Kinematic clues to the origins of starless HI clouds : dark galaxies or tidal debris ?

2.2 MB·

by  R. Taylor

Mon.Not.R.Astron.Soc.000,1–??(2016) Printed20January 2017 (MNLATEXstylefilev2.2)
Kinematic clues to the origins of starless HI clouds : dark
galaxies or tidal debris ?
7
1 R. Taylor1∗, J. I. Davies2, P. Jáchym1, O. Keenan2, R. F. Minchin3, J. Palouš1,
0
4 1
2 R. Smith , R. Wünsch

n 1Astronomical Institute of the Czech Academy of Sciences, Bocni II 1401/1a, 141 31 Praha 4
a 2School of Physics & Astronomy, Cardiff University,QueensBuildings, The Parade, Cardiff CF24 3AA, U.K.
J 3Arecibo Observatory, HC03 Box 53995, Arecibo, Puerto Rico 00612
9 4Yonsei University,Graduate School of Earth SystemSciences-Astronomy-Atmospheric Sciences, Yonsei-ro 50, Seoul 120-749, Korea;
1

]
A
2016
G
.
h ABSTRACT
p IsolatedHicloudswithnoopticalcounterpartsareoftentakenasevidenceforgalaxy-
-
galaxy interactions, though an alternative hypothesis is that these are primordial
o
r ‘darkgalaxies’whichhavenotformedstars.Similarly,certainkinematicfeaturesinHi
t streams are also controversial,sometimes taken as evidence of dark galaxies but also
s
a perhaps explicable as the result of harassment. We numerically model the passage
[ of a galaxy through the gravitational field of cluster. The galaxy consists of SPH

particles for the gas and n-bodies for the stars and dark matter, while the cluster
1
includes the gravitationaleffects ofsubstructureusing 400subhalos(the effects ofthe
v
1 intracluster medium are ignored). We find that harassment can indeed produce long
6 Hi streams and these streams can include kinematic features resembling dark galaxy
3 candidates such as VIRGOHI21. We also show that apparent clouds with diameter
5 <20 kpc and velocity widths <50 kms−1 are almost invariably produced in these
0 simulations, making tidal debris a highly probable explanation. In contrast, we show
1. that the frequency of isolatedclouds of the same size but velocitywidth >100kms−1
0 is negligible - making this a very unlikely explanation for the observed clouds in the
7 Virgo cluster with these properties.
1
: Key words: galaxies:evolution - surveys: galaxies.
v
i
X
r
a
1 INTRODUCTION cloudswiththoseparticularpropertiesisextremelydifficult
and the only similar features produced in our simulations
AswedescribedinTaylor et al.(2016),hereafterT16,star-
are transient, seldom lasting longer than 50 Myr.
lesscloudsofneutralatomichydrogen(Hi)appeartobevery
WealsotestedanalternativehypothesisinT16thatthe
rare in the local Universe, but they are not entirely absent.
cloudscouldbeprimordial‘darkgalaxies’:rotatingdiscsof
The leading hypothesis is that themajority of those clouds
gas embedded in dark matter halos. These have been pro-
which do exist are tidal debris produced during galaxy-
posedasapossiblesolutiontothewell-known‘dwarfgalaxy
galaxy encounters, with simulations by Bekkiet al. (2005)
problem’, e.g. Moore et al. (1999), Davies et al. (2004). We
(B05)andDuc& Bournaud(2008)(DB08)supportingthis.
demonstratedthatsuchobjectscaneasilyexplainthesmall
However we also showed that the clouds in the Virgo clus-
size and high velocity widths of the observed clouds, and
terdescribedinTaylor et al.(2012)(T12) andTaylor et al.
would be stable against the effects of harassment both in
(2013) (T13) have properties which make them extremely
terms of disruption and triggering of star formation. This
difficult toexplain inthisway (asimilar cloud was recently
supportsthespeculation that recentlydiscovered ‘ultradif-
reportedinSorgho et al.2016).Specificallytheyareisolated
fuse’ galaxies (which have similar baryonic masses) require
(>100 kpc from the nearest galaxy), compact (<17 kpc in
a high dark matter content in order to survive in cluster
diameter) and of high velocity width (>150 kms−1). Addi-
environments(vanDokkum et al. 2015,Koda et al. 2015).
tionally, there are no indications of any more extended Hi
The nature of the clouds could have important
features in their vicinity. We demonstrated that producing
consequences for cosmological models. The idea that
some dark halos do not accrete enough baryons to
∗ Email:[email protected] form stars has recently been revived in new sim-
2 R. Taylor, J. I. Davies,P. Jáchym, O. Keenan, R. F. Minchin, J. Palouš, R. Smith, R. Wünsch
ulations (e.g. Schayeet al. 2014) and extremely dark the parent galaxies would appear disturbed - an important
matter dominated galaxies appear to be common in parameter since the AGES clouds are all near undisturbed
clusters, e.g. van Dokkumet al. 2015, Koda et al. 2015, galaxies. It also allows us to test if the clouds are optically
van derBurg (2016), Davies, Davies & Keenan (2016), dark or if significant stellar material is also removed, ex-
Muñoz et al. (2015). If, on the other hand, the clouds are amine the possibility that compact clouds may be removed
stripped gas from galaxies, they might explain why so few withouttheformation ofalongerstream,anddirectlymea-
HistreamsarecurrentlyobservedinVirgodespitethelarge suretheir separation from their parent galaxy.
numbers of gas-depleted galaxies (see T16 for a full dis- Our basic procedure is the same as in T16, except in-
cussion). Thus we continue our goal of T16 to establish stead of dropping toy model gas cylinders into a simulated
whether it is more likely that the clouds are primordial or cluster, we now use galaxies on multiple trajectories. Our
non-primordial objects. goalisnottodetermineifthereissomecarefullyfine-tuned
The natureof our simulations in T16 and those of B05 encounter which can reproduce the observed clouds, but
andDB08areslightlydifferent.Thepreviousworksdemon- whether or not such clouds form naturally (i.e. on random
strated that interactions between two galaxies could pro- trajectories), how frequently, and for how long they would
duce starless Hi features by gas stripping directly out of match theparameters of the observed clouds.
