In this proceeding we make use of the two-dimensional stellar kinematics of a
representative sample of E and S0 galaxies obtained with the SAURON
integral-field spectrograph to reveal that early-type galaxies appear in two
broad flavours, depending on whether they exhibit clear large-scale rotation or
not. We measure the level of rotation via a new parameter LambdaR and use it as
a basis for a new kinematic classification that separates early-type galaxies
into slow and fast rotators. With the aid of broad-band imaging we will
reinforce this finding by comparing our kinematic results to the photometric
properties of these two classes.Comment: 4 pages, 2 figures, to appear in "Pathways Through an Eclectic
  Universe", J. H. Knappen, T. J. Mahoney, and A. Vazedekis (Eds.), ASP Conf.
  Ser., 200

Bacon, R.

Cappellari, M.

Davies, R. L.

de Zeeuw, P. T.

Emsellem, E.

Falcón-Barroso, J.

Krajnovic, D.

Kuntschner, H.

McDermid, R. M.

Peletier, R. F.

Sarzi, M.

van de Ven, G.

English

arXiv

Original paper can be found at: http://www.astrosociety.org/pubs/cs/381.html Copyright ASPIn this proceedings paper we make use of the two-dimensional stel- lar kinematics of a representative sample of E and S0 galaxies obtained with the SAURON integral-field spectrograph to reveal that early-type galaxies appear in two broad flavors, depending on whether they exhibit clear large-scale rotation or not. We measure the level of rotation via a new parameter (λR) and use it as a basis for a new kinematic classification that separates early-type galaxies into slow and fast rotators. With the aid of broad-band imaging we will reinforce this finding by comparing our kinematic results to the photometric properties of these two classes

Falcon-Barroso, J.

Davies, R.L.

de Zeeuw, P.T.

McDermid, R.M.

Peletier, R.F.

