomponentdoesnot
theareacontainedwithinthemismuchlessinthe evenstretchmuchtowardsgalaxyB beforeitfalls
leftpanel,whichhighlightsthestrongcompression back to the A disk plane. However, it remains
resulting from the collision. outside the A disk and projected onto the bridge
in the x-y view through most of the simulation.
3.2.3. The Bridge (Feature 5) This can be seen in the upper left panel of Figure
11,whichprovidesthe remainingtwoviews ofthe
The HI maps of Paper II suggested that the
bridge gas at intermediate times. In the alternate
bridgebetweenthetwogalaxiesisacomplexstruc-
modelthisAbridgedoesstretchtowardsgalaxyB
ture. In particular, there is a measureable offset
(see Figures 12 and 11). At late times it transfers
between the star and HI gas centroids. To begin
gas directly into the core of B. Figure 11 high-
to understand this let us consider the origin and
lights this difference between these two otherwise
development of the bridge. A plot like Figure 10,
very similar models. This difference is mostly due
but for bridge particles (not shown), tells us that
to the different companion tilt angles. In the best
most bridge particles originate in the outer parts
model the companion cuts through more sharply,
oftheirparentdisks,andonthesidethatisclosest
notsplashingasmuchgas,justgravitationallylift-
to the other galaxy at closest approach.
ing the nearby disk slightly. Because the gas is
The y-z plots of Figure 11 provide good views projected onto the bridge in both cases, observa-
of the development of several bridge components tionsmaynotdistinguishbetweenthe twomodels
(whicharelabeledinthesecondpanel). Thefigure in this regard.
showstheresultsofHydramodelsattwointerme-
There is also a fourth component of the bridge
diate times. The first model is the usual ‘best’
(iv), which can be seen as the sharp linear fea-
model, while the second is one we will refer to as
ture stretching from the B center toward the A
the ‘alternate.’ The alternate model differs from
center in the top right panel of Figure 11. It
the best model by having a different tilt of the B
is also present in the best model, though harder
galaxy,andhigherorbitalangularmomentum(see
to distinguish in the top left panel of Figure 11.
Table 3). It is representativeof a groupof models
This feature mergeswith the late bridgeby about
with slightdifferences fromthe best model. Com-
t=100Myr. Graphsatothertimes (notshown),
parisons between the best and alternate models
revealthatthisstructureistheremnantofanearly
help us understand which structures are common
or pre-collisiontidal transfer stream from B to A,
7
whichappearstotransferarespectableamountof Another possibility is that these star clusters
material(seediscussionattheendofthissection). are formed in the NGC 7715 tail, which experi-
Thus, B is a remarkable case of a galaxy with a ences compression in the segment projected onto
longtidaltail,andtwo(!) tidalbridgesoriginating thebridgeataboutthesametime. Delayedstrong
on opposite sides of the galaxy. The multifaceted compression was found to occur in many of the
structureofthisbridgealsohelpsexplainhowsuch models of Wallin (1990). Kinematic observations
a large mass of HI gas can be located outside the might allow this hypothesis to be tested. We will
disks of the two galaxies. describe the kinematic predictions of the simula-
Thedifferentcomponentsofthebridgehavedif- tion below.
ferentdegreesofgas/staroffset. TheBtailhases-
3.2.4. The NGC 7715 Tail (Feature 6)
sentiallynooffsetbetweenstarsandgas. InFigure
13weshowtwoviewsofgasandstarparticlesthat
NextwewillconsidertheNGC7715(stellarand
originateinAinthe bestmodel. Itis clearinthis
gas) countertail. As we have already discussed,
figure that the stars are much more broadly dis-
our models indicate that this tail curves around
tributed than the gas particles. It is also evident
behind NGC 7715 and the bridge (Fig. 8). Ac-
from the top panels that the center lines of the
tually, in the best model it is located a bit be-
star and gas distributions are offset.
low the bridge in the x-y plane. In the alternate
Moreover,thelasttwopanelsofFigure12show model it is slightly above the bridge (at the same
howthebridgegasparticlesfromBcompressinto time). The inclination of the companion disk was
a relatively thin filament in the Hydra models. changedin the best model to give a more edge-on
This compression includes a merger of the ‘early’ appearance in the x-y view (see Table 3). With
and ‘late’ B bridges. In the gas this process is a somewhat smaller change it should be possible
dissipative, but the stars are dissipationless. This to obtain a quite edge-on appearance, while also
also helps explain the offset. superimposingthetailontheline-of-centersofthe
From Figure 11 (and Figure 15 below) we de- two galaxies.