the galaxies. Our simulations examined the evolution of a We also investigate whether our simulations produce
stripped gas stream in a cluster environment. We dropped featuressimilar totheobject knownasVIRGOHI21,which
a stream of gas through the simulated gravitational field of is quite similar to the T13 clouds except that it is found
a Virgo-mass cluster (extracted from a n-body cold dark in the middle of a 250 kpc stream (Minchin et al. 2007,
matter (CDM) simulation) on 27 different trajectories, the Hayneset al. 2007). The VIRGOHI21 object is a sharp
aim being to assess the probability (as opposed to the pos- ‘kink’inthevelocitygradientofthestream,promptingspec-
sibility) of producing isolated clouds, especially those with ulationthatitwasakinematicallydistinctdarkgalaxy.The
propertiessimilartotheT13clouds.B05,DB08andT16all DB08 model explains this object as the result of a high ve-
neglect therole of any external medium. locity encounter disturbing the gas of NGC 4254. However,
The results of T16 support the conclusion that some theirmodelofNGC4254wasratherunusual,soitisworth-
isolated clouds can indeed be explained by purely gravita- while to demonstrate if the same results can be obtained
tional interactions. Compact clouds with velocity widths < with more typical spiral galaxies. Thus we investigate two
50 kms−1 were almost ubiquitous in our simulations - but kinds of potentially fake dark galaxies : those embedded in
thosewithwidths>100kms−1wereonlyfound0.2%ofthe long Hi streams, and those which appear to beisolated.
time.WhilstthesimulationsofB05andDB08producedfea- ItisimportanttoemphasisethatasinT16,oursimula-
tureswithhighvelocitywidths,asdiscussedinT16thiswas tions are still very limited, particularly with regards to the
only true for relatively large features > 100 kpc in extent. very important intracluster medium (ICM). This is partly
Thus although it is common to cite B05 and/or DB08 as due to the technical challenge of including the harassing
a possible explanation for starless clouds (e.g. Serra et al. galaxiesandICMinaself-consistentsimulation,andpartly
2013, Cannon et al. 2015), one should be very careful - the becausethepreviousanalyseshavebeeninterpretedtoclaim
velocitygradientofthecloudsisnotatrivialdetailbutapa- that the observed features can be explained solely by tidal
rameterwhichverystronglydetermineswhethertidaldebris interactions. Ultimately we also need to incorporate the
is a likely explanation or not. ICM, heating and cooling, and perhaps gas in the harass-
OursimulationsinT16producedresultsconsistentwith ing galaxies, but it is sensible to increase the complexity of
thoseofB05andDB08,oftenproducinglargefeatureswith thesimulationsgraduallytotryandunderstandwhicheffect
highvelocitywidths.Howeverwealsoemphasisedthatsmall is themost significant.
featureswithsuchwidthsareveryrare,animportantdetail Thethreequestionswewilltryandanswerinthiswork
whichhashithertobeenneglected.Forexample,Wong et al. can be expressed as follows :
(2015)statethatB05andDB08‘demonstratedthatthema- 1) Can the Hi clouds described in T13 be produced by ha-
jorityofgascloudsidentifiedwithnoknownopticalcounter- rassment of a spiral galaxy, and if so, how probable is this
partcanbeeasilyreproducedbygalaxy-galaxyinteractions’. scenario ?
Similarly Pisano et al. (2007), citing B05, state that, “all 2) How probable is the production of a system similar to
claimsofintergalacticHicloudswithoutassociatedstars,or VIRGOHI21 (i.e. with a sharp velocity kink) by harass-
‘darkgalaxies’...haveturnedouttobe,oncloserinspection, ment?
either low surface brightness galaxies... or tidal debris con- 3) How do these results depend on the properties of the
nected with a bright galaxy.” Janowiecki et al. (2015) state harassed spiral galaxy ?
that, “many ‘dark’ galaxy candidates turn out to be tidal The remainder of this paper is organized as follows. In
features”, citing DB08. Ourmodels in T16 agreed with this section2wedescribethesimulationsetupandanalysistech-
interpretation regarding large features but showed that the niques, in section 3 we describe the results, and in section
tidal debris hypothesis has significant problems explaining 4 we discuss ourconclusions. Throughout this paperwe as-
smaller structures. sumeadistancetotheVirgoclusterof17MpcandaHubble
In T16 we modelled thestripped gas as a simple cylin- constant of 71 kms−1 Mpc−1.
der.Thisisclearlyunrealistic,soitispreferabletomodelthe
gasremovalprocessaswellasitssubsequentevolution.Here
wereplacetheinfallinggasstreamwithamodelgalaxy,with
stars, gas and dark matter all represented by particle dis-
tributions.Thisenablesustoquantifiablyestimatewhether
Isolated HI clouds or dark galaxies ? 3
2 NUMERICAL SIMULATIONS
Both our simulation code and analysis methods are de-
scribedindetailinT16.Thereforeweonlybrieflysummarise
them here, except for the description of the spiral galaxy
whichisobviouslyverydifferentfrom thestreamsanddark
galaxies we used in T16.
2.1 Simulation setup
2.1.1 The cluster
Our model of the cluster is identical to that already de-
scribed in T16 and in detail in Smith et al. (2015) and
Warnick & Knebe (2006). Briefly, the original simulation
was a pure n-body CDM simulation using 5123 particles.
The simulation domain was 64 h−1 Mpc on a side, and
the effects of cosmological expansion were included. Re-
simulating with this many particles is prohibitively compu-
tationallyexpensive,sothiswasreducedto400subhalosus-
ingthestructure-findingalgorithmofGill, Knebe& Gibson
(2004). In the simulations described here, each subhalo
was approximated by a Navarro-Frenk-White potential
Figure 1. Distribution of the masses of the NFW subhalos in
(Navarro, Frenk& White 1996) based on their measured
thesimulation.Theopenredhistogramshowsthedistributionat
massandconcentration.Thepropertiesofeachsubhaloare
the beginning of the simulation and the blue hatched histogram
allowed to vary with time based on the original simulation
shows the distribution after 5 Gyr of evolution. Subhalos tend
data. In the interests of computation time we only use 5 tomerge withthe maincluster potential (not shown)over time,
Gyrfrom theoriginal 6.8Gyrofsimulation time.Theover- hence the mass in the substructure is less by the the end of the
all cluster mass and virial radius at z = 0 are compara- simulation.
ble to thereal Virgo cluster (1.1×1014M⊙ and 973 kpc; see
Smith et al. 2015 and references therein); the distributions
of the subhalo masses are shown in figure 1. The velocity
dispersiondependsonthechoiceofthedirectionofmeasure-
ment but along the (arbitrary) x axis averages 485 kms−1,
which compares favourably (though slightly smaller than)
the observed Virgo value of 530 kms−1(Mei et al. 2007).
