NGC 4608 and NGC 5701: Barred Galaxies without Disks?

6 Pages·0.41 MB·English

by  D. A. Gadotti

DraftversionFebruary2,2008
PreprinttypesetusingLATEXstyleemulateapjv.11/12/01
NGC 4608 AND NGC 5701: BARRED GALAXIES WITHOUT DISKS?
D. A. Gadotti and R. E. de Souza
DepartamentodeAstronomia,InstitutodeAstronomia,Geof´ısicaeCiˆenciasAtmosf´ericas,
UniversidadedeSa˜oPaulo
RuadoMata˜o,1226–Cid. Univers. CEP05508-900, Sa˜oPaulo–SP,Brasil
[email protected]
[email protected]
Scheduled to The Astrophysical Journal LettersFebruary 1st issue
ABSTRACT
We haveperformedforthe firsttime inthe literaturea2Dstructuralanalysisonthe barredlenticular
3 and face–on galaxies NGC 4608 and NGC 5701. The results indicate that these galaxies either have
0 neverhadlargedisksortheirdiskswerealmostcompletelydestroyedbytheirstrongbars,duetosecular
0
evolution processes. We discuss these surprising conclusions checking for signs of secular evolution,
2
considering bar forming instabilities, and suggesting, based on N-body simulations, a new mechanism

n to form bars in spheroids, which includes non-spherical halos. Quantitative predictions from our new
a mechanism are compared with those from other recent models for bar formation and evolution.
J
Subject headings: galaxies: evolution — galaxies: halos — galaxies: individual (NGC 4608,NGC 5701)
6
— galaxies: structure — methods: N-body simulations

1
v 1. introduction The two galaxies discussed in this Letter were observed
2
in optical (B, V, R, I) and near-infrared (Ks) broadbands
7 Themostwidelyacceptedmechanismtoproducebarsin
in the Steward Observatory 61” and Bok telescopes, re-
0 galaxiesisbasedonaglobalinstabilityincoolstellardisks,
spectively. After standard reduction procedures, the im-
1 where cool means rotationally supported (see, e.g., Bin-
ageswereusedina2Dstructuralanalysisalgorithmdevel-
0 ney & Tremaine (1987)). However, it was already argued
3 by Toomre (1981) that a high central density disk would oped by de Souza (1997). The algorithm was constructed
0 inordertomodelthesurfacebrightnessprofilesofgalaxies
not form a bar since an Inner Lindblad Resonance (ILR)
/ usingtwocomponents: abulgeobeyingtheS´ersicluminos-
h would stop the bar instability (Sellwood 2000). There are
ity profile and an exponential disk. Besides of being thor-
p nevertheless several evidences for the presence of dense
oughly tested in model galaxies, the algorithm has been
- centres and ILR’s in galaxies,including barred ones (Sell-
o already applied in a sample of 39 Sbc galaxies (Gadotti
wood 2001). Recently, Sellwood & Moore (1999) have
r 1999; dos Anjos & Gadotti 2003) and 51 ellipticals and
t numerically confirmed that even dynamically cool disks
s lenticulars(deSouzaetal.2003)providingvaluablestruc-
a can not develop a bar if it has the observed high central
tural information.