ducethatmostofthebridgematerialiscontained In the models, at t ≥ 100 Myrs., the gas in
within tidalstructures. In PaperII we speculated this tail extends about 25 kpc to the southwest
that the bridge might contain a mixture of splash of NGC 7714 (Figures 5, 8, and 14). The long
and ‘tidal swing’ material. Dissipation in the col- HI plume to the southwest of NGC 7714, seen in
lisionbetweendisksdoesseemtobe animportant low resolution HI maps (Figure 3b in Paper I) is
factor in the development of the A to B compo- very likely the observational counterpart to this
nent, which is important in determining the gas- modeltail. Thisfeaturedoesnothaveanobserved
star offset. However, this ‘splash’ effect is more optical counterpart, consistent with it being the
indirect than originally envisioned. Dissipation in end of an HI-rich tidal tail. As shown in Figure
the later compression of the early and late B to 14,almostallofthis tailmaterialoriginatedinan
A components also plays a role (see also Mihos annulusintheouterpartoftheNGC7715disk,on
(2001)). the side nearest the NGC 7714 disk before closest
Theseconsiderationssuggestanunusualexpla- impact. As discussedbelow,the observedvelocity
nationfortheyoungstarclustersinthebridge. As structure of this tail is also reasonably consistent
noted,shortlybeforethepresentthelateBbridge with the model NGC 7715 tail.
swingseastandovertakestheremnantoftheearly
bridge. The gas in the latter is compressed, and 3.2.5. The Bridge Extension
this compression may trigger star formation. The
IntheArpAtlasphotograph(Figure1),asmall
Hα observations of Paper II suggest that these
stubby plume is visible on the west side of NGC
clusters are truly young, and this is the only dy-
7714, exactly in line with the bridge on the east
namical event that occurs in the region within a
side. This feature lies north of (above) features 2
few times 107 yrs.of the present. However,we do
and3inthefigure. Itseemslikelythatthisplume
not have sufficient particle resolution in the simu-
was formed by bridge material that has fallen
lation to detect clump compression, so we cannot
through the NGC 7714 disk plane. The model
directly confirm this conjecture.
8
bridges do not have enough particles to confirm the tail/loop as in the observations. At this time,
this, and the available HI data do not have suf- this structure is in the NW quadrant, rather than
ficient resolution to distinguish this feature from the SW, but otherwise generally matches the ob-
the disk. servations well. In the true x-y view the plus
signs lie on top of the HI loop contours. The
3.2.6. TheInnerSouthwestNGC7714Tail(Fea- small rotation (about 5◦) about the y-axis used
ture 2) to make the fourth panel easily separates the two
features, without greatly changing other charac-
The inner SW tail (Feature 3 in Figure 1) is
teristics. Thus, the position of this feature seems
veryunusualinseveralrespects. First,itdoesnot
to be a very sensitive function of viewing angle.
seem to be part of the southern tidal tail of NGC
TheinnerSWtailisnotreproducedaswellinthe
7714 (which is also the HI loop), but rather an
other Hydra models, though it is still present.
extra inner tail paralleling feature 2. Moreover,it
has a high surface brightness, and contains knots
3.2.7. The Inner Disks and Mass Transfer
of recent star formation. It also does not appear
to be an extension of the bridge, nor material ac- Figure 16 shows, as a function of time, the gas
creted out of the bridge. masscontainedinasphericalvolumeofradius0.03
Figure 15 provides a plausible solution to the model grid units (about 3 kpc) normalized to its
mystery. The first panel shows how mass transfer initial mass centered on the nuclei of the galax-
is well underway at the time of closest approach. ies. The figure also shows the gas fraction in each
Inner SW tail particles were first identified in the galaxy transferred from the other galaxy. Note
third panel of Figure 15. That is, all gas parti- thatpeaksinthemasstransferattimesnearclos-
cles from B lying within a rectangular region be- est approach (t = 0) are largely due to the prox-
tween the A disk and the HI loop were selected imity of the two disks, and temporary incursions
andmarkedwithplussigns. Thissomewhatcrude into the spherical volumes measured.
procedure misses a number of the inner tail par- At times from t = 0 on, ring-like waves have
ticles, and includes some that are not truly part a significant effect on the total gas curves for the
ofthatstructure,andtravelpastitatlatertimes. primarygalaxiesinalltheHydramodels. Astime
Nonetheless, the procedure is accurate enough for goes on, there is an increase in the mass of gas in
illustrative purposes. The location of the marked the primary core. This is most likely due to com-
particles in the first panel confirms that they are pressionresulting from angular momentum trans-
part of the earliest mass transfer. Already by the port in the spiral waves, and angular momentum
time of the second panel (t = 40 Myr.) they are mixing with transferred gas.
part of a plume located behind the nascent HI The delayed central gas buildup after closest
loop. approach is interesting, and agrees with the ob-
At the time of the second panel the point of servationsofdelayedstarburstsininteractingsys-
origin of the early transfer stream is near the top tems (see e.g., Bernl¨ohr (1993)). The delay is
of the companion disk, on the far side relative to long enough that the tidal transfer streams are
A, and this stream has become very thin (just to much reduced before the central buildup gets un-
the left of the label). In the meantime, a second, derway. Most of the gas transferred to A at the
vigoroustransferstreamhasdevelopedonthecur- earliesttimesfallsontotheouterpartsofthedisk.