The time evolution of the velocity dispersion is shown in
figure2.
We then construct a spiral galaxy using the smooth
particle hydrodynamicscode ‘gf’ (Williams & Nelson 1999)
with properties described in the next section. Full de-
tails of the code are given in Williams (1998), see also
Williams & Nelson (1999). As with the streams in T16, we
positionthegalaxyat27initialpositions(atthecornersand
mid-pointsofacube;seeT16figure6).Thesizeofthiscube
isset sothatthegalaxy isinitially ≈500kpcfrom theclus-
tercentre1.Thegalaxythenundergoesradialinfallthrough
theclusterasitevolves.Thusthegalaxyexperiencesagrav-
itational field which is a reasonable approximation to the
overall properties (though of course not the precise details)
of theVirgocluster. Theessentially random trajectories al-
low us to evaluate what should actually happen to an in-
falling galaxy; rather than fine-tuningthe initial conditions Figure2.Evolutionoftherms velocitydispersionofthecluster
toproveaparticular result is possible, we want toestimate alongthreedifferent(arbitrary) axes:xinsolidred,y indotted
theprobability of a given result occurring naturally. blue,andzindashedgreen.Timeismeasuredfromthebeginning
of the simulation. The mean velocity dispersion after the first 1
Gyr (when the velocity dispersion is still rising) is 490 kms−1
along the x axis, 420 kms−1 along the y axis, and 410 kms−1
1 WefoundinT16that infallfrom1Mpcdoes notsignificantly alongthez axis.
affecttheevolutionofagasstream-thesamechangesoccur,with
theonlydifference beingthatthistakeslongerfrom1Mpcthan
500 kpc. However this mightnot bethe case foramore massive
and/ormoreextendedcluster.
4 R. Taylor, J. I. Davies,P. Jáchym, O. Keenan, R. F. Minchin, J. Palouš, R. Smith, R. Wünsch
2.1.2 The infalling spiral galaxy
OurmodelspiralgalaxyconsistsofSPHparticlesforthegas
and n-bodies for the stars and dark matter. For all models
we use 50,000 dark matter particles, initially 20,000 stars
and 20,000 gas particles. The numbers are chosen to give
high enough resolution to study features with the mass of
thedarkcloudsweareinterestedin,whichwould consist of
about100particlesgiventhetotalgasmass(seebelow),but
lowenoughtobeacceptablycomputationallycheap(though
ahigherresolutionwouldbepreferableforreasonsdiscussed
in section 2.2 - this is one advantage of the DB08 model
which used 1,000,000 particles for each component of the
galaxy). As in T16 we use a gravitational softening length
of100pcandthegasissettobeisothermal.Asdiscussedin
T16, this resolution should be sufficient for modelling fea-
tures similar to the T13 clouds, which are likely to be at
least 2 kpc in diameter in order to remain optically dark
(withanupperlimitof17kpcdiameter)asbelowthistheir
column density would exceed the threshold for star forma-
tion2.Thedarkmatterdistributionin thesimulationsisan
isothermal spheretruncated at a radius of 50 kpc.
StarformationisemployedusingaSchmidtlawwithan
index of 1.5. Star formation in gf is not very sophisticated
(see Smith et al. 2010 for details) but is useful to indicate
whethergaswouldremainopticallydarkornot.Forstarfor-
mation to occur thedensity of gas must exceed a threshold
of2.5×10−3M⊙pc−3.Abovethisthresholdstarparticlesare
createdatarategivenbytheSchmidtlawwhilethemassof Figure 3. NGC 4254 Hi contours in white overlaid on an RGB
the associated gas particles is decreased (total mass is con- image from the SDSS. The contours are from a moment 0 map
served). We set the mass of the new star particles to equal integrated over the velocity range 2,200 - 2,600 kms−1, ranging
theoriginal star particles. Ourmodels typically havea star from0.1to1.0Jykms−1instepsof0.1Jykms−1.Theredcross
formation rate of ∼0.6 M⊙yr−1 in isolation. indicatestheHicentreofthegalaxyasdeterminedwithmbspect.
Aposition-velocitydiagramofthissystemisshowninfigure9.
Toensurethediscisstable,weallowallmodelstoevolve
in isolation for 2 Gyr. After some slight redistribution of
mass in the first few hundred megayears, the disc quickly
ral arm clearly indicate that the galaxy has experienced
settles down and the final radial profile remains very close
an unusual event, the nature of that event is controver-
to the initial parameters. Examples of the evolution of the
sial. Sofue et al. (2003) show that the stellar arm (see fig-
radialprofilesareshowninfigures4and5at0,1and2Gyr.
ure 3) could be the result of ram pressure stripping, while
Since the disc has achieved stability by 1 Gyr, we use this
the model of Davies (2008) shows that it could be induced
as theinitial conditions for theharassment runs.
by a low-speed tidal encounter. Signatures of ram pres-
As discussed, our goals are partially motivated by the
sure stripping have been found in the radio continuum by
DB08study,whichattemptedtoreproducetheVIRGOHI21
Kantharia et al.(2008)aswellasinthedisplacementofthe
feature associated with the spiral galaxy NGC 4254. Since Hi and optical discs (figure 3), despite the rather high (1
we wish to see if sucha feature can arise in ourharassment
Mpc)projecteddistancefromtheclustercentre.Thismakes
simulations, we choose NGC 4254 to provide the basic pa-
itespeciallydifficulttodeterminethecharacteristicsofNGC
rametersofourgalaxy,i.e.thegasandstellarmass,circular
4254priortoitsinteraction.Thereforeweusearangeofval-
velocity and radial density profile. However, many of the
uescompatible with theconstraints from theobservations.
parameters of NGC 4254 are not well determined observa-
tionally and it is worthwhile to examine how changing the
Gas in NGC 4254
parametersinfluencestheendresult-inparticular,howthe
Estimates of the gas mass of NGC 4254 vary. Af-
resultschangeusingparameters similar tothoseadoptedin
ter correcting for our assumed distance of 17 Mpc,
DB08comparedtothosemotivateddirectlyfromtheobser-
Davies& Lewis (1973)’s flux measurement gives an Hi
vations (see below).