: densities. Thus, how galaxies form bars remains a ques-
v The reader is referred to the above cited references for
tion to be properly answered. Possibilities are discussed
i details on the algorithm. Essentially, the galaxy images
X in the papers by Sellwood cited above, but all are easily
are transformed into a matrix in which each point repre-
rejected in the same papers. The problem is even worse
r sents a pixel value. The user provides initial values for
a for lenticular galaxies, since these are not cool stellar sys-
the structuralparametersofbulge anddisk (e.g.,effective
temsandgenerallyspeakinghavethemostmassivebulges
surface brightness, ellipticity, S´ersic index etc.) and the
among disk galaxies in the Hubble sequence. But the ex-
algorithm then tries to fit the galaxy image with model
istence of barred lenticulars is indisputable! For instance,
images of bulge and disk, by varying their structural pa-
in the Revised Shapley–Ames Catalog (Sandage & Tam-
rameters,minimizingtheχ2deviationateachpixeluntilit
mann1981),1/4ofthelenticulargalaxiesbrighterthan14
reaches a convergence limit. A total of 11 parameters are
intheBbandarebarred. Arelatedissueconcernstherole
needed to fully describe the model and these are obtained
ofhalosinbarformation,whichwereoncethoughtto sta-
from the image fitting. Using the fitted parameters, one
bilize disksagainstthe formationofabar. Recent results,
can build a model galaxy and construct a residual image
however, indicate that, while producing smaller growth
which can be useful in showing hidden sub-structures.
ratesofbar-formingmodes,haloscaninducestrongerbars
Animportantpointtoclarifynowisthat,inthe caseof
(Athanassoula 2002, and references therein). In this Let-
strongly barred galaxies,like those studied here, the code
ter, we will present results of a detailed structural anal-
performs a decomposition in 2 components that may be
ysis on two barred lenticulars, which indicate that either
thoughtasthe bar/bulgeandthe disk. The readershould
their disks have almost completely disappeared, probably
keep this in mind when we refer to “bulge” hereafter.
by secular evolution processes, or that bars were formed
NGC 4608iscataloguedasaSB0galaxyinthe Revised
without disks. Moreover, we present numerical simula-
Shapley–Ames Catalog (Sandage & Tammann 1981) and
tions whichshowhowthe laterpossibility canbe attained
by van den Bergh, Pierce & Tully (1990) with CCD im-
with non-spherical halos.
ages. Previous surface photometry analysis was done by
Benedict (1976), with 1D fitting on shallow photographic
2. structural analysis plates,and by Wozniak & Pierce (1991),fitting ellipses to
1
2 Gadotti & de Souza
theisophotesonCCDframes. Bothstudiesconcludedthat between them corresponds to the bulge luminosity, while
this galaxy presents a low surface brightness disk. NGC the luminosity in the residual images may be attributed
5701isdescribedashavinganintermediatemorphological totheassumeddisk. Thuswecalculatedwhatwouldhave
type between SB0 and SBa by a number of authors (see, been a bulge/disk luminosity ratio for these galaxies be-
e.g., Sandage & Tammann (1981)). fore the secular evolution processes took place. This ratio
When applied to NGC 4608 and NGC 5701 the al- is≈2andthereforeitiscompatiblewiththesecularevolu-
gorithm retrieves a very low surface brightness disk. tion hypothesis since the bulge/disk ratio for normal S0’s
The major component is indubitably represented by the is around 2 (Binney & Merrifield 1998).
bar/bulge. Thus one is left to conclude thatthere is prac- While bar formation without disks seems to be a quite
ticallynodiskpresentinthese galaxies. Thepresenceofa unusual possibility, it is not at all trivialthat secular pro-
largeFreemandiskliketheonesobservedinspiralscanbe cesses can be so strong as to destroy a disk almost com-
readily discarded. Figure 1 shows the 2 galaxiesand their pletely. We suggest, in the next section, how bars could
residualimagesaftersubtractionofthemodelimages. The be formed in galaxies without disks.