University of Hertfordshire Research Archive

Pathways through an Eclectic UniverseASP Conference Series, Vol. 390, c© 2008J. H. Knapen, T. J. Mahoney, and A. Vazdekis, eds.Towards a New Classification of Early-type Galaxies:An Integral-field ViewJesu´s Falco´n-Barroso,1 Roland Bacon,2 Michele Cappellari,3Roger L. Davies,3 P. Tim de Zeeuw,4 Eric Emsellem,2Davor Krajnovic´,3 Harald Kuntschner,5 Richard M. McDermid,4Reynier F. Peletier,6 Marc Sarzi,7 and Glenn van de Ven81European Space and Technology Centre, Keplerlaan 1, NL-2200 AGNoordwijk, The Netherlands2Universite´ de Lyon 1, CRAL, Observatoire de Lyon, 9 av. CharlesAndre´, F-69230 Saint-Genis Laval, France3Sub-Department of Astrophysics, University of Oxford, DenysWilkinson Building, Keble Road, Oxford OX1 3RH, UK4Sterrewacht Leiden, Universiteit Leiden, Postbus 9513, NL-2300 RALeiden, The Netherlands5Space Telescope European Coordinating Facility, European SouthernObservatory, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany6Kapteyn Astronomical Institute, University of Groningen, NL-9700 AVGroningen, The Netherlands7Centre for Astrophysics Research, University of Hertfordshire, Hatﬁeld,Herts AL10 9AB, UK8Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540,USAAbstract. In this proceedings paper we make use of the two-dimensional stel-lar kinematics of a representative sample of E and S0 galaxies obtained with theSAURON integral-ﬁeld spectrograph to reveal that early-type galaxies appear intwo broad ﬂavors, depending on whether they exhibit clear large-scale rotationor not. We measure the level of rotation via a new parameter (λR) and use it asa basis for a new kinematic classiﬁcation that separates early-type galaxies intoslow and fast rotators. With the aid of broad-band imaging we will reinforcethis ﬁnding by comparing our kinematic results to the photometric properties ofthese two classes.1. IntroductionThe origins of the morphological classiﬁcation of galaxies date back from earlywork by Jeans in 1929. The latter addition of the S0 galaxies as a new classby Hubble (1936) resulted in the “tuning-fork” diagram that we know and usetoday. The Hubble classiﬁcation is a continuous sequence between elliptical (E),lenticular (S0), and spiral (S) galaxies, with the S0s occupying the transitionregion. Elliptical and lenticular galaxies are usually gathered into the so-calledearly-type category, given the large number of global photometric propertiesthey share (de Vaucouleurs et al. 1991).227228 Falco´n-Barroso et al.Figure 1. Stellar velocity ﬁelds for four elliptical galaxies in the SAURONsample of early-type galaxies. The maps are sorted from left to right byincreasing value of the λR parameter. The ﬁrst two galaxies (on the left)belong to the slow rotator class, whereas the last two are fast rotators. Notethe diﬀerent kinematic substructures despite belonging to the same ellipticalclass. Numbers on the top right corner of each map indicate the velocity cutsapplied to the data, which have been adjusted so as to properly emphasizethe observed velocity structures.More recently, with the advent of CCD imaging, there have been several at-tempts to revise the current scheme to introduce a more physical description ofthe objects and therefore go beyond a purely descriptive tool. With this goal inmind Kormendy & Bender (1996, hereafter KB96) updated the Hubble sequenceby sorting ellipticals in terms of photometric quantities, used as a proxy for theimportance of rotation. They used the diskiness or boxiness of the isophotes todeﬁne reﬁned types: E(d) galaxies (for disky ellipticals) making the link betweenS0s and E(b) galaxies (for boxy ellipticals). This extension of the Hubble typeshas the merit of upgrading our view of Es and S0s via some easily accessibleobservable parameter that, at the same time, introduces some physics into thesorting criteria. It did, however, use a photometric indicator as an attempt toquantify the dynamical state of the galaxy, which may be deemed unreliable.In this contribution, we revisit the early-type galaxy classiﬁcations above usingthe available full two-dimensional kinematic information coming from the uniquedata set obtained with the SAURON integral-ﬁeld spectrograph. A more com-pelling study of the kinematics for this sample with a discussion of the kinematicclassiﬁcation presented here can be found in Emsellem et al. (2007).2. A Kinematic Classification of GalaxiesThe velocity ﬁelds for our sample of 48 E and S0 galaxies, the basis for thisstudy, were presented in Emsellem et al. (2004). They revealed a large variety ofkinematic structures including decoupled cores, velocity twists, misalignments,cylindrical or disk-like rotation. A close examination of the maps suggests thatearly-type galaxies come in two broad ﬂavors: one which exhibits a clear large-scale and rather regular rotation pattern, and another which shows perturbedvelocity structures (e.g., strong velocity twists) or central kinematically decou-pled components with little rotation in the outer regions. Here, we illustratethese features in Fig. 1 using four elliptical galaxies (NGC4486, NGC4458,NGC4621, NGC2974). From the ﬁgure, it is easy to recognize the large diﬀer-Towards a New Classiﬁcation of Early-type Galaxies 229ences in kinematic substructure between the galaxies, despite their belonging tothe same E morphological class.In order to build a robust classiﬁcation based on the observed kinematics weneed a simple measurable parameter that quantiﬁes the global dynamical stateof a galaxy, and that can be applied to all the galaxies in our sample. Theideal tool would be a physical parameter that captures the spatial informationincluded in the kinematic maps. Since we wish to assess the level of rotationin galaxies, this parameter should follow the nature of the classic V/σ: orderedversus random motion (see Emsellem et al. 2007 for a detailed explanation of thepitfalls of the V/σ parameter as a reliable classiﬁcation parameter). Followingthis idea, we have deﬁned a new quantity λR, such thatλR ≡ 〈R |V |〉〈R√V 2 + σ2〉 ,measures the amount of speciﬁc (projected) angular momentum from the veloc-ity maps. The parameter has been deﬁned such that it is insensitive to smallfeatures in the maps and therefore provides a robust measurement of the globalrotation. As we go from galaxies with low to high λR values (from left to rightin Fig. 1), the overall velocity amplitude naturally tends to increase. More im-portantly, there seems to be a