While its long Hi stream and single prominent spi- mass of 1.0×1010M⊙. More recent values have tended to
be lower : Huchtmeier& Richter (1989) gives 7.0×109M⊙,
whilePhookun, Vogel & Mundy(1993)andsubsequentoth-
2 Also, as in T16, to be self-bound by their Hi gas alone, the ersgivevaluesaround5.0×109M⊙ (e.g.Minchin et al.2007,
Chunget al. 2009, Giovanelli et al. 2008, Chemin et al.
cloudswouldhavetobesosmalltheircolumndensitieswouldbe
severalordersofmagnitudehigherthantheHiinmostgalaxiesso 2016). Since the Westerbork data given in Minchin et al.
they shouldbe star-forming. This means that we are only inter- (2007) iswell-resolved andweusethistodeterminethegas
estedinunboundobjects,sohigherresolutionsimulationswould distribution,weusetheirvalueof5.5×109M⊙,whichisvery
notchange theconclusions. close tothe otherrecent values.
Isolated HI clouds or dark galaxies ? 5
Figure4.Evolutionofthesurfacedensityprofileofthegascom- Figure 5. Evolution of the surface density profile of the stel-
ponentinourM2simulation.Thethickredlineshowstheinitial larcomponent inourM2simulationinisolation,usingthesame
conditions, which are taken from an azimuthally-averaged pro- colourschemeasfigure 4.
fileofNGC4254fromtheWesterbork datacube(Minchinetal.
2007).Thethingreen solidlineshowstheprofileextracted from
the simulation with the galaxy in isolation after 1 Gyr and the Dynamical mass of NGC 4254
dashedbluelineshowstheprofileat2Gyr.Theblackdottedline Dynamical mass is determined by the velocity width of
shows themedianprofile ofall27 simulationsafter 5Gyr inthe the galaxy, which must be corrected for inclination angle.
cluster. Estimates vary widely, to some extent depending on the
component used for the measurement. The lowest estimate
is 20◦ in Chemin et al. (2016) and Makarov et al. (2014),
The molecular gas in the galaxy is substantial, esti- basedonthestellarcomponent;Gavazzi et al.(2003)optical
mated to be between 1.0×109M⊙ (Obreschkow& Rawlings data gives 28◦; Huchtmeier& Richter (1989), Chung et al.
2009) and 6.5×109M⊙ (Wilson et al. 2009); Chemin et al. (2009), Sofue et al. (2003) and Wilson et al. (2009) say 28-
(2016)estimates5.0×109M⊙.Includingthemoleculargasis 30◦ using optical and CO data; Kranz, Slyz& Rix (2003),
notpossible ingf andwould requireextremelyhighresolu- Phookun,Vogel & Mundy(1993)andSofue et al.(2003)all
tion,andinanycasewouldbeunlikelytoaffecttheamount give 41 or 42◦ using NIR or Hi data. These are significant
ofstrippedgassignificantly(sincethemoleculargasismuch variations,causingtheestimateddeprojectedcircularveloc-
morecentrallyconcentratedthantheHibutmuchlessmas- ity to vary by a factor of two, and therefore the dynamical
sive than the stellar component). Since we wish to create mass estimate to varyby a factor of four.
synthetic observations to quantify how much Hi we would The difficulty is that the galaxy is experiencing some
detect in any resulting features (see section 2.2), we choose kind of interaction : not only is there a single prominent
to ignore the molecular gas. spiral arm but the Hi and optical centres are offset (by
The observed Hi shows a roughly flat profile (figure 4) approximately 1′ or 5 kpc), and the Hi extends well be-
within the stellar disc (a radius of approximately 13 kpc) yond the optical disc but only towards the north (see fig-
thenanexponentialdecreaseouttoaround25kpc(thelimit ure 3). The nature of the interaction is unclear (e.g. ram
atwhichtheHicanbemeasuredfrom theWesterborkdata pressure stripping according to Kantharia et al. 2008 and
due to sensitivity). We use this measured profile to set our Sofue et al. 2003; various sorts of tidal encounteraccording
initialprofileforthegas,scalingthesurfacedensityaccord- to Vollmer, Huchtmeier& van Driel 2009, Davies 2008 and
ing to thetotal gas mass used in each of ourmodels. DB08), making it impossible to estimate how it may have
affected the Hi inclination angle and line width. For our
Stars in NGC 4254 standardmodelswemeasureaW20of270 kms−1 from the
As with the gas, estimates for the stellar mass Westerbork data (using the mbspect task from the miriad
vary. Kranz,Slyz & Rix (2003) estimate 2.0-3.5×1010M⊙, data analysis package. This would give a rotation velocity
Chemin et al. (2016) says 4.2×1010M⊙. We estimate a range from 210 kms−1 at 40◦ inclination to 395 kms−1 at
slightly higher value of 6.1×1010M⊙ based on (somewhat 20◦.
crude)SDSSgandibandphotometry(seeT12).Weusethe
profileshowninfigure5,whichwasextractedfromanSDSS Our chosen parameters for NGC 4254
g band image, with a stellar mass of 3.5×1010M⊙ (the up- Themajoruncertaintyintheobservationsistheinclination
perlimitfrom Kranz, Slyz& Rix2003,whoappeartohave angle(andhencecircularvelocityanddynamicalmass),and
donethe most detailed investigation of thisparameter). (arguably) the initial gas distribution. There is not much
6 R. Taylor, J. I. Davies,P. Jáchym, O. Keenan, R. F. Minchin, J. Palouš, R. Smith, R. Wünsch
ously know the gas particle mass, and the Hi line intensity
Table1.Parametersofourmodelgalaxies,asfollows:(1)Name
ofmodel;(2)Himassinsolarmasses;(3)ShapeoftheHiprofile; dependsonly on theHi mass.