directimagesatlefthaveanisophotalmapoverlayedwith
0.5 mag interval between each contour level. These im- 3. n-body simulations
agesreferto the Rbroadbandbut similarresults wereob-
tained in all the other bands. It is worth noting that not The idea of having a bar forming without a disk is very
even in Ks a disk was found, which could be a possibility powerful given its simplicity. It is based on the assump-
if the disk was hidden by dust or contained mostly very tionthatnon-sphericaldarkmatterhalosshouldexist. As
old population stars. Our surface photometry reaches the noted by Frenk (1988) model dark halos are generally tri-
level of about 26 mag/arcsec2 in B and 23 mag/arcsec2 axialandmaybe prolate. Moreover,their axialratioscan
in Ks, with comparable values reached in the other pass- bequiteextreme,reachingvaluesaround3,whilearatioof
bands. Thus our photometry is deep enough to indicate 2iscommon. Giventhatthesehalosarelargeandmassive,
that the absence of a disk is real. Even a faint disk would itisreasonablethattheycouldexertastronginfluenceon
be detected. If there is a disk in these galaxies it has to the dynamics of a stellar system which is embedded in
havea luminosity far lowerthanthe onemeasuredinnor- suchahalo. Weperformednumericalsimulationstocheck
mal S0’s. Moreover, we do detect disks in other galaxies if such a configuration could make a spheroid turn into a
with images taken during the same nights, with the same bar, i.e., a triaxial (or prolate) eccentric structure.
exposure times and applying the same analysis. Thus, we simulated the evolution of a Plummer sphere
Looking at the images at left in Fig. 1, one can al- embeddedina rigidhalo representedbya logarithmicpo-
ready notice what seems to be a somewhat empty region tential, which produces flat rotation curves and can be
around the bar in both galaxies. These empty regions ap- usedtoeasilymodifythehalocoreradius,themasswithin
pear much clearer in the residual images at right. In the itandthe haloellipticity (Binney &Tremaine 1987). The
case of NGC 4608 the residual image shows clearly the Plummer sphere has a characteristic radius of 2 kpc, ex-
bar and the lens. For NGC 5701, one is left with the bar, tends to 17 kpc, and a total mass of 1.2×1011M⊙. The
the outer ring and the inner ring, which is not visible in haloparametersmodifiedweretheaxialratios(from1to4,
the direct image. The important point to stress here is prolateandtriaxial),thecoreradius(from6kpcto10kpc)
that, after the subtraction of only a bulge model, there is and the core mass (from 0.9×1011M⊙ to 1.2×1011M⊙).
no clear sign of the presence of a disk. We should remark These values for the core properties are usually found in
thattheseobjectsaregenuineS0galaxiesasmanyauthors the literature (e.g., Begeman et al. (1991)).
have argued and because the conspicuous bars should be The simulations were performed with the nemo pack-
a disk perturbation. Thus, one must face the possibility age(Teuben1995)with105 particles,usingBarnes&Hut
of formationand maintenance of a bar in the almosttotal (1986) algorithm. The softening parameter was ≃ 0.03,
absence of a disk! and the opening angle was 0.7. Energy and the center of
Another possibility is that secular evolutionary pro- mass were conserved better than 0.1%, typically.
cesses induced by bars in their host galaxies (Gadotti & Several experiments were performed varying the halo
dosAnjos2001,andreferencestherein)havedisturbedthe parameters. We noticed that: (i), the sphere is stable
structure of an originally healthy normal disk. In princi- when the axial ratios are low, remaining approximately
ple,thesestrongbarscouldhavetransferredmaterialfrom spherical for any typical values for the core radius and
the initialdiskto the bulge. Then,whatseems nowalens mass; (ii), oval distortions, weak and strong bars are
in NGC 4608 could be the outer remains of the disk. The formed when raising the axial ratios from ≈ 2 to ≈ 4,
outer ring in NGC 5701 could also be the signature of a notdepending substantiallyonthe othercoreparameters;
pre-existent disk. For this galaxy too, there is a hint of and (iii), these results do not depend whether the halo is
inner disk remaining in the residual image. prolate or triaxial. Figure 2 shows a clarifying example.