rent near side of the companion. The bridge nar- There is some direct transfer from the bridge into
rows with time, and the streams are merging by the central regions at later times, and some ma-
the time of the third panel (t=70 Myr.). In this terial from the outer parts is transferred inward.
viewtheinnerSWtailisvisibleasahorizontalset The transfer history of B is more extreme. The
ofparticleslyingbelowtheprimary,andabovethe ‘bridge’ from A initially misses B almost entirely
long curved contour of the HI loop. inthealternatemodel,andneverreallyleavesthe
The final panel of Figure 15 shows a nearly x- parent galaxy in the best model. In the former
y view at a time near the present, with the fea- case, gas accumulates at the end of this bridge,
ture labeled. In this plot, the plume is offsetfrom and later, falls en masse onto B.
9
3.2.8. Structure Summary VLA21cmobservationsofthissystem(presented
inPaperII).InFigure18weshowthecorrespond-
Table 4 provides summary comparisons of the
ing map for the model. More precisely, the figure
gas masses in different components in the obser-
showscontoursofthe z-componentofthe velocity
vations and the best model at a time of 170Myr
across the x-y plane, and the contour values have
after closest approach. The comparisons are gen-
been scaledto be comparableto the observedval-
erally very good. Specifically, the post-collision
ues as described in the caption. Both Figures 17
model disks contain gas fractions that are quite
and 18 also show the velocity dispersion map in
close to those observed. (Note that about 20%
gray-scale.
of the observed HI flux is not contained in the
TheobservationalcontoursinFigure17arenot
identifiedstructures,sothe modelfiguresmustbe
calculated at column densities less than a fixed
renormalized to compare to observation.) The B
minimum value, and the model contours (and ve-
disk contains about twice the observed fraction,
locitydispersions)areonlycomputedforbinscon-
but this is still much lower than the initial value.
taining at least 5 particles. The fact that these
That is, it has lost 2/3 of its original gas. NGC
cutoffs are notidentical resultsin some minor dif-
7715 is a post-starburst galaxy, so some of its gas
ferences, for example,the contours connectacross
may have been lost in a starburst wind, which is
the bridge in the model plot, but not in the ob-
notmodeled. Alternately,itmaysimply havehad
servationalone. The resolutionof the two plots is
lessgasinitiallythanassumedinthemodel. Even
comparable. Themodelcontoursarecomputedfor
though the A disk receives a substantial amount
databinned onascaleof0.01gridunits, orabout
of mass transfer, its gas fraction is also greatly
1 kpc. The observational effective beam width is
reduced by the collision.
about6′′, orabout1.1kpc,withthe assumedsys-
Thebridgegasfractionofthemodelappearsto
tem distance of 37 Mpc.
besubstantiallylessthanobserved,evenincluding
Thelimitedresolutionandsignaltonoiseinthe
the superposedgalaxy B tail. However,this num-
observations (and models) of the bridge preclude
ber is a very sensitive function of time and of the
any detailed kinematic analysis there. However,
post-collisiondeceleration. Att=100Myrinthis
Figures 17, and 18 do show general agreement
model, the gas fraction in the bridge was nearly
between models and observations in the bridge.
twice the value shown in Table 4. Between these
Specifically, as the contours go from NGC 7714
two times, substantial amounts of gas falls out of
across the bridge they go from fairly horizontal
the bridge (and mostly onto the A disk). If, for
(constant declination), to mixed vertical and hor-
example, the dark halo of A was somewhat less
izontal, and ultimately more vertical.
concentrated than in the model, then the galax-
ies would not have decelerated as quickly after Similarly, in the primary galaxy (NGC 7714),
closestapproach,andwouldhaveseparatedmore, the model and observational contours are similar.
stretchingthebridge,andmakingforalongerfall- However, the LOS velocity range of Figure 18 is
back time. slightly greater than observed. Also the model
galaxy A contours are more horizontal than the
TheHIloopisalittletoomassiveinthemodel,
observational contours, but they tilt more toward
perhaps indicating that the model A disk is too
thevertical,andcompresstogether,atlatertimes.
large. On the other hand, faint parts of this dif-
fusestructurewouldnotbeidentifiedintheobser- The agreement in the companion (NGC 7715)
vations, so the model result may in fact be more isalsoquitegood,withmostlyhorizontalcontours
correct than it first appears. in the bulk of the galaxy, which curve upward on
the bridge side in both Figures 17 and 18. Fig-
3.3. Kinematics ure 18 shows that the velocity range is too large
in the model galaxy. This is probably the com-
3.3.1. Velocity Maps
bined result of several effects. However, because
In this section we compare the model and ob- it is common to all the Hydra models, we suspect
served line-of-sight (LOS) kinematics. Figure 17 that the dominant effect is the compact halo dis-
providestheLOSvelocitymapforthemulti-array tribution.
10