(4)Stellarmassinsolarmasses;(4)Deprojectedcircularvelocity Theonlylimitation ofthismethodistheparticlenum-
in kms−1, assuming a W20 of 270 kms−1; (3) Dynamical mass ber. For an AGES-class survey, with a spatial resolution of
in solar masses at 25 kpc radius. For all galaxies we use a total 3.5′, a velocity resolution of 10 kms−1 and a sensitivity of
stellar mass of 3.5×1010M⊙ with a radial profile matching the 0.6 mJy, a S/N of 4.0 (which we set as our threshold for
observations ofNGC4254. detectability) in a single cell would require just 6 particles.
Thus, unfortunately, the detectable3 low-mass features will
Model HiMass HiProfile Vcirc Mdyn bestrongly influenced by individualparticles.
M1 8.5E9 Flat+linear 210 2.6E11 We visualise both the particle and gridded data using
M2 5.5E9 Observed 270 4.2E11 frelled, as described in Taylor (2015). We produce grid-
M3 5.5E9 Observed 395 9.1E11 ded P-V cubes of each simulation for each timestep, and
although these are used in the automatic parameter mea-
surementsit isobviouslyimpractical tovisually inspect the
resulting 5,400 data cubes. Instead, each cube is clipped to
marginforerrorinthegasorstellarmass.Thereforeweuse
removeanythingbelowaS/Nof4.0ineachvelocitychannel,
galaxies corresponding to three different inclination angles
for NGC 4254 : 20, 30 and 40◦ for our M1, M2 and M3 andintegratedalongthevelocityaxistocreateadetectabil-
ity map for each timestep.
models respectively.
While the realtime view in frelled displays particles
Our M2 model represents a typical spiral galaxy, with
as simple points, for the figures they are shown as diffuse
an observationally-derived gas distribution (albeit being
circular Gaussians. A minor change to the frelled code
rather gas rich) and typical circular velocity - we take this
was to make the size of the Gaussians dependent on the
as our standard model. M3 is similar to M2 except for the
highercircularvelocityandcorrespondinglygreaterdynam- SPH kernel size. This greatly enhances the visibility of the
low-densitymaterialwhilepreservingthesmallerstructures
ical mass.
in dense regions, though it can have the adverse affect of
Our M1 model has the lowest inclination angle, giv-
ing a circular velocity similar to that used in DB08. The making the discs appear more disturbed than they really
are.
DB08approachisdifferenttoours,adoptingparametersfor
NGC4254theybelievewereconsistentwiththegalaxyprior
to its interaction. They used a slightly higher gas mass of
8.5×109M⊙ with a more extended density profile - flat to 3 RESULTS
a radius of 25 kpc then linearly decreasing to zero at 30
As the simulations use 90,000 particles in multiple compo-
kpc. We argue that this combination of a more extended
nents, they are considerably more computationally expen-
gas disc with a low dynamical mass may make the galaxy
sivethanthesimulationsofT16. Thereforeweonlyrunthe
more susceptible to gas stripping and thus more readily al-
full set of 27 simulations using our ‘M2’ model, which has
lows the formation of a relatively massive pure gas feature
the parameters of a typical spiral galaxy. We visually in-
such as VIRGOHI21. Therefore we investigate this possi-
spectthesimulationsandselect 9initialpositionswhichwe
bility by using the DB08 gas mass and profile for the M1
then use for our more extreme examples of theM1 and M2
model.
models. These initial positions are selected on the basis of
Thefullparametersaregivenintable1.Inthiswaywe
what happens to the galaxy during its infall : we choose 3
comparetheeffectsofharassmentonnormal(M2),gas-rich
in which there is negligible disturbance to the galaxy, 3 in
(M1),andverymassive(M3)spiralgalaxies.Wenote,how-
which the galaxy is slightly disturbed (i.e. a grand design
ever,thattheM1modelisfarfrom atypicalspiral galaxy:
structure is evident in the disc, and the gas becomes dis-
its gas content would be exceptionally high for a galaxy of
tributed into tails or other structures of comparable size to
this size, and the flat distribution of its gas is atypical (as
theinitialdiscorsmaller), and3inwhichthegalaxyshows
shown in Bigiel & Blitz 2012, which show that gas profiles
arenormallyexponentialratherthanflat).AlthoughtheHi a major disturbance (i.e. with tails longer than 100 kpc).
The idea is to test whether the more extreme cases show
intherealNGC4254is,asdiscussed,slightlyoffsetfromthe
behaviourwhich is significantly different from thestandard
stellardiscandmoreextendedtowardsthenorth,overallits
galaxy.
surface density profile (see figure 4) is not at all unusual -
flat within the stellar disc then decreasing exponentially.
3.1 The M2 model
3.1.1 Global properties
2.2 Simulation analysis
The results of our standard galaxy can be seen in figure 6,
Weusethesame techniquesas thosefully described in T16
which shows theparticle distribution andthegridded data.
section 4.2. Inbrief, while weinspect therawparticle data,
A wide variety of effects can be easily seen from a visual
our main technique is to grid the data to create virtual
position-velocitydatacubes.Sincethesecanbeofarbitrary
resolution and sensitivity we can not only directly compare 3 As discussed in T12 and T13, this is a subjective value owing
the simulations with existing data, but also (in principle) to the typical search methods used - a single-channel 4σ peak
predict what future, higher resolution observations should might be detected by an automatic program, but a human ob-
detect. This is a straightforward procedure since we obvi- serverwouldprobablyrejectthisfromitscatalogue.
Isolated HI clouds or dark galaxies ? 7
inspection. Quite unlike the much less massive discs we ex- bycomparing figures7(b)and (c) - more extremestripping
aminedinT16,someofthesegalaxiesshowverylittledam- eventsoccur more frequently for the gas than for the stars.
agefrombeingintheclusterevenforthefull5Gyr.Others The star formation rate of the galaxy also does not vary
show only minor changes, for example temporary evidence dramatically,withthemediantotalnumberofstarparticles
of grand design structures or short tails which quickly dis- being around 21,500 after 5 Gyr while in the most extreme
sipateorfall backintothedisc. Afewshowratherstronger case this only rises to 22,500 (star formation tends to dom-
disturbances,with tails producedthat are > 100 kpcin ex- inate over stellar removal, at least for the first 2-3 Gyr, so
tentwhichareharassed intoanassortmentofverydifferent the stellar mass in the disc increases). These numbers cor-
shapes. There areoccasionally stellar streams devoidof gas respondtoaveragestarformationratesof0.5-0.9M⊙yr−1.
andgasstreamswithlowstellarcontent,aswellasone-sided While there are a few instances where the star formation
tails and isolated clouds. rate briefly exceeds this, overall the amount of gas affected
Galaxies can move between all three of these phases by harassment is simply not enough to cause any major al-
- long tails eventually disperse and become undetectable, terations to the star formation activity of the galaxy. Ha-
while grand design structures fade in a few rotations of rassment neither induces a significant density increase nor
the galaxy after the perturbation (for a detailed discus- adecrease, thusthestarformation rate(andtheamountof
sion on the formation of spiral arms in galaxy clusters see gas converted into stars) is largely unaltered.