Totesttheevolutionaryhypothesis,wemayconsiderthe ItshowshowaPlummerspherecanbetransformedintoa
question whether the stars we see today in the bar once bar within the dynamical influence of a triaxial halo with
belonged to a disk. Could the luminosity we detect in axial ratio equals 3, after 1 Gyr. Comparing Fig. 2 with
the bar make up a disk if distributed accordingly? To an- the left panels of Fig. 1, one can see that the bar formed
swerthat, we havedetermined the luminosity inallbands in our simulations is a good representation of the bars in
within a radius equal to the bar length (that was deter- NGC 4608 and NGC 5701. Also, the size of the bars in
mined examining the ellipticity and position angle radial thesegalaxiesandinoursimulationsissimilar(≈10kpc).
profiles)forthedirectandresidualimages. Thedifference Our new bar formation mechanism needs halos with
axial ratios around 3 and the results from cosmological
Barred Galaxies without Disks? 3
simulations by Frenk (1988) indicate that at least some erature (Jungwiert et al. (1997)) and from analysing sin-
halos must have these high values for their axial ratios. gle dish HI velocity measurements (Haynes et al. (1998)).
More recent cosmological simulations by, e.g., Warren et The main difference between the behaviour of the Fourier
al. (1992), show that the axial ratios of halos have a wide componentsintheATHandGDSmodelsisthatthereisa
distributiongoingtovaluesashighas3,whilethe simula- maximumintheformerbutnotinthelater. Thusthedata
tions of Bullock (2001) indicate a typical value of around from NGC 4608 favours the ATH model. The position of
1.5, but with extreme values going as high as 5. Con- the maximum is not relevant here since it varies consider-
cerningobservations,recentresultsindicatevaluesranging ably from one simulation to another, depending on the Q
from 1.25–2 (Buote et al. (2002); Sackett (1999)) to more Toomre parameter (see, e.g., Binney & Tremaine (1987)).
than 3 (Sackett & Sparke (1990)). The maximum is nearer to the center in dynamically cold
systems (Athanassoula, private communication). How-
4. discussion and conclusions
ever,NGC5701doesnotseemtohaveamaximum,which
Therearethustwopossiblescenariosthatcouldexplain supports the GDS model, unless a maximumis found in a
the existence of bars in systems (almost) devoid of discs. verydeepimage. Thereadermustbearinmindthatboth
The first one is that bars can form in diskless systems, models are in no way a specific fit to the galaxies under
as described in the previous section. The second one is study,buttheyshowthatextremecasesofbarredgalaxies
that such systems are extreme examples of the evolution- withhardlyanydiscarepossible. Itisclearthatadefinite
aryscenarioproposedrecentlybyAthanassoula(2003). A conclusion should not be taken at this stage. More data
weak bar forms initially, and grows by losing angular mo- (includingspectroscopy)arenecessaryonthesetwogalax-
mentum to the external disk and the halo, via resonant ies and S0’s in general. On the other hand, simulations
stars(see Athanassoula(2002);Athanassoula& Misiriotis may now be directed to address this question further.
(2002)). In extreme cases it could “consume” all or most Nonetheless,twonotnecessarilymutuallyexclusivecon-
of the disk material, so that the end product would be a clusions can be tentatively postulated: (i) - bars can be
bar in a halo, with very little, if any, disk left, i.e., what formedinspheroidsthroughthedynamicaleffectsofasuf-
we have found for NGC 4608and NGC 5701. Thus, these ficiently eccentric halo, without the need of a stellar disk,
galaxies can well be extreme examples of this evolution- and(ii)-secularevolutioninbarredgalaxiescanbestrong
ary scenario. With this mechanism, strong halos lead to enough to almost completely destroy their disks.
strong bars, which is also the case. The first possibility puts some hope in our struggle to
Let us now evaluate the predictions of the Athanas- understand how bars form in galaxies, since other mech-
soula’smodel(ATH)andourmodelofbarformationwith- anisms have several serious drawbacks. Of course, this
out disk (GDS) and make a quantitative comparisonwith mechanism alone can not be responsible for all the ob-
the results for NGC 4608 and NGC 5701. For that, snap- served bars, since it needs very eccentric halos (and there
shots of the final result from both models were scaled in are disk galaxies!), but could be at least for some barred
size and intensity to allow for a meaningful comparison lenticulars. Also, it not excludes the possibility of other
with the observed galaxies. Figure 3 shows radial profiles bar forming instabilities acting together and thus can be
of the ellipticity of the two galaxies and of both models. exploredtoovercomesomeofthedifficulties. Ontheother
OneseesthatGDSprovidesaverygoodfittothegalaxies hand,itisbasedonverysimplisticnumericalexperiments.