qualitative change in the observed stellar velocitystructures. Rotators with λR < 0.1 exhibit low stellar mean velocities at largeradii with very perturbed stellar kinematics, and all have large-scale kinemat-ically decoupled components. In Fig. 2 (left) we show the distribution of ourgalaxies as a function of λR. There are 36 fast rotators and 12 slow rotators (75%and 25% of the total sample), their median λR values being ∼0.44 and 0.05 re-spectively. Within the class of slow rotators, three galaxies have λR signiﬁcantlybelow 0.03 (their mean stellar velocity maps being consistent with zero rotationeverywhere). These are among the brightest galaxies in our sample (NGC4486,NGC4374, and NGC5846).3. Comparing Photometry and Kinematics: Misalignments andTwistsIn order to assess whether our kinematic classiﬁcation based on the λR param-eter is solid, it is desirable that the structural diﬀerences seen in the kinematicsare also reﬂected in the photometry. In Fig. 2 (right) we show that slow and fastrotators display clear diﬀerences in the global alignment between their photo-metric and kinematic major axes (Ψ ≡ |PAphot − PAkin|). The ﬁgure illustratesthat all fast rotators, except one, have misalignments Ψ below 10◦. The onlyexception is NGC474, which is an interacting galaxy with well-known irregularshells (Turnbull, Bridges, & Carter 1999). In fact, the few galaxies that have5◦ < Ψ < 10◦ (NGC3377, NGC3384, NGC4382, NGC4477, and NGC7332)are most probably barred. In contrast, more than half of all slow rotators haveΨ > 10◦, and none of these exhibits any hint of a bar. This diﬀerence in themisalignment values of slow and fast rotators cannot be entirely due to the ef-fect of inclination, mainly because even the roundest fast rotators do not exhibitlarge misalignment values.230 Falco´n-Barroso et al.Figure 2. Left: Distribution of galaxies as a function of λR. The hori-zontally striped bar indicates the bin for slow rotators, whereas diagonallystriped histogram corresponds to fast rotators. Right: λR versus thekinematic misalignment Ψ between the global photometric major-axis andthe kinematic axis within the SAURON ﬁeld. Slow rotators are representedby ﬁlled circles, and fast rotators by solid triangles. The vertical dashed linescorrespond to Ψ = 5◦. Nearly all fast rotators have small Ψ values (< 10◦),the only exception being NGC474, the photometry of which is perturbedby the presence of irregular shells. This contrasts with slow rotators, whichshow signiﬁcantly non-zero Ψ values.Independent evidence in support for our kinematic classiﬁcation comes fromthe observed velocity twists in the SAURON kinematic maps: only six out of48 galaxies exhibit strong velocity twists larger than 30◦ outside the inner 3′′,with three out of these having large-scale counter-rotating stellar components(NGC3414, NGC3608, and NGC4550). All these galaxies are in fact slow rota-tors. This implies that only fast rotators have a relatively well deﬁned (apparent)kinematic major-axis, which in addition is roughly aligned with the photometricmajor axis. As emphasized above, slow and fast rotators exhibit qualitativelybut also quantitatively diﬀerent kinematic properties. This suggests that slowrotators are not just scaled down versions of fast rotators.Acknowledgments. JFB would like to thank the organizers for a veryfruitful meeting and wishes to congratulate J. Beckman for his outstanding con-tribution to astronomy in particular, but science in general, over the last 40years.Referencesde Vaucouleurs, G., de Vaucouleurs, A., Corwin Jr, H. G., Buta, R. J., Paturel, G., &Fouque, P. 1991, Third Reference Catalogue of Bright Galaxies (Springer-Verlag:Berlin)Emsellem, E., et al. 2004, MNRAS, 352, 721Emsellem, E., et al. 2007, MNRAS, 379, 401Hubble, E. 1936, The Realm of the Nebulae (New Haven: Yale University Press)Kormendy, J., & Bender, R. 1996, ApJ, 464, L119Turnbull, A. J., Bridges, T. J., & Carter, D. 1999, MNRAS, 307, 967Towards a New Classiﬁcation of Early-type Galaxies 231DiscussionKnapen: Can you indicate to what extent SAURON results for ellipticals arerepresentative for the general population?Falco´n-Barroso: This is diﬃcult to answer properly. As you know, the SAURONsurvey selection criteria intended to obtain a representative sample for the E/S0population. Then, later on, we found the result I have summarized today onthe slow/fast rotator classes. Since we did not expect to ﬁnd this result, it isdiﬃcult to predict what the impact is of our selection on the intrinsic distributionof slow/fast rotators. It is precisely for that reason that we have started a newproject, i.e., ATLAS3D, where we will observe a complete sample of galaxies.This new sample should be free of any particular bias and will give us the truefraction of slow/fast rotators in the nearby Universe.Erwin: 1) What happens if an S0 galaxy, which ought to be a fast rotator, isviewed close to face on? Does it turn into a slow rotator? 2) You have madea good case for a distinction between slow and fast rotators. But is not therestill a confusion within the fast rotator class, in that it’s a mixed bag of whatmorphologists would call ellipticals and S0s?Falco´n-Barroso: 1) This is indeed an obvious question to ask. Since ΛR isa projected quantity one may wonder what the dependence on inclination is.To understand this dependence we have built two-integral models and see howΛR varies with inclination. What we saw is that a galaxy with a measuredΛR = 0.05, typical for a slow rotator, would need to be at a nearly face-oninclination (∼20 degrees) to reach an intrinsic ΛR value ∼0.15 and thereforebecome a fast rotator. The probability that slow rotators are truly fast rotatorsis small and cannot explain the current population we see. 2) That is correct,however, remember that the slow/fast rotator classiﬁcation is able to separateintrinsic properties of these galaxies unambiguously (something that cannot bedone at present with the E/S0 classiﬁcation). This is in fact the reason for ournew scheme.From left to right: Sergio Fernandez Acosta, Gelys Trancho, and Jesu´s Falco´n-Barroso.

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Towards a New Classification of Early-type Galaxies: An Integral-field View

Towards a new classification of early-type galaxies: an integral-field
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Towards a new classification of early-type galaxies: an integral-field view

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