Semczuk,Lokas & del Pino 2016). As a crude estimate, we Theradial profileevolution of thegas andstars can be
madeavisualinspectionofallthesimulationstodetermine seen in figures 4 and 5 respectively. Interestingly, while the
how much time each galaxy spends in each phase. On av- gas profile remains broadly simlar to the initial conditions
erage, galaxies spend about 60% of their time in the quies- evenafter5Gyrinthecluster,thereissignificantevolution
centstate,25% inamildlydisturbedphase,and15% being in the stellar profile. While initially there is a clear change
strongly disturbed. Such extended Hi features are certainly in the gradient at r≈ 9 kpc, by 5 Gyr the profile is well-
not seen in anything like 15% of galaxies in the real Virgo described by an exponential with a scale length of 4 kpc
cluster. As noted in T16, there are around 350 late-type eventor>40kpc.Thischangein thestellar profileisslow
galaxies in Virgo but only four long Hi streams. This sup- and steady over the 5 Gyr. Despite this, according to this
ports our earlier assertion that the process of gas removal modeltherealNGC4254couldhavebeenintheclusterfor
must be more complicated than simply displacing the gas 1-2 Gyr with no discernible effects on its stellar or gaseous
from its parent galaxy. discs.
Theevolutionoftheoverallpropertiesofthesimulated As noted above the morphology of the gas which is
galaxy can be seen in figure 7. The stripped gas mass typ- stripped is another story, with a wide variety of structures
ically reaches 4.0×108M⊙ (increasingly linearly with time, produced which are reminiscent of the results of T16. As
on average, but with a very strong scatter). This is some- in our previous study, isolated clouds are not uncommon -
what higher than in the DB08 simulation (2.0×108M⊙) however,thosewhichmatchthespecificcriteriaoftheAGES
and only slightly less than in the real tail containing VIR- clouds (< 17 kpc diameter, > 100 kpc from the nearest Hi
GOHI21(5.0×108M⊙ according to Hayneset al. 2007), the detection,W50>100kms−1)areagainextremelyrare.The
other known long streams in Virgo (see T16 table 1) or the strippedgasmassiscomparabletothelow-mass streamsin
low-mass streams used in our previous simulations in T16. T16, and consequently the evolution of the streams here is
However it typically takes several gigayears of being in the verysimilar.
clustertostripthismuchmaterial,soitseemsveryunlikely Those isolated clouds which do form have velocity
that a single encountercould beresponsible. widthsduetostreamingmotionsalongthelineofsight(just
All galaxies in these simulations experience some gas as in the B05 and DB08 models, and the low-mass streams
loss, but typically they retain 90-95% of their original gas inT16),ratherthantheself-gravitatingfeaturesseeninthe
content.Theobserveddeficiency ofthesegalaxies neverex- more massive streams of T16. They are generally not truly
ceeds +0.2, even with the (very generous) assumption that isolated - they are slight overdensities in streams which are
theiroriginalgascontentcorrespondstoadeficiencyofzero. just below the AGES sensitivity limit. They are basically
Infact,asdiscussed,theseinitialconditionsrepresentapar- similar in nature to the clouds in the low-mass streams in
ticularlygas-richgalaxywithadeficiencyof-0.5.Theeffects the T16 simulations. This is not surprising since the same
ofharassmentarenowherenearstrongenoughtoexplainthe argument we have mentioned against the observed clouds
strongdeficienciesofmanygalaxiesintherealVirgocluster being self-bound also applies to simulated clouds : at the
(>+0.5 in T12), which likely requires ram-pressure strip- high velocity widths we are interested in, their size would
ping (e.g. Vollmer et al. 2001, Roediger & Brüggen 2007). havetobeso small theircolumn densitieswould besohigh
Rather,theseresultsareconsistentwiththeobservationthat that they would rapidly form stars.
most long streams in Virgo are not associated with highly Asshowninfigure8,onlyforsixtimestepsforthewhole
deficientgalaxies(e.g.Koopmann et al.2008,Minchin et al. run of 27 simulations did any clouds ever equal or exceed a
2007), and those which are have masses insufficient to velocity width of 100 kms−1 (0.1% of the total simulated
explain the presumed parent galaxy’s strong deficiency time). Each simulation timestep is 2.5 Myr though for rea-
(Oosterloo et al. 2005). sons of disk space we only output every tenth interval, so
Perhaps surprisingly, there is not so much differencein each output timestep (which we measure) contains 25 Myr
the typical fraction of gas and stars which are stripped. By of evolution. Thus the six timesteps for which the clouds
theendofthesimulation themedianfractions remainingin are produced is equivalent to a maximum of 150 Myr out
thediscsare92%forthegasbut95%forthestars.However, of the total 135 Gyr (27 simulations each of 5 Gyr). Or, to
thereisconsiderablymorevariationinthegas,asisevident putitanotherway,25ofour27simulationsneverproduced
8 R. Taylor, J. I. Davies,P. Jáchym, O. Keenan, R. F. Minchin, J. Palouš, R. Smith, R. Wünsch
Figure 6. Snapshots of three representative cases of the M2 simulations, each after 4.2 Gyr inthe cluster. Each pair of images shows
theparticle distribution (greyscale) ontheleftwiththe gridded data(each pixel isequivalent to3.5’across) inred ontheright, set to
show anything with a S/N > 4.0 to an AGES-class survey. The green diamond on the far right panel shows an Hi cloud detectable to
AGES at least 100 kpc and 500 kms−1 in projected space/velocity from the nearest other Hi detection. The left pair of images shows
an example of an undisturbed galaxy and the middle panel a moderately disturbed case, with a field of view 120×200 kpc. The right
panelshowsaheavilydisturbedgalaxy,withafieldofview300×500kpc.Thecompletesetofsimulations,withmoviesforthecomplete
5Gyrofevolution,areshowninfigures A6-A7.