in mostof the profile, while ATH fits reasonablywellonly More realistic simulations can be performed, e.g., includ-
the last fifth part of it. Figure 4 presents intensity cuts ing a live halo and gas, which can then account for other
along the major and minor axes of the bar in the models observedsub-structures,liketheringsinNGC5701. How-
and the galaxies. One can see that these cuts are bet- ever, the essential idea, we showed now, seems to be cor-
ter predicted by the ATH model. Another strong point rect. Thesecondpossibleconclusionshowshowstrongsec-
favouring the secular evolution hypothesis is the fact that ular evolutionary processes may be and so how seriously
a boxy–peanut shape develops in the ATH model when they should be considered in models of galaxy formation
viewededge–on,asweknowisthe caseformanyobserved and evolution.
bars(e.g.,Bureau&Freeman(1999)). Thisdoesnotoccur
in the GDS model.
Another powerful quantitative comparison of the face– We are indebted to the referee, Lia Athanassoula, for
on shapes may be performed if one calculates the Fourier many suggestions and comments that helped to greatly
components of the intensity distribution projected onto improve our work, and for letting us use her results prior
the equatorialplane, like in, e.g., (Athanassoula & Misiri- topublication. ItisapleasuretothankRonButaandIvo
otis 2002). Figure 5 shows the results of the Fourier anal- Buskofor help ondeprojecting galaxyimages andFourier
ysis. An inclination angle of 15 degrees was assumed for analysis. DAG would like to thank Peter Teuben for in-
both galaxies to be deprojected before the components valuable help on nemo. Financial support from FAPESP
werecalculated,inagreementwithvaluesfoundinthe lit- grant 99/07492-7is acknowledged.
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Fig. 1.—Results fromthe structural analysis. Toppanels referto NGC4608 whilebottom panels refertoNGC 5701. Atleftwepresent
directRimageswithanisophotalmap. Atrightweshowresidualimagesafterthesubtractionofabulgemodelonly. Notetheabsenceofa
disk.
Barred Galaxies without Disks? 5
Fig. 2.—FinalstructureformedfromaPlummersphereunderthedynamicalinfluenceofaneccentrichaloafter1Gyr. Comparewiththe
barsintheleftpanelsofFig. 1.
0.8
0.6
a
b/ GDS
1-0.4
N5701
0.2
N4608
ATH
0
0 20 40 60 80 100
R (pix)
Fig. 3.—EllipticityradialprofilesforNGC4608andNGC5701, aswellasforboththeGDSandATHmodels.
I
-40 0 40 -40 0 40
1000 major 1000 major gds
ath
100 100
N4608
10 N5701 10
1000 minor 1000 minor gds
ath
100 100
N4608
10 N5701 10
-40 0 40 -40 0 40 R
Fig. 4.—Intensity profilesalongthemajorandminoraxesoftheobservedgalaxiesandthemodels.
6 Gadotti & de Souza
1.4
N4608 N5701
1.2
1
0.8
0.6
s
t
n 0.4
e
n
o 0.2
p
m
0
o
c ATH GDS

r
e 1.2
i
r
u
o 1
F
0.8
0.6
0.4
0.2
0
0 20 40 60 80 0 20 40 60 80 100
Radius (pixels)
Fig. 5.— Radial profiles of the Fourier components of the observed galaxies and the models. The solid line refers to m = 2, while the
dotted linetom=4,thedashedlinetom=6andthedash–dotted linetom=8.