(a) (b) (c) (d)
Figure7.EvolutionofthemajorpropertiesofthestarsandgasintheM2model.Inallcaseswethedefine,‘thegalaxy’tobeasphere
of radius 25 kpc centred onthe medianparticle position. Panel (a) shows the gas mass outside the galaxy; (b) shows the measured Hi
deficiency of the galaxy assuming its initial deficiency is zero; (c) shows the total stellar mass within the galaxy; (d) shows the total
stellarmassoutsidethegalaxy.Thethickredlineindicatesthemedianvaluewhilethethincoloured linesshowindividualsimulations.
such clouds at all. Of the two that did, one had high width dividual simulations, isolated clouds (detectable to AGES)
cloudsforjust1%ofthetimewhiletheotherdidsofor2%of with velocity widths <50 kms−1 were present for as much
itsduration.Therewasnevermorethanasingledetectable as 36% of thetime. Clouds with low velocity gradients (i.e.
high width cloud in any simulation at any time, and then totalvelocitywidthperunitphysicalsize) canbeeasily ex-
only very rarely. plained by the tidal debris hypothesis - it’s theclouds with
Once again altering theinitial conditions has madelit- high velocity gradients that thismodel has difficulty with.
tle difference to main result : it is almost impossible to Unlike in T16, we are now able to examine the condi-
produceclouds likethis purely by tidal encounters. Techni- tion of the parent galaxy. We inspected the simulations for
cally these new simulations have made the conclusion from those six timesteps when a cloud has W50 > 50 kms−1. In
T16 stronger, decreasing thefraction of timesteps featuring three cases there were also extended Hi features visible in
AGES-like dark clouds from 0.2% (in T16) to 0.1%. The thesyntheticobservations(spanningthreeormore Arecibo
reasons are that in T16 every simulation contained an ini- beams at S/N > 4.0) that clearly indicated the disturbed
tialstreamandonlyastream,whereasheretheybeginwith natureofthegalaxy,whileintheotherthreecasestheHiof
a galaxy - not every simulation forms a stream, and when thegalaxy was notvisibly perturbed.Thesituation is simi-
they do form, the galaxy is always detectable so it is more lar for the low velocity width clouds (which are much more
difficulttoformanisolatedcloud.Inanycasetheconclusion frequentsoweonlyexaminedasmall randomsubset),with
fromT16wasalreadydecisiveandtheseresultsdonotalter thegalaxy being clearly disturbed in some cases but not in
that. others. Wealso found that theclouds of any velocity width
ItshouldbeemphasisedthatasinT16,itisonlyclouds wouldremainopticallydark,withonlyonecloudhavingtwo
with properties similar to the AGES clouds which are rare. star particles and all therest being puregas.
Cloudswithvelocitywidths<50kms−1aremuchmorecom-
mon, appearing for 9% of the total simulated time. In in-
Isolated HI clouds or dark galaxies ? 9
(a) (b) (c)
Figure8.Evolutionoftheunresolved,isolatedHicloudsasseenwithanAGES-classsurveyfortheM2model.AsinT16weplotonly
theproperties ofthecloudwiththehighestvelocitywidthforeachsimulation(each plottedasadifferent colouredline),sincethehigh
velocity widths appear to be the limitingfactor in reproducing clouds similarto those described in T12 and T13. Panel (a) shows the
detected massinthecloud;(b)showsitssignaltonoiseratio;(c)showsitsvelocitywidth.
3.1.2 Kinky curves as fake dark galaxies a tidal structure is vindicated, though we strongly caution
that this does not necessarily mean this is the true expla-
Wenowturnourattentiontothe‘kinkiness’ofthestreams.
nation (see also section 4). These VIROGHI21-mimics are
As in T16, high velocity width features are common in the transient butcan last for appreciable timescales, ∼250Myr
detectable streams but rare in isolation. One might think
in the case of the example of figure 10(a). It is also worth
that perhapsthis means thereare potentially far more fake
notingthatwesawfeatureswithevenhighervelocitywidths
darkgalaxieslurkinginthestreams,butthisisnotso.Iniso- (> 1,000 kms−1) and sudden gradient changes, but an ob-
lation,asinglehighvelocitywidthfeaturecouldbemistaken
serverwouldprobablynotregardtheseasbeingdarkgalaxy
forrotation.Thisisnotnecessarily truewithinastream :if
candidates. They appear in systems where there are many
the adjacent pixels are of similar velocity width, no part of other Hi streams present and are not isolated features, and
the stream will appear as kinematically distinct, nor would
theirsheerwidthwouldwarnanobserver(iftheywereeven
therebeany reason toassume thestream could potentially
detectable) that they donot resemble galaxies.
fragmentintoseparatehigh-widthfeatures.Rather,itisthe
The origin of these sharp velocity kinks varies and it
change in velocity gradient of the stream that determines
notalwayseasilyattributabletoasingleinteraction.Forin-
whether an observer might mistake a feature for a dark
stanceinfigure10(b),thekinkoccurs asthestream closely
galaxy.
approaches (< 50 kpc) the cluster centre and is strongly
WearguedinT16thattheresultsofDB08donotactu-
accelerated. Thehighvelocitywidth ofthekinkquicklyex-
ally reproduce the steep velocity gradient (or sharp change pands,reaching 1,000 kms−1 in approximately 125 Myr.In
in gradient) of VIRGOHI21, which can be seen in figure
contrast,thefeatureinfigure10(a)hasaquitedifferentori-
9. It therefore seems worthwhile to examine the results of
gin. The overdensity arises as the stream climbs out of the
our own simulations to see if such steep gradients are pro-
cluster potential and begins to fall back in, but there is no
duced.Parametrisingthesharpnessofthechangeingradient
single clear interactor that can be identified as the cause of
isnon-trivial,fortunatelyitisalsounnecessaryforthisinves-
the high velocity width. This feature is rather more persis-
tigation. A cursory glance in position-velocity space at one
tent than that in panel (b),having a similar velocity width
simulation showing a long tail revealed that sharp ‘kinks’ (and small physical size, never being much more than ∼20
in the velocity profile were common. We therefore visually
kpcacross in any dimension) for around 250 Myr.
inspectedallthesimulationsinP-VspacealongsidetheP-V
Neither of the two features discussed is a perfect ana-
diagram of VIRGOHI21 (set to the same scale as the sim-
logueoftherealVIRGOHI21,thoughbothreproducesome
ulations) for reference. Hereweexamined theparticle data,
aspects of the system quite closely. Feature (a) is an over-
not the synthetic observations, as the goal was only to es-
density at the end of a long stream with a sharp change in
tablish if features of this type were produced at all rather
thevelocitystructure.Itisalsofoundataconsiderabledis-
than assess whether they are detectable. We searched only
tancefrom theclustercentre(400 kpc),thoughnot as high
for sharp velocity gradient changes, not precise analogues
astherealVIRGOHI21(1Mpc).Themajordifferencecom-
to the whole NGC 4254/VIRGOHI21 system - as we shall
pared to the real VIRGOHI21 is that the velocity gradient
show, it isnot necessary toproducetheexactP-V shapeof
of the stream does not change sign, however, it could still
theVIRGOHI21stream to create a fake dark galaxy.
appearasaconvincingdarkgalaxycandidate.Feature(b)is
We found that such sharp velocity kinks are common
achangeinthesignofthevelocitygradientofthestream,as
(vergingonubiquitous)whereverlongstreamsareproduced
in the real VIRGOHI21, and the stream is also much more
- we show a couple of examples in figure 10. Thus the no-
asymmetricalthanfeature(a).Themaindifferencefromthe
tionthatthesteepvelocitygradientofVIRGOHI21could be
10 R. Taylor,J.I. Davies,P. Jáchym,O. Keenan,R. F.Minchin,J. Palouš,R. Smith, R.Wünsch
realsystemisthat thedensityof thestream isroughlyuni-
form, so the S/N of the kink would be lower than the rest
of the stream because of its high velocity width - which is
contrary to the observations. It is also very much closer to
the cluster centre than the real VIRGOHI21. Additionally,
both features are at the end of a stream rather than in the
middle as in the real system. We leave modelling a more
exact recreation of VIRGOHI21to a futurework.
3.1.3 Tidal history of the galaxy
One inherent difficulty, already briefly mentioned, of sim-
ulating the gravitational field of both the cluster and its
substructureis that it is very difficult to identify which en-
countersarecausingthegasstrippingandinfluencingthere-
sultingstreams.UnlikeinB05andDB08theparticlegalaxy
is interacting with 400 other subhalos simultaneously, and
often (on visual inspection) there is no single clear interac-
torresponsible.Thesituationisevenmoredifficultandper-
hapsfundamentallyimpossibleforthestrippedgas:streams
(a) (b)
>100 kpc often have different subhalos simultaneously af-
Figure 9. Comparison of the P-V diagrams of the real VIR- fecting different parts of them by different amounts - it is
GOHI21 system (panel (a), taken from the publicly available
not always possible to say that one particular encounter is
Westerbork data cube described inMinchinetal.2007) andthe
responsible, it’s the cumulative effect of many interactions
simulation of DB08 (panel (b), stretching their figure 6 to have
whichcausestheresultingstructures.Therefore,ratherthan
the same aspect ratio as panel (a)). The DB08 model does not
attemptingtodisentanglewhich particularsubhaloislikely
reproduce the verysharp change invelocityseeninthereal sys-
tem or our simulations (figure 10), although their use of a third to be most responsible for the gas stripping and cloud for-
objectdidsuggestthiswaspossible. mation, which is not the goal of the current work (but see
Smith et al.2010foradetaileddiscussion),weparameterise
the interactions in a simple way by examining the objects
thatcomewithin100kpcofthegalaxy.Weshowthedistri-
bution of these in figure11.
Althoughthetimingoftheencountersvariesdepending
on the precise orbit of the galaxy, in broad terms the tidal
history ofthegalaxies tendtobesimilar. Interaction veloc-
ities are typically high, around 1,500 kms−1 though with a
strong scatter. Velocities ∼1,000 kms−1 (as in DB08) are
common,withinteractionsovertherange800kms−1 <v<
1,200kms−1 occuringonaverage(median)8timesforeach
galaxyoverthesimulated5Gyr(hereclassingthewholedu-
rationforwhichasubhalospendswithinthe100kpcradius
asasingleencounter,ratherthaneachtimestepasinfigure
11). Encounters with galaxies as massive as the interloper
inDB08(1.4×1012M⊙)arerare,asexpectedgiventhemass
distribution of the subhalos as shown in figure 1, with the
(a) (b) typical interloper mass being around 1.0×1010M⊙ - lower
Figure10.ExamplesofVIRGOHI21analoguesseeninoursim- than the 1.4×1011M⊙ in B05. DB08-like encounters, with
the same velocity range as above but an interactor mass
ulationsofharasseddiscs,withsimilarvelocitywidthsandsharp
changes of velocity gradients. Panel (a) shows the simulation 1.1×1012M⊙ < M < 1.0×1013M⊙ are very rare, occurring
which began at the initial position -500.0, 0.0, -500.0 kpc from only twice in thefull 27 simulations.
the cluster centre. The tail on the left is truncated by the field DB08-likeencountersarerareprimarilyduetothehigh
of view, it has a similar length to the visible tail but does not mass of the interloper rather than the relative velocity or
show any sharp velocity changes. (b) Shows a system of the ini- close proximity.Encounterssimilar tothose inB05 are also
tialposition0.0,-500.0,-500.0kpcfromtheclustercentre.Unlike rare, but here the limiting factor is proximity. B05 do not
panel(a)thefieldofviewisnottruncated-thesystemhasalong state the interaction velocity, however, we find only three
one-sidedtailsimilartoVIRGOHI21aswellasthesharpvelocity
encounters (at any interaction velocity) within 50 kpc with
kink. interloper masses 1.0×1011M⊙ < M < 5.0×1011M⊙. Both
theinteractionsdescribedinDB08andB05dooccasionally
occur,butarenotcommon.Alargerparameterstudywould
be required to quantify their expected frequency more pre-
cisely.